RECORD OF PALEOGENE SEDIMENTARY BASIN AND FLUVIAL SYSTEM DEVELOPMENT IN SOUTHERN NEVADA, USA

Rocks deposited above the regional sub-Cenozoic unconformity in the Cordilleran hinterland of Nevada provide data critical to understanding the timing and causes of the transition from Mesozoic shortening to Cenozoic extension. Such rocks, deposited prior to volcanic rocks of the southwest-migrating middle Cenozoic ignimbrite flare-up, are sparse. Few have been characterized and most lack absolute age control. We characterize and provide detrital zircon U-Pb maximum depositional ages (MDAs) for Cenozoic rocks in southern Nevada that were deposited below the ca. 28–27 Ma Monotony Tuff and younger volcanic rocks. Although most of these deposits in southern Nevada are thin (around 100 m), sections in the southern Fallout Hills and the east flank of the Pintwater Range are >1 km thick. Deposits vary vertically and laterally, consisting of conglomerate, diamictite, sandstone, and limestone, often containing clasts of Neoproterozoic and Paleozoic rocks similar to units exposed nearby. Although these deposits lack visible volcanic detritus, their zircon age distributions suggest that they contain material derived from volcanic centers to the north and delivered to the study area via ash fall and/or fluvial transport. For example, a sample from the lowest 10 m of a 100 m-thick measured section in the southeast Pahranagat Range yielded a MDA of ca. 36.0 ± 0.3 Ma, similar to eruptive ages in northern Nevada. A higher sample in the same section yielded a younger MDA of ca. 32.7 ± 0.5 Ma, suggesting a source in central Nevada. The uppermost ~45 km of that section consists of tabular, laminated limestone indicative of ponding. As with similar sections in southern Nevada, it is overlain, apparently conformably, by locally erupted ignimbrites. In the southern Delamar Mountains, ~35 km to the southeast, the upper ~10 m of a >80 m section shows evidence for southeastward flow. A sample from near the top of the section yielded a MDA of 37.0 ± 0.4 Ma. About 75 km further northeast, on the west side of Chief Mountain, northwest of Caliente, a sample from the middle of a ~120 m thick section yielded a MDA of ca. 33.3 ± 0.2 Ma. Together, these data show that basin development and fluvial deposition occurred in parts of southern Nevada shortly before Oligocene volcanism, suggesting that south-migrating volcanism modified topography ahead of its front.