Sedimentary Structures Continued
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. Laminae thicken into areas with higher deposition rates. 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 stratification/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: Sedimentary Structures Lab and Homework Resources
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:
- no transport
- faint planar lamination - the lamination is poorly developed because the sediment is often poorly sorted and not much transport is occurring
- dunes - the flow is strong enough to erode at the attachment point
- upper planar lamination
In contrast, the sequence of structures in silt is:
- no transport
- upper planar lamination
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). 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 very faint laminae that dip upstream, and they migrate upstream (anti normal dune behavior). Sediment is still transported downstream; it is just the peak of the dune itself that moves upstream. At even higher flow, the waves on the surface of the water break, and the dunes become very irregular. Antidunes are rarely preserved in the rock record because they are reworked into other sedimentary structures as the flow speed decreases.
Other Types of Flows - Not all flows are uniform in one direction. For example, waves move water back and forth, transporting sand back and forth. Because the transport direction varies through time, the orientation of cross laminations vary through time. Compare the ripple types at http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/ARipples.html Note that wave ripple lamination dips in two directions and the ripple crests are symmetric rather than steeper on the lee slope than on 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 above. 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) 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.
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.
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
What features do we see in the photo of the beach? How should we divide those into facies? We can compare them to what we would see in the rock record. Take a look at photos of Scott Creek Beach stratification again: http://mygeologypage.ucdavis.edu/sumner/gel109/sedstructures/Beach.html. Describe some of the facies.
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 (or actualism). 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.