Abstract
Certain Proterozoic quartzites (including, Baraboo, Barron, and Sioux) and associated metasedimentary rocks in the upper midwest represent a distinct epicontinental depositional environment. New data indicate that argillite, chert, iron formation, micaceous conglomerate, and rhyolite are much more abundant and significant in this sequence than previously reported. The entire group of rocks was deposited during one period in a series of tectonic episodes (magmatism and uplift) related to epeirogeny. None of these episodes included overt rifting. Sedimentation began on subsiding continental crust during the waning stages of anorogenic magmatism 1760 m.y. ago. Following deposition, a regional event 1630 m.y. ago is interpreted as a time of uplift and mild deformation with no magmatic expression in the region. Epeirogenic tectonism was culminated in the widespread intrusion of 1500 m.y.-old alkalic plutonism, which metamorphosed and further deformed the sedimentary rocks and surroundings.
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