In order to make the following guides of roadside geology of interest to as wide an audience as possible I have added explanatory notes at the end of some stops. Advanced students may ignore these notes.
The Huronian Supergroup of Ontario is a 12,000 m thick belt of Middle Precambrian (1)rocks extending; from the east shore of Lake Superior, north of Sault Ste Marie, across the North shore of Lake Huron, as far east as the Quebec border. It is comprised mainly of sedimentary rocks although volcanic rocks and mafic intrusions are locally significant near the base of the sequence. The rocks of the Huronian Supergroup have been important both in terms of mineral wealth and for what they have revealed about the ancient Earth's climate and atmosphere. Rocks correlative with the upper formations of the Huronian Supergroup have been identified in the Marquette area of Michigan.
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(1) The Huronian Supergroup is a sequence of rock formations containing four Groups- The Elliot Lake Group, The Hough (huff) Lake Group ,The Quirke Lake Group, the Cobalt Group.
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All of the rocks of the Huronian Supergroup have undergone low to medium grade regional metamorphism. This means that for most rocks, especially the mafic volcanic rocks, the original mineral assemblages, and some small-scale structures may have been modified or destroyed to be replaced by minerals in equilibrium with the conditions of temperature and pressure produced by deep burial and tectonic forces.
The mineralogy of the sandstones and conglomerates however, have undergone much less modification than that of the volcanic rocks. Most of the original textures and primary sedimentary structures such as bedding and pebble size and shape can easily be recognized. For the most part however, recrystallisation and welding at points of contact have; the clasts of the sandstones and conglomerates break "across the grains" indicating that they are "quartzites". For the purpose of this discussion I will not use the prefix; "meta" when referring to metamorphosed sedimentary rocks of low metamorphic grade.
In the Sudbury area the Huronian is about 12 000 m thick, and it is at least 6 000 m thick north of the town of Thessalon on the North Shore of Lake Huron. The thickness of rocks that was actually deposited during Huronian time cannot be accurately determined because the top of the Huronian is not exposed. Also, faulting, folding and unconformities within the Huronian make the present estimates uncertain.;
In recent years Geochronologists have become successful; in determining the age of many igneous rocks within a few million years (However, the time of deposition; of sedimentary rocks has been , with few exceptions, impossible to determine.) Zircons in a; rhyolite body in the Sudbury area were; found; to have crystallized; about 2.45Gabp; (Billion years before the present). This rock, the "Copper Cliff Rhyolite" is the only accepted Huronian rock which for which an absolute age date has been determined, but the date is accurate to only within 25 million years. A number of mafic layered intrusions between Sudbury and Elliot Lake have given dates of within 5 million years of 2.45 Ga. The location, stratigraphic position; and composition has led; many geologists believe that these mafic intrusions are magma chambers for the basaltic rocks near; the base of the Huronian Supergroup between Sault Ste Marie and Elliot Lake. If this is true (and it probably is) then most rocks; the Huronian Supergroup are younger than 2.45 Ga.
Everywhere they are found the Huronian rocks are intruded by sills and dikes of gabbroic rocks generally referred to as "Nipissing Diabase". The Nipissing diabase has been dated as 2.22 Ga.. Since the Nipissing diabase cuts across all of the known Huronian rocks, we can conclude all Huronian rocks are older than 2.22 Ga.
In contrast to some earlier writers, the author considers all volcanic rock of the Elliot Lake Group between the Elliot Lake are and Sault Ste Marie were one a contiguous flood basalt field and correlative with; the Thessalon Formation. Any correlation with the Huronian volcanic rocks of the Sudbury area will not be discussed here.
Available chemical analyses indicate that the Thessalon Formation can be subdivided into a basaltic member, comprised mainly of tholeiitic metabasalt, and an underlying andesite member of more varied composition, including metamorphosed basaltic andesite, mugearite, hawaiite, and rhyolite. In the Sault Ste Marie are the basaltic member is about XXXm thick and the andesitic member is about xxx m thick. In the Elliot Lake area the basaltic member is very thin or missing while the underlying andesitic member is well represented north of Ten Mile Lake.
The general absence of pillow structures in the Thessalon Formation, along with local thin interbeds of sandstone and quartz pebble conglomerate, suggest a terrestrial rather than a marine setting for the volcanic assemblage.
Over much of the area between the Sault Ste Marie area and Elliot Lake, conglomerate and sandstones of the Livingstone Creek Formation unconformably overlies the Archean basement rocks, most of which are over 2.7 Ga old. These ancient; rocks are predominantly pale-pink to reddish granite, granite gneiss and migmatite. In a few places there are relatively small Archean greenstone belts which are even older than the granitic rocks (Map 1).
It is commonly found that immediately below the rocks of the Elliot Lake Group the granitic rocks are commonly a pale gray color; while a few hundred meters or less away from the unconformity (ca. up to tens of meters stratigraphically) the granitic rocks are more likely to be the typically pinkish color.;
It is the opinion of the writer that the tiny amounts of ferric iron in the feldspars of the granite has been reduced to the less colorful; ferrous iron as a result of deposition in an environment lacking free oxygen.
Two major Early Proterozoic 2454+- 2 Ma( Heaman, 1988) north to northwest striking dike swarms intrude the Archean basement rocks north of the Huronian belt. The Hearst and Matachewan swarms; are quartz diabase generally containing saussuritized plagioclase phenocryts or aggregates of phenocrysts up to 20 cm across. Both swarms have similar ages of; 2454+- 2 Ma( Heaman, 1988).
A series of five gabbro-anorthosite intrusions extend eastward from the Huronian rocks of Quirke Lake Syncline into the Grenville Supergroup south east of the Sudbury Igneous Complex. These rocks have returned dates of 2491+-5 Ma and 2480+10-5Ma. The orientation and distribution of these bodies suggest that they are high-level magma chambers to the volcanic rocks of the Thessalon Formation.;
The lowermost formations of the Huronian Supergroup are those of; the Elliot Lake Group, and it; is within this group that the only Huronian volcanic rocks are found. Whereas most younger Huronian formations are continuous over all or most of the Huronian belt, the formations of the Elliot Lake Group are less widely distributed and show greater variations in thickness.;
The Livingstone Creek Formation is the lowermost, and probably the oldest, of Huronian Formations. In most areas the basal units of the Livingstone Creek Formation is clast-supported and polymicitic conglomerate containing megaclasts which generally reflect the gray color of the rock types of the immediately underlying; Archean basement. In some areas; the sandstone member lies directly on the basement rocks.;
The sandstone member of the Livingstone Creek Formation is everywhere a gray, cross-bedded, calcareous sandstone of fine to medium sand-sized clasts. No mudstone or beds of coarse sandstone were noted anywhere in the sandstone member of the Livingstone Creek Formation. The limited range of clast size suggests that the sandstone may have an eolian origin, or has; at least; been sorted by wind at some stage.
Most of the Livingstone Creek rocks are covered by younger Huronian Formations. The limited exposure limits any quantitative description of the distribution and thickness of the formation but the geological mapping and diamond drill hole data indicate that the preserved thickness of the Livingstone Creek Formation is probably greatest in the Sault Ste Marie area. Patches of clast-supported polymictic conglomerate overlying Archean rocks in the Elliot Lake area have been tentatively correlated with the Livingstone Creek Formation by the author.;