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Keweenawan sedimentary rocks associated with the Midcontinent Rift System form a thick, dominantly red-bed sequence of fluvial-lacustrine origin in eastern Minnesota and northwestern Wisconsin. The strata can be divided into two major sequences on the basis of mineral composition and tectonic setting. The older sequence includes the Oronto Group in northwestern Wisconsin and the Solor Church Formation in southeastern Minnesota. Although these lithostratigraphic units are not entirely correlative, they were derived largely from within the rift system. Both contain variable amounts of quartz, feldspar (plagioclase > K-feldspar), rock fragments (basalt > granite), and other labile constituents. The younger sequence includes the Bayfield Group in Wisconsin, and the Fond du Lac Formation and Hinckley Sandstone in Minnesota. These correlative lithostratigraphic units, derived largely from outside the rift system, contain abundant quartz, feldspar (K-feldspar > plagioclase), and rock fragments (granite > basalt). The Oronto Group and Solor Church Formations were deposited in part contemporaneously with basalt in several grabenlike basins along the axis of the expanding rift system. Subsequently, relative uplift of the axial zone formed a series of half-grabenlike basins along the flanks of the rift system, in which the Bayfield Group and its equivalents in Minnesota were deposited. The transition from Oronto-like to Bayfield-like rocks marks a transition from dominantly extensional to dominantly vertical tectonic processes. This relationship between tectonism and sedimentation may be useful in unraveling the stratigraphic history of the sedimentary rocks in those parts of the Midcontinent Rift System buried by Paleozoic and younger strata.

Abstract Precambrian rocks in the Lake Superior region underlie all or parts of Minnesota, Wisconsin, and Michigan, an area along the southern margin of the Superior province of the Canadian Shield (Fig. 1). Except on the north, adjacent to Canada, the Precambrian rocks are overlapped by sedimentary strata of Paleozoic and Mesozoic age, which constitute a thin platform cover of relatively undisturbed rocks that thicken to the west, south, and east. Inliers of Precambrian rocks are exposed locally in southern Minnesota and Wisconsin, mainly in the flat valleys of major rivers, where erosion has cut below the Phanerozoic strata. The present landscape is subdued, and is inherited largely from Pleistocene continental glaciations, which produced a variety of erosional and depositional landforms. The glacier ice scoured the bedrock in the northern parts of the region, in much the same way as throughout most of Canada, and deposited materials of diverse lithology and provenance, as much as 200 m thick, over much of the remainder of the region. The Precambrian rocks in the region record an extended interval of crustal development and evolution that spans nearly 3 b.y. of earth history. This interval of geologic time is not continuously recorded in layered and intrusive units, but instead is punctuated by specific rock-forming and tectonic events that can be deduced from geologic relations and placed in a chronometric framework by isotopic dating. (Fig. 2, also see correlation chart for Precambrian rocks of the Lake Superior region, Morey and Van Schmus, 1986; and Bergstrom and Morey, 1985.)

Abstract This volume of 25 field guides plus one paper on field instruction was prepared in conjunction with the 2011 GSA Annual Meeting in Minneapolis, Minnesota. The diverse slate of field trips spans a geologically broad range of topics, including the Precambrian geology of the southern Canadian Shield; the economic geology of the Lake Superior region; Phanerozoic strata in Minnesota, Wisconsin, Iowa, and North Dakota; glacial geology; hydrogeology and limnology; undergraduate and K12 geoscience field education; archaeological investigations in the upper Mississippi River valley; and geology by bicycle.