Petrography and Sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, Western Lake Superior Region: Midcontinent Rift System

Abstract

Detailed sedimentological descriptions and petrographic analysis of the upper Keweenawan Nonesuch Formation were accomplished for selected Bear Creek drill cores from Ashland, Bayfield, and Douglas Counties,Wisconsin. These data, coupled with the information from outcrops in northwestern Wisconsin and Upper Michigan, provide evidence on source rocks, environment of deposition, and the tectonic framework of the Nonesuch Formation in the Midcontinent Rift System. Lower Keweenawan felsic, intermediate, and mafic volcanic units were the major contributors of detritus to the formation. Younger Keweenawan volcanic and granitic intrusive rocks were minor sources. Detritus from Early Proterozoic and Archean crystalline rocks increases in abundance upsection as older source rocks outside the rift were unroofed. Sedimentary structures and stratigraphic facies relationships suggest that deltaic processes, sheetfloods, density and turbidity currents, and suspension settling were the primary mechanisms of deposition in a thermally stratified perennial lake. Rapid fluctuations in water levels were brought on by changes in tectonism or climate. The gradational contacts of the Nonesuch Formation with the underlying Copper Harbor Formation and the overlying Freda Formation, along with outcrop and drill core facies data, suggest that the site of Nonesuch deposition was adjacent to, and some- times upon, a prograding alluvial fan complex. The Nonesuch Formation in the Bear Creek drill cores was divided into six sedi- mentational intervals (1, 2, 3, 3a, 4, and 5, lowest to highest) based on textures, sedi- mentary structures, and color. Light gray to black rocks predominate in all of the intervals and indicate that deposition of Nonesuch sediments was in a predominantly euxinic environment. The facies assemblages represented in these sedimentational intervals record clastic deposition that occurred during the initial transgressive and final regressive stages of the Nonesuch Lake over the contemporaneous alluvial fan complex of the upper Copper Harbor Formation and the subaerial fluvial plain environment of the lower Freda Formation. Examination of the genetic relationship of the Nonesuch sedimentational intervals, the textures within each facies type, petrographic data, paleocurrent analyses, and the regional interpretation of the western Lake Superior rift structure, suggest that most sediment was transported northward into the rift zone from the southern *Present flank of the basin. Less important sources were present within the rift zone but on the northern side of the Nonesuch basin. These data also suggest that the Nonesuch Formation in the Bear Creek cores was deposited in a basin that was partially restricted, or perhaps completely isolated, from areas containing Nonesuch Formation farther east in Wisconsin and Upper Michigan.