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 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 1,850 million years ago.

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