Upper Jurassic Oxfordian sediments of Wyoming, Montana, and eastern Idaho were deposited in part of an elongate inland seaway extending southward from the present Arctic. This seaway contained a complex of tectonic elements which influenced significantly the geographic distribution of depositional environments. On the western edge of the Oxfordian seaway (in eastern Idaho), a clastic wedge sequence is developed. Farther east, toward the center of the seaway, complex sequences of marine bars and channels are represented around the edges of the Wind River, Big Horn, and Powder River basins. On the north in Montana, and east in the Black Hills, more typical prodeltaic sequences are developed.
Sediment dispersal directions in this seaway were complex due to the interaction of tides, regional currents, depositional environment, and storms. Of these 4 controlling factors, storms may have had the most profound influence, particularly in marine-bar and channel environments. Evidence for storms may be found in coquinoid sandstones. These sandstones are of 2 distinct geometries: (1) sheetlike lags interbedded with silty shales, representing the deposition of reworked shallow benthos in deeper water areas; and (2) channel-fill lags incised into marine bar sediments, representing scours in a shallow marine environment.
Vertical successions of Oxfordian sediments display an initial transgressive sequence followed by a well-marked regressive sequence.