Sedimentology and Depositional Environments of the Emery Sandstone Member of the Mancos Shale, Emery and Sevier Counties, Utah

Abstract

Results of a sedimentological study of the Emery Sandstone Member of the Mancos Shale (Upper Cre- taceous) in southern Castle Valley, Utah, are presented. Depositional environments, paleogeography, textural and mineralogrcal characteristics and possible sediment sources are determined. Tidal flat deposits are dominant in the Emery, but subtidal (shoreface) and offshore deposits also occur. The paleotidal range is estimated to have been between 1 3 and 1.7 m Many asymmetrrc, transgressive-regres- srve cycles of two drfferent magnitudes and periods are present. They formed in response to minor fluctua-tions in sea level combrned wrth slight varrations m the subsidence rate. The Emery Sandstone was deposited m the foreland of the Sevier orogenic belt. The average orientatron of the paleoshoreline, as determrned by paleocurrent analysis, was N. 9" W. Sediment was probably transported southward from the Utah-Idaho- Wyomrng border area by longshore currents. Well-sorted, subrounded to subangular, very fme-grained subarkose is the dominant rock type in the Emery Dolomite and calcrte are the major cements Average porositres, based on thin sectron analysis, are less than 2 percent. Abundant chert grains and reworked authigenrc quartz overgrowths suggest a sedimentary source terrain. The observed amounts of feldspar could have been derived from Mesozorc sedimentary rocks exposed in the Sevier orogenrc belt