Geological Survey Water-Supply Paper 1475-O Hydrologic and Geologic Reconnaissance of Pinto Basin Joshua Tree National Monument, Riverside County, California

GENERAL FEATURES

Pinto basin is a large alluvium-filled desert valley, about 15 miles long and 8 miles wide, bordered by the Pinto Mountains on the west and north, by the Coxcomb Mountains on the east, and by the Eagle Mountains on the south. In the central and major part of the valley floor is a large almost flat area, which is traversed by an eastward-draining sandy wash. The wash discharges through a gap between the Eagle and Coxcomb Mountains into Chuckwalla Valley to the south. This flat area is bordered on the south by the steep north face of the Eagle Mountains. To the west, north, and east, the flat area grades into the extensive alluvial fans and pediment cover that border the Pinto and Coxcomb Mountains.

The altitude of the central part of the flat valley area ranges from about 1,000 feet at the downstream (east) end to about 1,250 feet at the west end. Locally the alluvial fans and pediment cover extend to altitudes of about 1,800 feet. The altitudes of crests of the surrounding mountains are generally about 3,000 feet, and a few peaks in the Coxcomb Mountains (outside the area shown on pl. 28) are more than 4,000 feet in altitude.

Pinto basin is a structural depression formed by downfaulting along the north side of the Eagle Mountains and along the west side of the Coxcomb Mountains. The faulting along the north side of the Eagle Mountains is pronounced and is indicated by a steep eastward-trending scarp. The brecciated fault zone may be observed in the vicinity of the Mystery mine where basalt has been extruded to the surface along the fault zone. The eastern extension of this major fault is concealed beneath the alluvial deposits of the Pinto formation of Scharf (1935), that are deformed into an anticlinal fold where they overlie the extension of the fault. Movement of the fault during the Pleistocene probably caused this folding. The extension of the fault probably lies between wells 1 and 2, as indicated by the discontinuity of water levels between the wells.

Other hydrologic evidence, discussed in the section on ground water, also indicates that these wells are not in direct hydraulic continuity. Water levels for wells are shown in tables 3 and 4.

The major fault along the west side of the Coxcomb Mountains is concealed beneath the alluvial fans that extend into the valley, and the exact position of the fault trace is not known. Faulting along the northeast side of hill 1430 at the outlet of Pinto basin does not constitute a major structure, but its presence adds complexity to the area through which ground water discharges from Pinto basin by subsurface outflow to Chuckwalla Valley.

GEOLOGIC UNITS

In this report the geologic units of Pinto basin are divided into unconsolidated deposits and consolidated rocks. The reconnaissance geologic map (pl. 28) shows the areal distribution of these two groups. The map was prepared primarily to aid the estimating of the ground water storage capacity of the basin.

The unconsolidated deposits of Pinto basin are Quaternary in age and consist of alluvial and lacustrine accumulations of tufa, clay, silt, sand, and gravel that are locally deformed and interbedded with flows of basalt at their base (Pinto formation of Scharf, 1935). In the central and lower parts of the basin the unconsolidated deposits yield nearly all the usable ground water that is pumped.

Tables 1 and 2 include drillers' logs of four wells in the alluvium and the rates at which the alluvium yields water to them as shown by pumping tests. Drillers' tests, in general, indicate maximum specific capacities because they usually are not of long enough duration for the drawdown of water level to become effectively stabilized. The tests (table 2) conducted by the Geological Survey on wells 1 and 9 indicate specific capacities of 18 and 19 gpm (gallons per minute) per ft of drawdown, respectively. These tests probably are more reliable than those recorded for wells 2 and 10. The Geological Survey did not test wells 2 or 10.

TABLE 1.—Drillers' logs of wells

TABLE 2.—Summary of pumping-test¹

The consolidated rocks of Pinto basin are pre-Tertiary and may range in age from Paleozoic to Mesozoic. The rocks that form the Coxcomb Mountains are predominantly granitic and are chiefly Jurassic in age or older (Jenkins, 1938). The rocks of the Pinto and Eagle Mountains are igneous, metaigneous, and metasedimentary rocks that are pre-Cretaceous in age (Jenkins, 1938). Locally the older consolidated rocks are intruded by younger volcanic rocks, but they are undifferentiated on plate 28.

None of the consolidated rocks bear significant quantities of ground water, although small amounts of water may percolate through joint system and other fractures. These rocks underlie the alluvium at depth and crop out to form the hills and mountains around the main valley. They receive most of the precipitation within the drainage area. Runoff from this marginal and mountain area flows onto the alluvial fans and contributes most of the recharge to the ground-water body. The structural basin, which is filled with the alluvium that contains the main ground-water body of the valley, is formed by consolidated rocks of the basement complex.