Keweenawan sedimentary rocks of the Lake Superior region consist of four major sequences: pre-volcanic sedimentary rocks, interflow sedimentary rocks, and post-volcanic sedimentary rocks consisting of the Oronto Group and equivalents and the overlying Bayfield Group and equivalents. The oldest pre-volcanic rocks of concern here are those of the Sibley Group (1350–1300 m.y. old), which includes quartzose sandstone, mudstone, and dolomite. Whereas the quartzose sandstone may be fluvial, the finer clastic units and the dolomite appear to be lacustrine (or marine?). Other quartz sandstone units, the Puckwunge Formation, the “Nopeming Formation,” the basal sandstones of the Osier Group, and the Bessemer Quartzite, are probably younger—about 1200–1100 m.y. old. The Bessemer and the lower Sibley are normally polarized, whereas the other units are reversely polarized and on that basis are probably younger. Most of these sandstones probably originated on braided alluvial plains, but the Bessemer, which has a bimodal-bipolar paleocurrent pattern, apparently originated in a standing body of water that was either influenced by tides or had opposing longshore current systems. Immature interflow sedimentary rocks, mostly fluvial sandstones, were derived in large part from the 1200–1100 m.y. old Keweenawan volcanic sequence of which they are a part. They are interbedded with volcanic rocks in several sub-basins along the rift zone. The post-volcanic sedimentary rocks, the Oronto and Bayfield Groups and their equivalents, constitute a thick red bed sequence of conglomerate, sandstone, and siltstone deposited in a large, elongate basin created by tectonic activity along the rift zone. The Oronto Group (Copper Harbor Conglomerate, Nonesuch Shale, and Freda Sandstone in ascending order) is present in northern Wisconsin, western Upper Michigan, and on Isle Royale. The Copper Harbor Conglomerate (to 2100 m thick) is dominantly a fining upward alluvial fan-fluvial clastic wedge. The gray cupriferous, pyritiferous, and carbonaceous Nonesuch Shale is an argillaceous siltstone unit (75–225 m thick) which accumulated under reducing conditions, probably in a lacustrine (and deltaic) environment. The Freda Sandstone (3600 m thick) is a fluvial (and lacustrine?) unit. The Oronto Group was largely derived from Keweenawan volcanic rocks on the basin flanks. In Wisconsin, the more steeply dipping Oronto Group is overlain (unconformably?) by the more mature subhorizontal Bayfield Group, which includes, in ascending order, the feldspathic Orienta Sandstone, the quartzose Devils Island Sandstone, and the feldspathic Chequamegon Sandstone. Geophysical evidence suggests that the group may be as much as 2100 m thick. The Devils Island Sandstone (100 m thick) is apparently the result of the lacustrine reworking of Orienta Sandstone fluvial detritus during a significant pause in, or cessation of, tectonic activity. The source rocks for the Bayfield Group were dominantly pre-volcanic granitic basement rocks, although reworking of Oronto Group detritus also may have been important. In eastern Minnesota, three formations are present—the feldspathic-lithic Solor Church Formation (to 1000 m thick), the feldspathic Fond du Lac Formation (120–600 m thick), and the quartzose Hinckley Sandstone (to 300 m thick). The Solor Church, found only in the subsurface, was deposited southwest of Lake Superior along the rift zone. The Solor Church Formation can be correlated with the Oronto Group, the Fond du Lac with the Orienta Sandstone, and the Hinckley Sandstone with the Devils Island Sandstone. The Solor Church and the Fond du Lac were deposited in a meandering stream-floodplain environment, whereas the Hinckley appears to have been formed by the reworking of Fond du Lac detritus in the same lacustrine environment in which the Devils Island Sandstone was formed. In Michigan, the feldspathic to quartzose Jacobsville Sandstone is a northward-thickening, fault-bordered wedge of regionally variable fluvial sedimentary rocks. The maximum drilled thickness is 868 m, and the geophysically inferred thickness is 3000 m. Most of the conglomerate clasts were derived from deeply weathered source areas to the south and southeast. The Jacobsville also is found at the east end of Lake Superior and beneath Lake Superior. Correlation with the Bayfield Group is likely but uncertain. Paleocurrent data indicate basinward transport of sediment from both the northern and southern flanks of the basin throughout Keweenawan time. The Proterozoic Sibley Group is the oldest of the pre-volcanic sedimentary units in the Lake Superior region, having been dated at about 1340 m.y. ago. The Sibley consists of more than 400 m of strata that may have been deposited in a failed rift basin; part of the unit may be marine, but the sandstones of the Pass Lake Formation at the base of the group may have at least a partial fluvial origin. The Bessemer Quartzite, “Nopeming Formation,” Puckwunge Formation, and the lower sandstones of the Osier Group are lithostratigraphic equivalents, all having been deposited before Keweenawan volcanism began in their respective outcrop areas. Although paleomagnetic data indicate that the Bessemer is older than the other three units, none can be correlated with certainty. The Bessemer Quartzite may be as old