Friday Field Foto #50: Swaley cross-stratified sandstone
Today’s Friday Field Foto is from the Cretaceous of Utah and is a nice example of a type of cross stratification called swaley cross stratification. The key to recognizing this type of sedimentary structure is that the angle of the laminae decrease upwards. In the photo below, you’ll notice the very obvious scour surface cutting down through roughly horizontal laminae. The fill of that depression has sandy laminae that start off steep and then systematically decrease in steepness upwards. I will admit that this criterion is not always foolproof … you can sometimes get tricked by the cut of the outcrop and such. In this case, you’ll have to take my word for it as there were plenty of examples in this stratigraphic section.
This sedimentary structure is thought to be a product of a combination of both bedload sediment transport and sediment falling out of suspension. Reworking and resuspension of sand as a result of vigorous storm waves are a common interpretation of swaley cross-stratified sandstone. Another structure that is, in a way, the inverse of this – a mounded structure, called hummocky cross-stratification – can be quite common in close stratigraphic association (to see a nice example of hummocky bedding, check out front page of The Dynamic Earth blog).