Large-scale Tectonic Controls on the Origin of Paleozoic Dark-shale Source-rock Basins

Abstract

Recent plays like the Middle Devonian Marcellus Shale and possible prospects like the Upper Ordovician Utica Shale point out the significance of dark-shale source rocks in the Appalachian Basin. Mapping the distribution of such shales in space and time throughout the basin shows that periods of dark-shale deposition coincided with orogenies and the related formation of foreland basins. The fact that foreland basins form and become repositories for organic-rich dark-shale source rocks is mostly the result of deformational loading in the adjacent orogen. Tectonism mostly exerts its control through the flexural effects of deformational loading and subsequent relaxation in the orogen. These flexural processes generate sedimentary responses in the foreland basin that are reflected in a seven-part unconformity-bound cycle, of which dark shales are a major component. Because orogenies comprise a series of smaller deformational events, or tectophases, and each tectophase generates a similar cycle, many foreland basins typically exhibit a cyclic array of dark-shale and intervening clastic units, called tectophase cycles. Thirteen such third-order tectophase cycles, formed during four orogenies, are present in the Appalachian Basin. Using examples of foreland-basin dark-shale units formed during the Ordovician–Silurian Taconian and Devonian–Mississippian Acadian/Neoacadian orogenies, the timing of cycles and migration of successive dark-shale units within them relative to the progress of orogeny are presented as evidence of causal relationships between tectonism and dark-shale sedimentation. However, tectonic influence may extend well beyond the confines of the foreland basin in the form of far-field tensional and compressional forces. This may impel the yoking of foreland and intracratonic basins as well as the reactivation of foreland basement structures—the former allowing dark-shale depositional conditions to move from one basin to the other, and the latter, inaugurating new basins for dark-shale accumulation.