Sedimentology of Upper Cretaceous Cody-Parkman Delta, Southwestern Powder River Basin, Wyoming

Abstract

The Upper Cretaceous Cody Shale and Parkman Sandstone in the southwestern Powder River basin, Wyoming, were deposited during progradation southeastward of a wave-dominated, high-destructive type of delta. The Cretaceous cordillera in eastern Idaho and southwestern Montana was the source of the homogeneous group of litharenite and feldspathic litharenite sandstones. The source rocks were pre-Belt crystalline rocks, the Belt Supergroup of argillite and quartzite, Paleozoic and Mesozoic sedimentary rocks, and Cretaceous volcanic rocks and ash falls. The upper several hundred feet of the 2,500-ft Cody Shale comprise a flyschlike sequence deposited on the upper prodelta slope. The lower 220 ft of the overlying Parkman Sandstone accumulated on the delta platform and along the strandline. Abundant pillow structures occur in sandstone beds deposited at the top of the prodelta slope and on the outer delta platform. Mapping of structural elements in pillow horizons shows that the subaqueous delta surface dipped southeast with southwest-northeast contours. The Cody graded beds were deposited by ocean-bottom currents that flowed northeast, perpendicular to the paleoslope, rather than by downslope turbidity currents. The sandstone beds on the delta platform were deposited by strong waves and subordinate tidal currents, both of which were highly variable in direction. Cross-bedding consistently dips landward and along shore in the strandline sandstones; they were deposited in a complex of beaches, swash bars, spits, and sub-tidal areas swept by alongshore drift. The upper 200 ft of the Parkman Sandstone are delta-plain carbonaceous mudstones, lensing channel sandstones, and a few thin lignite beds. The region was one of low relief and southeast gradient, with floodplains, meandering stream courses, lakes, swamps, and a coastal marsh with tidal creeks. Lowering of the delta surface by compaction of the thick sequence of prodelta mud resulted in local deposition of 12 to 60 ft of marine sandstone at the top of the Parkman Sandstone which represents the destructional phase of the delta. Subsequent transgression of an unnamed marine tongue of the Cody Shale terminated the delta cycle.