AGE CONSTRAINTS AND DEPOSITIONAL SETTING OF THE EARLIEST CENOZOIC SEDIMENTARY ROCKS DEPOSITED IN SEVERAL RANGES THROUGHOUT SOUTHERN NEVADA, USA

Sedimentary rocks deposited above the regional sub-Cenozoic unconformity in Nevada are important for evaluating the Late Cretaceous to early Cenozoic tectonic and paleotopographic history of the western U.S. Cordilleran hinterland. We report preliminary detrital zircon U-Pb age distributions for 15 samples obtained from these rocks in several ranges in southern Nevada. Such rocks are sparsely exposed throughout southern Nevada and southeast California, where they typically form thin deposits on the order of 100 m thick. New detrital zircon maximum depositional ages (MDAs) from 9 of the samples indicate that most were deposited ≤10 Myr before eruption of the ca. 28–27 Ma Monotony Tuff, the lowest Cenozoic volcanic unit in the area. One succession, exposed between the Sheep and southeast Pahranagat Ranges, along the Arrowhead Mine fault, yielded upward-younging Eocene–Oligocene MDAs of ca. 36.0 ± 0.3 Ma, 32.7 ± 0.5 Ma, and 29.1 ± 0.2 Ma. In contrast, in the Pintwater and Spotted Ranges, northwest of Las Vegas, the thickness of the Cenozoic succession beneath the first tuffs exceeds 1 km, and MDAs are appreciably older. Deposits on the east side of the Pintwater Range yielded Eocene MDAs of ca. 47.6 Ma and 42.4 Ma for samples within the lower half of the succession. A sample from ~350 m above the base of correlative deposits on the northeastern flank of the nearby Spotted Range returned an Eocene MDA of 39.3 ± 0.2 Ma. The new age constraints demonstrate that depocenters potentially as old as middle Eocene in age existed across more of southern Nevada than previously thought, recording the development of accommodation as much as 20 Myr before ignimbrite flare-up volcanic rocks at this latitude.