Wednesday, March 14, 2012

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 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.  

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

Cretaceous-Tertiary Boundary Example
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!)










Tuesday, March 13, 2012

Regional Strat Column Correlations

Guest lecture by Cara Harwood on regional correlations of stratigraphic columns across the Precambrian-Cambrian boundary in the western US

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Geology 109: Sediments and Strata
March 12, 2012
Cara Harwood

Integrated Stratigraphic Analysis:  Correlating multiple columns using lithology, biostratigraphy, and chemostratigraphy


Recap:  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.  


Today:  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. 
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.  
Sedimentologists, stratigraphers, paleontologists, have studied this interval to try to learn about: 
  • the time of the boundary, and correlating the boundary globally
  • timing of evolutionary events
  • what environments were like where these events were happening
  • how environments were distributed
They studied stratigraphic columns and looked at lithologic, paleontologic, and chemostratigraphic data.  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. 

Stratigraphy Types
Biostratigraphy: characterization and correlation of rocks based on their fossil content, based on the principle that organisms have undergone successive changes through geologic time
Chemostratigraphy:  characterization and correlation of rocks based on their chemical composition, based on the principle that certain chemical signatures occur globally through geologic time
Why is studying the pC-C boundary in California interesting? 
  • The boundary is defined by a trace fossil that is present in siliciclastic rocks
  • It is also defined by a specific chemical signature in carbonate rocks
  • 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
Paleogeographic map of North America 
550 Ma:
http://www2.nau.edu/rcb7/namPC550.jpg
510 Ma:
http://www2.nau.edu/rcb7/namC510.jpg
Regional map showing positions of the craton margin relative to the Death Valley region and White-Inyos
From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf
 
Look at White-Inyo stratigraphic column (on left of the following figure) and talk through the whole thing - lithology, fossils including trace fossils...
From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf
Formations around the Precambrian-Cambrian boundary:
  • Wyman Formation: interbedded mudrock, siltstone, quartzite; interbedded carbonate layers that increase in number upsection --> shallow marine deposition
  • Reed Dolomite: 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. 
  • Deep Spring Formation: siliciclastic carbonate couplets, with ripple laminated quartzites with hummocky cross stratification, cross bedded oolites, intraclastic grainstone --> high energy shallow water depositional environment.  
Each of these is a formation: a mappable 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.  

Treptichnus pedum - 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.   Show examples of this trace fossil.

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.

Index fossils generally - 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.    

Look at how the White-Inyo section is correlated with other sections in the region - Death Valley and craton margin sections.  


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.  


Look at the 510 Ma paleogeographic map again to see the broad environments where each of these columns are from.  

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. 

What general observations can we make about correlating stratigraphic columns across a region? 
  • sediment packages thicken as we move from the craton (land) out into the basin
  • more unconformities (can happen when exposed, above sea level) towards land
  • facies shift from being a mix of non-marine and marine to mostly marine and deeper water facies
  • correlating long distances - can’t do lithostratigraphy (i.e. correlating sands to sands)
  • correlations based on fossil/trace fossil occurrence are robust
Look at chemostratigraphic signature (C isotopes) from ‘offshore’ (White-Inyos) to ‘onshore’ (Death Valley and craton sections):
From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf
Correlations across a region (and globally!) can also be made based on chemostratigraphy.   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. 
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. 

Other elements also have global trends and can be used for correlating based on chemostratigraphy.  Show examples of Sr and O isotope curves.  

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.

Wrap up - this is an example of using stratigraphy.  Key points: 
  • Looking at mixed carbonate-siliciclastic facies allows us to combine data types that occur in just one
  • We get a complete picture of the time period by looking at a combination of data sets (lithostratigraphy/facies, biostratigraphy, and chemostratigraphy)...
  • ...and how they are distributed across the region - looking at multiple stratigraphic columns

Wednesday, March 7, 2012

Interpreting Stratigraphic Columns


Interpreting Stratigraphic Columns

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

  • Hummocky Cross Stratification - waves plus currents (storms)
  • Wave Ripples (vs current ripples) - waves (standing water)
  • Herringbone Cross Stratification - bidirectional flow over hours or longer (tides)
  • Reactivation Surfaces - reshaping of bedforms due to changes in flow (tides)
  • Mud Drapes in sandstone - flow stops (tides)
  • Bouma Sequence - rapid flow slowing down (turbidity current)
  • Mud Cracks - mud contracts (exposed to air)
  • Root Casts - from plants, usually land plants (land)
  • Faint Ripple Cross Lamination with reverse grading - (eolian ripples)
  • Meter-high Dunes in fine sand - (eolian dunes)
  • Diamictites - laminar flow - (debris flows, mud flows, melting ice)
  • Diamictites with Facetted Clasts and Striations - (glacial)
  • Lone (or drop) Stones in laminated shale - large grains rafted over quiet environment (icebergs; trees possible)

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 or Stratification

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 your 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.


