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Lewis Shale, South-Central Wyoming: Shelf, Delta Front, and Turbidite Sedimentation
Core evidence, log cross-sections, and outcrop data show the Lewis Shale of south-central Wyoming to consist of a thin, largely transgressive mudstone overlain by a thicker section of coarser, delta-related clastics. The entire formation is up to about 762 m (2500 ft) thick. Shale of the lowermost Lewis is black, carbonaceous, and bioturbated, and shell debris, in places, is abundant. Depositional site is inferred to have been a shallow oxygenated shelf. Transgression occurred in the area during the Baculites eliasi interval (Maestrichtian) of the Western Interior biostratigraphic zonation. Maximum westward and northward extent of the transgression was to areas of the Rock Springs and Wind River uplifts. The Lewis sea opened eastward into the main part of the North American Interior seaway.
Clastics above the transgressive interval were deposited in delta front, interdistributary, and relatively deep basin settings. The basin center subsided from shallow depths to sub-storm wave base with onset of the regressive sedimentation phase. Deposition was largely controlled by distributary systems which entered the basin first from the northeast and later apparently from the south. The northeastern delta system was active during the Baculites grandis to B. clinolobatus interval; the southern delta was important during the B. clinolobatus interval.
Many hydrocarbon-producing Lewis sandstones, including those in the Hay Reservoir and Wamsutter Fields, are thickly bedded and massive; several show fluid-escape structures. These beds were apparently deposited in the center of the Lewis sea by high-density turbidity currents. Interbedded thinner sandstones are characterized by grading, sole markings, and Bouma sequences. These were also deposited by sediment gravity flows. Shale interbedded with sandstone in the Wamsutter field are slightly burrowed by a restricted fauna or are unburrowed indicating the existence, at times, of dysaerobic and anaerobic conditions on the basin floor. Water depths were likely 150-200 m (about 500-650 ft) or more for these conditions to have occurred.
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