Geologic Map of the Corona South 7.5' Quadrangle, Riverside and Orange Counties, California, USGS OFR 02-21

The data set for the Corona South 7.5' quadrangle was prepared under the U.S.
Geological Survey Southern California Areal Mapping Project (SCAMP) as part of
an ongoing effort to develop a regional geologic framework of southern
California, and to utilize a Geographic Information System (GIS) format to
create regional digital geologic databases. These regional databases are being
developed as contributions to the National Geologic Map Database of the
National Cooperative Geologic Mapping Program of the USGS.

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.