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Depositional History of Dakota Sandstone, Moxa Arch and Vicinity, Southwestern Wyoming--Implications for Early Evolution of Cretaceous Foreland Basin

Thomas A. Ryer, Jon J. McClurg, Margaret M. Muller

The Dakota Sandstone in the vicinity of the Moxa Arch is divided into upper and lower parts using an unconformity identified on the basis of petrographic evidence and facies relationships. The unconformity is believed to be of subaerial origin and came into being during a pronounced lowering of relative sea level during the late Albian. The lower Dakota consists predominantly of shoreline sandstone and offshore marine shale on the northern part of the Moxa Arch; it consists predominantly of fluvial strata on the southern part of the arch. Meander belts of the lower Dakota trend north-northeastward toward the west-northwest-trending shoreline of the Thermopolis Sea.

The upper Dakota consists predominantly of strata deposited in low-energy, restricted marine paleoenvironments that came into being during gradual transgression of the Shell Creek/Mowery Sea. Barrier-island sandstones bodies are elongate toward the northeast, indicating that the shoreline trended in that direction.

The reorientation of the shoreline from west-northwest-trending in the lower Dakota to northeast-trending in the upper Dakota is attributed to acceleration in the rate of subsidence in the foreland basin. The Shell Creek Sea advanced down the eastern side of the foreland basin, transgressing over lacustrine deposits that accumulated there during the lowstand of sea level. The Moxa Arch appears to have served as the eastern hinge of the foreland basin during the Dakota; only later, in the Late Cretaceous, did it assume the characteristics of a foreland welt.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.