U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY PRELIMINARY GEOLOGIC MAP EMPHASIZING BEDROCK FORMATIONS IN ALAMEDA COUNTY, CALIFORNIA: A DIGITAL DATABASE By R.W. Graymer(1), D.L. Jones(2), and E.E. Brabb(1) Open-File Report 96-252 1996 This report 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 "Preliminary geologic map emphasizing bedrock formations in Alameda County, California: A digital database," has been approved for release and publication by the Director of the USGS. 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 condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. (1) Western Region Mapping Group, U.S. Geological Survey, 345 Middlefield Rd., M/S 975, Menlo Park, CA 94025 (2) Department of Geology and Geophysics, University of California, Berkeley, CA 94720 Introduction This Open-File report is a digital geologic map database. This pamphlet serves to introduce and describe the digital data. There is no paper map included in the Open-File report. The report does include, however, PDF files of a geologic map sheet and an explanation sheet. This digital map database, compiled from previously open-filed U.S. Geological Survey reports (Graymer and others, 1994), other published and unpublished data (see Sources of Data in algeo.txt), and substantial new mapping by the authors in about two- thirds of the county, represents the general distribution of rocks and geologic structures in Alameda County. Together with the accompanying text file (algeo.txt), it provides current information on the stratigraphy and structural geology of the area covered. The database delineates map units that are identified by general age and lithology following the stratigraphic nomenclature of the U.S. Geological Survey. Quaternary units are almost all combined into an undifferentiated unit, with the exception of some artificial deposits (Qar), some early Holocene or late Pleistocene gravel deposits (Qt, Qoa) and a few large landslides (Qls). More specific information about the units may be available in the original publications. The scale of the source maps limits the spatial resolution (scale) of the database to 1:50,000 or smaller. The content and character of the database, as well as three methods of obtaining the database, are described below. For Those Who Don't Use Digital Geologic Map Databases For those interested in the geology of Alameda County who do not use an ARC/INFO compatible Geographic Information System (GIS), two PDF files containing images of much of the data in the digital database have been included. Database Contents The digital database consists of the geologic map database itself and the supporting data, including a fault map, base maps, map explanation, geologic description, and references. The digital maps, or coverages, along with their associated INFO directory have been converted to ARC/INFO export files. ARC export files promote ease of data handling, and are usable by some Geographic Information Systems in addition to ARC/INFO (see below for a discussion of working with export files). The ARC export files and the associated ARC/INFO coverages and directories, as well as the additional digital material included in the database, are described below: The geologic map database consists of two ARC coverages, which have been converted to uncompressed ARC/INFO export files: ARC/INFO Resultant Description of Coverage export file Coverage -------------- ----------- -------------------------------- al_um-py.e00 al_um-py/ Faults, depositional contacts, and rock unit identities al_um-sr.e00 al_um-sr/ Fold axes, strike and dip information The database directory also includes the following supporting directory, ARC coverages, and files: Directory: info/ INFO directory containing files supporting the databases. This directory is not included in the database release, but is created in the process of converting the export files into ARC coverages. ARC Coverages, which have been converted to uncompressed ARC/INFO export files: ARC/INFO Resultant Description of Coverage export file Coverage -------------- ----------- -------------------------------- al_um-flt.e00 al_um-flt/ Map of faults and fold axes in Alameda County with additional database items containing fault name (if any). al_so.e00 al_so/ Index map of sources of data (see algeo.txt for the corresponding list of sources of data). al_as.e00 al_as/ Index map of Assemblages in Alameda County (see algeo.txt for explanation of Assemblages). al_quad.e00 al_quad/ Index map of quadrangles in Alameda County. al_corr.e00 al_corr/ Correlation table for the units in this map database. al_dr.e00 al_dr/ Drainage base map. al_cu.e00 al_cu/ Cultural base map. al_topo.e00 al_topo/ Topographic contours base map. ASCII text files, including explanatory text, ARC/INFO key files, PostScript plot files, and a ARC Macro Language file for conversion of ARC export files into ARC coverages: algeo.txt A text only computer file containing detailed unit descriptions and geological information, plus sources of data and references cited. aldb.txt This file. alkey.un Together, these key files produce a alkey.ln plottable or displayable map explanation alkey.mrk and key. import.aml ASCII text file in ARC Macro Language to convert ARC export files to ARC coverages in ARC/INFO. Database Release Format The database was compiled in ARC/INFO, a commercial Geographic Information System (Environmental Systems Research Institute, Redlands, California), and is stored in uncompressed ARC export format (ARC/INFO version 6) in a compressed UNIX tar (tape archive) file. Tar and uncompress utilities are therefore required to extract the database from the tar file. These utilities are included in most UNIX systems, and can be obtained free of charge via the Internet from Internet Literacy's Common Internet File Formats Web page (http://www.matisse.net/files/formats.html). ARC/INFO export files (files with the .e00 extension) can be converted into ARC/INFO coverages (see below) and can be read by some other Geographic Information Systems, such as MapInfo via ArcLink. The digital compilation was done using version 7.0.3 of ARC/INFO with version 3.0 of the menu interface ALACARTE (Fitzgibbon and Wentworth, 1991, Fitzgibbon, 1991, Wentworth and Fitzgibbon, 1991). Extracting the Database from the Tar file If you obtained the