Foreland-basin sequence response to collisional tectonism

Abstract

As structural salients and recesses evolved from reentrants and promontories along the collisional continental margin associated with the Taconic orogeny, cross- strike structural features provided a mechanism for transferring zones of relative subsidence and uplift across the Appalachian foreland basin. The regional distribution of Lower Silurian clastic sequences reflects this tectonic influence. Thick, aggradational sequences formed in areas corresponding to salients in response to high rates of sediment supply and creation of sediment accommodation. As the rate of sediment supply exceeded the rate of accommodation added, shoreline progradation onto the distal foreland ramp produced upward-coarsening sequences. In areas of structural recesses, accommodation was created by erosion during sea-level fall and lowstand. Upward-fining sequences formed as the topographic lows were filled during subsequent sea-level rise. Results from this investigation indicate that predictable variations in foreland-basin deposition and in the resulting stratigraphic pattern occur along regional tectonic strike as well as in the dip direction. The thickness of foreland-ramp sequences is greater in areas of salients than in recesses, whereas the ratio of sandstone to total thickness is greater in the recesses. Aggradational sequences grading laterally into upward-coarsening progradational sequences of the distal ramp characterize areas of relative subsidence, which provides a mechanism for creating sediment accommodation. In contrast, deep erosion, common unconformities, and incised valley fills are present in areas corresponding to recesses, where the rate of eustatic fall commonly exceeds the subsidence rate. These along-strike stratigraphic variations in response to collisional tectonism should be considered in the interpretation of other foreland-basin successions.