Geologic Map of the Telegraph Peak 7.5 min. quadrangle, San Bernardino County, California, USGS OFR 01-293

The data set for the Telegraph 7.5' quadrangle was prepared under the U.S.
Geological Survey Southern California Areal Mapping Project (SCAMP) and the
California Division of Mines as part of an ongoing effort to develop a regional
geologic framework of southern California, and to 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 Telegraph 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, San Bernardino National Forest, may use the map and database as
a basic geologic data source for soil studies, mineral resource evaluations,
road building, biological surveys, and general forest management. The database
is not suitable for site-specific geologic evaluations at scales greater than
1:24,000 (1 in = 2,000 ft).

This data set maps and describes the geology of the Telegraph 7.5' quadrangle,
San Bernardino County, California. Created using Environmental Systems Research
Institute's ARC/INFO software, the data base consists of the following items:
(1) a double precision 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), Description of Map Units (DMU), and the graphic produced by the
PostScript plot file.

The Telegraph Peak quadrangle is located in the eastern San Gabriel Mountains
part of the Transverse Ranges Province of southern California. The generally
east-striking structural grain characteristic of the crystalline rocks of much
of the San Gabriel Mountains is apparent, but not well developed in the
Telegraph Peak quadrangle. Here, the east-striking structural grain is
somewhat masked by the northwest-striking grain associated with the San Andreas
Fault zone.

Faults within the quadrangle include northwest-striking, right-lateral
strike-slip faults of the San Andreas system. The active San Andreas Fault,
located in the northern part of the quadrangle, dominates the younger
structural elements. North of the San Andreas Fault is the inactive Cajon
Valley Fault that was probably an early strand of the San Andreas system. It
was active during deposition of the middle Miocene Cajon Valley Formation.
South of the San Andreas, the Punchbowl Fault, which is probably a
long-abandoned segment of the San Andreas Fault (Matti and Morton, 1993), has a
sinuous trace apparently due to compression in the eastern San Gabriel
Mountains that post-dates displacement on the fault. The Punchbowl Fault
separates two major subdivisions of the Mesozoic Pelona Schist and is
left-laterally offset by a northeast-striking fault in the northwestern part of
the quadrangle. Within the Punchbowl Fault zone is a thin layer of highly
deformed basement rock, which is clearly not part of the Pelona Schist. To the
southeast, in the Devore quadrangle, this included basement rock attains a
thickness of several hundred feet. Along strike to the northwest, Tertiary
sedimentary rocks are included within the fault zone. South of the Punchbowl
Fault 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, and probably
include the eastern part of the San Gabriel Fault.

The Vincent Thrust of late Cretaceous or early Tertiary age separates the
Pelona Schist in the lower plate from a heterogeneous basement complex in the
upper plate. Immediately above the Vincent Thrust is a variable thickness of
mylonitic rock generally interpreted as a product of displacement on the
thrust. The upper plate includes two Paleozoic units, a schist and gneiss
sequence and a schist, quartzite, and marble metasedimentary sequence. Both
sequences are thrust over the Mesozoic Pelona Schist along the Vincent Thrust,
and intruded by Tertiary (late Oligocene) granitic rocks, granodiorite of
Telegraph Peak, that also intrude the Vincent Thrust. The Pelona Schist
consists mostly of greenschist to amphibolite metamorphic grade meta-basalt
(greenschist and amphibolite) and meta-graywacke (siliceous and white mica
schist), with minor impure quartzite and marble, in which all primary
structures have been destroyed and all layering transposed. Cretaceous
granitic rocks, chiefly tonalite, intrude the schist and gneiss sequence, but
not the Pelona Schist or the Vincent Thrust.

North of the San Andreas Fault, bedrock units consist of undifferentiated
Cretaceous tonalite, here informally named tonalite of Circle Mountain, with
some included small boldies of gneiss and marble. These basement rocks are the
westward continuation of rocks of the San Bernardino Mountains and not rocks of
the San Gabriel Mountains south of the San Andreas Fault. Also north of the
San Andreas Fault are the Oligocene Vaqueros Formation, middle Miocene Cajon
Valley Formation, and Pliocene rocks of Phelan Peak. The latter two formations
are divided into several conglomerate and arkosic sandstone subunits. In the
northeastern corner of the quadrangle, the rocks of Phelan Peak are
unconformably overlain by the Quaternary Harold Formation and Shoemaker Gravel.
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 Telegraph 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.