tag:blogger.com,1999:blog-21039608343610553222024-02-19T10:40:59.991-08:00Dawn's Sed Strat Lecture NotesThis blog includes my lecture notes for sedimentology and stratigraphy related classes. The focus right now is on UCDavis GEL109.Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.comBlogger53125tag:blogger.com,1999:blog-2103960834361055322.post-66002372616658471692013-09-03T22:47:00.000-07:002013-09-03T22:47:01.318-07:00Beautiful Standing Waves / Antidunes!Just think about the sediment transport as you watch the fun!<br />
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http://www.dailymotion.com/video/xgtvdp_unbelievable-surf-video-how-to-create-a-standing-wave_sport<br />
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<br />Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com2tag:blogger.com,1999:blog-2103960834361055322.post-42126846700663734302013-03-11T12:27:00.001-07:002013-03-11T12:27:51.023-07:00ChronostratigraphyThe idea behind chronostratigraphy is to correlate rocks that formed at the same time. This is useful for reconstructing events and depositional environments in earth history as well as finding resources like oil. There are several techniques that can be used for chronostratigraphy, including: event stratigraphy, magnetostratigraphy, chemostratigraphy, biostratigraphy, and sequence stratigraphy. Here, I will address event stratigraphy, magnetostratigraphy, and biostratigraphy. Sequence stratigraphy is very powerful, and lots of resources on it can be found at: <a href="http://sepmstrata.org/">http://sepmstrata.org/</a><br />
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<b>Event Stratigraphy</b><br />
Event stratigraphy involves identifying the sedimentary effects of an unusual event in multiple stratigraphic columns. If one can demonstrate the the effects were all produced by the same even, one can reasonably interpret the effects to have happened at the same time in the different columns. For example, if a volcano erupts and deposits ash over a broad region, that ash is preserved in the stratigraphy, and a geologist can demonstrate that the ash in multiple sections came from the same eruption, then the geologist can create a chronostratigraphic correlation among the sections. Other events that are useful for event stratigraphy can include impact debris layers (for example at the Cretaceous-Tertiary boundary), tsunami deposits, and sometimes large storms. <br />
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There are some shortcomings of event stratigraphy. First, there has to be an event that affects the stratigraphy. For it to be useful, it needs to be something that affects multiple depositional environments in a way that produces a distinctive set of features that can be distinguished from the normal depositional processes. Second, for a specific event to be useful for correlations, it has to have affected the stratigraphy at the sites of interest. For example, a tsunami that affected the west coast of North America might help one correlate Pleistocene coastal deposits in Oregon and Washington. However, it would not be helpful for correlating Pleistocene rocks in Florida because it did not influence them. Third, if there are multiple events, the geologist has to sort out which correlate with each other. For example, if there are multiple volcanic eruptions at different times, the geologist needs to evaluate which eruptions the ash beds might represent. It can become complicated to correlate many events. Sometimes correlations are more reliable if there are fewer events, but then there are not as many potential temporal ties between the stratigraphic columns. Even with these complications, event stratigraphy is a very valuable tool. When the events are volcanic, the ash beds can often be dated, providing a precise age for a segment of the stratigraphic column.<br />
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<b>Magnetostratigraphy</b><br />
Magnetostratigraphy uses preserved magnetization of rocks for correlation. The magnetization comes from the alignment of magnetic minerals in sedimentary rocks (and other types of rocks) with the earth’s magnetic field. Small magnetic minerals, especially clay-sized minerals, align like little magnets, and when the sediment is lithified, that magnetization can be preserved. Under the right conditions, samples can be collected and the direction of magnetization measured. Data can be used to reconstruct the direction of the earth’s magnetic field. This magnetic field can reverse directions due to the dynamics of circulation in the core. In other words, sometimes the magnetic field is aligned such that magnets point north (as they do now, and called “normal” in the scientific literature) and sometimes it is aligned such that magnets point south (called “reversed”). The earth’s magnetic field changes at close to the same time globally, so the effects are seen everywhere. Paleomagnetists have studied well dated sedimentary and volcanic rocks and have mapped out the times in earth history where the magnetic field was normal and reversed (see: <a href="http://www.geosociety.org/science/timescale/">http://www.geosociety.org/science/timescale/</a>). This provides a reference that can help correlate other stratigraphic sections.<br />
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To correlate a suite of stratigraphic sections using magnetostratigraphy, one would collect samples, measure their magnetic properties using a variety of techniques, and evaluate whether or not they have been remagnetized. If they have not been remagnetized, changes in the direction of earth’s magnetic field can often be interpreted from the results. If a geologist has multiple sections that were deposited at the same time, they can interpret the changes in the direction of earth’s magnetic field to have happened at the same time. Unfortunately, however, one can not necessarily independently tell the many normal intervals apart from each other nor the many reversed intervals apart. Thus, the geologist needs additional information to make reliable chronostratigraphic correlations.<br />
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<b>Biostratigraphy</b><br />
Biostratigraphy is an extremely powerful tool for chronostratigraphic correlation. Life evolves through time, with new species emerging and other species going extinct. For time intervals and species that are well studied, the process of evolution provides a detailed temporal framework for correlating stratigraphic columns. The basic idea is for the geologist to identify fossils in the stratigraphic columns, compare them to the ranges of those organisms know from previous studies, and then interpret the age of the rocks from documented extinction and species origination events. This is an extremely powerful approach to correlating stratigraphic columns because each species is unique and changes through time. However, not all organisms are useful for biostratigraphy. <br />
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Good biostratigraphic species: 1) have short geological ranges, e.g. they did not live for millions of years, and evolved quickly; 2) were distributed over a large region of the earth; 3) were easily preserved; and 4) were abundant. They also need to be well studied.<br />
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Zones of well documented species with distinct origination and extinction times can be defined a number of ways. A zone could consist of the total time of existence of a fossil, it could consist of the time where two or more fossils coexist, it could be defined as the time between the origination of one fossil and the extinction of a different fossil, etc. An example of a biostratigraphic zone chart, combined with magnetostratigraphic reversals can be found at: <a href="http://www-odp.tamu.edu/publications/189_IR/chap_02/c2_f6.htm">http://www-odp.tamu.edu/publications/189_IR/chap_02/c2_f6.htm</a><br />
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<br />Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-51796855996851336312013-03-06T12:37:00.000-08:002013-03-06T12:37:03.880-08:00Interpreting Stratigraphic Columns<b>Step 1: Look for sedimentary structures that are characteristic of a specific environment or process</b><br />
Examples: <br />
<list><br />
</list><br />
<li>hummocky cross stratification - waves plus currents (storms)<br />
</li>
<li>wave ripples (vs current ripples) - waves (standing water)<br />
</li>
<li>herringbone cross stratification - bidirectional flow over hours or longer (tides)<br />
</li>
<li>reactivation surfaces - reshaping of bedforms due to changes in flow (tides)<br />
</li>
<li>mud drapes in sandstone - flow stops (tides)<br />
</li>
<li>bouma sequence - rapid flow slowing down (turbidity current)<br />
</li>
<li>mud cracks - mud contracts (exposed to air)<br />
</li>
<li>root casts - from plants, usually land plants (land)<br />
</li>
<li>faint ripple cross lamination with reverse grading - (eolian ripples)<br />
</li>
<li>meter-high dunes in fine sand - (eolian dunes)<br />
</li>
<li>diamictites - laminar flow - (debris flows, mud flows, melting ice)<br />
<list><br />
</list></li>
<li>with facetted clasts and striations - (glacial)<br />
</li>
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<li>lone (or drop) stones in laminated shale - large grains rafted over quiet environment (icebergs; trees possible)<br />
</li>
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<b>Step 2: Evaluate how these distinctive structures relate to each other in the stratigraphic column to develop a tentative environmental interpretation</b><br />
<list><br />
</list><br />
<li>Are there several indicators of waves or storms?<br />
</li>
<li>Are there several indicators of tides?<br />
</li>
<li>Are there several indicators of wind-deposited sediment?<br />
</li>
<li>Are there several indicators of glacial activity?<br />
</li>
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<b>Step 3: Compare the tentative interpretation to flow implied by other sedimentary structures in the column and evaluate whether they are consistent with your tentative environmental interpretation.</b><br />
Examples of other sedimentary structures:<br />
<list><br />
</list><br />
<li>Trough cross stratification<br />
</li>
<li>Planar cross stratification<br />
</li>
<li>Current ripple cross lamination<br />
</li>
<li>Planar lamination<br />
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<b>Step 4: Evaluate how the vertical sequence of sedimentary structures changes to refine or correct your environmental interpretations.</b><br />
Do structures occur in a distinctive pattern that suggests a depositional environment?<br />
<list><br />
</list></li>
<li>Is there an erosion surface followed by dune stratification followed by ripple lamination followed by a rooted horizon? (Then it might be migrating river channels or tidal channels if there are indicators of tidal currents.)<br />
</li>
<li>Do the structures suggest an environment that shallows upward into a river system? (Then it might be a delta building out into standing water.)<br />
</li>
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<b>Step 5: Use Walther's Law to refine your environmental interpretations and to test whether or not they are reasonable. </b><br />
Try to sketch neighboring environments and interpret how they shifted through time. Are your interpreted vertical changes in environments consistent with neighboring environments horizontally? Does your interpretation require any jumps in environments or imply an unconformity? Revise your interpretation until it is consistent with your data. <br />
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Often, there is some ambiguity about the depositional environment(s) represented in real rocks. By going through this process, you can reach a reasonable interpretation that is well supported by the data. You will also understand where the ambiguities are. This is particularly helpful if it is your own data and you can make more observations by doing more field work. <br />
Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-15476256773211416862013-03-04T12:38:00.002-08:002013-03-04T12:38:50.728-08:00Carbonates - A very brief introductionCarbonate rocks form from ions in seawater. Thus, their deposition and accumulation is somewhat different than it is for siliciclastic sediments. They do not require that sediment is transported into the environment. Rather, they require specific chemical, temperature, and biological conditions in the environment where they form.<br />
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Most carbonates (during Phanerozoic time) are created by living organisms as shells and skeletons. (During Precambrian time, microbial communities strongly influenced carbonate mineral precipitation.) Corals, snails, clams, etc. are good examples. The reaction to form the carbonate minerals calcite or aragonite (which have the same mineral formula) is:<br />
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Ca<sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> = CaCO<sub>3</sub><br />
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Because HCO<sub>3</sub><sup>-</sup> is more abundant in seawater than CO<sub>3</sub><sup>2-</sup>, the actual reaction that takes place is:<br />
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Ca<sup>2+</sup> + 2HCO<sub>3</sub><sup>-</sup> = CaCO<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub><br />
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The = means the reaction can go either way. If it goes from left to right, calcium carbonate minerals - calcite or aragonite - form. If it goes to the left, calcium carbonate minerals dissolve.) The chemical reaction to form the minerals is most likely in warm water, particularly in warm agitated waters. Breaking waves help get rid of the CO<sub>2</sub> produced by the reaction, which makes the reaction proceed even faster. Also, some corals contain photosynthetic organisms within their tissues, and those organisms consume CO<sub>2</sub>, which also helps with the mineralization process. Thus, carbonate minerals form in warm, shallow seawater. The accumulation of carbonates creates “carbonate platforms” around many tropical islands (e.g. Caribbean islands, Bahamas, Hawaii, etc.) and along tropical shorelines (Florida, Great Barrier Reef of Australia, etc.). <br />
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One major source of carbonate sediments is from calcifying algae. These organisms are very abundant. They produce the minerals as sand or mud grains (depending on the species) within their tissues. When the algae die, the carbonate grains are released into the environment to be transported by waves and currents. At this point, the grains behave more or less the same as siliciclastic grains, with coarser sediment requiring high flow speeds to be transported, and mud-sized grains requiring very low flow speeds to settle from suspension. In shallow environments, muds accumulate in deeper areas of lagoons or get transported off shore into deeper waters. The grains get concentrated into shoals where water speeds slow down, for example, where water is channeled through a reef into a lagoon. These grains can also grow through carbonate mineral precipitation forming coated grains, or ooids. Thus, they get coarser with time.<br />
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A second distinctive feature of carbonates is the growth of reefs. Corals and other skeletal organisms grow well in high energy zones with breaking waves. Their skeletons make them resistant to erosion, and the breaking waves enhance carbonate mineral formation. Also, the precipitation of more carbonate as cements makes the structures hard and very resistant to erosion even though they are in high energy zones with breaking waves. These reef ecosystems can grow very quickly, creating a topographic high located off shore. This high induces more breaking waves, changing the energy distribution across the carbonate platform. The distribution of grain sizes around a reef depend on the flow speeds, similar to the dependence for siliciclastic grains, but the reef itself is cemented in place and provides a unique environment. Grains that are broken off tend to be transported to the inside or outside of the reef where water depths increase and flow speeds slow.<br />
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As reefs grow upward, they create very steep slopes, sometimes almost vertical slopes. These slopes can be unstable long-term, and they can fail, creating breccia in deep water and inducing turbidites as in siliciclastic sediments.<br />
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See figure 15.12 in Nichols, Edition 2 for the distribution of environments across a reef. Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-22426564639581882902013-02-20T12:19:00.000-08:002013-02-20T12:19:21.649-08:00Marine ShorelinesShorelines are the interface between the land and the oceans. Their characteristics vary depending on the balance of sediment supply and transport processes. When the sediment supply from rivers is large compared to the rate at which transport processes redistribute the sediment, deltas form, building out into the ocean. If sediment supply is low compared to the rate of sediment transport seaward of the shoreline, the shoreline erodes back. When sea level rises, river valleys can become flooded with marine water, creating estuaries. When sea level falls, rivers tend to erode downward into the previously coastal sediments. <br />
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The balance between tides and waves also affects the geometry of shorelines. Wave-dominated shorelines tend to have beaches, whereas tide-dominated shorelines tend to have broad marshy flats. Either can be erosional if the offshore transport of sediment is higher than the sediment supply or constructional if offshore transport is lower. They can shift back and forth through time if sediment supply or transport processes change. Thus, most shorelines are dynamic environments that vary significantly on human time scales.<br />
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<b>Wave Influenced Shorelines</b><br />
Waves have very specific sediment transport characteristics, with the highest energy flows near the breaker zone and lower flows both onshore and offshore. The onshore flows transport sediments to form beaches. The swash zone is the area that forms the primary beach. During storms, the waves are commonly higher, and, if sufficient sediment is available, they carry sediment farther up the beach, creating a berm. This gives the beach a characteristic slope up away from the shore, a crest, and then a slope downward. In some cases, the beach can extend off the coastline, creating a barrier bar or barrier island. A lagoon then forms between the beach and the main coastline. When there is a large sand supply, these barrier bars and islands can grow to be quite large. However, waves also transport sand off shore, going from the high energy breaker zone to the lower energy deep water. If the sand supply is low, more sand can get transported offshore than is delivered to the beaches. This causes beaches, barrier bars, and barrier islands to erode. <br />
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<b>Tide Influenced Shorelines</b><br />
Tidal currents flow on and off shore every day or twice a day. When tidal ranges are high, tidal currents can be strong, redistributing sediment either onshore or offshore. These tidal currents often become channelized, and they begin to act like rivers, with meanders, etc. <br />
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<b>Constructional Shorelines: Deltas</b><br />
<i>Deltas</i> form at the mouths of rivers that transport enough sediment to build outward. (<i>Building outward</i> is a key component of the definition of a delta. Rivers where the ocean or lake floods the river valley flow into <i>estuaries</i>.) Deltas require substantial accumulation of sediment, in contrast to estuaries which do not build outward. Sedimentary facies are similar to other depositional environments, but the association of subenvironments are recognizable as deltas. Some of the sub environments include: river facies with all the associated sub environments; shore line deposits including beaches, marshes/swamps, etc.; submarine shelf and slope facies, including storm deposits and turbidites; etc. <br />
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I will draw cross section and map views of a delta showing the delta plane, delta slopes, and prodelta. Rivers flow through delta planes and slow when reaching water, producing a mouth bar. Grain size decreases with distance away from the river mouth.<br />
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<b>Progradation</b> - Because deltas are sites of sediment building outward from the coast, they are progradational; the landward depositional environments move seaward over more marine/lacustrine deposits. Thus, delta sequences in the rock record start with deep water, marine, fine grained sediments and grade upward into shallower water, possible more freshwater, coarser grained sediments. This is one of the distinguishing aspects of deltas that let you define them in the sedimentary record. These changes in grain size and environment typically occur over 1’s to 100’s of meters in the rock record and include many beds.<br />
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<b>Sediment Transport Type</b> - All deltas (by definition) have their sediment transported to the delta by rivers. Thus, riverine deposits are always associated with them. In addition, depending on marine (or lacustrine) conditions, waves and tides can redistribute the riverine sediment changing the morphology and facies of deltas. There are three main end member categories of deltas when characterized by processes: 1) River dominated; 2) Wave influenced; and 3) Tide influenced.<br />
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<b>River Dominated Deltas</b> - River dominated deltas have very low wave energy and a very small tidal range. Delta top deposits are well developed and are very similar to meandering river deposits, including channel, levees and overbank deposits. Overbank areas are commonly heavily vegetated and result in peat and coal deposition. Channels build out into the ocean (or lake) on top of their mouth bars. This leads to a coarsening upwards of grain sizes within the mouth bars as well as a change from some marine processes to unidirectional river flow. Avulsion of the rivers is common due to low gradients on the delta plain. Lobes of the delta become abandoned creating a “bird’s foot delta”. Sheltered bays are common between the lobes, and are filled with overbank deposits from floods as well as marshy deposits. The Mississippi River Delta is a classic river dominated delta.<br />
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<b>Wave Influenced Deltas</b> - Waves redistribute the sediment deposited by the rivers. Progradation of channels is limited because mouth bars are reworked by waves into shore parallel sand bars and beaches. Spits of sand are also common. The waves sort the sediment better than rivers and, if the grains are not already well rounded, the waves will round them. The big differences for wave influenced deltas are that beach facies are abundant and channel fill and overbank facies are less common. The Niger River Delta is a wave influenced delta.<br />
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<b>Tide Influenced Deltas</b> - Tides rework sands into elongate bars perpendicular to shore (vs. waves). These bars are analogous to mouth bars, but they contain tidal sedimentary characteristics including bi-directional flow indicators and slack tide mud drapes. Overbank areas can include tidal flats. The Ganges-Bramhaputra delta in Bangladesh is a tide dominated delta. <br />
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<b>Constructional Shorelines: Coastal Planes</b><br />
Coastal planes are broad areas where there is sufficient sediment for the land to build seaward, but it is not localized at a single delta mouth. Examples of coastal planes include the Everglades area of Florida and the coast of the Carolinas. Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-22211704988676827192013-02-20T12:16:00.000-08:002013-02-20T12:16:19.545-08:00Marine Processes<b>Marine Deposition -</b> Most of Earth is covered with oceans, there is abundant life in the oceans, most sediments eventually get transported into the oceans, and shallow marine deposits are the most abundant in the in sedimentary record due to their large volume and the low erosion rates in shallow marine environments. You need tectonics to uplift them above sea level to get significant erosion. This happens commonly, so that we can also see them exposed. <br />
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<b>Processes -</b> Several processes are unique to shallow marine deposition (and some large lakes): Waves, storms, and tides<br />
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<b>Waves -</b> Waves have oscillating current directions every few seconds. The flow in both directions is equal in deep water, but not necessarily near shore. Draw a picture of wave water motion. (Water at the top of the wave moves in the direction the waves move.)<br />
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<i>Wave Ripples -</i> Wave ripples are like current ripples, except that they experience transport in both directions. Draw a picture with the laminar boundary layer, etc. At low flow, the boundary layer doesn’t have enough speed or momentum to remove the crest of the ripple and deposition of the grains that are moved are deposited right on the upper part of the lee slope. Thus, crests are sharp. At higher flow, the crests erode due to the higher speeds and momentum and deposition occurs farther down the lee slope. Thus, high flow ripples have rounded crests. Wave ripples can be recognized in rocks by their symmetric shape (if flow in each direction is the same speed) and most importantly, the presence of x-laminations dipping in two directions. This is the truly distinctive feature and can be present even if the ripples are not very symmetric. <br />
