Program, Abstracts, and Guidebooks - University of Minnesota Duluth

a
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at Disraeli Lake and near Stewart Lake belong to the group Conophyton
(Hoffman, 1969), formed of vuggy columnar "cone in cone" structures
(Plate lb). The chert facies ranges to 10 feet in thickness at Ross—
port, and is composed of finely laminated grey to black chert and
brown dolomite. Anthraxolite, accumulations are common along the base
of this unit.
The overlying limey red mudstone contains less than 20 per cent
coarse microcline with less than 2 per cent hematite. The clay
expands itt ethylene glycol, and is a mixed layer chlorite—montmor—
illonite, similar to corrensite (Peterson, 1961). The feldspar is
very fine—grained (less than 10 p diameter) and is probably authigenic.
The overlying purple mudstone unit is finely laminated,
moderately but irregularly fissile, and is composed of approximately
40 per cent each of corrensite and microcline, with less than 4 per
cent hematite, less than 15 per cent quartz, and minor calcite. Less
than 10 per cent coarse clastic material is present in most of this
unit.
The uppermost limestone unit is grey to buff, poorly bedded,
and crops out only north and west of Nipigon.
The Sibley Group is distributed over both the edge of an older
mobile belt (the Penokean orogenic deformation of Aphebian rocks)
and the stable Archean craton. Deposition occurred in a basin restricted
on the south and west by uplifted Aphebian rocks. A shallow,
periodically dry, basin transgressed northward over the craton.
Material was derived from both the Aphebian highlands and adjacent
Archean granitic rocks in a semi—arid, warm environment, (Franklin,
1970).
NEOHELIKIAN
Osler Grçp
Volcanic and sedimentary rocks of the Osler Group disconformably
overlie the Sibley Group and are exposed on an arcuate belt of islands
parallel to the shore of Lake Superior, and on Black Bay Peninsula
(Ont. Dept. Mines Map 2137). The lavas are similar to those of the
Portage Lake Lava Supergroup (DuBois, 1962) and are composed of thin,
laterally extensive sheets of vesicular, tholeiitic flood basalt with
minor interf low greywacke beds, & rhyolite (quartz porphyry) bodies.
Intrusive Rocks
The four types of intrusive rocks present in this area are as
follows:
(a) Logan sills: laterally extensive thin diabase sheets,
cutting Archean, Aphebian, and Paleohelikian rocks
(b) northeast—trending gabbro dykes, parallel to the
shore of Lake Superior, extending from Pigeon Point to Edward Island