Pleistocene geology and geomorphology of the San Pedro River Valley, Cochise County, Arizona

Abstract

Most of the area from the foot of the Whetstone Mountains eastward to the San Pedro River, Cochise County, Arizona, an area of about 150 square miles, is the site of Pleistocene valley filling by lacustrinelike, fluvial, and alluvial sedimentation0 This report is concerned with the distribution and relationships of the valley-filling sediments and the geomorphic development of the area as indicated by its geologic history. The Quaternary deposits range in age from probable Kansan to Recent. The oldest Quaternary rock exposed is a probable Kansan valley4ill deposit0 It varies from fine-grained, dark=red-brown silt and clay near the valley center to a series of brown to orange silty paleosols separated by paleocaliche layers mountainward0 A fault occurred obliquely across the southcentral part of the area during pre-Illinoian and post-Kansan time that raised the Kansan surface at least 100 feet south of the fault2 reduced the surface gradient south of the fault, and restricted Illinoian deposition in this area0 llhinoian fan alluvium was deposited in other places on the valley-fill deposit. This formation varies from fine-grained buff sands near the valley center to coarse gravels and boulders near the mountains. A deeply weathered, dark-red soil was formed on this deposit during Sangamon time0 Younger alluvial fans of very coarse, dark-colored alluvium were deposited during Wisconsin time at the base of the Whetstone Mountains. Scouring during Wisconsin time and pedimentation by tributaries since has formed a 100-foot scarp cut into the fan alluvium and valley-fill deposit, parallel to the San Pedro River. This scouring produced a lowland in the center of the valley where a constructional flood-plain and cienega surface of redeposited valley-fill and fan alluvium deposits was formed. This surface dates from the time of Early Man cultures0 A post-1880 erosion cycle has cut a channel into the constructional floodplain surface in which a modern flood plain of low relief has been deposited.