Gunflint chert

Gunflint chert
Stratigraphic range: 1.88 Ga[1]
Microfossils of microbes similar to cyanobacteria, Gunflint Formation, north shore of Lake Superior, 1.9 billion years old
Type	Geological formation
Lithology
Primary	Banded iron formation
Location
Region	Minnesota  Ontario
Type section
Named for	Gunflint Range

The Gunflint chert (1.88 Ga^[1]) 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 Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic.^[2] The Gunflint Chert is composed of biogenic stromatolites.^[3] At the time of its discovery in the 1950s, it was the earliest form of life discovered and described in scientific literature, as well as the earliest evidence for photosynthesis.^[4] 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 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 cryptocrystalline quartz as chert or jasper, along with some minor silicate minerals.

The Gunflint Iron Formation (exposed as the Gunflint Range) spans northwestern Ontario and northern Minnesota along the shores of Lake Superior. The type locality of the Gunflint Iron Formation is at Schreiber, ON near Lake Superior’s Thunder Bay.^[5]

Geologist Stanley A. Tyler first examined the area in 1953 and noticed its 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 and concluded that "they were indeed structurally preserved unicellular organisms."^[6] In 1965 the two scientists published their landmark finding and named the first variety of Gunflint flora.^[2] This created an academic "stampede" to explore Precambrian microfossils from similar Proterozoic environments. While older microfossils have since been described, the Gunflint microfauna is a historic geologic discovery and remains one of the most robust and diverse microfaunal fossil assemblages from the Precambrian.