TECTONIC ROLES IN THE ORIGIN OF MARINE, EPICONTINENTAL BLACK SHALES

The origin of marine, epicontinental black shales is still controversial with most discussion centering on the roles of bottom-water anoxia or high organic productivity. Clearly, these are important factors, but if, as some suggest, normal productivity is nearly always and everywhere sufficient to generate the organic matter necessary for forming major black-shale deposits, then other factors may be equally compelling. One such factor may be availability of suitable repositories in which to collect and preserve the sedimented organic matter. In epicontinental settings, such repositories are tectonic basins generated by compressional, extensional or strike-slip mechanisms, commonly expressed paleogeographically as marine embayments. Moreover, at times in the development of these basins, the tectonic regime and resulting paleogeography generate conditions that are favorable for sediment starvation, restriction through depth and geographic isolation, and enhanced nutrient influx.

An exercise to test the validity of such tectonic controls examined the distribution of black-shale deposits on North America during its assembly into and dispersal from Pangea, reasoning that black shales should be more abundant during tectonically active periods of assembly and disassembly, when basins are forming, than during intervening periods of tectonic quiescence. Results show that all black shales occupied tectonic basins, but were more abundant during Carboniferous assembly in compressional basins and during Jurassic disassembly in extensional basins than during Permo-Triassic times of stasis, when tectonism was minimal.

Although global and regional controls on organic productivity accompanying supercontinent assembly may have been important, the fact that productivity was sufficient to produce some major black-shale deposits during every period examined, and across diverse climatic zones, suggests that organic productivity was not the only controlling factor. Instead, the exercise suggests that availability of suitable, tectonically generated repositories may have been just as important, and that factors like sediment starvation, restriction, reduced oxygenation and enhanced organic productivity are naturally inherent at some time in the development of most tectonic basins.