NEW OUTCROP INSIGHTS FROM THE LACUSTRINE FLAGSTAFF FORMATION, UINTA BASIN AND WASATCH PLATEAU, UTAH

The lacustrine Paleogene Flagstaff Formation is in tectonically separated geologic provinces spanning the eastern Basin and Range, Wasatch Plateau, and Uinta Basin. The Flagstaff Formation is also a prospective unconventional hydrocarbon target in the deep Uinta Basin. This study provides updated interpretations with detailed measured sections, drone photogrammetry models, and geochemical data. Results showcase highly variable lithological assemblages reflecting distinct lacustrine-dominated environments. The Flagstaff Formation in the western study area (e.g., Pahvant Range) is dominated by basin-margin siliciclastic facies including conglomerate, sandstone, and paleosols, intercalated with pedogenically modified shale and carbonate. The Wasatch Plateau exhibits thick (750+ ft, 229+ m) sequences of limestone and shale, although complete outcrop sections are limited. The western Uinta Basin (e.g., Price Canyon) consists predominantly of shale interbedded with limestone containing oncoids and mollusks. A new 400-ft-thick (122 m) section from the southern Uinta Basin represents the southeastern margin of Lake Flagstaff and consists of mudstone, paleosols, channelized sandstone, and limestone. This section showcases qualitative paleoclimate indicators (e.g., faunal assemblages and karst features), alters previous interpretations of maximum lake extent, and provides new insights into bounding relationships with the underlying North Horn Formation and overlying Colton Formation. With limited geochronological data, temporal and lithostratigraphic correlation of these unique facies assemblages across the tectonically separated outcrop study areas is difficult and relies on stratigraphic interpretation of depositional environments and paleogeographic reconstructions across central Utah. Two new U-Pb ages from a condensed section of Paleogene strata overlying the Flagstaff Formation in the southern Wasatch Plateau pose new questions on the depositional timing of Lake Flagstaff and may indicate a time-transgressive lake body (or multiple lake bodies) with shifting depocenters throughout the Paleogene. This presentation aims to highlight the knowns and unknowns of this relatively unstudied and potentially economically significant unit, with a focus on showcasing new stratigraphic questions developed from ongoing research.