USING BLACK SHALES TO DISCERN BASIN YOKING

Basin yoking is a largely unrecognized flexural process that effects the migration of depositional regimes from a foreland basin into adjacent intracratonic basins and is best illustrated by mapping the distribution of distinctive sedimentary units, like black shales, in space and time. Considering that basins and intervening arches or bulges reflect long-wave-length fluctuations of the crust, yoking represents the cratonward migration of very long, wave crests (bulges) and troughs (basins) due to foreland deformational loading in an adjacent orogen. As a result, a phase of migrating basinal subsidence may effectively “depress” an inter-basinal arch due to destructive interference between basins and bulges, thereby allowing foreland-basin depositional conditions to migrate into an adjacent intracratonic basin. Much of the uplift and subsidence involved in basin yoking may reflect flexural movement in viscous parts of the crust that facilitate reactivation of old basement fault zones in uppermost, brittle parts of the crust, and it is the resulting movement along these faults that apparently accounts for much of the yoking. Some basin yoking, like that during the Blountian tectophase of the Taconian Orogeny, involved an irregular path of subsiding basement structures with infilling black shales, whereas in the Late Ordovician Taconic tectophase, tilting of northwestern parts of the Utica black-shale basin merely joined the Michigan Basin between two simple structures during the Late Ordovician Eden–Mayville transition. In contrast, the Devonian Illinois Basin started with its own distinctive Middle–Late Devonian black-shale sequence, which was abruptly replaced by a later Devonian (Acadian), Appalachian-Basin-type sequence, indicating that yoking is a way to extend the deposition of source-prone black shales into adjacent basins. Moreover, where more than one intracratonic basin yoked to a foreland basin, the sequence of yoking events progresses in line with the migration of orogeny in the orogen. Although foreland and yoking-related subsidence is mainly controlled by large-scale kinematic processes during convergence, much of the subsidence clearly occurred along basement structures that probably controlled the local timing, nature and distribution of the involved black-shale units.