Shallow-Water and Sub-Storm-Base Deposition of Lewis Shale in Cretaceous Western Interior Seaway, South-Central Wyoming

Abstract

Shallow-and deep-water (below storm base) clastics of the Lewis Shale were deposited in south-central Wyoming during a part of the Late Cretaceous (Maestrichtian). Core, log, and outcrop data indicate that the Lewis Shale consists of a thin, transgressive, marine shale overlain by a thicker, coarser, progradational interval. The initial transgression of the Lewis sea occurred during the Maestrichtian (Baculites eliasi range), and the maximum westward and northward extent of submergence was in areas of the Rock Springs and Wind River uplifts. The sea in which the Lewis Shale was deposited opened eastward into the North American Western Interior seaway. Shale in the lowermost Lewis is black, carbonaceous, bioturbated, and commonly contains shell debris and was deposited in hallow water. The central basin floor changed from shallow to sub-storm-wave and sub-storm-current depths, seemingly in response to eustatic sea level rise and coeval tectonic subsidence. Deepening of the basin began at the time of dominance of Baculites eliasi to B. grandis (Maestrichtian). Sandstone and shale of the upper Lewis Shale are mostly related to distributary systems that entered the basin first from the northeast (during B. grandis to B. clinolobatus range) and later possibly from the south (during B. clinolobatus). Southwest progradation from a northeastern delta suggests significant uplift in the Wind River upthrust area concurrent with Lewis sedimentation. Many hydrocarbon-producing sandstones in the Lewis Shale are thickly bedded and massive, and some show fluid-escape structures. Thin sandstones are typically characterized by grading, Bouma sequences, and sole markings. Turbidity currents are believed to be responsible for deposition of all these sandstones. Unburrowed shale and shale containing burrows made by a restricted infauna are interbedded with reservoir sandstones at Wamsutter field. The shale is interpreted to indicate anaerobic to dysaerobic conditions and water depths of 500-650 ft (150-200 m) or more.