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In shallow water, currents along the bottom from the waves are strong enough to flatten out the ripples, but they are not consistent enough in one direction to form dunes. Thus, the sedimentary surface tends to be planar or broadly scalloped as the waves are focused into certain areas. This produces a flat lamination (not upper planar lamination) where waves are in very shallow water relative to their height, e.g. from the breaker zone towards the shore.<br />
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KEY POINT FOR WAVES: Bi-directional flow every few seconds<br />
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<b>Storms -</b> Storms produce both large waves and strong, irregular currents. The combination and interference of these produces some unique deposits which can be used to recognize the importance of storms in a given marine sequence. Storms generally start far from shore and can approach through time. Then they either die out or move on. Thus, deposits that storms affect, i.e. those on continental shelves, tend to start out with low energy flows, increase to erosional (if strong enough) and then decrease back to lower energy flows. For example, sharp crested wave ripples might transition into round crested wave ripples, followed by cross stratification due to large waves and strong currents, followed by erosion, deposition of the coarsest sediment, and a reverse of the sedimentary structures. However, because there is usually little sediment being deposited at the beginning of a storm because there is not much sediment in motion and because flow speeds are increasing, there is usually no record of the first half of this sequence in the rock record. It is only the second half that gets preserved. <br />
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<i>HCS -</i> The cross stratification that is deposited as a combination of strong currents and large waves is unique to storms (and is found only in medium to fine sands). It is called hummocky cross stratification (HCS) and swaley cross stratification. When currents are washing eroded sand into an area with strong oscillatory flow, rounded mounds or hummocks of sand develop on the sea floor separated by lows (swales). These mounds are a few to 10 cm high and 10’s of cm across. See Figures 14.3 and 4 in Nichols. Variations in current strength cause erosion locally, and the locations of the hummocks and swales change through time. This produces erosional surfaces which truncate the older laminae (note that Fig 14.2 has the wrong laminae truncated). HCS is characterized by low angle laminae truncated by low angle surfaces. There are abundant concave and convex up laminae and many fewer flat laminae. <br />
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<i>Storm Sequence -</i> A sample stratigraphic column consists of: Mud, scoured surface, sole marks, (gravel at base), normally graded, HCS, flat laminae or wave rippled top, return to suspension settling. Contrast this to a turbidite - I will ask you to do this!<br />
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KEY POINT FOR STORMS: Multi-directional flows over seconds, low to high to low energy in deep water<br />
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<b>Tides -</b> Two key characteristics that are unique to tides: 1) flow changes direction 1 or 2 times per day; and 2) The speed of flow is cyclical with flow going onshore, stopping at hight tide, then flowing offshore, and stopping at low tide. There is lots of variability in tides depending on geography. Flow speeds vary, producing different sedimentary structures. In the Bay of Fundy, which has the highest tides recorded in the world (up to 16m - a 5 story building), the water moves up to 15 km/hr (417 cm/sec) which is fast enough to transport boulders and is well above the upper flat lamination zone for smaller grain sizes. At the low end, tidal currents are essentially non-existent. Also, there are times of slack tides when the water is essentially still or wave-dominated. Thus, the range of sedimentary structures is wide, including dunes (often called tidal bars when very large) and ripples. The main characteristic to look for, though, is variations in flow speed and DIRECTION.<br />
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<i>Tidal sedimentary structures -</i> Due to changing flow directions, two sediment transport directions are common, one for onshore flow and one for offshore flow. Often the onshore and offshore flows are not in the same location, but they shift around. This gives rise to current ripples showing transport in two directions and dune migration in two directions producing herringbone cross stratification. See figures 11.6 and 11.7 in Nichols. If the dunes are small and sedimentation rates are very high, you can get herringbone cross stratification in one tidal cycle in a modern environment. It is usually not preserved in the geological record because it is eroded prior to lithification. It is almost always the longer term changes in current locations that gives rise to preserved herringbone cross stratification. Dunes migrate in one direction for a while, and then currents patterns change and they migrate in the other direction. Herringbone cross stratification is almost always due to tidal processes, although it is not all that common in the sedimentary record. Commonly, one tidal current is much stronger than the others or the flow locations aren’t systematically shifting, so tabular cross stratification is more common. It is not unique to tidal environments, however.<br />
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Reactivation Surfaces - Reactivation surfaces form when flow in one direction is stronger than the other, but the other flow is strong enough to modify the bedform shape. See figures 11.6 and 11.9 in Nichols. Reactivation surfaces are erosion surfaces within the sets of cross stratification. They look like irregular surfaces that are similarly oriented to the foresets, but usually do not dip quite as steeply. Also, the foresets above and below the reactivation surface commonly have a slightly different orientation. Reactivation surfaces indicate varying flow directions, which is very common in tidal environments.<br />
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Mud Drapes - Flow speeds are also cyclical. During slack tides (low or especially high), fine grained sediment can fall out of suspension draping tidal bedforms with mud. Because mud is cohesive, it does not necessarily erode during the next tidal flow, particularly in the separation zone where flow is slow, e.g. at the bases of ripples and dunes. Thus, sand foresets coated with mud are very common in tidal environments as well. See figures 11.6 and 11.8 in Nichols.<br />
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KEY POINT FOR TIDAL PROCESSES: Bi-directional flow with varying speeds over hoursDawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com2tag:blogger.com,1999:blog-2103960834361055322.post-70646700460491236142013-02-04T12:51:00.000-08:002013-02-04T12:51:42.739-08:00Rivers<b>Transport Capacity</b><br />
Erosion by water occurs when water is flowing across a surface and the flow is capable of transporting more sediment than is currently moving as bedload. This is called the sediment transport “capacity”. A certain number of grains of a certain size can be picked up by the Bernouli effect for a given flow. If there are too many grains, they start colliding and the characteristics of sediment transport change. Grains are directed back toward the bed and up into the flow. Eventually, more go back to the bed and are deposited, leaving fewer grains in the flow even at high flow speeds because there are more grains than the transport capacity of the flow.* In contrast, if there is a shortage of grains of a size that can be moved by the flow, e.g. the flow is moving all of grains present, any new grains will be eroded off the bed as soon as they are available. The flow then has excess transport capacity. <br />
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* Think about dumping a truck load of fine sand into a fast moving river, it takes time to move all that sediment even if the flow speed is theoretically fast enough to erode fine sand. <br />
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One of the most common times for a flow to have excess transport capacity is when the flow is speeding up. We know from the Hjulstrom diagram that faster flows transport larger grains. They can also transport more grains. Thus, water flowing downhill commonly speeds up, has excess capacity and erodes sediment. When it slows down, sediment is deposited. In floods, the water speeds up, erodes sediment, and transports it. As the flood ends, the water slows down and deposits the excess sediment. In general, erosion occurs when flows are speeding up or when they go from an environment with low sediment (e.g. a dam spillway) to an environment with more sediment (e.g. a river bed). <br />
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<b>Rivers</b><br />
Rivers are responsible for most sediment transport from mountains to lowlands and the oceans. They do the most to even out the topography that tectonic processes create. Rivers consist of channel, bank and overbank or floodplain deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds.<br />
<br />
<i>River Types -</i> <br />
Straight (rare, except for ones humans have modified)<br />
Meandering (high sinuosity)<br />
Braided (many branches within a channel)<br />
Anastomosing (rivers with branching and merging channels)<br />
<br />
The form of the river is controlled by the gradient of the river bed (steep = braided, gently dipping = meandering), local vegetation that stabilizes banks and limits the number of channels, sediment grain size, particularly the ratio of suspended versus bedload sediment, and sediment volume. A high bedload gives rise to abundant bars, which promotes formation of braided rivers.<br />
<br />
<b>Braided Rivers</b> <br />
Braided rivers develop when the proportion of bed load sediment is high, which produces abundant bedforms and promotes the development of bars, and thus, the braided character of the river. The sediment is commonly coarse, which requires fast flow and steep gradients for the sediment to be transported. Much of the geometry of braided rivers is shaped by the highest flows, e.g. spring floods, when the bars are covered in water. Many braided rivers have exposed bar tops for much of the year.<br />
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Flow speeds and transport capacity vary dramatically within a braided river. Friction with the riverbed tends to slow down the flow, particularly where the flow is shallow. Thus, the Reynolds number in shallow areas is relatively low (but still high enough that the flow is turbulent) and the transport capacity is low. In contrast, the transport capacity and Reynolds number are much higher in the deeper middles of channels in the river. Thus, the coarsest sediment is transported here, whereas finer sediment gets deposited in shallow areas. Also, bars block the flow on the upstream sides, and like dunes, the upstream sides tend to erode. Areas of low flow and eddies form on the downstream sides of bars, and they are usually sites of net deposition. Thus, bars migrate downstream through time. If we summarize the processes:<br />
<br />
<i>Sediment Transport: </i> <br />
1) The coarsest sediment is only transported in the middle of the flow where the Reynolds number is highest. (All grain sizes that can be moved are transported where Re is high.)<br />
2) Bars are eroded upstream where the bars deflect the flow. Sediment is deposited on downstream side of bars and some on the flanks of bars where flow is slower, particularly on the insides of bends.<br />
3) Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels. <br />
<br />
Sedimentary structures include:<br />
1) trough x-bedding in channels, due to the migration of irregular dunes<br />
2) coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes<br />
3) sediment on the edges of bars fines upward because the flow is shallower and slower, e.g. has a lower Reynolds number. Sedimentary structures can include anything from upper planar to ripple laminations.<br />
<br />
<i>Braided River Facies</i><br />
Channels migrate back and forth leaving a sheet of sand with abundant cross stratification. These sheets of sand tend to fine upward. General characteristics of braided river deposits include: <br />
1) Scoured surface at the base of a channel<br />
2) Gravel lag at base of channel<br />
3) Trough x-bedded sands deposited just off the center of channels<br />
4) Occasional tabular x-stratification from migrating bars<br />
5) Sand deposited at slower speeds (ripple cross lamination possible)<br />
6) Overbank deposits from floods mostly composed of sand and silt, with some mud<br />
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The large scale geometry of the deposits includes sheets of sand with various grain sizes representing bar migration separated by floodplain deposits. <br />
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Example of a braided river in Alaska: <a href="http://g.co/maps/wrk9n">http://g.co/maps/wrk9n</a> It is cutting through glacial morraines deposited as a glacier retreated up the valley. Follow the river downstream (to the north and east) to <a href="http://g.co/maps/q5kq7">http://g.co/maps/q5kq7</a>. How does the channel geometry change?<br />
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<b>Meandering Rivers</b> <br />
Meandering rivers have a low gradient and thus slower flow, and usually have a high proportion of suspended sediment relative to the amount of bedload. A meandering river channel has curves that meander back and forth on a gently sloping floodplain. The flow speed in the channel varies with the geometry of the meanders. Water has to travel faster on the outside of bends than on the insides of bends. We know from the relationships between Reynolds number and bed shear stress that higher flow speeds mean that more and coarser sediment can be transported at higher flow speeds. Thus, we can predict that:<br />
<br />
1. there is more erosion on the outsides of bends<br />
2. the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available<br />
3. sediment will accumulate on the insides of bend and this sediment will be finer grained. <br />
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If we look at a channel in cross section, it is asymmetric, representing the sites of erosion and deposition. Variation in flow speed also produce different sedimentary structures. Upper planar lamination and dune cross stratification are common where Re is highest, and ripple cross lamination is common where Re is lower. <br />
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The main parts of the channel include eroding bank, the thalweg (the deepest point of the flow) and the point bar (on the inside of the bend where most sediment is accumulating). As the channel migrates due to erosion and deposition, a distinctive suite of sedimentary structures accumulate. The deepest part is coarser and has upper planar lamination or dune cross stratification. This is overlain by finer sediment with current ripple lamination. <br />
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As meandering rivers migrate, the meanders tend to increase. Eventually, the channel forms almost a circle, and the meander gets cut off, often during a flood. This straightens the channel temporarily and produces an ox bow lake in the abandoned meander. The lake accumulates mud and organic matter. <br />
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Watch this cartoon of a meander migration in France: <a href="http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm</a><br />
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<i>Levees and Floodplains</i> - When a river floods, it goes from a confined flow in the channel which is very rapid to a widespread flow across the floodplain. It slows down very quickly and the water becomes shallower, both of which cause a decrease in Re. Thus, the water can not transport as much sediment on the floodplain as it does in the channel. Thus, finer sands that may be in suspension during a flood are transported as bedload or rapidly deposited once the river tops its banks. This produces levees. The finer silts and especially clays remain in suspension much longer and settle out on the floodplain as the flood waters dry up. <br />
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Watch this model of a meandering river flood: <a href="http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm</a><br />
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Over time, the levees build up and provide a higher bank for the channel than the level of the floodplain. Thus, the channel bottom can aggrade (fill in) until the bottom of the channel is as high or higher than the floodplain. When the next flood comes along, the river avulses and does not go back into its old channel which is higher than a new one on the floodplain. This results in the downstream part of the channel being completely abandoned.<br />
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<i>Meandering River Channel Facies:</i><br />
1. Scoured base of flow <br />
2. Lag deposit with mud rip-up clasts and the coarsest grains being transported<br />
3. Fining upward sands with trough cross stratification <br />
4. Rippled sands <br />
5. Sigmoidal cross stratification from migrating point bars <br />
<br />
<i>Floodplain Facies</i><br />
1. Fine sand with climbing ripples<br />
2. Mudstone/shale with mud cracks<br />
3. Soils<br />
4. Root casts<br />
<br />
<i>Ox Bow Lake Facies</i><br />
1. Mudstone/shale without mud cracks<br />
2. Organic-rich deposits, including coal<br />
3. Anoxic water indicators (especially in fossils and absence of trace fossils)<br />
<br />
<br />
<b>Differences between braided and meandering river deposits:</b><br />
<br />
1. Braided river deposits are commonly coarser grained <br />
2. Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on floodplains.<br />
3. Overbank deposits are better developed and finer grained in meandering river systems.<br />
4. Bar migration is much more regular in direction in meandering rivers because there is a well defined, single thalweg towards which the bars migrate. In contrast, braided river bar migration occurs in multiple directions. Thus, meandering rivers produce a more regular geometry of tabular cross bedding, when preserved. <br />
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<b>General Characteristics of Fluvial Sediments:</b><br />
1) On a large scale, river deposits consist of sheets and lenses of sand deposited in channels associated with flat laminated shales and silts with rare rippled sand beds deposited on floodplains.<br />
2) Fining upward sequences of beds in the sands with sedimentary structures that indicate decreasing flow speeds.<br />
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification.<br />
4) Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop since they are rarely preserved<br />
5) Soil development in associated shales deposited in the floodplain environment.<br />
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Look at pictures of fluvial rocks at <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html</a><br />
<br />Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-44341782768540805052013-01-30T12:26:00.000-08:002013-01-30T12:26:17.744-08:00ErosionOnce sediment is produced by weathering, it is available for transport. The two main forces in erosion are fluid flow and gravity. <br />
<br />
Fluid flow is what we talk about most, e.g. glacial erosion of sediment, wind blown sediment, and mostly water flow. Flowing water is the biggest influence in erosion because it is very common and effective at transporting sediment.<br />
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Erosion by water occurs when water is flowing across a surface, and the flow is capable of transporting more sediment than is currently moving as bedload. This is called the sediment transport “capacity”. A certain number of grains of a certain size can be picked up by the Bernouli effect for a given flow. If there are too many grains, they start colliding and and the characteristics of sediment transport change. Grains are directed back toward the bed and up into the flow. Eventually, more go back to the bed than are lifted into the flow, and sediment is deposited. Sediment can be deposited even at high flow speeds when there are more grains than the transport capacity of the flow.* In contrast, if there is a shortage of grains that are small enough to be transported by the flow, e.g. the flow is moving all of grains present, any new grains will be eroded off the bed as soon as they are available. The flow then has excess transport capacity. <br />
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* Think about dumping a truck load of fine sand into a fast moving river, it takes time to move all that sediment even if the flow speed is theoretically fast enough to erode fine sand. <br />
<br />
One of the most common times for a flow to have excess transport capacity is when the flow is speeding up. We know from the Hjulstrom diagram that faster flows transport larger grains. They can also transport more grains. Thus, water flowing from a shallower slope to a steeper slope commonly speeds up, has excess capacity and erodes sediment. When it slows down, the transport capacity decreases, and sediment is deposited. In floods, the water speeds up, erodes sediment, and transports it. As the flood ends or flood waters extend over larger areas, the water slows down and deposits the excess sediment. In general, erosion occurs when flows are speeding up or when they go from an environment with low sediment (e.g. a dam spillway) to an environment with more sediment (e.g. a river bed). <br />
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<b>Gravity Transport of Sediment</b><br />
Gravity pulls sediment down steep slopes through creep, rock or debris falls, landslides and slumps. These processes are really important for the hills in coastal California where there is enough water for extensive weathering, but there is little runoff of water most of the time. <br />
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Here are some videos of gravity-transported sediment:<br />
<iframe allowfullscreen="" frameborder="0" height="315" src="http://www.youtube.com/embed/videoseries?list=PL93B0D9433B8C4275&hl=en_US" width="560"></iframe><br />
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Usually, the concentration of sediment is very high in these flows, which means that the grains collide with each other frequently. Grain collisions can help keep the flow moving. The collisions also keep the different grain sizes from being sorted out, and many of gravity flow deposits are poorly sorted. If lots of mud-sized sediment is present, the viscosity of the flow is high, and the flows can be laminar, which produces very poorly sorted deposits. These are often called mud flows or debris flows. Watch the videos again, and predict the characteristics of the sediment that would be deposited by the different processes. Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-2320252023476582812013-01-30T12:24:00.000-08:002013-01-30T12:24:37.378-08:00Weathering<b>Origins of Sediment</b><br />
Sediment comes from the break down of rocks into smaller, transportable components. This occurs via two processes: <b>physical weathering</b> and <b>chemical weathering</b>. Physical weathering consists of breaking apart rocks and crystals. The results of physical weathering are smaller components of the same material that is being weathered. There is no change in composition. In contrast, chemical weathering consists of changing the composition of at least some components of the rock that is weathering. The sediment does not have the same composition as the original rock.<br />
<br />
<b>Physical Weathering</b>:<br />
Physical weathering occurs via:<br />
<br />
1) Freeze-thaw action. Water in cracks expands when it freezes, putting force on the cracks. The cracks grow, and eventually crystals and pieces of rock break off into smaller components. Obviously, this process is most important in environments where temperatures cycle across the freezing point of water. <br />
<br />
2) Salt crystal growth. When water evaporates, salts precipitate. If this happens in fractures in rock, the growth of the salt crystals can put pressure on the cracks, causing them to grow. This process is most important near oceans where rocks are exposed to lots of salt water spray and in arid environments where water evaporates rapidly. <br />
<br />
3) Temperature changes. Minerals contract and expand as temperature decreases and increases, respectively, and different parts of the rock are heated different amounts. Those in direct sunlight expand as they heat, whereas the interiors and shaded areas do not. Differential expansion and contraction produces stresses which can result in cracks and physical weathering. This process is most important when temperatures change dramatically from day to night, a characteristic of many desert environments. <br />
<br />
