USING THE CONFLUENCE OF FLEXURAL MODELS, BIOSTRATIGRAPHY AND ABSOLUTE DATING TO DETERMINE THE DURATION OF CLASTIC-WEDGE FORMATION IN TWO APPALACHIAN TECTOPHASES

The Appalachian Basin is a composite foreland basin filled largely with clastic sediments that accumulated during five orogenies, represented by 13 tectophase cycles. Orogenies typically occur as a series of pulses or tectophases, during which unconformity-bound sequences (tectophase cycles), which reflect flexural responses to deformational loading in the orogen, accumulate. In subduction-related orogenies, flexural models indicate that cycles exhibit an active-loading stage, during which deformation loads the crust, forming a rapidly subsiding foreland basin; this is followed by stages in which the crust accommodates the stress through subsidence and rebound, during which major clastic wedges form. Determining duration of the tectophase cycles or of stages within them has proven difficult, because later erosion has destroyed upper parts of each cycle or because of biostratigraphic imprecision. However, in the last decade, the confluence of biostratigraphy, absolute dating and isotope curves has enabled a better correlation between absolute dating and biostratigraphic zonation. Using these improved correlations with the two most complete Appalachian tectophases, the Taconic and Neoacadian tectophases, suggests that most of an orogenic event—at least as indicated by its sedimentary sequence—comprises relaxation, or the later accommodation of the crust to loading. Of the 7.5 m. y. Taconic tectophase, only about 1 m. y. involved active loading; the other 6.5 m. y. reflect relaxation, which generated the so-called Queenston Delta clastic wedge. Of the 37 m. y. Neoacadian tectophase, only about 3 m. y. involved active loading, followed by 34 m. y. of relaxation that generated the Borden-Price-Pocono-Grainger and Pennington-Mauch Chunk clastic wedges. Clearly, how we define an orogenic event is important; but based on this analysis, a brief active loading stage, during which a basin is generated, is followed by relaxation stages, nearly 10 times longer, during which clastic wedges are generated to infill the basin.