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Voyageurs National Park

Geologic Setting

Figure 3. A schematic drawing of subprovinces of the Superior Province. Contacts between subprovinces are fault zones.
Figure 3. A schematic drawing of subprovinces of the Superior Province. Contacts between subprovinces are fault zones.

The park is at the southern end of the Canadian Shield, a large area of exposed basement rock containing some of the oldest Precambrian rocks in North America and forming the ancient core of the continent (Hemstad et al. 2002).

The Precambrian is divided into two eons: Archean (3,800+ to 2,500 million years) and the Proterozoic (2,500 to 540 million years) Most of the rocks in Voyageurs NP are igneous and metamorphic rocks of Archean age. The metamorphic rocks, mostly schists and gneisses, are exposed in the west and central portions of Voyageurs and igneous granitic rocks are exposed in the east and southeast areas of the park. These units belong to the Quetico subprovince of the Superior Physiographic Province (Day 1990; Harris et al. 1995; Kiver and Harris 1999; Davis et al. 1994). Voyageurs National Park straddles the transition from volcanogenic greywacke and granite of the Quetico subprovince to the metamorphosed sedimentary and volcanic rocks of the Wabigoon subprovince (figure 3) (Day 1990; Weeks and Andrascik 1998).

Metamorphic rocks of the more northern Wabigoon subprovince form the islands in the extreme northwestern segment of the park and are part of a northeast trending greenstone belt. The Rainy Lake - Seine River fault zone, a major northeast- southwest trending right- lateral strike- slip fault zone, marks the contact between the two subprovinces. The rocks at Voyageurs have been deformed by at least three episodes of Precambrian folding and faulting. Each episode has resulted in a specific pattern of deformation. The metamorphic and igneous rocks reveal a history of moving lithospheric plates, ancient subduction zones, mountain building episodes, and extensive volcanic activity (Kiver and Harris 1999).

During the Ice Age in the Pleistocene Epoch, which began about 190,000 years ago, the region experienced at least four periods of glaciation that shaped and carved the bedrock into the landscape seen by visitors today (Davis et al. 1994; Weeks and Andrascik 1998; Kiver and Harris 1999). The Precambrian was exposed during the most recent period of glaciation, the Wisconsin period, which lasted from 50,000 to 11,000 B.C.

Glaciers scoured the region scooping- out dozens of lake basins while scratching and polishing rock surfaces by dragging loose rocks over the surface leaving lakes, outwash (mostly sand and gravel), and till deposits. Today the rugged and varied topography includes rolling hills interspersed with irregular slopes and outcrops of bedrock between bogs, beaver ponds, swamps, islands, small lakes, and the four large lakes.

Lake elevations are about 1,100 feet (335 m) above mean sea level and land elevations in the west and north sections of the park rarely exceed the lake elevations by more than 100 feet (30 m) (Weeks and Andrascik, 1998). In the east section of the Kabetogama Peninsula and along the south shores of Kabetogama, Namakan, and Sand Point lakes, land elevations are commonly 100 to 200 feet (30- 60 m) higher (Weeks and Andrascik 1998; NPS 1994).

Biotite schist is the most widespread rock type in the park and forms the bedrock for most of the Kabetogama Peninsula (Day 1990; Hemstad et al. 2002). Outcrops of schist are relatively low and flat because the glaciers more easily eroded the schist than the surrounding granitic rocks.

A broad belt of mixed gneiss trends through the middle of Kabetogama and Namakan Lakes. The gneiss lies between biotite schist to the north and massive granite to the south and consists of layers and irregular masses of biotite schist separated by sheets of granite. Most outcrops contain highly contorted folds. Granitic rocks compose the terrane south of Kabetogama Lake and Sullivan Bay. The mineral composition and texture vary greatly over short distances so that the rocks range from true granites to hornblende quartz diorite. Glaciers rounded and smoothed the granitic outcrops and marked them with glacial striations, chattermarks, and glacial polish.

Mafic dikes, common in the area, are the youngest rocks at Voyageurs. The dikes are primarily composed of gabbro and diorite and cut through older granitic rocks.


References:

Davis, S.R., Hite, A.G., and Larson, W.S., 1994, Mineral occurrences and development potential near Voyageurs National Park, Minnesota: U.S. Department of the Interior, Bureau of Mines, MLA 5- 94, Intermountain Field Operations Center, Denver, CO., 153 p.

Day, W.C., 1990, Bedrock geologic map of the Rainy Lake area, northern Minnesota: USGS Map I- 1927, scale 1:50,000.

Harris, A.G., Tuttle, E., Tuttle, S.D., 1995, Geology of National Parks: Kendall/Hunt Publishing Co., Dubuque, IA, p. 247- 258.

Hemstad, C.B., Southwick, D.L., and Ojakangas, R.W., 2002, Bedrock geologic map of Yoyageurs National Park and Vicinity: Minnesota Geological Survey and University of Minnesota, Map M- 125, scale 1:50,000.

Kiver, E.P., and Harris, D.V., 1999, Geology of U.S. Parklands: John Wiley & Sons, Inc., New York, p. 177- 189.

NPS, 1994, Natural Resources Management Plan and Environmental Assessment, Voyageurs National Park, MN.: Department of the Interior, National Park Service, p. I- 3 – I- 38, II- 00- 1 – II- 00- 22, II- 03- 6 – II- 03- 12.

Weeks, D.P., and Andrascik, R.J., 1998, Voyageurs National Park, Minnesota, Water Resources Scoping Report: National Park Service, Technical Report NPS/NRWRS/NRTR- 98/201, 51 p.

updated on 07/17/07  I   http://nature.nps.gov/geology/parks/voya/geol_setting.cfm   I  Email: Webmaster
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