Physical weathering tends to produce mostly sand-sized sediment and larger grains because most of the fracturing occurs along mineral boundaries. Physical weathering of fine grained or finely crystalline rock can produce abundant very fine grains, but most of the sediment from these rock types consists of rock fragments (called lithic clasts).<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjcvxSd5RobQW-DLsuVeV7n4MRNn3rDfH4xvwzFqAARjTBd3tzu3YKEx-AnP7C3ITmgFmnitgKcNPqZeHmm_fBa94bwG3Uud7XM5LTDPA0TnRYZ6KEUuxMI4V6upRE7lJx4sw0upb0YYMo/s1600/Weathering_Physical_Makarora.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="240" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjcvxSd5RobQW-DLsuVeV7n4MRNn3rDfH4xvwzFqAARjTBd3tzu3YKEx-AnP7C3ITmgFmnitgKcNPqZeHmm_fBa94bwG3Uud7XM5LTDPA0TnRYZ6KEUuxMI4V6upRE7lJx4sw0upb0YYMo/s320/Weathering_Physical_Makarora.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Image of physically weathered rocks in New Zealand (from Lab 1)</span></td></tr>
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<b>Chemical Weathering</b>:<br />
<br />
Chemical weathering occurs via:<br />
<br />
1) Dissolution of minerals. Some minerals like halite and other evaporites dissolve very easily in water. Other minerals, particularly silicates, do not dissolve easily. Carbonates are in between and dissolve in acidic waters. (Rain water has a pH of ~5.7 due to dissolved CO<sub>2</sub>, even without “acid rain” pollution.) The results of dissolution are ions in water that are transported downstream. Ions are not deposited until the water evaporates, they react with other minerals, or organisms use them to make shells. Often, only part of a rock dissolves, leaving sediment that can be transported by wind, water, etc. <br />
<br />
2) Alteration of minerals. Silicate minerals do not dissolve very easily, but they do react with water to form new minerals. Feldspars react with water to form clay minerals and ions, olivine reacts with water and O<sub>2</sub> to form oxides, clay minerals and ions, pyrite reacts with water and O<sub>2</sub> to form oxides and sulfate ions. Iron oxides, such as hematite, are commonly red, giving weathered rocks a rusty hue. Alteration of minerals is one of the main sources of clay minerals and mud-sized grains.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgH2ADc1p26-BNtkFZsbKXC7L9fQllEo8z9TjysqqxB4fFH3krvz03M2xIZZZKXGmLRGh7x058ddBS-zE7-mKQWe7gTGvLg4O0L6K07JB_8QbOfJRRFvdPEufoKQlaVmXo7Qg9Osfb_lY/s1600/Weathering1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgH2ADc1p26-BNtkFZsbKXC7L9fQllEo8z9TjysqqxB4fFH3krvz03M2xIZZZKXGmLRGh7x058ddBS-zE7-mKQWe7gTGvLg4O0L6K07JB_8QbOfJRRFvdPEufoKQlaVmXo7Qg9Osfb_lY/s320/Weathering1.jpg" width="240" /></a></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhV0PBD-ZJtZNMLMQM0bN1o-v3RXZlQ9PSpPWLGgSHi0xmMa1rEhMfgI3lXJHaDNlEcXS82vilTK5oYHTmroZQMetFEeOCJh3OGlzr5b0d1kWnvwXL-_pVomT5hnvCGzwC8_lrQ1lqD294/s1600/Weathering2.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhV0PBD-ZJtZNMLMQM0bN1o-v3RXZlQ9PSpPWLGgSHi0xmMa1rEhMfgI3lXJHaDNlEcXS82vilTK5oYHTmroZQMetFEeOCJh3OGlzr5b0d1kWnvwXL-_pVomT5hnvCGzwC8_lrQ1lqD294/s320/Weathering2.jpg" width="240" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span style="font-size: small; text-align: start;">Images of chemically weathered rocks in New Zealand. These outcrop originally had the same composition as the rock shown in the previous photograph, but have been exposed to much more water as well as plant-assisted soil processes.</span></td></tr>
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<br />
<b>Mineralogy of Weathered Rocks</b><br />
Sediments that have been subjected primarily to physical weathering have a mineralogy that is similar to that of the parent rock. If the sediments have been subject to extensive chemical weathering, it is much harder to characterize the source rocks because the composition has changed extensively. Overall, the composition of the resulting sediment depends on the mineralogy of the rock, how it is transported, and the weathering environment. <br />
<br />
Some minerals alter more quickly than others. Quartz is difficult to dissolve and is hard, so it commonly lasts through both chemical and physical weathering and is the most common mineral in sand on Earth. In contrast, minerals like Ca-feldspar and olivine react to form new minerals quickly. They are substantially less common in sediments. Thus, mafic rocks (which contain Ca-feldspar, olivine and pyroxenes) tend to alter to clay minerals very easily and produce little sand and abundant mud. In contrast, granites (quartz, K-feldspar, Na-feldspar, mica) contains minerals that react more slowly and tend to produce sand-sized grains, especially quartz. <br />
<br />
The following list includes minerals from most reactive (rarely found in sediments) to least reactive (common in sediments): Olivine, Ca-feldspar, Pyroxene, Amphibole, Na-feldspar, Biotite, K-feldspar, Muscovite, and Quartz<br />
<br />
The other main control on sediment mineralogy is the hardness of the grains. During transport, grains hit each other. Softer grains tend to be damaged when they collide with harder grains, and this damage can cause them to break into smaller grains. Thus, soft grains become smaller very quickly when they are transported with hard grains. Quartz is the most common mineral in sandstones because it is hard and unreactive. Clay minerals are also very common because they are too small to damage much during collisions and they are the product of the alteration of other minerals. <br />
<br />
<b>Controls on Weathering</b><br />
The extent and style of weathering is mainly controlled by climate. <b>Water</b> is extremely important, even for physical weathering. The more water present, the faster weathering occurs. <b>Temperature</b> is also important, as discussed for physical weathering. Warmer temperatures also promote faster reactions, so chemical weathering is more effective in warm climates. Thus, warm, humid climates tend to have the most rapid weathering (and poor outcrop). Finally, <b>vegetation</b> has a strong influence on weathering. Plants tend to increase the extent of chemical weathering by producing organic acids that help break down rocks into soil through both dissolution and alteration. They also help soil retain moisture, increasing the availability of water for weathering, and their roots can help widen cracks. <br />
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<b>Mars</b><br />
Some of the big questions we have about Mars right now are about the extent and timing of chemical weathering. There is no vegetation. Right now, the climate is very cold and very dry, so the rate of chemical weathering now is extremely low. However, most of the rocks on the surface of Mars (that we have characterized) are basalts, which have glass and mafic minerals that should alter “quickly” if they are in contact with water. What do you expect a chemically weathered basalt to produce? What should we look for as evidence of past chemical weathering on Mars? How might products from basalt weathering look different if one looks at the weathered basalt itself or at sediment that was eroded from weathered basalt? (Basalt is mostly plagioclase (calcium feldspar) and pyroxene, with or without olivine, and has less than 20 percent quartz.)Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-53956238686072778782013-01-28T12:21:00.000-08:002013-01-28T12:21:24.115-08:00Stratigraphy and Time<b>Stratigraphy </b>is the study of sedimentary rocks in space and time. It is the basis of interpreting what happened in the past. We use facies to interpret depositional environments from the rocks. Changes in facies both vertically and horizontally allow us to interpret changes in ancient landscapes and processes. <br />
<br />
<i>Example:</i> Beach Facies. Beach environments grade laterally into each other. The offshore areas grade into the swash zone of the foreshore. The foreshore grades into the berm (the highest point of the beach) and backshore (if present). Eolian (wind) dunes, marshes or erosional cliffs can be present landward of the beach. Rock facies similarly grade into each other because they were deposited in different depositional environments. If the depositional environments stay in exactly the same place through time, a stratigraphic column in each place would consist of a uniform facies, but each stratigraphic column would have a different style of rock (facies). However, depositional environments tend to migrate back and forth as sea level rises or falls, basins fill in with sediment, etc. Thus, facies in stratigraphic columns tend to change upward. They also vary laterally. See figure 19.8 on pg. 308 of Nichols or this figure: <a href="http://www.ocean.odu.edu/~spars001/geology_112/laboratory/session_04/walthers_law.jpg">http://www.ocean.odu.edu/~spars001/geology_112/laboratory/session_04/walthers_law.jpg</a> from this page discussing stratigraphic correlations: <a href="http://www.ocean.odu.edu/~spars001/geology_112/laboratory/session_04/handout.html">http://www.ocean.odu.edu/~spars001/geology_112/laboratory/session_04/handout.html</a><br />
<br />
Changes in sea level and depositional environment lead to variations in stratigraphic columns both laterally and vertically. If you compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. <b>lithostratigraphy</b>, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. <b>chronostratigraphy</b>, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time. <br />
<br />
Lithostratigraphic correlations are easy because you can directly observe rock type. These correlations are very useful for studies of reservoir properties, where one might want to identify a porous sand that acts as a water or hydrocarbon reservoir. However, these correlations do not help you interpret ancient depositional environments because they do not represent an ancient landscape. Chronostratigraphic correlations tell you the most about depositional environments and their distribution through time, but they can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. Sometimes volcanic ash beds or other depositional events allow you to directly observe which rocks were deposited at the same time, but these events are rare. Often, chronostratigraphic correlations require detailed facies analyses and an understanding of how depositional environments change through time.<br />
<br />
<b>Walther’s Law</b> is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record <b>if there is no break in sedimentation</b> (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits are river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments.<br />
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<b>Chronostratigraphy -</b> Chronostratigraphy enhances the interpretation of the stratigraphic record in terms of Earth history. Even when one has a detailed map of the distribution of depositional environments, it is difficult to say exactly how to correlate section in terms of time. In real rocks, there are a number of tools that you can use to get correlations of various accuracy. These include: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as elemental isotope ratios in carbonates; geological instantaneous depositional events such as huge storms, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end. <br />
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<b>Distribution of Rock and Time -</b> One might think that sections can be correlated based on assuming that the same amount of sediment gets deposited in all places in the same amount of time. This is a BAD assumption, although many researchers are forced to use it. It is important to understand that the preserved rock does not represent all of time. What I mean is that time is not evenly represented by rock thickness. For example, with turbidites, the sandstones may have been deposited in a couple hours to a day at most, whereas the shales (Bouma E) represent 100’s to 1000’s of years of fine grains settling out. Thus, most of the "time" is represented in the much thinner shales. In addition, there is erosion at the base of some of the turbidites. Thus, there is a significant amount of time that is only represented by an erosional surface which produces a gap in the rock record. Generally, sedimentation is thought of as a continuous processes. This is NOT true. Sedimentation is episodic and there are unconformities in the stratigraphic record spanning all time ranges from minutes to millions of years. Gaps of minutes might occur in a river if there is a burst of strong flow that is erosive rather than depositional. Gaps of hours occur at low tides when the intertidal zone is exposed. Gaps of years to thousands of years can occur in land environments where there is no source of sediment or the topography is too high to collect sediment. Gaps of millions of years also occur in terrestrial environments, especially if there is erosion. The longer time gaps usually represent regional changes in deposition and can be very useful for correlating rocks chronostratigraphically. Also, different depositional environments accumulate sediment at different rates: thickness does not equal time!Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-19098509542725859992013-01-28T11:58:00.002-08:002013-01-28T12:01:39.986-08:00Turbidites<b>Turbidites</b><br />
Turbidites provide a good summary of the ideas we have been talking about, e.g. facies and sedimentary structures related to flows. Turbidites are deposited from slurries of sediment and water in any standing body of water (lakes, oceans). <br />
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1) Turbidity flows start with slope failure in soft sediment. Slopes become oversteepened where sedimentation rates are very high, such at the mouths of rivers. Because flow speeds are very low in standing water, the sediment does not get washed downslope. Rather, it builds up until there is a subaqueous slope failure. Earthquakes can trigger these slides, too.<br />
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2) Sediment and water mix creating a “fluid” that is denser than the surrounding water because of the entrained sediment. Thus, it flows downhill even if the slope is very low (1°).<br />
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3) The base of the flow is commonly erosional on steep slopes, so more sediment is entrained in the flow.<br />
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4) Enough sediment is entrained that erosion stops. Deposition begins as the slope gets shallower or the flow starts to slow down. Initially, the coarsest grains are deposited (remember the Hjulstrom diagram) and then finer grains, so the sediment is “graded”. However, the sediment is usually poorly sorted because the flow is a slurry of water and sediment so hydraulic sorting is reduced. (Facies = Bouma a)<br />
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5) Sediment concentration decreases with deposition, so one gets more hydraulic sorting. The flow is very fast so the sediment has upper planar lamination. (Facies = Bouma b)<br />
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6) As the flow slows more, grain size decreases and ripples start to form. Dunes do not usually form for two reasons: a) often only fine sand and finer grains are left in the flow by this point; and b) dunes do not have time to develop. (Facies = Bouma c)<br />
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7) Eventually, the flow slows to the point that bedload transport stops and deposition is mostly settling of silt and then clay. The progressive settling of coarser and then finer grains produces a faint lamination, but it is not as strong as the planar laminations in Bouma b. (Facies = Bouma d)<br />
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8) Mud settles out producing shale. This can look identical to background settling of clays brought into the lake/ocean as suspended sediment. (Facies = Bouma e)<br />
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Bouma divisions a-d can take hours or a day or so to be deposited. However, division e, which is usually the thinnest, commonly accumulates over months or longer (e.g. hundreds of years) depending on how frequent turbidites are in the area. <br />
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Watch these movies of turbidites in flumes:<br />
<a href="http://faculty.gg.uwyo.edu/heller/SedMovs/middletonturb.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/middletonturb.htm</a><br />
<a href="http://faculty.gg.uwyo.edu/heller/SedMovs/Turbidity%20ignition.html">http://faculty.gg.uwyo.edu/heller/SedMovs/Turbidity%20ignition.html</a><br />
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<i>Changes in Character Downslope</i> - The parts of turbidites that are deposited change downslope and usually only a few of the subdivisions are preserved. In the most proximal (upslope) environments, divisions a and b are most common. In the more distal areas, all of the coarser sediment has already been deposited upstream, so divisions d and e are most common. Generally, there are also channels which fan out producing variations in rock types that change in space and through time. <br />
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<i>Turbidite Facies Models</i> - Over the decades, sedimentologists have described and interpreted sedimentary rocks and defined generalized facies and facies associations that are characteristic of different depositional environments. These generalized facies and associations are called Facies Models. Each depositional environment or system has its own facies model. This is a VERY powerful tool for interpreting ancient environments. See my video summary: <a href="http://www.youtube.com/watch?v=G05juwK2OTI">http://www.youtube.com/watch?v=G05juwK2OTI</a><br />
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A nice, hour long lecture on turbidites in the Monterrey Bay canyon, CA, can be found at: <a href="http://online.wr.usgs.gov/calendar/2010/jun10.html">http://online.wr.usgs.gov/calendar/2010/jun10.html</a> The actual lecture starts about 5 minutes into the video.<br />
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<b>Extra on Turbidites</b> - Turbidite facies analysis and the resulting facies model led to the discovery of a new process. Sedimentologists had characterized turbidites all over the world. They all had the same flow characteristics consisting of a very strong erosive flow, deposition of a normally graded bed which was massive, followed by upper plane bedding, rippled finer sands, coarsely laminated silts, then shales. Comparisons with known flows showed that this sequence of deposits must come from a strong initial flow that slowed through time to still water. And this repeated again and again. The associated facies and the succession of different facies in these sequences suggested that the deposits had to be in deep water. For example, the fossils were all characteristic of deep water, shales were abundant and only settle from still water (shallow or deep), and they were sometimes associated with deep water storm deposits. Thus, the sedimentologists proposed slope failure and turbid currents flowing downslope and called them turbidity currents. A process like this had not been observed in modern depositional environments, so the idea was controversial. Many geologists did not believe that you could generate strong enough currents underwater to get those flow characteristics. Eventually in 1964, two geologists Heezen and Drake realized that an event in 1929 provided strong evidence for turbidity currents. In 1929, without satellites, under water telegraph cables were strung from Newfoundland to Europe. In November, about 30 cables broke in order from farthest north and shallowest to farther south and deeper water. At the time, people did not know why they broke, but Heezen and Drake suggested that a turbidity current was triggered by an earthquake and the cables broke as the turbidity current passed over them (they are strong flows!). Because they were continuously used for communication, the time each cable broke was very well known. Heezen and Drake calculated that the front of the flow traveled at 250 km/h (36,000 cm/s) when it first formed and then slowed to around 20 km/h (7000 cm/s) by the time the last cables broke 500 km from the source. This was a fast, strong flow and may be typical of turbidites. These speeds are above the upper end of the Hjulstrom diagram and are very erosive. It is only after the turbidite slows down even more that you get deposition. The characteristics of the flow seen by the breaking cables fit the flow characteristics proposed by the sedimentologists, and now turbidity currents and the facies model developed for turbidites are widely accepted and often treated as an ideal example of rocks that closely reflect flow characteristics. Turbidites and their interpretation are almost an ideal example of a good Facies Model.<br />
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<b>Extra on Dense Sediment Flows</b><br />
Sometimes with slope failures on land or under water, much more sediment can be put into motion than the flow would normally erode. Depending on the amount of water mixed with the sediment, the flow characteristics are different. When abundant water is present, the sediment can form a thick slurry with a higher density than sediment-free water, commonly leading to a higher <b>Re</b> and more turbulent flow (<b>Re=u*l*r/µ</b>). Also, collisions between grains become extremely important. Both of these tend to keep the sediment moving. Grain-to-grain collisions also have an important effect on grain sorting. The collisions tend to make sorting much less efficient and the sediment that gets deposited tends to consist of whichever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes.<br />
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If there is very little water associated with a clay-rich sediment flow, the flow can be very viscous due to the charge attraction among clay particles. The high viscosity makes the flow laminar (<b>Re=u*l*r/µ</b>). Debris flows with lots of cohesive mud are like this. In laminar flows, there is no mixing of the water or grains (or ice) and there is no sorting of grain sizes. Thus, the sediment remains mixed up with large grains, sometimes boulders, “floating” in mud. They flow down hill pulled by gravity until the flow seizes up and stops. This can be due to too low a slope or loss of water. Underwater debris flows can also be diluted by water that gets incorporated at the edges of the flow and become less viscous and more turbulent. <br />
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There also are dry sediment flows in which air is present between grains. For example, rock avalanches and some pyroclastic flows from volcanoes lack water. For these to move significant distances, large amounts of energy from either gravity or explosions are necessary to keep the sediment in motion. <br />
<br />Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-78711470892878487432013-01-23T12:51:00.000-08:002013-01-23T12:51:13.227-08:00Facies - Groupings of Rock or Sediment Based on Shared Characteristics<b> Environments and Facies</b><br />
Look at the photo of Scott Creek Beach at:<br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg</a><br />
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Note that the antidunes are forming in one part of a creek. The middle of the creek has upper planar lamination flow speeds, and the closest part is very shallow and has some antidunes again. (I know some of this from being there more than from looking at the photo.) Note that there is a faint lamination present in the eroding bench on the far side of the creek. This lamination mimics the beach surface. It is lamination from the waves swashing and transporting sediment on the beach. If all sediment transport stopped immediately, one would see a suite of sedimentary structures: Antidunes and upper planar laminae next to each other in the creek, an erosional surface overlying planar stratification that undulates like a beach. The association of these features would tell you that the sediment was deposited in an environment with a variety of flow conditions. <br />
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The suite of structures forms a <b>facies</b>. A facies (Latin for aspect or appearance) is a body of rock (i.e. a sequence of beds, etc.) or sediment marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock/sediment above, below and adjacent to it. A sedimentary facies has a characteristic set of properties that makes it distinctive, which the geologist defines. Usually facies are defined based on a suite of characteristics in rocks/sediment. <br />
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<b>Facies vs Environments</b> - By grouping characteristics of the rocks into facies, the depositional environments can be more easily compared and interpreted. It is important to remember that the sedimentary environment is the combination of physical, chemical and biological processes that influence sediment deposition, whereas sedimentary facies are the characteristics of the rocks/sediments after deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross stratification; the stratification is the result of high flow speed, but they are not the same. <br />
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<b>Example Facies</b><br />
Facies are groupings of rock types based on similar features. We use these groupings to generalize individual properties into useful, genetically related categories. Some examples include:<br />
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<i>Facies based on grain size:</i><br />
Coarse-grained sandstone with 1-5% pebbles (suggests high flow speeds)<br />
Fine-grained, well-sorted sandstone (suggests low flow speeds with either only one size sediment source or a consistent flow speed)<br />
Mudstone (suggests standing water)<br />
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<i>Facies based on sedimentary structures:</i><br />
Fine-grained sandstone with current ripple cross lamination<br />
Fine-grained sandstone with upper planar lamination<br />
