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<br />13 <br /> <br />Mancos-Mesa Verde and some shaly parts of the Green River Formations, <br />~ <br />--I herein mentioned in ascending stratigraphic sequence), the Upper <br />I-"- <br />00 Cretaceous Mancos and Mesa Verde Shales are the thickest in the <br /> <br />succession, reaching a total thickness of 1000 m and covering bands <br /> <br />10-20 miles in width. Figure 1.6 is a map of a portion of the state <br /> <br />of Colorado, showing the location of Mancos Shale outcrops, <br /> <br />The identification of specific soluble minerals by X-ray diffrac- <br /> <br />tion and chemical analyses, as well as the spatial trends in SMC <br /> <br />are discussed in detail in the Appendix (A-I). <br /> <br />The occurence of Mancos Shale in this area is mostly restricted <br /> <br />to the lowlands (e.g" Grand Valley and the Delta-Montrose area), <br /> <br />which suggests that these low-lying semiarid areas are the largest <br /> <br />potential contributors to the salinity of the Colorado River. In <br /> <br />'fact, the Environmental Protection Agency (1971) clearly identified <br /> <br />these upper basin areas as being the Price, San Rafael, Dirty Devil, <br /> <br />McElmo and Mancos River Basins, including the Grand Valley-Montrose <br /> <br />region. In view of the preceding discussion on the simultaneous <br /> <br />location of saline outcrops and semi-aridity, the Chaco Wash and <br /> <br />Paria River Basins may also be included among the major contributors. <br /> <br />Figure L 7, derived from Reir and Wadell's (1973) investigations, <br /> <br />shows the quality of surface waters in the Upper Basin. It can be <br /> <br />noted that the highest salinities arc associated with low-lying <br /> <br />semiarid areas where extensive outcrops of saline deposits occur, <br /> <br />This map is useful in showing large-scale trends in water quality, <br /> <br />mostly associated with the Green, Grand and San 'Juan main stems and <br /> <br />their large tributaries, However, it is not useful for a precise <br /> <br />delineation of those areas contributing mostly to the salinity of <br /> <br />." <br />