Reconnaissance Geology of the Mesozoic and Lower Cenozoic Rocks of the Southern Papago Indian Reservation, Arizona: A Preliminary Report

Reconnaissance geologic mapping and uranium-lead isotopic geochronology indicate that the Mesozoic and lower Tertiary rocks of the southern Papago Indian Reservation consist of four broad units: lower Mesozoic rhyolitic to andesitic volcanic rocks, and conglomerate, sandstone, and siltstone; Middle and Upper Jurassic granitic to granodioritic plutons; upper Mesozoic sandstone, conglomerate, and subordinate intermediate volcanic rocks; and leucocratic garnet-biotite-muscovite granites of Late Cretaceous and early Tertiary age. Precambrian and Paleozoic rocks are rare. Rocks of the first three units have been variably affected by a Late Cretaceous and earliest Tertiary episode of regional metamorphism accompanied by thrust faulting and intrusion of the garnet-two-mica granites. Several areas of the southern Reservation have also been affected by a core-complex metamorphic episode of early and middle Tertiary age.