Quetico Geology

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Quetico Park lies within the Canadian Shield, a vast region of exposed Precambrian (more than 550 million year old) bedrock that extends from the Arctic Islands in the north to Minnesota in the south and from Great Slave Lake in the west to the coast of Labrador in the east. The early stages in the formation of the Canadian Shield are controversial. Nevertheless, it is thought to have formed as the result of plate tectonic processes as smaller continental landmasses drifted and collided over a period of hundreds of millions of years.

Prior to 2.7 billion years ago the rocks of northern Ontario formed a series of E-W trending belts of mainly volcanic and plutonic (or intrusive) rocks separated by oceans. Two of these are called the Wawa and Wabigoon belts. The Wawa belt crosses the southeastern part of Quetico Park, whereas the Wabigoon belt is found a short distance to the north of the Park. Between the Wawa and Wabigoon belts, and occupying the northwestern two-thirds of the Park, is the Quetico belt, composed largely of sedimentary rocks deposited in the intervening oceanic basin at about 2.7 billion years ago. Within the Park the sediments of the Quetico belt were intruded by large granitic plutons shortly after deposition. During ocean closure and collision of the belts, the rocks of the Quetico area were deformed and metamorphosed at elevated temperatures and pressures.

The large continental landmass that resulted from the amalgamation of the various volcano-plutonic and sedimentary belts is called the Superior craton. Today, it occupies most of northern Ontario and northern Quebec. Between 2.0 and 1.8 billion years ago, the Superior craton and several other large cratons collided with one another to form the nucleus of the Canadian Shield.

Geological Map of Quetico Park - click to see enlarged version in a new window

The sedimentary rocks of the Quetico belt are mainly grey to buff-coloured wacke, a variety of sandstone, that occurs over a large area in the western part of the Park, especially between Beaverhouse Lake and the Maligne River. However, much of the rock exposed in the Park belongs to the granite plutons that intruded the sedimentary rocks of the Quetico belt. Two major types of granite are present. One is a white or grey-coloured muscovite granite found over large areas in the northern and central portion of the Park, including the area between Sturgeon and Pickerel lakes. The second is a pink-coloured biotite granite, typical of the southwestern portion of the Park in the vicinity of Lac La Croix.

Iron formation occurs in both the Wabigoon belt just north of the Park and in the Wawa belt in the southeastern part of the Park. From the 1940's to 1980, a huge open pit mine operated at Steep Rock Lake, near Atikokan, on the southern edge of the Wabigoon belt. To reach the hematite orebody it was necessary to divert the Seine River and drain a large portion of the lake. In the Wawa volcanic belt within the southeastern part of the Park, iron formation occurs in outcrops along the shores of Sheridan, That Man, Other Man and Otter Track lakes. It is generally composed of hematite (or magnetite), jasper and white quartz. In some areas, such as Otter Track Lake, the iron formation, which is quite magnetic, can disorient a magnetic compass.

Faults are fractures in the rock where one side has been displaced relative to the other. They occur at a number of localities in the Park and are often responsible for prominent topographic features, such as the cliffs along the west shore of Agnes Lake. The narrow lakes along the Man Chain and the Kahshahpiwi, Keefer, Sark, and Cairn chain of lakes lie along old faults.

Glaciers, which completely covered Quetico Park as recently as 13,000 years ago, had a major effect on the landscape. As they advanced across the Park, they removed soil and some of the underlying bedrock. As the temperature rose at the end of the ice-age, the receding glaciers left behind deposits called moraines. Moraines consist of ground up rock varying in size from microscopic particles to large boulders. As a result of these processes, drainage patterns in the Park were rearranged. Lakes filled depressions that were gouged out of the bedrock by the glaciers, and glacial deposits blocked and diverted earlier rivers and creeks. The glaciers also left scratches or glacial striae on rock surfaces throughout the Park. For example, they can be seen on the shores of Sturgeon Lake west of Sturgeon Narrows, Knife Lake, Otter Track Lake, Saganagons Lake and the west shore of Cairn Lake.

Sources:
V.B. Meen, Quetico Geology (The Quetico Foundation, 1959);
P.C. Thurston, H.R. Williams, R.H. Sutcliffe and G.M. Stott (editors), Geology of Ontario. Ontario Geological Survey, Special Volume 4, 1991;
Thanks to Trustee Ken Buchan for input on the text.

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