Abstract

Uplift of the basement-cored San Rafael structure in eastern Utah influenced the subsidence rate, drainage patterns, and lacustrine facies distribution in the western Roan Cliffs during the Paleocene. The area northwest of the uplift began to subside in the middle Paleocene, and the area north and northeast of the uplift was subsiding by the late Paleocene. Middle and late Paleocene deposition in these areas, represented by the undifferentiated Flagstaff Member of the Green River Formation and the North Horn Formation, occurred in alluvial and lacustrine environments. Small meandering streams flowed across extensive low-gradient floodplains and discharged into shallow, quiet-water lakes. The floodplains graded downdip and laterally into broad lacustrine plains dominated by mud flats. Clastic-sediment-rich nearshore lacustrine facies graded offshore into carbonate-dominated facies. The carbonate-dominated lake environment was calm, shallow, vegetated, muddy, and inhabited primarily by grazing snails, including species of the genera Lioplacodes, Physa, Valvata, Hydrobia, and Viviparus. Recurrent, laterally continuous beds of limestone reflect frequent lake level fluctuations. The litharenite and sublitharenite sandstone compositions and the predominantly eastward paleocurrent directions indicate that the thrust belt to the west was the main source of clastic material. East-flowing rivers were diverted to the southeast around the northern part of the San Rafael uplift. Local presence of feldspar-bearing sandstone and northward to northeastward paleocurrent directions indicate a contribution of clastic material from the San Rafael uplift. In the latest Paleocene, a major influx of clastic material from the south and southeast established an entirely alluvial environment that initiated deposition of the Colton Formation.