GEOLOGY OF THE SIDEWINDER MOUNTAIN AND BLACK MOUNTAIN AREA, WEST CENTRAL MOJAVE DESERT, SAN BERNARDINO COUNTY CALIFORNIA

The Sidewinder Mountain, Black Mountain area is located in the west central Mojave Desert area, and was mapped at a scale of 1:6,000. This work presents the most detailed mapping of the area, and provides additional details regarding bedrock units and complex geologic structures.

Bedrock includes roof pendants of Late Proterozoic and Paleozoic miogeoclinal metasedimentary formations, and is the type location of the unconformably overlying Fairview Valley Formation (FVF), unconformably overlying quartzite, and unconformably overlying Lower Sidewinder volcanic rocks (LSV) all Early to Mid Jurassic age. Triassic, Jurassic and Cretaceous plutonic rocks and dikes are also exposed. Alluvial units of PrePleistocene to recent age are also present.

Structure includes multiple Mesozoic age metamorphic, intrusive, contractional and extensional deformational events, and younger deformation that continued to present time. Mesozoic deformation includes polyphase folding, faulting and metamorphism of Paleozoic rocks likely during Permo-Triassic time, followed by intrusion of Triassic monzonite, uplift and erosion. Lower Jurassic FVF unconformably overlies deformed Paleozoic metasediments and Triassic intrusive. The FVF was faulted and folded prior to unconformably overlying quartzite. Unconformably overlying quartzite is Mid Jurassic LSV, hypabyssal intrusives and subordinate sedimentary rocks. During and following deposition of the LSV were multiple caldera collapse events and associated faulting, intrusion of hypabyssal intrusives and unconformities. Several generations of extensional deformation occurred prior to and after intrusion of Late Mid Jurassic plutonic rocks. Northwest trending felsic and mafic dikes correlate with the Late Jurassic Independence dikes and cut older rocks. Late Jurassic Upper Sidewinder volcanics followed. Quartz monzonite plutonic rocks were intruded during Cretaceous time.

A prominant feature is the northwest trending Helendale fault zone. The fault zone is postulated to have several miles of movement. Alluvial and bedrock units indicate right lateral faulting. Gravity data suggest right stepping en echelon fault strands may have formed a pull apart basin up to two miles wide in which many faults are present, some of which have been active during recent time.