Abstract

Modal analysis of framework-grain composition of Morrison-Cloverly sandstones in and surrounding the Wind River Basin reveals changes from dominantly subarkoses to litharenites from the Upper Jurassic to Lower Cretaceous. Comparison with western equivalent Gannett Group sandstones and conglomerates also suggests relatively feldspar-rich sediments overlain by chert dominated sediments. However, jasper is generally absent in the Cloverly but abundant in those Gannett Group conglomerates present directly west of the Wind River Basin. This suggests a more southwestern source for the Cloverly where similar gray and green cherts are found such as in southwestern Wyoming.

The Cloverly Formation in the Wind River and southern Bighorn Basins can be separated into three stratigraphic intervals based on the geometry of conglomeratic sandstone bodies and their petrographic composition. The two stratigraphically lowest sandstones are present only in the western Wind River and southwestern Bighorn Basins. Each of these conglomeratic sandstones define a mappable petrofacies, one dominated by detrital chert (basal or A-interval) and one dominated by intraformational limestone with chert (B-interval). The third conglomeratic sandstone (C-interval), present only in the eastern Wind River and southeastern Bighorn Basins, is similar to the A-interval petrographically. These mappable petrofacies variations are indicators of changes in intrabasinal subsidence patterns which influenced local reworking by fluvial systems and the positioning of lacustrine and pedogenic carbonate during initial filling of the incipient Sevier foreland basin in central Wyoming.