Abstract

The Jurassic Curtis, Summerville, and Stump formations of Utah and Colorado consist of marine and marginal-marine strata deposited along the southern margin of the Western Interior Basin. These rocks consist of conglomerates, sandstones, mudstones, and limestones, which crop out along the flanks of the Uinta Mountains in northeast Utah and northwest Colorado, and in the San Rafael Swell in central Utah. In this study, we present new biostratigraphic data demonstrating the stratigraphic equivalency of the Curtis and Summerville formations with the Stump Formation. In the Curtis and Stump formations, palynological samples and ammonite specimens indicate an early Oxfordian age of deposition. When combined with the age of the overlying Morrison Formation, these new data bracket the age of Curtis/Summerville and Stump deposition from early to late Oxfordian time (~161-156 Ma).

Detailed sedimentological field data allow formulation of a sequence-stratigraphic model that indicates these formations were deposited during a single transgressive-regressive sequence. Outcrop gamma-ray logs and industry gamma-ray logs were utilized to trace the interpreted sequence stratigraphic surfaces beneath the Uinta Basin. The new sequence stratigraphic model for Curtis, Summerville, and Stump deposition helps clarify the stratigraphic architecture of the Middle and Upper Jurassic rocks of Utah and northwest Colorado, and allows for detailed paleoenvironmental reconstructions of the final stage of the Jurassic Western Interior Basin deposition.