as, but is probably somewhat younger than, the Sibley Group. The dispersion of cross-bedding patterns in the Nopeming and the Puckwunge is suggestive of deposition in a fluvial environment, probably on a braided alluvial plain with streams flowing southward into a shallow basin. Paleocurrent data are sparse for the lower sandstones of the Osier Group, but location and stratigraphic position suggest an environment similar to that of the Puckwunge. The Bessemer Quartzite, however, has a contrasting bimodal-bipolar paleocurrent pattern, indicative of a tidally influenced marine or estuarine environment, or a lacustrine environment with opposing current systems. All of the units under consideration here contain some detritus derived from older Proterozoic sedimentary rock units of the Lake Superior region, but the bulk of the quartz-rich sediment may have been derived from the Archean granite-greenstone terranes north and south of the basin. The stable tectonic framework that produced the quartz-rich sediment was ended by the formation of a shallow basin that heralded the onset of volcanism. The coarse, immature, polymictic, red-bed clastic rocks interbedded within the Keweenawan lava flows provide insight about the relationship between these accumulations and the surrounding borderland during rifting. The interflow sedimentary rocks were derived from three major sources: subjacent lava flows, uplifted Keweenawan volcanic and intrusive rocks located some distance from the exposed interflows, and pre-Keweenawan rocks exposed along the periphery of the Keweenawan lavas. The composition of the interflow sedimentary rocks can be used to determine their source. Previous studies of the interflow sedimentary rocks indicate that the rocks were deposited by streams that flowed predominantly on surfaces mantled by Keweenawan lava flows. Paleocurrent data indicate that the streams flowed generally toward the present Lake Superior Basin from surrounding highlands. The site of these highlands, in some instances, coincides with the location of numerous Keweenawan intrusions that may have served as feeder fissures for the lavas. The Keweenawan-age Oronto Group of northern Michigan and Wisconsin includes the Copper Harbor Conglomerate, Nonesuch (Shale) Formation, and Freda Sandstone. These formations are part of a volcanic-clastic sequence created in response to the formation of the Midcontinent Rift System. Although intercalated volcanics are found in the lower one-third of the Oronto Group, a sedimentary depositional regime was dominant. Along the Keweenaw Peninsula, paleocurrent indicators for all three formations show that the predominant depositional directions were northerly. On Isle Royale, the opposite side of the rift, sedimentary structures in the Copper Harbor Conglomerate indicate that flow was to the south and east. Lithologically, the Copper Harbor Conglomerate is a red-brown, basinward-thickening wedge of volcanogenic clastics and subordinate volcanics that fines distally and upsection. Maximum thickness for this formation is about 1830 m. The dominant sandstone type is lithic graywacke. Conglomerate facies are primarily clast-supported and comprised of volcanic clasts with a ratio of mafic to silicic + intermediate clasts of about 2:1. The heavy-mineral suite for the Copper Harbor Conglomerate (as well as the other Oronto Group formations) mainly consists of ilmenite and similar opaque minerals and epidote. Depositionally, the Copper Harbor Conglomerate represents a prograding alluvial fan complex. Interfingering with the Copper Harbor Conglomerate is the Nonesuch (Shale) Formation, an unoxidized sequence of gray-black siltstone, shale, and sandstone with a maximum thickness of 215 m. Besides having been deposited in a reducing environment, the Nonesuch differs from the enclosing redbed sequences by its increased textural maturity and its sulfide and hydrocarbon content. The heavy-mineral suite of the Nonesuch also differs from that of the redbeds only by relative enrichment of chlorite. The Nonesuch is perceived as a rift-flanking lacustrine environment, probably initiated through disruption of existing drainages by alluvial, volcanic, or tectonic processes. As with the underlying Copper Harbor Conglomerate, the contact with the overlying Freda Sandstone is gradational in character. The Freda Sandstone is a ferruginous, lithic sequence of cyclic sandstone and mudstone exceeding 3660 m in maximum thickness. Although similar in appearance to some sandstones of the Copper Harbor Conglomerate, the Freda, overall, is of greater compositional maturity, and conglomerate facies are uncommon. The Freda is dominantly fluvial in origin and appears to have “overridden” the Nonesuch environments. Although complex in detail, the overall depositional model for the Oronto Group is one of simple transgressive-regressive relationships between alluvial fan/lacustrine/fluvial environments. Important aspects of such a model are that (1) all the Oronto Group formations are genetically related with no major unconformities between them; and (2) the intervening Nonesuch Formation is, at least in part, equivalent in age to the upper Copper Harbor and the lower Freda. 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. 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.

