Abstract: Sedimentary Environments of Tidal Flat Created by Shoreline Erosion in Tidal Estuary

Richard M. Scott, James D. Howard

Cabbage Island tidal flat is in Wassaw Sound, a tidal estuary on the Georgia coast. Within the area of 2 sq km, more than 150 oriented can cores and vibrocores have been collected to evaluate the erosional/depositional origin of the flat, and to characterize animal-sediment relations. Primary physical and biogenic sedimentary structures were determined by examination of X-ray radiographs and epoxy relief casts. Biologic communities have been determined by field observations, examination of biogenic structures within cores, and by collection of organisms. Since the late 1800s the tidal flat has increased in width from 0.15 to 0.6 km by erosion of the adjacent salt marsh. Remnant marsh sediments underlie parts of the flat and crop out locally. An attached sand spit at the southern end of the tidal flat has retained its approximate position throughout this period of erosion, but has increased in length.

Sediment types, topographic relief, benthic organisms, and internal structures indicate the presence of five subenvironments on Cabbage Island tidal flat: (1) fine-sand shoals, (2) muddy fine-sand flats, (3) muddy sand-interflat channels, (4) current megarippled coarse-sand shoals, and (5) marginal fine-sand and shelly fine-sand flats.

Distribution of benthic organisms is dependent on topographic relief and sediment type. Tidal currents are the dominant transport agents on and adjacent to the flat and most cores exhibit bidirectional current structures. Fine-sand shoals are flood-tide features and indicate landward transport of fine sand onto the flat. Morphology is controlled by ebb-tidal currents from the bordering tidal rivers and by two smaller tidal creeks in the central-flat region.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA