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. o CD t.u 04 . . .1 SPECIAL STUDIES (Continued) The Paradox Member of the Hermosa Formation of Pennsylvanian age underlies a large area in western Colorado and eastern Utah. The few surface exposures of the Paradox are grouped with other rocks in geo- hydrologic unit 2, which consists of rocks that generally yield nonsaline to slightly saline runoff. However, the Paradox, where buried beneath younger rocks, contains large accumulations of salt. Locally, some of the salt is dissolved by ground water, carried in solution to shallower aquifers, and eventually discharged into streams. The most notable example is where the Dolores River crosseS the Paradox Valley to western Colorado (Figure 7). Data compiled by the U.S. Bureau of Reclamation (1975) indicate that the annual salt pickup of the Dolores River in (kfe Paradox Valley is approximately 200,000 tons (181,000 t). . 2. Geologic Structure Nearly all the rocks in the Upper Colorado River Basin have under- gone some structural deformation since their emplacement or deposition. In the mountainous areas, many of the rocks have been complexly folded and faulted; whereas, those in the Green River, Washakie, Uinta, Piceance, and San Juan Basins have been folded into broad synclinal troughs with some associated faulting and secondary folding. Even the relatively flat-lying rocks in the Canyonlands have been tilted, folded, or faulted to some extent. Because of the small map scale, Figure 7 shows only a small fraction of the faults that are known to exist in the basin. Geologic structues has significant influence on diffused sources and point sources of salinity in the basin. For example, faults asso- ciated with the formation of salt domes and the collapse of leached-out areas in the Paradox Member of the Hermosa Formation are the principal conduits along which saline ground water flows upward and eventually discharges to streams in and around the Paradox,Valley. Some thermal springs are significant point sources of salinity. Water moves downward deep into the earth along fractures and bedding planes. The temperature of the water increases with depth, signifi- cantly increasing its ability to dissolve mineral constituents. These thermal waters return to the surface along faults and discharge large amounts of salt into streams at various points in the Colorado River Basin. Hagen and others (1971, p. 70) estimated that the annual salt discharge of the major thermal springs in the basin exceeds 500,000 tons (454,000 t) and that the salt (~tscharge of Glenwood Springs alone is nearly 214,000 tons (194,000 t). In ,the Green River, Washakie, Uinta, Piceance, and San Juan Basins, milch of the ground water is under artesian presure. In the Piceance Basin, for example, highly saline water from a deep confined aquifer in the Green River Formation moves upward under artesian pressure into 115