SEQUENCE STRATIGRAPHY OF THE SEGO SANDSTONE AND ITS RELATIONSHIP WITH SEVIER FOLD AND THRUST BELT TECTONICS

The Upper Cretaceous Sego Sandstone is a member of the Mancos Shale deposited in the Western Interior Seaway and has been well documented in the Book Cliffs area, UT. The two main models that have been proposed for the Sego Sandstone’s depositional environment include incised valley fill to a tide dominated delta system. However, these two models make vastly different predictions of relative sea-level, depositional environment and stratigraphic architecture. Further investigation of the Sego Sandstone will determine its depositional environment and how tidal systems vary along strike. Furthermore, previous researchers speculated that sequence boundaries within the Sego Sandstone are related to Sevier tectonism; however, little constraint exists on the time relationships between Sevier deformation and erosion and Sego Sandstone deposition.

Our focus is on Sego Sandstone outcrops north of Rangely, CO, northeast and along strike of the well documented, Book Cliffs outcrops. We measured and correlated 40 stratigraphic sections along a 19 km dip exposure that allow us to document the Sego Sandstone’s stratigraphic architecture. Even though these outcrops are tide influenced to tide dominated, like their Book Cliffs counterpart, there are distinct facies from the ones observed in the Book Cliffs area. In our study area the Sego Sandstone transitions from a tide dominated, brackish environment to a wave dominated platform. One sequence boundary was identified with two coarsening upward parasequences on top. Above these coarsening upward parasequences, the Sego Sandstone becomes aggradational. These outcrops display few distributary channels. Furthermore, to constrain cooling ages of the source material for the Sego Sandstone, and to investigate the relationship between source erosion and distal foreland deposition, we use low-T- thermochronology and zircon U-Pb geochronology.

Detrital zircon U-Pb geochronology constrains the top of the Sego Sandstone to ~76 Ma. Preliminary zircon (U-Th)/He data from clasts from time equivalent Upper Castlegate in Utah show that the source material never exceeded the 160-200 °C partial retention zone, implying ~<10 km exhumation since the Upper Cretaceous. Detrital and in situ apatite fission track analyses are undergoing and will determine the time of source exhumation.