Abstract

The Tidwell Member at the base of the Upper Jurassic Morrison Formation in east-central Utah consists of nine lithofacies in the study area: gypsiferous mudstone, rippled sandstone, thin-bedded sandstone, siltstone, lenticular sandstone, micritic limestone, brecciated limestone, and mudstone. These lithofacies are dominated by floodplain sediments that were deposited distal to the Salt Wash Member alluvial complex. Sedimentologic characteristics of some fluvial sheet sandstones and pond limestones indicate that water resources were ephemeral. Petrologic composition and paleocurrent data indicate that Tidwell Member sediments were derived from a western recycled orogenic source and were transported northeastward onto the distal alluvial plain.

The Tidwell Member contains the type specimen of one of the oldest known sauropod dinosaurs from the western United States (Dystrophaeus viaemalae Cope 1877; USNM 2364), which is preserved in a thin fluvial channel sandstone of the lenticular sandstone facies. The remains are partially articulated, indicating that transport prior to burial was minimal. This rare fossil occurrence in the Tidwell Member may be the result of: 1) a scarcity of vertebrate faunas in arid alluvial plain environments; 2) early depositional and diagenetic conditions that were unfavorable to the preservation of organic material; or 3) historical sampling biases related to poor exposure of the slope-forming Tidwell Member. Facies analysis and interpretation of Tidwell Member depositional environments provides a context for future studies investigating early Late Jurassic sauropod paleoecology.