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Buried basement rocks of the central United States are mainly plutonic granitic rock, mafic and felsic metamorphic rock, and diabase. Rhyolite, granitic rock, and gabbro-diabase form a discontinuous belt from eastern New Mexico to eastern Missouri. The 0.5 b.y. rocks in the Wichita Mountains, Oklahoma, are the result of the last major igneous event. The Ouachita structural belt is the southern limit of petrographic knowledge of plutonic rocks.
None of these rock groups bears a simple relationship to basement topography and isotopic age. The Black Hills uplift-Cambridge arch-Central Kansas uplift is underlain dominantly by metamorphic rock with ages ranging from 1.7 b.y. in the north to 1.2 b.y. in the south. Siouxia arch contains 1.4 b.y. granitic and metamorphic rock. Nemaha uplift is underlain by 1.2-1.5 b.y. granite. Diverse rock types of 1.2-1.4 b.y. underlie Amarillo uplift and Red River-Matador arch.
Several large gravity anomalies correspond to major basement structures. The Williston basin is bounded on the south and west by a series of major west- and northwest-trending gravity anomalies and on the east by a belt of south-trending gravity anomalies extending from Canada into the central Dakotas that coincides with the boundary between the 2.5 b.y. Superior and the 1.7 b.y. Churchill Provinces of the Canadian Shield. The Sioux formation (minimum age 1.2 b.y.) lies along the west-trending anomaly in southeastern and central South Dakota. Keweenawan basaltic and sedimentary rock coincides with the mid-continent gravity anomaly and extends nearly continuously from Lake Superior to northeastern Kansas. The prominent gravity feature along the Red River-Matador arch coincides with the boundary between 1.2-1.4 b.y. rocks in the central United States and the 1.0 b.y. rocks in Texas.
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