Here are some example stratigraphic columns to think about:










Monday, March 5, 2012

Marine Shorelines and Interpreting Stratigrahic Columns

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.

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.

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.

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.

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.

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.

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.

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.

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.

Each of these processes creates distinctive features in stratigraphic columns.


Wednesday, February 22, 2012

Fluvial Review, Deltas and Marine Processes Part 1

Review: 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

http://gigapan.org/gigapans/67910
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialCycles.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezDebrisFlow.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezGravelChannel.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/VasquezMudCracks.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHRiverXStrat.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHDuneChannelXStrat.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/OHUpBar2.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughXStrat3.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/TroughMap.jpg
http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Lg/FluvialX1.jpg

Deltas and Estuaries - Introduction

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.

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 end in 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 come back to these delta types after discussing the marine processes.

Marine Depositional Processes


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

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

Wednesday, February 15, 2012

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 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.

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.

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:

Sediment Transport:
1) The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest.
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.

Sedimentary structures 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 Reynolds number. 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 center of channels
4) Occasional tabular x-stratification from migrating bars
5) Sand deposited at slower speeds (ripple cross lamination 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 with various grain sizes representing bar migration separated by floodplain deposits.


Example of a braided river in Alaska:  http://g.co/maps/wrk9n  It is cutting through glacial morraines deposited as a glacier retreated up the valley.  Follow the river downstream (to the north and east) to http://g.co/maps/q5kq7.  How does the channel geometry change?


Meandering Rivers
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:

1. there is more erosion on the outsides of bends
2. the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available
3. sediment will accumulate on the insides of bend 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 and the coarsest grains being transported
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)


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 sedimentary structures that indicate decreasing flow speeds.
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 since they are rarely preserved
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

Monday, February 13, 2012

Alluvial Fans and Glacial Environments

First a review:

Playa Facies
1. Grain Size: usually mud-sized, especially in the middle of the playa; wind blown sand may also be present.
2. Sedimentary Structures: mud cracks, planar lamination, bioturbation is rare.
3. Composition: often clay minerals, salts from water evaporation. Carbonate, gypsum and halite are common “salts”, but others can be present.

Eolian Dune Facies
1. Grain Size: usually fine sand with some medium sand present.
2. Grain Characteristics: Well sorted, rounded, frosted
3. Sedimentary Structures: large dune cross stratification, sometimes meters to 10’s of meters high; ripple cross lamination rare. Rare root casts, more frequent in coastal dunes than desert dunes.
4. Composition: depends on source of sand, but often quartz.

Thought question: Sandstones that fit eolian dune facies often have 5-50 cm-thick layers between dune deposits that contain mud-sized grains, planar lamination, and mudcracks. They also can contain salts. Would you want to call those interbeds “playa facies” or not? Would you want to define a new facies or include those in your Eolian Dune Facies? What does this suggest about neighboring environments?

Flash Flood Deposits

When it rains in deserts, it often floods because there is little vegetation to trap water in soils and slow the runoff. Two environment types dominated by flash flood sediment transport are common: valleys with ephemeral rivers (wadis) and alluvial fans. Alluvial fans form in areas with a steep gradient from a drainage catchment to the basin floor whereas wadis in valleys form where the gradients are much lower. Tectonic activity it typically required to maintain steep slopes because they erode to lower slopes through time. The Basin and Range Province in eastern California and Nevada is an area with abundant examples of alluvial fans. Less structurally active deserts where deposition is dominated by flash floods, such as eastern Egypt, tend to have wadis (which is an Arabic word).

Alluvial fans in Death Valley: http://g.co/maps/n2udd
A wadi in Egypt: http://g.co/maps/369c4

Alluvial fans - 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 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.

