7E Jacobsville sandstone

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