Abstract

Both before and after North America’s inclusion in Pangea, the continent was a site of extensive epicontinental dark-shale deposition, but from Carboniferous to Jurassic time, when North American was an integral part of Pangea, the extent of dark-shale deposition declined, reaching its minimum in Late Permian and Triassic times. Although this decline could reflect a general deterioration of global climate and associated organic productivity brought on by conditions related to the supercontinent state, mapping the distribution of dark shales in North American parts of Pangea in time and space suggests that availability of suitable repositories for organic-rich sediments may have been a more immediate cause. In fact, mapping shows that the extent of North American Pangean dark shales was greatest during Carboniferous assembly of the continent, when foreland-basin-type repositories were abundant, and again during Late Jurassic fragmentation, when rift-basin-type repositories were abundant. During Permian and Triassic times, when Pangea had been largely assembled, neither orogeny nor crustal extension were major factors in North America and the extent of dark-shale deposition reached a minimum. However continued presence of some major dark-shale deposits during the time of minimum suggests that low organic productivity was not a primary cause of decline, and points to the significance of active supercontinent assembly and breakup in generating repositories conducive to preservation of the organic matter that was nearly always present in quantities sufficient to form major dark-shale deposits.