Abstract

Synorogenic foreland basin deposition during deformation in the Wyoming Overthrust belt occurred along the active western margin, along the axis of the subsiding trough, and along the eastern cratonic margin. Debris derived from rising thrusts comprise a nonmarine sequence which is underlain and overlain by preorogenic and synorogenic marine formations. In northwestern Wyoming, basal shallow marine units include, in ascending order, the Twin Creek, Preuss, and Stump formations, which pass eastward into the generally coeval Sundance Formation of central Wyoming. Synorogenic nonmarine units of the Gannett Group in northwestern Wyoming pass cratonward into the generally coeval Morrison and Cloverly Formations. Synorogenic marine units consist of basal transgressive elastics overlain by black marine shales. These comprise the marginal marine “Rusty Beds” overlain by the Thermopolis Shale. These grade westward into the lower part of the Wayan group which overlies the Gannett.

Whereas exposures in the Overthrust belt proper have produced extensive documentation of sedimentation patterns along the active basin margin, excellent exposures along the Gros Ventre River in Teton County, Wyoming, allow examination of nonmarine depositional systems across the basin axis during initial subsidence. The preorogenic Stump formation consists of fine-grained glauconitic sandstone and limestone with abundant tabular and trough cross bedding. Flow directions are strikingly bimodal, recording deposition in low-relief coastal settings. Overlying nonmarine units are lithologically intermediate between typical Gannett Group lithologies which occur to the west and typical Morrison-Cloverly lithologies which occur to the east. These consist predominantly of numerous fining-upward sandstone channels isolated in variegated siltstone and mudstone. These were deposited in low-gradient meandering fluvial channel and related flood-plain settings, respectively. The Draney Limestone in the upper part of Gannett Group extends eastward across this area into the top of the “Lilac Beds” of the Cloverly Formation, demonstrating synchroniety of deposition across the foreland basin. Nonmarine units are overlain by rippled, crossbedded, and burrowed transgressive nearshore sands of the “Rusty Beds,” which are in turn overlain by the normal marine dark shales of the Thermopolis Formation.

Paleoflow directions in nonmarine axial foreland basin units are predominantly to the north, ranging from northwest to northeast. As such, these fine-grained meanderbelt facies demonstrate that while synorogenic rivers of the tectonically active basin margin carried sediment of the Gannett Group eastward toward the basin axis, rates of axial basin subsidence exceeded rates of fluvial input, even during the earliest stages of deformation. Synorogenic rivers, bounded on the west by the rising orogen and the east by the stable craton were constrained to flow northward, parallel to the belt of orogenic deformation. As such, major fluvial systems of the Western Interior were analogous to those of other foreland basin settings such as the Paleozoic Michigan River system which paralleled and flowed to the west of the rising Appalachians, and the east-flowing Quaternary Indogangetic system south of and parallel with the rising Himalayas.

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