Geological Survey Water-Supply Paper 1475-H Availability of Additional Water for Chiricahua National Monument Cochise County, Arizona

Sedimentary and volcanic rocks ranging in age from Paleozoic to Cenozoic comprise the stratigraphic units of the Chiricahua National Monument (Sabins, 1957a). The rocks consist of complexly faulted, generally westward-dipping strata (Enlows, 1955; Sabins, 1957b). Paleozoic and Mesozoic sedimentary rocks crop out in the extreme northeast corner of the area and dip steeply to the south and west under a thick sequence of Tertiary volcanic rocks that forms most of the outcrops in the monument area. The Paleozoic and Mesozoic sedimentary rocks may be water bearing locally. In the vicinity of the monument headquarters these sedimentary rocks are thought to be at a considerable depth below the land surface. The volcanic rocks, which include the Nipper formation (Sabins, 1957b) and the Faraway Ranch and Rhyolite Canyon formations (Enlows, 1955), consist mostly of rhyolitic welded tuff and associated pyroclastic rocks and are probably more than 2,000 feet thick (Sabins, 1957b). All the reported seeps and springs in the monument have their sources in the volcanic rocks.

The most recent deposit in the Chiricahua National Monument is the alluvial fill in the major stream canyons. The fill consists of silt, sand, and gravel. The thickness of the alluvial fill is unknown, but in the NE1/4NE1/4NE1/4 sec. 35, T. 16 S., R. 29 E., a test well penetrated 115 feet of fill without reaching bedrock. The six wells within the monument all obtain their water from the alluvium.

Several high-angle faults cut the volcanic rocks in the mapped area. These faults trend north-northwestward and are downthrown to the east. One of these faults, which crosses Bonita and Rhyolite Canyons east of the monument headquarters, has a displacement of about 150 feet. The seep in Bonita Canyon in the SW1/4NW1/4 sec. 25, T. 16 S., R. 29 E., is immediately northwest of this fault. The volcanic rocks are characterized by prominent vertical jointing. The joints and fractures of the rocks locally may be sufficiently large and interconnected to allow considerable vertical and lateral movement of ground water and the infiltration of small amounts of surface water.