Geologic map and digital database of the Porcupine Wash 7.5 minute quadrangle, Riverside County, California By Robert E. Powell1 Digital preparation by Pamela M. Cossette1 Pamphlet, version 1.0 Open-File Report 01-30 Online version 1.0 http://geopubs.wr.usgs.gov/open-file/01-30 2001 U.S. Department of the Interior U.S. Geological Survey Prepared in cooperation with National Park Service California Division of Mines and Geology A product of the Southern California Areal Mapping Project ____________________ 1 U.S. Geological Survey, Western Region Earth Surface Processes Team W904 Riverside Avenue, Spokane, WA 99201-1087 For database limitations, see following page DATABASE LIMITATIONS Content This database is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This database, identified as "Geologic map and digital database of the Porcupine Wash 7.5 minute quadrangle, Riverside County, California," has been approved for release and publication by the Director of the U.S. Geological Survey. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on the condition that neither the USGS nor the United States Government may be held responsible for any damages resulting from its authorized or unauthorized use. Spatial Resolution Use of this digital geologic map should not violate the spatial resolution of the data. The Porcupine Wash database was developed using digital orthophotograph quarter quadrangles (DOQQs) as a base. DOQQs have a pixel resolution of 1 m and are accurate to a scale of 1:12,000 (1 in = 1,000 ft). Any enlargement beyond 1:12,000 exceeds the spatial resolution of the geologic data and should not be used in lieu of a more detailed site-specific geologic evaluation. Similarly, the digital topographic base map is derived from the U.S. Geological Survey, 1:24,000-scale Porcupine Wash 7.5 minute quadrangle (provisional edition, 1986); any enlargement beyond 1:24,000 exceeds the spatial resolution of the topographic data. Where the geologic data is used in combination with the topographic data, the resolution of the combined output is limited by the lower resolution of the topographic data. Where this database is used in combination with other data of higher resolution, the resolution of the combined output will be limited by the lower resolution of these data. Table of contents DATABASE LIMITATIONS ................................................. ii Content ........................................................ ii Spatial Resolution ............................................. ii INTRODUCTION ......................................................... 1 Overview ....................................................... 1 Purpose ........................................................ 2 Approach to assembling the database ............................ 2 DIGITAL GEOLOGIC MAP SPECIFICATIONS ................................. 5 Map coordinates and projection ................................. 5 Digital geologic data .......................................... 5 Data acquisition ......................................... 5 Map nomenclature and symbols ............................. 6 Faults ................................................... 6 Digital base layer data ........................................ 6 Topographic base map ..................................... 6 Digital orthophotgraph quarter quadrgangle base .......... 7 Geologic map with topographic base ............................. 7 Locational accuracy ............................................ 7 Spatial resolution ............................................. 8 GEOSPATIAL OBJECTS AND RELATIONSHIPS ................................. 9 Lines (arcs) ................................................... 10 Areas .......................................................... 11 Polygons .................................................. 11 Regions .................................................. 12 Points ......................................................... 13 ACKNOWLEDGMENTS ....................................................... 14 LIST OF URLs CITED ................................................... 15 U.S. Geological Survey websites ................................ 15 Software websites ............................................. 16 REFERENCES CITED ..................................................... 17 INTRODUCTION Overview Open-File Report 01-30 is a digital geologic data set that maps and describes the geology of the Porcupine Wash 7.5 minute quadrangle, Riverside County, southern California. The Porcupine Wash database is one of several quadrangle databases that are in preparation for Joshua Tree National Park and vicinity. These quadrangles are a subset of digital quadrangle maps being generated for the Southern California Areal Mapping Project (SCAMP), a cooperative project sponsored jointly by the U.S. Geological Survey (USGS) and the California Division of Mines and Geology (CDMG). The SCAMP maps are, in turn, part of the nation-wide digital geologic map coverage being developed by the National Geologic Map Database Project (NGMDB) of the USGS. The Porcupine Wash data set consists of a digital geologic map database accompanied