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Tectonic and Structural Control of Ordovician Black-Shale Distribution during the Taconian Orogeny, Northern Appalachian Basin, U.S.A.: Utica and Related Black Shales


In the U. S. Appalachian area, black shales comprise parts of most flexural, foreland-basin sequences and reflect major tectonic and structural control during loading-related, foreland subsidence. Mapping the distribution of Middle to Upper Ordovician black shales suggests that Taconian orogeny proceeded in a diachronous fashion from south to north along the eastern Laurentian margin. In fact, Late Ordovician (late Sandbian–Hirnantian) changes in the distribution of 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 that created a narrow foreland basin along reactivated Iapetan basement structures and cratonward activation of the Sebree Trough. Abruptly, however, in late Chatfieldian–early Edenian (early Katian) time, subduction vergence apparently changed to the west, generating a regionally compressional regime that was accompanied by subsidence and change in regional dip, such that black shales and an underlying unconformity migrated westwardly. By Maysvillian (mid-Katian) time, the distribution of Utica and Utica-equivalent black shales show that the Appalachian and Michigan basins merged into one large, yoked basin. Coinciding changes in basin shape and migration with shift in subduction polarity suggests a causal relationship. The approximate time of polarity change is well-known from other sources, but is also well-constrained by the biostratigraphic ages of and changes in the distribution of the effected black shales.