WHAT CONSTITUTES A BELIEVABLE MAXIMUM DEPOSITIONAL AGE (MDA)? AN EXAMPLE USING NEW ZIRCON U-PB AGES FOR THE CENOZOIC STRATA OF BAT MOUNTAIN, DEATH VALLEY REGION, CALIFORNIA

As the acquisition of detrital zircon (DZ) data has become more cost-effective, increasing emphasis has been placed on using the youngest grains in DZ distributions to estimate the maximum depositional ages (MDAs) of host sediments. This in turn has prompted ongoing discussions regarding the “best” use of DZ ages for this purpose, with many arguments focused on the number of young grains that should be considered, which young grains (if any) should be ignored, which approaches hold statistical rigor, and ultimately, which approaches (and results) are geologically reasonable. We compare commonly used methods for estimating MDA [i.e., the YSP, YC1σ(2+), YC2σ(3+), YPP, and MLA metrics] for five sandstone samples that bracket the Cenozoic strata exposed on Bat Mountain in the southern Funeral Range of California. Importantly, the entire stratigraphic succession was deposited coeval with Cenozoic volcanism of the Great Basin and contains intercalated ash fall tuffs. Every sandstone sample yielded abundant Cenozoic zircon U-Pb ages that formed unimodal, near-normal distributions that were clearly distinguishable from the next-oldest grains in each sample. Similar to prior studies, we find that more ad hoc MDA metrics [e.g., the YSP, YC1σ(2+), YC2σ(3+), YPP] tend to drift toward unreasonably young or old values, a finding that is strongly supported by benchmarking the MDAs against the zircon U-Pb ages of ashes in the Bat Mountain succession. We instead prefer either (1) the maximum likelihood age (MLA) of the entire distribution or (2) the central age of the youngest age distribution as more statistically sound and geologic appropriate estimates of MDA that do not require arbitrary omission of any young grain ages. Using these DZ MDA metrics and ash ages, we develop a new chronostratigraphy for the siliciclastic strata of Bat Mountain: The Oligo-Miocene Amargosa Formation was deposited prior to the onset of regional Basin and Range extension between ~29 and 21 Ma, whereas the Miocene Bat Mountain Formation was deposited synchronous with Basin and Range extension between ~15.5 and 13 Ma.