Abstract

The easternmost exposure of the Lower Triassic Dinwoody Formation in the southern Wind River Basin is within the Conant Creek anticline, Fremont County, Wyoming. To the east of Conant Creek, Dinwoody-equivalent rocks in the western Rattlesnake Range of Natrona County are assigned to the upper part of the Goose Egg Formation (Freezeout and Little Medicine members) and the lower part of the Red Peak Formation of the Chugwater Group. The change in stratigraphic nomenclature between these two localities reflects significant lithologic differences previously described by several geologists. A review of the literature, however, disclosed that no agreement exists on the thickness of the Dinwoody Formation at Conant Creek anticline. Because of the importance of this exposure to regional Early Triassic studies, we revisited Conant Creek and measured a composite Dinwoody section that totals 18.5 m (60.7 feet). The basal 12.5 m (41 feet) of the sequence disconformably overlies the Upper Permian Ervay Member of the Park City Formation, and is dominated by resistant, thin-bedded, very fine-grained, tan, calcareous sandstone. The upper 6 m (19.7 feet) is poorly exposed, olive-green to light-brown, calcareous claystone and silty shale. The contact with overlying red siltstone of the Lower Triassic Red Peak Formation is gradational.

Correlation of the newly measured Dinwoody section with equivalent rock on the western end of the Rattlesnake Range supports the interpretation that the Little Medicine Member of the Goose Egg Formation is not an eastward projecting tongue of the Dinwoody Formation.