Abstract: Alluvial Valley Fill Architecture of Lower Coastal Plain Rivers

Samuel R. Johnson

Alluvial valley-fill sequences of two lower coastal plain river systems, the Edisto and Combahee rivers, located in South Carolina, were studied in detail to determine the heterogeneity of Holocene aggradational fill within meander loops. Both study areas are approximately 20 km inland of the St. Helena Sound and are influenced by 1 to 1.5 m. semidiurnal tides. A total of 37 vibracores, to depths of 12 m, and the regional geomorphology were used to determine the three-dimensional architecture.

Sandbody geometries within the meander loops exhibited sharp contrasts in complexity. The Edisto River contains a homogenous coarse-grained point bar capped by fresh water, organic-rich, muddy, marsh deposits. The Combahee River contains stacked (coarse to fine sand) point bar sequences separated by local paleosol horizons. C¹⁴ dates of the paleosols and overlying sediment are used to relate types of sediment accumulations to changes in sea level. Present day sedimentation within the rivers varies as well, with a more pronounced tidal influence in the Combahee River.

The pronounced change in sediment character is thought to be more a function of river avulsion processes and local sediment source rather than the influence of sea level rise and stabilization. The effects of present-day tidal influence on sedimentation within the river systems is variable.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994