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

The Browns Park Formation (fig. 13) was named for the valley of Browns Park astride the Utah-Colorado State line, where it is widespread and is exceptionally well exposed (Powell, 1876, p. 44, 186). From Browns Park it extends like a blanket east and south to Elk Springs and southeast into the Maybell—Lay—Axial Basin area, retaining a thickness of more than 490 m (Dyni, 1968; McKay and Bergin, 1974). Since Powell's time it has been described in considerable detail in many published reports, partly because it has been host to commercial uranium deposits.

The Browns Park Formation seems to have begun to accumulate in some places soon after the Bishop Conglomerate was deposited, perhaps with no great stratigraphic hiatus; the evidence is inconclusive. In most places, however, millions of years intervened. The region was in the midst of tectonic deformation, and sediments were accumulating in favorable locations while rock was being eroded nearby. The lithology of the formation, therefore, is varied (Luft and Thoen, 1981), but it consists chiefly of light-colored to nearly white, loosely cemented, generally calcareous sandstone and light-gray to white, vitric and ashy to earthy, friable to firm, rhyolitic tuff. Locally the sandstone is tan, has a pale greenish cast, or is rusty brown. Some of the sandstone is tuffaceous, and some of the tuff is sandy. Eastward from the Lay—Maybell area the appearance changes, and the formation more resembles the North Park Formation or the Troublesome than typical Browns Park.

Much of the sandstone in the more easterly area is eolian (fig. 18) and is intensely cross-stratified, especially in the area between Elk Springs and Maybell (Dyni, 1968, 1980; McKay and Bergin, 1974; Honey and Izett, in press). The source of the sand is undetermined, but deflation of nearby loose, sandy, upper parts of the Bishop is a likely possibility. The Bishop at that time was more widespread than it is now.

FIGURE 18.—Eolian crossbedding in Browns Park Formation near U.S. Highway 40 a few kilometers west of Maybell, Colo. Tangentially bedded upper set rests on a planar truncation surface on the lower set. Exposure is about 3 m high. Viewed toward the northwest.

In the west, much of the sandstone in the Browns Park is cross-laminated or is obscurely bedded and appears to be fluvial. Some of it is too friable to be called rock; it is easily crushed between the fingers and would be called soil or overburden by engineers. Tuff is abundant in the western part of the area, particularly from Vermillion Creek west into Utah. There, more than 35 percent of the material in measured sections has been identified as tuff or tuffaceous sandstone (Hansen, 1965, fig. 40). The formation also contains bedded chert in its lower part near Vermillion Creek and East Boone Draw; it contains minor amounts of limestone, especially in the Douglas Draw—Smelter Hill area south of Greystone, some of which is oolitic; it contains siltstone and lacustrine clay; and, in the western part of Browns Park, it has abundant locally derived conglomerate at varied stratigraphic levels. All these rock types are mentioned to highlight the lithologic differences between the Browns Park Formation and the Bishop Conglomerate.

In the western Browns Park area and just to the west in Red Canyon, the formation has greater lithologic diversity than elsewhere, mostly because of a varied depositional environment engendered by strong topographic relief. I regard patches of well-cemented sandstone and conglomerate along and below the rims of Red Canyon as remnants of alluvial fans. Most of the constituent material was derived from the Uinta Mountain Group, and the resulting deposits range from fine-grained sandstone to heterogeneous bouldery conglomerate, of which the larger boulders exceed 3 m in diameter. In the Dutch John area, sandstone and conglomerate interfinger with tuff. The overall color of the Browns Park at Red Canyon is tawny to light gray, exemplified by large exposures near the right (south) abutment of Flaming Gorge Dam. It contrasts, therefore, with the somber reds of the adjacent Uinta Mountain Group. Although some clasts retain the red color of the parent Uinta Mountain Group, most of them are partly bleached or mottled. Some have bleached shells covering fresh red cores. Calcium carbonate is the chief cementing material, but some sandstone is cemented with silica.

The tuffs are of two general types, vitric and earthy. Some of the vitric ones are well bedded or crossbedded, and some even contain climbing ripples that suggest rapid sedimentation in standing water (fig. 19). Others appear to have been reworked by winds into eolian crossbeds. Many tuffs, though, are massive or only faintly laminated; these perhaps settled quietly from still air. All the vitric tuffs have low bulk densities. The earthy tuffs are mostly well bedded, commonly having laminations that look lacustrine (fig. 19). Such a mode of deposition would facilitate alteration of the glass to produce the earthy texture. Some beds have load casts, ball-and-pillow structures, contortions, and broken, disrupted laminations of a sort that should form in standing water. As much as 20 percent of the material in several measured sections is lacustrine clay (Hansen, 1965, fig. 40), some of which contains diatoms and sparse ostracods.

