Geologic map of the San Bernardino North 7.5' quadrangle, San Bernardino County, California

Abstract

This data set maps and describes the geology of the San Bernardino North 7.5' quadrangle, San Bernardino County, California. Created using Environmental Systems Research Institute's ARC/INFO software, the data base consists of the following items: (1) a map coverage containing geologic contacts and units, (2) attribute tables for geologic units (polygons), contacts (arcs), and site-specific data (points). In addition, the data set includes the following graphic and text products: (1) A PostScript graphic plot-file containing the geologic map, topography, cultural data, a Correlation of Map Units (CMU) diagram, a Description of Map Units (DMU), an index map, a regional geologic and structure map, and a key for point and line symbols; (2) PDF files of this Readme (including the metadata file as an appendix), Description of Map Units (DMU), and the graphic produced by the PostScript plot file. The geologic map covers a part of the southwestern San Bernardino Mountains and the northwestern San Bernardino basin. Granitic and metamorphic rocks underlie most of the mountain area, and a complex array of Quaternary deposits fill the basin. These two areas are separated by strands of the seismically active San Andreas Fault. Bedrock units in the San Bernardino Mountains are dominated by large Cretaceous and Jurassic granitic bodies, ranging in composition from monzogranite to monzodiorite, and include lesser Triassic monzonite. The younger of these granitic rocks intrude a complex assemblage of gneiss, marble, and granitic rock of probable early Mesozoic age; the relationship between these metemorphic rocks and the Triassic rocks is unknown. Spanning the Pleistocene in age, large and small alluvial bodies emerge from the San Bernardino Mountains, and and fill the San Bernardino basin. In the southwestern part of the quadrangle, Cajon Wash carries sediments from both the San Bernardino and San Gabriel Mountains, and Lytle Creek heads in the eastern San Gabriel Mountains. Limited bedrock areas showing through the Quaternary sediments of the basin consist exclusively of Mesozoic Pelona Schist locally intruded by Tertairy dikes. Youthful-appearing fault scarps discontinuously mark the traces of the San Andreas Fault along the southern edge of the San Bernardino Mountains. Unnamed Tertiary sedimentary rocks are bounded by two strands of the fault between Badger Canyon and the east edge of the quadrangle. Young and old high-angle faults cut bedrock units within the San Bernardino Mountains, and the buried, seismically active San Jacinto Fault traverses the southwestern part of the quadrangle. The geologic map database contains original U.S. Geological Survey data generated by detailed field observation and by interpretation of aerial photographs. This digital Open-File map superceeds an older analog Open-File map of the quadrangle, and includes extensive new data on the Quaternary deposits, and revises some fault and bedrock distribution within the San Bernardino Mountains. The digital map was compiled on a base-stable cronoflex copy of the San Bernardino North 7.5' topographic base and then scribed. This scribe guide was used to make a 0.007 mil blackline clear-film, which was scanned at 1200 DPI by Optronics Specialty Company, Northridge, California; minor hand-digitized additions were made at the USGS. Lines, points, and polygonswere subsequently edited at the USGS using standard ARC/INFO commands. Digitizing and editing artifacts significant enough to display at a scale of 1:24,000 were corrected. Within the database, geologic contacts are represented as lines (arcs), geologic units as polygons, and site-specific data as points. Polygon, arc, and point attribute tables (.pat, .aat, and .pat, respectively) uniquely identify each geologic datum.