GEOLOGY OF PRECAMBRIAN BASEMENT ROCKS IN IOWA AND THE SOUTHERN PARTS OF WISCONSIN AND MINNESOTA

The well known Precambrian rocks of the Lake Superior region record geologic events in the evolution of Laurentia from 3.6 Ga to 1.1 Ga. Similar rocks extend southward beneath Paleozoic and Cretaceous strata in southern Minnesota and Wisconsin and throughout Iowa. Using aeromagnetic and gravity maps of the region, and several hundred basement-penetrating drill holes, we have produced a geologic map and preliminary tectonic interpretation of the Precambrian subcrop for this region. The map extends the well known Superior Province (Archean) and Penokean orogenic belt (Paleoproterozoic) of the Lake Superior region southward to a fundamental boundary, shown clearly by aeromagntic patterns, named the Spirit Lake-Trempealeau discontinuity (SLTD). This feature appears to mark the southern limit of Archean rocks and may also be the southern limit of Penokean age crust. South of this discontinuity, rocks at the subcrop are dominated by subaerial potassic rhyolite and epizonal granite, formed at about 1.75 Ga, and ultra-mature quartzite, such as the Baraboo Quartzite, which lies unconformably on them. Gneisses and mafic volcanic rocks, probably basement rocks from which the rhyolites formed by partial melting, are inferred from gravity and magnetic highs to be at subcrop in several areas, but their lithology and age are not well constrained. Because the SLTD transects Penokean trends at a low angle, it appears to be a post-Penokean feature and may mark the northern margin of the Yavapai orogenic belt. To date, no rocks older than 1.75 Ma have been identified south of the SLTD, but data are still sparse. All of these rocks were strongly deformed at about 1.63 Ga during the Mazatzal orogeny. Major granitic plutons were emplaced into this terrane in the interval 1.50-1.43 Ga. The final major Precambrian event in the region was formation of the Midcontinent Rift at 1.1 Ga. The rift transects older terranes at a high angle and consists of more than 10 km of flood basalts accumulated in and near a series of grabens, and slightly younger clastic rocks. Its location in the subsurface is clearly shown by very pronounced geophysical anomalies. The geologic framework produced by this study offers a series of new and testable hypotheses that can be investigated through geochronologic and geophysical investigations related to the Earthscope program.