This section highlights the map units (i.e., rocks and unconsolidated deposits) that occur in Voyageurs National Park and puts them in a geologic context in terms of the environment in which they were deposited and the timing of geologic events that created the present landscape.

Some of the oldest rocks in the world are exposed in Voyageurs National Park. About 2.5- 4.5 billion years ago (Ba), plate- tectonic processes generated a number of isolated areas of continental crust. In North America, rocks formed by these early tectonic events are found in Minnesota, Wyoming, parts of Canada, and Greenland.

These bedrock units were shaped and carved by at least four periods of glaciation. The most recent continental glaciation in North America ended about 10,000 years ago. All evidence of the geologic history that has occurred between these two widely separated periods is missing from Voyageurs and most of the Canadian Shield.

Figure 2. Geologic time scale

Precambrian
Archean Eon (2.5-3.0 Ba)
Relatively little is known about the early Precambrian since there were few fossils preserved and hard- bodied organisms are not found prior to about 550 million years ago at the beginning of the Cambrian Period (figure 2). However, excellent exposures of Archean rocks in northern Minnesota, including the Voyageurs area, reveal a protracted and complex structural evolution (Davis et al. 1994). The Archean basement complex of intensely metamorphosed rocks of sedimentary, volcanic, and intrusive igneous origin, dated at 2.6 Ba and older, is commonly referred to as pre- Kenoran. The earliest gneisses and schists, rich in mica, may have originally been sandstones (Harris et al. 1995). Following the emergence of the continental rocks of the Superior Province about 2.7- 3.0 Ba, structural deformation occurred over more than 100 million years.

In late Archean time (2.8- 2.5 Ba), repeated episodes of continental collision and north- south directed compression and subduction generated a mountainbuilding episode known in North America as the Algoman Orogeny. This orogeny formed the gneisses, schists, and granites exposed today in Voyageurs National Park (Miller et al. 1987; Kiver and Harris 1999). Subduction of oceanic crust beneath the cratonic crust generated volcanic islands in the Precambrian sea. Zones of thick, basaltic lava accumulated in linear basins (backarc basins) formed between the less- dense continental crust and the volcanic islands. These lava- filled basins created today's greenstone belts (Kiver and Harris 1999). The oldest known rock in Minnesota, the Ely greenstone, formed at this time.

During the Algoman Orogeny, a great number of tectonic faulting and folding events deformed the rocks in the north- central Minnesota part of the Superior Province. Today, these event are seen in the three geologically distinct subprovinces, Wabigoon ,Quetico, and Wawa (figure 3).

Major fault zones form the boundaries between the three subprovinces. The northeast- southwest trending Rainy Lake- Seine River fault zone separates the Quetico subprovince from the Wabigoon subprovince to the north. South of Voyageurs, the Vermillion fault zone marks the boundary between the Quetico subprovince and the Wawa subprovince (figure 3). The Quetico subprovince has been interpreted as a classical sedimentary basin (fore- arc or back- arc basin) developed between two active volcanic island chains (volcanic arcs) represented by the Wabigoon and Wawa subprovinces (figure 4) (Davis et al. 1994).

These ancestral fault zones developed oblique, northwesterly components of movement that caused extensive strike- slip motion. Locally, segments on these faults were displaced by more than 20 miles (32 km), with the north side of faults displaced to the east (Davis et al. 1994).

At least three phases of deformation folded the bedrock of Voyageurs (Day 1990). The first episode produced southwest - plunging en echelon folds. The second phase of deformation was the most intense in the area and is recognized by small- scale folds, a strong schistosity in the rocks, mineral lineations, and a progressive transition in ductile deformation from the first phase to the second phase. The third phase produced high- angle faults and shear zones. Regional folding appears to have occurred prior to, concurrent with, and after regional metamorphism.

Sedimentary units were complexly folded along eastwest axes and commonly have very steep to vertical dips. North- south directed compression generated regional, arch- like convex folds, or flexures, in sedimentary strata (antiformal syncline or anticlinoria) along eastnortheasterly trends. Regional metamorphism accompanied compressive north- south oriented thrusting and folding.

Later in the orogeny, the granitic plutons found in Voyageurs intruded into the area. Mineral- rich fluids from these plutons produced dikes, pegmatites, and quartz veins that concentrated metallic minerals in parts of the greenstone belts.