database from the web page: uncompress al_g1.tar.Z -makes a 97 MB uncompressed tar file al_g1.tar cd local_directory2 -go to the directory that will hold the directory algeo (if different from local_directory) tar xvfb {path to tar file}/al_g1.tar 20 -extract the algeo directory from the tar file. This process will create a directory "/algeo" that contains the ARC export files and supporting files as described above. Converting ARC export files ARC export files are converted to ARC coverages using the ARC command IMPORT with the option COVER. In order to ease conversion and to maintain naming conventions, we have included an ASCII text file in ARC Macro Language that will convert all of the export files in the database into coverages and create the associated INFO directory. From the ARC command line type: Arc: &run import.aml ARC export files can also be read by some other Geographic Information Systems. Please consult your GIS documentation to see if you can use ARC export files and the procedure to import them. Digital Compilation The geologic map information was digitized from stable originals of the geologic maps at 1:24,000 scale for newly mapped areas and 1:62,500 scale for the rest of the county. The author manuscripts (ink on mylar) were scanned using a Tektronix 4991 monochrome scanner with a resolution of 304.8 dots per inch. The scanned images were vectorized and transformed from scanner coordinates to projection coordinates with digital tics placed by hand at quadrangle corners. The scanned lines were edited interactively by hand using ALACARTE, color boundaries were tagged as a contact or fault as appropriate, and scanning artifacts visible at 1:50,000 were removed. Base Maps Base Map layers were prepared from scale-stable printing negatives of the U.S. Geological Survey San Francisco (1978 edition), Stockton (1989 edition), and San Jose (1978 edition) 1:100,000 topographic maps, which have a 50 meter contour interval. Scanned and vectorized images were transformed from scanner coordinates to projection coordinates with digital tics placed by hand at map corners. The images were then trimmed interactively by hand using ALACARTE to conform to the area of the geologic coverages, and the four portions were combined. Small mismatches at the boundaries caused by slight differences in the original scans remain in the three base map coverages. These base map layers are digital images but no information other than location is attached to the lines. The base maps are provided for reference only. Faults and Landslides This map is intended to be of general use to engineers and land-use planners. However, its small scale does not provide sufficient detail for site development purposes. In addition, this map does not take the place of fault-rupture hazard zones designated by the California State Geologist (Hart, 1988). Similarly, the database cannot be used to identify or delineate most landslides in the region, because only some large landslides are shown. For a more complete depiction of landslide distribution, see Nilsen and others (1979). Spatial Resolution Uses of this digital geologic map should not violate the spatial resolution of the data. Although the digital form of the data removes the constraint imposed by the scale of a paper map, the detail and accuracy inherent in map scale are also present in the digital data. The fact that this database was edited at a scale of 1:50,000 means that higher resolution information is not present in the dataset. Plotting at scales larger than 1:50,000 will not yield greater real detail, although it may reveal fine-scale irregularities below the intended resolution of the database. Similarly, 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. Note that in contrast to the geologic coverages, the base map layers have a resolution of 1:100,000, so significant discrepancies with the geologic coverages are possible. The base map layers are provided for reference only. Acknowledgments We are grateful to Kristin R. McDougall, William V. Sliter, David Bukry, John A. Barron, and William P. Elder of the U.S. Geological Survey for the paleontologic determinations necessary to help establish the ages of rocks in Alameda County. Scott D. Hill, Superintendent of Watershed, kindly provided access to lands owned by the East Bay Regional Park District. Leo T. Bauer, Alameda Division Manager, provided access to lands owned by the San Francisco Water Department. G. Doug Myron, U.S. Geological Survey, arranged for access to some of the private land in the study area. Carl M. Wentworth, Todd T. Fitzgibbon, and Geoffrey A. Phelps graciously provided assistance with Alacarte and ARC/INFO. James J. Lienkaemper shared some of his knowledge of the creeping strand of the Hayward fault, and David. H. Oppenheimer provided maps showing the location of epicenters. Larry J. Dickerson and John M. Parker, U.S. Geological Survey volunteers, provided extensive support in the field. We are grateful to AmeriCorps for sponsoring Dickerson, Parker, and Brabb. References Cited Fitzgibbon, T.T., 1991, ALACARTE installation and system manual (version 1.0): U.S. Geological Survey, Open-File Report 91-587B. Fitzgibbon, T.T., and Wentworth, C.M., 1991, ALACARTE user interface - AML code and demonstration maps (version 1.0): U.S. Geological Survey, Open-File Report 91-587A. Graymer, R.W., Jones, D.L., Brabb, E.E., and Helley, E.J., 1994, Preliminary geologic map of the Niles 7.5-minute quadrangle, Alameda County, California: U.S. Geological Survey Open-File Report 94-132, 3 sheets, 1:24,000 scale. Hart, E.W., 1988, Fault-rupture hazard zones in California; Alquist-Priolo Special Studies Zones Act of 1972 with index to special studies zones maps: California Division of Mines and Geology Special Publication 42. Lienkaemper, J.J., 1992, Map of recently active traces of the Hayward fault, Alameda and Contra Costa Counties, California: U.S. Geological Survey Misc. Field Studies Map MF-2196, 13 p., 1 sheet, 1:24,000 scale. Nilsen, T.H., Wright, R.H., Vlasic, T.C., and Spangle, W.E., 1979, Relative slope stability and land-use planning in the San Francisco Bay region, California: U.S. Geological Survey Professional Paper 944, 96 p. Wentworth, C.M., and Fitzgibbon, T.T., 1991, ALACARTE user manual (version 1.0): U.S. Geological Survey, Open-File Report 91-587C.