Duluth complex

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The shaded relief image shows the Duluth Complex, which stretches in an arc from Duluth to Pigeon Point, interrupting the Mesabi Range and the Gunflint Range

The Duluth Complex , the associated Beaver Bay Complex (which is often considered part of the Duluth Complex), and the associated North Shore Volcanic Group (also known as North Shore Volcanics ) are rock formations that make up a large part of the basement in the northeast of the US state of Minnesota . Duluth and Beaver Bay Complex are intrusive rocks that formed during the so-called Midcontinent Rift (also Keweenawan Rift ), in which a rift opened 1.1 billion years ago in the Mesoproterozoic in the North American continent . The intrusive complexes are contiguous with the volcanic rocks of the North Shore Volcanic Group that formed during the same event. The three geological units are part of the physiogeographical region of the American Superior Uplands , which is connected to the Laurentian Upland of the Canadian Shield - the heart of the North American Craton .

location

The Duluth Complex occupies a large part of the north of the Lake Superior located and how an arrowhead shaped Arrowhead region a of Minnesota. From the west end near Duluth (Minnesota) it curves in an arc to the north and northeast to about the 48th parallel south of Knife Lake , from there it runs to the east, about 5 to 20 kilometers south and parallel to the Canadian border, finally touching it on the Pigeon River (Minnesota-Ontario) to eventually extend along the border to Lake Superior.

Near Lake Superior, the rocks of the Duluth and Beaver Bay complexes mix with those of the North Shore Volcanics to form a complex geological mosaic. The two complexes extend a short distance below Lake Superior south of today's shoreline, but are overlaid for the most part along the shore by the rocks of the North Shore Volcanics .

education

The Sawtooth Mountains on Lake Superior
North Shore Volcanics : Palisade Head in the foreground in front of Shovel Point, both rhyolithic rocks of the Midcontinent Rift ; the Sawtooth Mountains on the horizon

About 1100 million years ago ( mya ) the North American craton began to break up on the Midcontinent Rift . Over a period of around 15-22 million years, magma rose in the earth's crust, separating the older rocks at the edge of the rift and forming new rocks. The rock sequences that emerged during this event are summarized under the name Keweenawan Supergroup . These include not only North Shore Volcanics , Duluth and Beaver Bay Complexes with a thickness of more than 20,000 m , but also more than 10,000 m, predominantly red-colored, continental sediments .

The North Shore Volcanics

The North Shore Volcanics emerged from around 1109–1096 mya from hundreds of individual lava effusions, which form six distinguishable, partially overlapping plateaus of more than 8000 m in thickness. These fall towards the center of the hollow under Lake Superior. This becomes clear, for example, in the Sawtooth Mountains , the surface of which depicts this uniform dip (see picture). The majority are basalts , subordinate to rhyolites and other volcanic rocks . As part of the Keweenawan Supergroup, the North Shore Volcanics are one of the oldest and largest and well-preserved examples of plateau basalts .

The Duluth Complex

The volcanic rocks form the host rock for the mafic formations of the Duluth complex. These formed after 1102 mya, with the oldest parts exposed near Duluth and the younger in the northeast near Tofte . The ascending magma slowly cooled down as a 16-kilometer-thick storage corridor and formed coarsely crystalline rocks, predominantly gabbro with not inconsiderable proportions of anorthosite as well as other, intermediate to granitic rocks.

The Duluth Complex is one of the largest gabbroider rocks on earth, and one of the largest known layered intrusions . Its area is 4715 km 2 . The lower part of the intrusion on the northwestern edge of the complex consists of ultramafic accumulations with secretions of nickel , copper and platinum metals . The upper, differentiated parts contain ilmenite- containing labradorite anorthosites.

Along its northern border, the Duluth Complex abuts the older rock formation of the archaic Ely Greenstones , which were previously believed to be the oldest exposed rocks on earth. Today older rocks are known from Minnesota ( Morton gneiss ), Greenland ( Isua gneiss ), Northwest Territories ( Acasta gneiss ) and Québec ( Nuvvuagittuq greenstone belt ). The complex is also adjacent to the ore-bearing Mesabi Range and the Gunflint Range , which were formed during the Penoke Orogeny , a orogeny of the Paleoproterozoic . The two mountain ranges probably formed a coherent unit in the past, until they were interrupted by the establishment of the Duluth complex.

