New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/32 Ancient drainage changes in and south of Unaweep Canyon, southwestern Colorado S. W. Lohman, 1981, pp. 137-144 in: Western Slope (Western Colorado), Epis, R. C.; Callender, J. F.; [eds.], New Mexico Geological Society 32nd Annual Fall Field Conference Guidebook, 337 p. This is one of many related papers that were included in the 1981 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. New Mexico Geological Society Guidebook, 32nd Field Conference, Western Slope Colorado, 1981 137 ANCIENT DRAINAGE CHANGES IN AND SOUTH OF UNAWEEP CANYON, SOUTHWESTERN COLORADO S. W. LOHMAN U.S. Geological Survey Denver, Colorado 80225 INTRODUCTION and Hunt's concept (1969, fig. 63) of the possible ancestral pat- On the first day of the field trip our route takes us through spec- terns, are shown in Figure 1. tacular Unaweep Canyon athwart the high Uncompahgre Plateau Hunt (1956a, p. 45) first postulated that most of the laccolithic —the now "high and dry" abandoned gorge that I believe once car- mountains were of late Miocene or early Pliocene age, but later ried the combined flows of the ancestral Colorado and Gunnison (1969, p. 81) he assumed an early Miocene age for the Sleeping rivers. From Gateway, at the southwestern end of the canyon, we Ute and La Sal Mountains, and other laccolithic mountains in the will go up the Dolores and San Miguel rivers to Uravan and Van- Colorado Plateau. Radiometric age determinations by the K-Ar corum, then traverse Paradox Valley along most of its northwest- (potassium-argon) method by Armstrong (1969) for rocks from ward trending longitudinal axis. At about the middle of the valley, seven of the eight groups of laccolithic mountains on the Colo- near Bedrock, Colorado, we will cross the Dolores River, which rado Plateau (Witkind, 1975, p. 245) indicated a Miocene or older "paradoxically" cuts across the roughly canoe-shaped valley in- age for the La Sal Mountains, but indicated a Late Cretaceous age stead of following the longitudinal axis, as do most normal for Sleeping Ute Mountain. Fission-track and K-Ar dating by Cun- streams, hence the name Paradox Valley. ningham and others (1977, p. 5) determined an average age of Evidence to be presented strongly suggests: that the Dolores about 70 million years for 5 samples of rock from Sleeping Ute River once joined the ancestral San Juan River to the southwest but Mountain, indicating a Late Cretaceous age. was diverted northward from about the present town of Dolores, Regardless of the exact age of the rocks involved, I am in entire Colorado, to join the ancestral San Miguel River; that the com- agreement with Hunt that the ancestral Dolores River once flowed bined flows of these two ancestral rivers once joined with the an- southwestward, probably to the ancestral San Juan River, but was cestral Colorado River at the southwestern end of ancestral Una- later diverted to its present northward course. If Sleeping Ute weep Canyon, near the present town of Gateway, Colorado; and Mountain was intruded as early as the Cretaceous, the downcut- that the ancestral upper Colorado River then joined the present ting of the ancestral Dolores may have been impeded by the resis- Colorado River southwest of the present confluence of the Dolo- tant igneous rocks, and the diversion may have been aided also by res and Colorado Rivers near Dewey Bridge, in eastern Utah. warping that accompanied later epeirogenic uplift of the Colorado Although most of the details of these suggested drainage Plateau. Regardless, the diversion allowed the meandering stream changes have already been published, it is hoped that this brief to cut downward into relatively flat-lying Cretaceous sediments, summary, which includes some new findings, will be of interest to oblivious to the structures that lay beneath. most participants on the field trip. As these possible drainage In a series of eight cross sections, Cater (1970, fig. 13) has clearly changes probably span more than 50 million years, they will be shown the gradual growth and eventual erosion and collapse of discussed in approximate chronological order. the Gypsum Valley and Paradox Valley salt anticlines, the products As will be noted below, there is not universal agreement con- of erosion and solution having been carried away by the Dolores cerning some of these suggested drainage changes, and a new and River and its tributaries. Additional geologic and hydrologic data quite different interpretation of the changes in and near Unaweep on the salt anticlines, including the Spanish Valley-Moab-Seven Canyon is given in this guidebook by Sinnock. Mile faulted anticline, which we will traverse near Moab, Utah, are given by Hite and Lohman (1973, p. 53-57; see also Lohman, DIVERSION OF THE ANCESTRAL DOLORES RIVER 1975). Early Diversion Later Diversion The present Dolores River heads on the south side of Lizard Figure 1 shows not only Hunt's concept of the diversion of the Head Pass in the San Miguel Range, in Dolores County, Colorado, ancestral Dolores River, but also the slight monoclinal slip of the and flows southwestward in a relatively straight consequent San Miguel River southwestward off the flank of the embryonic course to the town of Dolores in Montezuma County. There it Uncompahgre arch (fig. 2), the confluence of the Dolores and San abruptly turns about 135 degrees to the north and flows in an en- Miguel rivers, and in turn tieir confluence with the combined trenched meandering course some 110 km north to its confluence flows of the ancestral Gunnison and Colorado rivers heading with the San Miguel River in Montrose County. southwestward through ancestral Unaweep Canyon to the Colo- When I worked in the Dolores-Dove Creek area during the rado River near its confluence with the Green River. 1940's, this abrupt change in the direction of the Dolores River ap- It is perhaps interesting to note that even though the San Miguel peared to me to be quite anomalous, but I did not know the rea- probably is older than the diverted reach of the Dolores River, the son for this about face until Hunt (1956a, p. 45) suggested that the headward part of the ancestral Dolores River once flowed south- *This group of mountains was first referred to as the El Late Mountains by westward to join the ancestral San Juan River, but was diverted Holmes (1877, p. 237-276). Later the name was changed to Ute Mountain northward by the doming and uplift that accompanied the intru- when the range became the principal part of the Ute Mountain Indian Reservation. Still later the name was changed to Sleeping Ute Mountain sion of the laccolithic Sleeping Ute Mountain", about 30 km south- because when viewed from the east it resembles a human body lying on west of Dolores (see Ekren and Houser, 1965). its back, complete with head, folded arms, knees, and toes. The geology The drainage patterns of the Dolores River and other streams, of the mountain has been mapped by Ekren and Houser (1965). name of the latter was applied to the lower reach between the 1, possibly along a route connecting the present Fisher Valley salt confluence and the Colorado River, presumably because the anticline, the faulted salt anticline along the Onion Creek Canyon, longer Dolores had the greater flow. and the broad Professor Valley of the Colorado River (Lohman, The flow of the Dolores River has been reduced somewhat by a 1975, fig. 7). small diversion dam about 1.7 km west of the town of Dolores, A search of the route has revealed no residual gravels, so if any from which water flows southward through a tunnel and aqueduct were present they have long since been removed by erosion, as to supply the city of Cortez and westward in canals to irrigate would be expected. The bedrock geology of the La Sal Mountains croplands. If and when the McPhee Dam is completed about 15 has been studied by Hunt (1958) and the surficial geology was km downstream from the town of Dolores, the flow of the Dolo- studied by Richmond (1962), neither of whom found any residual res will be reduced still more.