Wednesday, January 23, 2013

Sedimentary Structures Part 2

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

A sketch of a ripple or dune like the one in lecture:

Remember where the separation point and attachment point are located. The geometry of the flow tracks these points. Erosion can only occur where there the bed shear stress is high enough to move sediment. In other words, the the main flow must be near the sediment surface. Sediment accumulates into a deposit in the flow shadow downstream of the ripple or dune crest; sediment accumulates in the flow detachment zone. Laminae are visible where deposition occurs due to variations in flow speed which cause variations in grain sizes transported and deposited.

Sets of laminae are separated by erosion surfaces which form on the upstream side of the ripples or dunes. They represent deposition on the downstream side. The shape of the laminae reflects the shape of the depositional surface and the geometry of sediment accumulation. If the depositional surface is curved, the base of the laminae is curved. Areas with higher deposition produce thicker laminae. Also, the maximum distance between erosion surfaces is less than the maximum height of the ripple or dune; since the erosion surfaces form on the upstream sides, they are closer to the underlying erosion surfaces than the ripple/dune crests. Thus, the maximum separation of erosion surfaces represents a minimum height for the ripple or dune.

Dunes and ripples behave similarly at the level of detail that I have been describing them. Their cross stratification geometries are similar. However, dunes are larger than ripples. If the distance between erosion surfaces defining cross sets is greater than a few centimeters, the cross stratification has to be from a dune. Ripples are only a few centimeters tall, and they cannot create laminae that are higher than the ripple crest-to-trough distance. Thus, if cross sets are greater than a few centimeters high, the cross stratification must be from dunes. However, if the cross sets are only one centimeter high, the cross stratification could be due to either ripples or dunes. It is possible for ALL sediment to be eroded as a dune migrates, leaving no cross stratification. If only a small amount of sediment accumulates, the cross sets might be only a centimeter high, much like ripples. In the field, grain size variations and changes in cross stratification along an outcrop can help you distinguish between ripples and dunes in a case like this. For example, you could look for an instance where the cross stratification is more than a few centimeters high. If you did not find one, that might suggest ripple cross lamination rather than dune cross stratification. Or maybe the grain size is wrong for one or the other.

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:

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 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 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.

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