Seismic Facies of Shelf, Slope, and Submarine Fan Environments of the Lewis Shale, Upper Cretaceous, Wyoming
The Maestrichtian Lewis Shale of south-central Wyoming was deposited on the west side of the Sevier foreland basin. Well-log, core, and outcrop data document a vertical succession consisting of a condensed shelf shale overlain by bioturbated, probable turbidite sheet sandstone, deeper water sandy turbidites, and a progradational sandstone-shale interval of turbidite and deltaic deposits. Basin deepening and turbidite deposition were controlled by tectonic subsidence during the Sevier orogeny or early stages of the Laramide orogeny, and was possibly affected by eustatic sea-level rise.
The Lewis Shale-Fox Hills Sandstone interval can be subdivided into three zones based on seismic-reflection data: (1) A lower interval of parallel reflections correlates to a condensed shale section of the lower Lewis Shale. (2) A middle zone of parallel to sigmoidal clinoform reflections exhibiting shelf aggradation and progradation correlates with turbidite, slope, and deltaic sedimentation of the middle Lewis. (3) An upper unit of parallel reflections and tangential clinoforms represents mostly progradational deltaic, delta-front, and turbidite facies of the upper Lewis Shale and Fox Hills Sandstone.
Seismic facies observations allows mapping shelf edges and sandstone-rich submarine fans in the aggradational/progradational and progradational units. Large submarine fans in the middle Lewis Shale entered the basin from the north and west. Smaller, more numerous submarine fans of the upper Lewis Shale entered the basin from the north and south.
Broad folds, defined by thinning of the regional condensed shale and by seismic onlap, formed local subbasins during early Lewis time. In particular, a large northwest-southeast fold across the west-central Red Desert/Washakie Basin area affected shelf and submarine fan sedimentation.
The Lewis Shale condensed shelf shale and upper Almond Formation are interpreted as a transgressive systems tract, whereas the middle and upper Lewis Shale, Fox Hills Sandstone, and lower Lance Formation represent a highstand systems tract. The occurrence of abundant turbidite sandstone during a highstand can be attributed to tectonism and high sediment supply during Lewis Shale deposition.
KeywordsForeland Basin Seismic Line Shelf Edge Highstand System Tract Transgressive System Tract
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