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Exploring Cross Stratification

Overview

By working through this page, you will gain a deeper understanding of how cross stratification reflects the geometry of migrating bedforms.  We will use information from David M. Rubin, who published a book called Cross-Bedding, Bedforms, and Paleocurrents, which is available from SEPM with free online movies. His great movies demonstrate how various types of cross stratification are created. You can use these movies to help interpret various styles of cross stratification.

Goal

The insights you gain from working through this page will help you predict the type of cross stratification a ripple or dune is creating as well as interpret the ancient flows that produced cross stratification you see in rocks.  

Straight Bedforms

A computer model generated these movies. Each starts with a bedform shape, which is constant throughout the movie. It is defined by the grey shaded surface. "Laminae" are deposited by moving the surface downstream and slightly up for each time step. Deposited laminae are represented by white lines. If the surface intersects the laminae, they are eroded, just like if laminae are exposed to an erosive current, they are eroded. Thus, the laminae change length and geometry as the "bedform" (surface) migrates.

All the movies associated with the publication are all listed at http://walrus.wr.usgs.gov/seds/bedforms/animation-toc.html. They are categorized by the shapes of the bedforms and photographs of real bedforms are also included. This is an EXCELLENT resource.

First, watch a model-produced version of the migration of a linear ripple or dune. Note that the crests of the ripples or dunes are straight.  They produce dipping surfaces on the lee side of the bedform (where sediment is deposited and shown by tan lines). These are preserved if the crest migrates upward through time, e.g. sediment accumulates.  Note that the laminae look flat when viewed looking directly upflow or downflow, as in the image here.  

Questions to Ask Yourself:

  • How does the geometry change looking at different angles? (Watch carefully between 30 and 50 sec in the video.)  
  • What is the geometry of the laminae on top of the deposit? (Watch after 50 sec.)

Curved Bedforms

Watch a movie of ripples or dunes with curved crests. After migration of the bedforms, the cross stratification is rotated to show how different surfaces show different cross stratification styles. Drag the marker at the bottom of the movie back and forth to explore these variations. Note how important it is to have a 3-D view of the cross stratification to interpret bedform geometry even when it is relatively simple!  The movie ends with the top surface of the ripples/dunes getting eroded flat. The tan lines represent the laminae that intersect the upper surface. Note how complexly shaped they are.

Questions to Ask Yourself:

  • How is the geometry of the cross stratification different than that of bedforms with linear crests when viewed perpendicular to the flow direction?
  • How is the geometry of the cross stratification different than that of bedforms with linear crests when viewed looking directly upflow or downflow?
  • How does the geometry change at intermediate angles?
  • How is the geometry of the top look different? Which lines represent the laminae and which represent the bounding surfaces?

Real Ripples

Next, watch a movie of real current ripples in an experimental flume. This will give you a sense of how irregular ripples can be. https://walrus.wr.usgs.gov/seds/bedforms/extras/movies/ripple_flume.html In nature, sometimes ripples are this variable, and sometimes they are more linear.

Make a Prediction:

  • Predict the geometry of the cross stratification produced by the bedforms in the flume!

Sediment Accumulation

The amount of sediment that accumulates as a ripple or dune migrates past changes whether or not sediment accumulates. Compare these two movies: No sediment accumulation and very abundant sediment accumulation. The amount of sediment that accumulates depends on how the sediment transport capacity of the flow is changing. If the flow is speeding up or getting deeper, its transport capacity is increasing, and erosion will occur. If the flow is staying exactly the same, all the sediment that is moving will bypass the sediment surface, and nothing will be preserved. If the flow is slowing down or getting shallower, it has to reduce the amount of sediment that it is carrying, so sediment remains behind and preserves the passage of the bedform.

Questions to Ask Yourself:

  • If there is no sediment accumulation, will there be any record of the bedform passing?
  • How is the geometry of the cross stratification different when there is so much sediment accumulating that there is no erosion on the stoss side of the bedform than when there is?
  • What types of conditions promote the accumulation of sediment and thus the preservation of the bedform passing? Where are these conditions likely to occur?

What's Next

One goal of the modeling effort is to reproduce complex cross stratification that is observed in the field. The following movie produces a simplified version of cross stratification observed in a river bar. Here is movie that shows how the model can fit complicated structures observed in the field. This movie begins with an image of cross stratification from the Colorado River.  Then it shows a computer-animated sequence that generates compound crossbedding. "After the computer-generated structure is constructed and rotated through a variety of outcrop orientations, the image morphs into a natural outcrop of the same kind of structure. The natural example was photographed on a horizontal surface that was excavated into a fluvial bar in Grand Canyon. This movie is the best one for illustrating the origin of moderately complicated crossbedding." (Dave's description)

Lots of other movies are available to explore as you wish. There are also LOTS of photos of different model-produced stratification styles and some outcrop photos associated with the online book (see https://walrus.wr.usgs.gov/seds/bedforms/animation-toc.html). The USGS web site also provides software to make your own movies by defining a bedform geometry. Have fun!