The Midterm is next Wednesday, Feb. 8. Use the study guide I sent out earlier today and posted on SmartSite.
Facies
Facies are groupings of rocks based on similar features. 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. See http://youtu.be/ydpJOgZYNBM.
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, too.
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 do not usually form for two reasons: a) often only fine sand and finer grains are left in the flow by this point; and b) dunes do not have time to develop. (Facies = Bouma c)
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
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.
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. My summary can be found at: http://youtu.be/G05juwK2OTI
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!
A discussion of time in sedimentary rocks can be found at: http://youtu.be/9ch-6HiOAW4
Monday, January 30, 2012
Wednesday, January 25, 2012
Sedimentary Structures cont. and Facies
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:
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: 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 ScottAntidunes.jpg below). 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 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:
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.) or sediment marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock/sediment above, below and adjacent to it. A sedimentary facies has a characteristic set of properties that makes it distinctive, which the geologist defines. Usually facies are defined based on a suite of characteristics in rocks/sediment.
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/sediments after deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross stratification; the stratification is the result of high flow speed, but they are not the same.
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: Beach.html. Predict some of the facies.
See you at 8 am Sunday for the field trip!
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:
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: 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 ScottAntidunes.jpg below). 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 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:
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.) or sediment marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock/sediment above, below and adjacent to it. A sedimentary facies has a characteristic set of properties that makes it distinctive, which the geologist defines. Usually facies are defined based on a suite of characteristics in rocks/sediment.
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/sediments after deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross stratification; the stratification is the result of high flow speed, but they are not the same.
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: Beach.html. Predict some of the facies.
See you at 8 am Sunday for the field trip!
Monday, January 23, 2012
Cross Stratification
Key Points of Sediment Transport
1) Faster flows have more bed shear stress. Thus, faster flows move larger grains (when considering sand sizes and larger).
2) Sediment is transported as bedload and in suspension. Bedload consists of rolling and saltating grains.
3) Grain size and flow strength (Re) determine how grains are transported.
4) As flow strength changes, grains are eroded or deposited. These relationships are represented in the Hjulstrom diagram.
A Few Definitions:
1) "Stratification" - layers in rocks; stratified rocks are those organized into beds
See Grand Canyon Beds: Grand Canyon Beds
2) “Beds” are separated by “bedding planes” - cm to m thick units of sedimentary rock that were deposited approximately horizontally (beds) and are separated by horizontal planes (bedding planes); the rocks typically weather more along these planes. Beds are usually fairly uniform or change gradationally in composition. Bedding planes usually represent breaks in sedimentation or changes in grain size. In other words, they usually represent changes in flow characteristics.
See Cache Creek Turbidite Beds and Bedding Planes: turbidites
3) "Laminae" are color, composition, or grain size variations defining surfaces within a bed. They typically represent variations in flow velocity, sediment supply, sediment composition, etc. Planar Laminae are parallel to bedding, e.g. planar.
4) "Cross Lamination”, "Cross Stratification" or "Cross Bedding" are laminations or layers that are oriented obliquely to bedding. They truncate older laminae and are truncated by younger laminae. The erosional surfaces that separate “sets” of similarly oriented laminae are called “bounding surfaces”. There are lots of subdivisions of cross stratification; different types represent different types of bedforms and different flow conditions.
See Burns Cliff on Mars: Mars cross stratification
 |
| (Image courtesy of NASA/JPL and the MER mission!) |
The upper part of the image has planar lamination, and the lower part to the far left has cross lamination or stratification.
See: trough cross stratification and other examples at: dune cross stratification
See ripple cross lamination on Mars: martian ripples
 |
| (Image courtesy of NASA/JPL and the MER mission!) |
Bedforms
When sediments get deposited from turbulent flows, the sediment interacts with the geometry of the flow. Depending on the flow speed, turbulence, and sediment characteristics, different structures or bedforms develop.
See: Ripples Movie
Bed Geometry and Flow Separation - Until now, we have been implicitly assuming that the bases of beds are flat and smooth, but if sediment is present, they are not. If you start with a smooth bed of sand and increase water speed above it, irregularities form from irregularities in the flow and develop into ripples. First, a few grains pile up. Once the height of the pile is several grains high, there is a flow shadow down stream of them, and the laminar sublayer detaches from the base of the flow. The water has enough momentum that it does not hug the bed surface and instead, goes shooting out over the top. This point is called the separation point. The water flows forward and downward and reconnects with the bed at the attachment point. At the attachment point, water is flowing directly towards the sediment with a lot of force. This force moves the grains and causes erosion. In contrast, the area between the separation point and the attachment point has very low flow. In fact there are back eddies, where the flow is upstream. Thus, sediment transport is very irregular along the bedding surface at a local scale.
Sediment Transport Over a Ripple - Sediment grains are mobilized at the attachment point - more so than in normal flow because the water is shooting directly into the sediment - and the grains are moved downstream by saltation and traction. As the flow becomes parallel to the sediment surface again, its ability to transport sediment decreases. Thus, the grains tend to pile up and a new mound forms. This gives a periodic chain of mounds - the beginnings of ripples. As flow continues, grains roll and saltate up the stoss (upcurrent) side of the ripples. Once they pass the crest, they reach the low flow on the lee side of the ripple. The larger grains settle out and roll partway down the slope; this is the site of net deposition. As the process of deposition on the lee side and erosion on the stoss side continues, the ripples migrate downstream. If there is net deposition of sediment, the ripples leave behind distinctive dipping layers between two erosional surfaces that can be preserved in the rock record. These layers slope downstream and are one type of cross lamination.
A sketch of a ripple or dune like the one in lecture: ripple sketch
Watch the USGS bedform movies described at: USGS Bedforms page
Bedforms and Flow Velocity- The size and shape of subaqueous bedforms depends on flow strength and grain size and can be used to interpret ancient flow characteristics in a depositional environment from looking at sedimentary rocks. See Nichols (2009, Sedimentology and Stratigraphy, section 4.3) for bedform, flow speed, and grain size relationships.
Ripples (crest-to-crest differences of less than 50 cm and heights of less than 4 cm) – The minimum flow for ripples is determined by the minimum velocity for sediment transport. Once this flow speed is reached, ripples form if the sediment is transported as bedload. The maximum flow speed for ripples depends on the location of the attachment point on the stoss side of the ripples. As flow gets faster, too much erosion occurs at the crests of the ripples - the point of attachment is too far up the stoss side of the ripple- and the ripples flatten out. Dunes develop.
