Geologic Map and Digital Database of the Conejo Well 7.5 Minute Quadrangle, Riverside County, California, USGs OFR 01-31

The data set for the Conejo Well quadrangle has been prepared by the Southern
California Areal Mapping Project (SCAMP), a cooperative project sponsored
jointly by the U.S. Geological Survey and the California Division of Mines and
Geology. The Conejo Well data set represents part of an ongoing effort to
create a regional GIS geologic database for southern California. This regional
digital database, in turn, is being developed as a contribution to the National
Geologic Map Database of the National Cooperative Geologic Mapping Program of
the USGS. The Conejo Well database has been prepared in cooperation with the
National Park Service as part of an ongoing project to provide Joshua Tree
National Park with a geologic map base for use in managing Park resources and
developing interpretive materials.

The digital geologic map database for the Conejo Well quadrangle has been
created as a general-purpose data set that is applicable to land-related
investigations in the earth and biological sciences. Along with geologic map
databases in preparation for adjoining quadrangles, the Conejo Well database
has been generated to further our understanding of bedrock and surficial
processes at work in the region and to document evidence for seismotectonic
activity in the eastern Transverse Ranges. The database is designed to serve as
a base layer suitable for ecosystem and mineral resource assessment and for
building a hydrogeologic framework for Pinto Basin.

This data set maps and describes the geology of the Conejo Well 7.5 minute
quadrangle, Riverside County, southern California. The quadrangle, situated in
Joshua Tree National Park in the eastern Transverse Ranges physiographic and
structural province, encompasses part of the northern Eagle Mountains and part
of the south flank of Pinto Basin. It is underlain by a basement terrane
comprising Proterozoic metamorphic rocks, Mesozoic plutonic rocks, and Mesozoic
and Mesozoic or Cenozoic hypabyssal dikes. The basement terrane is capped by a
widespread Tertiary erosion surface preserved in remnants in the Eagle
Mountains and buried beneath Cenozoic deposits in Pinto Basin. Locally, Miocene
basalt overlies the erosion surface. A sequence of at least three Quaternary
pediments is planed into the north piedmont of the Eagle Mountains, each in
turn overlain by successively younger residual and alluvial deposits.

The Tertiary erosion surface is deformed and broken by
north-northwest-trending, high-angle, dip-slip faults in the Eagle Mountains
and an east-west trending system of high-angle dip- and left-slip faults. In
and adjacent to the Conejo Well quadrangle, faults of the northwest-trending
set displace Miocene sedimentary rocks and basalt deposited on the Tertiary
erosion surface and Pliocene and (or) Pleistocene deposits that accumulated on
the oldest pediment. Faults of this system appear to be overlain by
Pleistocene deposits that accumulated on younger pediments. East-west trending
faults are younger than and perhaps in part coeval with faults of the
northwest-trending set.

The Conejo Well database was created using ARCVIEW and ARC/INFO, which are
geographical information system (GIS) software products of Environmental
Systems Research Institute (ESRI). The database consists of the following
items: (1) a map coverage showing faults and geologic contacts and units, (2) a
separate coverage showing dikes, (3) a coverage showing structural data, (4) a
point coverage containing line ornamentation, and (5) a scanned topographic
base at a scale of 1:24,000. The coverages include attribute tables for
geologic units (polygons and regions), contacts (arcs), and site-specific data
(points). The database, accompanied by a pamphlet file and this metadata file,
also includes the following graphic and text products: (1) A portable document
file (.pdf) containing a navigable graphic of the geologic map on a 1:24,000
topographic base. The map is accompanied by a marginal explanation consisting
of a Description of Map and Database Units (DMU), a Correlation of Map and
Database Units (CMU), and a key to point-and line-symbols. (2) Separate .pdf
files of the DMU and CMU, individually. (3) A PostScript graphic-file
containing the geologic map on a 1:24,000 topographic base accompanied by the
marginal explanation. (4) A pamphlet that describes the database and how to
access it. Within the database, geologic contacts , faults, and dikes are
represented as lines (arcs), geologic units as polygons and regions, and
site-specific data as points. Polygon, arc, and point attribute tables (.pat,
.aat, and .pat, respectively) uniquely identify each geologic datum and link it
to other tables (.rel) that provide more detailed geologic information.

Map nomenclature and symbols

Within the geologic map database, map units are identified by standard geologic
map criteria such as formation-name, age, and lithology. The authors have
attempted to adhere to the stratigraphic nomenclature of the U.S. Geological
Survey and the North American Stratigraphic Code, but the database has not
received a formal editorial review of geologic names.

Special symbols are associated with some map units. Question marks have been
added to the unit symbol (e.g., QTs?, Jmi?) and unit name where unit
assignment based on interpretation of aerial photographs is uncertain. Question
marks are plotted as part of the map unit symbol for those polygons to which
they apply, but they are not shown in the CMU or DMU unless all polygons of a
given unit are queried. To locate queried map-unit polygons in a search of
database, the question mark must be included as part of the unit symbol. In
some polygons, multiple units crop out in individual domains that are too small
or too intricately intermingled to distinguish at 1:24,000, or for which
relations are not well documented. For these polygons, unit symbols are
combined using plus (+) signs (e.g., Qyaos + Qyas2) in the LABL and PLABL
items.

Geologic map unit labels entered in database items LABL and PLABL contain
substitute characters for conventional stratigraphic age symbols: Proterozoic
appears as 'Pr' in LABL and as '<' in PLABL, Triassic appears as 'Tr' in LABL
and as '^' in PLABL. The substitute characters in PLABL invoke their
corresponding symbols from the GeoAge font group to generate map unit labels
with conventional stratigraphic symbols.