COMMISSURE ORIENTATION AND ASSESSMENT OF PALEOCURRENT DIRECTION FROM OYSTER DEPOSITS OF THE FERRON SANDSTONE

Thick accumulations of oysters (Crassostrea) are found in the upper Ferron Sandstone near Emery, Utah. These beds lie only a few meters below the transgressive marine deposits of the Blue Gate Shale. The Crassostrea accumulations can be up to 1.5 meters thick and tens of meters in lateral extent.

Three subfacies characterize the oyster accumulations. The first subfacies comprises dense accumulations (generally >40% shell material) that contain shell debris in the mudstone between larger valves. This is interpreted to represent the most reworked reef deposits. The second subfacies type shares similar densities, but contains a mudstone matrix. Subfacies 2 exhibits larger valves than Subfacies 1. Most valves in Subfacies 2 are paired and are oriented parallel or slightly oblique to bedding. These are interpreted to represent less reworking and possible packed life positioning. Although vertically oriented Crassostrea valve pairs are rarely observed, most occur within the third subfacies, which is dominantly mudstone with 5-20% oysters in inferred life position.

Orientations taken from the commissure alignment of Subfacies 3 shell pairs show strong north-south orientations that match north-trending paleocurrent data from closely related (geographically and stratigraphically) channel sandbodies. This suggests that even in strata with limited exposed vertical shell pairs, that commissure orientation of Crassostrea can be useful for determining paleocurrent patterns.