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 formations is geographically divided into the Gunflint Range, the Mesabi and Vermilion ranges, and the Cuyuna Range. On the map, the Animikie Group is the dark gray northeast-trending belt which ranges from south-central Minnesota, U.S., up to Thunder Bay, Ontario, Canada. The Gunflint Iron Range is the linear black formation labeled G, the Mesabi Iron Range is the jagged black linear formation labeled F, and Cuyuna Iron Range is the two black spots labeled E. The gabbro of the Duluth Complex, intruded during the formation of the Midcontinent Rift, separates the Mesabi and Gunflint iron ranges; it is shown by the speckled area wrapping around the western end of Lake Superior.
Banded-iron formations are iron formations which formed about 2,000 million years ago and were first described in the Lake Superior region. Sediments associated with the last stage of the Great Lakes tectonic zone contain banded-iron formations. These sediments were deposited for two hundred million years and extend intermittently along roughly the same trend as the Great Lakes tectonic zone, from Minnesota into eastern Ontario, Canada, and through upper Wisconsin and Michigan. They are characterized by bands of iron compounds and chert. Enough oxygen had accumulated in seawater so that dissolved iron was oxidized; iron reacts with oxygen to form compounds that precipitate out - including hematite, limonite and siderite. These iron compounds precipitated from the seawater in varying proportions with chert, producing banded-iron formations. These iron formations are abundant in the Lake Superior region. The Sudbury Impact event occurred 1,850 million years ago; it is theorized that this caused the end of the banded-iron deposits. The results of the impact affected concentrations of dissolved oxygen in the sea; the accumulation of banded-iron formations suddenly ended .
The Gunflint Range consists of a basal conglomerate, then the Gunflint Iron formation and the Gunflint Chert with the Rove Formation deposited on top. The Mesabi Range consists of the basal Pokegama Quartzite layer, then the Biwabik Iron Formation with the Virginia Formation deposited on top. The Vermilion Range consists of the basal Ely Greenstone, then the Soudan Iron formation with various granites on top. The Cuyuna Range consists of the basal North Range group, then the Trommald Formation with the Thomson Formation deposited on top.
Age, location and size
The Animikie Group sediments were deposited between 2,500 and 1,800 million years ago, in the Animikie Basin. Deposition of sediments began after the Algoman orogeny and continued through the Great Lakes tectonic zone rupture from 2,200 to 1,850 million years ago.The Animikie Group formations are in east-central and northeastern Minnesota, and the Thunder Bay District of Northern Ontario; they are geographically divided into the Gunflint Range, the Mesabi and Vermilion ranges, and the Cuyuna Range. The Animikie Basin was an extensional basin which developed over a basement consisting of the 2,750- to 2,600-million-year-old Superior province to the north and the 3,600-million-year-old Minnesota River Valley subprovince to the south. The extension was caused by the east-northeast-trending Great Lakes tectonic zone; it separates the Superior province from the Minnesota River Valley subprovince. The sediments were deformed, metamorphosed and intruded by the plutonic rocks of the 1860 ± 50-million-year-old Penokean orogeny.
The rocks of the Animikie Basin form a sequence up to thick and show a complete transition from a stable shelf environment to deep-water conditions. Irregularities in the basement influenced the thickness of the sequence. The by basin is an elongated oval parallel to and straddling the Great Lakes tectonic zone.
Development of Animikie Basin
Twenty-seven hundred million years ago the Algoman orogeny formed mountains; these bare mountains eroded for several hundred million years to a broad level peneplain. A sea invaded central Minnesota and extended eastward through northern Wisconsin and the Upper Peninsula of Michigan. Sediments composed of quartz-rich sand were deposited along the shoreline of this sea; these were succeeded by thick iron-rich layers and eventually kilometers of mud and muddy sand. The deposition of sedimentary strata on top of the Archean basement formed the Animikie Group.The next tectonic event was the Great Lakes tectonic zone which began with compression caused by the collision of the Superior province and the Minnesota River Valley subprovince during the Algoman orogeny about 2,700 million years ago; it continued as a pulling apart rift from 2,450 to 2,100 million years ago, followed by a second compression which deformed the rocks in the Lake Superior region during the Penokean orogeny which lasted from 1,900 to 1,850 million years ago. The first deposits occurred during the initial stages of extension of the Great Lakes tectonic zone in the continental crust. As the crust expanded it thinned, and magma was intruded through fissures in the thinned crust. Sedimentation stopped during this transitional period because the elevation was now above sea level. During later stages, the spreading center was adding oceanic crust - which is heavier than continental crust - so the area subsided, seas returned, and the second layer of sediments were deposited unconformably on the basin fill.