Wadis - Wadis are similar to braided river deposits, which we will talk about next week. They have a high sediment load for the amount of water.

Flow types - Three types of flows are common: 1) debris flows, 2) sheet flows, and 3) channelized 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. (Diamictites are defined as very poorly sorted sedimentary rocks with no grain size sorting within them. They are characteristics of laminar flow deposits.) In most cases, debris flow deposits are unsorted and lack any form of stratification. They are laterally restricted because they do not spread out too much, and they are commonly an even thickness throughout, with steep edges to the flows.

Sheet flows 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.

Channelized and other types of flows - A number of other flow types are also common on fans. For example, if there is insufficient 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, which we will talk about later. 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.

Alluvial Fan Facies
1. Poorly sorted beds (diamictites) that are of an approximately uniform thickness but of limited lateral extent, deposited by debris flows;
2. Moderately to well sorted sandstone beds, often normally graded with pebbles at the base deposited in ephemeral channels; these show some cross stratification due to turbulent flow dynamics;
3. Normally graded sandstone beds that are laterally extensive deposited by sheet flows;
4. Average grain size decreases down slope and the abundance of debris flow deposits decreases down slope.

Side note about facies: Each of list items 1-3 above could be described as a subfacies of the Alluvial Fan Facies, with its own grain sizes, characteristics, sedimentary structures, etc. Each of those could also be considered a facies, and the overall alluvial fan deposit could be assigned to an “Alluvial Fan Facies Assemblage”. If I was studying the sedimentation in all of Death Valley, I would probably be most interested in distinguishing between alluvial fan facies, playa deposits, and eolian deposits, so I would use the “Alluvial Fan Facies” I defined above. In contrast, if I wanted to highlight variations in the depositional processes on one fan in Death Valley, I would probably define the different types of alluvial fan deposits as different facies, so that my facies would emphasize the differences in debris flows, sheet flows, and channelized flows. I would then have an “Alluvial Fan Facies Assemblage” that would be distinct from my “Playa Facies”.


Glacial Environments

Glacial environments are defined as those where ice is a major transport process. Liquid water and wind can also transport sediment in these environments. Wind transport is common when there is little vegetation. Liquid water transport occurs when the ice melts.

As you all remember, the high viscosity of ice makes all ice transport of sediment laminar. Thus, grain sizes are not sorted. All of the sediment is transported together, with the ice, and it is deposited when the ice melts. There are several features that are characteristic of glacial environments, including the process of erosion.

Erosion
Erosion in glacial environments is dominated by physical processes:
1) ice freezing in cracks in rocks, breaking them up
2) flow of glaciers “plucking” rocks up from the base of the flow
3) grinding of rocks against each other and against the floor of the glacial valley as the ice flows

These processes produce some distinctive sedimentary features including:
1) facetted clasts, e.g. rocks with smoothed off faces from dragging against other rocks
2) striations and grooves in rocks from dragging against other rocks
3) flat valley floors called glacial pavements that are smoothed off due to glacial flow
4) rock flour, which is clay size lithic grains formed from the bits of rock that are abraded off as facets, striations, grooves, and glacial pavements form.

There is often little chemical weathering in glacial environments because temperatures are cold.

Deposition
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, forming a diamictite. 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. If it 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”. These commonly are deposited by melting ice bergs that carry large grains out over lakes or the ocean, where they are deposited in (nearly) standing water.

As glaciers melt over land, melt water commonly reworks glacial till into braided river deposits. In arid environments, much less reworking of the sediment takes place.

Glacial Facies Assemblage
For illustrative purposes, I am describing the glacial facies as an assemblage of other facies.

Morraine Facies
1. Composed of diamictite; no sedimentary structures.
2. Diamictite in mounds.
3. Clast composition mostly lithic fragments, including silt and clay-sized rock flour.
4. Clasts mostly angular, some with facets and striations.

Till Sheet Facies
1. Composed of diamictite; sedimentary structures suggestive of turbidites in rare sandstone interbeds.
2. Diamictite in sheets with rare shale and sandstone interbeds.
3. Clast composition mostly lithic fragments, including silt and clay-sized rock flour.
4. Clasts mostly angular, some with facets and striations.

