LATE PROTEROZOIC, PALEOZOIC, AND MESOZOIC ROCKS AND GEOLOGIC STRUCTURES IN THE VICTORVILLE AND HELENDALE 7.5' QUADRANGLES WEST CENTRAL MOJAVE DESERT, CALIFORNIA

The geology of the Victorville, California area was described in 1977 by Elizabeth Miller. New 1:6,000-scale bedrock mapping of the Victorville and Helendale 7.5’ quadrangles expanded on Miller’s work by delineating additional detailed stratigraphic and structural relations, which improved the understanding of the complex geologic history of the area. From this new mapping, the California Geological Survey prepared two new 1:24,000-scale geologic maps.

Miogeoclinal strata of Late Proterozoic and Paleozoic age in the Quartzite Mountain area were complexly folded (F-1, F-2), faulted and metamorphosed prior to or during Permo-Triassic time. Early Jurassic Fairview Valley Formation and a transitional sequence of volcaniclastic and quartz-rich sandstones were deposited and then locally folded (F-3) and metamorphosed. Lower Sidewinder Volcanics were subsequently deposited during Early Middle Jurassic time.

Detailed mapping at Sparkhule Mountain identified an allochthon of about 3 km² consisting of Paleozoic limestone and Fairview Valley Upper limestone conglomerate resting discordantly on lowest Sidewinder andesite and ignimbrites. The allochthon was emplaced during Lower Sidewinder deposition, perhaps as a gravity slide related to caldera collapse.

New mapping also redefined the detailed stratigraphy of the Sidewinder Volcanics and associated deformation events, previously described by Schermer and Busby (1994). Younger Lower Sidewinder ignimbrites and flows were deposited, followed by episodes of north- and NW-dipping low-angle extensional faulting. After Lower Sidewinder deposition, Jurassic granitics were emplaced, followed by north-trending high-angle faulting, hydrothermal activity, and uplift and erosion.

Emplacement of Late Jurassic Upper Sidewinder shallow intrusive plugs and dikes and formation of NW-trending, west-dipping (east-verging) foliation/cleavage was followed by NW-trending folding (F-4) and faulting. This occurred prior to intrusion of Late Cretaceous batholithic rocks.

The Jurassic events described above occurred in a short period of time (163 - 148 m.y.) and are roughly coeval with Mid-Jurassic compression events in other parts of the Mojave region, and similar to extension coeval with shortening in numerous convergent margin settings on a global scale.