Gunflint Formation

A value of S C = —33.2%o has been established for the free carbon from the slates of the Biwabik formation (Perry and Tan, 1973), and for the slates of the Thompson and Gunflint formations, —30.5 and —30.3, respectively (Hoering, 1967). These values are reported for the carbon of the schists of cherty iron-formations —34.9 and — 31.4%o (graphite schists of the Upper suite of the Krivoy Rog group) and — 23.3%o (carbonaceous-biotite-sericite schist of the Kursk group, Belgorod district, KMA). These 6 C values fall within those characteristic of the carbon of organic matter. 

The masses of laminated chert characteristic of all Precambrian BIF s occur in a wide variety of external forms comprising just about every form type described for modem and ancient stromatolites. Hofmann (1969) devised a complex form taxonomy of stromatolites from the Gunflint Formation of the Lake Superior region, and has tied this classification into those of Logan et al. (1964), Rezak (1957), and Soviet geologists (e.g. Korolyuk, 1960 Krylov, 1963 Komar, 1966) which are based mainly on carbonate stromatolites. 

Fig. 2. Carbon isotopic values (with error bars) of individual Precambrian microfossils from single stratigraphic horizons of three geologic units measured by ion microprobe mass spectrometry compared with those of the carbonate and total organic carbon measured in bulk samples of the same geologic units. Values plotted for carbonate and total organic carbon are from Strauss and Moore (1992) for microfossils from the Bitter Springs and Gunflint Formations, from House et al (2000) and those for microfossils from the Dresser Formation, from Ueno et al. (2001).

The oldest generally accepted microbiota occur in the 2.0 Ga old Gunflint iron formation in Ontario, Canada. There are reports that morphologically simpler microfossils are present in various older rocks, back to 3.8 Ga ago. Thus, it would be beneficial to gain paleobiochemical data supportive of these very old and simple microstructures. The Gunflint is obviously a suitable model on which the chemical and structural features of ancient life can be demonstrated. 

Mason (19 /1) points out that even in very old Archean formations slates are found which contain free carbon of possibly organic origin. In particular, such slates occur in the Gunflint BIF (Lake Superior), with an age of about 1.9 b.y. 

Fig. 34. Relict organic structures in Precambrian cherty iron-formations (after La Berge). I = type A structure, well preserved due to carbonaceous matter in chert matrix note fibrous surface and clear interior (lower chert unit of Biwabik Iron Formation, Pilotac mine, Minnesota) 2 = type A structure, largely replaced by greenalite 3 = type A structure, preserved due to unidentified brown substance (hematite ) in chert matrix (Belcher Islands iron-formation) 4 = type B structure (Eosphaera tyleri), well preserved due to carbonaceous matter in chert matrix (Gunflint district) 5 = type B structure in which most of the organic matter has been replaced by extremely fine hematite (Vicar mine, Gogebic district) ...

The data of Cloud and Licari (1968) support La Berge s conclusions. Spherical structures 4-10 fim in size, similar to Huroniospora , are found even in the very old rocks of the Soudan BIF. In the Proterozoic formations of Gunflint and Biwabik (Lake Superior) and in the Pretoria series (South Africa), spherical forms which probably were pelagic organisms living in the water layer near the bottom of the photozone are extensively developed in the banded iron-formations. 

Barghoorn, E.S. and Tyler, S.A., 1965. Microorganisms from the Gunflint Chert. Science, 147 563-577. Barghoorn, E.S., Knoll, A.H., Dembicki, H.Jr. and Meinschein, W.G., 1977. Variation in stable carbon isotopes in organic matter from the Gunflint iron formation. Geochim. Cosmochim. Acta, 41 425-430. 

Floran, R.J. and Papike, J.J., 1975. Petrology of the Gunflint Iron Formation, Ontario-Minnesota Pt. I, The low grade rocks. Bull. Geol. Soc. Am., 86 1169-1190. 

Winter B. L. and Knauth L. P. (1992) Stable isotope geochemistry of cherts and carbonates from the 2.0 Ga Gunflint Iron Formation implications for the depositional setting, and the effects of diagenesis and metamorphism. Precamb. Res. 59, 283-313. 

Goodwin, A.M., 1956. Facies relations in the gunflint iron formation. Econ. Geol., 51 565-595. 

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