STRUCTURE AND PETROLOGY OF THE NORTHEAST PART OF THE SODA MOUNTAINS, SAN BERNARDINO COUNTY, CALIFORNIA

Abstract

The geology of about 200 square miles in the northeast part of the Soda Mountains just west of Baker in the Mojave Desert of California was mapped on air photographs of scale 1:13,000. An important N. 25° W. right-lateral fault, herein named the Soda-Avawatz fault, divides the region into two distinctly different geologic areas. In the eastern area the following rocks occur: (1) metasedimentary rocks of probable Precambrian age, (2) the Prospect Mountain quartzite which is thought to be Lower Cambrian, (3) dolomite of Middle Paleozoic(?) age, and (4) a complex of plutonic rocks ranging from diorite to granite. The western area consists of: (1) Mississippian-Pennsylvanian(?) limestone and hornfels, (2) Lower Permian limestone of the Bird Spring formation, (3) Lower Triassic limestone and shale, (4) Triassic-Jurassic andesite and quartzite herein named the Soda Mountain formation, and (5) Upper Mesozoic intrusive rocks different from those in the eastern area. In the eastern area, Lower Cambrian(?) quartzite of restricted extent constitutes an isolated thrust complex, and intensely deformed Middle Paleozoic(?) dolomite occurs in klippen which are intruded by plutonic rocks. In the western area, highly disordered allochthonous Upper Paleozoic limestones occur in fragmentary blocks. These are irregularly downfaulted into an equally disordered para-autochthon of Mesozoic volcanic rocks. The thrust complex was passively intruded by Upper Mesozoic quartz diorite, quartz monzonite, and granite. The intrusions effected unusually large zones of contact metamorphism (garnetization) and regional hydrothermal alteration (albitization and epidotization). Several thousand feet of the lower Pliocene Avawatz formation consisting of fan-glomerate, sandstone, and monolithologic limestone breccia occurs in the district and is prevalent along the Soda-Avawatz fault zone. The limestone breccia originated by landsliding and normal fanglomerate accumulation coeval with oblique-slip movement of the Soda-Avawatz fault. Deformation took place during the Middle and Late Mesozoic and is considered part of the Nevadan-Laramide orogeny. This resulted in isoclinal folding, thrusting, and block faulting. The upper plates of the thrusts may have moved westward with respect to the lower blocks, but the evidence is inconclusive. Granitic intrusion followed deformation and was locally guided by pre-established vertical and low-angle faults. The Soda-Avawatz fault zone is the most prominent structure in the Soda Mountains. Ranging in width from 1 to 2 miles, it consists of three principal through-going faults and numerous branching faults characterized by wide zones of granulation and rock slivering, near-horizontal slickensides, and apparent reversal of throw along the strike. A large anticline is squeezed up within the fault zone. Most of the movement occurred during Late Cenozoic, but features indicating recent displacement are lacking. The strongest evidence for right-lateral displacement is the echelon arrangement of folds and secondary faults. Since specific geologic features in the Soda Mountains cannot be matched across the fault, the amount of displacement is unknown. The Soda-Avawatz fault continues northward along the east side of the Avawatz Mountains and becomes the Death Valley fault zone. Together they form a regionally significant northwest-trending right-lateral fault zone. More recent movement on the east-west Garlock fault has forced an eastward bulge in the Death Valley-Soda-Avawatz fault where the faults join on the northeast side of the Avawatz Mountains. During most of the Late Paleozoic and Mesozoic eras, the Soda Mountains area lay in a transition zone between miogeosynclinal environment to the east and eugeosynclinal environment to the west. The Death Valley-Soda-Avawatz fault seems to coincide in part with this transition zone and was perhaps localized by it.