Geological Survey Professional Paper 1356 Neogene Tectonics and Geomorphology of the Eastern Uinta Mountains in Utah, Colorado, and Wyoming

A recent reevaluation of the Bishop Conglomerate in the eastern part of the Uinta Mountains of Utah and Colorado helps clarify how the conglomerate relates to the Browns Park Formation and, in turn, how both formations relate to Tertiary tectonic activity and late Tertiary and Quaternary drainage adjustments. Field relations indicate divergent geomorphic histories for the two formations, yet over broad areas they are nearly coextensive and, as will be shown, the Browns Park locally rests directly on the Bishop. This latter relationship has led to the widely held but contentious belief that the Bishop is merely the basal conglomerate of the Browns Park Formation, a hypothesis first advanced by Sears (1924a, p. 289) but later vigorously disputed by his associate Bradley (1936, p. 182) and subsequently by others. Although Sears abandoned the idea, it has been revived in recent years, and it now appears that Sears was at least partly correct—correct in identifying as Bishop the thick conglomerate beneath much of the Browns Park Formation but incorrect in assigning it to the Browns Park. Some of the conglomerate beneath the Browns Park Formation surely is Bishop, but a hiatus is indicated between the two formations. Sears, moreover, showed that the Bishop is stratigraphically below the Browns Park and, hence, is older. The Browns Park is now well dated as largely Miocene; the Bishop is Oligocene. Earlier geologists had thought the Bishop to be the younger, and some of Sears' contemporaries had even believed that the Browns Park was a facies of the Eocene Bridger Formation, despite the fact that Powell (1876, p. 168) had described the unconformity between the Browns Park and the Bridger many years before. In their type areas the Bishop and the Browns Park are separated by great topographic discontinuity, the Bishop capping high mesas and the Browns Park filling deep adjacent valleys. Because of its wide geographic extent and relatively uniform character, the Bishop is an excellent datum for use in reconstructing tectonic and geomorphic events in the Uinta region.

REGIONAL SETTING

The area discussed in this report centers around the eastern part of the Uinta Mountains—here called simply "the Eastern Uinta Mountains"—and includes parts of the adjacent Tertiary basins in Utah, Colorado, and Wyoming (figs. 1 and 2). For a clear understanding of regional geographic relationships in this broad area, the 1:250,000-scale topographic maps of the Vernal and Rock Springs quadrangles are invaluable aids. These maps are culturally obsolescent, but they portray well most of the larger scale topographic features. More detailed maps of local areas appear in the following text from place to place as needs arise.

FIGURE 1.—Regional setting of the Uinta Mountains and principal rivers. Tint indicates mountainous areas. (click on image for a PDF version)

FIGURE 2.—Locations of pertinent features in the Eastern Uinta Mountains area. (click on image for a PDF version)

The Uinta Mountains trend generally eastward from a junction with the Wasatch Range east of Salt Lake City. The crestline of the range is broadly arcuate, concave to the south; from the Wasatch Range junction it trends roughly N. 80° E. for about 90 km, then swings gradually around to about S. 60° E. at the Colorado State line.

Structurally the range is a large compound anticline whose axis extends west beyond the Uinta Mountains through the Wasatch Range (Butler and others, 1920, p. 251; Billingsley, 1933, p. 19), emerging at the Wasatch Front as the axis of the Cottonwood uplift (Crittenden and others, 1952, 1973; Eardley, 1968). In the opposite direction the axis projects southeast toward the White River uplift in Colorado, merging into a plateau region of multiple but smaller folds (Tweto, 1976), a total axial length of at least 350 km (fig. 3).

FIGURE 3.—Regional structural setting of the Uinta anticline. (click on image for a PDF version)

Although the term Uinta arch or even Uinta anticline, is sometimes applied to the Cottonwood uplift, a clear distinction should be drawn, as these features are structurally distinct, even though they are collinear. The Cottonwood uplift is a semicircular, east-plunging half dome, cored by Precambrian rocks and middle Tertiary intrusives, truncated on the west by the Wasatch fault, and separated from the Uinta anticline by a deep, north-trending synclinal valley partly filled with Tertiary volcanics (Boutwell, 1912, p. 43; Hintze, 1913, p. 130; Bromfield and others, 1977). The Uinta anticline proper is about 260 km long. Its west plunge at the town of Kamas, Utah, is well defined topographically as well as structurally. Its east plunge, equally well defined, is near the mouth of the Little Snake River a few miles west of Maybell, Colo.

In a general way this large fold coincides with the mountainous mass of the range, although some of the outer hogbacks and cuestas of the fold extend several miles out from the mountains. Near the east end of the range the anticlinal axis passes beneath the thick basinal fill of the Browns Park Formation, which buries much of the northeast limb. Just east of the Little Snake River, the short but abrupt Cross Mountain anticline straddles the Uinta axis. Curiously, its crestline trends nearly north. Farther southeast the Uinta axis reemerges at the Axial Basin anticline (fig. 3; see also fig. 25).

