BLACK SHALES AND BASIN YOKING IN THE APPALACHIAN FORELAND

Foreland basins, intracratonic basins and the arches separating them are dynamic, changing features related to flexural processes in adjacent orogens. One of these processes is basin yoking, or the coupling of a foreland basin with an adjacent intracratonic basin. Considering that basins and intervening arches or bulges reflect long wave-length deflections of the crust, yoking reflects the cratonward movement of a loading-related deflection to the point that resulting destructive interference effectively “depresses” any inter-basinal arch, thereby allowing depositional regimes from one basin to migrate into another. Much of the subsidence involved in basin yoking reflects flexural movement in viscous parts of the crust that facilitates reactivation of old basement fault zones in uppermost, brittle parts of the crust, and it is the resulting movement along these faults that apparently facilitates much basin yoking. The occurrence of yoking in the geologic record, however, is difficult to discern unless distinctive depositional regimes can be traced from one basin to another, and the most easily traced depositional regime is the deeper-water, anoxic/dysoxic environment reflected in black shales, and it is the migration of black-shale deposition in space and time that is most useful in discerning the occurrence and nature of basin yoking. Basin yoking first occurred in the Appalachian foreland during the Blountian tectophase of the Taconian orogeny when the Appalachian and proto-Illinois basins yoked via a tortuous path of subsiding basement structures, known as the Seebree Trough, which filled with dark Maquoketa, Pt. Pleasant, and Utica shales. Later during the Taconic tectophase, tilting of northwestern parts of the Martinsburg-Utica foreland basin yoked it with the Michigan Basin between two simple structures during the Late Ordovician Eden–Mayville transition. Other intracratonic basins, like the Devonian Michigan and Illinois basins, started with their own distinctive Middle–Late Devonian dark-shale sequences, only to be abruptly replaced with later Devonian (Acadian), Appalachian-Basin-type, black-shale sequences due to yoking. Although foreland, yoking-related subsidence is mainly controlled by large-scale kinematic processes during convergence, upper-crust basement structures probably controlled the timing and nature of yoking events.