Sedimentologic and Stratigraphic Interpretation of Sand Bodies in a Tidal Embayment: ABSTRACT

A depositional model for intertidal sand bodies indicative of tidal embayments was developed from 20 vibracores and 25 can cores taken at St. Helena Sound, South Carolina. This V-shaped embayment located 35 km south of Charleston, South Carolina, has a tidal range of 2.0 to 2.8 m. The intertidal shoals are formed and reworked by opposing tidal currents. Ebb currents usually exceed 100 cm/sec in the deep adjacent channels and produce the long linear features on the shoals. Flood currents rarely exceed 75 cm/sec and are dominant across the broad seaward sand flats. The range of sedimentary features gradually changes from a dominance of physical sedimentary structures on the exposed seaward sand flats to a dominance of biogenic sedimentary structures on the protected sand flats. The distribution of each feature is controlled by their relative position on the sand flats to maximum wave energy. Where biogenic sedimentary structures are abundant, protection from wave energy is afforded by the shoal crest. Laterally the shoals grade into ebb channels or lower subtidal mixed sand and mud flats. The shoals display a coarsening-upward sequence of wavy-bedded to flaser-bedded clays and sands overlain by clean well-sorted, cross-bedded to burrowed sands. The sands are composed of fine to very fine subangular quartz grains. The depositional history of the intertidal sand bodies indicates a vertical buildup of sediments and subsequent lateral accretion. Subtidal sand bodies were first deposited on preexisting bay-fill muds. With a decline in sea-level rise, an increase in vertical deposition occurred, producing incipient intertidal bars. As the bars became fully emergent, increasing wave energy and tidal currents reworked the shoals into their present shape. Continued sand deposition occurred as lateral accretion and infilled adjacent channels. The shoals are up to 10 m thick and cover an area of 1 to 4 km2. They extend 3 to 5 km seaward and are as much as 1 to 2 km in width. Because most of the shoals are subtidal to intertidal, preservation potential is high. As the embayment fills, prograding salt marshes will eventually cap the sand bodies. End_of_Article - Last_Page 622------------