Stromatolites Steep Rock

Figure 3 (a) The surface of the 3 Ga Earth, Steep Rock, NW Ontario, Canada. The hill-face is very close to a 3 Ga unconformity surface, and the rocks (granitoids and mafic dikes) exposed on the hill-face are immediately helow the unconformity. Above them are assorted sediments, including thick stromatolitic limestones, (b) Stromatolitic limestone, Steep Rock, Ontario, Canada (ca. 3 Ga old). The palaeohorizontal surface dips 70°. Stromatolitic domes are up to 4-5 m long and 2 m high. 

Wilks, M. E. Nisbet, E. G. 1988. Stratigraphy of the Steep Rock Group, northwest Ontario a major Archaean unconformity and Archaean stromatolites. Canadian Journal of Earth Sciences, 25, 370-391. 

The carbonate horizons of the Cheshire and Manjeri Formations contain extensive well-preserved stromatolites. These were formed in shallow or intertidal waters, as demonstrated by interbedded ripple-marked and mud-cracked silts. Texturally, the limestones show many indications of organic activity and gas release structures (Martin et al. 1980). The simplest uni-formitarian explanation is that the structures were built by cyanobacteria. Isotopically, carbon from kerogen in the stromatolites typically has 6 C = -25 to -30%o. This implies but does not prove fractionation by rubisco of carbon captured from the atmosphere-ocean system. Carbonate in Cheshire limestones is typically close to 0%o, suggesting that carbon in the atmosphere-ocean system was modulated by rubisco and dominated by oxygenic photosynthesis on a planetary scale (Fig. 1) at least by this date. It should be noted that the c. 3 Ga Steep Rock stromatolites are similar (work by Abell, Grassineau and Nisbet). 

Late Archaean stromatolites Younger stromatolites have been described from 3.0 Ga rocks at Steep Rock in Canada (Wilks St Nisbet, 1985), from the 2.7 Ga Cheshire formation of the Belingwe Greenstone belt (Martin et al., 1980) and from 2.52 Ga shelf carbonate sediments in the Campbellrand subgroup, South Africa. In this latter locality there are also well-documented calcified microfossils of cyanobacteria (Kazmierczak St Altermann, 2002). It is also possible that at this locality the mineralization of the dead bacteria was the result of the action of het-erotrophic bacteria. 

Wilks, M.E. and Nisbet, E.G., 1985. Archaean stromatolites from the Steep Rock Group, northwestern Ontario, Canada. Can. f. Earth Sci., 22, 792-9. 

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