Geochemical Environments of Precambrian Sedimentation

Abstract

Distribution of carbonate sediments of different composition in the Precambrian section is reviewed. It is shown that calcium carbonates prevailed in the Archean–initial Early Proterozoic, and iron carbonates dominated in the banded iron formations. Dolomites and magnesites were more widespread in the Proterozoic. Such distribution suggests an acid reductive geochemical environment in the Archean–Early Proterozoic, which determined the corresponding character of continental weathering: appearance of dissolved forms of Fe and silica that were transported to ancient oceans and deposited, resulting in the formation of ferruginous–quartzites or jaspilites (banded iron formations). Large-scale and almost explosive development of photosynthesizing cyanobacteria at the Archean–Proterozoic boundary stipulated, first, the utilization of carbon dioxide and, correspondingly, the change of acid environment into alkaline, leading to the formation of magnesian carbonates; and, second, to the appearance of free oxygen.