The range is flanked in many places by high plateaus or mesas, mostly capped by remnants of the once more extensive Bishop Conglomerate. In the western part of the range many of these plateaus exceed 3,000 m in altitude, and some even reach 3,660 m, but toward the east they are progressively lower, finally dropping below 2,000 m at Elk Springs Ridge just west of Elk Springs, Colo.

The eastern part of the range is modified on the south flank by several large subsidiary anticlines that increase its breadth and complexity. These folds center around Dinosaur National Monument, and their impressive exposure lends a dramatic dimension to the local scenery. The main Uinta anticline itself is compound in that it consists of two elongate domes, nearly equal in size, alined on a single east-west axis (Hansen, 1957b; 1965, p. 137). These domes are expressed structurally by strike and dip changes in the Uinta Mountain Group and by inward bowings of the flanking formations (fig. 4). They are thus separated by a shallow structural swale across the fold axis, roughly in line with the towns of Manila and Vernal. Both domes are also outlined by relative positive gravity anomalies (Behrendt and Thiel, 1963), which probably result from dense Precambrian rock at shallow depth beneath the range and from density contrasts between the Uinta Mountain Group and the younger rocks of the basins (Behrendt and Thiel, 1963; Cook and others, 1975). The Laramide history of the two domes was roughly the same, but the eastern dome rose higher and has been more deeply dissected, exposing the Early Proterozoic or Archean Red Creek Quartzite. Starting in late Oligocene time, the histories of the two domes took separate turns as the eastern dome began to tilt and subside. In consequence, the summits of the Eastern Uinta Mountains are about 1,200-1,500 m lower than their western counterparts. The highest point in the Eastern Uintas is Diamond Peak, an outlier off the north flank, which is capped by Bishop Conglomerate and reaches an altitude of 2,960 m.

FIGURE 4.—Generalized structure map of the Uinta Mountains showing compound form of Uinta anticline. Modified from Hansen (195Th, fig. 1; 1965, fig. 45.) Concealed thrust faults in Uinta Basin after Ritzma (1969). A similar concealed thrust fault may border the range on the north. (click on image for a PDF version)

Though most maps show the Uinta Mountains as a single, continuous mountain range, they actually comprise two structurally and topographically distinct parts, which are differentiated here as the "Eastern Uinta Mountains" and the "Western Uinta Mountains." I draw the boundary between the two at a pass north of Vernal drained by Cart Creek on the north and Reader Creek, a tributary of Little Brush Creek, on the south (Hansen, 1969a, p. 13). Utah Highway 44 traverses the pass, joining Vernal to Manila. Within a few kilometers of this pass, the altitude, physiography, climate, and vegetation change drastically from the lofty, well-watered Western Uintas to the lower, more arid Eastern Uintas. The purpose of this report is to evaluate the Eastern Uinta Mountains, their Tertiary physiographic development, their Neogene structural history, and the ensuing drainage adjustments.

ACKNOWLEDGMENTS

I am pleased to express my thanks to the many people who helped make this report possible. Peter D. Rowley and Paul E. Carrara shared geologic-mapping responsibilities in the Eastern Uinta Mountains in the late 1970's, and they prepared and reviewed maps and articles that bear on my conclusions. Bruce Bryant, who recently remapped the Western Uinta Mountains at a scale of 1:250,000, shared many stimulating discussions with me about the Uinta Mountains in general and the Bishop Conglomerate in particular. Bryant and Carrara both critically reviewed this report and offered many useful suggestions. Peter L. Martin carefully edited the entire report and ferreted out many weaknesses. John R. Dyni, Glen A. Izett, and Stanley J. Luft contributed valuable information from their expert knowledge of the Browns Park Formation and its regional geologic setting. M. Dean Kleinkopf gave helpful advice on structural and geophysical interpretations. Henry W. Roehler cleared up many of my uncertainities about the lower Tertiary rocks. Richard B. Taylor and Raymond L. Parker arranged for photo-reconnaissance flights in fixed-wing aircraft over the Eastern Uinta Mountains, flights piloted by Parker and Robert D. Miller. Karen S. Bailey supervised numerous iterations of the manuscript. Bernard W. Hawkins drafted the illustrations. Charles B. Hunt, down through the years, has generated a flood of provocative ideas that have greatly influenced my thinking. All of the above people are or have been affiliated with the U.S. Geological Survey. The National Park Service furnished logistical assistance in the rugged canyon country of Dinosaur National Monument. Andrew E. Godfrey, of the U.S. Forest Service, provided enthusiastic intellectual and physical support in the field, including the use of a helicopter.