Abstract

The Cretaceous Lewis Shale of southern Wyoming is an excellent outcrop example of a submarine fan deposited basinward of a coeval, prograding margin. A regional cross section constructed from outcrop and subsurface data reveals several large-scale attributes of the system. Regionally continuous, condensed sections in the Lewis Shale define southward-prograding clinoforms that are related to a shelf-slope-basin physiography during deposition. The condensed sections form the boundaries of fourth-order stratigraphic cycles/parasequences. The average height of the clinoforms is ~400 m (~1300 ft), which is interpreted to reflect the minimum water depth during deposition. Strata with 50% or more sandstone are located in fluvial-deltaic deposits on the topset of the clinoforms and submarine-fan strata on the bottomset of the clinoforms. Slope strata on the foresets of clinoforms contain 15–20% sandstone. Although the sandiest strata are located in shelf and base-of-slope strata, the depocenter of each fourth-order cycle is consistently located in muddy slope strata.

The prograding Lewis Shale depositional system was initiated by uplift and associated denudation and bypass in the Lost Soldier and Granite Mountains areas, which was immediately followed by regional subsidence. During deposition of submarine-fan strata, sediment accumulation rates in the thickest parts of clinoforms exceeded basin-subsidence rates by more than twofold, resulting in rapid progradation. A vertical profile through the Lewis Shale contains basin-floor strata in the lower part, slope strata in the middle, and shelf strata in the upper part. This vertical succession results from the southward progradation of the genetically related shelf-slope-basin system.

Shelf edge, slope, and base-of-slope exposures were studied to address changes in strati-graphic architecture and facies by physiographic position. Shelf-edge strata are ~50% sandstone and are composed of channelform bodies, prograding bar forms, mudstone sheets, and large slumps. Slumps appear to be related to sea-floor instability at the shelf edge. Incised valleys are not present in any shelf or shelf-edge exposures. Slope strata are ~15–20% sandstone and are composed of mudstone that is locally truncated by channelform bodies that display architectural and facies asymmetry. This asymmetry is interpreted to reflect channel sinuosity. A large proportion of the mudstone in slope strata is interpreted to be levee deposits. Submarine canyons are not present in any of the slope exposures. Base-of-slope strata are ~50% sandstone and are composed of sandy submarine-fan deposits consisting of slumps, sheetform bodies, and amalgamated channelform bodies.