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<br />-24- <br /> <br /><:) <br />.I:io <br />~ <br />00 <br /> <br />mountains is of very high quality. The nonmountainous <br />regions of the basin have been subjected to intermittent <br />inundations by great inland seas. Many of the thick sedi- <br />mentary rock formations underlying the basin were deposited <br />at the bottom of the seas and are consequently high in <br />residual salts. Figure 4 illustrates the areal extent of <br />the main sedimentary deposits. These sedimentary deposits <br />are also the location of the oil, oil shale, coal, uranium, <br />and other large potential energy resources of the basin. <br />The most significant formations in terms of salinity <br />contribution from irrigated areas are the Mancos Shales of <br />the Cretaceous Age. These shale, sandstone and mudstone <br />deposits form a thick formation that lies between the under- <br />lying Dakota sandstones and the overlying Mesa Verde forma- <br />tions. The thickness of the Mancos Shale usually varies <br />from between 900 to 1,500 meters. Due to its great thickness <br />and its ability to be easily eroded, this shale forms many <br />of the large irrigated valleys of western Colorado and <br />eastern Utah. <br />The effect of these shales is illustrated by Bently <br />et al. (1978). They report that sampling runoff from Spring <br />Creek in the Price River drainages above and below a Mancos <br />Shale outcrop resulted in a threefold increase in dissolved <br />solids concentrations. Irrigation return flows on Mancos <br />Shales in the Huntington Creek in the San Rafael and Muddy <br />Creek in the Dirty Devil River drainages result in as much <br />as a tenfold increase in dissolved solids concentrations. <br /> <br />.. <br /> <br />" . <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />