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The Geological Exploration of Arizona: The Role of State and Federal Surveys and the Geologic Map of Arizona

Article Author(s): 

Steve Reynolds
Jon Spencer
Stephen Richard
Phil Pearthree

The 1988 Geologic Map of Arizona

The 1969 map was compiled following completion of the Arizona County map series (1:375,000 scale) which made extensive use of aerial photographs. While the outlines of ranges were more accurately represented than on the 1924 map, the ages of many igneous rock units were still not well known. The field of isotope geochronology was in its early days of development and dating techniques had not been widely applied. This is especially apparent over large areas of western Arizona that were shown on the 1969 map as underlain by Cretaceous volcanic rocks, whereas these units are now known to be ~25-15 Ma.

In 1986, we concluded that enough new mapping and geochronologic data existed to begin compilation of a new state geologic map. Some large areas were still not sufficiently mapped, so it was decided to compile a somewhat generalized version at a scale of 1:500,000 and to print the map at 1:1,000,000. The AZGS took the lead in compiling the map, relying heavily on published and unpublished mapping by USGS and AZGS geologists and others. Such unpublished mapping included large areas of Proterozoic geology in the Transition Zone mapped by E. DeWitt and P. Anderson. Some areas around Phoenix were mapped or compiled directly at 1:100,000 by AZGS geologists. The compilation of the 1988 map occurred before widespread use of personal computers and so involved generalization of detailed maps onto partially transparent overlays, with the geology drawn using large pens and markers so that these lines would remain visible after being reduced by several generations using photocopy machines. The procedures used in compiling this map, and some of the main differences between this map and previous ones, are summarized elsewhere (Reynolds, 1989) and so are not repeated here. The 1:1,000,000-scale Geologic Map of Arizona was published in 1988 (Reynolds, 1988). The differences between the different versions of the Geologic Map of Arizona can be dramatic (Fig. 5).

Figure 5: Portrayals of the Little Harquahala and western Harquahala Mountains of west-central Arizona (from Reynolds, 1981).

As was done with the first two state geologic maps, it was decided that the map should be accompanied by a treatise on the geology of Arizona, conveying the state of knowledge for the time corresponding to the new geologic map. The Arizona Geologic Society and Bureau worked together to assemble such a summary, and the resulting book, Geologic Evolution of Arizona, was published as Arizona Geological Society Digest 17. This digest, commonly referred to simply as “the big red book,” contained papers on all the geologic time periods and many of the key topical issues relevant to Arizona (Jenney and Reynolds, 1989). This book remains the first place to begin when studying the geology of Arizona.

Geologic Studies Since 1988

The National Geologic Mapping Act was enacted in 1992 to replace and expand the COGEOMAP program. STATEMAP is the component of the act that funds State geological surveys to do new geologic mapping, with funds derived equally from State and Federal sources. This program has led to approximately 165 new 1:24,000 scale geologic maps in the past 20 years (Fig. 6), and a greatly improved understanding of the geology of large parts of Arizona. In addition to STATEMAP, funding for geologic mapping has occasionally been provided by diverse government agencies and mining companies (Fig. 7).

Figure 7. Geologic maps produced by the AZGS with finding from diverse sources not including STATEMAP, COGEOMAP, or internal (AZGS). Geologic map base from Richard et al. (2002).Figure 6. Geologic maps produced under the STATEMAP program, including maps proposed for new mapping in 2015. Geologic map base from Richard et al. (2002).

The 1988 State geologic map, which incorporated a large amount of geochronologic data, is an accurate representation of most of Arizona bedrock. This map was superseded by the 2000 map, which added detail and made changes to a few areas where rock ages or fault locations had become better known. Unlike previous maps, the 2000 map was compiled digitally, used shading instead of contours for topography, distinguished Quaternary faults from older, inactive faults, and included depth-to-bedrock contours for Cenozoic basins. The 2000 map also incorporated USGS mapping of 1:100,000 sheets (30 x 60-minute quadrangles) by G. Billingsley, B. Bryant, and others, which represented significant advances in understanding and a coherent view of large tracts of ground.

Very little new surficial geologic mapping was incorporated into the 2000 state geologic map, however, due to the limited time available to produce the map and the relative youth of the surficial mapping program. Because of this, most of the new surficial geologic mapping done at the AZGS since 1988 is not yet represented on a statewide geologic map of Arizona. Although no effort to produce a new State geologic map is currently planned, the next iteration of the State map will include a greatly improved representation of the Quaternary geology of Arizona, and will incorporate the results of the extensive geologic mapping that has been done since 2000.


The Geologic Map of Arizona Enters the Digital Era

Arizona Geological Survey use of computer technology in the management and presentation of geologic map information began in 1994 with acquisition of a digital version of the 1988 geologic map that had been prepared by the Bureau of Land Management using the MOSS Geographic Information System. Survey geologist Stephen Richard worked in collaboration with the University of Arizona Advanced Resource Technology laboratory (ART lab) to convert the data into ESRI ArcInfo format, using computers at the ART lab. The following years were a time of learning and experimentation with the new technology.

pcArcInfo was installed in 1996 when the AZGS received funding from the AASG-USGS National Cooperative Geologic Mapping Program to compile its first digital geologic map databases. The maps to be compiled were the four 1:100,000-scale USGS quadrangle that intersect to cover the Phoenix Metropolitan area-- Phoenix North, Phoenix South, Salome, and Little Horn. In 1997-1998, the Arizona Geological Survey did further refinement of the digital data for the Geologic Map of Arizona in preparation of AZGS Map 33-The Geologic Highway Map of Arizona. The geology on the highway geologic map was slightly generalized with the combination of some geologic units to simplify the legend. The supply of printed 1988 Geologic Maps began to run low in 1999 and a decision was made to update the map with new information for a new edition of the map. The survey had a digital database for the existing version and the GIS software and experience necessary. The digital technology offered a much more streamlined workflow from data compilation to a digital map, and especially in the pre-press work necessary to go from computer map layout to the color separation films necessary to print paper copies of the map.

In areas for which new maps were available, the database was updated with new information, particularly in the central part of the state from Morenci to Phoenix to the Colorado River, with updates in northwestern Arizona and the Inner Gorge of the Grand Canyon, and minor updates in many other parts of the map. The entire map was reviewed and edited for spatial accuracy by comparison with high-resolution satellite imagery and scanned images of source geologic maps.

Much work was done on the cartography. A shaded-relief base was generated using a digital elevation model provided by the USGS in Flagstaff to modulate the map-unit colors. The hydrography and road network base maps were slightly generalized from those created for the Highway Geologic Map. Depth-to-bedrock contours were added from a digital compilation generated in 1999 for another project. Because much of the necessary data was available in digital form, the entire update, pre-press preparation, and printing of the map was completed in about one year.

Senior Geologist
Arizona Geological Survey


Research Geologist
Arizona Geological Survey

Steve Reynolds

Arizona State University School of Earth and Space Exploration

Steve Richard

Research Geologist
Arizona Geological Survey

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