Duluth Complex
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The Duluth Complex, the related Beaver Bay Complex, and the associated North Shore Volcanic Group are rock formations which comprise much of the
basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...
bedrock In geology, bedrock is solid Rock (geology), rock that lies under loose material (regolith) within the crust (geology), crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface mater ...
of the northeastern part of the
U.S. state In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sover ...
of
Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...
in central North America. The Duluth and Beaver Bay complexes are intrusive rocks formed about 1.1 billion years ago during the Midcontinent Rift; these adjoin and are interspersed with the
extrusive Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contras ...
rocks of the North Shore
Volcanic Group A volcanic group is a stratigraphic group consisting of volcanic strata. They can be in the form of volcanic fields, volcanic complexes and cone A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (freque ...
produced during that same geologic event. These formations are part of the
Superior Upland The Laurentian Upland (or Laurentian Highlands) is a physiographic region which, when referred to as the "Laurentian Region" or the Grenville geological province, is recognized by Natural Resources Canada as one of five provinces of the larger ...
physiographic region of the United States, which is associated with the
Laurentian Upland The Laurentian Upland (or Laurentian Highlands) is a physiographic region which, when referred to as the "Laurentian Region" or the Grenville geological province, is recognized by Natural Resources Canada as one of five provinces of the larger ...
of the
Canadian Shield The Canadian Shield (french: Bouclier canadien ), also called the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), the anc ...
, the core of the
North American Craton North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' is ...
.


Location

The Duluth Complex includes much of Minnesota's
Arrowhead Region The Arrowhead Region is located in the northeastern part of the U.S. state of Minnesota, so called because of its pointed shape. The predominantly rural region encompasses of land area and includes Carlton, Cook, Lake and Saint Louis counties. ...
north of
Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...
. From the west near
Duluth, Minnesota , settlement_type = City , nicknames = Twin Ports (with Superior), Zenith City , motto = , image_skyline = , image_caption = Clockwise from top: urban Duluth skyline; Minnesota ...
, it arcs north and northeast to about 48° north latitude south of Knife Lake, proceeds east at that latitude some five to twenty kilometers distant from and south of the Canada–US border to about 90° west longitude where it joins the border at the Pigeon River, and thence runs east near and along the border to Lake Superior. The Duluth and Beaver Bay complexes lie south of this line. Near Lake Superior these intrusive formations intermingle in a complex mosaic with the rocks of the associated North Shore Volcanics, which also are relics of the Midcontinent rifting event. The Duluth and Beaver Bay Complexes extend a short distance under Lake Superior south of the present lakeshore, but in most places along and near that shore their southern reaches are overlain by the North Shore Volcanic Group.


