Chronostratigraphy

The 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: http://sepmstrata.org/

Event Stratigraphy
Event stratigraphy involves identifying the sedimentary effects of an unusual even 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.

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.

Magnetostratigraphy
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: http://www.geosociety.org/science/timescale/). This provides a reference that can help correlate other stratigraphic sections.

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.

Biostratigraphy
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.

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.

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: http://www-odp.tamu.edu/publications/189_IR/chap_02/c2_f6.htm

Interpreting Stratigraphic Columns

Here is an outline of how to approach interpreting the depositional environment represented by a stratigraphic column.

Step 1: Look for sedimentary structures that are characteristic of a specific environment or process
Examples:

HCS

wave ripples (vs current ripples)

herringbone cross stratification

reactivation surfaces

mud drapes in sandstone

bouma sequence

mud cracks

root casts

ripple cross lamination with reverse grading

meter-high dunes in fine sand

diamictites with facetted clasts and striations

lone (or drop) stones in laminated shale


Step 2: Evaluate how these distinctive structures relate to each other in the stratigraphic column to develop a tentative environmental interpretation

Are there several indicators of waves or storms?

Are there several indicators of tides?

Are there several indicators of wind-deposited sediment?

Are there several indicators of glacial activity?


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.
Examples of other sedimentary structures:

Trough cross stratification

Planar cross stratification

Current ripple cross lamination

Planar lamination

Step 4: Evaluate how the vertical sequence of sedimentary structures changes to refine or correct your environmental interpretations.
Do structures occur in a distinctive pattern that suggests a depositional environment?

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.)

Do the structures suggest an environment that shallows upward into a river system? (Then it might be a delta building out into standing water.)


Step 5: Use Walther's Law to refine your environmental interpretations and to test whether or not they are reasonable.
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 you interpretation require any jumps in environments or imply an unconformity? Revise your interpretation until it is consistent with your data.

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.

Alluvial Fans (in brief)

Alluvial fans are cone-shaped accumulations of coarse sediment deposited at the transition from confined flow in a canyon to unconfined flow in a basin. This also corresponds to a break in slope. As the slope shallows and the flows spread out, the flows slow down and deposit much of the sediment that they were able to transport in the canyon. (Think about the Hjulstrom diagram.) Fan geometry is determined by the rate of deposition. At the canyon mouth, it is steeps (up to 15°) due to rapid deposition of coarse sediment. It shallows to about 5° over the main part of the fan and shallows even more to 1-2° at the toe. Only the suspended sediments are transported beyond the toe, along with dissolved ions. If the water can pond, the fine grains settle out and the water evaporates forming minerals like gypsum and halite, and creating playa lake deposits. Deposition on a given alluvial fan is very rare - one event occurs about every 300 years on most fans in the southwestern US.

Flow types - Two types of flows are common: 1) debris flows and 2) sheet flows. Debris flows are slurries of mud, rock debris, and just enough water to make the sediment into a viscous flow. Due to the high viscosity, the flow is laminar, like a glacier, and like a glacier, there is no significant sorting of grain sizes. Debris flows can transport very large blocks. Debris flows continue to move until the internal friction of the flow due to viscosity exceeds the flow’s momentum when it freezes into place. This can occur due to either the loss of water or lower slope. The resulting deposits show little sorting and would be classified as a mud supported breccia or a diamictite. In most cases, debris flow deposits are unsorted and lack any form of stratification. They are laterally restricted because they don’t spread out too much, and they are commonly an even thickness throughout, with steep edges to the flows.

Sheet Flows - Sheet floods are turbulent flows with significantly more water and less mud than debris flows. Since the flows are turbulent, there is significant grain sorting and normally graded, fining upward deposits are common. Once a flow reaches the mouth of the canyon, the flow spreads out and the coarsest rocks are deposited first. Finer grains are deposited later and farther down the fan and later in time. This produces normally graded beds, but deposition is very rapid and the grading is commonly poor. The suspended load may make it to the toe of the fan if the water doesn’t filter into the fan first. Sheet flood deposits produce broad deposits that are clast supported, with some imbrication of clasts. Unlike a debris flow, sheet flows commonly cover 1/3 to 1/2 of a fan surface.

