Geochronology of the midcontinent region, United States: 2. Northern area

Radiometric age determinations for samples from wells drilled to the basement in the northern part of the midcontinent region of the United States permit new interpretations of the Precambrian history. Remnants of Early Precambrian rocks in eastern and western South Dakota suggest that the region originally was underlain by crustal rocks connecting the ancient Precambrian rocks of Minnesota with those of Wyoming and Montana. Middle Precambrian rocks in western South Dakota were folded and intruded by granite approximately 1700 m.y. ago. The Black Hills orogeny is essentially synchronous with the Penokean orogeny of the Lake Superior district. The orogenic belt extends into North Dakota and Saskatchewan and probably merges with the Hudsonian orogenic belt of the Churchill province on the Canadian shield. It continues to the southeast in the basement rocks of Nebraska into Kansas. A period of granite intrusion in Nebraska is dated at 1490 m.y. A somewhat younger group of granites in South Dakota and Nebraska is suggested by Rb-Sr feldspar ages at 1430 m.y. The long belt of middle Keweenawan flows and related rocks extending from Lake Superior, in the subsurface of Minnesota, Iowa, and Nebraska, to Kansas was formed during the Keweenawan igneous activity approximately 1100 m.y. ago.