Brain, over at Clastic Detritus ,has posted a nifty picture of some sweet, sweet Swaley Cross-Stratification. To show some solidarity, I too will post a couple of pictures of some SCS I’ve collected. Actually, both of these pictures are from the Book Cliffs, as well. If you ever find yourself in and around Price, Utah, I suggest you grab a map, your rock hammer, and a camera, and just spend some time wandering around all the BLM land and its fantastic geology.
The picture below has some SCS nicely exposed in the lower right of the outcrop; actually, if you trace the lower bounding surface over to the left a ways, you can just start to see it transition into hummocky cross-stratification. As Brian pointed out in his post, these two bedforms are often associated with one another. In fact, these bedforms are so characteristic, that they have often been evoked as “typical” of storm deposits. In some of the literature, HCS and SCS are often interpreted as unequivocal evidence for tempestites.
This next picture shows another nice little swale, hanging out with some structureless and planar laminated sands.
The SCS is all good and well, and some may be tempted to slap an interpretation of a storm-bed on there, but darned if some of the surrounding sands aren’t lookin’ an awful lot like some T a-b turbidites (with just the massive and planar laminated sections preserved of the more traditional Bouma sequence). But I thought SCS meant storms!?! Aren’t we in the middle-to-upper shore-face!?!
As Brian points out in his post, the flow hydrodynamics of these bedforms are related to high energy and rapid sedimentation. Dumas and Arnott (2006, Geology, p. 1073-1076) have further gone on to quantify the specific flow hydrodynamics related to these settings, demonstrating that oscillatory-dominant combined flow (meaning both back-and-forth AND directional currents) are responsible for the production of these bedforms. While it is true that storm currents can (and do) produce these bedforms, these same hydrodynamic conditions can also be produced by other processes, such as turbidity currents! The picture above is actually from a portion of the Book Cliffs recording the progradation of one of the famous clastic wedges, and there are turbidites and (potential) hyperpycnites associated with this interval.
Things are never as easy as they seem, and that goes double for facies models.