The Role of Waves and Tidal Currents in the Development of Tidal-inlet Sedimentary Structures and Sand Body Geometry: Examples from North Carolina, South Carolina, and Georgia

Abstract

ABSTRACT Morphologic variability in tidal inlets along the southeastern coast of the United States has been considered with respect to the distribution of large-scale sand bodies, intertidal and subtidal bedforms and internal sedimentary structures. Data indicate that the morphologic variability in these inlets can be largely explained as a response to waves and tides. Other factors (tidal prism, inlet cross-sectional area and shape, the nature of the back-barrier bay, the degree of flood or ebb dominance, fresh water input, relative changes in sea level and sediment supply) exhibit lesser controls and their effects are less easily quantified. Three types of inlets are identified: tide-dominated, wave-dominated and transitional. 1) Tide-dominated inlets are characterized by a deep, ebb-dominant main channel flanked by long, linear channel-margin bars. Flood-tidal deltas are poorly developed or non-existent. Sand bodies landward of the inlet throat are confined to tidal point bars further landward in the marsh creek system. 2) Wave-dominated inlets are characterized by large, lobate flood-tidal deltas building into wide, open lagoons. The ebb-tidal delta is small and extends only a short distance from the beach. Tidal channels are generally shallow (less then 6 m) and often bifurcate landward and seaward of the throat. 3) In transitional inlets, major sand bodies are typically concentrated in the inlet throat. These inlets v ry widely in morphology and sand body geometry. Logically, this variability should be expressed in the rock record. In a vertical section through a tide-dominated inlet channel, a coarse base, overlain by bidirectional trough cross stratification from the deep channel and ebb-oriented, planar and trough cross stratification from the shallower channel should be expected. Swash-generated, horizontal plane laminations or slightly inclined accretion beds formed along the channel margins are less likely to be preserved. In contrast, a wave-dominated inlet sequence would contain primarily landward-oriented, planar and cross stratification from the shallower channel bottom, overlain by dominantly horizontal or slightly inclined plane laminations from the shallow channel sides. Transitional inlets would produce a variety of sequences, the xact nature of which would reflect the relative importance of waves and tides.