Mesozoic stratigraphy of the Sierrita Mountains, Pima County, Arizona

Mesozoic rocks in the Sierrita Mountains, Pima County, Ariz., comprise more than 20,000 feet of terrestrial sedimentary and volcanic rocks and are divided into eight formations.Three formations are newly named in this report.These eight formations are not all found at any one place, however, and none has yielded closely datable fossils.Local geologic relations, radiometric age determinations, and regional correlations suggest that three of the formations are Triassic, one is Upper Triassic or Lower Jurassic, one is Lower Cretaceous (?), one is Lower Cretaceous, and two are Upper Cretaceous.The lower Mesozoic formations are the Ox Frame Volcanics, Tascuela Redbeds, Rodolfo Formation, and Stevens Dl D2 MESOZOIC STRATIGRAPHY IN SOUTHEASTERN ARIZONA disconformably by the Angelica Arkose near the type localities of the two formations, but the contact is gradational and conformable in an isolated exposure about 5 miles to the east.Because of the locally gradational contact and also the presence in the Angelica Arkose of quartzite beds similar in lithology to the Whitcomb, the Whitcomb Quartzite is assigned to the Lower Cretaceous (?) with recognition of the possibility that the formations in the isolated outcrop have been misidentified and that the Whitcomb could be a pre-Cretaceous equivalent of the Stevens Mountain Rhyolite.The Angelica Arkose (new) overlies the Whitcomb Quartzite and is a thick Lower Cretaceous sequence of interbedded feldspathic sandstone, siltstone, conglomerate, and a little quartzite and limestone.A basal conglomerate unit, locally more than 500 feet thick, contains pebbles and cobbles of various Mesozoic and Paleozoic formations.The succeeding unit, as much as 2,000 feet thick, consists of interbedded siltstone and well-sorted arkosic sandstone in sharply defined beds mostly less than 2 feet thick.Overlying this unit with sharp but interfingering contact is a thick-bedded unit as much as 3,000 feet thick that is characterized by coarsegrained ill-sorted arkosic grit and pebble conglomerate containing recognizable fragments of Jurassic granite.Locally the grit unit contains a thick quartzite bed and sparse lenticular beds of limestone.It is overlain with angular unconformity by the Demetrie Volcanics.Upper Cretaceous rocks comprise the Demetrie Volcanics and the Red Boy Rhyolite.The Demetrie Volcanics, a sequence of andesitic and dacitic breccias and flows having a local basal conglomerate and two intercalated rhyolitic tuff units, lies with angular unconformity on formations as old as the Ox Frame Volcanics and as young as the Angelica Arkose.The Demetrie is apparently about 8,000 feet thick south of the Sierrita Mountains, but it wedges out beneath the Red Boy Rhyolite in the core of the mountains.The Red Boy Rhyolite is 700 to perhaps 1,000 feet thick and consists of nonwelded and welded rhyolitic tuffs and tuff-breccias and minor rhyolitic flows.The formation lies unconformably on the Demetrie Volcanics, on Jurassic and Triassic(?)intrusive rocks, and on the Ox Frame Volcanics.South of the mountains the Red Boy is overlain unconformably by gravels and volcanic rocks of middle Tertiary age.