Dunes (60 cm-100’s m wavelength and 10’s of cm to meters in height) - Dunes develop as ripples flatten out because large scale irregularities start to develop. The basic ideas of dune and ripple formation are the same. The difference is that the area of flow separation is much larger (see Fig 4.17, Nichols 2009). Roller vortexes (e.g. upstream flows along the lee sides of dunes) are common, and the upstream flow can be strong enough to form ripples that migrate upstream. As flow speeds increase, the dunes start to flatten out.
Planar/Flat Lamination – Planar lamination forms when the flow is strong enough that the beds flatten out. The momentum of the transported grains and fluid are high enough that they tend to move horizontally, eroding any irregularities in the bed. This zone of planar lamination is called “upper flow regime”. (Why “upper”? - there is a zone of planar lamination in coarse grained sediment at low flow velocities.)
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 Dawn’s ripple image page. 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.
Bedforms and Grain Size - Bedforms also vary with grain size (see Figure 4.20, Nichols, 2009). Very fine sand and silt are very easy to transport and erode. They form nice ripples, but do not form dunes when transported by water. Instead, ripples transition into planar laminae. Coarse sand and larger sediment is too hard to transport and erode to get ripples. The erosional force at the reattachment point is not strong enough to erode the coarse grains and produce the erosional surfaces on the backs of ripples. Without this erosion, troughs do not form and without troughs, crests do not form. The sequence of structures in granules with increasing flow is:
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
Extra
High Sediment Loads - 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 which ever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes.
If there is very little water associated with a clay-rich sediment flow, the flow can be very viscous due to the charge attraction among clay particles. The high viscocity makes the flow laminar (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.
Wednesday, January 18, 2012
Fluid Flow II
Lecture 3: Fluid Flow and Sediment Transport
Key Concepts from Lecture 2
Reynolds Number - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Re = (fluid inertial forces)/(fluid viscous forces) = l*u*r/µ where the variables are flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (r), and fluid viscosity (µ). Turbulent flow has Re is greater than 2000 and laminar flow has Re is less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well.
Boundary Layer and Laminar Sublayer - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer.
Sediment Transport
Bed Shear Stress: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. It also depends on the slope. If the slope is steep, gravity helps pull grains down the slope, increasing bed shear stress. Also, the roughness of the bed is a factor. A rough bed deflects flows and increases turbulence, which increases the bed shear stress, particularly in places where flow is directed into the sediment and the boundary layer is compressed.
The Bernoulli Effect
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect.
Which Grains Move? Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is a grain's position relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average.
Bedload and Suspended Load Transport
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain settles out versus how turbulent the water is (back to Re...). Bedload refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in traction. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be saltating. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water.
Here is a playlist with movies related to sediment transport: Sumnerd’s Sed Transport Playlist and an extra movie at
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb)
Suspended sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow.
Photo of suspended sediment in a Costa Rica River:
http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg
YouTube video of white clay in a turbulent flow in a flume: http://tinyurl.com/78kg3z The pulsing in the flow is (probably) due to the pump that is making the water flow.
Hjulstrom Diagrams
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams. Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the critical velocity. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the critical threshold for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done!
A copy of the Hjulstrom Diagram from the book: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/Hjulstrom.jpg
Deeper flows can move larger grains at the same flow velocity because they are more turbulent: Re=u*l*r/µ and l is larger. This is because deeper flows can have larger variations in flow speed and the laminar flow layers are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than just Hjulstrom diagrams, which summarize a lot of characteristics into two axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess.
Silt and Clay - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clay minerals tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode.
In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix).
A few more words about saltation: Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur.
Watch grains transported by saltation and traction in these movies: http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb)
http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm (14 Mb)
Deposition: Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and starts to leave grains on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments.
Ripples and Other Bedforms
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser laminae that dip downstream.
Next Time: We will talk about the details about sedimentary structures. Homework is due on Monday.
Key Concepts from Lecture 2
Reynolds Number - Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Re = (fluid inertial forces)/(fluid viscous forces) = l*u*r/µ where the variables are flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (r), and fluid viscosity (µ). Turbulent flow has Re is greater than 2000 and laminar flow has Re is less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well.
Boundary Layer and Laminar Sublayer - There is boundary layer at the edge of every flow where flow speed decreases due to friction. Within the boundary layer, right next to the surface, the flow speed is very low, creating a laminar sublayer.
Sediment Transport
Bed Shear Stress: The boundary layer determines the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below the grain which tend to lift them off the bed. Bed shear stress is related to the thickness of the laminar sublayer. The narrower it is, the more bed shear stress. It also depends on the slope. If the slope is steep, gravity helps pull grains down the slope, increasing bed shear stress. Also, the roughness of the bed is a factor. A rough bed deflects flows and increases turbulence, which increases the bed shear stress, particularly in places where flow is directed into the sediment and the boundary layer is compressed.
The Bernoulli Effect
A pressure difference “pulls” grains off the bed. The pressure difference comes from a difference in water (or air) speed above and below the grain. As water flows faster, there are fewer collisions between the water and a surface it flows over than there are between standing water and a similar surface. Pressure is due to collisions. Thus, fewer collisions means lower pressure. The upstream side of a grain experiences the most collisions because the water is flowing into it. The downstream side experiences the fewest collisions, and the sides of the grain experience fewer collisions where flow is faster and more where the flow is slower. The net result is a low pressure zone above and slightly downstream of a grain. If the force exerted by this pressure difference is larger than the force of gravity, the grain will lift off the bed. This lift due to the pressure difference is the Bernouli Effect.
Which Grains Move? Which grains get entrained in the flow depends on their size and density (how much they weigh) because that determines the force of gravity holding them down. It also depends on the shape of the grain. One with a large area to experience the low pressure (like a plate) will be more susceptible to being picked up than a round grain of the same mass (although flat grains may see a smaller flow difference from top to bottom if the boundary layer is thick, and flat grains may experience a lower Bernoulli Effect per unit area.) The other thing that really matters is a grain's position relative to surrounding grains. If a grain is sandwiched between larger grains, i.e. in their flow shadows, it will not experience as big a pressure difference. Also, if a grain is upstream of a big grain, it has to be lifted over it, so a larger pressure difference is needed. Thus, things can get complicated if you are trying to predict the behavior of a specific grain. However, we have some general guidelines based on experiments and theory that nicely predict how grains behave on average.