Fine-grained sandstone lacking cross stratification, but with abundant burrows<br />
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<i>Facies based on grain composition:</i><br />
Coarse-grained sandstone with 25% lithic fragments, 25% feldspar, and 50% quartz<br />
Coarse-grained sandstone with 80% quartz, 10% mica, and 10% feldspar<br />
Coarse-grained sandstone with 99% quartz and trace gold flakes<br />
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<i>Beach Facies</i> What features did we see on the field trip to Bodega Bay beaches? How should we divide those into facies? We can compare them to what we would see in the rock record. Take a look at photos of Scott Creek Beach stratification again: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Beach.html">http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Beach.html</a> Predict some of the facies.<br />
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<b>From Sediment Transport to Rocks</b> - We have been talking about sediment transport and structures. These are processes that influence sedimentary rocks. What we really need is to be able to use our understanding of the processes to interpret ancient rocks when we can no longer see the processes in action. As I mentioned in the first class, we can use the modern processes as a model for interpreting past processes, which is the Principle of Uniformitarianism. However, it is often very different to see a process going on than it is to look at the ultimate deposited rock and interpret the process. For example, with bed forms, the entire shape of the structure you see as it migrates is rarely preserved. Instead, you only see a small part of it, if you get any sediment accumulation at all. Thus, we can also start the interpretation from the rock end by describing the general characteristics of the rocks and interpret flow from things like grain size, preserved cross stratification, and biogenic components. Then we can evaluate which environments are consistent with those characteristics. Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-2646445591985422932013-01-23T12:48:00.000-08:002013-01-23T12:48:17.596-08:00Sedimentary Structures Part 2<i>Ripples and Dunes</i> (A review with a bit of additional information)<br />
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A sketch of a ripple or dune like the one in lecture: <br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/duneXStrat.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/duneXStrat.jpg</a><br />
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Remember where the separation point and attachment point are located. The geometry of the flow tracks these points. Erosion can only occur where there the bed shear stress is high enough to move sediment. In other words, the the main flow must be near the sediment surface. Sediment accumulates into a deposit in the flow shadow downstream of the ripple or dune crest; sediment accumulates in the flow detachment zone. Laminae are visible where deposition occurs due to variations in flow speed which cause variations in grain sizes transported and deposited. <br />
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Sets of laminae are separated by erosion surfaces which form on the upstream side of the ripples or dunes. They represent deposition on the downstream side. The shape of the laminae reflects the shape of the depositional surface and the geometry of sediment accumulation. If the depositional surface is curved, the base of the laminae is curved. Areas with higher deposition produce thicker laminae. Also, the maximum distance between erosion surfaces is less than the maximum height of the ripple or dune; since the erosion surfaces form on the upstream sides, they are closer to the underlying erosion surfaces than the ripple/dune crests. Thus, the maximum separation of erosion surfaces represents a minimum height for the ripple or dune. <br />
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Dunes and ripples behave similarly at the level of detail that I have been describing them. Their cross stratification geometries are similar. However, dunes are larger than ripples. If the distance between erosion surfaces defining cross sets is greater than a few centimeters, the cross stratification has to be from a dune. Ripples are only a few centimeters tall, and they cannot create laminae that are higher than the ripple crest-to-trough distance. Thus, if cross sets are greater than a few centimeters high, the cross stratification must be from dunes. However, if the cross sets are only one centimeter high, the cross stratification could be due to either ripples or dunes. It is possible for ALL sediment to be eroded as a dune migrates, leaving no cross stratification. If only a small amount of sediment accumulates, the cross sets might be only a centimeter high, much like ripples. In the field, grain size variations and changes in cross stratification along an outcrop can help you distinguish between ripples and dunes in a case like this. For example, you could look for an instance where the cross stratification is more than a few centimeters high. If you did not find one, that might suggest ripple cross lamination rather than dune cross stratification. Or maybe the grain size is wrong for one or the other.<br />
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<i>Variations in Geometry and Bedform</i><br />
Dunes and ripples are often irregular in plan view. This affects the geometry of the cross stratifcation/lamination. The laminae are always approximately parallel to the dip on the lee sides of the ripples or dunes. If the direction that these dip varies, the orientation of the laminae also varies. When looking at deposited cross stratification/lamination, these variations appear as variable dips in the laminae because you are viewing them at different angles. <br />
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Watch the USGS bedform movies described at: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/labs/USGSBedforms.html">http://mygeologypage.ucdavis.edu/sumner/gel109/labs/USGSBedforms.html</a><br />
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Remember that the structures also change with flow speed, both in terms of their geometry and which ones form. Grain size is also important. The sequence of structures in granules with increasing flow is:<br />
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1) no transport<br />
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring<br />
3) dunes - the flow is strong enough to erode at the attachment point<br />
4) upper planar lamination<br />
5) antidunes<br />
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In contrast, the sequence of structures in silt is:<br />
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1) no transport<br />
2) ripples<br />
3) upper planar lamination<br />
4) antidunes<br />
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<i>Antidunes</i> - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly (Putah Creek in flood; tidal channels; creeks flowing across beaches - see <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg</a>). Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases. <br />
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<b>Other Types of Flows</b> - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/ARipples.html">http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/ARipples.html</a> Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than the stoss slope. Flows can also be irregular due to combinations of currents and waves, etc. Some of these flows are very characteristic of specific environments, for example, storm-influenced beaches. The structures they produce are very useful for interpreting ancient rocks, and we will highlight them as we discuss different sedimentary environments. <br />
<br />Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-13201057571257731172013-01-16T12:24:00.001-08:002013-01-16T12:26:57.254-08:00Sedimentary Structures<b>Key Points of Sediment Transport in Water</b> <br />
1) Faster flows have more bed shear stress. Thus, faster flows move larger grains (when considering sand sizes and larger). <br />
2) Sediment is transported as bedload and in suspension. Bedload consists of rolling and saltating grains. <br />
3) Grain size, density and flow strength (Re) determine how grains are transported. <br />
4) As flow strength changes, grains are eroded or deposited. These relationships are represented in the Hjulstrom diagram.<br />
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<b>A Few Definitions: </b><br />
1) <i>"Stratification" - layers in rocks</i>; stratified rocks are those organized into beds<br />
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See Grand Canyon Beds: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L1/12GrandCanyon.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L1/12GrandCanyon.jpg</a><br />
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2) <i>“Beds” are separated by “bedding planes” </i>- cm to m thick units of sedimentary rock that were deposited approximately horizontally (beds) and are separated by horizontal planes (bedding planes); the rocks typically weather more along these planes. Beds are usually fairly uniform or change gradationally in composition. Bedding planes usually represent breaks in sedimentation or changes in grain size. In other words, they usually represent changes in flow characteristics.<br />
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See Cache Creek Turbidite Beds and Bedding Planes: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L1/13tiltedturbidites.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L1/13tiltedturbidites.jpg</a><br />
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3) <i>"Laminae" are color, composition, or grain size variations defining surfaces within a bed</i>. They typically represent variations in flow velocity, sediment supply, sediment composition, etc. <i>Planar Laminae</i> are parallel to bedding, e.g. planar.<br />
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4) <i>"Cross Lamination”, "Cross Stratification" or "Cross Bedding" are laminations or layers that are oriented obliquely to bedding.</i> They truncate older laminae and are truncated by younger laminae. The erosional surfaces that separate “sets” of similarly oriented laminae are called “bounding surfaces”. There are lots of subdivisions of cross stratification; different types represent different types of bedforms and different flow conditions. <br />
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See Burns Cliff on Mars observed by Opportunity: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L4/3BurnsCliff.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L4/3BurnsCliff.jpg</a> The upper part of the image has planar lamination, and the lower part to the far left has cross lamination or stratification.<br />
See: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughXStrat3.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughXStrat3.jpg</a> for trough cross stratification and other examples of dune cross stratification at: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Dunes.html">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Dunes.html</a><br />
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See ripple cross lamination on Mars: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L4/5MartianRipples.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L4/5MartianRipples.jpg</a><br />
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<b>Bedforms</b><br />
When sediments get deposited from turbulent flows, the sediment interacts with the geometry of the flow. Depending on the flow speed, turbulence, and sediment characteristics, different structures or bedforms develop.<br />
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See: <a href="http://faculty.gg.uwyo.edu/heller/SedMovs/mcbriderips.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/mcbriderips.htm</a><br />
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<b>Bed Geometry and Flow Separation</b> - Until now, we have been implicitly assuming that the bases of beds are flat and smooth, but if sediment is present, they are not. If you start with a smooth bed of sand and increase water speed above it, irregularities form from irregularities in the flow and develop into ripples. First, a few grains pile up. Once the height of the pile is several grains high, there is a flow shadow down stream of them, and the laminar sublayer detaches from the base of the flow. The water has enough momentum that it does not hug the bed surface and instead, goes shooting out over the top. This point is called the separation point. The water flows forward and downward and reconnects with the bed at the attachment point. At the attachment point, water is flowing directly towards the sediment with a lot of force. This force moves the grains and causes erosion. In contrast, the area between the separation point and the attachment point has very low flow. In fact there are back eddies, where the flow is upstream. Thus, sediment transport is very irregular along the bedding surface at a local scale.<br />
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<b>Sediment Transport Over a Ripple</b> - Sediment grains are mobilized at the attachment point - more so than in normal flow because the water is shooting directly into the sediment - and the grains are moved downstream by saltation and traction. As the flow becomes parallel to the sediment surface again, its ability to transport sediment decreases. Thus, the grains tend to pile up and a new mound forms. This gives a periodic chain of mounds - the beginnings of ripples. As flow continues, grains roll and saltate up the stoss (upcurrent) side of the ripples. Once they pass the crest, they reach the low flow on the lee side of the ripple. The larger grains settle out and roll partway down the slope; this is the site of net deposition. As the process of deposition on the lee side and erosion on the stoss side continues, the ripples migrate downstream. If there is net deposition of sediment, the ripples leave behind distinctive dipping layers between two erosional surfaces that can be preserved in the rock record. These layers slope downstream and are one type of cross lamination.<br />
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A sketch of a ripple or dune like the one in lecture: <br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/duneXStrat.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/duneXStrat.jpg </a><br />
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Watch the USGS bedform movies described at: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/labs/USGSBedforms.html">http://mygeologypage.ucdavis.edu/sumner/gel109/labs/USGSBedforms.html</a><br />
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<b>Bedforms and Flow Velocity</b>- The size and shape of subaqueous bedforms depends on flow strength and grain size and can be used to interpret ancient flow characteristics in a depositional environment from looking at sedimentary rocks. See Nichols (2009, Sedimentology and Stratigraphy, section 4.3) for bedform, flow speed, and grain size relationships.<br />
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<i>Ripples</i> (crest-to-crest differences of less than 50 cm and heights of less than 4 cm) – The minimum flow for ripples is determined by the minimum velocity for sediment transport. Once this flow speed is reached, ripples form if the sediment is transported as bedload. The maximum flow speed for ripples depends on the location of the attachment point on the stoss side of the ripples. As flow gets faster, too much erosion occurs at the crests of the ripples - the point of attachment is too far up the stoss side of the ripple- and the ripples flatten out. Dunes develop. <br />
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<i>Dunes</i> (60 cm-100’s m wavelength and 10’s of cm to meters in height) - Dunes develop as ripples flatten out because large scale irregularities start to develop. The basic ideas of dune and ripple formation are the same. The difference is that the area of flow separation is much larger (see Fig 4.17, Nichols 2009). Roller vortexes (e.g. upstream flows along the lee sides of dunes) are common, and the upstream flow can be strong enough to form ripples that migrate upstream. As flow speeds increase, the dunes start to flatten out.<br />
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<i>Planar/Flat Lamination</i> – Planar lamination forms when the flow is strong enough that the beds flatten out. The momentum of the transported grains and fluid are high enough that they tend to move horizontally, eroding any irregularities in the bed. This zone of planar lamination is called “upper flow regime”. (Why “upper”? - there is a zone of planar lamination in coarse grained sediment at low flow velocities.)<br />
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<i>Antidunes</i> - Antidunes form at flow speeds greater than planar lamination when shallow water moves very quickly (Putah Creek in flood; tidal channels; creeks flowing across beaches - see <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg</a>). Irregularities form on the planar beds, but there is no flow separation. Instead, the water surface mimics the bedding surface. On the down flow side of the antidunes, there is a very strong erosional force (from the Bernoulli Effect) and sediment gets plastered onto the upstream side. Thus, antidunes produce laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases. <br />
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<b>Other Types of Flows</b> - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/ARipples.html">http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/ARipples.html</a>. Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than the stoss slope. Flows can also be irregular due to combinations of currents and waves, etc. Some of these flows are very characteristic of specific environments, for example, storm-influenced beaches. The structures they produce are very useful for interpreting ancient rocks, and we will highlight them as we discuss different sedimentary environments. <br />
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<i>Bedforms and Grain Size</i> - Bedforms also vary with grain size (see Figure 4.20, Nichols, 2009). Very fine sand and silt are very easy to transport and erode. They form nice ripples, but do not form dunes when transported by water. Instead, ripples transition into planar laminae. Coarse sand and larger sediment is too hard to transport and erode to get ripples. The erosional force at the reattachment point is not strong enough to erode the coarse grains and produce the erosional surfaces on the backs of ripples. Without this erosion, troughs do not form and without troughs, crests do not form. The sequence of structures in granules with increasing flow is:<br />
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1) no transport<br />
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring<br />
3) dunes - the flow is strong enough to erode at the attachment point<br />
4) upper planar lamination<br />
5) antidunes<br />
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In contrast, the sequence of structures in silt is:<br />
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1) no transport<br />
2) ripples<br />
3) upper planar lamination<br />
4) antidunes<br />
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<b>Extra</b><br />
<b>High Sediment Loads</b> - Sometimes with slope failures on land or under water, much more sediment can be put into motion than the flow would normally erode. Depending on the amount of water mixed with the sediment, the flow characteristics are different. When abundant water is present, the sediment can form a thick slurry with a higher density than sediment-free water, commonly leading to a higher <b>Re</b> and more turbulent flow (<b>Re=u*l*r/µ</b>). Also, collisions between grains become extremely important. Both of these tend to keep the sediment moving. Grain-to-grain collisions also have an important effect on grain sorting. The collisions tend to make sorting much less efficient and the sediment that gets deposited tends to consist of which ever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes.<br />
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If there is very little water associated with a clay-rich sediment flow, the flow can be very viscous due to the charge attraction among clay particles. The high viscocity makes the flow laminar (<b>Re=u*l*r/µ</b>). Debris flows with lots of cohesive mud are like this. In laminar flows, there is no mixing of the water or grains (or ice) and there is no sorting of grain sizes. Thus, the sediment remains mixed up with large grains, sometimes boulders, “floating” in mud. They flow down hill pulled by gravity until the flow seizes up and stops. This can be due to too low a slope or loss of water. Underwater debris flows can also be diluted by water that gets incorporated at the edges of the flow and become less viscous and more turbulent. <br />
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There also are dry sediment flows in which air is present between grains. For example, rock avalanches and some pyroclastic flows from volcanoes lack water. For these to move significant distances, large amounts of energy from either gravity or explosions are necessary to keep the sediment in motion. Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-82986562320303278192013-01-14T11:35:00.000-08:002013-01-14T11:35:43.671-08:00Homework: Fluid Flow 1
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1. Calculate the Reynolds number, flow speed, or water depth for the following flows using the Reynolds equation. The density of water is 1000 kg/m3, and it has a viscosity of 0.001 kg/(m*s). (6 points)</div>
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<span class="s1">a) Calculate the Reynolds number for a flow with speed = 1 m/s and depth = 1 m. Is the flow laminar, transitional, or turbulent?</span></div>
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<span class="s1">b) Calculate the Reynolds number for a flow with speed = 0.1 m/s and depth = 0.1 m. Is the flow laminar, transitional, or turbulent?</span></div>
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<span class="s1">c) What is the maximum water depth for a laminar flow if the flow speed is 0.01 m/s?</span></div>
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<span class="s1">2. Where in any water flow is there laminar flow even if the flow speed is 0.45 m/s and the depth is 10’s of centimeters? Why is it laminar? (Assume that the bed it is flowing over is smooth. You can watch this video: <a href="http://tinyurl.com/7lcc4o">http://tinyurl.com/7lcc4o</a> for a big hint!) (4 points)</span></div>
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<span class="s1">3. Watch the video at <a href="http://youtu.be/TKH1DyV9vNU">http://youtu.be/TKH1DyV9vNU</a> called Sports Car Aerodynamics: Spoiler Alert! Using the description of pressure in airflow over cars with spoilers vs. airfoils vs neither, sketch the laminar and turbulent areas of flow over a round sedimentary grain sitting on a flat surface. Show the areas of low and high pressure and the forces on the grain. (6 points)</span></div>
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4. In words, briefly describe the process (Bernouli Effect) that causes grains to be picked up off the bed and entrained in the flow (at least temporarily). This description should be consistent with your sketch in question 3. (4 points)<span class="s1"></span></div>
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Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-78550798612236779942013-01-14T11:15:00.000-08:002013-01-14T12:46:00.138-08:00Fluid Flow and Sediment Transport<b>Key Concepts from Lecture 2</b><br />
<i>Reynolds Number</i> - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. <b>Re</b> = (fluid inertial forces)/(fluid viscous forces) = <b>l*u*r/µ</b> where the variables are flow velocity (<b>u</b>), characteristic length (<b>l</b>) which represents flow geometry, say river depth, fluid density (<b>r</b>), and fluid viscosity (<b>µ</b>). Turbulent flow has <b>Re is greater than 2000</b> and laminar flow has <b>Re is less than 500</b>. Flow with <b>Re</b> between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well. <br />
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<i>Boundary Layer and Laminar Sublayer</i> - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer. <br />
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<b>Sediment Transport</b><br />
<i>Bed Shear Stress</i>: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. It also depends on the slope. If the slope is steep, gravity helps pull grains down the slope, increasing bed shear stress. Also, the roughness of the bed is a factor. A rough bed deflects flows and increases turbulence, which increases the bed shear stress, particularly in places where flow is directed into the sediment and the boundary layer is compressed. (See <a href="http://en.wikipedia.org/wiki/Sediment_transport#Bed_shear_stress">http://en.wikipedia.org/wiki/Sediment_transport#Bed_shear_stress</a> for more detailed information.)<br />
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<b> The Bernoulli Effect</b><br />
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect.<br />
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<i>Which Grains Move?</i> Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is the position of a grain relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference as if it is on a flat surface. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average.<br />
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<b>Bedload and Suspended Load Transport</b><br />
