Stratigraphy and Sediment Characteristics of a Mesotidal Ebb-Tidal Delta, North Edisto Inlet, South Carolina

Abstract

ABSTRACT Ebb-tidal deltas can form extensive strandline sand deposits on barrier-island shorelines. This paper describes the diagnostic textures, sedimentary structures, and geometry of an ebb-tidal delta in a mesotidal setting at North Edisto Inlet, South Carolina. Bedforms and surface textures at North Edisto Inlet reflect typical ebb-tidal delta circulation patterns. The main ebb channel is dominated by large ebb-oriented bedforms, and channel sediments become finer grained in the seaward direction. At the inlet throat, the coarsest available sediment forms a lag deposit. Marginal flood channels adjacent to the barrier islands are dominated by floodoriented bedforms composed of medium-grained sand. Channel-margin linear bars, flanked by bedforms with opposing orientations, develop where flood channels approach the main ebb channel. The extensive swash platform is dominated by wave energy, which effectively sorts swash platform sediments. Ebb-tidal delta stratigraphy is dominated by sequences deposited by shifting tidal channels and by the migration of swash bars on the swash platform. Proximally, the ebb-tidal delta sequence is dominated by relatively thick tidal-channel deposits which fine upward and have a sharp, disconformable lower contact. Main ebb-channel deposits up to 20 m thick flank the active channel. These deposits are composed primarily of well-sorted, planar-bedded, fine-grained sand. Flaser-bedded sands directly overlie medium to coarse-grained, seaward-directed, crossbedded sands at the base of the sequence. Adjacent to barrier islands, marginal flood-channel deposits dominate the ebb-tidal delta sequence. These are composed of moderately to intensely burrowed, wavy to flaser-bedded sand and mud. Barrier island accretion in the vicinity of inlets often involves flood-channel switching; marginal flood channel deposits could thus comprise a significant portion of the barrier island lithosome in the vicinity of inlets. Distally, wave-formed swash platform deposits dominate the ebb-tidal delta sequence. These are characterized by very well sorted, fine-grained sand; planar bedding; landward-oriented crossbeds; coarse shell-hash layers; and extensively burrowed zones. Swash platform deposits interfinger with shoreface deposits laterally and at depth. Distal ebb-tidal delta deposits are thus relatively thin and have an overall coarsening-upward grain-size trend.