Geologic Map of the Corona South 7.5' Quadrangle, Riverside and Orange Counties, California

This data set maps and describes the geology of the Corona South 7.5' quadrangle, Riverside and Orange Counties, 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 Corona South quadrangle is located near the northern end of the Peninsular Ranges Province. Diagonally crossing the quadrangle is the northern end of the Elsinore Fault zone, a major active right-lateral strike-slip fault zone of the San Andreas Fault system. East of the fault zone is the Perris block and to the west the Santa Ana Mountains block. Basement in the Perris block part of the quadrangle is almost entirely Cretaceous volcanic rocks and granitic rocks of the Cretaceous Peninsular Ranges batholith. Three small exposures of very low metamorphic grade siliceous rocks correlated on the basis of lithology with Mesozoic age rocks are located near the eastern edge of the quadrangle. Exposures of batholithic rocks is restricted to mostly granodiorite of the Cajalco pluton that underlies extensive areas to the east and north. There are limited amounts of undifferentiated granitic rock and one small body of gabbro. The most extensive basement rocks are volcanic shallow intrusives and extrusives of the Estelle Mountain volcanics. The volcanics, predominantly latite and rhyolite, are quarried as a source of crushed rock.
West of the Elsinore Fault zone is a thick section of Bedford Canyon Formation of Jurassic age. This unit consists of incipiently metamorphosed marine sedimentary rocks consisting of argillite, slate, graywacke, impure quartzite, and small pods of limestone. Bedding and other primary sedimentary structures are commonly preserved and tight folds are common. Incipiently developed transposed layering, S1, is locally well developed. Included within the siliceous rocks are small outcrops of fossiliferous limestone than contain a fauna indicating the limestone formed in a so-called black smoker environment. Unconformably overlying and intruding the Bedford Canyon Formation is the Santiago Peak Volcanics of Cretaceous age. These volcanics consist of basaltic andesite, andesite, dacite, rhyolite, breccia and volcanoclastic rocks. Much of the unit has been hydrothermally altered; the alteration was contemporaneous with the volcanism. A minor occurrence of serpentine and associated silica-carbonate rock occurs in association with the volcanics.
Sedimentary rocks of late Cretaceous and Paleogene age and a few Neogene age rocks occur within the Elsinore Fault zone. Marine sandstone of the middle Miocene Topanga Formation occurs within the fault zone southeast of Corona. Underlying the Topanga Formation is the nonmarine undivided Sespe and Vaqueros Formation that are predominantly sandstone. Sandstone, siltstone, and conglomerate of the marine and nonmarine Paleocene Silverado Formation extends essentially along the entire length of the fault zone in the quadrangle. Clay beds in the Silverado Formation have been an important source of clay. In the northwest corner of the quadrangle is a thick, faulted, sedimentary section that ranges in age from Cretaceous to early Pliocene-Miocene.
Emanating from the Santa Ana Mountains is an extensive alluvial fan complex that underlies Corona and the surrounding valleys. This fan complex includes both Pleistocene and Holocene age deposits.
The Elsinore Fault zone at the base of the Santa Ana Mountains splays in the northwestern part of the quadrangle; beyond the quadrangle boundary the name Elsinore Fault is generally not used. The southern splay takes a more western trend and to the west of the quadrangle is termed the Whittier Fault, a major active fault. The eastern splay continues on strike along the east side of the Chino (Puente) Hills north of the quadrangle where it is termed the Chino Fault. The Chino Fault appears to have very limited displacement.
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.