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Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes region

By
Grant M. Young
Grant M. Young
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Published:
January 01, 1983

Paleocurrents, facies, and thickness variations suggest that volcanic and sedimentary rocks of the lower Huronian succession accumulated in an easterly-trending fault-bounded trough. The upper Huronian, here taken to include the glaciogenic Gowganda Formation and younger Huronian units, is much more widespread and records deposition under conditions of more general downwarping of the crust. In the Lake Superior region, rocks of the Chocolay Group, which includes tillites and extensive orthoquartzites, are considered to be part of this depositional phase. Paleocurrents in both the Huronian Supergroup and Chocolay Group are generally eastward-directed during these two depositional phases.

A strong deformational event next affected rocks of the Lake Huron region. This event is dated at about 2.1 Ga by contemporaneous intrusion of the Nipissing diabase. There is some evidence that much of the Huronian Supergroup was semilithified during this phase of deformation (McGregor phase). A similar tectonic episode may be recorded on the south shore of Lake Superior by intrusion of a porphyritic red granite about 2.0 Ga ago and by deformation and uplift of rocks of the Chocolay Group prior to deposition of the Menominee Group. The name Michigan phase is proposed for this event. In Minnesota, diabase dykes of similar age to the Nipissing diabase of Ontario were apparently intruded before deposition of the Menominee Group.

The next depositional episode is characterised, in the Lake Superior region, by deposition of basal conglomerate and orthoquartzite followed by iron-formation. Equivalent rocks are not known in the Lake Huron region which may have been weakly emergent in the aftermath of the McGregor phase. In the Lake Superior region, the final phase of recorded early Proterozoic sedimentary history is represented by a succession of carbonaceous mudstones followed in a coarsening-upward sequence by turbidites. This phase may be represented in the Lake Huron area by mudstones and proximal turbidites of the Whitewater Group, preserved uniquely in the Sudbury Basin. Turbidites of the Whitewater Group were transported westward; there is some evidence of similar transport directions in the corresponding rocks in the Lake Superior region, but significant amounts of clastic detritus were drawn from the north and south margins of the basin.

Subsequent deformation and low pressure, intermediate temperature metamorphism during the Penokean orogeny affected early Proterozoic rocks throughout the northern Great Lakes region. By analogy with other Proterozoic and Phanerozoic depositional basins, the sedimentary and tectonic history of early Proterozoic rocks of the northern Great Lakes area is tentatively interpreted in terms of the aulacogen model; a corollary of this interpretation is that the aulacogen opened into an ocean lying somewhere to the east, in the area now occupied by the Grenville province.

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GSA Memoirs

Early Proterozoic Geology of the Great Lakes Region

L. G. Medaris, Jr.
L. G. Medaris, Jr.
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Geological Society of America
Volume
160
ISBN print:
9780813711607
Publication date:
January 01, 1983

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