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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 74 (1990)

Issue: 11. (November)

First Page: 1695

Last Page: 1717

Title: Depositional History of the Maastrichtian Lewis Shale in South-Central Wyoming: Deltaic and Interdeltaic, Marginal Marine Through Deep-Water Marine, Environments (1)



The Maastrichtian Lewis Shale in south-central Wyoming was deposited during the final major transgression and regression of the Late Cretaceous North American epicontinental sea. Information on this formation was obtained from outcrops along the margins of several basins and from well logs and cores within those basins. Regional correlations were made using biostratigraphic control, which also allowed relatively fine scale resolution of the approximately 71 to 69 m.y. ages of the Lewis Shale. At least two branches of the sand-rich Sheridan delta system supplied sediments that either were deposited directly within deltaic environments, or were reworked and redeposited in laterally equivalent interdeltaic environments. Facies recognized within the study region are interpret d to represent nearshore, shell, and basin marine depositional environments. Throughout the study region, facies representing shallow-water environments are Iocated on the north, and those representing deep-water deposits are on the south. This distribution, combined with data on the ages of stratigraphic intervals, supports the conclusion that the Lewis Shale was deposited during north to south progradation. The Lewis Shale contains significant hydrocarbon reserves. Understanding the distribution of facies and the paleogeography of the formation is crucial for designing hydrocarbon exploration and development strategies for the region.

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