Surficial geologic map of the Newman Peak Quadrangle, Pinal County, Arizona

Surficial geologic maps of the Picacho Basin. The Picacho basin is a large and complex graben surrounded by horsts and half-horsts, which are now the Picacho, Casa Grande, Silverbell, and Sacaton mountains. It formed mainly in response to late Miocene extension. Internal drainage probably persisted until about 3 million years ago. Several thousand meters of sediments fill the basin; 2000 m of evaporites and claystone form the bulk of the basin fill (Scarborough and Pierce, 1978). The upper 200 meters or so of basin fill was deposited by a gradually aggrading, regionally integrated drainage system. Young alluvium of the Santa Cruz River is up to 30 m thick and is found within 3.2 km of the modern channel (Pool, 1986). Although the Picacho Basin does have external drainage, extremely low stream gradients and a lack of long-term downcutting have resulted in playa-like sediments, eolian deposits, and a lack of significant elevation differences between "terraces. Unlike basins upstream, the Picacho basin has not been incised significantly during the Quaternary.Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu