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The Green River Desert-Cataract Canyon region includes about 2,150 square miles of sparsely populated, semiarid, canyon and plateau country in Emery, Wayne, and Garfield Counties, Utah. Situated west of the Green and Colorado Rivers, it extends from the Dirty Devil River on the south to the Denver and Rio Grande Western Railroad on the north and includes the eastern part of the San Rafael Swell. The rivers and most of their tributaries flow through deep canyons, which have been cut into a northward-sloping upland surface, and the huge dome of the San Rafael Swell rises above this upland surface with an abrupt eastward-facing escarpment. The altitudes of surface features above sea level range from about 3,500 feet at the mouth of the Dirty Devil River to a maximum of about 7,000 feet at the southern margin of the upland surface and on the summit of the San Rafael Swell; the altitude of the upland surface decreases to about 4,100 feet at the north boundary of the area. The region abounds in unusual scenic features especially in that part adjacent to the Green, Colorado, and Dirty Devil Rivers, where vari-colored rocks, practically unobscured by soil and vegetation, crop out in the walls of numerous canyons, including Cataract Canyon of the Colorado River, and in high steep cliffs, and in many domes, mesas, and spires. Green River, the only town in the area, is located on the main line of the Denver and Rio Grande Western Railroad. Only about 25 people live in the area beyond-the limits of the town. Farming and stock raising are the principal industries, but the farming is limited to small areas of irrigated land near the town of Green River and. along the San Rafael River. The aggregate average thicknesses of the formations exposed in the area is 6,500 feet, and they range in age from Pennsylvanian to Upper Cretaceous. The oldest exposed rocks crop 'out in Cataract Canyon as plugs of gypsum, derived from the Paradox formation of lower Pennsylvanian age, which have been forced through a thin cover of the overlying rocks. Only the upper part of the overlying Pennsylvanian Hermosa formation is exposed. It consists ^of marine, dominantly gray, massive sandstone and limestone and some shale.. The overlying Permian Rico formation consists of interbedded sandstone, shale, siltstone, and limestone; it is about 575 feet thick in Cataract Canyon but may be absent in the San Rafael Swell in the western part of the area. Permian rocks younger than the Rico in the eastern part of the area are included in the Cutler formation, about 1,100 feet thick, which is composed of massive buff to gray sandstone interbedded with red siltstone and sandstone, but in the San Rafael Swell the post-Rico Permian is divided into the thick massive buff to gray Coconino sandstone, nearly 700 feet thick, and the thin overlying Kaibab limestone. The Lower Triassic Moenkopi formation rests upon an erosion surface at the top of the Permian; it has a maximum thickness of nearly ,700 feet and consists of thin regularly bedded, dominantly red sandstone and shale and in the San Rafael Swell contains the Sinbad limestone 2 GREEN RIVER DESERT-CATARACT CANYON REGION member in the lower part. The relatively thin Shinarump conglomerate of Upper Triassic age unconformably overlies the Moenkopi formation and grades upward into the Upper Triassic Chinle formation, which has a maximum thickness of about-450 feet aifd consists of variegated shale with subordinate sandstone and conglomerate. Conformably overlying the Chinle formation is the Jurassic (?) Glen Canyon group, which includes at the base the massive reddish-tan Wingate sandstone, about 300 feet thick, and at the top the massive gray to buff markedly cross-bedded Navajo sandstone about 500 feet thick separated by the irregularly bedded red to gray sandstone of the Kayenta formation with a thickness of about 275 feet.. The overlying San Rafael group of Upper Jurassic age includes the Carmel formation at the base 80 to 230 feet thick consisting of red to gray silty sandstone and shale, with some limestone and locally with abundant gypsum, overlain successively by (1) the red to gray Entrada sandstone, about 400 feet thick; (2) the Curtis formation, which has a maximum thickness of 235 feet and is composed of greenish-gray glauconitic sandstone and shale; and at the top of the group (3) the Summerville formation, 100 to 200 feet thick consisting of red and gray shale, mudstone, and subordinate platy sandstone in thin regular beds. The Morrison formation, about 600 feet thick, is the youngest Jurassic formation in the area and unconformably overlies the Summerville, locally with angular discordance; the lower Salt Wash sandstone member consists of irregularly bedded gray to brown, somewhat conglomeratic sandstone interbedded with red to gray shale and mudstone and some gypsum, and the upper part of the formation consists of variegated mudstone and siltstone with several beds of siliceous conglomeratic sandstone. The Dakota (?) sandstone at the base of the Upper Cretaceous is thin and discontinuous and fills depresssions on an eroded surface of the Morrison; it consists of bright yellowish-brown conglomerate and sandstone with some gray and black shale. The overlying Mancos shale is composed of blue-gray shale with a few thin beds of sandstone. The most conspicuous structural feature in the area is the steeply dipping east flank of the San Rafael Swell, which is a huge north trending regional upwarp about 100 miles long. The southern part of the area includes part of the gently dipping northern end of the Monument upwarp, which is a structural feature comparable in size and trend to the San Kafael Swell. A low structural ridge extends southeastward from the San Rafael Swell in the vicinity of the Emery-Wayne County line. North of this structural ridge the rocks dip gently northward into the Uinta Basin; southwest of it they dip gently into the large synclinal basin containing the Henry Mountains. The rocks are broken by numerous normal faults most of which are steeply dipping, have small displacements, and trend N. 45° to 65° W. The most important period of folding, during -which most of the surface rocks acquired their present attitudes, is believed to have occurred in late Cretaceous time, but important structural movements also occurred at other times. Scanty evidence from drilled wells suggests that Pennsylvanian sediments overlap upon an eroded surface of folded pre-Pennsylvanian rocks. Arching of the rocks and faulting along the upper part of Cataract Canyon occurred very recently in geologic time after the surface of the region had acquired essentially its present form. This structural disturbance is ascribed to flowage of the plastic material of the Paradox formation from areas adjacent to the canyon, where it was under the pressure of the weight of a thickness of 2,000 feet or more of overlying formations, into the arch along the canyon, where it was confined by only a thin cover of the overlying rocks. Several wells have been drilled for oil or gas within the area, but commercial production has not been obtained. Showings of oil or gas have been found at several horizons, especially in the upper part of the Permian and in the lower part of the Lower Triassic. Wells have been drilled into rocks believed to be of Mississippian age and a well drilled on the San Rafael Swell reached granitic rocks after passing through Mississippian (?) and possibly older formations.

Geology of the Green River Desert-Cataract Canyon region, Emery, Wayne, and Garfield Counties, Utah