Bedload and Suspended Load Transport
Two things can happen once a grain is lifted into the flow: 1) it can fall back down or 2) it can stay there. It depends on how quickly the grain settles out versus how turbulent the water is (back to Re...). Bedload refers to the grains that are transported along the sedimentary bed, e.g. grains that are rolling and being lifted off the bed, but they fall back quickly. The name bedload comes from the fact that the grains moving by traction and saltation never get too far from the bed and “load” is an engineering term for the amount of sediment transported by a river. Rolling grains are in traction. Grains that are pulled off the bed with the Bernoulli effect but are large enough that gravity causes them to fall “quickly” back to the bed are said to be saltating. (The word saltating refers to the way salt from a salt shaker bounces when it is shaken onto a hard surface. The word is derived from a Latin word meaning dance.) Bedload grains are the ones that form sedimentary structures in flowing water.
Here is a playlist with movies related to sediment transport: Sumnerd’s Sed Transport Playlist and an extra movie at
http://faculty.gg.uwyo.edu/heller/SedMovs/bedload.htm (7.8 Mb)
Suspended sediment consists of grains that are light enough that they do not settle out of the water; the turbulent bursts of water keep them in the flow. The more turbulence in the water, e.g. the higher the Reynolds number, the larger the grains in suspension will be. The upward motions of turbulent flow are faster than the rate these grains settle, so gravity is counteracted and they stay “floating” in the water even though they are denser than the water. Very small grains do not settle out of flows unless the Reynolds number is low, which means that the flows need to be standing or very shallow.
Photo of suspended sediment in a Costa Rica River:
http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/CostaRicaRiver.jpg
YouTube video of white clay in a turbulent flow in a flume: http://tinyurl.com/78kg3z The pulsing in the flow is (probably) due to the pump that is making the water flow.
Hjulstrom Diagrams
The flows that are required to pick up grains of certain sizes have been extensively studied in experiments and the results are plotted in Hjulstrom diagrams. Hjulstrom diagrams show grain entrainment on a plot of log grain size versus log flow speed. This diagram shows the areas where grains of different sizes are left on the bed, where they get moved sometimes (this is the gray zone), and where they get lifted up often and eroded away. Note that larger grains require higher flows - in general. The water speed that is required to transport a grain is call the critical velocity. This is important. If there is gravel in a sedimentary deposit, you can say that the water flow had to be above the critical threshold for it to get there! That might require a fast flowing river or strong wave action, thus, a large part of narrowing down the depositional environment has already been done!
A copy of the Hjulstrom Diagram from the book: http://mygeologypage.ucdavis.edu/sumner/gel109/Lectures/L3/Hjulstrom.jpg
Deeper flows can move larger grains at the same flow velocity because they are more turbulent: Re=u*l*r/µ and l is larger. This is because deeper flows can have larger variations in flow speed and the laminar flow layers are very thin. They can have bursts of very rapid flow relative to the average flow speed and these bursts can pick up larger grains. Actual flow characteristics are much more complex in detail than just Hjulstrom diagrams, which summarize a lot of characteristics into two axes. However, like a lot of people, we will use the diagram anyway, because it is very useful as a rule of thumb. Just remember that it is not a completely accurate representation of what will happen - it represents a reasonable guess.
Silt and Clay - Notice that for the small end of grain size, the speed of flow required for erosion actually increases. One reason small grains are hard to erode is that they tend not to stick up through the laminar sublayer; they are just too small. Thus, thinner boundary layers are necessary to roll them or for the pressure differences to pick them up off the bed. Also, the surfaces of clay minerals tend to be charged and the grains stick together. This is most obvious when big clumps of mud stick to your shoes. That just does not happen with sand (unless there is something gross in it). The stickiness of the clay grains makes them difficult to erode, so faster water flows (a greater pressure difference or larger turbulent burst down to the sediment surface) are required to move them. The smaller the grains, the more surface charges stick the grains together, thus the stronger the flow needed to erode them. The stickiness of the clay grains also depends on the amount of water between them and the mineralogy, so there is a big gray zone where a clay may or may not erode.
In the Hjulstrom diagram, there is an interesting area where the flow is not strong enough to move any of the particles on the bed, but those that are in the suspended load do not settle out either. This zone includes many of the waters on the surface of the earth. In flows with low velocity or that are very deep, Re is high enough to keep some clay in suspension. Clay deposition usually occurs very slowly, e.g. when the rate of settling is just slightly faster than the average rate at which turbulence moves clay particles upward or when the clays clump together to form larger grains (which is common when fresh and salty waters mix).
A few more words about saltation: Saltation is a very interesting and important process in sediment transport, because the force of the impact when the grains land tends to knock new grains up into the flow even if the flow is not fast enough to lift them with the Bernoulli Effect. These new grains can kick up more grains when they land, etc. This increases the rate of sediment transport above the amount the flow can lift grains off of the bed. This is one of the causes of the gray zone in the Hjulstrom diagram at larger grain sizes. Once saltation starts, it can trigger sediment transport that would not otherwise occur.
Watch grains transported by saltation and traction in these movies: http://faculty.gg.uwyo.edu/heller/SedMovs/Dietrich.htm (11 Mb)
http://faculty.gg.uwyo.edu/heller/SedMovs/sand_sheet.htm (14 Mb)
Deposition: Deposition is the accumulation of grains. If a flow starts slowly and gains speed, it will start to move larger and larger grains. As it slows down, it can only move the smaller ones. Deposition happens when a flow slows down and starts to leave grains on the bed. The combination of changing average flow speeds and local variations in flow speed caused by topography on the bed give rise to very informative sedimentary structures – including cross stratification - which are extremely useful for interpreting depositional environments.
Ripples and Other Bedforms
Structures form on the surface of a bed when topography influences the strength of the flow (and thus the strength of the Bernouli Effect). Erosion occurs where flow is strongest and directed into the bed. Deposition occurs where flow is slower. Deposition almost always creates laminae that are parallel to the depositional surface. Thus, laminae preserved in rocks reflect the shape of the ancient depositional surface. Small ripples have small laminae that dip downstream because that is where deposition occurs; flat beds have flat laminae; large dunes have coarser laminae that dip downstream.
Next Time: We will talk about the details about sedimentary structures. Homework is due on Monday.
