Darton Ridge, Pennsylvanian Feature of Wyoming Shelf

Abstract

Despite the apparent uniform nature of the Tensleep Sandstone and its genetically related time equivalents, petrologic analysis of sections measured in the Big Horn basin of Wyoming yields evidence of statistically significant variability in texture and composition. This variability provides a basis for interpretation of the nature and circulation pattern of shallow-marine currents which transported and deposited the well-sorted, very fine-grained orthoquartzite as a thin blanket sand on the Pennsylvanian cratonic shelf in Wyoming and adjoining areas. Approximately half of the sediment comprising the Tensleep Sandstone was accumulated under the influence of low-velocity longshore currents that moved from northwest to southwest along the shoreline. These currents produced thinly and evenly bedded deposits that display essentially unimodal grain-size distribution, but contain a small silt The other half of the detritus may have been transported by high-energy, storm-related bottom currents whose mean flow direction was southwest. Deposits produced by these currents display planar cross-lamination and are characterized by a polymodal grain-size distribution curve with a coarse tail. Although cross-bedded strata are approximately equal in volume to the evenly laminated beds with which they are intercalated, bottom-current trans ort probably dominated the depositional regime only 10 per cent of the time, and longshore movement was most important during the remaining 90 per cent. An elongate bathymetric prominence, here named the Darton Ridge, significantly affected the distribution pattern of Tensleep thickness, texture, and composition in the Big Horn basin-Big Horn Mountains area by its interaction with current flow. Oriented essentially northeast-southwest across north-central Wyoming, the approximately 45-by-90+-mile Darton Ridge created throughout the Pennsylvanian Period a shallow-water area of reverse by-passing, swept by bottom currents and characterized by a thin accumulation of evenly bedded, unimodal deposits. Strata surrounding the Darton Ridge are thicker and contain a much higher percentage of coarsely bimodal, cross-laminated beds. Feldspar and metamorphic grains, derived from a northeastern source, show characteristic concentration along the north-facing slope of the ridge.