The third tectonic event was the Penokean orogeny which is dated 1,850 million years ago. The intense, northward-directed compression folded the shale and greywacke of the southernmost unit - the Thomson Formation - and metamorphosed the shale into slate. The Animikie strata on the Gunflint and Mesabi ranges were far enough away so they escaped this deformation and metamorphism; they contain some of the oldest unmetamorphosed sedimentary deposits in the world.
About a fourth tectonic event occurred in the Lake Superior region. A hotspot of magma from the Earth's mantle beneath present-day Lake Superior rose, causing the crust to dome and break apart. This zone of crustal thinning and fracturing is the Midcontinent Rift System; it extends in a boomerang shape for over from northeastern Kansas northward through Iowa, under the Twin Cities of Minnesota, beneath Lake Superior, and then south through the eastern Upper Peninsula of Michigan and beneath the central Lower Peninsula of Michigan. As the crust was being stretched thin and more magma flowed out from below, the center of the rift was continuously subsiding. The vast quantities of rising magma created a vacuum under the crust, the weight of the solidified magma on the surface caused the crust to subside into that vacuum so the edges of the rift tilted toward the center. The rifting stopped after a few million years; one reason could be that the Grenville orogeny stopped the rift process when that collision occurred. Subsidence continued for several million years after the lava flows had ceased; immense volumes of sediments - sand, gravel and mud - were eroded off the barren landscape into the still-sinking basin along the rift axis. As much as of sedimentary rocks accumulated in the center before the sinking stopped and the region stabilized. A north-northeast trending branch of the Midcontinent Rift System separated the Animikie Basin into two distinct segments; the present-day Animikie Group and the Marquette Range Supergroup; the historical name for the Marquette Range Supergroup is the Animikie Series.
Banded-iron formations
Oceanic sediments associated with the last stage of the Great Lakes tectonic zone contain the banded-iron formations. Banded-iron formations are iron formations which formed about 2,000 million years ago and were first described in the Lake Superior region. They are characterized by interlayers - bands - of iron minerals and chert. These sediments were laid down for two hundred million years and extend intermittently along roughly the same trend as the Great Lakes tectonic zone, from Minnesota into eastern Canada, and through upper Wisconsin and Michigan.Change in atmospheric oxygen levels
Banded-iron sediments record the introduction of abundant free oxygen into earth's atmosphere. Microbial life played an important role in changing atmospheric conditions by releasing free oxygen as a waste product of photosynthesis. Free oxygen was taken up by elements with strong affinities for it - hydrogen, carbon and iron. Evidence for the change in oxygen levels is that the sediments of the earlier Archean are dark brown and black caused by unoxidized carbon, iron sulfide, and other elements and compounds. As oxygen levels increased in the atmosphere and oceans, the sediments changed. In the late Archean, sediments went through a transitional stage with the banded-iron formations; after this transition they demonstrate an oxygen-rich environment - shown by iron oxide-stained siltstones or mudstones called red beds.Enough oxygen had accumulated in seawater so that dissolved iron - which had earlier eroded from the surrounding land - was oxidized. Oxygenated water has low levels of dissolved iron because iron reacts with oxygen to form compounds that precipitate out; the compounds include hematite, limonite and siderite. These iron compounds precipitated from the sea water in varying proportions with chert, producing banded-iron formations. Banded-iron formations occur in several ranges around the margins of this basin, five of which contained sufficient concentrations of iron to be economically mined. These banded-iron formations have been one of the world's greatest sources of iron ore since mining began in the area during the late 19th century. Major iron formations in different parts of the basin represent either nearly contemporaneous shelf sedimentation on either side of the main basin, or deposits formed simultaneously in isolated sub-basins of the main basin.