The northern midcontinent region includes the buried basement of the interior platform and the southernmost exposed Precambrian rocks of the Canadian Shield (see index map). Structurally, it is part of a continent-wide "tectonic collage" consisting of (1) Archean cratonic elements (>2500 Ma) and remnants of Early Proterozoic (-1,900-2,100 Ma) cratonic cover, (2) anastomosing Early Proterozoic orogenic belts culminating at 1,850 Ma and -1,650 Ma, which partly enclose and marginally affect the Archean cratons, and (3) intracratonic igneous and sedimentary rocks, mainly of Middle Proterozoic age but including an older rhyolite-granite terrane (-1,760 Ma) and somewhat younger, scattered quartzite, assigned by Dott (1983) to the "Baraboo interval".
The map was compiled as part of a cooperative federal-state project from 1:500,000-scale maps submitted by respective state geological surveys showing basement drill holes, lithotypes or sketchy geologic map units, and basement topography, contoured at 200-foot intervals. In compiling the map, available aeromagnetic (Zietz, 1982; Burchett, 1985) and gravity anomaly maps (Hildenbrand and others, 1982) were utilized to define insofar as possible the trend, extent, and boundaries of individual rock bodies, and all available isotopic age data were used.
The principal geologic contribution resulting from the compilation is the delineation of a major buried, northwest-trending Early Proterozoic orogen, named the Central Plains orogen (Sims and Peterman, in press). It extends from Nebraska through Kansas into Missouri, where it is overlapped by Middle Proterozoic rhyolite-granite terranes; and it sharply truncates Archean rocks and an older Proterozoic orogenic (Penokean) belt in the inner part of the craton.
