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.

Abstract Most of the economic gold placers of the late Archean Witwatersrand Supergroup of South Africa have been found to be of fluvial origin. A downstream transition from the fluvial paleoenvironment to coastal environments is expected, but such facies changes have not been observed within the previously studied areas. In this paper, the sedimentology of the uppermost formations of the Supergroup in a distal part of the preserved basin, namely the Vredefort structure, is described and evidence for transition from fluvial through tidal to shallow marine depositional conditions is submitted. The study area comprises the exposed arcuate northeastern to southwestern sector of the Vredefort structure, where overturned strata dip radially towards its center. In the Vredefort structure, the upper subgroup of the Witwatersrand Supergroup, the Turffontein has been subdivided into five formations, for which the codes Rt1 through Rt5 are used. The succession between formations Rt1 through Rt3 is the subject of this study. Only the upper part of the basal formation, Rt1, is exposed in the study area and comprises coarse-grained, trough and planar cross-bedded arenites, which are texturally moderately mature. Paleocurrent distributions are mostly unimodal, except in the extreme eastern region, where the distribution is polymodal. Formation Rt2 disconformably overlies formation Rtl and consists of interbedded conglomerate and coarse-grained, cross-bedded arenites. Pebble size is largest in the northwestern parts of the study area and decreases towards the east. Paleocurrent distributions are unimodal in the western parts of the study area, but bimodal to polymodal in the central and eastern parts. Formation Rt3 conformably overlies formation Rt2 and comprises predominantly coarse-grained, moderately mature, trough and planar cross-bedded quartz wackes, with scattered small pebbles and thin conglomerates. Paleocurrent distributions of this formation, too, are unimodal in the western part of the study area and mostly bimodal in the remainder. A number of planar cross-bed sets with foreset packages separated by mudstone or immature sandstone laminae were found in the outcrops of the central and eastern parts of the study area. In some cases the thicknesses within series of consecutive foreset packages vary cyclically. A planar cross-bed set with mud-draped foreset packages was found towards the top of the formation in the eastern parts of the study area. The mudstone-bounded foreset packages described are interpreted as tidal bundles and indicate a tidal setting for those paleogeographic parts of the formations where they occur. Bimodal paleocurrent distributions provide additional evidence for tidal deposition. West of an imaginary point near the center of the study area, no indicators of tidal sedimentation have been found and all the formations were probably deposited by fluvial processes here. However, to the east of this point, which indicates the position of a paleo-bayline, formations Rt2 and Rt3 exhibit characteristics indicative of tidal deposition. Paleocurrents, and the decrease in pebble size of the conglomerates of formation Rt2 towards the east, suggest that its sediment was derived from a western source area, most likely in response to tectonic uplift and tilting here, and transported into a tidal environment by fluvial processes. The basal disconformity of this formation is thought to be a type 2 unconformity and the result of progradation of coarse-grained gravel across a paleo-surface comprising sand.