Depositional framework, hydrostratigraphy, and uranium mineralization of the Oakville Sandstone (Miocene), Texas Coastal Plain

Abstract

The Oakville Sandstone (Miocene) of the Texas Coastal Plain comprises a major sand-rich fluvial system composed of deposits of several major and minor rivers that originated within Texas and surrounding states. Broad bed-load fluvial axes, including the Hebbronville, George West, and New Davy trends, lie south of the San Marcos Arch and host significant reserves of uranium. To the north, the Moulton streamplain consists of deposits of numerous small, flashy to ephemeral streams that drained the inland margin of the Coastal Plain. The Burton/Penn mixed-load fluvial axes traverse the Coastal Plain in the area of the modern Colorado and Brazos Rivers. Each fluvial axis consists of diagnostic facies deposited in channel, crevasse splay, and floodplain environments. Sand percentage, sand-body dimensions and lateral relationships, and composition vary systematically among the axes. Overall transmissivity of the Oakville Formation, which closely corresponds to the Jasper aquifer system in conventional hydrostratigraphic terminology, correlates directly with mapped facies composition and trend. Uranium deposit size relates directly to associated aquifer transmissivity. Commercial uranium deposits lie within channel and interbedded sheet-splay sands in or along the margin of major fluvial belts, typically near shallow faults. Mineralization occurs along narrow, elongate fronts separating altered, but commonly resulfidized, hostmore » sand from epigenetically sulfidized reduced sand. Deposits show pronounced spatial zoning of trace metals and iron sulfide, carbonate, and locally, clay mineral phases across the mineralization front. 62 references, 34 figures, 6 tables.« less