The Precambrian basement of Wyoming consists predominantly of the Archean Wyoming craton (province), and includes small parts of two Paleoproterozoic accreted orogens—the Trans-Hudson, on the east, and the Colorado, on the south. The Trans-Hudson is a major lithospheric unit in North America composed mainly of oceanic-arc rocks (1.9-1.84 Ga), but includes substantial volumes of Archean rocks. It is mainly known from exposures in northern Saskatchewan and Manitoba, in Canada. Within the United States, rocks of the orogen are exposed only in the Black Hills uplift of South Dakota. The Trans-Hudson orogen separates the Wyoming province from the Superior province. The Colorado orogen, formerly called the Colorado province, is a >500-km-wide belt of oceanic arc rock (1.78-1.65 Ga) that extends southward into New Mexico, and composes a major part of the Transcontinental Proterozoic provinces of southwestern United States. Formerly, the Wyoming sector of the Colorado orogen was called the Medicine Bow orogen. The eastern extension of the Colorado orogen into the High Plains is named the Central Plains orogen. The boundary between the Colorado orogen and the Wyoming craton is the Cheyenne belt, a well-documented 5-km-wide mylonitic shear zone that verges northward. The Cheyenne belt transects and cuts off the older Trans-Hudson orogen on the south.

The Archean and Proterozoic rocks in Wyoming crop out mainly in the cores of the state’s several mountain ranges. In adjoining basins the Precambrian rocks are buried beneath younger Phanerozoic rocks as much as a few thousand meters thick. In the Yellowstone region in northwestern Wyoming, a veneer of flat-lying Cenozoic and recent volcanic rocks nearly completely obscures the Precambrian basement.

A new aeromagnetic anomaly map provides a means of seeing through the sedimentary and volcanic cover to reveal magnetic signatures from buried Precambrian rocks and structural features in these rocks. Magnetic lows do not correlate closely with the positions of major basins, so the anomalies primarily reflect variations in the lithology of the Precambrian basement rocks. Thus, the magnetic anomaly map provides a useful tool for understanding the geologic framework and regional tectonic features of Wyoming.

Revised January 27, 2009

First posted July 25, 2001