EDMAP SPONSORED GEOLOGIC MAPPING OF THE PHIL PICO MOUNTAIN QUADRANGLE, UINTA MOUNTAINS, UTAH

The Phil Pico Mountain quadrangle is located along the north flank of the Uinta Mountains in northeastern Utah and southwestern Wyoming. The Uinta Mountains were uplifted during the Laramide orogeny forming a large compound anticline bounded on the north and the south by high angle thrust faults. Recent mapping in the Phil Pico Mountain quadrangle at 1:24,000 scale, has provided additional data concerning the geology of the north flank of the Uinta Mountains. The map was compiled using a high resolution, digital 3-D mapping system. The most important findings from recent mapping of this quadrangle include: 1) more accurate placement of the North Flank/Henry's Fork thrust fault, 2) description and differentiation of the Paleogene conglomeratic units within the quadrangle, and 3) the nature of the lithologic changes occurring across the quadrangle in the Neooterozoic Uinta Mountain Group.

The Paleogene sedimentary deposits on Phil Pico Mountain contain key information pertaining to the tectonic and erosional history of the north flank of the Uinta Mountains. However, the relationship between these units (e.g. Bishop Conglomerate, Bridger Formation, and the Wasatch Formation) has been unclear. Detailed mapping has shown that Phil Pico Mountain is largely composed of the Wasatch and Bridger Formations. These formations are separated and bounded by the Henry's Fork fault, which has placed the older Wasatch Formation next to younger Bridger Formation. The fault cuts through the northern section of Phil Pico Mountain. South of the fault is Wasatch Formation, and north is Bridger Formation. The Bishop Conglomerate is not present in the quadrangle.

Our mapping of the Neoproterozoic Uinta Mountain Group has shown that resistant sandstone beds can be traced across the quadrangle. However, the youngest formation of the group, the Red Pine Shale, appears to thin to the east across the quadrangle. This thinning appears to be due to the progressive truncation at the unconformity with the overlying Paleozoic rocks. Stratigraphic changes may also play a role in this thinning.