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Black shales are integral parts of most foreland-basin deposits and, because they typically reflect maximum basin subsidence, their distributions serve as proxies for the extent of foreland-basin development. In the United States Appalachian area, the distribution of Middle–Upper Ordovician black shales suggests that the Taconian Orogeny proceeded from south to north along the eastern Laurentian margin and that Taconian tectophases were mediated by convergence at continental promontories. In the Late Ordovician Taconic tectophase, changes in the distribution of the Martinsburg and Utica black shales support a reversal of subduction polarity that effected the reactivation of basement structures and basin migration sufficient to yoke the Appalachian foreland basin with adjacent intracratonic basins. Shale distribution suggests that early Chatfieldian (late Sandbian–early Katian), east-verging subduction early in the tectophase generated a cratonic extensional regime with a narrow foreland basin that developed along reactivated Iapetan basement structures. Abruptly, in late Chatfieldian–early Edenian (early Katian) time, westwards migration of basinal Utica black shales and an underlying unconformity suggests change to a compressional regime and westwards subduction vergence. The coincidence of changes in basin shape and migration with the shifts in subduction polarity suggests a causal relationship.

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