Fluvial Evolution between the Salt Wash and Brushy Basin Members of the Upper Jurassic Morrison Formation, South-Central Utah

Abstract

Abrupt changes in fluvial deposition between the Salt Wash.Member and Brushy Basin Member of the Upper Jurassic Morrison Formation, in western interior North America, have been documented across a seemingly basin-wide depositional unconformity that resulted from poorly understood base level changes. Detailed field observations, paleopedologic analysis, and petrographic analysis of proximal deposits in the Henry Basin in south-central Utah, yield no depositional hiatus in the proximal basin during formation of the unconformity in the distal basin. Rather, the proximal stratal architecture suggest continuous deposition between the two members that likely resulted from a combination ofrelatively high basin subsidence rates and an increase in gravel fraction in the sediment source. Computed subsidence rates in the southern Henry Basin from recent radiometric dates document a time-averaged subsidence rate of 0.121 mm per year, while petrographic evidence suggests an up-section increase in chert-rich sediment source. Collectively these resulted in starving the distal basin of sediment and the formation of a distal depositional hiatus, while the proximal basin accumulated prograding sediments. A south-to-north transect near the western margin of the Morrison Basin documents a rapid pinch out of 150 meters of fluvial gravelly-sandstones of the Salt Wash Member between the southern Henry Basin to the south and the Emery High region 75 miles to the north. Mature paleosols at the base of the Morrison Formation in the north suggest that the lack of Salt Wash Member deposits were due to non-deposition rather than to post depositional erosion or incision. This suggests that syndepositional basin subsidence in the south channeled deposits there, while regions to the north underwent little to subsidence and deposition. The disparity in basin subsidence rates between closely spaced regions in proximal regions, combined with petrographic evidence of increased gravel-fraction in the sediment, suggests that tectonic thrusting to the west may have contributed an important role in producing continuous aggradation of sediment in the proximal basin, while contemporaneously starving the distal basin and forming a depositional unconformity. Incised valley-fill conglomerates at the contact between the two members also suggest that the change in sedimentation was influenced by a drop in base level that followed deposition of the Salt Wash Member.