Wednesday, January 11, 2012
Fluid Flow
Lecture 2: Walther’s Law and Fluid Flow
Key Concepts from Lecture 1 The Principle of Uniformitarianism – the processes that formed ancient deposits are the same as those that form modern deposits.
The Principle of Original Horizontality - strata (or sedimentary rock layers) are deposited in a nearly horizontal position. If they are no longer horizontal, later deformation much have changed their orientation.
The Law of Superposition - younger sediments overlie older sediments.
Walther’s Law
Key Concept: (Walther’s Law) Depositional environments vary in space and time such that “The facies [rock types] that occur conformably* next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894).
Images of environments and a Google Earth tour: Google Earth kmz files (You must have the program Google Earth installed on your computer. Download and open this file.):
Environments from Davis to San Francisco: DavisSFOEnvironments.kmz
Tidal Environments near Derby, Western Australia: TidalEnvironmentsDerby.kmz
One of the most important implications of Walther’s Law is that rocks of the same type are not necessarily deposited at the same time. There is a BIG difference between correlating rocks based on having the same lithology and rock being deposited at the same time. This is a critical conceptual idea that we will focus on throughout the class.
Summary of Walther’s Law: http://www.youtube.com/watch?v=ZSsULiPouTo
Sediment Transport
Most sediment transport is due to gravity. Things fall down hill in slumps, debris flows, and mudflows, and are transported downhill by fluids, like water, ice, and air. In some cases, processes like waves, currents, and wind transport sediment up a slope such as a beach or up mountain sides. This transport goes against gravity and is driven by the processes of fluid dynamics. Fluid dynamics is the main topic of today's and Wednesday’s lectures. We will come back to mass wasting processes when we talk about erosion. Mass wasting is important for transporting large volumes of sediment short distances, but fluid transport is required to move sediments long distances and is responsible for most sediment transport. To understand sediment transport, it is essential to understand the mechanics of fluid flow.
Fluid Flow There are two end member ways fluids flow: 1) laminar flow and 2) turbulent flow. There is a wide gradation between these two end members, specifically "transitional" flows.
Laminar Flow - In laminar flow, water molecules move in straight, parallel lines down current. If you add a dye to water that is in the laminar flow regime, the dye would not mix into the water; it would streak out in an approximately straight line. Laminar flow is characteristic of very slow moving, shallow water, which is uncommon in nature. It is also characteristic of flows in "fluids" that are very viscous, like glacial ice or mud flows that have little water.
Turbulent Flow - In contrast, turbulent flow is characterized by complex motion of water (or other) molecules. Molecules move in all directions in bursts of upward, downward, and forward motion, and even some backward movement. There is abundant mixing in the flow because neighboring molecules move in different directions, and an added dye mixes into the water very quickly. Most water and air flows are turbulent, at least to some degree. Turbulence is important for sediment transport in water and air because it makes grains easier to transport and tends to keep them moving longer.
Transitional Flow – Transitional flows have some characteristics of laminar flow and some of turbulent flow. For example, dye may take some time to mix into the flow, but it does mix.
Movies of Laminar and Turbulent Flow: YouTube Fluid Dynamics Playlist
Image of rivers mixing: CostaRicaRiver.jpg
and New Zealand:
Images of glaciers: GlacierTrails.jpg and http://visibleearth.nasa.gov/view.php?id=6534
and in Pearse Valley, Antarctica:
Is the flow in glaciers laminar or turbulent? How can you tell? What properties of ice make it behave differently than water in terms of the amount of turbulence?
Reynolds Number - The Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid.
Re can be viewed as inertial forces divided by viscous forces. Inertia is the resistance to change in motion, and inertial forces tend to make a bit of the fluid keep flowing in its own direction if it is misdirected from the main flow direction. Thus, high inertial forces tend to cause more turbulence. In contrast, viscous forces tend to suppress turbulence by damping out variations in motion through friction. Thus, a flow with a high viscosity (ice) tends to have less turbulence than a low viscosity flow (air).
The magnitude of Re gives an idea of whether the flow is turbulent or laminar. Turbulent flow has Re greater than 2000 and laminar flow has Re less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well. In most cases, water and air flows have high Re because l is large, u is high and µ is low. Rivers and wind storms are good examples of turbulent flow. In contrast, ice has a high µ and flows slowly (u is low), so it is usually laminar. Also, very thin, slow flows of water, such as water flowing off a smooth cement parking lot, have low Re because l and u are small. Thus, it can be laminar. Laminar flow also occurs locally in turbulent flows right at the contact between the fluid and a smooth surface because u becomes very slow. This is really important for sediment transport, and we'll talk more about it in a few minutes.
It is useful to think about which variables are important for different comparisons. When comparing ice and water, the main difference is viscosity; the viscosity of ice ranges from about 103 kg/(m*s) up to more than 1020 kg/(m*s) depending on temperature. In contrast, the viscosity of water is ~10-3 kg/(m*s). The density of both is very close to 1000 kg/m3. Thus, ice is almost always laminar but water is usually turbulent, although it can be laminar. When considering water flows, the flow speed and water depth are both very important. The viscosity and density change a little bit with temperature, but variations in flow speed and water depth are typically much larger effects.
Images of glaciers: low ice viscosity: http://earthobservatory.nasa.gov/IOTD/view.php?id=3421 and high ice viscosity: http://tinyurl.com/yhyrob9
For air, both the density and viscosity are low, so does Re tend to be high or low? The density of dry air at 1 atm at 15°C is 1.225 kg/m3, and its viscosity is 1.8x10-5 kg/(m*s), giving an inverse “kinematic viscosity” of p/µ=6.8x105 s/m2 for air versus 1.0x106 s/m2 for water. Thus, air would have a slightly lower value for Re than water for the same flow depth and speed. However, the thickness of typical air flows (meters to 100’s of meters) promotes turbulence. p/µ for ice is 1 to 10-17 s/m2, which is why it is essentially always in a laminar flow regime.
Here is a video of a gust of wind that probably has a maximum u somewhere around 40 m/s: LINK That’s me putting rocks on the fly of the tent. Note the turbulence and the snow transport.
Boundary Layer
There is boundary layer at the edge of every flow, and it is illustrated in the video above. Flows have an average speed in the middle, but friction with immobile surfaces slows down the speed of the flow right at the surface. This creates a boundary layer that has different flow characteristics than the rest of the flow. Right at the surface, the water does not move, but as you go higher into the flow it starts to move more like the average flow. The area of the flow that has a reduced speed is called the boundary layer. The thickness of the boundary layer depends on Re (i.e. the amount of turbulence) and the roughness of the surface the flow is moving past. If the main water flow is very turbulent, it changes the velocity distribution because more of the high speed water is mixed down into the lower speed areas. Thus, the boundary layer tends to be thin. In less turbulent flow, there is little mixing of water from the center of the flow toward the edge of the flow, so the boundary layer tends to be thicker.
Viscous/Laminar Sublayer - Within the boundary layer, right next to the surface, there is a laminar sublayer. Re=u*l*ρ/µ - remember this defines the difference between laminar and turbulent flow. Because u (water speed) is very low at the base of the boundary layer, the Re is low there and the flow is laminar. The laminar flow part of the boundary layer is called the viscous or laminar sublayer, “viscous” because the viscous properties of the fluid are more important than the inertial effects. The fluid is NOT more viscous here; rather, the inertia of the fluid is lower because u is lower, and the viscous properties dominate the behavior of the fluid. Farther up in the flow, u is higher, so the inertial properties of the flow dominate, and the flow is typically turbulent. If grains do not extend above the top of the laminar sublayer, they do not “see” much turbulence, and they are less likely to be transported. If they do stick up beyond the laminar sublayer because the laminar sublayer is thin or the grains are large, the grains feel the force of the turbulent flow.
Bed roughness or the characteristics of the surface also affect the boundary layer by affecting the amount of water that interacts with the surface. A very smooth bed, say one made of mud, does not deflect the water at all, so there is less mixing and less turbulence. There is a well developed laminar sublayer. In contrast, a bed with pebbles or boulders disrupts the direction of water flow in the boundary layer. The water gets deflected around the pebbles. Water from above tends to take its place. Since it is moving faster, the average water speed in the boundary layer increases. Thus, a rough bed reduces the thickness of the boundary layer much like a more turbulent flow does. A rough bed also disrupts the laminar sublayer by forcing the flow to move around objects. The laminar sublayer is developed locally, but in general, rough beds increase the amount of turbulence in a flow.
Sediments and Flow
Key Concept: The boundary layer strongly affects the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below grains, which tend to lift them off the bed.
Bed Shear Stress - Sediments are affected by the difference in flow speeds from the bottom to the top of the boundary layer, gravity, and friction with the ground. Bed shear stress is a measure of these differences; it is the differential force that a grain feels from top to bottom. In a thick boundary layer, the speed of water flow at the top of the grains is not much different from the bottom, so bed shear stress is lower, and sediment is less likely to move. In a thin boundary layer, bed shear stress is much higher, and grains are likely to roll down flow. Thus, more turbulent flow (with a thinner boundary layer) results in more sediment transport. Bed shear stress increases with increasing fluid density, slope, and turbulence (water depth and flow speed). For example, water is better at moving sediment than air because it has a higher density and exerts a larger bed shear stress than air can. Deep, fast rivers move more sediment than shallow, slow rivers because of more turbulence and higher flow speeds in the boundary layer in fast rivers.
Next Time: The Bernouli Effect, which causes grains move, the Hjulstrom Diagram, and sediment transport. Read Chapter 4 again.
Key Concepts from Lecture 1 The Principle of Uniformitarianism – the processes that formed ancient deposits are the same as those that form modern deposits.
The Principle of Original Horizontality - strata (or sedimentary rock layers) are deposited in a nearly horizontal position. If they are no longer horizontal, later deformation much have changed their orientation.
The Law of Superposition - younger sediments overlie older sediments.
Walther’s Law
Key Concept: (Walther’s Law) Depositional environments vary in space and time such that “The facies [rock types] that occur conformably* next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894).
Images of environments and a Google Earth tour: Google Earth kmz files (You must have the program Google Earth installed on your computer. Download and open this file.):
Environments from Davis to San Francisco: DavisSFOEnvironments.kmz
Tidal Environments near Derby, Western Australia: TidalEnvironmentsDerby.kmz
One of the most important implications of Walther’s Law is that rocks of the same type are not necessarily deposited at the same time. There is a BIG difference between correlating rocks based on having the same lithology and rock being deposited at the same time. This is a critical conceptual idea that we will focus on throughout the class.
Summary of Walther’s Law: http://www.youtube.com/watch?v=ZSsULiPouTo
Sediment Transport
Most sediment transport is due to gravity. Things fall down hill in slumps, debris flows, and mudflows, and are transported downhill by fluids, like water, ice, and air. In some cases, processes like waves, currents, and wind transport sediment up a slope such as a beach or up mountain sides. This transport goes against gravity and is driven by the processes of fluid dynamics. Fluid dynamics is the main topic of today's and Wednesday’s lectures. We will come back to mass wasting processes when we talk about erosion. Mass wasting is important for transporting large volumes of sediment short distances, but fluid transport is required to move sediments long distances and is responsible for most sediment transport. To understand sediment transport, it is essential to understand the mechanics of fluid flow.
Fluid Flow There are two end member ways fluids flow: 1) laminar flow and 2) turbulent flow. There is a wide gradation between these two end members, specifically "transitional" flows.
Laminar Flow - In laminar flow, water molecules move in straight, parallel lines down current. If you add a dye to water that is in the laminar flow regime, the dye would not mix into the water; it would streak out in an approximately straight line. Laminar flow is characteristic of very slow moving, shallow water, which is uncommon in nature. It is also characteristic of flows in "fluids" that are very viscous, like glacial ice or mud flows that have little water.
Turbulent Flow - In contrast, turbulent flow is characterized by complex motion of water (or other) molecules. Molecules move in all directions in bursts of upward, downward, and forward motion, and even some backward movement. There is abundant mixing in the flow because neighboring molecules move in different directions, and an added dye mixes into the water very quickly. Most water and air flows are turbulent, at least to some degree. Turbulence is important for sediment transport in water and air because it makes grains easier to transport and tends to keep them moving longer.
Transitional Flow – Transitional flows have some characteristics of laminar flow and some of turbulent flow. For example, dye may take some time to mix into the flow, but it does mix.
