REVISED EARLY MIOCENE STRATIGRAPHY AND YOUNGER STRUCTURES IN THE LANE MOUNTAIN-NORTH CALICO MOUNTAINS AREA, CENTRAL MOJAVE DESERT, CALIFORNIA

Detailed geologic mapping and geochronology provide new insights into Early Miocene deposition and subsequent structural history in the Lane Mountain - North Calico Mountains area north of Barstow, California. The recent work revises the early Miocene stratigraphy, depositional setting and adds structural data.

McCulloh (1960) defined and named the early Miocene Jackhammer and Pickhandle Formations that unconformably overlie eroded Paleozoic and Mesozoic rocks. The Jackhammer and Pickhandle Formations are volcanic clastic sequences and hypabyssal intrusives and domes thought to have accumulated in a northwest trending hangingwall basin of the Central Mojave Metamorphic Core Complex (CMMCC) (Fillmore and Walker 1996). McCulloh interpreted the Lane Mountain Rhyolite as the youngest (Pliocene?) of the sequences. Radiometric dating by Burke et.al. (1982) determined the Lane Mountain rhyolite was early Miocene (23.1 Ma) and thought to be the oldest Miocene rocks. Recent radiometric dating (22.3 ma) by Stone et.al. (2019) confirms the Early Miocene age.

Detailed mapping identified >50 meters of Jackhammer Formation that underlies Lane Mountain Rhyolite, and the Lower Jackhammer is the oldest of the Miocene rocks. The Jackhammer has a very distinctive non volcanic basal conglomerate throughout the area. At Lane Mountain, tilted Jackhammer is unconformably overlain by Lane Mountain rhyolite (23 Ma). Younger Miocene rocks in the area include Upper Jackhammer, the Pickhandle Fm. and numerous 18-19.3 ma dacite flows, plugs, and sills that intrude the Pickhandle, and possible younger (17 ma) dacite domes (Burke et. al. 1982, Singleton and Gans 2008). Based on thermochronology CMMCC related extension started ~21 ma (Gans et. al. 2005). Thus, Lower Jackhammer and Lane Mountain rhyolite pre-date onset of CMMCC related extension and may be pre-extensional deposits related to local volcanic and non-volcanic sources.

Deformation of the Miocene rocks includes tilting, angular unconformities, abundant syn- and post-depositional faulting, gravity slides, and localized intense folding of Miocene rocks adjacent to some faults, within roof pendants, and recumbent folds adjacent to dacite sills. Several potentially active north and northwest trending strike slip faults with displacement up to 2.0 km were also mapped.