THE INFLUENCE OF LAURENTIAN PROMONTORIES AND REENTRANTS ON APPALACHIAN, FORELAND-BASIN SEDIMENTATION AND STRATIGRAPHY

Orogenies are typically polyphase events that comprise two or more tectophases, or deformational episodes, commonly focused at different places and times along an orogen. The sedimentary record in the composite Appalachian foreland basin, along with available radiometric, deformational, and magmatic data, suggests that during four Paleozoic orogenies along the Laurentian margin, tectophase timing and occurrence were mediated by diachronous convergence at successive promontories. Projecting promontories served to localize tectonism because of greater shortening and more intense deformation, while intervening reentrants served as sediment sinks. The greater intensity of deformation and resulting deformational loading at promontories, moreover, insured greater flexural subsidence in adjacent parts of the foreland basin so that major basin depocenters or sub-basins are located just behind or adjacent to respective promontories. Hence, diachronous, oblique convergence with successive promontories generated tectophases during each orogeny that are reflected by distinct, unconformity-bound, flexurally related sedimentary cycles in the foreland basin. Thirteen such cycles are present in the Appalachian basin. Marine black shales initiate most cycles, are prominent and easily mapped, and mark the time of maximum flexural subsidence; by mapping the distribution of these shales in time and space, it is even possible to track the along-strike progress of orogeny relative to promontories. Moreover, the distribution of black-shale basins, distribution of unconformities, and clastic-wedge thicknesses are all commonly asymmetric toward the involved promontory. Because the Appalachian basin was largely a flexural response to phases of tectonism focused at various promontories, the promontories indirectly influenced basin sedimentation and stratigraphy. Consequently, understanding this relationship makes the sedimentary and stratigraphic record of the Appalachian basin a largely untapped source of information about the timing and nature of Paleozoic tectonism along the former Laurentian margin.