The geology and ground water investigation of the Tres Alamos Dam site area of the San Pedro River, Cochise County, Arizona

Abstract

Rocks of the mountains paralleling the San Pedro River Valley in the Tres Alamos Dam Site area range in age from Precambrian to Paleozoic. Granitic rocks which occur at the clam site are similar to those in the mountains to the west and east and are mapped as Precambrian in age. The oldest Tertiary unit exposed in the San Pedro Valley is the Granitic Conglomerate of Miocene (?) age which is overlain uncomfortably by the Black Conglomerate of Pliocene (?) age. A period of faulting and erosion followed the consolidation of the Black Conglomerate and parts of the units were removed. Erosion was followed by deposition of the Plio-Pleistocene Benson Beds. The flatbedded Benson Beds are generally fine grained and are reddish brown in color. The light brown Tres Alamos formation of Pleistocene age overlies the Benson Beds. The Tres Alamos unit is flat-bedded and consists of conglomeratic facies near the mountain fronts grading into fine-grained facies near the center of the valley. The youngest unit discussed is the floodplain deposits of the San Pedro River and its tributaries. The Tertiary conglomerates yield small to moderate amounts of unconfined water to wells penetrating those parts not overlain by Benson Beds. In the center of the valley where the conglomerates are overlain by Benson Beds, the unit contains artesian water at depths greater than 200 feet. The permeability of the Tertiary conglomerates probably ranges from 100 to 1,000 gpd/ft^2. The Tres Alamos formation yields small amounts of non-artesian water to wells penetrating the conglomeratic facies. The permeability of the coarse fades of the Tres Alamos formation is estimated to range from 10 to 1,000 gpd/ft^2. The Benson Beds do not yield water to any known well. The fine-grained beds of this unit form the confining layer for the underlying artesian system. The floodplain deposits yield moderate to large amounts of unconfined water to wells. The permeability of the floodplain deposits is believed to range from 1,000 to 5,000 gpd/ft^2. The granitic rocks exposed at the Tres Alamos Dam Site appear to form a ground water barrier to the artesian system. If this barrier existed in the past dividing the valley into two separate basins of deposition, it is possible that the sediments containing the artesian system are not continuous through the darn site area. It is possible that the artesian water may be moving through a buried channel to the west of the dam site, through buried remnants of the Tertiary conglomerates on the sides of the granitic rocks near the dam site, or through regional faults associated with the development of the Basin and Range province.