Preliminary Geologic Map of the San Bernardino 30' x 60' quadrangle, California

The San Bernardino 30'x60' quadrangle, southern California, is diagonally bisected by the San Andreas Fault Zone, separating the San Gabriel and San Bernardino Mountains, major elements of California's east-oriented Transverse Ranges Province. Included in the southern part of the quadrangle is the northern part of the Peninsular Ranges Province and the northeastern part of the oil-producing Los Angeles basin. The northern part of the quadrangle includes the southern part of the Mojave Desert Province. Pre-Quaternary rocks within the San Bernardino quadrangle consist of three extensive, well-defined basement rock assemblages, the San Gabriel Mountains, San Bernardino Mountains, and the Peninsular Ranges assemblages, and a fourth assemblage restricted to a narrow block bounded by the active San Andreas Fault and the Mill Creek Fault. Each of these basement rock assemblages is characterized by a relatively unique suite of rocks that was amalgamated by the end of the Cretaceous and (or) early Cenozoic. Some Tertiary sedimentary and volcanic rocks are unique to specific assemblages, and some overlap adjacent assemblages. A few Miocene and Pliocene units cross the boundaries of adjacent assemblages, but are dominant in only one. Tectonic events directly and indirectly related to the San Andreas Fault system have partly dismembered the basement rocks during the Neogene, forming the modern-day physiographic provinces.
Rocks of the four basement rock assemblages are divisible into an older suite of Late Cretaceous and older rocks and a younger suite of post-Late Cretaceous rocks. The age span of the older suite varies considerably from assemblage to assemblage, and the point in time that separates the two suites varies slightly. In the Peninsular Ranges, the older rocks were formed from the Paleozoic to the end of Late Cretaceous plutonism, and in the Transverse Ranges over a longer period of time extending from the Proterozoic to metamorphism at the end of the Cretaceous. Within the Peninsular Ranges a profound diachronous unconformity marks the pre-Late Cretaceous-post-Late Cretaceous subdivision, but within the Transverse Ranges the division appears to be slightly younger, perhaps coinciding with the end of the Cretaceous or extending into the early Cenozoic. Initial docking of Peninsular Ranges rocks with Transverse Ranges rocks appears to have occurred at the terminus of plutonism within the Peninsular Ranges. During the Paleogene there was apparently discontinuous but widespread deposition on the basement rocks and little tectonic disruption of the amalgamated older rocks. Dismemberment of these Paleogene and older rocks by strike-slip, thrust, and reverse faulting began in the Neogene and is ongoing. The Peninsular Ranges basement rock assemblage is made up of the Peninsular Ranges batholith and a variety of metasedimentary rocks. Most of the plutonic rocks of the batholith are granodiorite and tonalite in composition; primary foliation is common, mainly in the eastern part. Tertiary sedimentary rocks of the Los Angeles Basin crop out in the Puente and San Jose Hills along with the spatially associated Glendora Volcanics; both units span the boundary between the Peninsular Ranges and San Gabriel Mountains basement rock assemblages.
The San Gabriel Mountains basement rock assemblage includes two discrete areas, the high standing San Gabriel Mountains and the relatively low San Bernardino basin east of the San Jacinto Fault. The basement rock assemblage is characterized by a unique suite of rocks that include anorthosite, Proterozoic and Paleozoic gneiss and schist, the Triassic Mount Lowe intrusive suite, extensive deformed and undeformed Cretaceous granitic rocks, the Pelona Schist, and Oligocene granitic rocks. Internal structure of the assemblage includes the Vincent Thrust Fault, at least two old, abandonded segments of the San Andreas Fault system, and extensive areas of well-developed to pervasively mylonitized rocks.
The main body of the San Gabriel Mountains is bounded on the north by the San Andreas Fault and on the south by the Sierra Madre-Cucamonga Fault Zone. East of the San Jacinto Fault, the San Bernardino basin is an asymmetric pull-apart basin bounded by the San Andreas Fault on the east, and underlain by many of the same rock units that characterize the San Gabriel Mountains. Cretaceous and older rocks of the San Gabriel Mountains basement rock assemblage are divided into two structurally and lithologically distinct groups by the Vincent Thrust Fault, a regional, low-angle thrust fault that predates intrusion of Oligocene granitic rocks. The Vincent Thrust separates the Mesozoic Pelona Schist in its lower plate from highly deformed gneiss, schist, and granitic rocks in the upper plate. The fault, along with its far-offset, dismembered analogs in the Orocopia and Chocolate Mountains east of the Salton Sea, may underlie much of southern California.
