FINE-SCALE STRATIGRAPHIC AND TEMPORAL CALIBRATION OF OROGENIC STRATA ADJACENT TO THE UINTA UPLIFT USING THE WILKINS PEAK MEMBER OF THE GREEN RIVER FORMATION

Synorogenic, coarse-grained clastic deposits have long been recognized as a potentially rich source of uplift and denudation histories, but in the nonmarine basins commonly associated with continental intraplate deformation these histories are often hampered by a lack of precise age control. A prominent exception is the Wilkins Peak Member of the Eocene Green River Formation, an interval of lacustrine and associated nonmarine facies that contains numerous ash horizons dated to ~100 ky precision (2σ; Smith et al., 2003). A newly-constructed stratigraphic cross-section allows this temporal record to be extended up-dip, from near the center of the Bridger basin into the southern basin margin adjacent to the Uinta uplift. There, the Green River Formation intertongues with alluvial facies of the Cathedral Bluffs Tongue (Wasatch Formation). Using detailed correlation of tuffs, lacustrine facies, and fluvial sandstone intervals, we are able to establish a relatively precise chronology for the unroofing of the Uinta uplift. Stratigraphic correlation indicates that unroofing histories are markely diachronous over 20 kilometers along the strike of the Uinta Uplift. At Richards Mountain, near the intersection between the Uinta Uplift and the Rock Spring Uplift - Douglas Creek Arch structure, clasts of Proterozoic Uinta Mountain Group quartzite first appear in pre-Wilkins Peak strata dated at ~53 Ma. Ten kilometers to the west at Spring Creek, Uinta Mountain Group clasts appear first the middle of the Wilkins Peak Member (~51.5 Ma), and don’t occur until the upper Wilkins Peak (~50 Ma) at the Green River ten kilometers further west. The core of the Uinta uplift was thus breached diachronously from east to west in the study area over a 3 m.y. interval, in broad accordance with the mapping of Uinta Mountain Group strata by Hansen (1957) which indicated a structural culmination where the Uinta Uplift intersects the Rock Springs/ Douglas Creek structure. Barring the presence of significant local incision, this culmination was therefore present and directly affected the composition of conglomerates deposited adjacent to it during the Eocene.

Smith, M.E., Singer, B., and Carroll, A.R., 2003, ⁴⁰Ar/³⁹Ar geochronology of the Eocene Green River Formation, Wyoming: GSA Bull., v. 115, p. 549-565. Hansen, W.R., 1957, Structural features of the Uinta Arch, in IAPG 8th Annual Field Conf., p. 35-39.