Late Cenozoic History of Safford-San Simon and Bonita Creek Structural Basins, Graham County, Arizona: ABSTRACT

Abstract

Sedimentary rocks of the interconnected Safford-San Simon (S-SB) and lower Bonita Creek (BCB) basins are separated into: (1) lower Miocene volcaniclastic conglomerate, (2) upper Miocene? to Pleistocene? Gila Conglomerate, and (3) Quaternary alluvium and surficial deposits. The volcaniclastic conglomerate is locally interbedded with andesite and 19-Ma ash-flow tuff in BCB and is part of the bed rock deposited prior to formation of structural basins by Basin and Range extensional tectonics. The Gila Conglomerate consists of more than 1,200 m (3,900 ft) of basin-fill sedimentary rocks. Where exposed, most of the lower units are locally derived alluvial-fan deposits. Upper units of the Gila (Pliocene to lower Pleistocene?) include lacustrine beds interbedded with deposits of low-gradient alluvial fan covering 730 km2 (280 mi2) from its apex at the mouth of Bonita Creek, southwestward across the width of the S-SB. Thick sequences of halite and gypsum penetrated in wells in the S-SB indicate playa deposits at depth. Red granite clasts in the fan deposits indicate that Bonita Creek had eroded headward to the upper BCB and captured streams draining the Point of Pines area. The youngest sediments are chiefly Quaternary alluvium deposited after the Gila River developed through-flowing drainage. The initial course of the Gila River was along the north side of the low-gradient fan. This course disrupted the depositional balance of streams draining the Pinaleno Mountains southwest of the S-SB and caused deposition of range-front alluvial fans 1 0 m (390 ft) thick. These fans were abandoned as the Gila River migrated to the center of the basin and are now spectacularly exposed as erosional remnants. End_of_Article - Last_Page 938------------