To the south, near Lake Superior, the layers of the Duluth and Beaver Bay Complex lie beneath or intermingle with the North Shore Volcanics . The Beaver Bay Complex occupies the center of the North Shore Volcanics and is slightly younger than the Duluth Complex at around 1096 mya. It used to be assumed that the volcanic rocks and younger rocks under Lake Superior and also on the other side of the lake in Wisconsin - where gabbro formations also exist - are completely underlain by the Duluth Complex. The complex was thought to be a gigantic lopolith , a lenticular structure with a recessed center that linked the gabbro formations on either side of the lake. In fact, the Duluth complex was considered the archetype of a Loplith. Today it is known that it extends only a few kilometers beyond the lake shore to the south and has no connection to the occurrences on the other side. So he's not a Lopolith, but a very large warehouse corridor.

Landforms today

Narrow tower in Duluth, built from local Keweenawan rock, on a gabbro knoll (foreground)
The rugged Misquah Hills

The Precambrian bedrock of the Duluth Complex and the North Shore Volcanics are not buried under younger rocks, as is the case further south, but are close to or on the surface. Glaciers scraped off previous soils, and the new ones are thin and low-yielding, as is typical of the Canadian Shield, as they had to evolve from solid rock rather than from thick layers of boulder clay , which is irregular and thin throughout the region. For this reason, the bedrock is mostly well exposed, only in the catchment areas of Saint Louis and Cloquet Rivers in the west are younger deposits and bed debris found.

Gabbro outcrops form both ends of the complex and dominate the town that gave the complex its name. They are also part of Pigeon Point , the easternmost point of Minnesota. Between the gabbro outcrops lies the north shore of Lake Superior, which has been described as a contiguous outcrop of volcanic rocks from Duluth to the border, though interrupted by parts of the Beaver Bay complex such as the anorthosite cliffs and basalt casts at the Split Rock Lighthouse .

Outstanding remains of the volcanic rocks include the rhyolite cliffs at Palisade Head , the basaltic lava flows at Gooseberry Falls and the Sawtooth Mountains further to the east. Finds of quartz- banded Thomsonite and agates are possible on the beach of the lake , which were created by mineral fillings of gas bubbles when the lava cooled down.

Inside the complex, from the lake, are Eagle Mountain , Misquah Hills , the eastern part of the Superior National Forest and its Boundary Waters Canoe Area Wilderness (BWCA) of the surface pending formed Precambrian bedrock. Many of the region's distinctive features are found here. The smaller lakes on the gabbro intrusions lie in hollow forms that were created by the different resistance of the gabbro layers to erosion , especially glacier erosion. The excavated lowlands were dammed by moraine material left behind by the ice and formed the irregular lakes with rocky banks that are the hallmark of the Canoe Area Wilderness.

See also

Sources and individual references

  1. ^ A b c John Green: Volcanic and Sedimentary Rocks of the Southwestern Sequence of the North Shore Volcanic Group. Field Trip Guidebook ( Memento from August 21, 2011 in the Internet Archive ). In: Proceedings of the 50th Annual Meeting, Institute on Lake Superior Geology. Volume 2, 2004, pp. 47-52 (PDF; 32.9 MB).
  2. ↑ a b c Mark Jirsa, David Southwick: mineral potential and geology of the Duluth Complex. In: Mineral Potential and Geology of Minnesota , Minnesota Geological Survey, University of Minnesota, October 12, 2000.
  3. ↑ a b Physiographic Regions ( Memento of May 9, 2008 in the Internet Archive ). In: A Tapestry of Time and Terrain: The Union of Two Maps - Geology and Topography. United States Geological Survey (English).
  4. ↑ a b c Miron Hein Selman: The Boundary Waters Canoe Area Wilderness Ecosystem . University of Minnesota Press, Minneapolis 1996, ISBN 0-8166-2804-1 , pp. 12-14, 200, plate 1 .
  5. ↑ a b c d J.D. Miller, JC Green, MJ Severson, VW Chandler, DM Peterson: Geologic Map of the Duluth Complex and Related Rocks, Northeastern Minnesota (PDF; 5.6 MB) University of Minnesota. 2001. Archived from the original on February 26, 2009. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved January 2, 2008. @1@ 2Template: Webachiv / IABot / conservancy.umn.edu
  6. ↑ a b c d e f g h i j k Richard W. Ojakangas & Charles L. Matsch: Minnesota's Geology . University of Minnesota Press, Minneapolis 1982, ISBN 0-8166-0953-5 , pp. 49-57 ( Google Books ).
  7. ^ DA Schneider, DK Holm and V. Chandler: A Superior Swath: Proterozoic Geology of the North American Midcontinent . In: American Geophysical Union . Fall 2006. Abstract # T42A-06. bibcode : 2006AGUFM.T42A..06S .
  8. ↑ RW Ojakangas, GB Morey and JC Green: The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA . In: Sedimentary Geology . 141-142, 2001, pp. 421-442. doi : 10.1016 / S0037-0738 (01) 00085-9 .
  9. ^ Richard W. Ojakangas, Albert B. Dickas: The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior Region . In: Elsevier Science BV (Ed.): Sedimentary Geology . 147, No. 1-2, March 1, 2002, pp. 13-36. doi : 10.1016 / S0037-0738 (01) 00185-3 .
  10. ↑ a b Anthony J. Naldrett: Magmatic Sulfide Deposits: Geology, Geochemistry and Exploration . Springer-Verlag , Berlin 2004, ISBN 3-540-22317-7 , pp. 244 (Fig. 4.49), 246 .
  11. ^ S. Th. Schmidt: Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA . In: Blackwell Synergy (Ed.): Journal of Metamorphic Geology . 11, No. 3, May 1993, pp. 401-14. doi : 10.1111 / j.1525-1314.1993.tb00157.x .
  12. ^ Martin Frey, Doug Robinson: Low-Grade Metamorphism . Blackwell Science, Oxford 1999, ISBN 978-0-632-04756-7 , pp. 157-158 .
  13. ^ Lynn Topinka: America's Volcanic Past: Minnesota . United States Geological Survey. January 26, 2003. Archived from the original on January 10, 2009. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved January 4, 2009. @1@ 2Template: Webachiv / IABot / vulcan.wr.usgs.gov
  14. ↑ George M. Schwartz & George A. Thiel: Minnesota's Rocks and Waters . University of Minnesota Press, Minneapolis 1963, pp. 114 (LOC 54-6370).
  15. ^ A b John M. Guilbert and Charles F. Park, Jr .: The Geology of Ore Deposits . Freeman, 1986, ISBN 0-7167-1456-6 , pp. 314-346 .
  16. ^ Chandler: A Geophysical Investigation of the Ely Greenstone Belt in the Soudan Area . 2005. Archived from the original on July 13, 2010. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved January 2, 2009, pp. 4-5. @1@ 2Template: Webachiv / IABot / homepages.spa.umn.edu
  17. ^ A b Gene L. LaBerge: Geology of the Lake Superior Region . Geoscience Press, Tucson, AZ 1994, ISBN 0-945005-15-6 , pp. 157-160 .
  18. ↑ a b Constance Jefferson Sansome: Minnesota Underfoot: A Field Guide to the State's Outstanding Geologic features . Voyageur Press, Stillwater, Minnesota 1983, ISBN 0-89658-036-9 , pp. 20-25 .
  19. ↑ Generalized Density of Bedrock Outcrops and Drill Holes in Northeast Minnesota (map) (PDF; 3.2 MB) 2001. Archived from the original on July 20, 2011. Information: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved January 2, 2009. @1@ 2Template: Webachiv / IABot / conservancy.umn.edu
  20. ^ A b Richard W. Ojakangas & Charles L. Matsch: Minnesota's Geology . University of Minnesota Press, Minneapolis 1982, ISBN 0-8166-0953-5 , pp. 161, 167-168 ( Google Books ).
  21. ↑ Split Rock Lighthouse State Park Info . In: State Parks . Minnesota Department of Natural Resources . 2009. Archived from the original on February 11, 2009. Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved January 4, 2009. @1@ 2Template: Webachiv / IABot / www.dnr.state.mn.us

Web links

Coordinates: 47 ° 40 ′  N , 91 ° 0 ′  W