Sedimentologic and Tectonic Control of Uranium Mineralization, Upper Triassic Chinle Formation, Southeastern Utah: ABSTRACT

Abstract

Uranium deposits in the Upper Triassic Chinle Formation of the White Canyon, Capitol Reef, and Circle Cliffs areas occur in a succession of lithofacies that formed from sediments deposited under anoxic conditions. Anoxic conditions in the depositional environments of these lithofacies are indicated by the preservation of abundant organic matter and the drab (reduced) colors of the rocks. The coincidence of facies changes and the vertical sequence of rocks associated with isopach thicks suggests that certain depositional environments were localized by tectonic subsidence. Lake, marsh, and bog environments adjacent to anastomosing fluvial channel systems occur in synclinal areas that were actively subsiding during deposition. Deposition in these wetland environments, whic had high organic productivity, high water tables, and minimal clastic input, resulted in organic-rich rocks. The vertical sequence of lithofacies indicates that initially the rate of tectonic subsidence was greater than the rate of clastic sediment influx. Subsequently, as the rate of clastic influx exceeded the rate of subsidence, sedimentation on prograding deltas buried the wetland deposits with clastic sediments that also contained abundant detrital plant debris. The subaqueous decomposition of this organic matter produced anoxic conditions in the overlying deltaic sediments that protected underlying marsh and bog deposits from oxidizing meteoric waters. Preservation of the organic material incorporated in the bog and marsh deposits established the reducing chemical environment nece sary to precipitate uranium. Maintenance of this reducing environment also protected uranium deposits from later oxidation. Subsidence, concomitant with sedimentation, produced the hydrologic conditions conducive to plant production, accumulation, and preservation and established the reducing chemical conditions necessary to precipitate and preserve uranium. Identification of wetland environment sediments that accumulated in structurally controlled areas of subsidence is a useful guide for uranium exploration in the Chinle Formation. End_of_Article - Last_Page 936------------