by files containing a map plot, map graphics, and explanatory documents. The digital database was created using ARCVIEW, version 3.2, and ARC/INFO, version 7.2.1, commercial Geographical Information System (GIS) software designed by Environmental Systems Research Institute (ESRI), Redlands, California (http://www.esri.com). The data set includes the following files: * Readme file that describes how to obtain and access the data set and summarizes its contents; * This explanatory pamphlet that describes the approach used in building database, and discusses the purpose, content, and geologic framework of database; * Database files that represent the geology of the quadrangle in five map coverage layers, contain dictionaries of line and point symbols, and provide the symbolsets needed to plot the map; * FGDC-compliant metadata file that contains detailed technical descriptions of structure and content of the digital database * Geologic map: PostScript file that will plot a 1:24.000-scale, full-color geologic map of the Porcupine Wash quadrangle on its topographic base and accompanied by a map-margin explanation. The map margin includes a Description of Map and Database Units (DMU), a Correlation of Map and Database Units (CMU), and a key to point and line symbols. The map is displayed in a format that is similar to the USGS Miscellaneous Investigations (MF) map series, but has not undergone formal editing for strict conformity with technical layout guidelines. As yet, not all of the detailed information about geologic units contained in the map-margin explanation has been entered into the digital database; * Geologic map: Portable Document Format file that also contains a full-color geologic map of the Porcupine Wash quadrangle on its topographic base and accompanied by a map-margin explanation. The map graphic provides a full-resolution, navigable image for on-screen viewing and will also generate a paper plot; * Stand-alone versions of CMU and DMU in Portable Document Format files; * Topographic base map (Geotiff file): raster-scanned at 500 dpi; 1:24,000-scale Purpose The data set for the Porcupine Wash quadrangle has been prepared by SCAMP as part of an ongoing effort to create a regional GIS geologic database for southern California. This regional 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 Porcupine Wash database has been prepared in cooperation with the National Park Service as part of an ongoing project to provide Joshua Tree National Park (JTNP) with a geologic map base for use in managing Park resources and developing interpretive materials. The digital geologic map database for the Porcupine Wash 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 Porcupine Wash 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. Approach to assembling the database The Porcupine Wash geologic map and digital database has been assembled within the context of a broader effort to structure regional and national geologic map databases. At the national level, the USGS, in collaboration with the Association of American State Geologists, is in the process of developing a national geologic map database (http://ncgmp.usgs.gov/ngmdbproject/standards/). Necessary steps toward achieving a seamless national database include: (1) designing a data model to facilitate archiving, retrieving, and utilizing geologic information related to maps, (2) developing digital cartographic standards to represent that information, and (3) reaching consensus on science language used to describe the geologic content of the database. Simultaneously on a regional level, SCAMP, in cooperation with CDMG, is developing cartographic standards and science language for representing the varied and complex geology of southern Califonria (http://geology.wr.usgs.gov/wgmt/scamp/attrib/attribute.html). On a sub-regional level, SCAMP is producing the detailed geologic mapping (1:24,000-scale) that is needed by JTNP as part of its database for land-resource management and interpretive products. Science language and cartographic standards, developed as needed for SCAMP's sub-regional database for JTNP and vicinity, are applied consistently to the various quadrangle data sets that constitute the sub-regional database. Our approach in structuring the Porcupine Wash and other local digital geologic map data sets in JTNP has been guided by the choice to make preliminary versions available as soon as possible to the local user community. Subsequent versions of the Porcupine Wash geologic database will be produced as revisions are made to its content and structure. Geologic investigations in JTNP and vicinity are ongoing and the geologic content of the database is undergoing further evaluation, especially those features that relied largely on interpretation of aerial photographs. Moreover, pending completion and agency-wide adoption of a data model and of uniform standards and language, interim digital map products such as the Porcupine Wash database will lack the unifying structure anticipated for the national database. These interim products eventually will have to be made compatible with and part of regional and national databases. In