Other types of flows - A number of other flow types are also common on fans. For example, if there isn’t sufficient rain to produce a sheet flow, ephemeral rivers can flow down the surface of the fan - which is more common. This produces braided river type deposits. There is also a significant gradation between debris flows and sheet floods. They represent two end members, and there are lots of variations in mud content and water content which variously affect the viscosity of the flow and thus the resulting sedimentary deposits.

Laminar & Transitional Flows

Re= l*u*ρ/µ

Ice
Ice flows are laminar because they have very high viscosity. This can be seen in the ice cliffs along the edges of glaciers in Taylor Valley, Antarctica. The ice is particularly cold and is so viscous that it does not flatten out on the time scale of at least dozens of years. Because the flow is laminar, when the ice melts or sublimates, it dumps all grain sizes into one deposit, called a diamictite. (Diamictites are defined as very poorly sorted sedimentary rocks with no grain size sorting within them. Thus, debris flows also deposit.) If one knows that the diamictite was deposited by ice, it is then called till or tillite.

If the glacier melts on land, it leaves piles of till in moraines. Water from melting of the ice is turbulent (unless it contains lots of mud-sized sediment). Thus, the turbulent water from melting ice can sort sediments, usually associated with braided river facies due to a high proportion of bed load sediment.

If glacial ice melts over water, the debris is deposited into the water, commonly forming a till sheet. If only a few large clasts are deposited in the water, they are called “drop stones”. This is one way to get large grains in very deep, standing water.

Mudflows & Debris Flows
High concentrations of mud-sized grains, particularly if they are clay minerals, increase the viscosity of the flow and decrease the flow speed. Both of the reduce the Reynolds number, leading to laminar or, more commonly, transitional flows. These flows can transport boulders at very low flow speeds compared to a water flow with little mud. Also, the grains are not well sorted by size. All grain sizes are transported together and when the flow stops, they are deposited all at once. This creates a diamictite, like glacier ice melting does. (However, it does NOT produce a tillite, which has to have a glacial origin.)

Mudflows (few large clasts) and debris flows (many large clasts) commonly form when there is a very high supply of mud-sized grains relative to the amount of water. The ratio of mud to water affects viscosity and thus the level of turbulence in the flows. If sediment is deposited or water is added, they can become more turbulent through time. In contrast, if water drains out or more sediment is added to the flow through erosion, they become more viscous. If they become too viscous, they can “freeze” in place, leaving a thick layer of diamictite.

Flash floods and land slides are processes that can initiate mudflows and debris flows.

Here is a play list of laminar and transitional debris flows:
http://www.youtube.com/user/sumnerd#grid/user/93B0D9433B8C4275

Carbonates (in brief)

Carbonates
Carbonate 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.

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:

Ca²⁺ + CO₃^2- = CaCO₃

Because HCO₃^- is more abundant in seawater than CO₃^2-, the actual reaction that takes place is:

Ca²⁺ + 2HCO₃^- = CaCO₃ + H₂O + CO₂

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 CO₂ produced by the reaction, which makes the reaction proceed even faster. Also, some corals contain photosynthetic organisms within their tissues, and those organisms consume CO₂, 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.).

Channels south of Highbourne Cay, Bahamas:

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This atol NW of Hawaii consists entirely of a reef that built up to sea level and the extinct volcano that underlies it subsided:

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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.

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.

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.

See figure 15.12 in Nichols, Edition 2 for the distribution of environments across a reef.

Marine Shorelines

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.

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.

Wave Influenced Shorelines
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.

Sites with beaches and barrier bars include (I strongly recommend exploring the larger maps):
North Carolina Coastal Plane:

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Niger Delta:

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An estuary feeding into the Black Sea:

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Tide Influenced Shorelines
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.

Sites with tidal channels include:
An erosional tidal shoreline north of Derby, Western Australia:

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An estuary in Madagascar:

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The Ganges-Brahmaputra Delta:

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Constructional Shorelines: Deltas
Deltas form at the mouths of rivers that transport enough sediment to build outward. (Building outward is a key component of the definition of a delta. Rivers where the ocean or lake floods the river valley flow into estuaries.) 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.

In lecture, I drew cross section and map views of deltas 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 into the ocean/sea/lake.

Progradation - 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.

Sediment Transport Type - 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.

See my video summary of deltas at http://www.youtube.com/watch?v=_XOb07MDjAs

River Dominated Deltas - 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.

