FRINGE DEPOSITIONAL UNITS: A WAY OF TRACING THE MODERN TRANSGRESSION THROUGH GEOLOGIC MAPPING

Mapping of Coastal South Carolina at 1:24,000-scale and subsequent review of aerial photography produced conflicting interpretations of the marsh’s landward edge. Infrared photography shows salt-tolerant plants fringing the edge of upland maritime forests at the border between the modern estuarine system and Pleistocene upland. On-the-ground facies mapping identified salt marsh sediment away from the uplands, but not along all uplands and their maritime forest borders. Some areas have a gently sloping ramp of Pleistocene upland sediments colonized by salt-tolerant plants surrounding tree stumps. Although imagery indicates this is a marsh environment, not enough time has elapsed to allow deposition of the very fine-grained sediments characteristic of the marsh facies. Geologically, the area is Pleistocene upland sediment; but biologically, it is a salt marsh. A new map unit was defined to avoid the possibility of misinterpreting the geology from imagery. The fringe unit is biologically a marsh, but its coarser sediments are not modern marsh facies mud. This map unit has limited stratigraphic thickness (< 2ft) and it is principally a 2-D feature. The need for recognition of the fringe unit is important because it would not be preserved in the geological record as a salt marsh, but rather as an extension of the upland.

The map unit’s significance is that it represents a spatial transition between the marsh and upland and is characterized by the association of salt grasses surrounding tree stumps. Transition results as local relative sea-level rise kills off the freshwater vegetation and allows colonization of salt-tolerant plants. Salt-marsh plants are stunted because they grow poorly in dense Pleistocene fine-grained sand and clay and not the preferred soft marsh sediment. A cross section from the upland to the true salt marsh shows a gently sloping upland sandy sediment wedge that has Maritime forest overlying the tide line. Away from the upland, the wedge transitions to stunted salt grasses and tree stumps into a thickening layer of marsh sediment covering the stumps. Burial by marsh sediments is the leading edge of the modern transgression. Mapping the aerial distribution of this fringe unit through time follows the changes in the estuarine systems and can be a tool for predicting future physical change.