Relative Role of Waves and Tides in Development of Transgressive and Regressive Coastal Sequences: ABSTRACT

Abstract

Close examination of coastal morphology and related sediment bodies reveals that the commonly used terms and tide are in many places applied improperly. Such terminology provides little data that the geologist can use in establishing the various process parameters that prevailed in ancient coastal depositional environments. In addition to the absolute and relative size of waves and tides, it is also necessary to consider other factors which contribute to coastal morphology such as tidal prism, sediment availability, topographic relief, and rates of relative sea level change. Interaction of these factors may create a wide variety of stratigraphic sequences. Several of these additional factors differ strikingly depending on whether or not the setting is one of shoreline progradation or marine transgression. In the absence of a delta, progradation generally leads to a simple, relatively straight shoreline, which tends to limit tidal currents to inlets, if present. Transgression caused by a relative rise in sea level may promote shoreline irregularity and embayment, and thus the potential for a larger tidal influence. Progradation occurs under conditions of ample sediment supply relative to the rate of sea level change, whereas transgression causes entrapment of sediment in coastal bays and rivers, thus reducing the amount of sediment available to the open marine setting. Nondeltaic progradational deposits, accordingly, are likely to be considered wave dominated regardless of wave size or tidal range. Waves are more effective in distributing and reworking the sediment along a straight progradational coast than are tidal currents, and, in addition an ample amount of sand is available on the shoreface for this reworking. Shoreface deposits typically are the major component of a nearshore progradational sequence. Transgressive deposits, however, are more likely to be considered tide-dominated even where wave size and regional tidal range are unchanged from those prevailing under conditions of progradation. Coastal irregularity creates refraction patterns that dissipate wave energy over broad areas, and so tidal flow within embayments becomes an important process. Most of the available sediment resides in these embayments and bears the stamp of reworking by tidal currents. Shoreface deposits are likely to be sporadically distributed and of inconsequential volume. Deltas constitute a potential exception to the foregoing generalization. Many deltas are dominated by riverine processes and little influenced by either waves or tides. Moreover, deltas that form in areas of extreme tidal range can generate a complex shoreline during progradation and thereby mimic the embayed coast. Such tide-dominated deltas, however, typically form in areas of broader irregularity of coastline. Sufficient progradation may smooth this larger scale coastal configuration and thereby diminish the tidal influence. End_of_Article - Last_Page 439------------