Introduction
Manitoulin Island is reported to be the world’s largest freshwater island, being approximately 180 km long and 80 km wide. The Ojibway believe that Manitoulin Island was the home of Gitchi Manitou or the Great Spirit who inhabited its misty coves and tree-covered bluffs. Seventeenth century visitors included Champlain, Joliet, Marquette and LaSalle and the island soon formed part of the voyageur trade route to the west. The great Spirit would scarcely recognize his home today. The present landscape is a collage of mixed forest, fields separated by meandering split-rail fences, small communities, and cottages dotting the shoreline.
The first geologic reports of the island date back to the 1800’s. Numerous paleontolotical and stratigraphic studies generated a wealth of information on the rocks of the island and these aided significantly in understanding the geological setting of southern Ontario and correlative rocks in northern Michigan.
This field trip will take participants through selected outcrops of the early Proterozoic Huronian Supergroup and Ordovician-Silurian outcrops on and just north of Manitoulin Island. The nearly undisturbed, gently south-southwestward dipping Paleozoic sedimentary deposits of this area lap onto the Huronian strata of the Precambrian Shield (Fig. 1). The contact is exposed and accessible in several places. Except for a basal red-bed unit which is of uncertain age, the Paleozoic sediments begin with the Middle Ordovician and close with the Middle Silurian. Younger sedimentary rocks occur below the waters of Lake Huron. The Niagara escarpment stretches from New York State, crosses southern Ontario and the Bruce Peninsula, and reappears once again on Manitoulin Island. On the island, the escarpment is stepped, consisting of an Ordovician escarpment that is commonly the sight of spectacular water falls, and a Silurian escarpment that forms gleaming white cliffs.
Huronian stratigraphy
The Huronian Supergroup extends from Sault Ste. Marie in the west to the Ontario-Quebec border, a distance of about 340 km, and has a north-south extent of about 70 km. The age of the Huronian rocks in known from radiometric dating of the surrounding rocks. The underlying plutonic basement is 2.5 billion years old, thus providing a maximum age limit. The Huronian strata are intruded by Nipissing diabase dikes which are approximately 2.1 billion years old, thus providing a minimum age limit.
The Huronian sequence is about 10,700 m thick in the Espanola-Sudbury area. Current stratigraphic usage is shown in Figure 2. This excursion will give us the opportunity to examine the Mississagi, Bruce, Espanola, Serpent, Gowganda, and Lorraine formation. Part of the Huronian succession is cyclical. The cycle, clearly shown by the Hough Lake and Quirke Lake groups, involves three units which, in ascending order, are dominated by diamictite, mudstones, and cross-bedded arenites. The basal diamictites are usually considered to be glacial in origin but parts of these formations were evidently deposited by gravity flows. The succeeding fine grained units have been interpreted as deeper water deposits formed by post-glacial rise in sea level causing the shoreline to migrate northward. The Espanola Formation is unique among Huronian formations in that it contains abundant carbonate. The tops of the cycles are composed of thick sandstones that include the Mississagi, Serpent and Lorrain formations. These rocks are interpreted as prograding fluvial deposits, possibly related in part to isostatic rebound in the areas formerly under the ice cap. The most detailed sedimentological studies to date support a braided stream environment for most of these units.
The Huronian rocks underwent a complex series of deformational and intrusive events that are referred to collectively by some researchers as the "Penokean Orogeny". The earliest orogenic phase likely occurred while the Huronian sequence was still unlithified as suggested by intrusion of clastic dykes into folds, ragged "soft sediment" contacts along early faults, and the presence of magaslumps. The major east-trending folds seen in the main (Southern) part of the Huronian fold belt that resulted was contemporaneous with intrusion of Nipissing diabase dikes and the formation of the enigmatic "Sudbury breccia". The most recent tectonic model interprets the Huronian Supergroup as the complex filling of an easterly-trending fault-bounded trough or aulacogen that may have opened into an ocean in the region now occupied by the Grenville Province. This Proterozoic continental breakup in the Great Lakes region was contemporaneous with a major and protracted glaciation. The tectonic interpretation of these strate, however, remains problematical because original geologic relationships are only clearly displayed in a limited area. To the south of the Huronian outcrop belt in the Espanola-Sudbury area there is Paleozoic cover and the waters of Lake Huron. To the east, the easterly-trending fold belt is truncated by the Grenville Front.
