Petrography and Sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, Western Lake Superior Region, Midcontinent Rift System
1991-06
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Petrography and Sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, Western Lake Superior Region, Midcontinent Rift System
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1991-06
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Detailed sedimentological descriptions and petrographic analysis of the upper Keweenawan Nonesuch Formation was accomplished for selected Bear Creek drill cores (drilled in 1958, 1959 and 1960) from Ashland, Bayfield, and Douglas Counties, Wisconsin. These data, coupled with the information from outcrops in northwest 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. Middle 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 and/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 sometimes upon, a prograding alluvial fan complex. The Nonesuch Formation in the Bear Creek drill cores was divided into six sedimentational intervals (1, 2, 3, 3a, 4, and 5, lowest to highest) based on the occurrence of similar textures, sedimentary structures and color. Light-gray to black rocks predominate in all of the intervals and indicates that deposition of Nonesuch sediments was in a reducing environment. This is in contrast to the reddish-brown rocks of the underlying Copper Harbor and overlying Freda Formations, which were deposited in oxidizing environments. Sedimentational intervals 1 and 2 indicate a fining-upward and basinward fining trend consisting of conglomeratic sandstones to mudstones that occur as fining-upward sequences, and massive and normally graded beds deposited by turbidity and density currents in a shallow to deep water marginal lacustrine environment. The facies assemblages in intervals 1 and 2 record elastic deposition that occurred during the initial stages of development and transgression of the Nonesuch lake over the contemporaneous alluvial fan complex of the upper Copper Harbor Formation. Interval 3 of the Nonesuch consists predominantly of alternating varve-like beds of organic-rich mudstones and carbonate-rich siltstones deposited by suspension settling and bottom currents during periods of high water level in deep/quiet water areas of the central lake basin. Interval 3a consists of massive and normally graded turbidity and density current deposits and is completely enclosed by the carbonate laminite of interval 3. Interval 3a facies record a rapid and brief climatic and/or tectonic change, with either large-scale turbidity currents reaching the deeper areas of the basin, or a shift in environment of deposition from deep water central lake basin to shallower water marginal lacustrine. Sedimentational intervals 4 and 5 indicate a general coarsening-upward trend from deep water central lake siltstone and carbonate laminite mudstone, to shallow water marginal lacustrine facies with massive and normally graded beds and fining-upward sequences consisting of conglomeratic sandstone, siltstone and mudstone. Parallel beds, lenticular beds, small-scale trough cross-beds, and mud drapes with dessication cracks increase in abundance upsection. The facies in intervals 4 and 5 record elastic deposition during the final regressive stages of the Nonesuch lake and are contemporaneous with the lower Freda Formation, which records a subaerial fluvial plain environment to the south. Examination of the genetic relationship of the Nonesuch sedimentational intervals, combined with percentages of the various compositions and textures within each facies type, petrographic data, 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 flank of the basin. Less important sources occurred within the rift zone but on the northern side of the Nonesudh 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.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Thomas John Suszek in partial fulfillment of the requirements for the degree of Master of Science, June 1991.
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