Geologic map of the Steele Peak 7.5' quadrangle, Riverside County, California

Abstract

This data set maps and describes the geology of the Steele Peak 7.5' quadrangle, Riverside 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) a coverage containing structural data, (3) a coverage containing geologic unit annotation and leaders, and (4) 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), and a key for point and line symbols, and (2) PDF files of the Readme (including the metadata file as an appendix), and the graphic produced by the Postscript plot file. The Steele Peak quadrangle is located in the northern part of the Peninsular Ranges Province within the central part of the Perris block, a relatively stable, rectangular in plan area located between the Elsinore and San Jacinto fault zones. The quadrangle is underlain by Cretaceous and older basement rocks. Cretaceous plutonic rocks are part of the composite Peninsular Ranges batholith. A wide variety of mafic to intermediate composition granitic rocks occur in the quadrangle, and are mainly of tonalitic composition, but range from monzogranite to gabbro. Most rock units are faintly to intensely foliated, compositionally heterogenous, and contain varying amounts of meso-and melanocratic discoidal-shaped inclusions. Some rocks are composed almost wholly of inclusion material and some are migmatitic. Included within these granitic rocks are septa not shown on the geologic map of Paleozoic(?) schist of upper amphibolite metamorphic grade. Metamorphic rocks of primarily Mesozoic age occur in a discontinuous belt extending from the southeast to the northwest corner of the quadrangle. Most of these rocks are well foliated biotite-bearing schist. Near the southern edge of the quadrangle phyllitic rocks dominate. Northwestward, metamorphism increases from greenschist or sub-greenschist grade near the south edge of the quadrangle to sillimanite-bearing schist of upper amphibolite grade in the vicinity of Cajalco Road. Biotite-hornblende tonalite of the relatively large Val Verde pluton dominates the northeastern half of the quadrangle. In most places this tonalite has a northwest oriented crude to well developed planar fabric produced by oriented biotite and hornblende. Schlieren and massive clots of mafic tonalite locally occur. Discoidal- to pancake-shaped mafic inclusions are widespread and are oriented in the plane defined by the biotite and hornblende. This planar fabric typically dips moderately to the northeast, but locally shallows to a horizontal to subhorizontal planar fabric, or fades to an isotropic fabric. West of the Val Verde pluton are a number of plutons having fabrics ranging from massive isotropic to foliated. Compositions of these plutons range from monzogranite to pyroxene gabbro. Most of these granitic rocks fall within the composition range from monzogranite to tonalite, and are part of the composite Gavilan ring complex. Hypersthene is a characteristic mineral of most of the rocks of this complex, which includes black hypersthene-bearing monzogranite that has been quarried as a source of 'black granite' building stone. Several inactive gold mines, e.g., Goodhope, Gavilan, and Santa Rosa mines that constituted the Pinacate mining district, are located in the Gavilan ring complex. In the center of the Gavilan ring complex is the near circular Arroyo del Toro pluton, a massive-textured granodiorite essentially devoid of inclusions. Only the northern half of this pluton is located in the quadrangle. Some rock of this pluton was quarried for building stone. The southwestern corner of the quadrangle is underlain by siliceous volcanic and volcanoclastic rock considered to be coeval with the batholith and be the supra-part of the batholithic magmatism. Most of these volcanic rocks range in composition from rhyolite to andesite with latitic composition rocks predominating. In the northeastern part of the quadrangle is the proximal parts of a Pleistocene alluvial fan complex. The geologic map data base contains original U.S. Geological Survey data generated by detailed field observation recorded on 1:24,000 scale aerial photographs. The map was created by transferring lines from the aerial photographs to a 1:24,000 scale topographic base. The map was digitized and lines, points, and polygons were subsequently edited 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 are polygons, and site-specific data as points. Polygon, arc, and point attribute tables (.pat, .aat, and .pat, respectively) uniquely identify each geologic datum.