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 Big Eric’s Crossing (Fig. 1, Site 1) is a county bridge acrossthe Huron River (NW¼NW¼Sec.35, T.52N., R.30W.), Skanee, Michigan 15-minute Quadrangle, about 12 mi (19 km) northeastof L/Anse, Michigan, along the road that continues north fromMain Street. The second locality (Site 2), a small, southeastfacinggranite hill, (SE¼:SE¼Sec.26) is about 1 mi (1.6 km)nrtheast of the bridge and lies about 600 ft (200 m) south of the road. Big Eric/s Crossing is a famous fishing spot on the HuronRiver, and Baraga County maintains a campground at the bridge.The river is high from late September until late May, so theoutcrops along the river are best visited during the middle andlate summer. Sites 2 and 3 are on private land. However, so long as visitsare brief, and visitors continue to respect the surface and theexposures, property owners in the Upper Peninsula tend not topost their lands.

In the Lake Superior syncline the Jacobsville Sandstone is a thick (+900m) fluvial sequence of feldspathic and quartzose sandstones, conglomerates, siltstones, and shales, completely devoid of lava flows or cross-cutting dikes. On the north and south sides of Lake Superior, most of the sandstone occurs as inward-dipping, fault-bounded wedges, separated by regional faults from the Oronto and Bayfield Groups of similar red sandstones, situated on the inner side of the syncline. Sandstones that have been correlated with the Jacobsville at the east end of Lake Superior are probably upper Keweenawan. Similar sandstones in the subsurface in the Michigan Basin are probably Keweenawan, but their precise correlation with the Jacobsville remains uncertain. A lower age limit for the sandstone is established from large basalt clasts derived probably from the Portage Lake Volcanics, and from the occurrence of the sandstone as dikes in the volcanics. The upper age is based on its position below the late Cambrian Munising Sandstone. The sandstone can be inferred to be Precambrian, and probably upper Keweenawan on the proximity, and structural and lithologic similarities (and hence a similar tectonic environment) of the sandstone to the other red sandstones in the Lake Superior syncline. The most precise data is from paleomagnetics, which shows that the Jacobsville on Keweenaw Bay and the Oronto Groups have a similar upper Keweenawan pole position (estimated at 1,100 m.y.), and that the Jacobsville may be slightly younger than the Freda Sandstone. The Jacobsville Sandstone varies from subarkose to quartz sub-lithic arenite, and there are some beds of arkose and of quartzite. Quartz is derived from both metamorphic and volcanic source areas, and none shows overgrowths. Microcline is fresh everywhere whereas plagioclase is fresh to highly altered. Other clasts in sandstones are of mafic and of felsic volcanic rocks, quartz-staurolite schist, garnet, epidote, biotite, muscovite, chlorite, and shale. Conglomerates with abundant clasts of quartz and iron-formation are known from the base of the section, 30 km east of Lake Gogebic from where they increase in thickness and abundance in the section to some 40 km west of the lake. Jacobsville sedimentation was preceded by a long period of volcanic and tectonic quiescence and cratonic stability so that bedrock surfaces became blanketed by paleosols and a surface of chemically resistant debris was dominated by quartz and iron-formation. Erosion was initiated by late Keweenawan warping, perhaps accompanied by basement faulting during which the relative movement along the Midcontinent Rift System was down. Vigorous marginal fluvial systems developed on the south side from uplands dominated by ridges of iron-formation. The resistant debris was removed first, and became deposited in alluvial fans along the deeper basins. Streams flowed across the marginal basins with possibly some local influence on stream direction and sedimentation by faults along the outer margins of active basins. The major movement on the marginal reverse faults was in post-Jacobsville time. Still younger faults affect Paleozoic outliers. Subsequent to burial, the sandstone underwent low-grade alteration so that now the matrix mineralogy changes from microline-plagioclase-kaolinite-montmorillonite near the surface to microline-montmorillonite-illite(chlorite) at depth. If in Michigan the fluvial transport direction was paralleled by a similar groundwater flow direction northerly from ridges of iron-formation, the period of weathering preceding and coinciding with Jacobsville sedimentation may have been the time during which the soft iron ores in the iron ranges were produced by the oxidation and leaching by such groundwaters to depths in excess of 1,200 m.