INSIGHTS ON THE EVOLUTION OF A MIXED ENERGY INCISED VALLEY FILL SYSTEM, ACE BASIN, SOUTH CAROLINA

The analysis of the Quaternary evolution of this mixed energy valley fill system exhibits the stratigraphic complexity of multiple eustatic sea level cycles along a tectonically stable shoreline margin. The estuary covers approximately 1400 square kilometers and is 52 km long and 23 km wide at its entrance. By definition the estuary extends from its most seaward facies at its entrance to the up-dip limit of tidal influence. The estuary receives upland derived sediments from three river sources, the Ashepoo, Combahee, and Edisto Rivers (ACE Basin), all of which exhibit differences in drainage basin character ultimately resulting in mineralogical and textural differences within the system. Their sedimentological differences are seen not only in the alluvial valley architecture, but throughout the entire estuary to the seaward most facies. The system experiences both autocyclic and allocyclic processes with autocyclic sedimentation dominating.

The entrance of the mixed energy estuary is sand rich with multiple depositional environments and their associated facies. The mid-estuary is dominated by mud deposition that is marked by abundant tidal bedding and decreasing marine influence. The upper/fluvial portion of the estuary has numerous active and inactive channels. These channels represent multiple sea level events from both low stand incision and transgressive/high stand aggradation. Valleys are narrow, commonly less than 3 km with depositional environments rich in mud and organics. Channel fill sequences reflect the characteristics of their associated drainage basins.

The interrelationship of the older Pleistocene highstand deposits with younger Pleistocene and Holocene deposits form a complex stratigraphic assemblage. Much of the estuary has only a thin veneer of Holocene marsh deposited over older remnant topography. It appears that the framework of the Holocene estuary was controlled by pre-existing Pleistocene topography and the entrance of the system shows stratigraphic evidence of both sea level flooding and lateral movement of open marine and tidally bedded facies.