Fluvial terraces of the lower Susquehanna River

Fluvial terraces of the lower Susquehanna River offer a unique opportunity to investigate the late stage geologic and geomorphic evolution of the U.S. Atlantic passive margin. Petrography and elevation distinguish and provide a basis for correlation of two groups of terraces, the upland terraces and lower terraces, through the Piedmont, Newark Basin, and Great Valley. Downstream correlation to dated upper Coastal Plain and Fall Zone fluvial deposits, relative weathering, and soil profile development characteristics establish terrace age. Upland terraces (Tg1, Tg2, and Tg3), middle to late Miocene strath terraces 80 to 140 m above the present channel, occur only along the Piedmont reach. They are underlain by unstratified, texturally-mature, quartz-dominated roundstone diamictons. Lower terraces (QTg, Qt1–Qt6), Pliocene and Pleistocene strath and thin aggradational terraces within 45 m of the present channel, are underlain by stratified and unstratified, texturally and compositionally immature sand, gravel, and pebbly silt. Terrace age and longitudinal profiles suggest complex interactions among relative base level, long-term flexural isostatic processes, climate, and river grade. Our model for terrace genesis requires the Susquehanna River to attain and maintain a characteristics graded longitudinal profile over graded time. For the U.S. Atlantic margin, we propose that straths are continually cut along this graded profile during periods of relative base level stability, achieved by slow, steady, isostatic continental uplift acting in concert with eustatic rise. Change in an external modulating factor, such as eustatic fall or climate change, results in fluvial incision and subsequent genesis of strath terraces. Longitudinal profiles of lower Susquehanna River terraces, which converge at the river mouth, diverge through the Piedmont, and reconverge north of the Piedmont, contrast with their hypothesized, original concave-up profiles. Progressive and cumulative flexural upwarping of the Atlantic margin accounts for terrace profile deformation suggesting flexural isostasy as a first-order, regional deformation mechanism. These results offer new interpretations of terrace age, correlation, and geologic significance that require modification of previous studies suggesting uplifted or anticlinically-warped peneplains on the U.S. Atlantic margin.