Thursday, August 21, 2008
I spent a few days in Wyoming, at a conference and field trip focusing on clinoforms, organized by SEPM (Society for Sedimentary Geology). Clinoforms are sedimentary layers with a depositional dip of a few degrees that form packages of relatively large thickness (let's say more than a few meters; could be hundreds of meters in some cases. The point is that the foresets of ripples, sand dunes and other bedforms could be called clinoforms but they should not be). [Warning! - my definition]. After a couple of days of morning talks (many very good ones) and afternoon posters, we spent two additional days visiting some outcrops in southern Wyoming.
These photos come from exposures of the Maastrichtian Fox Hills Sandstone of the Eastern Washakie Basin, a sandstone of deltaic and fluvial origin that links through shaly clinoforms to turbidite sands and shales of the Lewis Shale, deposited in water depths of more than 400 meters (see reference below).
The photo above shows one set of smaller-scale clinoforms truncated by a cross-bedded sandstone unit above, probably of fluvial origin. This is a prograding shoreline. It was a matter of debate whether the erosional surface at the base the fluvial sands is a sequence boundary or not, and could be a blogworthy subject in itself, but I will refrain from discussing it here and now. What I think - at least visually - are more exciting are the water escape structures in the photograph below.
There are two sandy layers visible in the picture; the lower one is somewhat darker colored and more massive-looking than the upper one, which is more laminated and has an overall lighter color. The height of the rock surface covered in the photo is about 1.5 m.
A likely explanation for the structures is as follows (sorry for the arm-waving -- it would be nice to put some numbers here - sedimentation rates etc., but life is too short for that right now). Soon after the first (darker) layer was deposited, another flood of the river brought more sediment to this location, and started depositing sand, mostly along a flat bed that resulted in parallel lamination. The underlying sediment was still very porous and unconsolidated, and some of its pore water was trying to get to the surface as the weight of the overlying deposit increased. Thin layers of finer-grained and therefore less permeable sediment got in the way however; and the escaping pore water had to travel laterally until it found the most vulnerable spots to go again upward. There are two of these vertical water escape conduits in the photo. As all the water coming from the lower layer had to go through a limited number of these spots, the velocity of the pore fluid must have increased significantly, until it actually was able to fully suspend the sand it encountered. In other words, some of the sand along these vertical escape zones got fluidized and carried away. The white structureless patches of sand are sedimentary intrusions. The light color suggest that these sands are much 'cleaner' than the rest of the rocks; the finer grains (responsible for the darker color) were washed away.
One interesting detail is that the trough cross-bedded sand between the two intrusions thickens into the depression, suggesting that the water was trying to get out in real time, that is, at the same time as the upper layer was being deposited.
Below I linked in an amateurish-looking gigapan; and here is another post on water-escape structures.
Launch full screen viewer
Carvajal, C.R. & Steel, R.J. (2006), Thick turbidite successions from supply-dominated shelves during sea-level highstand. Geology, 34, p. 665-668.