POSSIBLE TECTONIC AND STRUCTURAL CONTROL ON ORDOVICIAN BLACK-SHALE DISTRIBUTION DURING THE TACONIAN OROGENY, NORTHERN APPALACHIAN BASIN, U.S.A

In the Appalachian Basin, black shales are major parts of most flexural, foreland-basin sequences and reflect the timing of major loading-related, foreland subsidence. Moreover, because of their distinctive character in the surface and subsurface, mapping their distribution can help track the progression of orogeny in space and time. In the U. S. Appalachian area, the distribution of Middle to Upper Ordovician black shales suggests that the Taconian Orogeny proceeded in a diachronous fashion from south to north along the eastern Laurentian margin, and that tectophases were mediated by convergence at successive continental promontories. In the Late Ordovician (late Sandbian–Hirnantian) 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 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 resulted in a relatively narrow foreland basin along reactivated Iapetan basement structures. 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, fully yoked basin. The coincidence of changes in basin shape and migration with the 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.