Sedimentary history of the Casper Formation (Wolfcampian), Powder River Basin, Wyoming

Abstract

Lower Permian sedimentary rocks of the Casper Formation in the Powder River basin, Wyoming, can be stratigraphically divided into upper and lower Wolfcampian sequences on the basis of unconformities and fusulinids. The sandstones were deposited in a variety of regressive marine environments dominated by an open sea with associated bar and lagoonal facies. These environments of deposition were delineated by vertical sequences of grain size and sedimentary structures. Following deposition of the Wolfcampian sediments, the upper surface of the Casper was dissected by erosion. Erosional remnants of the sandstone provided reservoirs for petroleum which is preserved by a combination of structural and stratigraphic traps. The Casper sandstones are quartzose with only minor amounts of feldspar and rock fragments and no matrix. This absence of unstable constituents is a result of the polycyclic history of the sands. The cyclicity is also reflected by the heavy mineral composition dominated by zircon and tourmaline. Cements are included within a three end-member series of calcite, anhydrite, and silica which were precipitated successively soon after deposition. After deep burial, secondary silica and calcite were again formed. Replacement among the cements and between the cements and detrital grains was common. ...