Distal Glacial-Lacustrine/Marine Facies
1. Shale with isolated large clasts and sandstone interbeds with sedimentary structures suggestive of turbidites.
2. Clast composition mostly lithic fragments.
3. Clasts mostly angular.
4. Frequency of large clasts decreases away from the glacier.

Braided River Facies
We will describe these later.

The parts of the glacial facies assemblage that are observed depends on whether the glacier ends on land or in standing water. Thus, the way I have described the facies here are particularly good for studying the environment for the glacier. If one wanted to determine how ice sheets around Antarctica have advanced and retreated through time, one would want to subdivide the Till Sheet Facies and Distal Glacial-Marine Facies into smaller groupings that would help locate the edge of the ice sheet. In contrast, if one wanted to distinguish between alluvial fan deposits and glacial deposits in Owens Valley, CA, one would want to pay particular attention to the geometry of the diamictites because they form in both environments, but the geometry of the deposition are different.

Monday, February 6, 2012

Arid Environments

Review of Details of Cross Lamination/Stratification Formation

There are several important details of cross lamination formation that I’d like to emphasize before the midterm. First, the geometry of the laminae within the cross stratification reflects the geometry of the surface during deposition. A layers of sediment is deposited on a surface, and the geometry of the bottom of the layer preserves the geometry of the depositional surface. If there is a change in slope on the surface, there will be a change of slope at the bottom of the laminae that mimics the surface. However, layers can vary in thickness. On the lee size of a ripple or dune crest, there is often more deposition on the steep slope because that is where the sediment lands when it is washed over the crest. It is the high depositional rate here that makes the slope steep. If the slope gets too steep, the grains avalanche downslope, and it is no longer quite as steep. The geometry of the ripple or dune is directly related to the distribution of deposition.

The geometry of laminae on the downstream side, e.g. where they merge with the erosion surface that forms on the stoss side of the next ripple/dune downstream, is also interesting. The laminae thin to zero thickness right where they “downlap” onto this surface. If deposition occurs beyond the steep slope of the ripple/dune, the laminae curve and gradually thin. If deposition only occurs on the steep slope, the laminae end more abruptly again the erosional surface. The geometry of the laminae tells you very precisely where deposition occurred relative to the ripple/dune trough.

These videos review these concepts:
http://youtu.be/r62qIKNBkos
http://youtu.be/0Zanh17ulXs
http://youtu.be/ogM-UqcYIfU

Arid Environments

Aridity - Aridity defines a desert, not temperature. An arid region gets less than 250 mm of rain/year. Our average in Davis is about 480 mm. That puts us barely in a semi-arid climate, which has 250-500 mm of rain/year on average. Arid environments are characterized by little vegetation.

Types of Deposits Typical of Arid Environments - 1) Wind blown sand (well sorted, texturally mature medium or finer sand); 2) Flash flood deposits (poorly sorted breccia, including debris flows); and 3) Playa lake deposits (silt, mud and evaporites). There are also very cold desert environments, such as the McMurdo Dry Valleys, Antarctica. These environments have glacial deposits left by glaciers that flow in from areas with higher precipitation (e.g. higher elevations) or the ice cap.

Flash Flood Deposits - When it rains in deserts, it often floods because there is little vegetation to trap water in soils and slow the runoff. Two environment types dominated by flash flood sediment transport are common: valleys with ephemeral rivers (wadis) and alluvial fans. Alluvial fans form in areas with a steep gradient from a drainage catchment to the basin floor whereas wadis in valleys form where the gradients are much lower. Tectonic activity it typically required to maintain steep slopes because they erode to lower slopes through time. The Basin and Range Province in eastern California and Nevada is an area with abundant examples of alluvial fans. Less structurally active deserts where deposition is dominated by flash floods, such as eastern Egypt, tend to have wadis (which is an Arabic word).

Alluvial fans in Death Valley: http://g.co/maps/n2udd
A wadi in Egypt: http://g.co/maps/369c4

Alluvial fans - 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 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.

Wadis - Wadis are similar to braided river deposits, which we will talk about next week. They have a high sediment load for the amount of water.

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 do not spread out too much, and they are commonly an even thickness throughout, with steep edges to the flows.

Sheet flows 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 is insufficient 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, which we will talk about later. 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.

