Phanerozoic environments of black shale deposition and the Wilson Cycle

摘要

The spatial and temporal distribution of black shales is related to thedevelopment of environments in which they accumulate and to a propitiouscombination of environmental variables. In recent years, much has been doneto improve our understanding of the mechanisms behind the temporaldistribution of black shales in the Phanerozoic and of the environmentalvariables that result in their deposition. However, the interpretation ofancient black shale depositional environments is dominated by anoversimplistic set of three depositional models that do not capture theircomplexity and dynamics. These three models, the restricted circulation, the(open) ocean oxygen minimum and the continental shelf models, are anoversimplification of the variety of black shale depositional environmentsthat arise and coexist throughout the course of a basin's Wilson Cycle, i.e. thedynamic sequence of events and stages that characterise the evolution ofan ocean basin, from the opening continental rift to the closing orogeny. Weexamine the spatial distribution of black shales in the context of theWilson Cycle using examples from the Phanerozoic. It is shown that thegeographical distribution of environments of black shale deposition and theposition of black shales in the basin infill sequence strongly depend onbasin evolution, which controls the development of sedimentary environmentswhere black shales may be deposited. The nature of the black shales that aredeposited, i.e. lithology and type of organic matter, also depends on basinevolution and palaeogeography. We propose that in studies of black shalesmore attention should be given to the sedimentary processes that have led totheir formation and to the interpretation of their sedimentary environments.