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Wave Influenced Deltas - 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.

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Tide Influenced Deltas - 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.

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Constructional Shorelines: Coastal Planes
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.

Marine Processes

Marine Deposition - 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.

Processes - Several processes are unique to shallow marine deposition (and some large lakes): Waves, storms, and tides

Waves - 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.)

Wave Ripples - 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.

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.

KEY POINT FOR WAVES: Bi-directional flow every few seconds

Storms - 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.

HCS - 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.

Storm Sequence - 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!

KEY POINT FOR STORMS: Multi-directional flows over seconds, low to high to low energy in deep water

Also see my video summary of waves and storms:  http://www.youtube.com/watch?v=ujLueo4JlCU

Tides - 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.

Tidal sedimentary structures - 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.

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.

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.

KEY POINT FOR TIDAL PROCESSES: Bi-directional flow with varying speeds over hours

See also my video summary of tidal processes: http://www.youtube.com/watch?v=k4yDvV_9ygQ

Rivers Continued, Intro to Deltas

Meandering River Facies

Meandering River Channel Facies:
1. Scoured base of flow
2. Lag deposit with mud rip-up clasts
3. Fining upward sands with trough cross stratification
4. Rippled sands
5. Sigmoidal cross stratification from migrating point bars

Floodplain Facies
1. Fine sand with climbing ripples
2. Mudstone/shale with mud cracks
3. Soils
4. Root casts

Ox Bow Lake Facies
1. Mudstone/shale without mud cracks
2. Organic-rich deposits, including coal
3. Anoxic water indicators (especially in fossils and absence of trace fossils)

See also:  http://www.youtube.com/watch?v=ZzwxQshIn1Q for a review of meandering rivers.

Braided Rivers - 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.

Sediment Transport:
1) The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest. (Like meandering rivers, the thalweg is the deepest point in the channel.)
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.
3) Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels, as seen in meandering rivers.

Sedimentary features include:
1) trough x-bedding in channels, due to the migration of irregular dunes
2) coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes
3) sediment on the edges of bars fines upward because the flow is shallower and slower, e.g. has a lower Re. Sedimentary structures can include anything from upper flat to ripple laminations.

Braided River Facies
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:
1) Scoured surface at the base of a channel
2) Gravel lag at base of channel
3) Trough x-bedded sands deposited just off the thalweg
4) Occasional tabular x-stratification from migrating bars
5) Sand deposited at slower speeds, finer grained (rippled possible)
6) Overbank deposits from floods mostly composed of sand and silt, with some mud

The large scale geometry of the deposits includes sheets of sand separated by floodplain deposits.


Differences between braided and meandering river deposits:

1. Braided river deposits are commonly coarser grained
2. Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on floodplains.
3. Overbank deposits are better developed and finer grained in meandering river systems.
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.

General Characteristics of Fluvial Sediments:
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.
2) Fining upward sequences of beds in the sands with decreasing flow sedimentary structures
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification
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
5) Soil development in associated shales deposited in the floodplain environment.

Look at pictures of fluvial rocks at http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html

Deltas and Estuaries - Introduction
Deltas form at the mouths of rivers that transport enough sediment to build outward. (Building outward is a key component of the definition of a delta. Rivers where the ocean or lake floods the river valley flow into estuaries.) 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.

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.

Progradation - 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.

Sediment Transport Type - 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 talk more about these after we discuss marine processes.

Rivers

Transport Capacity
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.

* 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.

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).

Rivers
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 a channel, bank and overbank or floodplain deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds.

River Types -
Straight (rare, except for ones humans have modified)
Meandering (high sinuosity)
Braided (many branches within a channel)
Anastomosing (rivers with branching and merging channels)

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.

Meandering Rivers -
Meandering rivers have a low gradient and thus slower flow, and often 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 slightly dipping plane. The flow speed in the channel varies with the geometry of the curves. 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, you should predict that there is more erosion on the outsides of bends, the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available, and the insides of bends will be were sediment is accumulating and this sediment will be finer grained. 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.

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.

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.

Watch this cartoon of a meander migration in France: 
http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm

Levees and Floodplains - 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.

Watch this model of a meandering river flood: 
http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm

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.

