Carbonate sedimentation during the Archean

Abstract

Archean (> 2400 m.y.) sediments of the Canadian Shield are principally composed of greywacke and shale of flysch facies that are preserved within slightly metamorphosed “greenstone” belts. These Archean sedimentary sequences contain only trivial proportions of carbonate rocks compared to those of Proterozoic and Phanerozoic age. Archean sediments from other parts of the world appear to have a similarly low proportion of limestone and dolomite. One possible explanation for this scarcity of carbonate rocks is that calcium (and magnesium) did not readily pass into solution upon weathering of primary rocks during the Archean. Another possible explanation is that calcium carbonate (or dolomite) was deposited as a minor component of Archean shales and greywackes rather than as discrete beds of carbonate minerals. Both of these explanations are discounted by chemical analyses of some 406 widely scattered samples of Archean shale from the Superior province of the Canadian Shield. These analyses indicate that mobile elements were removed by weathering during the Archean in approximately the same relative proportion as during later periods; and that the calcium thus liberated has not been redeposited with the shales. Explanations based on deposition of carbonates as shelf sediments and subsequent removal by erosion, or on an increased solubility of Ca 2+ in the Archean oceans, are also discounted. It appears that the thin crust of Archean time produced no extensive stable shelf or miogeosynclinal environments, which were important sites for carbonate deposition during the Proterozoic and Phanerozoic. It is suggested that carbonate deposition during the Archean took place largely in deep ocean basins, and that the mechanism for precipitation may have been photosynthetic reactions by algae living near the surface of the oceans. It seems likely that there were extensive oceanfloor deposits of carbonates during the Archean which were later resorbed into the mantle during ocean-floor spreading.