making a geologic map, it is principally through the recognition and definition of units, and the nature of the contacts between units, that the geologist is able to contribute to our understanding of earth processes and to apply that understanding to societal issues. The geologist chooses the units that best represent and communicate his or her interpretation of the geologic features being studied. In a digital geologic map database, areas in which geologic units are exposed are represented by polygons that are defined in polygon attribute tables. Because any geologic map consists of units, unit labels are expedient primary keys for identifying units in the database polygon attribute table. Attribution of polygons using unit labels as primary keys results in a digital version of a traditional geologic map. This approach is useful for generating local geologic map databases, although it is likely to prove untenable for assembling local databases into large regional or national databases. In the Porcupine Wash geologic map database, version 1.0, polygon features are attributed using the map unit label as the primary key. In the polygon coverage, geologic contacts are represented as arcs and map-units as polygons, all attributed to provide a searchable digital map database and to generate a traditional end-product geologic map as a plot-file. For the plot-file, the geologic map is drawn from the ARC/INFO coverages and combined with a map-margin explanation generated externally in Adobe Illustrator and imported into the plot-file. Polygons in the digital map coverage are attributed with the particular map unit labels chosen to represent the geology of the Porcupine Wash quadrangle in the plot-file map. In making a geologic map, however, the geologist may encounter a body of rock or a deposit that can be represented as different units, depending on the purpose of the map. For an analog geologic map, the geologist typically chooses the unit designation that best characterizes those aspects of the rock body or deposit that he or she wishes to emphasize. The digital database, unlike the analog geologic map, allows the geologist to attribute any given polygon with more than one unit designation. The digital database is thereby capable of supporting the display of a folio of geologic maps that represent various characteristics of the earth materials present in the study area. For the Porcupine Wash database, the polygon features that constitute the map coverage attributed in the polygon attribute table are also grouped and attributed as region features to show additional units that are not represented on the plot-file map. These regions, constituting subclasses of the polygon coverage, display different arrays of units and emphasize other aspects of the geology of the quadrangle than those displayed and emphasized in the plot-file map included in this open-file report. For example, granitic units in the area of the Porcupine Wash quadrangle were deeply weathered in the Tertiary and further weathered in the Quaternary. Uplift and erosion have exposed areas of saprolite, weathered rock, and relatively fresh rock. All the polygons that represent these rocks are granite; some of the polygons also represent regolithic units. In another example, augen gneiss units in and around the Porcupine Wash quadrangle can be traced laterally into undeformed porphyritic granite. In a third example, for some uses it may be sufficient to represent the multiple mapped units of young and very young alluvial deposits as one unit that includes all Holocene alluvial deposits. Similarly, it may be desirable to represent the various mapped lithosomes of Proterozoic gneiss simply as Pinto Gneiss. Polygons of mapped units have been grouped variously by metamorphic grade, geomorphic setting, degree of hydrothermal alteration, and so forth. To create a fully attributed digital database in the context of a geologic map data model, the geologist deconstructs the traditional end-product geologic map and organizes its observational components into relational tables that are the archives of the database. These tables store and relate several distinct classes of information: (1) spatial attributes that record the geographic coordinates and topologic definitions of map features; (2) descriptive attributes that describe the various geologic characteristics (lithology, composition, texture, age, etc.) that define a map feature; (3) classification attributes that define correlations and groupings among map features; (4) metadata attributes that describe the source, quality, and character of data contained within the database; and (5) legend attributes that define the cartographic representation of map features. In the data model being designed for the national database, the various data contained in the spatial and descriptive archives are related to unit classification tables and are used to generate maps by means of map-legend relational tables. The traditional end-product map can be generated from these tables, but that particular map is only one of many end-product maps that could be generated. The Porcupine Wash database, version 