The Petrology and Sedimentation of the Basal Keweenawan Sandstones of the North and South Shores of Lake Superior
1972-03
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The Petrology and Sedimentation of the Basal Keweenawan Sandstones of the North and South Shores of Lake Superior
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1972-03
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Abstract
The Puckwunge Formation of northeasternmost Minnesota is intermittently exposed for a 25-mile stretch beneath the Keweenawan North Shore Vo1canic Group. The formation consists of a basal conglomerate and an overlying orthoquartzite, with the maximum estimated total thickness of the formation being 300 feet. Its composition, texture, and structures indicate that the formation was derived from pre-Keweenawan rocks to the North, and was deposited in a high-energy, near-shore environment. The formation disconformably rests on the Middle Precambrian Rove Formation, and is disconformably overlain by Keweenawan lava flows. The Puckwunge Formation is inferred to be correlative with the nearby Sibley Formation of Ontario. The Sibley Formation is disconformably overlain by sedimentary rocks and lava flows of the Osler Formation, also Lower Keweenawan in age. Sedimentary rocks correlating with the Osler Formation are not present in northeasternmost Minnesota. At Nopeming, just west of Duluth, Minnesota, 28 feet of orthoquartzite and interbedded conglomerate are exposed beneath Keweenawan lava flows. Quartzite is the dominant lithology present, with minor metasiltstone interbedded with the quartzite just below the lava flows. The sediments were principally derived from pre-Keweenawan rocks of the area, and deposited by northward-flowing currents, with minor amounts of volcanic detritus being deposited in the metasiltstone. The sedimentary rocks overlie the Middle Precambrian Thomson Formation with an angular unconformity, and are conformably overlaid by Keweenawan lava flows. Deposition of the sediments took place in a high-energy, shallow water environment. The Bessemer Formation of Michigan and Wisconsin is intermittently exposed beneath Keweenawan lava flows along a 55-mile stretch. The dominant lithology is orthoquartzite; however, minor amounts of interbedded siltstone and a basal conglomerate are also present. The formation was principally derived from pre-Keweenawan rocks to the north, with deposition again having taken place in a shallow water, near-shore environment. The formation disconformably overlies the Middle Precambrian Tyler Formation, and is conformably overlain by Keweenawan lava flows. South of the Lake Superior Region, several lower Upper Precambrian quartzite formations are exposed, and may be correlative with some of the Lower Keweenawan formations to the north. Three orthoquartzites, the Barron, Baraboo, and Sioux Formations are very similar in composition, lithology, and sedimentation, and appear to have been deposited during the northward transgression of a sea into a stable region. The Sioux, Baraboo, Barron, Puckwunge, and Sibley Formations were deposited in a shallow water, near-shore, tectonically stable environment, before the outbreak of Keweenawan igneous activity in their depositional areas; while deposition of the Osler, "Nopeming," and Bessemer Formations, which contain minor amounts of volcanic rock fragments, was interrupted by extrusion of the Keweenawan lavas. The Sioux, Baraboo, Barron, Puckwunge, Sibley, and Bessemer Formations are inferred to be correlative with each other, while the Lower Keweenawan sediments at Nopeming and the Osler Formation appear to be younger in age.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Allen Francis Mattis in partial fulfillment of the requirements for the degree of Master of Science, March 1972. Allen Mattis was the department's first Master's oral exam, January 8, 1972.
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