Formation

Some 1,100 million years ago ( mya) the
North American craton North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' is ...
began to split apart in the Midcontinent Rift. Over a period of some 15 to 22 million years,
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
rose through the earth’s crust, separating the older formations and cooling into new rock in the area of the rift. The rock sequences thereby created are known as the
Keweenawan Supergroup The Keweenawan Supergroup is a supergroup of volcanic and sedimentary rocks that fill the Midcontinent Rift System in the U.S. states of Michigan, Wisconsin, and Minnesota. It is about thick and it formed about 1.1 billion years ago. Stratigraphy ...
. Rocks of this group north of Lake Superior are the layers of the North Shore Volcanic Group and the adjoining formations of the Duluth and Beaver Bay Complexes. The North Shore Volcanics originated c. 1109-1096 mya from hundreds of individual
lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
flows, forming six distinct tilted and partially stacked plateaus which total more than 8,000 meters in thickness. These tilt toward the
syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...
under Lake Superior, as shown in the adjacent picture of the Sawtooth Mountains, the slopes of which mirror those of the shoreline rocks. While principally
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
ic, these flows also include
rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
s and other types. As part of the Middle Proterozoic Keweenawan sequence, these volcanic layers are part of one of the oldest, largest, and best-preserved plateau lava provinces in the world. These volcanics created the "roof rocks" into which were emplaced the
mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
formations of the Duluth Complex. Primarily formed after 1102 mya, the oldest formations are near Duluth, and the youngest to the northeast near Tofte. Insulated by the overlying roof rock, upwelling magma cooled slowly, and the mafic rock into which it cooled therefore is coarse-grained. These intrusions formed a sill some 16 km thick, primarily of
gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is che ...
, but with significant amounts of
anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most c ...
and other related granitic rocks. The Duluth Complex is one of the largest intrusions of gabbro on earth, and one of the largest layered mafic intrusions known. It covers an area of 4715 km2.Guilbert, John M. and Charles F. Park, Jr., ''The Geology of Ore Deposits'', Freeman, 1986, pp. 314-346, . The upper differentiated portions of the intrusion include
ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...
-bearing
labradorite Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect ( schiller). Labradorite is an intermediate to calcic member of the plagioclase series. It ...
anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most c ...
s. The lower portion along the northwestern margin consists of
ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
cumulates Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group o ...
with associated segregations of nickel, copper and
platinum group elements The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...
. Those metallic ores have attracted the interest of resource companies, and their attempts to mine are opposed by conservationists. Along its northern margin, the Duluth Complex adjoins older structures, the Archaen Ely Greenstones (once believed to be the oldest exposed rock on earth), and the ore-bearing
Mesabi The Mesabi Iron Range is a mining district in northeastern Minnesota following an elongate trend containing large deposits of iron ore. It is the largest of four major iron ranges in the region collectively known as the Iron Range of Minnesota ...
and
Gunflint Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and start fires ...
iron ranges deposited as part of the
Animikie Group The Animikie Group is a geologic group composed of sedimentary and metasedimentary rock, having been originally deposited between 2,500 and 1,800 million years ago during the Paleoproterozoic era, within the Animikie Basin. This group of formati ...
from the
Penokean orogeny The Penokean orogeny was a mountain-building episode that occurred in the early Proterozoic about 1.86 to 1.83 billion years ago, in the area of Lake Superior, North America. The core of this orogeny, the Churchill Craton, is composed of terranes ...
, a mountain-building event from
Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6  Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
times. Those two Middle Precambrian ranges are thought to have comprised a single formation, but intruding magma of the Duluth Complex baked and engulfed the center of the mountain chain, separating it into the two ranges present today, as shown in the image at the top of this page. To the east, the complex abuts and intrudes into the
Rove Formation The Rove Formation, is a sedimentary rock formation of Middle Precambrian age underlying the upper northeastern part of Cook County, Minnesota, United States, and extending into Ontario, Canada. It is the youngest of the many layers of sedimentar ...
, an older structure of sedimentary rocks. Gabbro and diabase structures of the Duluth Complex trend generally from southwest to northeast, and differential erosion has left a series of ridges comprising these harder mafic rocks rising from the softer sedimentary rocks of the Rove Formation. Elongated lakes lie in many of these depressions. To the south near Lake Superior, rock strata of the Duluth and Beaver Bay complexes are interspersed with and underlie the extrusive rock of the North Shore Volcanic Group. The Beaver Bay Complex occupies the center of the North Shore Volcanics, and is slightly younger in age than the other mafic rocks of the Duluth Complex, dating from c. 1096 mya.Miller, Green, Severson, Chandler, & Peterson
Geologic Map of the Duluth Complex
(2001).
The volcanics and more recent sedimentary rocks were once thought to be underlain by the Duluth Complex across Lake Superior to Wisconsin, where gabbro formations also exist. The Duluth Complex was considered to be a giant
lopolith A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body. The term was first defin ...
, a lens-shaped structure depressed in the center, connecting gabbro exposures on opposite sides of the lake, but now is recognized to extend only a few kilometers south of Superior's North Shore.


