Origin of chert nodules in the Ediacaran Doushantuo Formation black shales from Yangtze Block, South China

Abstract

Abstract Chert nodules are widely developed in Ediacaran deposits of South China, yet the mechanism responsible for their formation remains disputed. Petrological and geochemical studies of chert nodules and host shale of the Ediacaran Doushantuo Formation of the Yangtze Block, South China, were carried out to more fully assess silicification of these deposits. Doushantuo chert nodules display a concentrically layered internal structure dominated by quartz and lesser concentrations of calcite, carbonate fluorapatite, organic matter, pyrite, and sphalerite. Individual layers become thinner from nodule centers to edges. Our results suggest that (1) Doushantuo host black shale accumulated under persistent anoxic bottom water conditions and (2) chert nodules formed as a consequence of the anaerobic oxidation of methane focused along sulfate-methane transition zones at shallow burial depth. Nodules likely originated from porous proto-nodules that consisted of 13C-depleted authigenic calcite produced by the anaerobic oxidation of methane. Decaying of organic matter within nodules helped to create porosity that was filled by authigenic calcite and also released phosphate and zinc ions to pore water, thus favoring precipitation of carbonate fluorapatite and sphalerite. Silica supplied principally from the diagenesis of clay minerals of the host shale cemented the nodules and replaced early formed calcite. The progressive infilling of residual porosity by 18O-depleted pore fluids associated with increasing burial depth is reflected in the strongly negative δ18Ocarb values documented from the nodules. Thus, the formation of Doushantuo chert nodules reflects a protracted diagenetic history initiated by bacterial sulfate reduction and anaerobic oxidation of methane close to and within the sulfate-methane transition zone and continued in association with the diagenetic release of silica from host shale as well hydrocarbon generation with increasing burial depth.