Introduction The San Pedro River Basin is an ecologically significant region, in part because the river hosts a major North American migratory bird flyway. The importance of surface flow in this river was acknowledged when Congress created the San Pedro National Riparian Conservation Area in 1988. Concern about regional water resources increased almost immediately with calls for the elimination of nearby Fort Huachuca Army base. The Upper San Pedro aquifer of southeastern Arizona is bracketed by crystalline and sedimentary rocks of the Huachuca Mountains on the west and the volcanic and sedimentary rocks of the Mule Mountains and Tombstone Hills on the east (Figure 1). Recharge of the Upper San Pedro Valley aquifer between them is believed to come primarily from the Huachuca Mountains–in fact, most of the aquifer lies to the west of the river on the Fort Huachuca side. Fort Huachuca and the adjacent city of Sierra Vista lie between the Huachuca Mountains and the river, and have been tapping this aquifer for over a century. The historical flow in the river has apparently been affected by water withdrawal by large copper mines in neighboring Mexico beginning in the late 1940s (Brown et al., 1966), but to what extent is unknown. Estimates of groundwater withdrawal versus recharge on the American side of the border, despite conservation efforts by the U. S. Army and local jurisdictions, still suggest a deficit (Pool and Coes, 1999).

The El Tule fault extends southeastward from the Huachuca Mountains area of Arizona to run along the base of the Sierra San José in northern Sonora. This fault, with the Jurassic and Cretaceous strata that lie on either side of it, helps to link the geologic histories of the Sierra San José, the Huachuca Mountains, and the Mule Mountains. This fault was active during the Jurassic and formed part of the southern and southwestern limit of a small Jurassic basin that accumulated Glance Conglomerate and contains the type locality of that unit; it was also active during the time of deposition of the upper part of the Bisbee Group, i.e., the Morita, Mural, and Cintura formations. This fault is an important marker for the northern margin of the Cananea high. Three evidences suggest that a genetic distinction should be made between the basin that collected the Glance Conglomerate and the younger, wider, superposing basin that collected upper Bisbee rocks: (1) There was a depositional pause after active deposition of the Glance (and therefore after active opening of the Glance basin). (2) The subsequent, finer-grained upper Bisbee deposits do not extend entirely over the Cananea high, but are otherwise regional in extent, forming a thick blanket over the inactive fault basin and other similar basins elsewhere in the region. (3) The axial trends of the Jurassic fault basin(s) and the upper Bisbee basin do not coincide. We offer the hypothesis that the Jurassic fault basins were caused by the same crustal stresses that caused the Mojave-Sonora megashear (their ages of activity and orientations are similar), and that the upper Bisbee basin was the probable result of crustal thinning associated with formation of the extensional Jurassic basins, and of eustasy. We offer this in contrast to the current hypothesis that the Bisbee basin is a rift basin, that is, the product of a single, ongoing Mesozoic process. Large-scale mass-gravity deposits are signal features both of the Jurassic fault basin (a result of crustal extension) and of the overlying local Laramide basin (which followed crustal contraction). Historically, these deposits have been confusing. A mountainous mass of megaclasts (La Negrita Hills) within the Glance Conglomerate has been mistaken for the hanging wall of a Laramide thrust (a contractional feature) instead of the product of Jurassic crustal extension that it is. The 91 Hills have been considered an in situ array of mid-Cretaceous patch reefs instead of the Late Cretaceous convulsive mass-gravity deposits (a product of Laramide contraction and inversion) that they are.

Abstract The Whetstone Mountains are in southeastern Arizona along the boundary between Cochise and Pima Counties, about 42 mi (70 km) southeast of Tucson (Fig. 1), Benson, 15 mi (25 km) to the northeast, and Sierra Vista, about the same distance to the south, are the nearest towns. Most of the Whetstone Mountains, including the Dry Canyon area, are in Coronado National Forest. Dry Canyon, on the lower southeast flank of the mountains, is in the Benson15-minute Quadrangle and the Apache Peak and McGrew Spring 7½-minute Quadrangles. Along the canyon, and particularly on the ridge on its south side, a thick sequence of paleozoic rocks is exposed, which is the focus of this report. The Dry Canyon area (Fig. 2) is reached via Arizona 90, which trends south toward Fort Huachuca and Sierra Vista from its junction with I-10, about 2.7 mi (4.5 km) west of Benson. At a point 13 mi (21.5 km) south of 1–10, a dirt road, marked by a simple ranch gate, leads westward a few mi (km) across public land into Dry Canyon. This road is best traveled using a four-wheel-drive vehicle, but it was passable with difficulty for passenger cars in 1982.