FIGURE 19.—Water-laid tuffs in the Browns Park Formation near Dutch John, Utah (in measured section B-2 of Hansen, 1965, p. 121). Top—Well-stratified, earthy tuffs overlain unconformably by massive, vitric airfall tuff. Scale is in inches (1 in. = 2.54 cm). Bottom—Climbing ripples, laminae in phase, suggest heavy suspended load and rapid sedimentation in gently oscillating water.

The source of the tuffs is unknown; explosive volcanic activity was widespread in the Western United States in Miocene time (Izett, 1968; Marvin and others, 1970; Christiansen and Lipman, 1972). Most of the nonvolcanic material in the valley of Browns Park was locally derived. Broad fans, consisting chiefly of pebbly cobbly alluvium derived from the Uinta Mountain Group but including Paleozoic limestone and older Precambrian metamorphic rocks, spread intermittently from the highlands enclosing Browns Park. The fans were buried from time to time by falls of volcanic ash, some of which was reworked into tuffaceous sandstone. Periodically, much of Browns Park was flooded by lake waters that deposited blankets of sand and clay. The result is a complex interbedding of conglomerate, sand, tuff, and clay. The tuffs and clays retain remarkable uniformity over considerable distances, but the sands and conglomerate thin from the sides toward the axis of the valley (Hansen, 1957a). Parts of the section were exposed to erosion at times, but for how long is unknown. Although unconformities have been recognized within the Browns Park Formation (Sears, 1924a, p. 296; Ritzma, 1965b, p. 131; Hansen, 1965, measured sections), their significance is undetermined. Sears noted an angular discordance near the east end of Browns Park.

Boulders of unusual size, some more than 5 m in diameter, occur in the Browns Park Formation at and near the SW corner sec. 21, T. 2 N., R. 24 E., near the head of the valley of Browns Park. There the formation fills a steep-sided channel cut into the Uinta Mountain Group just north of the existing trench of the Green River. These deposits pass into finer grained conglomerate which, in turn, interfingers with tuffaceous sandstone (Hansen, 1965, p. 118).

POST-BISHOP, PRE-BROWNS PARK CHANGE IN DRAINAGE DIRECTION

Other fanglomerates in the Browns Park Formation near the head of Browns Park thicken and coarsen markedly from the center of the valley toward the north margin and thus provide clear evidence of their source and direction of transport. Red quartzite from the Uinta Mountain Group is the chief constituent and, on lithologic grounds only, could have come from any direction, but abundant pebbles of white metaquartzite, amphibolite, and other rock types from the Red Creek Quartzite and occasional pebbles of gray Paleozoic limestone must have had sources to the north (Hansen, 1965, p. 118, 173). The Red Creek material obviously came from nearby areas just north of Browns Park. The limestone pebbles probably were recycled out of conglomeratic beds in the Wasatch Formation, also to the north, because all the Paleozoic limestone in that part of the Uinta Mountains had been removed by post Wasatch erosion and pre-Browns Park movements on the Uinta fault. The Wasatch, therefore, is the only likely source, and the nearest Wasatch outcrops are north of Browns Park near the Wyoming State line. Some cobbles might have been reworked from Bishop Conglomerate, but it, too, lay to the north. Thus, the direction of drainage across the area was reversed after the Bishop Conglomerate was deposited from the south and before the Browns Park was deposited from the north. The reversal was caused by regional tilting, described in detail later. The tilting and the drainage reversal required a significant interval of time after the Bishop was deposited and before the Browns Park Formation was deposited.

AGE OF THE BROWNS PARK FORMATION

Radiometric and fission-track datings indicate a wide age span for the Browns Park Formation in the Eastern Uinta Mountains region. The oldest date so far measured, 24.8±0.8 m.y., is a potassium-argon age obtained from a biotite separate of chalky white tuff collected by Izett and others (1970; Izett, 1975) from the northwest side of the Little Snake River, about 30 m above the top of the Bishop Conglomerate, and hence very close to the base of the Browns Park Formation. This tuff is about 4 m.y. younger than the tuff in the Bishop Conglomerate on the Diamond Mountain Plateau and is approximately equivalent in age with the lowermost Arikaree Formation of the Great Plains and the Wyoming Basin. A much younger zircon fission-track age of 9.9±0.4 m.y. was obtained by Naeser and others (1980, p. 24) from vitric tuff higher in the section at Vermillion Creek. Damon (1970, p. 52) determined a potassium-argon age of 11.8±0.4 m.y. for glass from a vitric tuff collected by G. R. Winkler in Browns Park in Utah. These ages thus span about 15 m.y., the latter two being equivalent with part of the Ogallala Formation of the Great Plains. Honey and Izett (in press) have recently reported an additional age of 11.3±0.8 m.y. from a site between Maybell and Cross Mountain. Whether or not deposition was continuous throughout that time is unknown, but local unconformities suggest episodic pauses of undetermined length. Physiographic constraints, discussed further below, argue against rocks much older than 12-15 m.y. existing in the Browns Park Formation within the valley of Browns Park.