Depositional Enviroment of the Morrison Formation in the West Poison Spider Field and Surrounding Areas, Southeast Wind River Basin

Abstract

The purpose of this research was to study evidence available about sandstone in the lower Morrison Formation, and to make a stable interpretation of its depositional environment. The interpretation was based upon evidence from petrographic analysis, core analysis, outcrop analysis, and analysis of general subsurface geology. The Morrison Formation of Wyoming has long been believed to have been deposited under fluvial and lacustrine conditions, but some workers have found implications of marine deposition. However, sandstone of the Lower Morrison studied in this report was interpreted as eolian. This conclusion was based on (1) well sorted and very fined grained grayish white sand in cores and outcrops (2) large-scale, high-angle dip of cross-bedding, (3) consistent stratigraphic position of the sandstone (4) absence of fluvial characteristics (5) bounding surfaces in the core that are suggestive of inter-dune deposits, (6) abundance of bioturbation, and (7) general composition, textures, and sedimentary structures. The sandstone is as thick as 186 feet. It was deposited on a low-lying surface that was formed after withdrawal of the last Sundance Sea. Wind across this surface deposited sand as dunes in and around the study area. Weathered Sedimentary rocks as well as flashy discharges from distant positive areas were also probable sources.The climate during sedimentation is believed to have been semi-arid to arid. An apparent change of climate led to stabilization of dunes; fluvial and lacustrine deposits overlie the dunes. In outcrop two distinct sand facies make up the Morrison Formation; lower is interpreted as eolian and the upper as fluvial.