PRECAMBRIAN BASEMENT MAP SHOWING GEOLOGIC-GEOPHYSICAL DOMAINS, WYOMING

The aeromagnetic map of Wyoming provides a tool to extend the Precambrian basement geology from outcrop areas to regions covered by younger sedimentary strata, and to determine the geologic framework of the Archean Wyoming craton (province). The craton underlies more than 90 percent of the state and is juxtaposed with two Paleoproterozoic convergent orogens: the Medicine Bow (1.78 Ga) and the older Trans-Hudson Island-arc rocks of the Medicine Bow orogen are intruded by anorthosite and granite plutons (~1.4 Ga), exposed mainly in the Laramie Mountains.

The Archean craton is composed of amphibolite facies gneisses (>2.6 Ga) and sparse supracrustal rocks intruded by potassium-rich granitic rocks (2.8--2.55 Ga). The gneisses and granitic rocks form alternating belts, crudely semi-circular in plan view, and open to the north. These rocks are expressed by alternating magnetic anomalies: positive for granite and negative for gneiss and metamorphosed volcanic-sedimentary rocks. The cratonic rocks were deformed during the Archean. Those in the Laramie Mountains and Hartville uplift in the southeast part of the exposed craton, were extensively deformed and metamorphosed in the foreland of the two convergent Paleoproterozoic orogens. Faults and shear zones related to these deformational events appear as linear anomalies and sharp gradients on the magnetic map. The craton consists mainly of rocks derived by reworking of older (3.1--2.8 Ga) gneisses and lacks significant volumes of juvenile materials and definitive evidence for accreted convergent terranes.

Aeromagnetic signatures of the Wyoming province differ greatly from those of the Superior province of the Canadian Shield. The contrasting geophysical data strongly supports previous conclusions of several investigators from geologic data that the two provinces were not joined during the Archean, then separated during the Proterozoic.