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Large-Scale Tectonic and Structural Controls on Ordovician Black-Shale Distribution (Utica and Martinsburg) During the Taconian Orogeny, Northern Appalachian Basin, USA


Organic-rich black shales are major parts of most foreland-basin sequences, and the organic-rich Utica Shale, deposited in the Taconian foreland basin, contains sufficient organic matter that it has become a major source of unconventional gas. Such organic-rich black shales are thought to reflect high organic productivity, but in most marine settings, organic productivity in the upper water column is almost always sufficient to generate abundant organic matter for preservation. Hence, organic productivity may not be such a limiting factor. More limiting, however, may be generating means to preserve that organic matter so that it persists into the geologic record, and large basin repositories, like foreland basins, may be critical for that preservation. Consequently, understanding connections between large-scale tectonic and structural controls on basin development and dark-shale deposition may be pivotal in fully exploiting black-shale resources. In point, mapping Ordovician black-shale distribution in the Appalachian area shows that black shales are parts of Taconian tectophase sequences and migrate in a diachronous fashion that tracks the progress of Taconian orogeny and even subduction polarity. In the Late Ordovician Taconic tectophase, 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 change black-shale distribution. Shale distribution suggests that early Chatfieldian, east-verging subduction generated a cratonic extensional regime that formed a relatively narrow foreland basin along reactivated Iapetan basement structures. Abruptly, however, in late Chatfieldian–early Edenian 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 time, distribution of Utica and Utica-equivalent black shales shows that the Appalachian and Michigan basins merged into one large yoked basin. The coincidence of changes in basin shape and migration with the shift in subduction polarity, as inidcated by black-shale distribution, suggests a causal relationship and shows how important large-scale tectonic and structural controls can be in generating suitable basin repositories for black shales.