New insights into basin development of the Miocene Barstow Formation through zircon U-Pb tephrochronometry and Lu-Hf petrochronology

The Miocene Barstow Formation, central Mojave, California, USA, preserves a detailed sedimentary, paleoclimatic, and paleontological record. Despite its importance in calibrating the Barstovian North American Faunal Stage and cataloging the onset and terminus of the Miocene Climatic Optimum, the precise depositional age, stratigraphic architecture, and tectonic significance of the Barstow Formation are unresolved. This uncertainty is a consequence of isolated exposures, structural complexity, and limited previous geochronologic constraints. To reconstruct a 4-D history of this basin, and elucidate the timing, rate(s) and duration of sedimentation, and tephra provenance, we dated zircon from 19 tuff beds in three separate well-exposed sections of the Barstow Formation. Of these, zircons from nine distinct tuffs, spanning the full depositional history of the basin, were analyzed for Hf isotopes. Results indicate that most of the exposed fluvio-lacustrine rocks of the Barstow Formation in the Mud Hills accumulated between 16.4 Ma and 13.0 Ma at sediment accumulation rates from 0.3 mm/yr to <0.1 mm/yr. While these ages are in broad agreement with extant geochronology, our results are generally more accurate and less vulnerable to alteration, and their local stratigraphic context is clearer. Our findings demonstrate along-strike variations in coeval lithofacies, thickness, and accumulation rates in the Barstow Formation. Zircon ɛHf(t) varies between +14.4 and −16 and abruptly decreases from a pre–15.5 Ma mean of +7.5 to a post–15.5 Ma value of −7. These ɛHf(t) values are consistent with source volcanic centers in Nevada and Idaho and appear to capture cratonic influence on melt generation in the source region after 15.5 Ma.