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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 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Animikie Group」の詳細全文を読む スポンサード リンク
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