CRATONWARD EXTENSION OF DARK, GAS-SHALE DEPOSITION THROUGH BASIN YOKING: EXAMPLES FROM THE ORDOVICIAN AND DEVONIAN OF NORTH AMERICA

Foreland basins can be major repositories for dark, organic-rich gas shales in epicontinental settings, and mapping the distribution of these dark-shale basins shows that they are clearly related to the migration of orogenic loads in space and time. However, mapping also shows that at certain times and places, some of the same dark shales migrated beyond the normal limits of the foreland basin into adjacent intracratonic basins, along with their respective depositional regimes. Such a flexural interaction between a foreland basin and an intracratonic basin has been called basin yoking, and it results from the cratonward movement of a foreland deformational load to the point that load-related subsidence effectively “submerges” any inter-basinal arch. Moreover, much of the subsidence involved in basin yoking occurred along old basement fault zones that were reactivated in ways that apparently facilitated flexural movement in upper viscous parts of the crust. Some of the yoking, like that during the Blountian tectophase of the Taconian Orogeny, involved a rather tortuous path of subsiding basement structures with accompanying dark-shale deposition, whereas in the Late Ordovician Taconic tectophase, northwestern parts of the Utica, dark-shale, foreland basin merely joined the Michigan Basin between two simple structures during the Eden-Maysville transition. Other basins, like Devonian Illinois Basin, started with their own distinctive Middle-Upper Devonian dark-shale sequences, which were abruptly replaced with later Devonian, Appalachian Basin-type sequences due to yoking. Although foreland basins may provide ideal circumstances for dark-shale deposition, basin yoking is a widespread, and largely unrecognized, way of extending that deposition to adjacent basins. Although foreland and yoking-related subsidence are 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 related dark shales.