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<br />~ <br /> <br />o <br />\ .~) <br />... <br />(.;) <br />CD <br />.;::. <br /> <br /> <br />" <br /> <br />""". <br /> <br /> <br /> <br /> <br />. ~ .';C. <br /> <br />Figure 7. Chinle shale, Painted Desert, Arizona. <br /> <br /> <br />During much of Jurassic time the region of the Colorado Plateaus <br />was a back shore area covered with magnificent sand dunes. Much of the <br />sand may have been moved southward from the shore of the sea, then <br />encroaching on the area from the north. The sea finally advanced far to <br />the south and carried the very gypsiferous sediments of the Carmel <br />Formation over the massive sands of the Glen Canyon Group. The <br />outcrop area of the Arepien shale. the Carmel equivalent to the west in <br />central Utah, exposes much halite and gypsum, but fortunately this area <br />drains to the Great Basin. The gypsiferous Cannel Formation probably <br />has its greatest affect on the quality of the Colorado through the San <br />Rafael River and the Dirty Devil River both of which have relatively large <br />areas of Carmel outcrop adjacent to them. <br /> <br />(.; <br /> <br />The Summerville Formation, is a thin-bedded colorful silty <br />formation well developed in central Utah in the vicinity of Hanksville. It <br />is the shore deposit of a second pulse of the Jurassic sea from the north <br />and it is highly gypsiferous. Its contributions go largely to the Dirty <br />Devil. Figure 8 shows an outcrop of the Summerville Formation, laced <br />with veinlets of gypsum, near Hanksville. <br /> <br />The Brushy Basin shale member of the Morrison Formation is like <br />the Chinle, highly colored, but it is relatively thin and thus without <br />extensive outcrops. It also is probably non-marine, and thus has much <br />less soluble matter to furnish to the runoff, than the marine shales. <br /> <br />" <br /> <br />7 <br /> <br /> <br /> <br />