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!

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