Stratigraphy and Petrology of Crow Mountain Sandstone Member (Triassic), Chugwater Formation, Northwestern Wyoming

ABSTRACT In northwestern Wyoming the Crow Mountain Sandstone Member (Triassic) of the Chugwater Formation is underlain by either the Alcova Limestone Member or the sandy facies of the Red Peak Member. An unnamed redbed unit is locally disconformable on the Crow Mountain. The boundaries of the Crow Mountain are redefined and the member is divided into a basal sandstone unit and an upper sandstone and siltstone unit. The basal sandstone unit is characterized by red and drab, massive, very fine-grained sandstone with medium-scale cross-stratification. The sandstone of the upper sandstone and siltstone unit is red and drab, flaggy to slabby, and very fine-grained. Well-sorted to poorly sorted siltstone beds contain abundant ripple marks and small-scale cross-stratification. Oil staining is common in the sandstone. Based on 22 modal analyses, the sandstone and siltstone are arkose and subarkose. Of 356 paleocurrent directions that were measured, 32 per cent are between 31° and 90°, and 31 per cent are between 181° and 270°. The two dominant directions were apparently at right angles to the shorelines. The Crow Mountain is interpreted to have been deposited during a westward regression of the sea. The principal environments of deposition were: (1) shallow marine, with moderate to strong currents during deposition of the basal sandstone unit; (2) tidal flat, with weak currents during deposition of most of the upper sandstone and siltstone unit; and (3) beach, during deposition of the uppermost part of the upper sandstone and siltstone unit. Deposits of associated tracts, such as dunes, marshes, and a few alluvial plains, probably occur.