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

Proterozoic Eon (2.5-0.5 Ba)
The Proterozoic Eon represents about 40 percent of all geologic time. By 1.8 Ba, isolated regions had joined into a single large craton that included what we see today as Greenland, central Canada, and the north- central United States. The Midcontinent Rift that forms the axis of Lake Superior formed during this time and is likely the surface expression of a rising plume of hot mantle material (Hauser 1996). Faults and igneous dikes mark the northwest arm of this rift. Lava flows poured from long fissures to fill the axes of synclinal basins. Magma that did not breach the surface crystallized into granitic plutons that are now exposed in the park. Deformation in the Voyageurs area seems to have terminated by the time the northwest- trending diabase dike swarm was emplaced around 2.12 Ba (Day 1990). The Proterozoic diabase dikes of the Kabetogama- Kenora Dike Swarm in Voyageurs formed in long, linear fractures between 2.2 and 2.1 Ba. Proterozoic schistose tectonite (fault rock) intrude the older Archean rocks.

Prolonged Erosion (570 Ma - 0.19 Ma)
By the end of the Precambrian, the long series of tectonic events that created the basement complex of the Canadian Shield was over. Millions of years of erosion left only the roots of ancient mountains exposed (Harris et al. 1995). Extensive seaways covered much of the craton during the Early Paleozoic, but during the Late Paleozoic, Mesozoic, and Cenozoic, the province stood above sea level and was extensively eroded. The topography was reduced to a flat rolling plain even before the advance of the Pleistocene glaciers. The glaciers removed most of the Cenozoic deposits and all of the Paleozoic and Mesozoic record. When the first continental glaciers advanced into the Voyageurs area, the landscape was probably of low relief with a fairly uniform soil cover supporting extensive forests (Harris et al. 1995).

Figure 4. Massive Precambrian rocks, like those pictured above, are exposed throughout Voyageurs National Park.

Quaternary
Pleistocene Epoch (1.6-0.01 Ma)
Throughout the Ice Age, beginning perhaps 190,000 years ago and extending intermittently in the Voyageurs region to approximately 11,400 years B.P., continental sheet glaciers covered north- central Minnesota, the Great Lakes Region, and most of Eastern Canada (Davis et al. 1994). The Illinoian stage (190,000 to 135,000 years B.P.) was the first glacial event and was followed by a warming period known as the Sangamonian Interglacial (135,000 to 119,000 years B.P.) The Wisconsin stage followed the Sangamonian Interglacial and advanced and retreated across the region from 135,000 years B.P. to 10,000 years B.P. when the Holocene Epoch, or "Recent" geological time period began (figure 2).

During the Pleistocene, glaciers scooped out Lake Superior's basin along the trend of the Midcontinent Rift. At the east end of Lake Superior, the bottom of the lake is 1,302 feet (397 m) below the surface at an elevation of 700 feet (213 m) below sea level (Kiver and Harris 1999).

To the west of Voyageurs, Lake Agassiz formed during glacial retreat. Meltwater from the last continental glacier, backed- up because the north- flowing rivers that drained into Hudson Bay were blocked by ice. Water began to collect in the Red River valley in western Minnesota about 12,000 years ago and then spread over eastern North Dakota and adjacent Canadian Provinces. Lake Agassiz covered the Voyageurs area during glacial retreat from about 11,400 to 9,500 years B.P. (Davis et al. 1994).

Holocene Epoch
Today unconsolidated sediments and thin soils have been superimposed on some of the oldest bedrock in North America. Streams have been reworking and sorting the glacial debris rather slowly due to the low relief in the area. Large basins are now occupied by lakes, ponds, bogs, wetlands, and interconnecting waterways. The tremendous weight of the ice sheet depressed the crust. When the ice melted, the crust rebounded and it is still rising today. Glacial rebound has caused some lakes to drain away and some lakes to become bogs and swamps. In the Boundary Waters region, the lakes occupy deeper parts of the basins once filled by Lake Agassiz. The land is rising at a rate of about 1 foot (0.3 m) per century (Kiver and Harris 1999).

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.

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

Miller, J.D., Jr., Morey, G.B., and Weiblen, P.W., 1987, Seagull Lake- Gunflint Lake area; A classical Precambrian stratigraphic sequence in northeastern Minnesota, in D.L. Biggs, ed., North- Central Section of the Geological Society of America: Geological Society of America, Centennial Field Guide, v. 3, p. 47- 51.