Basement in the Uintas: an enigma
The eastern Uinta Mountains, Utah-Colorado, contain the only exposures of the Precambrian Red Creek Quartzite (or Complex) and its contact with the Uinta Mountain Group (also Precambrian). Because of the higher metamorphic grade of the Red Creek and the relatively unmetamorphosed nature of the Uinta, the former has come to be regarded as the older unit, with the Uinta metasediments described as resting unconformably upon the Red Creek metamorphics, or faulted against them. The Uinta Mountain Group is more than 24,000 ft thick and comprises more than 99% of the outcropping Precambrian core of the Uinta Mountain arch (or anticline). Regional and local field relationships disprove existence of the unconformity. The so-called oldest rocks are exposed in a 15-mi/sup 2/ area on the north flank, instead of along the axis of the Uinta Mountain arch, in violation of the law of superposition. Xenoliths of younger rock occur within older rock, and the so-called basal conglomerate and as much as 3500 ft of Uinta Mountain Group bedding are transected by the Red Creek-Uinta contact. The Red Creek Complex rocks resulted from advance of a metamorphic front, with brecciation, melting, and reconstitution of the pre-existing sediments. The metamorphic contact ranges from sharp to gradational and diffuse. Faulting of post-Eocene(.) age has juxtaposed the two units in some places. The Red Creek has been dated at 1.55-1.65 b.y. (K-Ar). The Uinta metasediments must, therefore, be older, and are the oldest rocks in the range. No crystalline basement is exposed. Admittedly, this interpretation disagrees almost totally with more than a century of published geologic work on the Precambrian of the Uintas and has broad repercussions on long-standing concepts of regional Precambrian geology.
- OSTI ID:
- 5906106
- Report Number(s):
- CONF-870915-
- Journal Information:
- AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States), Vol. 71:8; Conference: American Association of Petroleum Geologists Rocky Mountain Section meeting, Boise, ID, USA, 13 Sep 1987
- Country of Publication:
- United States
- Language:
- English
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