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain settles out versus how turbulent the water is (back to Re...). <b>Bedload</b> refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in <b>traction</b>. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be <b>saltating</b>. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water.<br />
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Here is a playlist with movies related to sediment transport: <a href="http://www.youtube.com/playlist?list=PLF02560C499E2A9D4&feature=plcp">Sumnerd’s Sed Transport Playlist</a> and an extra movie at<br />
<a href="http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm"> http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb)</a><br />
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<b>Suspended</b> sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow.<br />
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Photo of suspended sediment in a Costa Rica River: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg</a><br />
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YouTube video of white clay in a turbulent flow in a flume: <a href="http://tinyurl.com/78kg3z">http://tinyurl.com/78kg3z</a> The pulsing in the flow is (probably) due to the pump that is making the water flow.<br />
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<b>Hjulstrom Diagrams</b><br />
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams. Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the <i>critical velocity</i>. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the <i>critical threshold</i> for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done! <br />
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A copy of the Hjulstrom Diagram can be found on Wikipedia: <a href="http://en.wikipedia.org/wiki/Hjulstr%C3%B6m_curve">http://en.wikipedia.org/wiki/Hjulstr%C3%B6m_curve</a><br />
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Deeper flows can move larger grains at the same flow velocity because they are more turbulent: <b>Re=u*l*r/µ</b> and<b> l</b> is larger. This is because deeper flows can have larger variations in flow speed and the laminar flow layers are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than just Hjulstrom diagrams, which summarize a lot of characteristics into two axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess.<br />
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<i>Silt and Clay</i> - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clay minerals tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode. <br />
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In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix). <br />
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<b>A few more words about saltation:</b> Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur. <br />
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Watch grains transported by saltation and traction in these movies: <a href="http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm</a> (11 Mb)<br />
<a href="http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm</a> (14 Mb)<br />
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You can also see bedload transport in a movie that describes why we think we have found evidence of a river on mars: <a href="http://www.youtube.com/watch?v=HYHc2alzdUk">http://www.youtube.com/watch?v=HYHc2alzdUk</a> My friend Sanjeev narrates it. He is a professor at Imperial College in London. More information and graphics are at <a href="http://mars.jpl.nasa.gov/msl/news/index.cfm?FuseAction=ShowNews&newsid=1360">http://mars.jpl.nasa.gov/msl/news/index.cfm?FuseAction=ShowNews&newsid=1360</a><br />
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<b>Deposition:</b> Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and starts to leave grains on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments. <br />
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<b>Ripples and Other Bedforms</b><br />
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser laminae that dip downstream. <br />
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<b>Next Time</b>: We will talk about the details about sedimentary structures.Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com1tag:blogger.com,1999:blog-2103960834361055322.post-4911983411165684962013-01-09T12:57:00.000-08:002013-01-14T11:32:44.158-08:00Fluid Flow Part 1<b>Key Concepts from Lecture 1</b><br />
The <i>Principle of Uniformitarianism</i> – the processes that formed ancient deposits are the same as those that form modern deposits.<br />
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The <i>Principle of Original Horizontality</i> - strata (or sedimentary rock layers) are deposited in a nearly horizontal position. If they are no longer horizontal, later deformation much have changed their orientation. <br />
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The <i>Law of Superposition</i> - younger sediments overlie older sediments.<br />
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<b>Walther’s Law</b><br />
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<b>Key Concept:</b> (Walther’s Law) Depositional environments vary in space and time such that “The facies [rock types] that occur conformably next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894).<br />
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One of the most important implications of Walther’s Law is that rocks of the same type are not necessarily deposited at the same time. There is a BIG difference between correlating rocks based on having the same lithology and rock being deposited at the same time. This is a critical conceptual idea that we will focus on throughout the class.<br />
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Let’s look at an alluvial fan in Gale Crater, Mars:<a href="http://tinyurl.com/be39pmv"> http://tinyurl.com/be39pmv</a><br />
Where are there differences in environment laterally? If the fan grew through time, how would you predict the deposits to change vertically?<br />
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For more information on ancient stream flows on Mars, see: <br />
<a href="http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1360">http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1360</a><br />
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<i>Summary of Walther’s Law:</i> <a href="http://www.youtube.com/watch?v=ZSsULiPouTo">http://www.youtube.com/watch?v=ZSsULiPouTo</a><br />
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<b> Sediment Transport</b><br />
Most sediment transport is due to gravity. Things fall down hill in slumps, debris flows, and mudflows, and are transported downhill by fluids, like water, ice, and air. In some cases, processes like waves, currents, and wind transport sediment up a slope such as a beach or up mountain sides. This transport goes against gravity and is driven by the processes of fluid dynamics. Fluid dynamics is the main topic of today's and Monday’s lectures. We will come back to mass wasting processes when we talk about erosion. Mass wasting is important for transporting large volumes of sediment short distances, but fluid transport is required to move sediments long distances and is responsible for most sediment transport. To understand sediment transport, it is essential to understand the mechanics of fluid flow.<br />
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<b>Fluid Flow</b><br />
There are two end member ways fluids flow: 1) laminar flow and 2) turbulent flow. There is a wide gradation between these two end members, specifically "transitional" flows. <br />
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<i>Laminar Flow</i> - In laminar flow, water molecules move in straight, parallel lines down current. If you add a dye to water that is in the laminar flow regime, the dye would not mix into the water; it would streak out in an approximately straight line. Laminar flow is characteristic of very slow moving, shallow water, which is uncommon in nature. It is also characteristic of flows in "fluids" that are very viscous, like glacial ice or mud flows that have little water. <br />
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<i>Turbulent Flow</i> - In contrast, turbulent flow is characterized by complex motion of water (or other) molecules. Molecules move in all directions in bursts of upward, downward, and forward motion, and even some backward movement. There is abundant mixing in the flow because neighboring molecules move in different directions, and an added dye mixes into the water very quickly. Most water and air flows are turbulent, at least to some degree. Turbulence is important for sediment transport in water and air because it makes grains easier to transport and tends to keep them moving longer. <br />
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<i>Transitional Flow</i> – Transitional flows have some characteristics of laminar flow and some of turbulent flow. For example, dye may take some time to mix into the flow, but it does mix.<br />
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Movies of Laminar and Turbulent Flow: <a href="http://www.youtube.com/view_play_list?p=23D938B8B55F49E8">YouTube Fluid Dynamics Playlist</a><br />
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<i>Image of rivers mixing</i>: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg</a> <br />
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<i>Images of glaciers</i>: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/GlacierTrails.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/GlacierTrails.jpg</a><br />
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Is the flow in glaciers laminar or turbulent? How can you tell? What properties of ice make it behave differently than water in terms of the amount of turbulence?<br />
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<i>Reynolds Number</i> - The Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid.<br />
<blockquote>
[<b>Viscosity</b> is a measure of the resistance of a material to flow, i.e. how “thick” and easily deformed it is. Viscosity is sort-of like the amount of friction within a substance. Walking through air is easy, because there is not much friction between air molecules. Air has a low viscosity. Swimming is more difficult because the water drags on your body. This is due to the “friction” between adjacent water molecules, i.e. higher viscosity. Ice is more viscous and impossible to move through because of the crystal bonds between the water molecules. It flows, but it does so slowly. It has a high viscosity relative to water and air (but low compared to most rock).] </blockquote>
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Back to the Reynolds number. The variables for the Reynolds number (<b>Re</b>) are: flow velocity (<b>u</b>), characteristic length (<b>l</b>) which represents flow geometry, say river depth, fluid density (<b>ρ</b>), and fluid viscosity (<b>µ</b>). The book uses <b>µ/ρ = v</b> (kinematic viscosity). <b>Re</b> = (fluid inertial forces)/(fluid viscous forces) = <b>l*u*ρ/µ</b>. The units for this equation are typically (length)*(length/time)*(mass/length<sup>3</sup>)/(mass/(length*time)). These all cancel out to form a unitless number, if you choose the same set of units for each variable, which you should always do. <br />
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<b>Re</b> can be viewed as inertial forces divided by viscous forces. Inertia is the resistance to change in motion, and inertial forces tend to make a bit of the fluid keep flowing in its own direction if it is misdirected from the main flow direction. Thus, high inertial forces tend to cause more turbulence. In contrast, viscous forces tend to suppress turbulence by damping out variations in motion through friction. Thus, a flow with a high viscosity (ice) tends to have less turbulence than a low viscosity flow (air). <br />
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The magnitude of <b>Re</b> gives an idea of whether the flow is turbulent or laminar. Turbulent flow has <b>Re greater than 2000</b> and laminar flow has <b>Re less than 500</b>. Flow with <b>Re</b> between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well. In most cases, water and air flows have high <b>Re</b> because <b>l</b> is large, <b>u</b> is high and <b>µ</b> is low. Rivers and wind storms are good examples of turbulent flow. In contrast, ice has a high <b>µ</b> and flows slowly (<b>u</b> is low), so it is usually laminar. Also, very thin, slow flows of water, such as water flowing off a smooth cement parking lot, have low <b>Re</b> because <b>l</b> and <b>u</b> are small. Thus, it can be laminar. Laminar flow also occurs locally in turbulent flows right at the contact between the fluid and a smooth surface because <b>u</b> becomes very slow. This is really important for sediment transport, and we'll talk more about it in a few minutes.<br />
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It is useful to think about which variables are important for different comparisons. When comparing ice and water, the main difference is viscosity; the viscosity of ice ranges from about 10<sup>3</sup> kg/(m*s) up to more than 10<sup>20</sup> kg/(m*s) depending on temperature. In contrast, the viscosity of water is ~10<sup>-3</sup> kg/(m*s). The density of both is very close to 1000 kg/m<sup>3</sup>. Thus, ice is almost always laminar but water is usually turbulent, although it can be laminar. When considering water flows, the flow speed and water depth are both very important. The viscosity and density change a little bit with temperature, but variations in flow speed and water depth are typically much larger effects.<br />
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<i>Images of glaciers:</i> <br />
<span style="background-color: white; color: #222222; font-family: Arial, Tahoma, Helvetica, FreeSans, sans-serif; font-size: 13px; line-height: 18px;">high ice viscosity: </span><a href="http://tinyurl.com/yhyrob9" style="background-color: white; color: #888888; font-family: Arial, Tahoma, Helvetica, FreeSans, sans-serif; font-size: 13px; line-height: 18px; text-decoration: initial;">http://tinyurl.com/yhyrob9</a><br />
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For air, both the density and viscosity are low, so does <b>Re</b> tend to be high or low? The density of dry air at 1 atm at 15°C is 1.225 kg/m<sup>3</sup>, and its viscosity is 1.8x10<sup>-5</sup> kg/(m*s), giving an inverse “kinematic viscosity” of p/µ=6.8x10<sup>5</sup> s/m2 for air versus 1.0x10<sup>6</sup> s/m2 for water. Thus, air would have a slightly lower value for Re than water for the same flow depth and speed. However, the thickness of typical air flows (meters to 100’s of meters) promotes turbulence. p/µ for ice is 1 to 10<sup>-17</sup> s/m<sup>2</sup>, which is why it is essentially always in a laminar flow regime. <br />
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Here is a video of a gust of wind that probably has a maximum <b>u</b> somewhere around 40 m/s: <a href="http://www.youtube.com/watch?v=iXlYEJaJ66A">http://www.youtube.com/watch?v=iXlYEJaJ66A</a> That’s me putting rocks on the fly of the tent. Note the turbulence and the snow transport. <br />
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<b>Boundary Layer</b><br />
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<iframe class="youtube-player" frameborder="0" height="312" src="http://www.youtube.com/embed/cUTkqZeiMow" title="YouTube video player" type="text/html" width="384"></iframe><br />
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There is boundary layer at the edge of every flow, and it is illustrated in the video above. Flows have an average speed in the middle, but friction with immobile surfaces slows down the speed of the flow right at the surface. This creates a boundary layer that has different flow characteristics than the rest of the flow. Right at the surface, the water does not move, but as you go higher into the flow it starts to move more like the average flow. The area of the flow that has a reduced speed is called the boundary layer. The thickness of the boundary layer depends on <b>Re</b> (i.e. the amount of turbulence) and the roughness of the surface the flow is moving past. If the main water flow is very turbulent, it changes the velocity distribution because more of the high speed water is mixed down into the lower speed areas. Thus, the boundary layer tends to be thin. In less turbulent flow, there is little mixing of water from the center of the flow toward the edge of the flow, so the boundary layer tends to be thicker. <br />
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<b>Viscous/Laminar Sublayer</b> - Within the boundary layer, right next to the surface, there is a laminar sublayer. <b>Re=u*l*ρ/µ</b> - remember this defines the difference between laminar and turbulent flow. Because <b>u</b> (water speed) is very low at the base of the boundary layer, the <b>Re</b> is low there and the flow is laminar. The laminar flow part of the boundary layer is called the viscous or laminar sublayer, “viscous” because the viscous properties of the fluid are more important than the inertial effects. The fluid is NOT more viscous here; rather, the inertia of the fluid is lower because <b>u</b> is lower, and the viscous properties dominate the behavior of the fluid. Farther up in the flow, <b>u</b> is higher, so the inertial properties of the flow dominate, and the flow is typically turbulent. If grains do not extend above the top of the laminar sublayer, they do not “see” much turbulence, and they are less likely to be transported. If they do stick up beyond the laminar sublayer because the laminar sublayer is thin or the grains are large, the grains feel the force of the turbulent flow.<br />
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<b>Bed roughness</b> or the characteristics of the surface also affect the boundary layer by affecting the amount of water that interacts with the surface. A very smooth bed, say one made of mud, does not deflect the water at all, so there is less mixing and less turbulence. There is a well developed laminar sublayer. In contrast, a bed with pebbles or boulders disrupts the direction of water flow in the boundary layer. The water gets deflected around the pebbles. Water from above tends to take its place. Since it is moving faster, the average water speed in the boundary layer increases. Thus, a rough bed reduces the thickness of the boundary layer much like a more turbulent flow does. A rough bed also disrupts the laminar sublayer by forcing the flow to move around objects. The laminar sublayer is developed locally, but in general, rough beds increase the amount of turbulence in a flow.<br />
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<b>Sediments and Flow </b><br />
<i>Key Concept:</i> The boundary layer strongly affects the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below grains, which tend to lift them off the bed.<br />
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<b>Bed Shear Stress</b> - Sediments are affected by the difference in flow speeds from the bottom to the top of the boundary layer, gravity, and friction with the ground. Bed shear stress is a measure of these differences; it is the differential force that a grain feels from top to bottom. In a thick boundary layer, the speed of water flow at the top of the grains is not much different from the bottom, so bed shear stress is lower, and sediment is less likely to move. In a thin boundary layer, bed shear stress is much higher, and grains are likely to roll down flow. Thus, more turbulent flow (with a thinner boundary layer) results in more sediment transport. Bed shear stress increases with increasing fluid density, slope, and turbulence (water depth and flow speed). For example, water is better at moving sediment than air because it has a higher density and exerts a larger bed shear stress than air can. Deep, fast rivers move more sediment than shallow, slow rivers because of more turbulence and higher flow speeds in the boundary layer in fast rivers.<br />
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<b>Next Time: </b>The Bernouli Effect, which causes grains move, the Hjulstrom Diagram, and sediment transport. Read Chapter 4 again.<br />
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<br />Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-13956775235569470562013-01-07T09:59:00.000-08:002013-01-07T09:59:06.657-08:00Introduction to Sedimentology and Stratigraphy<b>Sediments and Strata</b><br />
Sediments and sedimentary rocks cover most of earth (and large parts of mars), and weathering is occurring on the rest of it. The reshaping of the surface of the earth has had a huge influence on the planet, affecting everything from the evolution of life to the tectonics of mountain ranges. Sediments and sedimentary rocks record the events and processes that shaped the surface of earth – and other rocky planets. They provide the temporal framework that connects processes within the earth to those at the surface. They are important for:<br />
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1. Earth (and mars) history. Sedimentary rocks contain features that allow us to interpret ancient depositional environments, including the evolution of organisms and the environments they lived in, how climate has changed throughout earth history, where and when faults were active, etc.<br />
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2. Economic resources. Petroleum reservoirs have organic-rich, sedimentary source rocks that produced the petroleum when heated, most oil and gas migrates through sedimentary rocks, and most of the reservoirs are hosted in sedimentary rocks. Water aquifers are dominantly found in sedimentary rocks (although some are in fractured metamorphic and igneous rocks). The composition of the rocks strongly influences water quality due to water-rock interactions. (Why does Davis water taste bad?) Sedimentary rocks also host economic minerals such as gold and diamonds, which are eroded from other rocks and concentrated to specific areas during sediment transport.<br />
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3. Environmental geology. Sediments cover 2/3 of the continents and essentially all of the ocean floor, which totals 89% of the surface of earth. They host the biosphere, and they are most of the rocks we interact with directly and indirectly. Our actions as humans have an extremely strong effect on sedimentation and erosion. Understanding our impact on the environment - and the environment’s impact on us - must include deep appreciation for sediments and sediment transport.<br />
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<b>Sedimentology and Stratigraphy</b><br />
Sedimentology is the study of sediment transport processes and sedimentary rocks, and it is the focus of most of this course. If covers scales ranging from a single grain to entire planets, and focuses on processes. Stratigraphy is the study of the distribution of sediments and sedimentary rocks in space and time. It is essential for understanding earth history, reservoir properties, etc. We will place our sedimentological interpretations into a stratigraphic context with examples in lecture, the homework, and tests. <br />
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Sedimentology and stratigraphy are about as old as mineralogy as a field of study. Leonardo da Vinci provided one of the first environmental interpretations from sediments; he interpreted fossils in the Italian Apennines as evidence of an ancient ocean. He used the logic behind what we now call the <b>Principle of Uniformitarianism</b>: Similar organisms produce similar shells. The logic is, “If you see shells on the tops of mountains that look like those from organisms that live in the ocean today, the shells on the mountain tops were probably once in the ocean, too.” (Did the mountains go up or did the ocean go down? That is a question that was not thoroughly answered until we understood plate tectonics!) Here is a more formal statement of the Principle of Uniformitarianism:<br />
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<b>Key Concept:</b> The characteristics of sedimentary rocks can be used to determine the environmental conditions under which they were deposited, and the environmental conditions allow you to predict the characteristics of sediments that are likely to be deposited. This is the <b>Principle of Uniformitarianism</b> (formulated by James Hutton in the mid 1700’s) – the processes that formed ancient deposits are the same as those that form modern deposits.<br />
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Current ripples forming in a flume: <a href="http://youtu.be/rSzGOCo4JEk">http://youtu.be/rSzGOCo4JEk</a><br />
Wave ripples forming in a flume: <a href="http://youtu.be/MBDadRqSHOc">http://youtu.be/MBDadRqSHOc</a><br />
Modern Current Ripple Image: <a href="http://tinyurl.com/yjlw7gq">http://tinyurl.com/yjlw7gq</a><br />
Ancient Current Ripple Image: <a href="http://tinyurl.com/yh46jlm">http://tinyurl.com/yh46jlm</a><br />
Modern Wave Ripple Image: <a href="http://tinyurl.com/7dbwr82">http://tinyurl.com/7dbwr82</a><br />
Ancient Wave Ripple Image: <a href="http://tinyurl.com/7t8p2bz">http://tinyurl.com/7t8p2bz</a><br />
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In the much more recent past, for example in the 1970’s, the Principle of Uniformitarianism was interpreted by some as requiring continuous, incremental processes and as excluding dramatic, rapid events. For example, a meteorite impact was seen as a non-uniformitarian event. However, my view of uniformitarianism can encompass rare events. The basic idea is that catastrophic events also produce characteristic features. For example, a meteorite impact produces similar deposits no matter when it occurs in time. We can recognize impact spherules from Archean sedimentary rocks that formed and were deposited in essentially the same way as those from the Cretaceous impact that killed the dinosaurs. Or an impact on mars will produce features similar to those produced by an impact on earth. <br />