1.0, comprises a limited array of tables that combines groups of data that eventually may be reorganized within the framework of the national geologic map data model. In this version, most of the spatial, descriptive, classifying, and cartographic data are contained in feature-attribute tables and a few dictionary tables. These attributes are listed in fields that are designed to be consistent with those for adjoining quadrangle databases in JTNP so that the 7.5 minute quadrangle databases eventually can be merged into 30 x 60 minute quadrangle database. For the purposes of cross-reference, eventual integration into a regional database, and utilization of SCAMP symbolsets, attributes are also listed in fields that link to SCAMP dictionaries and symbolsets. Tabulating and relating all the observational data and inferences that go into a geologic map is a daunting task that can greatly delay publication of digital map databases. This process is especially time-consuming if the field data has been recorded in notebooks rather than in digital form. As time permits, this information will be tabulated for subsequent versions of the Porcupine Wash database. DIGITAL GEOLOGIC MAP SPECIFICATIONS Map coordinates and projection The Porcupine Wash quadrangle lies between 115° 45( and 115° 52( 30(( west longitude and 33° 52( 30(( and 33° 45' north latitude. The 7.5-minute quadrangle is subdivided in latitude and longitude by a 2.5-minute tic grid that is marked on the topographic map. For the digital coverages, only the four corner tics that represent the geographic extent of the quadrangle have been generated mathematically in ARC/INFO. Geologic map information entered into ARC/INFO has been spatially registered using the four tics that represent the geographic extent of the quadrangle. Both the geologic database and the topographic base map are represented in polyconic projection referenced to the NAD27 datum. (See metadata 'Identification Information: Spatial Domain' and 'Spatial Reference Information' sections for detailed coordinate and projection information.) Digital geologic data Data acquisition The geologic map database contains original USGS data generated by detailed field observation and by interpretation of aerial photographs, 1:24,000 and 1:36,000 color photographs and 1:40,000 NAPP and 1:80,000 NHAP infrared photographs. Using ARCVIEW and its Image Analysis extension (version 1.0), geologic contacts, faults, and dikes were mapped on georeferenced USGS digital orthophotograph quarter quadrangles (DOQQs). Geologic data integrated onto the DOQQs includes bedrock mapping originally carried out on 1:36,000 color aerial photographs (USGS, 1973), compiled on 1:62,500 topographic maps, and further reduced to 1:125,000 (Powell, 1981). Additional observations were mapped on the 1:36,000 photographs during the course of wilderness studies of BLM-managed land in the Eagle Mountains (Powell and others, 1984). Mapping of Quaternary surficial deposits and augmentation of the older bedrock mapping is based on interpreting the 1:24,000 color aerial photographs (USGS, 1998) and the DOQQs. The lines generated in ARCVIEW were assigned basic attributes and exported to ARC/INFO where a line-polygon coverage was generated for the geology layer. This coverage was exported back to ARCVIEW as line and polygon shape files. Polygons were assigned their basic attributes in ARCVIEW and re-exported to ARC/INFO for more complete attribution in the full coverage. (See metadata 'Lineage' section for detailed information about the processes by which digital data was acquired.) Geologic information is stored in the database in the following ways. In mapping real-world geologic features into the database, geologic contacts, faults, and dikes are represented as lines (arcs), geologic units as polygons and regions, and site-specific structural data as points. ARC/INFO records the spatial coordinates and topology of each feature and links it to a polygon, arc, or point attribute table (.pat, .aat, and .pat, respectively) that uniquely identifies the feature. Feature-attribute tables can be related to other tables that further describe and classify the geologic features. In version 1.0, this additional tabular information is limited to dictionaries that contain the definitions of points and lines as described by Matti and others (1997a,b,c). (See 'Database structure and content' below; also see Esri, 1992, 1994) For the purposes of plotting and viewing, the digital geologic map is combined with a traditional map-margin explanation, including Description of Map and Database Units, Correlation of Map and Database Units, key to map symbols, and an index figure. The map-margin explanation contains detailed descriptive and interpretive information about geologic units that has not been entered into database tables in version 1.0. This information can be viewed on-screen or as a paper plot, either as part of the geologic map (pwash_map.pdf, pwash_map.ps) or as stand-alone versions of the DMU and CMU (pwash_dmu.pdf, pwash_cmu.pdf). 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 USGS 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?,