Monday, March 1, 2010

Trough Cross-Stratification

This photograph was taken about a mile east of Butte Community College on the Durham-Pentz Highway.

I am the person in the photo.  Unfortunately, I am awful for  a scale due to perspective.

This is a roadcut and it clearly shows trough cross-stratification, perhaps from a stream.  The structures that clues us in are the abundance of smiley faces.  These are created as subaqueous dunes migrate and their troughs cuts off previously deposited dune peaks.

Streams have these kinds of structures at the thalweg or just above it.  The thalweg is the deepest, and often the fastest, part of a stream.  It is what develops cut banks. The fastest, outside portion of a stream's meander cuts into terraces creating a cut bank.  The interior portion of the meander develops a point bar.  If sediment were deposited above the trough cross-stratification, we might see fining upwards and current ripples.  That would indicate a pointbar.  If we could see sediment below this structure, we might see mud from overbank deposits.  That is Walther's Law: vertical succession of beds shows differences in lateral environments (providing unconformities do not exist!).

If you click on the photo, you should notice that the grain sizes are pretty apparent.  They are medium to coarse, though I lean towards coarse.  That would be an additional line of evidence that this was deposited by a stream.

I cannot say for certain which direction this stream flowed.  From this perspective it is either in or out of the photograph.  But I would hazard to guess water flows in to the photograph since the troughs seem to be dipping that way.

In summary, this is a cut-and-fill structure which represents the bottom of a stream!  Cool!

3 comments:

  1. In my experience its the stream migrating across the valley floor and reworking terraces that cause cut banks, not the thalweg.

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  2. Robin: from what I understand it's part of the meandering mechanism. It it is always in the fastest part of the stream which cuts into the bank. So it is at least an accessory to the crime. Then those seds can go downstream or be brought to the pointbar in some kind of cross-current. But one sedimentology class does not make me a hydrologist!

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  3. Oh, it's great for people to pick apart any of my interpretations and other statements. So bring them on; I'll learn from my mistakes.

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