Pinterest icon

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 1969 State Geologic Map

The Arizona Bureau of Mines, in cooperation with the U.S.G.S., started work on a new state geologic map in 1956. Both agencies agreed to map certain areas, with much of the state ultimately being mapped by Bureau geologists Eldred D. Wilson and Richard T. Moore. Due to the limited time and personnel available for the project, the mapping was still only broad reconnaissance in many areas. Most of the mapping was done from vehicles or on foot, although horses and aerial photographs were used on occasion. As always, geologic mapping was not without its hazards. One Sunday, two Bureau geologists had permission to map on the Air Force gunnery range in southwestern Arizona. Suddenly, they were strafed by machine-gun fire from military jets. Unsubstantiated reports suggest that the jet pilots missed the Bureau vehicle on purpose, intending only to scare away the "intruders." In spite of such incidents, the geologic mapping of southwestern Arizona and the rest of the state was completed by 1960. Between 1924 and 1969, nearly all areas of the state were remapped in more detail.

In order to release the mapping results as rapidly as possible, the Bureau decided to publish geologic maps of each county, or of two adjoining counties, as the mapping was completed. Bureau geologists spent months in the office, transferring the geology from their field maps onto county base maps. The county base maps available to the Bureau were somewhat inaccurate, and topography on the maps had to be substantially modified for many areas. Bureau geologists did all of the drafting and color separation for the published county maps. Bureau mineralogist, R. T. O'Haire, used modified phonograph needles and other improvised drafting tools to prepare the maps for printing. The entire folio of county geologic maps was published by 1960, only four years after initiation of the project. In 1962, Eldred Wilson's version of "A Resume of the Geology of Arizona" was published to accompany the county map series. This represents another classic publication on the geology of Arizona, a read worthwhile to modern geologists.

The U.S.G.S. was responsible for compiling a state geologic map from the county maps provided by the Bureau. The Survey modified the reconnaissance geology on the county maps with more recent mapping when it was available. The resulting "Geologic Map of Arizona," published in 1969 by the Arizona Bureau of Mines and the U.S. Geological Survey (Wilson and others, 1969), bore little resemblance to its 1924 predecessor.

Geologic Studies Between 1969 and 1988

In the decade after the publication of the 1969 Geologic Map of Arizona, most geologic mapping was done by geologists from the U.S. Geological Survey. This mapping was concentrated in southeastern Arizona, especially in mountain ranges near the large copper deposits and in other mountain ranges that had been mapped in only reconnaissance. Such mapping and associated studies included those of M. Kreiger, H. Drewes, P. Hayes, S. Creasy, C. Thorman, T. Theodore, N. Banks, and others. USGS geologists also worked on the geology of the Transition Zone, including the work by C. Conway and others in the region near Payson. Mapping and stratigraphic studies also continued on the Colorado Plateau, both on the layered Paleozoic and Mesozoic sedimentary sequences, including the important studies by Ed McKee (1982). Detailed USGS mapping and geochronology also focused on San Francisco volcanic field, published as a series of maps by E. Wolfe, G. Ulrich. R. Holm, and others (e.g., Wolfe and others, 1987).

The Arizona Bureau of Mines was reorganized in 1978 to become the Arizona Bureau of Geology and Mineral Technology (AZBGMT). The change was made partly because the mission of state geological surveys in general had become more diverse, incorporating such areas of investigation as earthquakes, flood hazards and earth fissures. In 1980, the Arizona Bureau of Geology and Mineral Technology, under the direction of State Geologist Larry Fellows, hired S. Reynolds to begin work on a new geologic map and in 1982 hired J. Spencer to conduct bedrock mapping and studies of minerals deposits and regional tectonics. The AZBGMT began a major program of detailed geologic mapping at this time, mostly at 1:24,000 scale. Between approximately 1980 and 1994, AZBGMT (AZGS after 1988) geologic mapping focused on western Arizona, especially around La Paz County, because this area had some remarkably complex and poorly understood geology, and because its detachment-fault related, iron-oxide – copper – gold (IOCG) deposits were a new deposit type that was also not well understood. In addition, Chris Menges and Roger Morrison conducted statewide reconnaissance mapping and related studies of Quaternary deposits and landforms in the early 1980’s.

Figure 4. Geologic maps produced under the joint AZGS-USGS COGEOMAP program and by the AZGS western Arizona mineral resource and geologic framework program. Geologic map base from Richard et al. (2002).The mapping efforts of the Arizona Bureau received a large boost in 1984 with the initiation of the Cooperative Geologic Mapping (COGEOMAP) Program (Reinhardt and Miller, 1987). This program, a cooperative cost-sharing effort between the USGS and state geological surveys, enabled the Bureau to hire seasonal (i.e., winter) mappers, who worked with and under the direction of S. Reynolds, who received much insight and guidance from longtime Bureau geologist H. Wesley Peirce. A series of excellent geologists on the COGEOMAP team concentrated on the mountains ranges west and northwest of Phoenix, including the Little Horn, Big Horn, Vulture, and Wickenburg Mountains (Fig. 4). At the same time, S. Reynolds and J. Spencer, along with their colleague S. Richard, conducted detailed mapping farther west, in the Granite Wash, Buckskin, Harcuvar, and Little Harquahala Mountains (Fig. 4). The Bureau mapping resulted in major discoveries, including previously unknown thrust faults, huge folds that overturned the entire Paleozoic section, regional mid-Tertiary detachment faults with tens of kilometers of displacement, and previously unstudied highly extended, volcanic sections. In addition, P. Pearthree was hired in 1988 and, with his colleagues, began a major program to map Quaternary units (Fig. 4).

At the same time, USGS geologists conducted geologic mapping, mostly in other parts of the state. USGS geologists G. Billingsley, K. Wenrich, and others worked on the Colorado Plateau, while studies in southern Arizona were more topical in nature, such as the recognition and study of calderas by P. Lipman and associates. Important mapping and compilation studies were also conducted by the USGS on officially designated Wilderness Study Areas. Such studies led to mapping of many “roadless” areas scattered across southern and western Arizona.

Several other important mapping projects involved tribal lands in Arizona. The largest was the USGS studies of the Tohono O’odham (Papago) tribal lands in south-central Arizona. This project resulted in important papers and maps by G. Haxel, F. Gray, and others. These documented a vast region composed of mostly Jurassic rocks, Tertiary volcanic rocks, and complex structure. A similar project was in the San Carlos Reservation, between Safford and Globe, Arizona. Only some of the mapping from these tribal projects was available for the 1988 geologic map.

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

Visit the Arizona Experience Store
Visit the Arizona Experience Store
AZGS Digital Document Repository
AZGS Earth Fissure Viewer
State Geothermal
Additional Files/Images: 
Image icon reynolds6.png
geologic map, surveys