Contemporary landforms

The Precambrian bedrock of the Duluth Complex and the North Shore Volcanics are not buried beneath layers of later sedimentary rock, as is common further south; much of this bedrock is close to or at the surface. Glaciers scoured away earlier soils, and as is typical of the Canadian Shield, the new topsoils are thin and poor, being derived from the rock beneath or nearby rather than from deep layers of
glacial till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
, which is intermittent and relatively shallow over most of the region. Consequently much of the bedrock is exposed, except for sediments and glacial till in the watershed of the Saint Louis and
Cloquet River The Cloquet River is a , accessed May 7, 2012 river in Minnesota, United States. It is the main tributary of the Saint Louis River. Name Cloquet ( ) River is known in the Ojibwe language as the ''Gaa-biitootigweyaag-ziibi'' ("River that parallel ...
s inland on the west. Gabbro outcroppings anchor both ends of the complex. They dominate the city which gave the Duluth Complex its name, and also form part of Pigeon Point, the easternmost point of Minnesota. In between, Superior's shoreline from Duluth to the international border has been likened to one long volcanic outcrop, albeit interrupted by parts of the Beaver Bay Complex, such as the
anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most c ...
cliffs at
Split Rock Lighthouse Split Rock Lighthouse is a lighthouse located southwest of Silver Bay, Minnesota, USA on the North Shore of Lake Superior. The structure was designed by lighthouse engineer Ralph Russell Tinkham and was completed in 1910 by the United States Ligh ...
adjacent to basalt flows. Prominent relics of volcanism include
rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
cliffs at
Palisade Head Palisade Head is a headland on the North Shore of Lake Superior in the U.S. state of Minnesota. It is within Tettegouche State Park but not contiguous with the rest of that park. Palisade Head is located at milepost 57 on scenic Minnesota Sta ...
, basaltic lava flows at
Gooseberry Falls Gooseberry Falls State Park is a state park of Minnesota, United States, on the North Shore of Lake Superior. The park is located in Silver Creek Township, about 13 miles (21 km) northeast of Two Harbors, Minnesota in Lake County on ...
, and the Sawtooth Mountains further east. Along the lakeshore can be found
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
-banded
thomsonite Thomsonite is the name of a series of tecto-silicate minerals of the zeolite group. Prior to 1997, thomsonite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a ser ...
and
agate Agate () is a common rock formation, consisting of chalcedony and quartz as its primary components, with a wide variety of colors. Agates are primarily formed within volcanic and metamorphic rocks. The ornamental use of agate was common in Ancie ...
gemstones created by mineral infilling of gas cavities formed when the lava flows cooled. Interior highlands include Eagle Mountain and the
Misquah Hills The Misquah Hills are a range of mountains in northeastern Minnesota, in the United States. They are located in or near the Boundary Waters Canoe Area Wilderness within Superior National Forest. Eagle Mountain, the highest point in Minnesota at ...
. Most of the eastern part of the
Superior National Forest Superior National Forest, part of the United States National Forest system, is located in the Arrowhead Region of the state of Minnesota between the Canada–United States border and the north shore of Lake Superior. The area is part of the grea ...
and its
Boundary Waters Canoe Area Wilderness The Boundary Waters Canoe Area Wilderness (BWCAW or BWCA) is a wilderness area within the Superior National Forest in the northeastern part of the US state of Minnesota under the administration of the U.S. Forest Service. A mixture of forests ...
(BWCA) is located on the Duluth Complex, and its exposed Late Precambrian bedrock formations are characteristic features of the region. The inland lakes lie in hollows formed by differential erosion of the gabbro intrusions. These depressions were given their final form by glacial scouring during recent ice ages, creating the irregularly shaped and rocky-shored lakes which are hallmarks of the wilderness.Ojakangas & Matsch (1982), p. 161; Heinselman (1996), pp. 12–13, 200.


See also

*
Geology of Minnesota The geology of Minnesota comprises the rock, minerals, and soils of the U.S. state of Minnesota, including their formation, development, distribution, and condition. The state's geologic history can be divided into three periods. The first per ...
* Coldwell Complex


References


Sources

* Chandler (2005)
''A Geophysical Investigation of the Ely Greenstone Belt in the Soudan Area''
Retrieved 2013-11-23. *
Generalized Density of Bedrock Outcrops and Drill Holes in Northeast Minnesota
(map),
University of Minnesota The University of Minnesota, formally the University of Minnesota, Twin Cities, (UMN Twin Cities, the U of M, or Minnesota) is a public university, public Land-grant university, land-grant research university in the Minneapolis–Saint Paul, Tw ...
(2001). Retrieved 2013-11-23. * Green, John (2004): Volcanic and Sedimentary Rocks of the Southwestern Sequence of the North Shore Volcanic Group
Field Trip Guidebook
''Proceedings'' 50, Vol. 2. Institute on Lake Superior Geology, 2004. Retrieved 2013-11-23. * Guilbert, John M. and Charles F. Park, Jr., ''The Geology of Ore Deposits'', Freeman, 1986, . * * * * Miller, J.D.; Green, J.C.; Severson, M.J.; Chandler, V.W.; and Peterson, D.M.
Geologic Map of the Duluth Complex and Related Rocks, Northeastern Minnesota
University of Minnesota The University of Minnesota, formally the University of Minnesota, Twin Cities, (UMN Twin Cities, the U of M, or Minnesota) is a public university, public Land-grant university, land-grant research university in the Minneapolis–Saint Paul, Tw ...
(2001). Retrieved 2013-11-23. * Miller, Jim (2011)
''Geology and Mineral Deposits of the Duluth Complex, Minnesota and why it will be mined someday''
USGS. * * * * * * * * * * * *


External links


Geologic map of the Duluth Complex
{{Chronostratigraphy of Minnesota Igneous petrology of Minnesota Proterozoic magmatism Layered intrusions Mesoproterozoic