Cenozoic stratigraphic and structural framework of southwestern Utah

Abstract

Cenozoic rocks of southwestern Utah chronicle a complex history of early Tertiary sedimentation followed by middle and late Tertiary and Quaternary volcanism and basin-fill sedimentation. Sedimenta tion in latest Cretaceous(?) to Oligocene time took place largely in two broad basins that developed east of a highland formed during the Sevier orogeny in Late Cretaceous time. Calc-alkalic volcanism began in early Oligocene time and continued into the early Miocene; many widely scattered, partly clustered, intermediate-composition stratovolcano complexes formed in two east-northeast-trending belts. The intervening lowlands were covered by thin but widespread ashflow tuff sheets. Little deformation took place during most of early sedimentation and calc-alkalic volcanism, but minor faulting can be documented locally, especially along several east-trending linea ments. The Basin and Range and Colorado Plateaus provinces appear to have begun to separate into identifiable structural entities about 26 m.y. ago. About 20 m.y. ago, east-west extension started to imprint a north erly striking pattern of basin-range faults onto the older easttrending igneous belts and underlying sedimentary rocks the present structural and topographic grain of southwestern Utah formed. Uplifted blocks were eroded and sediments were deposited in the adjacent developing basins. Concurrently a bimodal assemblage of mostly small volumes of basaltic and rhyolitic rocks was erupted. Basalt flows are very widely distributed, but the rhyolitic rocks were erupted from local centers, many of which are located along the east-trending lineaments within the older igneous belts.