THE IMPORTANCE OF DRAINAGE BASIN CHARACTER ON ALLUVIAL VALLEY ARCHITECTURE

The stratigraphy of three lower coastal plain rivers (Ashepoo, Combahee, Edisto) was studied to determine facies and textural characteristics regarding the evolution of the valley fills. The rivers investigated are largely autocyclic systems that empty into the ACE Basin Estuary in S. Carolina. Data collected from a total of 87 vibracore samples to depths of 12m, with selected ¹⁴C dating, in association with the valley’s geomorphology were used to construct geologic cross sections and sand and mud isopact maps.

The portions of the rivers chosen for study were located inland between 25 and 35 km from the present day coastline and experience semidiurnal tides that range between a 1-2m. The purpose of the study was to investigate the heterogeneity of the modern valley fills to be used as analogies for hydrocarbon exploration.

The Edisto River valley is composed of a medium to coarse grained homogenous deposit with thin isolated organic rich mud drapes capped by a fresh water organic rich rooted mud. The sand isopach at this site is skewed to the up-stream portion of the meander with thicknesses of up to 8m in contrast to the down-stream portion of the meander with mud up to 4m thick. In contrast, the Combahee River valley is a more complex fill with a fine grained mottled sand with multiple paleo-soil horizons at the base which dated as old as 9,870 years BP, capped by peat deposits which range between 3,590 and 4,890 years BP, capped by an organic rich rooted mud. The Ashepoo River is the most sand starved with very limited quantities present in our study area.

 The size of each river’s drainage basins varied considerably: Edisto 7,987; Combahee 3,392; Ashepoo 1,036 km² respectfully. The drainage basin character and associated provenance with the associated processes dictates the style of sedimentation within the valley. These valleys have experienced the same sea level fluctuations and exhibit a variety of stratigraphic architectures.