Banded iron-formations clastic-sedimentary

In some works a tendency toward convergence of the volcanogenicsedimentary and clastic-sedimentary hypotheses is noted. Belevtsev et al. (1966), who considered mainly the clastic-sedimentary hypothesis, postulate the extensive occurrence of acid waters in the Precambrian hydrosphere as the result of intensive volcanic activity. Tyapkin and Fomenko (1969) believe that the main source of iron and silica in the Precambrian was the basic rocks which were the chief constituent of the Earth s crust at that time, but that some was also derived from basaltic rocks erupted along abyssal faults and other products of basic volcanism. In this case it is impossible to deny the possibility that part of the iron and silica was supplied to the sea basins along with products of volcanic activity. In this scheme the role of volcanic activity in the formation of the BIF comes down chiefly to the creation of acid environments which promoted the leaching of iron compounds from basic rocks and its transport and subsequent accumulation. The primary banding is explained by periodic revival and extinction of volcanic activity, as a result of which the pH of the water basin varied, which ultimately led to deposition of iron or cherty sediments in turn. The periodicity of those cycles might have been of the order of several hundred years. 

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