Geologic Map of the Corona North 7.5' Quadrangle Riverside and San Bernardino Counties, California, USGS OFR 02-22

The data set for the Corona North 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 North 7.5'
quadrangle, Riverside and San Bernardino 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 North quadrangle is located near the northern end of the Peninsular
Ranges Province. All but the southeastern tip of the quadrangle is within the
Perris block, a relatively stable, rectangular in plan area located between the
Elsinore and San Jacinto fault zones. The southeastern tip of the quadrangle
is barely within the Elsinore fault zone.

The quadrangle is underlain by Cretaceous plutonic rocks that are part of the
composite Peninsular Ranges batholith. These rocks are exposed in a
triangular-shaped area bounded on the north by the Santa Ana River and on the
south by Temescal Wash, a major tributary of the Santa Ana River. A variety of
mostly silicic granitic rocks occur in the quadrangle, and are mainly of
monzogranite and granodioritic composition, but range in composition from
micropegmatitic granite to gabbro. Most rock units are massive and contain
varying amounts of meso- and melanocratic equant-shaped inclusions. The most
widespread granitic rock is monzogranite of the Cajalco pluton, a large pluton
that extends some distance south of the quadrangle. North of Corona is a body
of micropegmatite that appears to be unique in the batholith rocks.

Diagonally bisecting the quadrangle is the Santa Ana River. North of the Santa
Ana River alluvial deposits are dominated by the distal parts of alluvial fans
emanating from the San Gabriel Mountains north of the quadrangle. Widespread
areas of the fan deposits are covered by a thin layer of wind blown sand.

Alluvial deposits in the triangular-shaped area between the Santa Ana River and
Temescal Wash are quite varied, but consist principally of locally derived
older alluvial fan deposits. These deposits rest on remnants of older, early
Quaternary or late Tertiary age, nonmarine sedimentary deposits that were
derived from both local sources and sources as far away as the San Bernardino
Mountains. These deposits in part were deposited by an ancestral Santa Ana
River. Older are a few scattered remnants of late Tertiary (Pliocene) marine
sandstone that include some conglomerate lenses. Clasts in the conglomerate
include siliceous volcanic rocks exotic to this part of southern California.
This sandstone was deposited as the southeastern-most part of the Los Angeles
sedimentary marine basin and was deposited along a rocky shoreline developed in
the granitic rocks, much like the present day shoreline at Monterey,
California. Most of the sandstone and granitic paleoshoreline features have
been removed by quarrying and grading in the area of Porphyry north to Highway
91. Excellent exposures in highway road cuts still remain on the north side of
Highway 91 just east of the 91-15 interchange and on the east side of U.S. 15
just north of the interchange.

South of Temescal Wash is a series of both younger and older alluvial fan
deposits emanating from the Santa Ana Mountains to the southeast. In the
immediate southwest corner of the quadrangle is a small exposure of sandstone
and pebble conglomerate of the Sycamore Canyon member of the Puente Formation
of early Pliocene and Miocene age and sandstone and conglomerate of undivided
Sespe and Vaqueros Formations of early Miocene, Oligocene, and late Eocene age.

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.