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<i>Brief summary of how geologists identified the K-T impact:</i> <a href="https://www.e-education.psu.edu/earth501/content/p3_p5.html">https://www.e-education.psu.edu/earth501/content/p3_p5.html</a><br />
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The key point is that <b>similar processes produce similar products</b>. All processes are not active at all times (large meteorites are not continuously bombarding earth!), and some, like burrowing by worms for example, did not occur at specific times, e.g. before the evolution of worms. However, if a feature is present that is characteristic of a specific process, e.g. a thin tube-shaped area in a siltstone with a slightly different color and a specific geometry, it is reasonable to interpret that process, e.g. burrowing by a worm, as having produced the feature. This is how we extract earth history from rocks, e.g. the absence of worm burrows before 540 Ma allows us to state with confidence that worms did not exist before 540 Ma. However, it is often challenging to identify which processes produce which features. There is rarely the nice, exact correlation between features and processes that one would wish for. For example, a specific color variation in a rock could reflect a burrow or a water flow path or both if the burrow influenced water flow. One needs to understand the uncertainties in geological interpretations.<br />
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Right now, I am using the Principle of Uniformitarianism as a scientist on the NASA Mars Science Laboratory (<a href="http://mars.jpl.nasa.gov/msl">http://mars.jpl.nasa.gov/msl</a>) - a mission that is using the rover Curiosity to investigate ancient environment in Gale Crater on mars. We are looking at grains and rocks, trying to understand what the ancient environments were like. The only way we can do this is to assume that sedimentary processes on mars produce the same features they would produce if they occurred on earth. There may be some processes on mars that are very rare or do not occur on earth, and these might produce features we do not recognize. However, if we see a current ripple, we can infer that there was flowing water on mars. The details of the flow speed, water depth, etc. might have been different (and we could calculate that), but the basic process was similar.<br />
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<i>Summary of the Principle of Uniformitarianism:</i> <a href="http://www.youtube.com/watch?v=ifdlx_dFzPU">http://www.youtube.com/watch?v=ifdlx_dFzPU</a><br />
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There are two other really important concepts that were first articulated in the 1660’s that we use on both earth and mars: The <b>Principle of Original Horizontality</b> and the fact that <b>younger sediments overlie older sediments</b>. Nicolas Steno was the first to write down the idea that strata (or sedimentary rock layers) are deposited in a nearly horizontal position, an idea called the <b>principle of original horizontality</b>. Some layers are deposited exactly flat, but most layers follow the tilt of the depositional surface, which is not exactly horizontal. However, most sedimentary layers are close to horizontal for our purposes here. If layers are no longer horizontal, later deformation must have changed their orientation. <br />
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<b>Images "horizontal" and tilted strata:</b><br />
Grand Canyon "horizontal" strata: <a href="http://www.flickr.com/photos/grand_canyon_nps/sets/72157626136162880/">http://www.flickr.com/photos/grand_canyon_nps/sets/72157626136162880/</a><br />
Rainbow Basin tilted and folded strata: <a href="http://en.wikipedia.org/wiki/File:Rainbow_Basin.JPG">http://en.wikipedia.org/wiki/File:Rainbow_Basin.JPG</a><br />
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This idea is intimately associated with the idea of time in rocks.<br />
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<b>Key Concept:</b> Younger sediments overlie older sediments (if they are still approximately horizontal)– obviously, at least to us now. <br />
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Steno first wrote this down in 1667. The relative ages of sedimentary rocks gives us time. We can interpret changes in processes through time using the principle of uniformitarianism combined with the relative age (or stratigraphic succession) of rock layers. Steno’s work provided the intellectual framework for understanding relative time in a local areas. It was not until much later that the idea of “faunal succession” (articulated by William Smith, early 1880’s) provided a global time scale. Smith (and other geologists at the time) recognized that fossil organisms succeed one another in the stratigraphic record in an orderly, recognizable fashion. They were learning about a key component of evolution, although they did not yet have the intellectual framework of evolution by natural selection. They formulated the basic ideas that if organisms evolve through time, rocks containing similar organisms are approximately the same age. <br />
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<i>Discussion of the distribution of time in sedimentary rocks:</i> <a href="http://www.youtube.com/watch?v=fBjR_1vK9ug">http://www.youtube.com/watch?v=fBjR_1vK9ug</a><br />
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The MSL team is using these ideas to interpret the time history of events in Gale Crater. One of the reasons we chose to go to this landing site was that Mt. Sharp, which is a 5 km-high mountain in the crater, contains layers of rock. These layers record a history of processes and events that occurred in Gale Crater, they contain clues to what mars was like about 3 billion years ago. We do not yet know very much about the rocks. However, we do know that the earliest history is in the rocks at the bottom, and changes in the rocks going upward will reflect changes through time. In other words, the history book starts at the bottom.<br />
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<i>Strata in Mt. Sharp from the Curiosity landing site:</i> <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/Mars/MtSharp_M100.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/Mars/MtSharp_M100.jpg</a><br />
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On earth, geologists have studied the time succession of rocks to create the Geological Time Scale. This time scale is largely based on fossils (for rocks younger than mid-Neoproterozoic time), with radiometric ages providing absolute time and refinements on our understanding of evolution. Continual refinements in the time scale provide insights into the process and history of earth that are core to geology. In many ways, the geological time scale documents our substantial understanding of earth. In contrast, the Martian Geological Time Scale is just being built. We know very little about the history of mars, and right now, we can only use how old rocks look - the number of craters or how many other rocks are on top of them - to get relative ages. Some planetary scientists have noted a correlation between apparent age and mineralogy, with the oldest rocks having more clay minerals, medium aged rocks having more sulfate minerals, and the youngest rocks being mostly wind deposits. On average, this may be true, and this model provides a testable hypothesis, that we can test in part with Curiosity. Rocks in Mt. Sharp show spectral signatures of both clay and sulfate minerals. Using Curiosity, we will analyze the mineralogy of these rocks, in the context of their sedimentary structures, grain size, etc. to interpret the environments they formed in and their relative ages.<br />
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It was this process on earth that led to the definition of Cambrian, Ordovician, etc. Comparisons of the spatial distribution of similar features leads to many important insights. Da Vinci recognized that fossils can be used to interpret ancient rocks; he interpreted similar environments because the shells he saw were essentially identical to those in the modern oceans. To Smith and other English geologists, similar fossils suggested the rocks were similar ages. <br />
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These two ideas reflect the two key components of stratigraphy: rock types vary in both space and time, but the same type of rock can be deposited in different places at different times. These changes can be organized based on how different environments are distributed: <br />
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<b>Key Concept:</b> (Walther’s Law) Depositional environments vary in space and time such that “The facies [rock types] that occur conformably* next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894).<br />
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(*<i>Conformably</i> means that there is neither a break in sedimentation nor erosion between the two environments, e.g. there is no unconformity between them. Jumps in depositional environment can occur if the rocks do not provide a complete record of the environmental changes that occurred; rock types in a vertical succession separated by the unconformity do not necessarily represent neighboring environments.)<br />
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<i>Images of environments and a Google Earth tour:</i> Google Earth kmz files (You must have the program Google Earth installed on your computer. Download and open this file.): <br />
Environments from Davis to San Francisco: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/GoogleEarth/DavisSFOEnvironments.kmz">http://mygeologypage.ucdavis.edu/sumner/gel109/GoogleEarth/DavisSFOEnvironments.kmz </a><br />
Tidal Environments near Derby, Western Australia: <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/GoogleEarth/TidalEnvironmentsDerby.kmz">http://mygeologypage.ucdavis.edu/sumner/gel109/GoogleEarth/TidalEnvironmentsDerby.kmz</a><br />
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One of the most important implications of Walther’s Law is that rocks of the same type are not necessarily deposited at the same time. There is a BIG difference between correlating rocks based on having the same lithology and rock being deposited at the same time. This is a critical conceptual idea that we will focus on throughout the class. And what does this suggest for the martian time scale, which is based on mineral compositions?<br />
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<i>Summary of Walther’s Law:<i> <a href="http://www.youtube.com/watch?v=ZSsULiPouTo">http://www.youtube.com/watch?v=ZSsULiPouTo</a><br />
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<b>Next time - Sediment transport</b><br />
<i>Reading: </i><br />
Chapter 4 is the most important for Wednesday’s lecture.<br />
Chapters 2 and 3 cover classification of sediments and sedimentary rocks. We are not going to cover them explicitly in class, but knowing the terminology will be critical. Thus, read these chapters soon! Chapter 2 will be very useful for those of you in lab.</i></i>Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com2tag:blogger.com,1999:blog-2103960834361055322.post-60175152821125729112013-01-07T09:40:00.000-08:002013-01-07T09:40:18.107-08:00GEL 109 Winter 2013: Business: <b>Business</b><br />
The Course web site is the SmartSite for GEL109/109L and <a href="http://mygeologypage.ucdavis.edu/sumner/gel109">http://mygeologypage.ucdavis.edu/sumner/gel109</a> which is linked on the SmartSite.<br />
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<i>I am responsible for:</i><br />
1. Coming to lecture prepared to provide you with the opportunity to learn about sedimentology. <br />
2. Preparing instructive homework assignments and tests that will help you learn the most important material. My philosophy is to make it clear what I think is important and provide you with tools to learn the material. I provide old tests and detailed study guides so that you can thoroughly learn the material and demonstrate your knowledge of it on the various assignments. <br />
3. Grading your homework and tests in a fair and timely manner. <br />
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<i>You are responsible for:</i><br />
1. <b>Coming prepared for lecture by reviewing your previous lecture notes and reading the text book.</b> Reading the book is very important because it includes some information that I will cover only briefly in class, but I’ll expect you to know. For example, I will not discuss rock classification schemes in lecture because it is boring and the book explains the rock types well. However, I will use the terms sandstone, siltstone, shale, etc. because they are always used in geology, and you need to know them. In most cases, the material I cover in lecture and with the homework assignments are the concepts I think are most important, so the tests will focus on those. <br />
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2. <b>Asking questions when you have them in class, through e-mail, during office hours, when you see me in the hall, etc.</b> Asking questions is VERY important for two reasons. First, it will help you and your fellow students learn, which is the entire reason you are in this class. Second, it helps me gage how well I'm communicating information to you. The more I know about how much you are understanding and what you are thinking about, the better I can prepare explanations that are both clear and interesting. <br />
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<i>Grades</i><br />
Grades will be calculated using two formulas: 2 tests and 10 homework assignments with the midterm=33%, final=34%, and homework=33%; and 2 tests with the midterm=50% and final=50%. You will get the higher grade from the two formulas. I do not grade on a curve, so if you all thoroughly learn the material, you can all earn A’s. I have taught this class enough times that I know you can do so if you work hard and ask lots of good questions.<br />
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<i>Tests</i><br />
My tests are hard, but fair. They are hard because they ask you to think about the material. I will give you detailed study guides which will contain all of the material that will be on the tests (plus some). The homework will include questions like those on the tests, and practice tests are available. The only people who have failed the tests are those that did not take advantage of the study tools.<br />
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<i>Homework</i><br />
The homework assignments will be posted on SmartSite as pdf files. For some assignments, there are supplemental web materials. These materials are very useful, and you should plan to use them. Homework is due by the end of the date listed.<br />
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<i>Study Resources</i><br />
Over the years, I’ve accumulated numerous study resources including images and videos. I have posted summary videos on YouTube since 2007, and those videos are used by people all over the world to help learn sedimentology and stratigraphy concepts. I made them for you, my students in GEL109, and they emphasize the key concepts from many of the lectures. My channel is sumnerd. Use the videos!<br />
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<i>For people not taking the lab:</i><br />
The students who are also taking the lab (GEL109L) are spending an additional 6 hours per week working on sedimentology, so they will be more familiar with the material than you are. You can still do well, and it is particularly important that you ask questions. Also, there are two field trips required for the lab. I strongly recommend coming on them. You can learn much more in the field than in lecture or even lab!Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-25575741352278763292012-03-14T12:37:00.000-07:002012-03-14T12:38:18.945-07:00ChronostratigraphyThe idea behind chronostratigraphy is to correlate rocks that formed at the same time. This is useful for reconstructing events and depositional environments in earth history as well as finding resources like oil. There are several techniques that can be used for chronostratigraphy, including: event stratigraphy, magnetostratigraphy, chemostratigraphy, biostratigraphy, and sequence stratigraphy. Here, I will address event stratigraphy, magnetostratigraphy, and biostratigraphy. Sequence stratigraphy is very powerful, and lots of resources on it can be found at: <span class="s2"><a href="http://sepmstrata.org/">http://sepmstrata.org/</a></span><br />
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<span class="s1"><b>Event Stratigraphy</b></span></div>
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<span class="s1">Event stratigraphy involves identifying the sedimentary effects of an unusual event in multiple stratigraphic columns. If one can demonstrate the the effects were all produced by the same even, one can reasonably interpret the effects to have happened at the same time in the different columns. For example, if a volcano erupts and deposits ash over a broad region, that ash is preserved in the stratigraphy, and a geologist can demonstrate that the ash in multiple sections came from the same eruption, then the geologist can create a chronostratigraphic correlation among the sections. Other events that are useful for event stratigraphy can include impact debris layers (for example at the Cretaceous-Tertiary boundary), tsunami deposits, and sometimes large storms. </span><br />
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<span class="s1">There are some shortcomings of event stratigraphy. First, there has to be an event that affects the stratigraphy. For it to be useful, it needs to be something that affects multiple depositional environments in a way that produces a distinctive set of features that can be distinguished from the normal depositional processes. Second, for a specific event to be useful for correlations, it has to have affected the stratigraphy at the sites of interest. For example, a tsunami that affected the west coast of North America might help one correlate Pleistocene coastal deposits in Oregon and Washington. However, it would not be helpful for correlating Pleistocene rocks in Florida because it did not influence them. Third, if there are multiple events, the geologist has to sort out which correlate with each other. For example, if there are multiple volcanic eruptions at different times, the geologist needs to evaluate which eruptions the ash beds might represent. It can become complicated to correlate many events. Sometimes correlations are more reliable if there are fewer events, but then there are not as many potential temporal ties between the stratigraphic columns. Even with these complications, event stratigraphy is a very valuable tool. When the events are volcanic, the ash beds can often be dated, providing a precise age for a segment of the stratigraphic column.</span><br />
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<span class="s1"><b>Magnetostratigraphy</b></span></div>
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<span class="s1">Magnetostratigraphy uses preserved magnetization of rocks for correlation. The magnetization comes from the alignment of magnetic minerals in sedimentary rocks (and other types of rocks) with the earth’s magnetic field. Small magnetic minerals, especially clay-sized minerals, align like little magnets, and when the sediment is lithified, that magnetization can be preserved. Under the right conditions, samples can be collected and the direction of magnetization measured. Data can be used to reconstruct the direction of the earth’s magnetic field. This magnetic field can reverse directions due to the dynamics of circulation in the core. In other words, sometimes the magnetic field is aligned such that magnets point north (as they do now, and called “normal” in the scientific literature) and sometimes it is aligned such that magnets point south (called “reversed”). The earth’s magnetic field changes at close to the same time globally, so the effects are seen everywhere. Paleomagnetists have studied well dated sedimentary and volcanic rocks and have mapped out the times in earth history where the magnetic field was normal and reversed (see: <a href="http://www.geosociety.org/science/timescale/"><span class="s2">http://www.geosociety.org/science/timescale/</span></a>). This provides a reference that can help correlate other stratigraphic sections.</span><br />
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<span class="s1">To correlate a suite of stratigraphic sections using magnetostratigraphy, one would collect samples, measure their magnetic properties using a variety of techniques, and evaluate whether or not they have been remagnetized. If they have not been remagnetized, changes in the direction of earth’s magnetic field can often be interpreted from the results. If a geologist has multiple sections that were deposited at the same time, they can interpret the changes in the direction of earth’s magnetic field to have happened at the same time. Unfortunately, however, one can not necessarily independently tell the many normal intervals apart from each other nor the many reversed intervals apart. Thus, the geologist needs additional information to make reliable chronostratigraphic correlations.</span><br />
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<span class="s1"><b>Biostratigraphy</b></span></div>
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<span class="s1">Biostratigraphy is an extremely powerful tool for chronostratigraphic correlation. Life evolves through time, with new species emerging and other species going extinct. For time intervals and species that are well studied, the process of evolution provides a detailed temporal framework for correlating stratigraphic columns. The basic idea is for the geologist to identify fossils in the stratigraphic columns, compare them to the ranges of those organisms know from previous studies, and then interpret the age of the rocks from documented extinction and species origination events. This is an extremely powerful approach to correlating stratigraphic columns because each species is unique and changes through time. However, not all organisms are useful for biostratigraphy. </span><br />
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<span class="s1">Good biostratigraphic species: 1) have short geological ranges, e.g. they did not live for millions of years, and evolved quickly; 2) were distributed over a large region of the earth; 3) were easily preserved; and 4) were abundant. They also need to be well studied.</span></div>
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<span class="s1">Zones of well documented species with distinct origination and extinction times can be defined a number of ways. A zone could consist of the total time of existence of a fossil, it could consist of the time where two or more fossils coexist, it could be defined as the time between the origination of one fossil and the extinction of a different fossil, etc. An example of a biostratigraphic zone chart, combined with magnetostratigraphic reversals can be found at: <span class="s2"><a href="http://www-odp.tamu.edu/publications/189_IR/chap_02/c2_f6.htm">http://www-odp.tamu.edu/publications/189_IR/chap_02/c2_f6.htm</a></span></span><br />
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<b>Cretaceous-Tertiary Boundary Example</b></div>
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The end of Cretaceous time is marked by a major extinction, a meteorite impact, and reversals in the Earth's magnetic field. The following are some figures we'll use in class. I'll update the notes and reference the figures after class (my computer just crashed and I lost what I'd written - save often!)</div>
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<span class="s1"></span></div>Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-64994960066779692802012-03-13T15:38:00.001-07:002012-03-13T18:04:03.148-07:00Regional Strat Column CorrelationsGuest lecture by Cara Harwood on regional correlations of stratigraphic columns across the Precambrian-Cambrian boundary in the western US<br />
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<span class="s1">Geology 109: Sediments and Strata</span></div>
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<span class="s1">March 12, 2012</span></div>
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<span class="s1">Cara Harwood</span><br />
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<span class="s2"><b>Integrated Stratigraphic Analysis: </b></span><b>Correlating multiple columns using lithology, biostratigraphy, </b><b>and chemostratigraphy</b></div>
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<span class="s2">Recap</span><span class="s1">: You have been looking at interpreting stratigraphic columns based in lithology, distinctive structures, sequences of structures. We use Walther’s law to predict how rocks will be stacked vertically. </span></div>
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<span class="s1"><i>Remind yourself what Walter’s law is. (</i></span><i><a href="http://dawnssedstrat.blogspot.com/2012/01/uniformitanianism-original.html">http://dawnssedstrat.blogspot.com/2012/01/uniformitanianism-original.html</a>)</i></div>
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<span class="s2"><b>Today</b></span><span class="s1">: Looking at multiple stratigraphic columns from the same time period that correlate across a region. Focusing on the Precambrian-Cambrian boundary and how multiple columns are used along with biostratigraphy and chemostratigraphy to understand and recognize this key time period. </span></div>
