|Abstract:||This data set maps and describes the geology of the Pinto Mountain 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 parts of the northeastern Hexie Mountains, central Pinto Mountains, and central Pinto Basin.
The quadrangle is underlain by a basement terrane comprising Proterozoic metamorphic rocks, Mesozoic plutonic rocks, and Mesozoic and Mesozoic and (or) Cenozoic hypabyssal dikes. The basement terrane is capped by a widespread Tertiary erosion surface preserved in remnants in the Hexie and Pinto Mountains and buried beneath Cenozoic deposits in Pinto Basin. Locally, a cover of Miocene sedimentary deposits and basalt overlie the erosion surface. Quaternary and (or) Tertiary lacustrine deposits crop out in the center of Pinto Basin and interfinger laterally with sandstone, conglomerate, and debris flows originating in the Pinto and Hexie Mountains. A sequence of at least three Quaternary pediments is planed into the north piedmonts of the Hexie and Eagle Mountains, each in turn overlain by successively younger residual and alluvial, surficial deposits. The Tertiary erosion surface is deformed and broken by north-northwest-trending, high-angle, dip-slip faults in the Pinto and Eagle Mountains and an east-west trending system of high-angle dip- and left-slip faults along the range fronts facing Pinto Basin. In and around the Pinto Mountain quadrangle, faults of the north-northwest-trending set displace Miocene sedimentary rocks and basalt deposited on the Tertiary erosion surface and some of the faults may offset 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 Pinto Mountain database was created using ARCVIEW and ARC/INFO, which are geographical information system (GIS) software products of Envronmental Systems Research Institute (ESRI). The database comprises eight coverages: (1) a geologic layer showing the distribution of geologic contacts and units; (2) a structural layer showing the distribution of faults (arcs) and fault ornamentation data (points); (3) a layer showing the distribution of dikes (arcs); structural point data layers showing (4) bedding attitudes, (5) foliation attitudes, (6) lineations, (7) minor fold axes; and (8) cartographic map elements, including unit label leaders and geologic unit annotation. The dataset also includes a scanned topographic base at a scale of 1:24,000. Within the database coverages, geologic contacts , faults, and dikes are represented as lines (arcs and routes), geologic units as areas (polygons and regions), and site-specific data as points. Polygon, region, arc, route, and point attribute tables uniquely identify each geologic datum and link it to descriptive tables that provide more detailed geologic information.
The digital database is accompanied by two derivative maps: (1) A portable document file (.pdf) containing a navigable graphic of the geologic map on a 1:24,000 topographic base and (2) a PostScript graphic-file containing the geologic map on a 1:24,000 topographic base. Each of these map products is accompanied by a marginal explanation consisting of a Description of Map Units (DMU), a Correlation of Map Units (CMU), and a key to point and line symbols. The database is further accompanied by three document files: (1) a readme that lists the contents of the database and describes how to access it, (2) a pamphlet file that describes the geology of the quadrangle, and (3) this metadata file.