DEPOSITIONAL AGE AND SEQUENCE STRATIGRAPHY OF THE JURASSIC CURTIS, SUMMERVILLE, AND STUMP FORMATIONS, UTAH AND 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, which consist of conglomerates, sandstones, mudstones, and limestones crop out along the flanks of the Uinta Mountains in northeast Utah and northwest Colorado, and 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. In this study we present new data consisting of 36 biostratigraphic samples along with 26 measured stratigraphic sections showing that the Curtis and Summerville formations of central Utah are temporally equivalent with the Stump Fm. in NE Utah and NW Colorado. Within the basal Curtis and Stump formations palynology samples containing the index dinoflagellate cysts species Wanea fimbriata and Stephanelytron redcliffense indicate an early Oxfordian age of deposition. This early Oxfordian age designation is further supported by ammonite specimens collected from the basal portion of these formations identified as Quenstedtoceras (Pavloviceras) sp. 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). Using the detailed sedimentological data from the field within the context of a sequence-stratigraphic model, these formations are interpreted to represent a single transgressive-regressive sequence recording the final pulse sedimentation in the Jurassic interior seaway. This new data redefines the Middle-Upper Jurassic stratigraphy of the region and also shows the utility of using sequence stratigraphy to correlate the lithologically different units containing limited age control.