Skip to Main Content
Book Chapter

7: Proterozoic sedimentary rocks

By
Richard W. Ojakangas
Richard W. Ojakangas
Search for other works by this author on:
G. B. Morey
G. B. Morey
Search for other works by this author on:
Published:
January 01, 1982

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.

You do not currently have access to this article.

Figures & Tables

Contents

GSA Memoirs

Geology and Tectonics of the Lake Superior Basin

Richard J. Wold
Richard J. Wold
Search for other works by this author on:
William J. Hinze
William J. Hinze
Search for other works by this author on:
Geological Society of America
Volume
156
ISBN print:
9780813711560
Publication date:
January 01, 1982

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal