Depositional history of the Lewis Shale, Maestrichtian, south-central Wyoming, consists of a transgressive period followed by an interval of delta infill. Clastics are up to 760 m thick. The Lewis sea transgressed westward to areas of the Rock Springs and Wind River uplifts and opened eastward into the main part of the Interior Seaway. Transgressive shales are black, high in organic material, relatively silt-poor, and totally bioturbated. Depositional setting was a shallow, well-oxygenated shelf. Lewis clastics above the transgressive shale are coarser and are related mostly to deltas that entered the basin first from the northeast and later from the south. Sediments were deposited in delta-front, prodelta, deep-basin, and interdistributary areas. Lewis sandstones produce at Wamsutter and Hay Reservoir fields in addition to other areas. Wamsutter and Hay Reservoir sandstones were deposited beneath storm wave base by sediment gravity flows at the toe of the northern delta system. Sandstones may be thickly bedded and are commonly massive. Presence of fluid escape structures indicates that many thick beds underwent liquefaction during the final stages of deposition. Thinner sandstones are graded and many show Bouma sequences. Interbedded shales are unburrowed or are slightly burrowed by restricted faunas, indicating anaerobic to dysaerobic conditions and water depths of 150-200 m or more at times.

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