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REVISIONS TO UPPER CRETACEOUS STRATIGRAPHY NEAR HELL'S HALF ACRE, EASTERN WIND RIVER BASIN, CENTRAL WYOMING

Abstract Thick, steeply dipping, and generally conformable Upper Cretaceous and lower Paleogene strata are well exposed on the western Casper arch, central Wyoming. They record deposition prior to, during initiation of, and following onset of subsidence and basement-involved thrusting of the Laramide orogeny. Confusion has permeated the distributions, thicknesses, and ages of these strata, adjacent to the Wind River Basin's east-central margin. New lithostratigraphic mapping near Hell's Half Acre (HHA) revises identification of nonmarine units overlying the marine Lewis Shale (of Campanian and/or earliest Maastrichtian age). The homogeneous, mostly fine-grained Meeteetse Formation (MF) represents paludal conditions following deposition of the Lewis Shale's “lower tongue.” The local MF exceeds 3,200 feet (nearly 1.1 km) in thickness, almost 7.5 times greater than previously reported. The MF near HHA, although much thicker, exhibits characteristics strikingly similar to the Meeteetse elsewhere in the Wind River and Bighorn Basins. Most strata identified here as MF previously were considered as Lance Formation. The top of the MF near HHA matches the previously recognized contact with the Fort Union Formation (mostly Paleocene). Dominance of sandstone above that contact implies a more energetic fluvial regime that may, in part, represent latest Cretaceous time. Reflecting existing uncertainties, rocks above the MF and below the lacustrine Waltman Shale Member (Fort Union Formation) are designated here as “Fort Union (unnamed lower part) and Lance Formations, undifferentiated.” Upper surfaces of the Fort Union Formation were eroded early in Eocene time, and local scouring near HHA cut as deeply as the MF. Thus it is improbable that youngest parts of the Paleocene are recorded paleontologically in strata near HHA. The unexpectedly great thickness of the MF reflects onset of major, differential subsidence and provides evidence that the Laramide structural axis of the Wind River Basin originated several million years earlier than previously considered.