Meandering River Channel Facies:
1. Scoured base of flow
2. Lag deposit with mud rip-up clasts
3. Fining upward sands with trough cross stratification
4. Rippled sands
5. Sigmoidal cross stratification from migrating point bars

Floodplain Facies
1. Fine sand with climbing ripples
2. Mudstone/shale with mud cracks
3. Soils
4. Root casts

Ox Bow Lake Facies
1. Mudstone/shale without mud cracks
2. Organic-rich deposits, including coal
3. Anoxic water indicators (especially in fossils and absence of trace fossils)


Braided Rivers - 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.

Sediment Transport:
1) The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest. (Like meandering rivers, the thalweg is the deepest point in the channel.)
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.
3) Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels, as seen in meandering rivers.

Sedimentary features include:
1) trough x-bedding in channels, due to the migration of irregular dunes
2) coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes
3) sediment on the edges of bars fines upward because the flow is shallower and slower, e.g. has a lower Re. Sedimentary structures can include anything from upper flat to ripple laminations.

Braided River Facies
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:
1) Scoured surface at the base of a channel
2) Gravel lag at base of channel
3) Trough x-bedded sands deposited just off the thalweg
4) Occasional tabular x-stratification from migrating bars
5) Sand deposited at slower speeds, finer grained (rippled possible)
6) Overbank deposits from floods mostly composed of sand and silt, with some mud

The large scale geometry of the deposits includes sheets of sand separated by flood plane deposits.


Differences between braided and meandering river deposits:

1. Braided river deposits are commonly coarser grained
2. Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on floodplains.
3. Overbank deposits are better developed and finer grained in meandering river systems.
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.

General Characteristics of Fluvial Sediments:
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.
2) Fining upward sequences of beds in the sands with decreasing flow sedimentary structures
3) Abundant cross stratification in well sorted sands, particularly trough cross stratification
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
5) Soil development in associated shales deposited in the floodplain environment.

Look at pictures of fluvial rocks at http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html

Lecture 7: Stratigraphy Review, Weathering & Erosion

Lithostratigraphy and Chronostratigraphy (review)

Changes in depositional environments lead to variations in accumulation of sedimentary rock both laterally and vertically. If you look at ancient rocks and compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, 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. chronostratigraphy, 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; the beach is different than the offshore environment.

Lithostratigraphic correlations are easy because you can directly observe rock types, define facies, and match them between stratigraphic sections. Chronostratigraphic correlations can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. In real rocks, there are a number of tools that you can use to get correlations of various accuracy, including: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as isotopic ratios of certain elements in carbonates; geological instantaneous depositional events such as huge storms, tsunamis, 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.

Walther’s Law 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 if there is no break in sedimentation (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 might be 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.

Weathering:

Origins of Sediment
Sediment comes from the break down of rocks into smaller, transportable components. This occurs via two processes: physical weathering and chemical weathering. 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.

Physical Weathering:
Physical weathering occurs via:

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.

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.

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 contractions 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.

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).

Chemical Weathering:
Chemical weathering occurs via:

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 CO₂, 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.

2) Alteration of minerals. Silicates 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 O₂ to form oxides, clay minerals and ions, pyrite reacts with water and O₂ 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.

Mineralogy of Weathered Rocks
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.

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 composition of 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.

The following lists 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

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.

Controls on Weathering
The extent and style of weathering is mainly controlled by climate. Water is extremely important, even for physical weathering. The more water present, the faster weathering occurs. Temperature 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, vegetation has a strong influence on weathering. Plants tend to increase the extent of chemical weathering by producing organic acids which help break down rocks into soil through both dissolution and alteration. They also help soil retain moisture, increasing the availability of water for weathering.

Erosion
Once sediment is produced by weathering, it is available for transport. The two main forces in erosion are gravity and fluid flow. 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. 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.

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 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.

* 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.

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, 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).

Lecture 6: Turbidites and Stratigraphy

Facies (From Wednesday's lecture)
Facies are groupings of rocks based on similar features. The field trip will focus on making observations of the beach and defining facies. Rocks (and sediments) can be grouped by a suite of different characteristics, for example grain size, sedimentary structures, grain composition, grain rounding, etc. The goal of assigning rocks or sediments to facies is to provide a useful categorization scheme.

Turbidites
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).

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

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°).

3) The base of the flow is commonly erosional on steep slopes, so more sediment is entrained in the flow.

