SEDIMENTOLOGICAL CHARACTERIZATION OF THE SEGO SANDSTONE OF WESTERN COLORADO AND DETAILS ON FLOOD TIDAL DELTA DEPOSITS

The Sego Sandstone of Western Colorado is a tidally influenced member of the Mancos Shale lying on the western edge of the Cretaceous Interior Seaway. The Sego Sandstone was deposited in the Upper Campanian, at the end of the Sevier Orogeny and at the beginning of the Laramide Orogeny. Although regional studies have detailed time equivalent deposits in the Book Cliffs, UT, the facies observed in north of Rangely, CO are distinctly different from the Book Cliffs outcrops to the southwest. We present a detailed study of the tidal facies including bedform architecture, provenance, and grain size variations, as well as new details on flood tidal delta deposition.

Multiple depositional environments have been suggested for the Sego, but most agree that it is tidally influenced to tidally dominated. Three targeted tidal bodies spanning our study area, west, east, and south, are hypothesized to be potential flood tidal delta deposits. To test this hypothesis, we investigate bedform architecture, paleocurrent direction, and grain size within the three sand bodies. If these deposits represent a flood tidal delta environment we expect them to exhibit bidirectional paleocurrents, to vary in grain size due to overwash from storm deposits breeching the barrier island, and some kind of lobe shape. Our new field data show that flood tidal delta facies are 3-6m thick and range from ~90-500m long. Distinguishing features include: white, cross-bedded, upper-fine grained sand with subsidiary rippled beds and tidal bundles. Bioturbation is minimal to moderate and mud drapes and erosion surfaces are always present, usually in between discreet unidirectional cross-bedded intervals. We speculate that the erosional surfaces represent the opposite tidal direction, accounting for the missing bidirectional cross-beds.

Preliminary sandstone petrography data show that most of the composition is quartzitic, which is consistent with a Sevier source. In addition to correlating the stratigraphic architecture of the Sego Sandstone, we have applied zircon U-Pb geochronology to better determine the provenance and depositional age of the Sego Sandstone. Our data document a source characterized by 76.6-360 Ma, consistent with a source located in the Sevier hinterland and a maximum depositional age for the investigated samples of 76 ± Ma.