Petrography of the Faraway Ranch Formation, Chiricahua National Monument, Arizona

摘要

The Faraway Ranch Formation is a sequence of volcanic rocks and volcanic-derived sedimentary rocks forming much of the northern Chiricahua Mountains, Cochise County, Arizona. This formation rests in part upon Lower Cretaceous rocks of the Bisbee Formation and in part upon the eroded surface of an altered andesite, possibly intruded during Laramide magmatism. It is overlain by the middle Miocene Rhyolite Canyon Formation; both are thought to have resulted from the middle Cenozoic pulse of Basin and Range magmatism. The Faraway Ranch Formation has been subdivided into nine members comprising 1375 feet of strata: five are pyroclastic in origin, two are flows, and two are fluviatile sedimentary rocks formed of the debris from volcanic rocks. Andesite and basalt are present, but rhyolite and rhyodacite predominate. Although mafic volcanic debris is present in the sedimentary rocks, silicic debris predominates. Fanglomerates and coarse, fluviatile sedimentary rocks indicate a topography during Faraway Ranch time similar to that now existing.