ANALYSIS OF SOURCE AREA EVOLUTION THROUGH PETROLOGIC EXAMINATION OF SANDSTONE, CASPER FORMATION, SOUTHEASTERN WY, USA

The Pennsylvanian/Permian Casper Formation in southeastern Wyoming is a composed of a series of alternating limestone and sandstone beds that were formed over a 13 million year span of time. During the time that the sediments were being deposited, the environments present in the area were an offshore, shallowly-dipping carbonate platform and an onshore dune field located near tectonically active topographic highs. Because the areas surrounding the dune field were dynamic, it is possible that the source area for the sandstone units may have shifted during the course of time in which the sediments in-filled the basin. Unraveling this component of the formation of the Casper sandstone units will provide a better understanding of the paleogeographic evolution of this basin.

Field sampling was done during Summer, 2010 to obtain samples from each of the sandstone units contained in the Casper Formation. Some units were robust enough to yield multiple samples from various horizons; most units were too poorly exposed to extract more than one sample. Thin sections were created from each of the samples, and Gazzi-Dickinson point counts were done systematically at 1 mm increments on a mechanical stage affixed to a petrologic microscope. Analysis of the data demonstrates that the sandstones all were derived from a recycled orogenic provenance; however, there was an increase in percentage of lithics upsection as well as a change in the species of feldspar present. This suggests that the specific source area did not remain static throughout the deposition of the Casper sediments.