158
DISCUSSIONS
Middle to Late Ordovician rocky bottoms and rocky shores from the Manitoulin Islandarea, Ontario:1 Discussion
JAMES A. ROBERTSON
Mineral Development and Lands Branch, Ontario Ministry of Northern Development and Mines, 3rd Floor, 880 Bay St.,
Toronto, Ont., Canada M5S1Z8
Received August 28, 1989
Accepted October 20, 1989
Can, J. Earth Sci. 27, 158 (1990)
In their paper, Johnson and Rong describe the relationshipbetween the basal Ordovician and the underlying Precambrianquartzites at various localities near Highway 6 (formerly 68)between Whitefish Falls and Sheguiandah.
The physical descriptions and locations of the outcrops arecorrect, but the description of the Precambrian stratigraphyrequires correction.
The Precambrian rocks of the area are folded and metamor-phosed rocks of the Huronian Supergroup, which is subdividedinto groups and formations (Robertson et al. 1969). In the areain question there are three quartzite formations: the MississagiFormation of the Hough Lake Group; and the Lorrain Formationand the Bar River Formation, both of the Cobalt Group, theuppermost group of the Huronian Supergroup.
The distribution of these formations in the area under discus-sion has been mapped by Card (1976,1978), and Highway 6 hasbeen used for numerous field trips; the geology has been sum-marized in Robertson and Card (1988).
Johnson and Rong (Fig. 1 and p. 646) ascribed all quartziteoutcropping within their study area to the Lorrain Formation —admittedly the most spectacular and thus the best known unit.The correct designations are the following:
Birch Island, the north shore of Great La Cloche Island, andAmedroz Island are Mississagi Formation.
Dreamers Rock and isolated outcrops to the west as well asMcGregor Point and the southern portion of Frazer Point andBadgeley Point are Lorrain Formation, as are one outcrop onthe north shore of Hey wood Island and outcrops near the north-ern limit of Lots 4-8, Concession I, of Howland Township,Manitoulin Island.
'Paper by M. E. Johnson and Rong J.-y. 1989. Canadian Journal ofEarth Sciences, 26: 642-653.
The greater portion of Frazer Point - Killarney Point and thestring of outcrops running westerly from Pine Island throughBadgeley Island, Centre Island, Partridge Island, HeywoodIsland, and Manitoulin Island in the vicinity of Sheguiandah(south part of Concession I, Howland Township, and Conces-sions XII and XI, Bidwell Township) (Card 1976, Map 2318)are Bar River Formation.
Of the outcrops of particular interest to the Johnson and Rongpaper only those near Dreamers Rock are Lorrain Formation,whereas those at Sheguiandah are Bar River Formation.
The folding structure of the Precambrian is not discussed, andthe pattern chosen for the cross section (Fig. 7) gives the im-pression that the Precambrian comprises one formation dippingconsistently south, whereas the basement structure explains theprecise nature of the paleotopography and therefore ultimatelyof many features of the younger Paleozoic rocks.
I trust that those interested in this topic will find theclarification both significant and useful.
CARD, K. D. 1976. Geology of the McGregor Bay - Bay of Islandsarea, Districts of Sudbury and Manitoulin. Ontario Ministry ofNatural Resources, Geoscience Report 138 and Maps 2316, 2317,and 2318, scale 1 in. = 1/2 mi.
1978. Geology of Sudbury-Manitoulin area, Districts of Sud-bury and Manitoulin. Ontario Ministry of Natural Resources,Ontario Geological Survey, Report 166 and Map 2360, scale 1in. = 2 mi.
ROBERTSON, J. A., and CARD, K. D. 1988. Geology and scenery, northshore of Lake Huron region. 3rd ed. Ontario Geological Survey,Geological Guidebook 4. (First printed in 1972.)
ROBERTSON, J. A., FRAREY, M. J., and CARD, K. D. 1969. TheFederal-Provincial Committee on Huronian Stratigraphy. CanadianJournal of Earth Sciences, 6: 335-336.
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