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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


The Thrust Belt Revisited; 38th Annual Field Conference Guidebook, 1987
Pages 179-206

Dakota-Bear River Paleoenvironments, Depositional History and Shoreline Trends-Implications for Foreland Basin Paleotectonics, Southwestern Green River Basin and Southern Wyoming Overthrust Belt

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

Abstract

The Dakota Sandstone on the Moxa Arch includes strata that are equivalent to the Rusty Beds, Thermopolis Shale, and Muddy Sandstone of areas to the northeast. The Bear River Formation of the Overthrust Belt contains equivalents to these units, plus beds equivalent to the Shell Creek Shale and the upper part of the Cloverly Formation.

An unconformity is tentatively identified within the Dakota on the basis of petrographic evidence and facies relationships, and is used to divide the formation into lower and upper members. The unconformity is believed to be equivalent to that which separates the Muddy from the Skull Creek Shale in areas to the northeast. It resulted from a pronounced lowering of relative sea level during late Albian time. The unconfirmity is not recognized in the Bear River Formation, suggesting that the rate of basins subsidence in the area west of the Moxa Arch exceeded the rate of lowering of sea level, but did not on the crest and east of the arch.

The lower Dakota consists predominantly of shoreline sandstone and offshore marine shale on the northern part of the Moxa Arch and predominantly of fluvial strata on the southern part of the arch. Meanderbelts 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 during gradual transgression of the Shell Creek-Mowry Sea. Barrier-island sandstone bodies within the Upper-Dakota trend northeastward. The reorientation of the shoreline trend from west-northwest in lower Dakota time to northeast in upper Dakota time is attributed to acceleration in the rate of subsidence in the foreland basin and to an increase in the differential between subsidence rates west and east of the Moxa Arch. The Moxa Arch appears to have served as the eastern hinge of the foreland basin during Dakota-Bear River time; only later, in Late Cretaceous time, did it assume the characteristics of a peripheral bulge.


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