Middle Devonian-Mississippian Stratigraphy On and Near the Nevada Test Site: Implications for Hydrocarbon Potential

Abstract

Paleozoic strata on the Nevada Test Site and surrounding area are affected by the intersection of several important geologic trends, including the early Paleozoic rift margin and the Late Devonian-Early Mississippian Antler orogenic foredeep. Upper Paleozoic strata are lithologically diverse and include siliciclastic sediments (carbonaceous shale, clean quartzose sands, bedded chert, and chert-lithic conglomerate and sand) and various carbonates. These are interpreted to have been deposited in a range of environments and water depths. The resulting complex stratigraphy with its dramatic vertical and lateral changes is difficult to correlate, even between nearby surface and subsurface sections. Our detailed stratigraphic studies on and near the Nevada Test Site show that at least three distinct, and largely coeval, Mississippian facies assemblages can be recognized. When dated, correlated, and restored to their original relative geographic positions, these sections allow a new reconstruction of the Devonian and Mississippian geologic history that includes (1) delineation of the Antler foredeep basin and its clastic fill, (2) description of the muddy marginal shelf that lay inboard of the foredeep, and (3) correlation from that shelf to the outer fringes of the carbonate platform that characterizes the edge of the Mississippian epicontinental sea. The position and delineation of the mud-dominated marginal shelf environment is particularly important because shales deposited under such conditions elsewhere in the Great Basin are the hydrocarbon source rocks for the Pine Valley and Railroad Valley oil fields. On the Nevada Test Site, Mississippian shale probably generated hydrocarbons in the past, and still has an organic content and a thermal history appropriate for generating hydrocarbons.