STRATIGRAPHIC RELATIONSHIPS WITHIN THE DUCHESNE RIVER FORMATION AND BISHOP CONGLOMERATE: CONCLUSIONS FOR MAPPING THE VERNAL NW QUADRANGLE, UT

The Vernal NW quadrangle contains important evidence related to the Uinta uplift and basin fill proximal to the uplift. These evidences are recorded in the Eocene Duchesne River Formation (DRF) and its four members (Brennan Basin, Dry Gulch Creek, Lapoint, and Starr Flat) and the Oligocene Bishop Conglomerate. These formations have been mapped at 1:24,000 scale. In addition to mapping, volcanic ash from the Lapoint and Dry Gulch Creek members of the Duchesne River Formation (DRF) have yielded radiometric ages of about 39.5 Ma. These new ages constrain the timing of deposition and clarify stratigraphic relationships within the DRF; they suggest a significant unconformity of as much as 5 Ma between the DRF and the overlying Bishop Conglomerate which is 34-30 Ma.

Measured sections and clast point counts show that Paleozoic rocks were unroofed and clasts deposited first in the Brennan Basin Member, followed by a significant upward increase in Precambrian clasts. The presence of upper Paleozoic clasts at the top of the section in upper Starr Flat Member and Bishop Conglomerate suggests the reworking of older fan deposits or changes in upstream drainages. Mapped contacts within the DRF show a significant increase in coarse-grained deposits from the southern parts of the quadrangle (30-50% coarse) to the northern parts (75% coarse) nearer the Uinta uplift. To the north, the fine-grained Lapoint and Dry Gulch Creek Members of the DRF appear to pinch out completely. This can be explained by erosion of these fine-grained deposits or by lateral facies shifts. Starr Flat conglomerates were deposited above Lapoint siltstones representing progradation of alluvial fans. Similarities in composition and sedimentary structures has caused confusion surrounding the Starr Flat/Bishop contact. Within the Vernal NW quadrangle, we interpret this contact as an angular unconformity (the Gilbert Peak Erosion Surface) above the uppermost red siltstone of the Starr Flat Member. Thus, stratigraphic relationships (unroofing sequence DRF, progradation of Starr Flat, and the Gilbert Peak Erosion Surface) lead to the conclusion that there were at least 3 distinct episodes of uplift during the deposition of these formations.