Movies of Laminar and Turbulent Flow: YouTube Fluid Dynamics Playlist
Image of rivers mixing: CostaRicaRiver.jpg
and New Zealand:
Images of glaciers: GlacierTrails.jpg and http://visibleearth.nasa.gov/view.php?id=6534
and in Pearse Valley, Antarctica:
Is the flow in glaciers laminar or turbulent? How can you tell? What properties of ice make it behave differently than water in terms of the amount of turbulence?
 |
| What is the difference beween water and ice in this image? |
Reynolds Number - The Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid.
[Viscosity is a measure of the resistance of a material to flow, i.e. how “thick” and easily deformed it is. Viscosity is sort-of like the amount of friction within a substance. Walking through air is easy, because there is not much friction between air molecules. Air has a low viscosity. Swimming is more difficult because the water drags on your body. This is due to the “friction” between adjacent water molecules, i.e. higher viscosity. Ice is more viscous and impossible to move through because of the crystal bonds between the water molecules. It flows, but it does so slowly. It has a high viscosity relative to water and air (but low compared to most rock).]Back to the Reynolds number. The variables for the Reynolds number (Re) are: flow velocity (u), characteristic length (l) which represents flow geometry, say river depth, fluid density (ρ), and fluid viscosity (µ). The book uses µ/ρ = v (kinematic viscosity). Re = (fluid inertial forces)/(fluid viscous forces) = l*u*ρ/µ. The units for this equation are typically (length)*(length/time)*(mass/length3)/(mass/(length*time)). These all cancel out to form a unitless number, if you choose the same set of units for each variable, which you should always do.
Re can be viewed as inertial forces divided by viscous forces. Inertia is the resistance to change in motion, and inertial forces tend to make a bit of the fluid keep flowing in its own direction if it is misdirected from the main flow direction. Thus, high inertial forces tend to cause more turbulence. In contrast, viscous forces tend to suppress turbulence by damping out variations in motion through friction. Thus, a flow with a high viscosity (ice) tends to have less turbulence than a low viscosity flow (air).
The magnitude of Re gives an idea of whether the flow is turbulent or laminar. Turbulent flow has Re greater than 2000 and laminar flow has Re less than 500. Flow with Re between 500 and 2000 is transitional and has some characteristics of laminar flow, but some turbulence as well. In most cases, water and air flows have high Re because l is large, u is high and µ is low. Rivers and wind storms are good examples of turbulent flow. In contrast, ice has a high µ and flows slowly (u is low), so it is usually laminar. Also, very thin, slow flows of water, such as water flowing off a smooth cement parking lot, have low Re because l and u are small. Thus, it can be laminar. Laminar flow also occurs locally in turbulent flows right at the contact between the fluid and a smooth surface because u becomes very slow. This is really important for sediment transport, and we'll talk more about it in a few minutes.
It is useful to think about which variables are important for different comparisons. When comparing ice and water, the main difference is viscosity; the viscosity of ice ranges from about 103 kg/(m*s) up to more than 1020 kg/(m*s) depending on temperature. In contrast, the viscosity of water is ~10-3 kg/(m*s). The density of both is very close to 1000 kg/m3. Thus, ice is almost always laminar but water is usually turbulent, although it can be laminar. When considering water flows, the flow speed and water depth are both very important. The viscosity and density change a little bit with temperature, but variations in flow speed and water depth are typically much larger effects.
Images of glaciers: low ice viscosity: http://earthobservatory.nasa.gov/IOTD/view.php?id=3421 and high ice viscosity: http://tinyurl.com/yhyrob9
|
| Is this low or high viscosity ice? |
Here is a video of a gust of wind that probably has a maximum u somewhere around 40 m/s: LINK That’s me putting rocks on the fly of the tent. Note the turbulence and the snow transport.
Boundary Layer
There is boundary layer at the edge of every flow, and it is illustrated in the video above. Flows have an average speed in the middle, but friction with immobile surfaces slows down the speed of the flow right at the surface. This creates a boundary layer that has different flow characteristics than the rest of the flow. Right at the surface, the water does not move, but as you go higher into the flow it starts to move more like the average flow. The area of the flow that has a reduced speed is called the boundary layer. The thickness of the boundary layer depends on Re (i.e. the amount of turbulence) and the roughness of the surface the flow is moving past. If the main water flow is very turbulent, it changes the velocity distribution because more of the high speed water is mixed down into the lower speed areas. Thus, the boundary layer tends to be thin. In less turbulent flow, there is little mixing of water from the center of the flow toward the edge of the flow, so the boundary layer tends to be thicker.
Viscous/Laminar Sublayer - Within the boundary layer, right next to the surface, there is a laminar sublayer. Re=u*l*ρ/µ - remember this defines the difference between laminar and turbulent flow. Because u (water speed) is very low at the base of the boundary layer, the Re is low there and the flow is laminar. The laminar flow part of the boundary layer is called the viscous or laminar sublayer, “viscous” because the viscous properties of the fluid are more important than the inertial effects. The fluid is NOT more viscous here; rather, the inertia of the fluid is lower because u is lower, and the viscous properties dominate the behavior of the fluid. Farther up in the flow, u is higher, so the inertial properties of the flow dominate, and the flow is typically turbulent. If grains do not extend above the top of the laminar sublayer, they do not “see” much turbulence, and they are less likely to be transported. If they do stick up beyond the laminar sublayer because the laminar sublayer is thin or the grains are large, the grains feel the force of the turbulent flow.
Bed roughness or the characteristics of the surface also affect the boundary layer by affecting the amount of water that interacts with the surface. A very smooth bed, say one made of mud, does not deflect the water at all, so there is less mixing and less turbulence. There is a well developed laminar sublayer. In contrast, a bed with pebbles or boulders disrupts the direction of water flow in the boundary layer. The water gets deflected around the pebbles. Water from above tends to take its place. Since it is moving faster, the average water speed in the boundary layer increases. Thus, a rough bed reduces the thickness of the boundary layer much like a more turbulent flow does. A rough bed also disrupts the laminar sublayer by forcing the flow to move around objects. The laminar sublayer is developed locally, but in general, rough beds increase the amount of turbulence in a flow.
Sediments and Flow
Key Concept: The boundary layer strongly affects the amount of “Bed Shear Stress” which corresponds to the forces that tend to roll particles along the bed and the pressure differences above and below grains, which tend to lift them off the bed.
