Assembly and dispersal of Pangea: Large-scale tectonic effects on coeval deposition of North American, marine, epicontinental, black shales

Abstract

Both before and after inclusion of Laurussia in Pangea, the continent was a site of extensive epicontinental, marine, black-shale deposition, but from Pennsylvanian to Jurassic time when North America was an integral part of Pangea, the pattern of black-shale deposition was one of long-term decline. In North America the decline seems to have been greatest in Late Permian and Triassic times. Although this decline could have reflected a period of global cooling and a related decrease in organic productivity brought on by conditions associated with the supercontinent state, mapping the distribution of black shales in time and space on North American parts of Pangea suggests that the restricted availability of suitable repositories for organic-rich sediments may have been an equally important cause. In fact, mapping shows that the distribution of North American, Pangean, marine, black shales was greatest during Mississippian assembly of Laurussia when foreland-basin-type repositories were abundant and again during Late Jurassic fragmentation when rift-basin-type repositories were abundant. In both cases, tectonically conditioned basins formed the major repositories and promoted certain conditions that enhanced early basin anoxia. During Late Permian and Triassic time, when Pangea had been assembled, neither compressive orogenies nor crustal extension were major influences on North America. Consequently, suitable repositories were minimal and so was the extent of black-shale deposition. However, the continued presence of even a few major black-shale deposits during this time of minimum suggests that even low organic productivity was not a primary cause of decline and points to the possible significance of active continent assembly and breakup in generating tectonic-basin repositories conducive to accumulation and preservation of the organic matter that is nearly always present in quantities great enough to form major black-shale deposits.