Introduction
The geological exploration of Arizona was pioneered in the latter half of the 1800s by geologists, such as John Wesley Powell, who accompanied the early expeditions into the territory. These geologists found the region to be a wild and untamed frontier. They were confronted by enormous problems – uncharted mountains and canyons, lack of dependable transportation (mules included), and occasional encounters with unfriendly tribes. By necessity, many of the early geologists were as much interested in the weather, watering holes, wildlife, and human inhabitants of the region as they were in the geology. Their reports make fascinating reading and provide a vivid perspective on the Arizona of yesterday.
As the early geologists explored Arizona, they encountered many new and exciting geologic features that had not been previously described. In order to fully document the size, shape, and characteristics of these features, they constructed geologic maps, drew elaborate sketches (figure 1), and wrote pages of detailed descriptions. The geology of Arizona began to be understood through these efforts. Eventually, enough was known to produce a geologic map of the entire state. The first state geologic map was published in 1924 (Darton and others, 1924), only 12 years after Arizona's statehood. A revised, greatly improved version was published in 1969 (Wilson and others, 1969). It too became significantly outdated by more detailed geologic studies and more radiometric dates for rocks in Arizona, so a third version was published in 1988 (Reynolds, 1988). When the Arizona Geological Survey sold all of the available copies of the third version, a fourth version was released in 2000 (Richard and others, 2000). Currently, the map is maintained and updated in digital form (Richard and others, 2002; http://www.azgs.az.gov/services_azgeomap.shtml)
In this article, we present some of the highlights of the geological exploration of Arizona, focused on the role of early surveys by the U.S. Government and later geologic studies by the U.S. Geological Survey (USGS) and by the Arizona Geological Survey (AZGS) and its predecessors (the Arizona Bureau of Mines and Arizona Bureau of Geology and Mineral Technology). The first part of this article is largely derived from an article (Reynolds, 1981) originally published in Fieldnotes, a predecessor of Arizona Geology.
The Essence of a Geologic Map
A geologic map is a graphic representation of the rock units and geologic features that occur at the surface of the earth. Each area of the earth's surface is unique and must be individually examined and mapped. Reasonably detailed geologic mapping can be accomplished fairly quickly in regions of relatively uncomplicated geology, such as the Colorado Plateau of northern Arizona. In contrast, mapping can be excruciatingly slow in western Arizona where the geology is very complex. In either case, the information gained from geologic mapping is essential to our modern-day society and its dependence on things geologic (Peirce, 1981).
Some of the main attributes of a geologic map are illustrated by comparing Figures 2 and 3. Figure 2 is a three-dimensional perspective of geologic features in the San Francisco volcanic field north of Flagstaff, Arizona. The most striking feature is a dark-colored lava flow that dominates the center of the image. The nearly circular feature at the south end of the lava flow is SP Crater, a well-preserved scoria (cinder) cone from which the lava flow was erupted. Surrounding SP Crater are additional dark-colored hills; these are scoria cones that are older and not as well preserved as SP Crater.
Another obvious geologic feature on the photograph is the large, light-colored gray region in the left-center of the image. In this area, light-colored Kaibab Limestone is exposed at the surface. Surrounding the limestone exposures are dark gray volcanic rocks that are older than SP Crater and its associated lava flow. The older volcanic rocks and adjacent limestones are traversed by conspicuous, generally north-trending linear features, which are fault zones where the rocks have been broken and displaced. In the upper left (northwest) corner of the area is a monoclinal fold that uplifted rock layers in Gray Mountain relative to the same rock layers closer to SP Crater. The deep trough along the flank of this uplift is the large northeast-trending Mesa Butte fault.
Figure 3 represents the same area – Figure 3a is a vertically-taken satellite image, and Figure 3b is a geologic map of the same area. The map is highly simplified, but portrays the general geologic features of the area. It shows the distribution of the following four rock units: 1) volcanic rocks comprising the SP lava flow; 2) scoria (cinder) cones, including SP Crater; 3) other volcanic rocks; and 4) Permian Kaibab Limestone. In essence, the map outlines areas where each rock unit is exposed at the surface. Contacts between different rock units are depicted with a thin, unbroken line, whereas thicker lines show the location of fault zones. There are many faults in the map area; two of the most obvious faults are shown with bold black lines.
Figure 3b is but one example of a geologic map. Geologic maps can portray the geology of either large or small areas. For example, the geology of North America can be shown in a highly simplified manner on a single, standard-sized map; such a map is referred to as a small-scale map. On the other hand, a large-scale map may be needed to accurately depict the geology of a small, geologically complex hill. The scale chosen for a particular map is largely dependent upon its intended use. A small-scale map would be used to show the distribution of active volcanoes of North America, whereas a more detailed, large-scale map would be needed for evaluating the mineral potential (e.g., copper) of a small area. Most geologic maps are produced at a scale that is intermediate between the two extremes discussed above; geologic maps at the scale of standard U.S.G.S. 7.5’ (1:24,000 scale) and 30’ by 60’ (1:100,000 scale) quadrangles are perhaps most common.
There are two types of geologic mapping: reconnaissance and detailed. In reconnaissance mapping, a geologist has a limited amount of time in which to map the geology of a relatively large area. Around 1920, N. H. Darton of the U.S. Geological Survey mapped nearly one third of Arizona in a scant 20 months. Darton's mapping, by necessity, showed only the main geologic features of the region. However, other geologists are known to have spent their entire professional careers mapping in detail the geology of a single mine or mining district. The choice between a detailed map and a reconnaissance map is dictated by its intended use and by time and financial constraints. A detailed map provides more information than a reconnaissance map, but requires more time, effort and money.
Geologic maps are used for numerous purposes. A good geologic map is essential for evaluating potential geologic hazards, such as volcanic eruptions and earthquakes, because it helps identify sites of recent volcanism and faulting. Geologic maps also play a key role in exploration for energy, mineral, and water resources. For example, a geologic map might indicate where oil-bearing rocks are exposed at the surface or, if buried, how deep they might be. Areas with high geothermal energy potential might also be located by examining a standard geologic map. Engineering applications include siting roads, evaluating landslide potential, and many other applications.
Arizona is well known for its important copper industry. Nearly all of the large copper deposits in the state are associated with granites of a particular age (55 to 75 million years old). For the most part, granites of this age are specifically identified on the present Geologic Map of Arizona. Areas near these granites are probably most favorable for the discovery of additional copper deposits. Geologic maps are used in analogous ways for exploration of other types of mineral resources. Lastly, good geologic maps are important for reconstructing the natural history of the earth, including national and state parks, monuments, and wilderness areas.
