STRATIGRAPHY, AGE, AND TECTONIC SETTING OF THE MIOCENE BARSTOW FORMATION AT HARVARD HILL, CENTRAL MOJAVE DESERT, CALIFORNIA

Our detailed geologic mapping and geochronology of the Barstow Formation at Harvard Hill, CA, help to constrain Miocene paleogeography and tectonics of the central Mojave Desert. The study area is 30 km east of Barstow, CA and near the eastern boundary of a WNW-trending Miocene basin. A northern strand of the Quaternary ENE-striking, sinistral Manix fault divides the Barstow Formation at Harvard Hill into two distinct lithologic assemblages. Strata north of the fault consist of: a green rhyolitic tuff, informally named the Shamrock tuff; lacustrine sandstone; partially silicified thin-bedded to massive limestone; and alluvial sandstone to pebble conglomerate. Strata south of the fault consist of: lacustrine siltstone and sandstone; a rhyolitic tuff dated at 19.1 Ma (U-Pb); rock-avalanche breccia deposits; partially silicified well-bedded to massive limestone; and alluvial sandstone and conglomerate.

Our U-Pb zircon dating of the Shamrock tuff yields a peak probability age of 18.7 ± 0.1 Ma. Distinctive outcrop characteristics, mineralogy, remanent magnetization, and zircon geochemistry (Th/U) suggest that the Shamrock tuff represents a lacustrine deposit of the regionally extensive Peach Spring Tuff (PST) pyroclastic density current. When Shamrock tuff zircon age data are combined with zircon age analyses from three well-characterized PST samples, the peak probability age is 18.7 ± 0.1 Ma, thus providing new insight into the age of zircon crystallization in the PST rhyolite.

Results of our field studies show that Miocene strata at Harvard Hill mostly accumulated in a shallow lacustrine environment; the rock-avalanche breccias near the base of the exposed section indicate proximity to a steep basin margin, perhaps just after basin initiation. Our geochronology demonstrates that deposition of the Barstow Formation at Harvard Hill extended from before ~19.1 Ma until well after ~18.7 Ma, similar to timing of Barstow Formation lake deposition in the Calico Mountains but at least 3 million years older than comparable facies in the Mud Hills type section. These observations are consistent with either of two paleogeographic models: westward transgression of lacustrine environments within a single large basin, or sequential development of geographically distinct eastern and western sub-basins.