Abstract

Thirty-three measured sections of the Dinwoody Formation, including five from the literature, provide information on thickness, lithology, paleontology, and stratigraphic relations within the Wind River Basin and immediately adjacent areas of Wyoming. Most of these sections are in Fremont County, and some lie within the Wind River Indian Reservation. The Dinwoody becomes progressively thinner eastward, from a maximum thickness of 54.6 m in the northwestern Wind River Mountains to zero near the Fremont-Natrona county line. The formation is characterized by yellowish-weathering, gray siltstone and silty shale. Variable amounts of carbonate, sandstone, gypsum, and claystone are also present. Marine bivalves, gastropods, phosphatic brachiopods (Lingula), and conodonts are common in the western part of the study area, but are absent to the northeast in gypsiferous strata, and near the eastern limit of Dinwoody deposition.

The Dinwoody in the Wind River Basin area was deposited unconformably on the Middle Permian Ervay Member of the Park City Formation during the initial Mesozoic flood onto the Wyoming shelf during the Griesbachian, and represents the first of three Lower Triassic transgressive sequences in the western miogeocline. Conodonts of the Isarcica Zone document the rapid nature of this eastward transgression. The Permian surface underlying the Dinwoody rarely shows evidence of the long hiatus separating rocks of this age and earliest Triassic deposits. The Dinwoody transgression was followed by westward progradation of the Red Peak Formation of the Chugwater Group across the study area.