Sequence stratigraphy of the Curtis, Summerville and Stump Formations, Utah and northwest Colorado

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 Seaway. 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. Although previous investigations have documented the distribution, lithologic characteristics, and interpreted depositional environments of these units, they have been difficult to correlate regionally due to internal lithofacies variations and the lack of age-diagnostic fossils. Within this study I present new data consisting of 34 biostratigraphic samples and 25 measured stratigraphic sections demonstrating the stratigraphic equivalency of the Curtis and Summerville formations with the Stump Formation. Within the basal Curtis and Stump formations, palynological samples containing the index dinoflagellate cyst species Wanea fimbriata and Stephanelytron redcliffense indicate an early Oxfordian age of deposition. This early Oxfordian age designation is further supported by ammonite specimens (Quenstedtoceras (Pavloviceras) sp) collected from the basal portion of these formations. This new biostratigraphic data, when combined with the age of the overlying Morrison Formation, brackets the age of Curtis/Summerville and Stump deposition from early to late Oxfordian time (∼161-155 Ma).; Detailed sedimentological field data allows formulation of a sequence-stratigraphic model that indicates these formations were deposited during a single transgressive-regressive sequence. Outcrop gamma logs and petroleum industry borehole 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 formation deposition helps clarify the stratigraphic architecture of the Middle-Upper Jurassic rock of Utah and northwest Colorado, and allows for detailed paleoenvironmental reconstructions of the final stage of the Jurassic Western Interior seaway deposition. Additionally, the new model may serve as a guide for future sequence-stratigraphic interpretations of other tidally-influenced depositional systems.