Gunflint chert

The Gunflint chert (1.88 Ga) is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age. Stromatolite colonies of cyanobacteria that have been converted to jasper are found in Ontario. The banded ironstone formation consists of alternating strata of iron oxide-rich layers interbedded with silica-rich zones. The iron oxides are typically hematite or magnetite with ilmenite, while the silicates are predominantly quartz as chert or jasper, along with some minor silicate minerals.

Stanley A. Tyler examined the area in 1953, and noted the red-colored stromatolites. He also sampled a jet-black chert layer which, when observed petrographically, revealed some lifelike small spheres, rods and filaments less than 10 micrometres in size. Elso Barghoorn, a paleobotanist at Harvard, subsequently looked at these same samples. Barghoorn concluded that "they were indeed structurally preserved unicellular organisms." In 1965, the two scientists published their finding, and named a variety of the Gunflint flora. This created an academic "stampede" to explore Precambrian microfossils from similar Proterozoic environments.

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