UNDERSTANDING THE SPATIAL AND TEMPORAL DISTRIBUTION OF APPALACHIAN OROGENIES USING THE SEDIMENTARY RECORD OF FLEXURAL, FORELAND-BASIN, TECTOPHASE CYCLES: EXAMPLES FROM THE TACONIAN OROGENY

The Appalachian Basin is in large part a composite, retroarc, foreland basin, reflecting a series of stacked foreland basins, developed during a succession of five Paleozoic orogenies. These orogenies occurred as a series of pulses, commonly called tectophases, which in the Appalachian area were mediated by continental promontories. Unconformity-bound sedimentary sequences generated in the foreland basin during these tectophases were largely controlled by the flexural effects of deformational loading and subsequent relaxation in the orogen. These flexural processes generate a distinctive, third-order tectophase cycle of sedimentary units, comprising in ascending order: basinal black shales, flysch-like clastics, and marginal-marine, molasse-like clastics with redbeds, which reflect, respectively, the flexural processes of deformational loading, loading-type relaxation, and unloading-type relaxation. The bounding unconformities represent bulge uplift and moveout during the inception of each tectophase. Inasmuch as each orogeny represents a series of tectophases, the composite Appalachian basin is filled with third-order tectophase cycles. The basinal black shales are the most distinctive lithologies in each cycle, and because they represent a period of active deformational loading, mapping their distribution can track the spatial distribution of orogeny along the Laurentian margin. In the case of the Taconian Orogeny, the distribution of basinal black shales through three tectophases, shows that the orogeny was diachronous, progressing from south to north along the Laurentian margin, with tectophases concentrated at successive promontories. Moreover, the use of cycle stratigraphy with biostratigraphy and absolute dating from bentonites allows the addition of a temporal aspect to tectophase development and reveals that the larger part of each Taconian tectophase reflects relaxation, and not active deformation. Clearly, the sedimentary record of Ordovician tectophase cycles from the composite Appalachian Basin is providing additional information about the temporal and spatial distribution of the Taconian Orogeny. Up to ten younger tectophase cycles in the Appalachian Basin hold the possibility of providing similar information for the succeeding Appalachian orogenies.