Friday Field Foto #61: Hummocky cross stratification
This week’s Friday Field Foto highlights everybody’s favorite oscillatory-/combined-flow sedimentary structure … hummocky cross stratification!
I have to apologize for not having the specific location and formation information for this photograph. I took this several years ago – before I became anal-retentive about my geologic photography. I do know that it is in the Upper Cretaceous (~70-90 million yrs ago) of westernmost Colorado (not too far from the town of Rangely … maybe the Sego Sandstone, I don’t know).
Hummocky cross stratification, also known as HCS, is very cool … everybody gets excited when they see a good example. Check out the front page the blog The Dynamic Earth for an even better shot (also in the Cretaceous Western Interior Seaway succession). One of the key recognition criteria is the convex-up, mounded shape (going up from left to right in the photo; click on it for larger version).
Some of the nitty-gritty details about the formation of this sedimentary structure are still discussed and debated but, in general, it is thought to be formed by a combination of oscillatory flow (i.e., from waves going back and forth) with unidirectional currents that entrain sediment. I’ll leave it at that for now, but check out Myrow & Southard (1991) for more.
Another reason people get excited when they see HCS is that it’s a fantastic indicator of a particular environment — a subaqueous environment shallow enough to feel the effects of waves. Some also think this structure is indicative of particularly strong wave action associated with big storms. You will typically find them in a stratigraphic succession sandwiched between offshore shale/siltstone and shoreface and/or delta-front deposits.
Myrow, P.M. and Southard, J.B., 1991, Combined-flow model for vertical stratification sequences in shallow marine storm-deposited beds: Journal of Sedimentary Research, v. 61, p. 202-210.