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)

5) Sediment concentration decreases with deposition, so one gets more hydraulic sorting. The flow is very fast so the sediment has upper plane bedding. (Facies = Bouma b)

6) As the flow slows more, grain size decreases and ripples start to form. Dunes are not usually found 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)

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)

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)

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.

Watch these movies of turbidites in flumes:
http://faculty.gg.uwyo.edu/heller/SedMovs/middletonturb.htm
http://faculty.gg.uwyo.edu/heller/SedMovs/Turbidity%20ignition.html

And here are some photos of turbidites: http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Turbidites.html

Changes in Character Downslope - 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.

For HW #3, you will draw a stratigraphic column of a turbidite and then describe the facies characteristic of different environments.

Turbidite Facies Models - 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.

Extra on Turbidites - 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.

Extra on Dense Sediment Flows
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 Re and more turbulent flow (Re=u*l*r/µ). 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.

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 (Re=u*l*r/µ). 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.

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.

Other Resources
Here is a 6 minute summary of my turbidite lecture:  http://www.youtube.com/watch?v=G05juwK2OTI
A nice, hour long lecture on turbidites in the Monterrey Bay canyon, CA, can be found at: http://online.wr.usgs.gov/calendar/2010/jun10.html  The actual lecture starts about 5 minutes into the video.

Stratigraphy and Time:
Stratigraphy is the study of sedimentary rocks in space and time.

Stratigraphy 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.

Example: Beach Facies. Beach environments grade laterally into each other. The off shore areas grade into the swash zone of the foreshore. The foreshore grades into the berm and backshore (if present). Eolian (wind) dunes 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.

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. lithostratigraphy, 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. chronostratigraphy, 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.

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.

Walther’s Law 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 if there is no break in sedimentation (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.

Chronostratigraphy - 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.

Distribution of Rock and Time - 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!

Extra Resources

Here is a short video summary of the distribution of time in rocks:  http://www.youtube.com/watch?v=9ch-6HiOAW4

Lecture 5: Bedforms continued and Facies

Thanks to Tyler for giving today's lecture!  


Sedimentary Structures Continued

Homework #2 due today.

Ripples and Dunes (A review with a bit of additional information)

A sketch of a ripple or dune like the one in lecture:
http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/duneXStrat.jpg

Remember where the separation point and attachment point are located. The geometry of the flow tracks these points. Erosion can only occur where there is a bed shear stress sufficient 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. In other words, sediment accumulates in the flow detachment zone. Laminae are visible where deposition occurs due to variations in flow speed and thus the grain sizes transported and deposited.

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.

Variations in Geometry and Bedform
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.

Watch the USGS bedform movies described at: http://mygeologypage.ucdavis.edu/sumner/gel109/labs/USGSBedforms.html

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:

1) no transport
2) faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring
3) dunes - the flow is strong enough to erode at the attachment point
4) upper planar lamination
5) antidunes

In contrast, the sequence of structures in silt is:

1) no transport
2) ripples
3) upper planar lamination
4) antidunes

Antidunes - 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 http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg ). 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.

Other Types of Flows - 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 http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/ARipples.html 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.

Environments and Facies
Look at the photo of Scott Creek Beach at:
http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Lg/ScottAntidunes.jpg

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.

The suite of structures forms a facies. A facies (Latin for aspect or appearance) is a body of rock (i.e. a sequence of beds, etc.) marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock 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.

From Sediment Transport to Rocks - 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.

Facies vs Environments - 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 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.

Example Facies
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:

Facies based on grain size:
Coarse-grained sandstone with 1-5% pebbles
(suggests high flow speeds)
Fine-grained, well-sorted sandstone
(suggests low flow speeds with either only one size sediment source or a consistent flow speed)
Mudstone
(suggests standing water)

Facies based on sedimentary structures:
Fine-grained sandstone with current ripple cross lamination
Fine-grained sandstone with upper planar lamination
Fine-grained sandstone lacking cross stratification, but with abundant burrows

Facies based on grain composition:
Coarse-grained sandstone with 25% lithic fragments, 25% feldspar, and 50% quartz
Coarse-grained sandstone with 80% quartz, 10% mica, and 10% feldspar
Coarse-grained sandstone with 99% quartz and trace gold flakes

Beach Facies What features will 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: http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Beach.html