SHORELINE AND RIVERS IN ALABAMA DURING THE OLIGOCENE HIGHSTAND

The early Oligocene highstand was the greatest of the Cenozoic Era, and it left a distinct signature on the landscape of the southern Appalachians. The approximate position of the shoreline can be recognized in Alabama by extending the methods used by Staheli (1976, GSA Bull. 87: 450-452) in neighboring Georgia. Staheli noted a discontinuity in drainage patterns at the paleoshoreline. Trellis drainage survives in the modern inner Piedmont and Valley and Ridge, but the outer Piedmont and Valley and Ridge now have dendritic drainage, probably due to superimposition on a cover of Oligocene nearshore deposits. The Oligocene cover has vanished but the dendritic drainage persists. In Alabama, the outer Piedmont and Valley and Ridge show much lower relief than the inner Piedmont and Valley and Ridge, and rivers shift direction abruptly seaward at the paleoshoreline (Adams, 1929, J. Geol. 37: 193-203). Major ridges appear to have been planed flat at the shoreline.

Evidence from water gaps, wind gaps, fossil plants, and alluvium suggests that the Chattahoochee, Tallapoosa, Coosa, Cahaba, and Black Warrior Rivers have occupied their current valleys since the Late Cretaceous, with some Late Tertiary exceptions as a result of Miocene tectonism and blanketing of upland valleys with alluvium. In particular, the Coosa drainage has become larger and more trellised with time, supplanting an even earlier transverse drainage whose remnants include the upper part of Talladega Creek.