Phanerozoic black shales and the Wilson Cycle

Abstract

The spatial and temporal distribution of black shales is related to the development of the environments in which they accumulate and to a propitious combination of environmental variables. Whereas much has been done in recent years to improve our understanding of the mechanisms behind the temporal distribution of black shales in the Phanerozoic, the interpretation of the palaeogeographical distribution of black shales is still dominated by an oversimplistic set of three uniformitarian depositional models that do not capture the complexity and dynamics of environments of black shale accumulation. These three models, the restricted circulation, the (open) ocean oxygen minimum and the continental shelf models, are in fact a uniformitarian simplification of the variety of depositional environments that arise and coexist throughout the course of a basin's Wilson Cycle, i.e. the dynamic sequence of events and stages that characterise the evolution of an ocean basin, from the opening continental rift to the closing orogeny. We examine the spatial distribution of black shales in the context of the Wilson Cycle using examples from the Phanerozoic. It is shown that the geographical distribution of black shales, their position in the basin infill sequence and their nature (e.g. type of organic matter, lithology) depend on basin evolution because the latter controls the development of sedimentary environments where black shales may be deposited