Geologic Map of the Telegraph Peak 7.5' quadrangle, San Bernardino County, California

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.