Characteristics of Alluvial Fan Deposits

  1. Poorly sorted beds that are of an approximately uniform thickness but of limited lateral extent, deposited by debris flows;
  2. Moderately to well sorted beds, often normally graded with pebbles at the base deposited in ephemeral channels; these show some cross stratification due to turbulent flow dynamics;
  3. Normally graded beds that are laterally extensive deposited by sheet flows;
  4. Average grain size decreases down slope and the abundance of debris flow deposits decreases down slope.


Eolian Environments:
The viscosity of air is low so it is typically a turbulent flow (Re=ulρ/µ). Density is also low, but viscosity is a larger effect. Think back to the Bernoulli Effect. The lift force has to overcome gravity, but it also depends on the difference in density between the fluid and the grains. The lower the density of the fluid, the harder it is to lift the grains against gravity. Thus, wind speeds must be very high to transport grains, and wind tends to lift only medium sand or smaller grains into saltation or suspension even at 30 m/s (about 60 miles/hour). Larger grains can only roll along the ground, mostly due to impacts from saltating grains.

Saltation - Wind transports sand as bedload (traction and saltation) and in suspension, like water. The traction and saltation transport are slightly different, however, because the impacts between grains are more forceful. Water dampens the impacts by limiting grain speed by friction between the water and grains and the effects of viscosity. Air does much less so because of both lower density and much lower viscosity. Thus, impacts when saltating grains land are very forceful. This has 2 effects: 1) more grains are launched into the saltating layer than the fluid could lift. This leads to a positive feedback - once saltation starts, the number of saltating grains increases rapidly. 2) The landing grains push the grains they hit, leading to surface creep of grains. This processes can push much bigger grains up slopes than could be transported by the wind alone, even with traction transport.

Ripple Formation - As in water ripples, wind ripples form as an initial pile of grains once saltation has started. However, the mechanisms of growth are different. There is no separation of a laminar flow sublayer or back eddy as seen in water ripples, in part because air flow is very turbulent. Rather, the impacts from saltating grains push coarser grains up the back sides of ripples to the crests where they eventually avalanche off the lee slope. Most of the smaller grains get transported off the crests where wind speeds are the greatest. Some small grains may also accumulate in troughs, especially between the larger grains. This leads to one of the rare cases of reverse grading: The larger grains are more concentrated at the tops of ripples and smaller grains are more concentrated at the bases. If there is significant accumulation of sand, thin reversely graded layers can be preserved in the rock record and are indicative of aeolian transport.

Cross Lamination - Migration of ripples occurs due to erosion on the windward side and deposition on the leeward side, as in water ripples. However, cross lamination is rare because the sand in dunes tends to be well sorted, especially the sand that gets transported down the lee slope - which is where sediment accumulates. Sometimes cross lamination is preserved due to fluctuations in wind speed resulting in different grain sizes being deposited at different times.

Dunes and Draas - Larger bedforms also form due to wind transport of sand. Unlike water transported sand, dunes and draas (huge bedforms really observable only from the air) commonly have abundant smaller bedforms developed on them. Basic sand transport on both is the saltation of sand up their windward sides and avalanching down their leeward sides. Cross stratification is common and large scale. Meter-thick beds are common even though the tops of the bedforms are not preserved.

Sand Characteristics - Another aspect of strong collisions between grains is that they are commonly rounded very quickly and commonly have frosted surfaces due to collision damage. The collisions also break down softer grains, particularly lithic fragments. Thus, most dune sand consists of well rounded, well sorted quartz sand. Rare exceptions, such as the gypsum sands at White Sands National Park can persist due to a lack of hard dense grains to abrade the softer gypsum grains.

Characteristics of Eolian Deposits -
  1. Well sorted, rounded grains; 
  2. Little clay or silt sized grains; 
  3. Large bedforms, thus thick sets of cross strata; 
  4. Ripple stratification is rare; 
  5. Some reversely graded laminae (not beds); 

Wednesday, February 1, 2012

Weathering and Erosion

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 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.

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).

Image of physically weathered rocks in New Zealand:


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.

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.

Image of chemically weathered rocks in New Zealand:
This outcrop originally had the same composition as the rock shown in the previous photograph, but has been exposed to much more water as well as plant-assisted soil processes.

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 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.

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

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 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.

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

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 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.

* 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, 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).

Gravity Transport of Sediment
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.

Here are some videos of debris flows, mud flows, landslides, etc.:


Usually, the concentration of sediment is very high in gravity 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.