The 86th annual meeting of the Cordilleran Section of the Geological Society of America was held in Tucson, Arizona in March 1990. The occasion provided an opportunity for geologists studying central and southern Arizona and Sonora, Mexico to lead field trips to their study areas. Field guides for 16 of the 18 field trips run in association with this meeting are included in this special paper. Field guides for the two remaining trips may be purchased from the American Geophysical Union. A primary responsibility of the Arizona Geological Survey is to provide information on the geology and mineral resources of Arizona. This special paper was published to offer the public a guide to outstanding and generally accessible geologic features in the Southwest and an explanation of their significance. We believe that a better understanding of the geology and mineral deposits of the Southwest will lead to greater appreciation of their complexity and diversity and to more responsible stewardship of Arizona's land and mineral resources.

The 1:24,000 scale geologic map that is the basis for this report covers part of the central Mazatzal Mountains in the area where the Beeline Highway (State Highway 87) crosses the range (Figure 1). The map area includes all of the Reno Pass 7 ½' Quadrangle, the eastern third of the Lion Mountain 7 ½' Quadrangle, and the western third of the Kayler Butte 7 ½' Quadrangle. Parts of this area have been mapped and studied by several investigators (Figure 2), whereas other areas remain unmapped, in part because Arizona interior chaparral is difficult to pass through and may require crawling under and climbing over for large distances. This map is an attempt to compile and synthesize maps and unit descriptions from these diverse studies. A stratigraphic correlation diagram is included as figure 3.

Arizona is truly a land of scenic beauty and geologic marvels. The arid. climate and great relief combine to expose geology in a dramatic fashion equaled by few other places in North America. This guidebook is the fourth of a series being issued by the Arizona Bureau of Mines to acquaint tourists and residents of this state with its geology. It is hoped that the text will appeal to the amateur and layman as well as professional geologists and engineers. This is not a textbook, however, and the conceptual aspects of geologic phenomena are not treated in detail. The reader should consult introductory textbooks to learn such things as why volcanoes erupt and mountain ranges rise. What we have attempted to do is to call to the attention of the public specific examples of geology which occur along our highways. We hope this will make your trip a little more satisfying.

Tonto Basin is a Mid-Tertiary, west-tilted half-graben located in the Transition Zone tectonic province of central Arizona (Figure 1). Along with the northerly adjacent Payson Basin, Tonto Basin is bounded on the west by a major east-side-down normal fault that accomodates at least 3,000' of displacement. In the Payson basin, the basin-bounding fault was named Payson fault (Muehlberger, 1990) and it forms the abrupt, east-facing escarpment of the Mazatzal Mountains. The western basin-bounding fault of Tonto Basin is also referred to as Payson fault, even though there is an abrupt change of strike where the two basins merge (Figure 1). The eastern margin of both basins is defined by a gentle, west-dipping, depositional unconformity with Proterozoic rocks of the Sierra Ancha (Martin, 1990; Mayes, 1990). Tonto Basin is filled with Miocene and younger mudstone and conglomerate that in general dip gently to the west. Locally along the western margin of the Tonto Basin and in the center of Payson Basin (Muehlberger, 1988), an older sequence of east-tilted conglomerates are present suggesting a complex structural history along the Payson fault. A sequence of Pliocene and younger conglomerate and mudstone in the Payson basin overlap the Payson fault (Muehlberger, 1988), and are probably equivalent with a sequence of flatlying, post-tectonic, younger Tertiary conglomerates in the Tonto Basin which also bury the Payson fault. Report and geology of the Tonto Basin 7.5' Quadrangle, Gila and Maricopa Counties, Arizona. scale 1:24,000.