Shallow subsurface geology of part of the Savannah River alluvial valley in the upper Coastal Plain of Georgia and South Carolina

Abstract

The depth to which Coastal Plain rivers incise underlying formations is an important control on local and regional hydrologic flow systems. In order to clarify these stream/aquifer relations, a better understanding of the shallow subsurface geology of the Savannah River was necessary. To accomplish this, three drillhole transects were completed across a part of the Savannah River alluvial valley in September 1993, and five geologic sections were constructed from the data. The alluvium is coarser, more angular, and more poorly sorted than the underlying formations, and lithologic differences between the strata are readily apparent, especially in areas where the underlying strata are of marine origin. Inspection of the transects indicates an asymmetry to both the alluvial terrace complex and the underlying bedrock strath. The alluvium thins in a coastward direction; and similarly, bulk-grain size diminishes in a downstream direction. This phenomenon has remained constant over time and is most likely a function of the change in slope which occurs when the river traverses the Fall Line north of the study area. The maximum thickness of the alluvial valley fill is 50 ft. The elevation of the unconformity between the alluvium and the underlying formation is far below the lowest elevation of the modern-day thalweg, indicating that the alluvial system has aggraded to form the modern-day Savannah River Valley. Formerly, the Savannah River was located immediately adjacent to and east of the modern floodplain when the river valley was formed by a cyclic pattern of infilling and subsequent entrenchment that gave rise to an irregular bedrock surface beneath the depositional terrace system. After this depositional period, the river migrated to the southwest and began a period of downcutting that ended with the formation of the unconformity (erosional terrace) that lies some 45 ft. beneath the modern-day river. The protracted southwestward migration of the river system is perhaps the best indication that pre-historic tectonism exerts an influence on the modern-day alluvial system.