Bed Shear Stress - Sediments are affected by the difference in flow speeds from the bottom to the top of the boundary layer, gravity, and friction with the ground. Bed shear stress is a measure of these differences; it is the differential force that a grain feels from top to bottom. In a thick boundary layer, the speed of water flow at the top of the grains is not much different from the bottom, so bed shear stress is lower, and sediment is less likely to move. In a thin boundary layer, bed shear stress is much higher, and grains are likely to roll down flow. Thus, more turbulent flow (with a thinner boundary layer) results in more sediment transport. Bed shear stress increases with increasing fluid density, slope, and turbulence (water depth and flow speed). For example, water is better at moving sediment than air because it has a higher density and exerts a larger bed shear stress than air can. Deep, fast rivers move more sediment than shallow, slow rivers because of more turbulence and higher flow speeds in the boundary layer in fast rivers.
Next Time: The Bernouli Effect, which causes grains move, the Hjulstrom Diagram, and sediment transport. Read Chapter 4 again.
Monday, January 9, 2012
Uniformitarianism, Original Horizontality and Walther's Law
Lecture 1B: Sediments and Strata
Sediments and sedimentary rocks cover most of earth, and weathering is occurring on the rest of it. The reshaping of the surface of the earth has had a huge influence on the planet, affecting everything from the evolution of life to the tectonics of mountain ranges. Sediments and sedimentary rocks record the events and processes that shaped the surface of earth – and other rocky planets. They provide the temporal framework that connects processes within the earth to those at the surface. They are important for:
1. Earth history. Sedimentary rocks contain features that allow us to interpret ancient depositional environments, including the evolution of organisms and the environments they lived in, how climate has changed throughout earth history, where and when faults were active, etc.
2. Economic resources. Petroleum reservoirs have organic-rich, sedimentary source rocks that produced the petroleum when heated, most oil and gas migrates through sedimentary rocks, and most of the reservoirs are hosted in sedimentary rocks. Water aquifers are dominantly found in sedimentary rocks (although some are in fractured metamorphic and igneous rocks). The composition of the rocks strongly influences water quality due to water-rock interactions. (Why does Davis water taste bad?) Sedimentary rocks also host economic minerals such as gold and diamonds, which are eroded from other rocks and concentrated to specific areas during sediment transport.
3. Environmental geology. Sediments cover 2/3 of the continents and essentially all of the ocean floor, which totals 89% of the surface of earth. They host the biosphere, and they are most of the rocks we interact with directly and indirectly. Our actions as humans have an extremely strong effect on sedimentation and erosion. Understanding our impact on the environment - and the environment’s impact on us - must include a strong focus on sediments and sediment transport.
Sedimentology and Stratigraphy
Sedimentology is the study of sediment transport processes and sedimentary rocks, and it is the focus of most of this course. If covers scales ranging from a single grain to entire planets, and focuses on processes. Stratigraphy is the study of the distribution of sediments and sedimentary rocks in space and time. It is essential for understanding earth history, reservoir properties, etc. We will place our sedimentological interpretations into a stratigraphic context with examples in lecture, the homework, and tests.
Sedimentology and stratigraphy are about as old as mineralogy as a field of study. Leonardo da Vinci provided one of the first environmental interpretations from sediments; he interpreted fossils in the Italian Apennines as evidence of an ancient ocean. He used the logic behind what we now call the Principle of Uniformitarianism: Similar organisms produce similar shells. The logic is, “If you see shells on the tops of mountains that look like those from organisms that live in the ocean today, the shells on the mountain tops were probably once in the ocean, too.” (Did the mountains go up or did the ocean go down? That is a question that was not thoroughly answered until we understood plate tectonics!) Here is a more formal statement of the Principle of Uniformitarianism:
Key Concept: The characteristics of sedimentary rocks can be used to determine the environmental conditions under which they were deposited, and the environmental conditions allow you to predict the characteristics of sediments that are likely to be deposited. This is the Principle of Uniformitarianism (formulated by James Hutton in the mid 1700’s) – the processes that formed ancient deposits are the same as those that form modern deposits.
Movies and photos of ripples and ripple cross-lamination:
Current ripples forming in a flume: http://youtu.be/rSzGOCo4JEk
Wave ripples forming in a flume: http://youtu.be/MBDadRqSHOc
Modern Current Ripple Image: http://tinyurl.com/yjlw7gq
Ancient Current Ripple Image: http://tinyurl.com/yh46jlm
Modern Wave Ripple Image: http://tinyurl.com/7dbwr82
Ancient Wave Ripple Image: http://tinyurl.com/7t8p2bz
In the much more recent past, for example in the 1970’s, the Principle of Uniformitarianism was interpreted by some as requiring continuous, incremental processes and as excluding dramatic, rapid events. For example, a meteorite impact was seen as a non-uniformitarian event. However, my view of uniformitarianism can encompass rare events. The basic idea is that catastrophic events also produce characteristic features. For example, a meteorite impact produces similar deposits no matter when it occurs in time. We can recognize impact spherules from Archean sedimentary rocks that formed and were deposited in essentially the same way as those from the Cretaceous impact that killed the dinosaurs. Or an impact on mars will produce features similar to those produced by an impact on earth. The key point is that similar processes produce similar products. All processes are not active at all times (large meteorites are not continuously bombarding earth!), and some, like burrowing by worms for example, did not occur at specific times, e.g. before the evolution of worms. However, if a feature is present that is characteristic of a specific process, e.g. a thin tube with a specific geometry, it is reasonable to interpret that process, e.g. burrowing by a worm, as having produced the feature. This is how we extract earth history from rocks, e.g. the absence of worm burrows before 540 Ma allows us to state with confidence that worms did not exist before 540 Ma. However, it is often challenging to identify which processes produce which features. There is rarely the nice, exact correlation between features and processes that one would wish for. For example, a specific color variation in a rock could reflect a burrow or a water flow path or both if the burrow influenced water flow. One needs to understand the uncertainties in geological interpretations.
Summary of the Principle of Uniformitarianism: http://www.youtube.com/watch?v=ifdlx_dFzPU
There are two other really important concepts that were first articulated in the 1660’s: The Principle of Original Horizontality and the fact that younger sediments overlie older sediments. Nicolas Steno was the first to write down the idea that strata (or sedimentary rock layers) are deposited in a nearly horizontal position, an idea called the principle of original horizontality. Some layers are deposited exactly flat, but most layers follow the tilt of the depositional surface, which is not exactly horizontal. However, most sedimentary layers are close to horizontal for our purposes here. If layers are no longer horizontal, later deformation must have changed their orientation.