Crystalline rocks between the Mill Creek Fault and the main trace of the San Andreas Fault Zone range from highly deformed gneiss of unknown age to relatively undeformed Mesozoic biotite-hornblende diorite. They are overlain by Miocene sedimentary rocks and cut by the Wilson Creek Fault, that is considered to be an older segment of the San Andreas Fault system. Crystalline rocks of this basement assemblage are similar to rocks in the Little San Bernardino Mountains to the southeast, and appear to have been displaced about 50 km by the Wilson Creek and Mill Creek Faults.
About 80 to 85 percent of the San Bernardino Mountains bedrock assemblage in the quadrangle is Mesozoic granitic rocks, and the rest, highly metamorphosed and deformed Late Proterozoic and Paleozoic metasedimentary rocks. There is a pronounced gradient from east to west, and to a slightly lesser degree from south to north, in the magnitude of both deformation and metamorphism of the Late Proterozoic and Paleozoic metasedimentary rocks. In addition to the east to west gradient of increasing metamorphism and deformation, east of the quadrangle there appears to be a sharp break between highly deformed and relatively undeformed Late Proterozoic and Paleozoic rocks.
Late Proterozoic and Paleozoic units comprise a thick sequence of metasedimentary rocks generally consisting of a lower quartzitic sequence and an upper carbonate rock sequence. The entire lower quartzitic part is Late Proterozoic and Early Cambrian, and includes the Stirling Quartzite, Wood Canyon Formation and Zabriskie Quartzite; the upper carbonate rock sequence includes the Cambrian Carrara and Bonanza King Formations, the Devonian Sultan Limestone, the Mississippian Monte Cristo Limestone, and the Pennsylvanian Bird Spring Formation.
Mesozoic intrusive rocks in the San Bernardino Mountains and southern Mojave Desert include numerous Triassic and Jurassic plutons. The Triassic rocks are relatively alkalic and quartz deficient, and contrast with the voluminous, quartz-rich, calc-alkalic Cretaceous granitic rocks, which make up the largest part of the San Bernardino Mountains assemblage. The voluminous tonalitic rocks in the San Gabriel Mountains and Peninsular Ranges assemblages are essentially absent in the western San Bernardino Mountains. Many areas of dominantly Cretaceous granitic rocks are mapped as Mesozoic mixed-rock units, because they are extremely heterogeneous, and include large components of older rocks.
The relatively young, active San Andreas Fault system is by far the dominant structure in the San Bernardino quadrangle. Based on offsets of many of the rock units found in the San Bernardino quadrangle, different amounts of lateral displacement have been proposed for the San Andreas Fault system within and south of the Transverse Ranges. The Neogene evolution of the Transverse Ranges Province, and its relationship to the San Andreas Fault system in particular, are complicated by several abandonded segments and the shifting locus of the fault during the late Cenozoic. Most recent structural interpretations require relatively large rotations within the Transverse Ranges Province.
Other active faults in the quadrangle include the San Jacinto Fault and the reverse faults bounding and within the Transverse Ranges. Older faults considered to be abandoned segments of the San Andreas Fault system include the San Gabriel Fault, Punchbowl Fault, Mission Creek Fault, and Wilson Creek Fault. The Vincent Thrust and Squaw Peak Fault are both older faults, the Vincent probably of late Mesozoic to early Tertiary age.

1. Geologic map database
2. Plot files for five sheets:
  Sheet 1 Geologic map
  Sheet 2 Correlation of Map Units
  Sheet 3 List of map units
  Sheet 4 Fault map
  Sheet 5 Three figures, including index map, physiographic provinces map, and sources of data
3. A pamphlet containing:
  a. A detailed Description of Map Units
  b. A discussion of the regional geologic framework
  c. Photographs of many rock units and geologic features