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

In many places east of the Uinta Mountains a thick, loosely cemented conglomerate, here assigned to the Bishop, lies at the base of the Browns Park Formation. The existence of this rock has led to most of the speculation regarding the relationship of the Browns Park to the Bishop. This speculation arose, perhaps, out of the widely held assumption that the conglomerate is merely the basal unit of the Browns Park Formation. Sears' early conclusion (1924a) that the Bishop and the basal Browns Park are one and the same was subsequently challenged by Bradley (1936). This conglomerate has not generally been shown separately on maps, nor has it received close scrutiny in the literature; its detailed distribution and its relation to the Browns Park have largely escaped serious study. McKay (1974) assigned it to the Browns Park Formation but noted its significant lateral differences and mapped its outcrop separately for about 19 km west of the Little Snake River. Dyni (1980) described it in some detail but did not differentiate it on the map; most other workers in the area did neither.

Evidence now indicates that two separate and distinct conglomerates underlie the Browns Park Formation in the area east from Vermillion Creek to the limit of outcrop east of Maybell. One is indeed a basal conglomerate, and it passes upward gradationally into the overlying beds (Hancock, 1925, p. 24). The other, I believe, is the Bishop, disconformable with the Browns Park and genetically unrelated.

Sears (1924b, p. 295) actually touched close to the heart of the problem when, in describing the Browns Park Formation, he noted that "almost everywhere the base of the formation is marked by a conglomerate, which has very different characteristics in the eastern and western parts of the field." From Simsberry Draw (which is just east of the Little Snake River; fig. 13) east toward Juniper Mountain, the basal conglomerate of the Browns Park Formation is less than 1 m thick and has a light-gray overall tint. North of Sunbeam it is locally nonexistent. Just east of Juniper Mountain, near the Yampa River, it reaches about 15 m (Hancock, 1915, p. 186). East from Simsberry Draw to Juniper Mountain, it consists of small subrounded to subangular pebbles of mafic and felsic igneous and metamorphic rocks, red and white quartzite, varicolored chert, and milky quartz (McKay, 1974; McKay and Bergin, 1974; Sears, 1924b, p. 295). Its lithology indicates a Park Range provenance (Izett, 1975, p. 203; oral commun. 1975).

From the Little Snake River west toward Vermillion Creek, however, the conglomerate changes abruptly in thickness, color, and lithology to an aspect much the same as that of the classic Bishop Conglomerate: "in appearance the two conglomerates are practically identical" (Sears, 1924b, p. 289). Tracing it from the Little Snake River to Vermillion Creek, Sears (1924a, p. 295; 1924b, p. 285) reported a maximum thickness of 90 m. McKay (1974) reported 15-90 m. The average might be near 45 m. Locally, however, it is nonexistent—removed, I believe, by pre-Browns Park erosion. As seen at Sand Wash, it is a poorly sorted cobbly conglomerate that contains abundant boulders of red quartzite and gray limestone, some more than a meter in diameter, mostly some distance above the base, and clearly derived from the Uinta Mountains. Its overall color is pink. Its dip is southwesterly. At the Little Snake River, where it appears to be partly eroded, it is not as coarse or as thick as at Sand Wash, but there also its provenance is the Uinta Mountains, and it retains the overall pink color.

On the southeast side of the Yampa valley, between Elk Springs and Maybell, across the Browns Park—Lay syncline, the conglomerate reemerges from beneath the Browns Park Formation and forms a line of rolling hills on and flanking Elk Springs Ridge. There it has a northerly dip of several degrees. There, too, the provenance is clearly the Uinta Mountains, although the dip is toward the Uintas. Cobbles and small boulders of gray limestone predominate, but red quartzite is abundant, and gray chert is common. Notably lacking are igneous and metamorphic crystalline rocks from the Park Range. The assemblage is typical Uinta Mountains material, and the texture is much coarser than that of the Park Range derivatives at the base of the Browns Park Formation to the east and northeast. At Elk Springs the conglomerate is very well exposed in a large gravel pit just south of U.S. Highway 40, where it has a dip of about 20 degrees north.

West from Elk Springs, as the conglomerate emerges from beneath the white eolian sandstone of the Browns Park Formation, it flattens rapidly, then rises gradually westward. To the north the conglomerate slopes gently into the Lily Park syncline (Dyni, 1968). Although mapped as Browns Park Formation west and north of Elk Springs (Schultz, 1918, pl. 5; Sears, 1924b, p. 291; Dyni, 1968, 1980; McKay, 1974), it has the appearance of the Bishop conglomerate where it emerges from beneath the Browns Park sandstones. To the west it caps flat-topped remnants of a once broader surface that merges farther west with the high Gilbert Peak erosion surface of the south flank of the Uinta Mountains.