Images "horizontal" and tilted strata:
Grand Canyon "horizontal" strata: National Park photos
Rainbow Basin tilted and folded strata: http://en.wikipedia.org/wiki/File:Rainbow_Basin.JPG
This idea is intimately associated with the idea of time in rocks.
Key Concept: Younger sediments overlie older sediments (if they are still approximately horizontal)– obviously, at least to us now.
Steno first wrote this down in 1667. The relative ages of sedimentary rocks gives us time. We can interpret changes in processes through time using the principle of uniformitarianism combined with the relative age (or stratigraphic succession) of rock layers. Steno’s work provided the intellectual framework for understanding relative time in a local areas. It was not until much later that the idea of “faunal succession” (articulated by William Smith, early 1880’s) provided a global time scale. Smith (and other geologists at the time) recognized that fossil organisms succeed one another in the stratigraphic record in an orderly, recognizable fashion. They were learning about a key component of evolution, although they did not yet have the intellectual framework of evolution by natural selection. They formulated the basic ideas that if organisms evolve through time, rocks containing similar organisms are approximately the same age.
Discussion of the distribution of time in sedimentary rocks: http://www.youtube.com/watch?v=fBjR_1vK9ug
The observations leading to the definition of Cambrian, Ordovician, etc. were basically the same as those of da Vinci: Fossils can be used to interpret ancient rocks. In the case of da Vinci, he interpreted similar environments because the shells he saw were essentially identical to those in the modern oceans. In the case of Smith and other English geologists, similar fossils suggested the rocks were similar ages. These two ideas reflect the two key components of stratigraphy: rock types vary in both space and time, but the same type of rock can be deposited in different places at different times. These changes can be organized based on how different environments are distributed:
Key Concept: (Walther’s Law) Depositional environments vary in space and time such that “The facies [rock types] that occur conformably* next to one another in a vertical section of rock will be the same as those found in laterally adjacent depositional environments” (Johannes Walther, 1894).
(*Conformably means that there is neither a break in sedimentation nor erosion between the two environments, e.g. there is no unconformity between them. Jumps in depositional environment can occur if the rocks do not provide a complete record of the environmental changes that occurred; rock types in a vertical succession separated by the unconformity do not necessarily represent neighboring environments.)
Images of environments and a Google Earth tour: Google Earth kmz files (You must have the program Google Earth installed on your computer. Download and open this file.):
Environments from Davis to San Francisco: DavisSFOEnvironments.kmz
Tidal Environments near Derby, Western Australia: TidalEnvironmentsDerby.kmz
One of the most important implications of Walther’s Law is that rocks of the same type are not necessarily deposited at the same time. There is a BIG difference between correlating rocks based on having the same lithology and rock being deposited at the same time. This is a critical conceptual idea that we will focus on throughout the class.
Summary of Walther’s Law: http://www.youtube.com/watch?v=ZSsULiPouTo
Next time - Sediment transport
Reading:
Chapter 4 is the most important for Wednesday’s lecture.
Chapters 2 and 3 cover classification of sediments and sedimentary rocks. We are not going to cover them explicitly in class, but knowing the terminology will be critical. Thus, read these chapters soon! Chapter 2 will be very useful for those of you in lab.
Intro to GEL109: Business
Lecture 1A: Introduction
Business
The Course web site is the SmartSite for GEL109/109L and http://mygeologypage.ucdavis.edu/sumner/gel109 which is linked on the SmartSite.
I am responsible for:
1. Coming to lecture prepared to provide you with the opportunity to learn about sedimentology.
2. Preparing instructive homework assignments and tests that will help you learn the most important material. My philosophy is to make it clear what I think is important and provide you with tools to learn the material. I provide old tests and detailed study guides so that you can thoroughly learn the material and demonstrate your knowledge of it on the various assignments.
3. Grading your homework and tests in a fair and timely manner.
You are responsible for:
1. Coming prepared for lecture by reviewing your previous lecture notes and reading the text book. Reading the book is very important because it includes some information that I will cover only briefly in class, but I’ll expect you to know. For example, I will not discuss rock classification schemes in lecture because it is boring and the book explains the rock types well. However, I will use the terms sandstone, siltstone, shale, etc. because they are always used in geology, and you need to know them. In most cases, the material I cover in lecture and with the homework assignments are the concepts I think are most important, so the tests will focus on those.
2. Asking questions when you have them in class, through e-mail, during office hours, when you see me in the hall, etc. Asking questions is VERY important for two reasons. First, it will help you and your fellow students learn, which is the entire reason you are in this class. Second, it helps me gage how well I'm communicating information to you. The more I know about how much you are understanding and what you are thinking about, the better I can prepare explanations that are both clear and interesting.
Grades
Grades will be calculated using two formulas: 2 tests and 10 homework assignments with the midterm=33%, final=34%, and homework=33%; and 2 tests with the midterm=50% and final=50%. You will get the higher grade from the two formulas. I do not grade on a curve, so if you all thoroughly learn the material, you can all earn A’s.
Tests
My tests are hard, but fair. They are hard because they ask you to think about the material. I will give you detailed study guides which will contain all of the material that will be on the tests (plus some). The homework will include questions like those on the tests, and practice tests are available. The only people who have failed the tests are those that did not take advantage of the study tools.
Homework
The homework assignments are posted on SmartSite as “Quizzes” or pdf files. This is the first time I’m trying to use the Quiz feature for some of the homework. The main reason is to help with grading. Some of the problems require you to draw, however. Those problems will require you to hand in paper unless you choose to do the drawing on a computer. Please let me know how you feel about the format of the homework assignments.
For some assignments, there are supplemental web materials. These materials are very useful, and you should plan to use them. Homework is due by the end of the date listed, or as set on the SmartSite “Quiz”.
Study Resources
Over the years, I’ve accumulated numerous study resources including images and videos. I have posted summary videos on YouTube since 2007, and those videos are used by people all over the world to help learn sedimentology and stratigraphy concepts. I made them for you, my students in GEL109, and they emphasize the key concepts from many of the lectures. My channel is sumnerd and some of the play lists I’ve assembled can be found on the course web page. Use the videos!
For people not taking the lab:
The students who are also taking the lab (GEL109L) are spending an additional 6 hours per week working on sedimentology, so they will be more familiar with the material than you are. You can still do well, and it is particularly important that you ask questions. Also, there are two field trips required for the lab. I strongly recommend coming on them. You can learn much more in the field than in lecture or even lab!
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