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The growth in the use of Geographic Information Systems (GIS) has highlighted the need for digital geologic maps that have been attributed with information about age and lithology. Such maps can be conveniently used to generate derivative maps for manifold special purposes such as mineral-resource assessment, metallogenic studies, tectonic studies, and environmental research. This report is part of a series of integrated geologic map databases that cover the entire United States.
Three national-scale geologic maps that portray most or all of the United States already exist; for the conterminous U.S., King and Beikman (1974a,b) compiled a map at a scale of 1:2,500,000, Beikman (1980) compiled a map for Alaska at 1:2,500,000 scale, and for the entire U.S., Reed and others (2005a,b) compiled a map at a scale of 1:5,000,000. A digital version of the King and Beikman map was published by Schruben and others (1994). Reed and Bush (2004) produced a digital version of the Reed and others (2005a) map for the conterminous U.S. The present series of maps is intended to provide the next step in increased detail. State geologic maps that range in scale from 1:100,000 to 1:1,000,000 are available for most of the country, and digital versions of these state maps are the basis of this product.
The digital geologic maps presented here are in a standardized format as ARC/INFO export files and as ArcView shape files. Data tables that relate the map units to detailed lithologic and age information accompany these GIS files. The map is delivered as a set of 1:250,000-scale quadrangle files. To the best of our ability, these quadrangle files are edge-matched with respect to geology. When the maps are merged, the combined attribute tables can be used directly with the merged maps to make derivative maps.
The digital datasets that form the basis for this product were compiled and created using existing published and unpublished data. Alaska spatial and text databases in this series are all linked through the use of a field, called nsaclass, that is related to the age and lithology of the map units contained on each map. (Note: database field names are in italics). Two fields which have been added to the polygon attribute table (PAT) and also in the text databases, nsaclass and qclass, are used to store information that correlates individual map units between sources. Nsaclass is used to make regional unit assignments and generally reflects a known or an inferred correlation of map units. For example, all "Surficial deposits, undivided" are assigned an nsaclass code of 100. The schema for nsaclass was developed as regional maps throughout Alaska were compiled and reflects an iterative process. As new or additional information became available, the nsaclass code for a particular map unit may have changed, either to reflect lumping or finer separation of map units. Nsaclass is used to cover the entire geologic time scale, whereas the similar qclass is restricted to and provides finer detail for Quaternary map units. A source field and a field called nsamod are also added. Nsamod provides information with respect to hydrothermal alteration or contact metamorphism of a map unit, either for the entire unit or on an individual polygon basis. In this way, the nsaclass field needs only to store the primary map unit information. Source is a coded reference citation, indicating the manuscript or other source for the information. The format for source is XX###, where XX is the two letter quadrangle code (CAPITAL letters) and ### is a three digit number (using leading zeros) to indicate a specific reference. Finally, a field called lith2 is in the PAT as a scratch field; no uniform schema has been developed for this field.
The standardized attribute tables were generated by extracting information from the legends of the source maps and also data generated by the compilers of this regional map. Thus, the age and lithology information in the attribute tables may, in some cases, conflict with the information on the legends of the original source maps that may have been compiled decades ago. The standardized attribute tables record an abstracted map unit description, lithologic and age information, and references.
The spatial databases are provided in the native UTM projection of the sources as well as geographic coordinates. The UTM projection parameters are described in the metadata (note that the datum is NAD’27). Because of limitations of the UTM projection for regional maps, provided here are the parameters for the Albers Equal Area projection commonly used in Alaska for regional or state-wide products:
Projection: Albers equal Area
ArcView files can be viewed with the free viewer, ArcExplorer, which can be downloaded from http://www.esri.com/software/arcexplorer/.
King, P.B., and Beikman, H.M., 1974a, Geologic map of the United States: Reston, Va., U.S. Geological Survey, scale 1: 2,500,000.
King, P.B., and Beikman, H.M., 1974b, Explanatory text to accompany the geologic map of the United States: U.S. Geological Survey Professional Paper 901, 40 p.
Reed, J.C., Jr., Wheeler, J.O., and Tucholke, B.E., 2005a, Geologic map of North America: Boulder, Colo., Geological Society of America, Decade of North American Geology, 3 sheets, scale 1:5,000,000.
Reed, J.C., Jr., Wheeler, J.O., and Tucholke, B.E., 2005b, Geologic map of North America – Perspectives and explanation: Boulder, Colo., Geological Society of America, Decade of North American Geology, p. 1-28.
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