Abstract

The area of this report includes parts of the Chiricahua and the Dos Cabezas Mountains which together constitute a typical range in the Mexican Highland section of the Basin and Range province. The Precambrian basement complex consists of the Pinal Schist, with a massive quartzite member, and two granite intrusives: the foliated Sheep Canyon Granite which is a small synkinematic stock, and the nonfoliated Rattlesnake Point Granite which is a large postkinematic intrusive. All these units are cut by Precambrian aplite dikes.

The Paleozoic strata have an aggregate thickness in excess of 7600 feet and consist of the following units, in ascending order: Bolsa Quartzite (Middle(?) and Upper Cambrian), El Paso Formation (Upper Cambrian and Lower Ordovician), Portal Formation (Upper Devonian), Escabrosa Limestone (Lower Mississippian), Paradise Formation (Upper Mississippian), and the Naco Group (Pennsylvanian and Permian). The Naco Group is subdivided into the following formations, in ascending order: Horquilla Limestone, Earp Formation. Colina Limestone, Scherrer Formation, and Concha Limestone.

The Bisbee Group (Lower Cretaceous) is about 2600 feet thick and includes the Glance Conglomerate at its base. It unconformably overlies the older rocks. The Nipper Formation is here named for mafic volcanic rocks and associated sediments that unconformably overlie the older rocks. The rhyolitic flows of the succeeding Faraway Ranch Formation overlie the Nipper and Bisbee units, and are overlain by the welded tuffs of the Rhyolite Canyon Formation. These volcanic units were formed during the late Cretaceous to late Tertiary interval. All the pre-Rhyolite Canyon formations are cut by Tertiary intrusions of two major types: a light-colored quartz monzonite group and a dark-colored diorite and basalt group.

There is evidence for at least five periods of tectonic activity within the area. The Precambrian orogeny, which pre-dated the intrusion of the Rattlesnake Point Granite and the aplite dikes, caused the regional metamorphism and folding of the Pinal Schist. An epeirogenic uplift that began late in early Ordovician time resulted in the disconiormity between the Lower Ordovician strata and the Upper Devonian strata. Uplift and erosion occurred in the late Permian-early Cretaceous interval and supplied the clastic sediments for the Glance Conglomerate.

During the major post-Comanche to pre-Pliocene orogeny, strong southerly to southwesterly horizontal compression caused the following tectonic sequence. The autochthonous rocks along the northeast front of the range were overridden from the southwest by the first thrust sheet. Strike-slip displacement along the Emigrant fault cut the autochthonous block and the overlying thrust sheet, which was separated into the Fort Bowie plate and the Wood Mountain plate. The Fort Bowie plate was later folded to form the Marble Quarry syncline and was truncated by the younger Fort Apache reverse fault. Finally, the Whitetail plate overrode the Fort Apache fault block.

During the Basin and Range orogeny (Pliocene?) vertical displacement along high-angle faults adjacent to the range uplifted the mountains relative to the valleys on either side.

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