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.

The thickness of the unconsolidated sediment and the topography of the underlying bedrock surface of Lake Superior are interpreted from 8,000 km of high-resolution seismic reflection profiles taken during 1966 and 1967. A depth-to-bedrock map was constructed by combining the isopach map of unconsolidated sediments (prepared from our profiles) with the bathymetric map of the lake (Canadian Hydrographic Service Chart 885, 1973). Lake Superior can be divided into three morphologic regions on the basis of bathymetry and underlying bedrock: a western region composed of long linear valleys and gentle changes in relief, a central region composed of a single broad bathymetric depression, and an eastern region composed of a complex pattern of linear troughs and ridges. The western region is dominated by a nearly continuous bathymetric and bedrock valley paralleling the north shore from Thunder Bay, Ontario, to Duluth, Minnesota. The bedrock valley reaches depths of more than 800 m below lake level near Silver Bay, Minnesota, and has more than 500 m of overlying unconsolidated sediments. It probably resulted from differential glacial erosion, where the relatively erodible sandstone of the Proterozoic Bayfield Group comes in contact with Proterozoic rocks of the relatively resistant underlying volcanic rocks of the Keweenawan Supergroup and gabbro of the Duluth Complex. The central region, separated from the western region by a north-south basement ridge, consists of a broad valley with only 8 to 15 m of unconsolidated sediments overlying the bedrock surface. The complex pattern of troughs and ridges of the eastern region forms a north-south dendritic pattern with valleys as much as 100 km long but only 5 to 10 km wide. The bedrock surface is more than 600 m below lake level in some places and is overlain by as much as 300 m of unconsolidated sediments. These valleys are probably the result of erosion by subglacial streams. The stream erosion may well have followed a system of shear zones that have been observed in the shore exposures of the underlying Proterozoic sedimentary rocks of the Bayfield Group and Jacobsville Sandstone to the south. Most of these bedrock valleys are truncated about 15 km north of the south shore, where the lake floor rises abruptly to the coastline, although two valleys extend onshore. In general, the morphology of the bedrock surface in Lake Superior probably reflects the result of scour by glacial and subglacial streams, which in turn were localized by lithologic contacts, pre-existing topography, and shear zones. Over most of the lake, the acoustic impedance contrast across the contact of unconsolidated sediments with bedrock is high enough so that the seismic energy is reflected with little or no penetration into the bedrock. Despite this, there are some places where layering within the bedrock can be identified and apparent dips determined. Among the structural trends determined from these dips are the following: a southwest-plunging synclinal feature bordering the Bayfield Peninsula; a syncline lying between the Apostle Islands and the Keweenaw Peninsula that probably represents the center of the Lake Superior depositional syncline; a south-plunging syncline located between Michipicoten Island and Superior Shoals; and an apparent southward dip of the bedrock in the southeastern region of the lake.

The Lake Superior Basin and the Lake Superior syncline are differentiated as tectonic elements; the syncline is locally defined within and adjacent to the southwestern and southern portions of the basin. Formation of the basin apparently commenced with the extrusion of lava in sub-basins. As a result of rifting, the lava ponded and created the Keweenawan lava plateau about 1100 m.y. ago. Coalescence of subsiding sub-basins created the overall structure during Keweenawan time (1225 ± 50 m.y.-1100 ± 10 m.y. ago). Uncertainty exists as to the time of rift initiation dependent upon tectonic interpretation of the Sibley Group (1339 ± 33 m.y. ago) with respect to the rifting in the Lake Superior region. Faulting along sub-basin margins probably controlled subsidence locally. The development of the Lake Superior syncline was dependent upon subsidence of the Keweenaw Point-Isle Royale lava plateau, the youngest of such features in the Lake Superior area, together with folding of units at the eastern end of Lake Superior. Folding and faulting, particularly the development of major longitudinal and transverse faults, generally postdate volcanism and plateau subsidence. The basin appears to have been well developed by the time of deposition of Proterozoic Oronto and Bayfield Group sediments, although isostatic adjustments, which probably continued into the Phanerozoic, may have further accentuated the basin structure. The structural trends of Keweenawan units along the midcontinent gravity high are coincident with those in older Precambrian units in the Lake Superior area but cut the predominant east-west fabric found in older units at high angles in the Minnesota-Kansas and Michigan segments.

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.