Ordovician-silurian stratigraphy
The pre-Ordovician landscape of Manitoulin Island and northern Ontario probably consisted of low relief hills and valleys over which the spreading, oscillating Ordovician-Silurian (Tippecanoe) seas gradually transgressed. This landscape was probably very similar to that presently seen in the Whitefish Falls area where the rolling white hills of the Lorrain Formation dominate the scenery. The Paleozoic sedimentary cover is primarily carbonate rock, with minor shale and virtually no siliciclastic sand or conglomerate except at the very base. Stratigraphic columns for the Ordovician and Silurian are presented in Figure 3. Similar Paleozoic rocks occuring as outliers at Lake Nipissing, Lake Temiskaming, and other isolated outcrops demonstrate that the veneer of sediment was extensive but the whole eastern Precambrian Shield was not necessarily inundated. Overall thickness of the Paleozoic strata is less than 150 m in the Manitoulin area.
The Middle Ordovician saw a very critical development in marine benthic communities throughout the world and this is reflected clearly in the Manitoulin area. ‘Shelly’ benthos (brachiopods, gastropods, bivalves, cephalopods, etc.) became abundant for the first time. Concomitantly we see the introduction of early solitary rugose corals (streptelasmids), primitive colonial rugosans (columnariids), tabulates (Tetradium) and stromatoporoids (actinostromids, etc.) some of which occur in the form of biostromal beds rarely more than 30 cm thick. Biohermal (reefal) mounds occurred only during the middle Silurian time (Wenlock) on Manitoulin Island: some of these reefs are perhaps up to 30 m thick in their core, but most in this area seem to be less than 6 m high. Overall relief of these mounds during their deposition was probably less than 3m. Most bioherms were probably patch reefs sticking up only slightly above the inner-reefal, bedded carbonates. The Amabel Formation reef core and flank beds probably represent the best example of reef and reef-associated facies on Manitoulin Island, these being part of the Michigan Basin Silurian reef complex. Younger evaporites are absent on Manitoulin Island, but the reefs may have formed part of a barrier restricting circulation in the Michigan basin thus initiating evaporite precipitation there.
References
Card, K.D., Innes, D.G., and Debicki, R.L., 1977. Stratigraphy, sedimentology, and petrology of the Huronian Supergroup in the Sudbury-Espanola area. Ministry of Natural Resources, Ontario Division of Mines Geoscience Study 16, 99p.
Copper, P., 1978. Paleoenvironments and paleocommunities in the Ordovician-Silurian sequence of Manitoulin Island in Sanford, J.T. and Mosher, R.E., eds., 1978. Geology of the Manitoulin Area. Michigan Basin Geological Society Special Papers No. 3, p. 47-61.
Grawbarger, D.J., 1978. The Manitouwaning bioherm: and early Silurian patch reef, in Sanford, J.T. and Mosher, R.E., eds., 1978. Geology of the Manitoulin Area. Michigan Basin Geological Society Special Papers No. 3, p. 85-89.
Kobluk, D.R. and Brookfield, M.E., 1982. Lower Paleozoic rocks and paleoenvironments in southern Ontario. International Association of Sedimentologists Field Excursion 12A Guidebook, 62p.
Sandford, J.T. and Mosher, R.E., eds., 1978. Geology of the Manitoulin Area. Michigan Basin Geological Society Special Papers No. 3, 111p.
Young, G.M., 1982. Depositional environments and tectonic setting of the early Proterozoic Huronian Supergroup. International Association of Sedimentologists Field Excursion 13B Guidebook, 79p.
Young, G.M., 1985. The Gowganda Formation in the southern Part of the Huronian outcrop belt, Ontario, Canada: stratigraphy, depositional environments and regional tectonic significance. Precambrian Research, v. 29, p. 265-301.