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Abstract

The Wyoming province is the region in Wyoming and adjacent states underlain by rocks of Archean age (Plate 2). It is an Archean craton bordered on the east and south by younger Precambrian provinces (Plate 1). Precambrian rocks are only exposed in the cores of the Laramide (Late Cretaceous to Early Tertiary) uplifts, and outcrops constitute less than 10 percent of the area underlain by Archean basement. Between uplifts, basement rocks are covered by thick Phanerozoic strata, so that extrapolations of geology from uplift to uplift are generally tenuous. Reconstructing the Archean history of the Wyoming province is further complicated by deformation associated with the late Mesozoic fold and thrust belt along the western margin. Although the Archean record is fragmentary, the basement uplifts generally afford excellent exposure.

The northern and northwestern margins of the Wyoming province are poorly constrained. Archean rocks are known as far north as the Little Rocky Mountains of Montana (Peterman, 1981). According to King (1976), Archean and Early Proterozoic dates are intermingled at the northwest margin of the province in a wide zone that exhibits a gradational change from Archean dates in central Montana to Early Proterozoic dates to the northwest, reflecting increasing influence of post-Archean thermal events.

The southwestern and southern margins of the Wyoming province are also poorly constrained. Archean rocks have been reported from several ranges in the Cordilleran orogenic belt, such as the Albion and Raft River Ranges (Armstrong and Hills, 1967) and possibly the Ruby Mountains of Nevada (A. W. Snoke, personal communication, 1986).

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