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Age and Correlation of the Precambrian Baraboo Quartzite of Wisconsin

R. H. Dott Jr. and I. W. D. Dalziel
The Journal of Geology
Vol. 80, No. 5 (Sep., 1972), pp. 552-568
Stable URL: http://www.jstor.org/stable/30063976
Page Count: 17
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Age and Correlation of the Precambrian Baraboo Quartzite of Wisconsin
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Abstract

The Baraboo Quartzite and overlying metasediments in south-central Wisconsin long were assumed correlative with middle Precambrian Huronian and/or Animikean strata of the Lake Superior region-that is, presumably between 1,800 and 2,600 m.y. old. Like Animikie rocks in northern Wisconsin and Michigan, the pure quartzite at Baraboo is overlain by dolomite, banded iron "formation," and slate. But rhyolitic tuff breccias and welded tuffs that stratigraphically underlie the Baraboo metasediments have yielded a Rb-Sr whole-rock isochron date of $1,640 \pm 40 m.y. (\lambda\beta = 1.39 \times 10^{11} years^{-1})$ with an initial $Sr^{87}/Sr^{86}$ ratio of $0.7048 \pm 0.0064$. Previous isotopic dating indicates that similar quartzite at nearby Waterloo, Wisconsin can be no younger than 1,410-1,440 m.y.; this appears to be a younger limit for all of the Baraboo-type sediments. If the Rb-Sr isochron age is interpreted as approximately the time of eruption of the rhyolites, then overlying Baraboo metasediments would be of late Precambrian rather than middle Precambrian age as currently defined in the region. Rocks in the Baraboo region and in many other areas of central Wisconsin were deformed and metamorphosed during some event in the interval 1,300-1,500 m.y. ago, which seems to have had widespread effects in the central United States. If, as considerable circumstantial evidence indicates, the Sioux, Barron, and Bessemer Quartzites occurring to the west and north are equivalent to the Baraboo, then a region of thick quartz sand sedimentation extended at least from present Lake Michigan to western South Dakota about 1,500-1,600 m.y. ago. The sediments suggest accumulation on a subsiding shallow-water shelf, but it is not clear whether that shelf marked the southern margin of an embryonic North American continent or lay within some intracontinental belt.

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