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<span class="s1">The Precambrian-Cambrian transition records one of the most important intervals in the history of life. It encompasses appearance and diversification of metazoans, a change in style of bioturbation (organisms are moving to different types of environments within the sediment), and mineralization of skeletons. </span><br />
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<span class="s1">Sedimentologists, stratigraphers, paleontologists, have studied this interval to try to learn about: </span></div>
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<li class="li1"><span class="s1">the time of the boundary, and correlating the boundary globally</span></li>
<li class="li1"><span class="s1">timing of evolutionary events</span></li>
<li class="li1"><span class="s1">what environments were like where these events were happening</span></li>
<li class="li1"><span class="s1">how environments were distributed</span></li>
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<span class="s1">They studied stratigraphic columns and looked at </span><span class="s2">lithologic, paleontologic, and chemostratigraphic data.</span><span class="s1"> We will talk about each of these types of data and how they are correlated in the context of the pC-C boundary in southern California. </span><br />
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<span class="s2"><b>Stratigraphy Types</b></span></div>
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<span class="s1"><b>Biostratigraphy</b>: characterization and correlation of rocks based on their fossil content, based on the principle that organisms have undergone successive changes through geologic time</span></div>
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<span class="s1"><b>Chemostratigraphy</b>: characterization and correlation of rocks based on their chemical composition, based on the principle that certain chemical signatures occur globally through geologic time</span></div>
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<span class="s1">Why is studying the pC-C boundary in California interesting? </span></div>
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<li class="li1"><span class="s1">The boundary is defined by a trace fossil that is present in siliciclastic rocks</span></li>
<li class="li1"><span class="s1">It is also defined by a specific chemical signature in carbonate rocks</span></li>
<li class="li1"><span class="s1">Many regions that preserve this boundary do not have both siliciclastics and carboantes together, but they are both present in this region so we can see how they correlate</span></li>
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<span class="s1"><i>Paleogeographic map of North America </i></span></div>
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<span class="s1"><i>550 Ma:</i></span></div>
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<tr><td style="text-align: center;"><a href="http://www2.nau.edu/rcb7/namPC550.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="309" src="http://www2.nau.edu/rcb7/namPC550.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">http://www2.nau.edu/rcb7/namPC550.jpg</td></tr>
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<i>510 Ma:</i><br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://www2.nau.edu/rcb7/namC510.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="309" src="http://www2.nau.edu/rcb7/namC510.jpg" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">http://www2.nau.edu/rcb7/namC510.jpg</td></tr>
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<span class="s1"><i>Regional map showing positions of the craton margin relative to the Death Valley region and White-Inyos</i></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgl3DJI2xj6UteZK_LOdmDLJaSIGnLVvYj6blGI-KXWZPghdZjirLczotxwkmNqLFXlDA1Wp-LYeJU9PEM7xBhyqSTgc3wtM58s6rzP2Xx_v_orKr6gL8gTRBJLSwqXdGBKtJwYMvSWJrQ/s1600/pC-C_map.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="277" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgl3DJI2xj6UteZK_LOdmDLJaSIGnLVvYj6blGI-KXWZPghdZjirLczotxwkmNqLFXlDA1Wp-LYeJU9PEM7xBhyqSTgc3wtM58s6rzP2Xx_v_orKr6gL8gTRBJLSwqXdGBKtJwYMvSWJrQ/s320/pC-C_map.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf</td></tr>
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<span class="s1"><i>Look at White-Inyo stratigraphic column (on left of the following figure) and talk through the whole thing - lithology, fossils including trace fossils...</i></span></div>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhChn4eM204v0Mr4kI4loZ-wAbIimj1vKSkfKYGaTVSZpyjati4Iwror5FaH35hJB96JLP90ANGxGQh9-p52lQbJ78atiQN_q6Cr4PNLwVJmUceG3nruSp3ppH3ovDG5ngEH3BNHyv3d18/s1600/pC-C_stratcolumn_1.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="161" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhChn4eM204v0Mr4kI4loZ-wAbIimj1vKSkfKYGaTVSZpyjati4Iwror5FaH35hJB96JLP90ANGxGQh9-p52lQbJ78atiQN_q6Cr4PNLwVJmUceG3nruSp3ppH3ovDG5ngEH3BNHyv3d18/s320/pC-C_stratcolumn_1.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf</td></tr>
</tbody></table>
Formations around the Precambrian-Cambrian boundary:<br /><ul>
<li class="li1"><span class="s2">Wyman Formation</span><span class="s1">: interbedded mudrock, siltstone, quartzite; interbedded carbonate layers that increase in number upsection --> shallow marine deposition</span></li>
<li class="li1"><span class="s1"><i></i></span><span class="s2">Reed Dolomite</span><span class="s1">: cross bedded oolitic grainstone, stromatolites --> shallow marine, subtidal to intertidal; then hummocky cross bedded sandstone --> deposition below normal wave based, but above storm wave base. </span></li>
<li class="li1"><span class="s1"><i></i></span><span class="s2">Deep Spring Formation</span><span class="s1">: siliciclastic carbonate couplets, with ripple laminated quartzites with hummocky cross stratification, cross bedded oolites, intraclastic grainstone --> high energy shallow water depositional environment. </span></li>
</ul>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s1">Each of these is a <b>formation: </b>a mappable<b> </b>unit that can be defined by its lithology and stratigraphic position; has some degree of homogeneity in rock type, mineralogical composition, sedimentary structures, fossil content. </span><br />
<span class="s1"><br /></span></div>
<div class="p2">
<span class="s1"><span class="Apple-tab-span"> </span></span></div>
<div class="p1">
<span class="s1"><i>Treptichnus pedum</i> - pC-C boundary index fossil globally; it is one of the first (and once thought to be the first) complex trace fossils, indicating more complex organisms that made the trace. <i>Show examples of this trace fossil.</i></span><br />
<span class="s1"><i><br /></i></span></div>
<div class="p2">
<span class="s1"><i></i></span></div>
<div class="p1">
<span class="s1">Once we get into the Cambrian trilobites are good index fossils that are useful for biostratigraphy; specific time zones with unique assemblages of trilobite species.</span><br />
<span class="s1"><br /></span></div>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s2"><i>Index fossils</i></span><span class="s1"><i> generally</i> - can be trace fossils or body fossils; they mark specific time periods. Good index fossils are taxa that appear and disappear (evolve rapidly), that are widespread - global distribution, that are distinctive and abundant, and that are facies independent. **T. pedum is only found in siliciclastic rocks, so not found everywhere at the boundary. </span><br />
<span class="s1"><br /></span></div>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s1"><i>Look at how the White-Inyo section is correlated with other sections in the region - </i></span><i>Death Valley and craton margin sections. </i></div>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s1"><br /></span><br />
<span class="s1">Note that the scale of these columns is different than others that we have been looking at - the total thickness of the White-Inyo column is 2900 m, so we’re not looking at each individual bed, but rather looking at facies and dominant lithologies. </span><br />
<span class="s1"><i><br /></i></span><br />
<span class="s1"><i>Look at the 510 Ma paleogeographic map again to see the broad environments where each of these columns are from. </i></span><br />
<span class="s1"><i><br /></i></span></div>
<div class="p2">
<span class="s1"><i></i></span></div>
<div class="p1">
<span class="s1">Marine deposits in the west correlate to the east with non-marine facies: glacial deposits, non-marine sandstone and conglomerate, and there are more unconformities. Also notice how the overall thickness of the columns is becoming thinner to the east. </span><br />
<span class="s1"><br /></span></div>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s1">What general observations can we make about correlating stratigraphic columns across a region? </span></div>
<ul>
<li class="li1"><span class="s1">sediment packages thicken as we move from the craton (land) out into the basin</span></li>
<li class="li1"><span class="s1">more unconformities (can happen when exposed, above sea level) towards land</span></li>
<li class="li1"><span class="s1">facies shift from being a mix of non-marine and marine to mostly marine and deeper water facies</span></li>
<li class="li1"><span class="s1">correlating long distances - can’t do lithostratigraphy (i.e. correlating sands to sands)</span></li>
<li class="li1"><span class="s1">correlations based on fossil/trace fossil occurrence are robust</span></li>
</ul>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s1"><i>Look at chemostratigraphic signature (C isotopes) from ‘offshore’ (White-Inyos) to ‘onshore’ (Death Valley and craton sections):</i></span></div>
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<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6QWdCdCkYBoaPb_ZqVWk9ipqgsbEwN52Z3Z_Ve9QYs_ItQ9tF1Tp7OWoMRSK8BeA2tdKlfxvAPoZ-Z05etX_ij-MdCdB5cI2ZFhueAL_DjbAiBaVj_tvLgsYi9GOI_poZ1judZuVOBA4/s1600/pC-C_stratcolumn_2.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="146" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6QWdCdCkYBoaPb_ZqVWk9ipqgsbEwN52Z3Z_Ve9QYs_ItQ9tF1Tp7OWoMRSK8BeA2tdKlfxvAPoZ-Z05etX_ij-MdCdB5cI2ZFhueAL_DjbAiBaVj_tvLgsYi9GOI_poZ1judZuVOBA4/s320/pC-C_stratcolumn_2.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf</td></tr>
</tbody></table>
<div class="p1">
<span class="s1">Correlations across a region (and globally!) can also be made based on </span><span class="s2">chemostratigraphy. </span><span class="s1"> Even though the lithology is different, carbonate rocks track the concentration of ions in seawater, so the marine rocks everywhere can have the same signature. </span><br />
</div>
<div class="p2">
<span class="s2"></span></div>
<div class="p1">
<span class="s1">We’re not going to talk about what these chemical signatures mean, but notice how in the Lower Deep Spring Formation there is a positive 13 C value, and then around the pC-C boundary there is a negative value. This is a global trend during this time period. </span><br />
<span class="s1"><br /></span></div>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s1">Other elements also have global trends and can be used for correlating based on chemostratigraphy. <i>Show examples of Sr and O isotope curves. </i></span><br />
<span class="s1"><i><br /></i></span></div>
<div class="p2">
<span class="s1"><i></i></span></div>
<div class="p1">
<span class="s1">Using this integrated approach (chemostratigraphy and biostratigraphy) showed that the index fossil and the distinct chemical signature occur at the same time. With this time constrained, we can apply either one of these (chemical signature or fossil) to correlation in other sections, and use this to better understand this time period.</span><br />
<span class="s1"><br /></span></div>
<div class="p2">
<span class="s1"></span></div>
<div class="p1">
<span class="s1">Wrap up - this is an example of using stratigraphy. Key points: </span></div>
<ul>
<li class="li1"><span class="s1"><i></i>Looking at mixed carbonate-siliciclastic facies allows us to combine data types that occur in just one</span></li>
<li class="li1"><span class="s1"><i></i>We get a complete picture of the time period by looking at a combination of data sets (lithostratigraphy/facies, biostratigraphy, and chemostratigraphy)...</span></li>
<li class="li1"><span class="s1"><i></i>...and how they are distributed across the region - looking at multiple stratigraphic columns</span></li>
</ul>Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com1tag:blogger.com,1999:blog-2103960834361055322.post-2834801468195935652012-03-07T10:24:00.003-08:002012-03-13T16:41:41.908-07:00Interpreting Stratigraphic Columns<br />
<div style="text-align: center;">
<span style="font-size: large;">Interpreting Stratigraphic Columns</span><span style="font-size: large; font-weight: bold;"><br />
</span></div>
<div style="text-align: center;">
<b style="font-weight: bold;"><br />
</b></div>
<div style="text-align: left;">
<b style="font-weight: bold;">Step 1: Look for sedimentary structures that are characteristic of a specific environment or process</b></div>
Examples:<br />
<br />
<ul>
<li>Hummocky Cross Stratification - waves plus currents (storms)</li>
<li>Wave Ripples (vs current ripples) - waves (standing water)</li>
<li>Herringbone Cross Stratification - bidirectional flow over hours or longer (tides)</li>
<li>Reactivation Surfaces - reshaping of bedforms due to changes in flow (tides)</li>
<li>Mud Drapes in sandstone - flow stops (tides)</li>
<li>Bouma Sequence - rapid flow slowing down (turbidity current)</li>
<li>Mud Cracks - mud contracts (exposed to air)</li>
<li>Root Casts - from plants, usually land plants (land)</li>
<li>Faint Ripple Cross Lamination with reverse grading - (eolian ripples)</li>
<li>Meter-high Dunes in fine sand - (eolian dunes)</li>
<li>Diamictites - laminar flow - (debris flows, mud flows, melting ice)</li>
<li>Diamictites with Facetted Clasts and Striations - (glacial)</li>
<li>Lone (or drop) Stones in laminated shale - large grains rafted over quiet environment (icebergs; trees possible)</li>
</ul>
<br />
<b style="font-weight: bold;">Step 2: Evaluate how these distinctive structures relate to each other in the stratigraphic column to develop a tentative environmental interpretation</b><br />
<ul>
<li>Are there several indicators of waves or storms?</li>
<li>Are there several indicators of tides?</li>
<li>Are there several indicators of wind-deposited sediment?</li>
<li>Are there several indicators of glacial activity?</li>
</ul>
<b style="font-weight: bold;">Step 3: Compare the tentative interpretation to flow implied by other sedimentary structures in the column and evaluate whether they are consistent with your tentative environmental interpretation.</b><br />Examples of other sedimentary structures:<br />
<br />
<ul>
<li>Trough Cross Stratification</li>
<li>Planar Cross Stratification</li>
<li>Current Ripple Cross Lamination</li>
<li>Planar Lamination or Stratification</li>
</ul>
<br />
<b style="font-weight: bold;">Step 4: Evaluate how the vertical sequence of sedimentary structures changes to refine or correct your environmental interpretations.</b><br />
<br />
<ul>
<li>Do structures occur in a distinctive pattern that suggests a depositional environment?</li>
<li>Is there an erosion surface followed by dune stratification followed by ripple lamination followed by a rooted horizon? (Then it might be migrating river channels or tidal channels if there are indicators of tidal currents.)</li>
<li>Do the structures suggest an environment that shallows upward into a river system? (Then it might be a delta building out into standing water.)</li>
</ul>
<br />
<b style="font-weight: bold;">Step 5: Use Walther's Law to refine your environmental interpretations and to test whether or not they are reasonable. </b><br />
Try to sketch neighboring environments and interpret how they shifted through time. Are your interpreted vertical changes in environments consistent with neighboring environments horizontally? Does your interpretation require any jumps in environments or imply an unconformity? Revise your interpretation until it is consistent with your data.<br /><br />
Often, there is some ambiguity about the depositional environment(s) represented in real rocks. By going through this process, you can reach a reasonable interpretation that is well supported by the data. You will also understand where the ambiguities are. This is particularly helpful if it is your own data and you can make more observations by doing more field work.<br /><br />
<br />
Here are some example stratigraphic columns to think about:<br />
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</div>Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com4tag:blogger.com,1999:blog-2103960834361055322.post-82966183062557476352012-03-05T11:38:00.000-08:002012-03-13T18:05:01.741-07:00Marine Shorelines and Interpreting Stratigrahic Columns<b><span style="font-size: large;">Marine Shorelines</span></b><br />
<br />
Shorelines are the interface between the land and the oceans. Their characteristics vary depending on the balance of sediment supply and transport processes. When the sediment supply from rivers is large compared to the rate at which transport processes redistribute the sediment, deltas form, building out into the ocean. If sediment supply is low compared to the rate of sediment transport seaward of the shoreline, the shoreline erodes back. When sea level rises, river valleys can become flooded with marine water, creating estuaries. When sea level falls, rivers tend to erode downward into the previously coastal sediments. <br />
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The balance between tides and waves also affects the geometry of shorelines. Wave-dominated shorelines tend to have beaches, whereas tide-dominated shorelines tend to have broad marshy flats. Either can be erosional if the offshore transport of sediment is higher than the sediment supply or constructional if offshore transport is lower. They can shift back and forth through time if sediment supply or transport processes change. Thus, most shorelines are dynamic environments that vary significantly on human time scales.<br />
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<b>Wave Influenced Shorelines</b><br />
Waves have very specific sediment transport characteristics, with the highest energy flows near the breaker zone and lower flows both onshore and offshore. The onshore flows transport sediments to form beaches. The swash zone is the area that forms the primary beach. During storms, the waves are commonly higher, and, if sufficient sediment is available, they carry sediment farther up the beach, creating a berm. This gives the beach a characteristic slope up away from the shore, a crest, and then a slope downward. In some cases, the beach can extend off the coastline, creating a barrier bar or barrier island. A lagoon then forms between the beach and the main coastline. When there is a large sand supply, these barrier bars and islands can grow to be quite large. However, waves also transport sand off shore, going from the high energy breaker zone to the lower energy deep water. If the sand supply is low, more sand can get transported offshore than is delivered to the beaches. This causes beaches, barrier bars, and barrier islands to erode. <br />
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<b>Tide Influenced Shorelines</b><br />
Tidal currents flow on and off shore every day or twice a day. When tidal ranges are high, tidal currents can be strong, redistributing sediment either onshore or offshore. These tidal currents often become channelized, and they begin to act like rivers, with meanders, etc. <br />
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<b>Constructional Shorelines: Deltas</b><br />
<i>Deltas</i> form at the mouths of rivers that transport enough sediment to build outward. (<i>Building outward</i> is a key component of the definition of a delta. Rivers where the ocean or lake floods the river valley flow into <i>estuaries</i>.) Deltas require substantial accumulation of sediment, in contrast to estuaries which do not build outward. Sedimentary facies are similar to other depositional environments, but the association of subenvironments are recognizable as deltas. Some of the sub environments include: river facies with all the associated sub environments; shore line deposits including beaches, marshes/swamps, etc.; submarine shelf and slope facies, including storm deposits and turbidites; etc. <br />
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Deltas consist of the delta plane, delta slopes, and prodelta. Rivers flow through delta planes and slow when reaching water, producing a mouth bar. Grain size decreases with distance away from the river mouth.<br />
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<b>Progradation</b> - Because deltas are sites of sediment building outward from the coast, they are progradational; the landward depositional environments move seaward over more marine/lacustrine deposits. Thus, delta sequences in the rock record start with deep water, marine, fine grained sediments and grade upward into shallower water, possible more freshwater, coarser grained sediments. This is one of the distinguishing aspects of deltas that let you define them in the sedimentary record. These changes in grain size and environment typically occur over 1’s to 100’s of meters in the rock record and include many beds.<br />
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<b>Sediment Transport Type</b> - All deltas (by definition) have their sediment transported to the delta by rivers. Thus, riverine deposits are always associated with them. In addition, depending on marine (or lacustrine) conditions, waves and tides can redistribute the riverine sediment changing the morphology and facies of deltas. There are three main end member categories of deltas when characterized by processes: 1) River dominated; 2) Wave influenced; and 3) Tide influenced.<br />
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<b>River Dominated Deltas</b> - River dominated deltas have very low wave energy and a very small tidal range. Delta top deposits are well developed and are very similar to meandering river deposits, including channel, levees and overbank deposits. Overbank areas are commonly heavily vegetated and result in peat and coal deposition. Channels build out into the ocean (or lake) on top of their mouth bars. This leads to a coarsening upwards of grain sizes within the mouth bars as well as a change from some marine processes to unidirectional river flow. Avulsion of the rivers is common due to low gradients on the delta plain. Lobes of the delta become abandoned creating a “bird’s foot delta”. Sheltered bays are common between the lobes, and are filled with overbank deposits from floods as well as marshy deposits. The Mississippi River Delta is a classic river dominated delta.<br />
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<b>Wave Influenced Deltas</b> - Waves redistribute the sediment deposited by the rivers. Progradation of channels is limited because mouth bars are reworked by waves into shore parallel sand bars and beaches. Spits of sand are also common. The waves sort the sediment better than rivers and, if the grains are not already well rounded, the waves will round them. The big differences for wave influenced deltas are that beach facies are abundant and channel fill and overbank facies are less common. The Niger River Delta is a wave influenced delta.<br />
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<b>Tide Influenced Deltas</b> - Tides rework sands into elongate bars perpendicular to shore (vs. waves). These bars are analogous to mouth bars, but they contain tidal sedimentary characteristics including bi-directional flow indicators and slack tide mud drapes. Overbank areas can include tidal flats. The Ganges-Bramhaputra delta in Bangladesh is a tide dominated delta. <br />
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<b>Constructional Shorelines: Coastal Planes</b><br />
Coastal planes are broad areas where there is sufficient sediment for the land to build seaward, but it is not localized at a single delta mouth. Examples of coastal planes include the Everglades area of Florida and the coast of the Carolinas. <br />
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Each of these processes creates distinctive features in stratigraphic columns.<br />
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<span style="font-size: large;"><br /></span>Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-65263281615219365142012-02-22T12:44:00.000-08:002012-02-22T12:44:13.237-08:00Fluvial Review, Deltas and Marine Processes Part 1<b>Review: General Characteristics of Fluvial Sediments:</b><br />
1) On a large scale, river deposits consist of sheets and lenses of sand deposited in channels associated with flat laminated shales and silts with rare rippled sand beds deposited on floodplains.<br />
2) Fining upward sequences of beds in the sands with decreasing flow sedimentary structures<br />
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification<br />
4) Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop<br />
5) Soil development in associated shales deposited in the floodplain environment.<br />
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Look at pictures of fluvial rocks at <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html</a><br />
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<a href="http://gigapan.org/gigapans/67910">http://gigapan.org/gigapans/67910</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialCycles.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialCycles.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezDebrisFlow.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezDebrisFlow.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezGravelChannel.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezGravelChannel.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezMudCracks.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezMudCracks.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHRiverXStrat.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHRiverXStrat.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHDuneChannelXStrat.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHDuneChannelXStrat.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHUpBar2.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHUpBar2.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughXStrat3.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughXStrat3.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughMap.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughMap.jpg</a><br />
<a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialX1.jpg">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialX1.jpg</a><br />
<br />
<b>Deltas and Estuaries - Introduction</b><br />
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When rivers reach standing water such as a lake or the ocean, the flow speed slows down dramatically. And when flows slow down, sediment is deposited. Almost all of the sediment transported in a river is thus deposited close to the river mouth, with the exception of grains that are fine enough to remain in suspension. Lacustrine and marine processes can rework the deposited sediment to distribute it along shorelines. <br />
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<i>Deltas</i> form at the mouths of rivers that transport enough sediment to build outward. (<i>Building outward</i> is a key component of the definition of a delta. Rivers where the ocean or lake floods the river valley end in <i>estuaries</i>.) Deltas require substantial accumulation of sediment, in contrast to estuaries which do not build outward. Sedimentary facies are similar to other depositional environments, but the association of subenvironments are recognizable as deltas. Some of the sub environments include: river facies with all the associated sub environments; shore line deposits including beaches, marshes/swamps, etc.; submarine shelf and slope facies, including storm deposits and turbidites; etc. <br />
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I will draw cross section and map views of a delta showing the delta plane, delta slopes, and prodelta. Rivers flow through delta planes and slow when reaching water, producing a mouth bar. Grain size decreases with distance away from the river mouth.<br />
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<b>Progradation</b> - Because deltas are sites of sediment building outward from the coast, they are progradational; the landward depositional environments move seaward over more marine/lacustrine deposits. Thus, delta sequences in the rock record start with deep water, marine, fine grained sediments and grade upward into shallower water, possible more freshwater, coarser grained sediments. This is one of the distinguishing aspects of deltas that let you define them in the sedimentary record. These changes in grain size and environment typically occur over 1’s to 100’s of meters in the rock record and include many beds.<br />
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<b>Sediment Transport Type</b> - All deltas (by definition) have their sediment transported to the delta by rivers. Thus, riverine deposits are always associated with them. In addition, depending on marine (or lacustrine) conditions, waves and tides can redistribute the riverine sediment changing the morphology and facies of deltas. There are three main end member categories of deltas when characterized by processes: 1) River dominated; 2) Wave influenced; and 3) Tide influenced. We will come back to these delta types after discussing the marine processes.<br />
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<div style="text-align: center;">
<b><span style="font-size: large;">Marine Depositional Processes</span></b></div>
<b><span style="font-size: large;"><br /></span></b><br />
Most of Earth is covered with oceans, there is abundant life in the oceans, most sediments eventually get transported into the oceans, and shallow marine deposits are the most abundant in the in sedimentary record due to their large volume and the low erosion rates in shallow marine environments. You need tectonics to uplift them above sea level to get significant erosion. This happens commonly, so that we can also see them exposed. <br />
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<b>Processes -</b> Several processes are unique to shallow marine deposition (and some large lakes): Waves, storms, and tides<br />
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<b>Waves -</b> Waves have oscillating current directions every few seconds. The flow in both directions is equal in deep water, but not necessarily near shore. Draw a picture of wave water motion. (Water at the top of the wave moves in the direction the waves move.)<br />
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<i>Wave Ripples -</i> Wave ripples are like current ripples, except that they experience transport in both directions. Draw a picture with the laminar boundary layer, etc. At low flow, the boundary layer doesn’t have enough speed or momentum to remove the crest of the ripple and deposition of the grains that are moved are deposited right on the upper part of the lee slope. Thus, crests are sharp. At higher flow, the crests erode due to the higher speeds and momentum and deposition occurs farther down the lee slope. Thus, high flow ripples have rounded crests. Wave ripples can be recognized in rocks by their symmetric shape (if flow in each direction is the same speed) and most importantly, the presence of x-laminations dipping in two directions. This is the truly distinctive feature and can be present even if the ripples are not very symmetric. <br />
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In shallow water, currents along the bottom from the waves are strong enough to flatten out the ripples, but they are not consistent enough in one direction to form dunes. Thus, the sedimentary surface tends to be planar or broadly scalloped as the waves are focused into certain areas. This produces a flat lamination (not upper planar lamination) where waves are in very shallow water relative to their height, e.g. from the breaker zone towards the shore.<br />
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KEY POINT FOR WAVES: Bi-directional flow every few seconds<br />
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<b>Storms -</b> Storms produce both large waves and strong, irregular currents. The combination and interference of these produces some unique deposits which can be used to recognize the importance of storms in a given marine sequence. Storms generally start far from shore and can approach through time. Then they either die out or move on. Thus, deposits that storms affect, i.e. those on continental shelves, tend to start out with low energy flows, increase to erosional (if strong enough) and then decrease back to lower energy flows. For example, sharp crested wave ripples might transition into round crested wave ripples, followed by cross stratification due to large waves and strong currents, followed by erosion, deposition of the coarsest sediment, and a reverse of the sedimentary structures. However, because there is usually little sediment being deposited at the beginning of a storm because there is not much sediment in motion and because flow speeds are increasing, there is usually no record of the first half of this sequence in the rock record. It is only the second half that gets preserved. <br />
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<i>HCS -</i> The cross stratification that is deposited as a combination of strong currents and large waves is unique to storms (and is found only in medium to fine sands). It is called hummocky cross stratification (HCS) and swaley cross stratification. When currents are washing eroded sand into an area with strong oscillatory flow, rounded mounds or hummocks of sand develop on the sea floor separated by lows (swales). These mounds are a few to 10 cm high and 10’s of cm across. See Figures 14.3 and 4 in Nichols. Variations in current strength cause erosion locally, and the locations of the hummocks and swales change through time. This produces erosional surfaces which truncate the older laminae (note that Fig 14.2 has the wrong laminae truncated). HCS is characterized by low angle laminae truncated by low angle surfaces. There are abundant concave and convex up laminae and many fewer flat laminae. <br />
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<i>Storm Sequence -</i> A sample stratigraphic column consists of: Mud, scoured surface, sole marks, (gravel at base), normally graded, HCS, flat laminae or wave rippled top, return to suspension settling. Contrast this to a turbidite - I will ask you to do this!<br />
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KEY POINT FOR STORMS: Multi-directional flows over seconds, low to high to low energy in deep water<br />
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<b>Tides -</b> Two key characteristics that are unique to tides: 1) flow changes direction 1 or 2 times per day; and 2) The speed of flow is cyclical with flow going onshore, stopping at hight tide, then flowing offshore, and stopping at low tide. There is lots of variability in tides depending on geography. Flow speeds vary, producing different sedimentary structures. In the Bay of Fundy, which has the highest tides recorded in the world (up to 16m - a 5 story building), the water moves up to 15 km/hr (417 cm/sec) which is fast enough to transport boulders and is well above the upper flat lamination zone for smaller grain sizes. At the low end, tidal currents are essentially non-existent. Also, there are times of slack tides when the water is essentially still or wave-dominated. Thus, the range of sedimentary structures is wide, including dunes (often called tidal bars when very large) and ripples. The main characteristic to look for, though, is variations in flow speed and DIRECTION.<br />
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<i>Tidal sedimentary structures -</i> Due to changing flow directions, two sediment transport directions are common, one for onshore flow and one for offshore flow. Often the onshore and offshore flows are not in the same location, but they shift around. This gives rise to current ripples showing transport in two directions and dune migration in two directions producing herringbone cross stratification. See figures 11.6 and 11.7 in Nichols. If the dunes are small and sedimentation rates are very high, you can get herringbone cross stratification in one tidal cycle in a modern environment. It is usually not preserved in the geological record because it is eroded prior to lithification. It is almost always the longer term changes in current locations that gives rise to preserved herringbone cross stratification. Dunes migrate in one direction for a while, and then currents patterns change and they migrate in the other direction. Herringbone cross stratification is almost always due to tidal processes, although it is not all that common in the sedimentary record. Commonly, one tidal current is much stronger than the others or the flow locations aren’t systematically shifting, so tabular cross stratification is more common. It is not unique to tidal environments, however.<br />
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Reactivation Surfaces - Reactivation surfaces form when flow in one direction is stronger than the other, but the other flow is strong enough to modify the bedform shape. See figures 11.6 and 11.9 in Nichols. Reactivation surfaces are erosion surfaces within the sets of cross stratification. They look like irregular surfaces that are similarly oriented to the foresets, but usually do not dip quite as steeply. Also, the foresets above and below the reactivation surface commonly have a slightly different orientation. Reactivation surfaces indicate varying flow directions, which is very common in tidal environments.<br />
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Mud Drapes - Flow speeds are also cyclical. During slack tides (low or especially high), fine grained sediment can fall out of suspension draping tidal bedforms with mud. Because mud is cohesive, it does not necessarily erode during the next tidal flow, particularly in the separation zone where flow is slow, e.g. at the bases of ripples and dunes. Thus, sand foresets coated with mud are very common in tidal environments as well. See figures 11.6 and 11.8 in Nichols.<br />
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KEY POINT FOR TIDAL PROCESSES: Bi-directional flow with varying speeds over hoursDawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com0tag:blogger.com,1999:blog-2103960834361055322.post-26649068418787072602012-02-15T10:19:00.000-08:002012-02-15T10:19:49.338-08:00Rivers<b>Transport Capacity</b><br />
Erosion by water occurs when water is flowing across a surface and the flow is capable of transporting more sediment than is currently moving as bedload. This is called the sediment transport “capacity”. A certain number of grains of a certain size can be picked up by the Bernouli effect for a given flow. If there are too many grains, they start colliding and the characteristics of sediment transport change. Grains are directed back toward the bed and up into the flow. Eventually, more go back to the bed and are deposited, leaving fewer grains in the flow even at high flow speeds because there are more grains than the transport capacity of the flow.* In contrast, if there is a shortage of grains of a size that can be moved by the flow, e.g. the flow is moving all of grains present, any new grains will be eroded off the bed as soon as they are available. The flow then has excess transport capacity. <br />
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* Think about dumping a truck load of fine sand into a fast moving river, it takes time to move all that sediment even if the flow speed is theoretically fast enough to erode fine sand. <br />
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One of the most common times for a flow to have excess transport capacity is when the flow is speeding up. We know from the Hjulstrom diagram that faster flows transport larger grains. They can also transport more grains. Thus, water flowing downhill commonly speeds up, has excess capacity and erodes sediment. When it slows down, sediment is deposited. In floods, the water speeds up, erodes sediment, and transports it. As the flood ends, the water slows down and deposits the excess sediment. In general, erosion occurs when flows are speeding up or when they go from an environment with low sediment (e.g. a dam spillway) to an environment with more sediment (e.g. a river bed). <br />
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<b>Rivers</b><br />
Rivers are responsible for most sediment transport from mountains to lowlands and the oceans. They do the most to even out the topography that tectonic processes create. Rivers consist of channel, bank and overbank or floodplain deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds.<br />
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<i>River Types -</i> <br />
Straight (rare, except for ones humans have modified)<br />
Meandering (high sinuosity)<br />
Braided (many branches within a channel)<br />
Anastomosing (rivers with branching and merging channels)<br />
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The form of the river is controlled by the gradient of the river bed (steep = braided, gently dipping = meandering), local vegetation that stabilizes banks and limits the number of channels, sediment grain size, particularly the ratio of suspended versus bedload sediment, and sediment volume. A high bedload gives rise to abundant bars, which promotes formation of braided rivers.<br />
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<b>Braided Rivers </b><br />
Braided rivers develop when the proportion of bed load sediment is high, which produces abundant bedforms and promotes the development of bars, and thus, the braided character of the river. The sediment is commonly coarse, which requires fast flow and steep gradients for the sediment to be transported. Much of the geometry of braided rivers is shaped by the highest flows, e.g. spring floods, when the bars are covered in water. Many braided rivers have exposed bar tops for much of the year.<br />
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Flow speeds and transport capacity vary dramatically within a braided river. Friction with the riverbed tends to slow down the flow, particularly where the flow is shallow. Thus, the Reynolds number in shallow areas is relatively low (but still high enough that the flow is turbulent) and the transport capacity is low. In contrast, the transport capacity and Reynolds number are much higher in the deeper middles of channels in the river. Thus, the coarsest sediment is transported here, whereas finer sediment gets deposited in shallow areas. Also, bars block the flow on the upstream sides, and like dunes, the upstream sides tend to erode. Areas of low flow and eddies form on the downstream sides of bars, and they are usually sites of net deposition. Thus, bars migrate downstream through time. If we summarize the processes:<br />
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<i>Sediment Transport: </i> <br />
1) The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest. <br />
2) Bars are eroded upstream where the bars deflect the flow. Sediment is deposited on downstream side of bars and some on the flanks of bars where flow is slower, particularly on the insides of bends.<br />
3) Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels. <br />
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Sedimentary structures include:<br />
1) trough x-bedding in channels, due to the migration of irregular dunes<br />
2) coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes<br />
3) sediment on the edges of bars fines upward because the flow is shallower and slower, e.g. has a lower Reynolds number. Sedimentary structures can include anything from upper flat to ripple laminations.<br />
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<i>Braided River Facies</i><br />
Channels migrate back and forth leaving a sheet of sand with abundant cross stratification. These sheets of sand tend to fine upward. General characteristics of braided river deposits include: <br />
1) Scoured surface at the base of a channel<br />
2) Gravel lag at base of channel<br />
3) Trough x-bedded sands deposited just off the center of channels<br />
4) Occasional tabular x-stratification from migrating bars<br />
5) Sand deposited at slower speeds (ripple cross lamination possible)<br />
6) Overbank deposits from floods mostly composed of sand and silt, with some mud<br />
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The large scale geometry of the deposits includes sheets of sand with various grain sizes representing bar migration separated by floodplain deposits. <br />
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<div class="p1">
<span class="s1">Example of a braided river in Alaska: <a href="http://g.co/maps/wrk9n"><span class="s2">http://g.co/maps/wrk9n</span></a> It is cutting through glacial morraines deposited as a glacier retreated up the valley. Follow the river downstream (to the north and east) to <a href="http://g.co/maps/q5kq7"><span class="s2">http://g.co/maps/q5kq7</span></a>. How does the channel geometry change?</span></div>
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<b>Meandering Rivers</b><br />
Meandering rivers have a low gradient and thus slower flow, and usually have a high proportion of suspended sediment relative to the amount of bedload. A meandering river channel has curves that meander back and forth on a gently sloping floodplain. The flow speed in the channel varies with the geometry of the meanders. Water has to travel faster on the outside of bends than on the insides of bends. We know from the relationships between Reynolds number and bed shear stress that higher flow speeds mean that more and coarser sediment can be transported at higher flow speeds. Thus, we can predict that:<br />
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1. there is more erosion on the outsides of bends<br />
2. the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available<br />
3. sediment will accumulate on the insides of bend and this sediment will be finer grained. <br />
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If we look at a channel in cross section, it is asymmetric, representing the sites of erosion and deposition. Variation in flow speed also produce different sedimentary structures. Upper planar lamination and dune cross stratification are common where Re is highest, and ripple cross lamination is common where Re is lower. <br />
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The main parts of the channel include eroding bank, the thalweg (the deepest point of the flow) and the point bar (on the inside of the bend where most sediment is accumulating). As the channel migrates due to erosion and deposition, a distinctive suite of sedimentary structures accumulate. The deepest part is coarser and has upper planar lamination or dune cross stratification. This is overlain by finer sediment with current ripple lamination. <br />
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As meandering rivers migrate, the meanders tend to increase. Eventually, the channel forms almost a circle, and the meander gets cut off, often during a flood. This straightens the channel temporarily and produces an ox bow lake in the abandoned meander. The lake accumulates mud and organic matter. <br />
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Watch this cartoon of a meander migration in France: <a href="http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm</a><br />
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<i>Levees and Floodplains</i> - When a river floods, it goes from a confined flow in the channel which is very rapid to a widespread flow across the floodplain. It slows down very quickly and the water becomes shallower, both of which cause a decrease in Re. Thus, the water can not transport as much sediment on the floodplain as it does in the channel. Thus, finer sands that may be in suspension during a flood are transported as bedload or rapidly deposited once the river tops its banks. This produces levees. The finer silts and especially clays remain in suspension much longer and settle out on the floodplain as the flood waters dry up. <br />
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Watch this model of a meandering river flood: <a href="http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm">http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm</a><br />
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Over time, the levees build up and provide a higher bank for the channel than the level of the floodplain. Thus, the channel bottom can aggrade (fill in) until the bottom of the channel is as high or higher than the floodplain. When the next flood comes along, the river avulses and does not go back into its old channel which is higher than a new one on the floodplain. This results in the downstream part of the channel being completely abandoned.<br />
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<i>Meandering River Channel Facies:</i><br />
1. Scoured base of flow <br />
2. Lag deposit with mud rip-up clasts and the coarsest grains being transported<br />
3. Fining upward sands with trough cross stratification <br />
4. Rippled sands <br />
5. Sigmoidal cross stratification from migrating point bars <br />
<br />
<i>Floodplain Facies</i><br />
1. Fine sand with climbing ripples<br />
2. Mudstone/shale with mud cracks<br />
3. Soils<br />
4. Root casts<br />
<br />
<i>Ox Bow Lake Facies</i><br />
1. Mudstone/shale without mud cracks<br />
2. Organic-rich deposits, including coal<br />
3. Anoxic water indicators (especially in fossils and absence of trace fossils)<br />
<br />
<br />
<b>Differences between braided and meandering river deposits:</b><br />
<br />
1. Braided river deposits are commonly coarser grained <br />
2. Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on floodplains.<br />
3. Overbank deposits are better developed and finer grained in meandering river systems.<br />
4. Bar migration is much more regular in direction in meandering rivers because there is a well defined, single thalweg towards which the bars migrate. In contrast, braided river bar migration occurs in multiple directions. Thus, meandering rivers produce a more regular geometry of tabular cross bedding, when preserved. <br />
<br />
<b>General Characteristics of Fluvial Sediments:</b><br />
1) On a large scale, river deposits consist of sheets and lenses of sand deposited in channels associated with flat laminated shales and silts with rare rippled sand beds deposited on floodplains.<br />
2) Fining upward sequences of beds in the sands with sedimentary structures that indicate decreasing flow speeds.<br />
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification.<br />
4) Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop since they are rarely preserved<br />
5) Soil development in associated shales deposited in the floodplain environment.<br />
<br />
Look at pictures of fluvial rocks at <a href="http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html">http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html</a><br />
<br />Dawn Sumnerhttp://www.blogger.com/profile/15967361551408621044noreply@blogger.com1