MAPPING THE DRY FORK AND STEINAKER RESERVOIR 7.5' QUADRANGLES, VERNAL, UTAH

The EDMAP-sponsored study of a possible termination of the South Flank Fault, Uinta Mountains includes a range of geologic problems in the map area. Structure, stratigraphy, and geohazards have each been addressed with the goal of producing two geologic maps—the Steinaker Reservoir and Dry Fork 7.5’ Quadrangles north of Vernal, Utah. Some authors have illustrated the South Flank Fault as either a normal fault or a strike-slip fault, but had failed in text to explain that type. The cluster of faults in the indicated quadrangles have favored oblique-slip some 30˚ off horizontal in left-lateral motion. It is unclear whether this slip was primarily oblique or whether deformation was bi-modal. The Jurassic Glen Canyon Sandstone of Eastern Utah was named after the Glen Canyon Group of the Colorado Plateau. Stratigraphic work has yet to definitively correlate the Glen Canyon Sandstone to its southern counterparts. Both the Upper Glen Canyon Sandstone (Navajo equivalent) and the Lower Glen Canyon Sandstone (likely Wingate equivalent) are mappable at the 7.5’ scale and do not reflect the same depositional equivalents. The Permian Meade Peak Member of the Park City Formation is currently being mined in the map area. Local debate speculates whether or not other areas adjacent to the phosphate mine are economically feasible. Stratigraphic thicknesses mapped in the area show that they probably are. The Jurassic Morrison Formation and the Cretaceous Cedar Mountain Formation are separated in the map area by a disconformity spanning approximately 50 million years. The contact separating the two formations is near impossible to pick out because of their similar depositional environments. South of the map area, the formations are separated at the base by the Buckhorn Conglomerate. The Buckhorn Conglomerate was thought not to exist in the map area. Field work has revealed that it does; it is represented as East-West oriented channel stringers. Landslides in the area are larger and more numerous than previously thought. Mapping at the 7.5’ scale has shown the extensiveness of the landslides—some up to one mile across at the head and about two miles in length.