Geologic Map of the Cucamonga Peak 7.5' Quadrangle, San Bernardino County, California, USGS OFR 01-311

The data set for the Cucamonga Peak 7.5' quadrangle has been 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, andto utilize a Geographical 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.

The digital geologic map database for the Cucamonga Peak 7.5' quadrangle has
been created as a general-purpose data set that is applicable to other
land-related investigations in the earth and biological sciences. For example,
the U.S. Forest Service and the San Bernardino National Forest may use the map
and data base as a basic geologic data source for soil studies, mineral
resource evaluations, road building, biologicalsurveys, and general forest
management. The Cucamonga Peak database is not suitablefor site-specific
geologic evaluations at scales greater than 1:24,000 (1in = 2,000 ft.).

This data set maps and describes the geology of the Cucamonga Peak 7.5'
quadrangle, San Bernardino County, California. Created using Environmental
Systems Research Institute's ARC/INFO software, the database consists of the
following items: (1) a map coverage containing geologic contacts and units, (2)
a coverage containing site-specific structural data, (3) a coverage containing
geologic-unit label leaders and their associated 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) and the graphic produced by the PostScript plot file.

The Cucamonga Peak quadrangle includes part of the boundary between two major
physiographic provinces of California, the Transverse Ranges Province to the
north and the Peninsular Ranges Province to the south. The north part of the
quadrangle isin the eastern San Gabriel Mountains, and the southern part
includes an extensive quaternary alluvial-fan complex flanking the upper Santa
Ana River valley, the northernmost part of the Peninsular Ranges Province.

Thrust faults of the active Cucamonga Fault zone along the the south margin of
the San Gabriel Mountains are the rejuvenated eastern terminus of a major old
fault zone that bounds the south side of the western and central Transverse
Ranges (Morton and Matti, 1993). Rejuvenation of this old fault zone, including
the Cucamonga Fault zone, is apparently in response to compression in the
eastern San Gabriel Mountains resulting from initiation of right-lateral slip
on the San Jacinto Fault zone in the Peninsular Ranges. Within the northern
part of the quadrangle are several arcuate-in-plan faults that are part of an
antiformal, schuppen-like fault complex of the eastern San Gabriel Mountains.
Most of these arcuate faults are reactivated and deformed older faults that
probably include the eastern part of the San Gabriel Fault.

The structural grain within the San Gabriel Mountains, as defined by basement
rocks, is generally east striking. Within the Cucamonga Peak quadrangle, these
basement rocks include a Paleozoic schist and gneiss sequence which occurs as
large, continuous and discontinuous bodies intruded by Cretaceous granitic
rocks. Most of the granitic rocks are of tonalitic composition, and many are
mylonitic. South of the granitic rocks is a comple assemblage of
Proterozoic(?) metamorphic rocks, at least part of which is metasedimentary.
This assemblage is intruded by Cretaceous tonalite on its north side, and by
charnockitic rocks near the center of the mass. The charnockitic rocks are in
contact with no other Cretaceous granitic rocks. Consequently, their relative
position in the intrusive sequence is unknown. The Proterozoic(?) assemblage
was metamorphosed to upper amphibolite and lower granulite grade, and
subsequently to a lower metamorphic grade. It is also intensely deformed by
mylonitization characterized by an east-striking, north-dipping foliation, and
by a pronounced subhorizontal lineation that plunges shallowly east and west.

The southern half of the quadrangle is dominated by extensive, symmetrical
alluvial-fan complexes, particularly two emanating from Day and Deer Canyons.
Other Quaternary units ranging from early Pleistocene to recent are mapped, and
represent alluvial-fan, landslide, talus, and wash environments.

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 supercedes 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
Gabriel Mountains. The digital map was compiled on a base-stable cronoflex
copy of the Cucamonga Peak 7.5' topographic base and then scribed. This scribe
guide was used to make a 0.007 mil blackline clear-film, from which lines and
point were hand digitized. Lines, points, and polygons were 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.