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Dynamic Diversion: Influence of Longshore Current-Tidal Flow Interaction on Chenier and Barrier Island Plains
Thomas W. Todd
Chenier and barrier island plains are characteristic products of post-Pleistocene sedimentation along the northern and western coasts of the Gulf of Mexico, but similar features may be recognized throughout the world. These plains have been formed by net progradational displacement of the sea throughout the latest 3000 to 5000 year period of sea level stability. They consist of low, narrow, well-sorted shell and sand ridges that are elongate parallel with the former shoreline, and which are separated by tidal mud flats or zones of poorly sorted, fine sand.
A correlation has been established in the vicinity of the Mississippi River delta, by means of radiocarbon dating, between low rate of fine sediment discharge into westward moving longshore currents, stability or minor recession of the downdrift shoreline, and slow accretion of well-sorted chenier beach ridge sand. High rate of sediment discharge into nearshore currents, however, coincides with rapid progradation of sandy, silty mudflat deposits along the downdrift shoreline. It is suggested that the presence of a high content of fine sediment in longshore currents changes their fluid dynamic characteristics in such a way as to enhance traction transport, inhibit separation of differing grain size modes, and thus promote deposition of unsorted detritus. Light sediment loads halt rapid progradation and induce reworking of previously accumulated sediment. Thus no change in general wave regime is required to account for alternation of chenier sand accumulation with interchenier mudflat deposition.
Barrier island plains closely resemble chenier plains, except the barrier island beach ridges are separated by clayey, silty, poorly-sorted fine sand, but cheniers are separated by unsorted, sandy, shoreface mud deposits. Comparison of sublittoral beach profiles show that, rising from equivalent depths of water, the barrier island profile has a seaward gradient that is approximately twice that of the non-barred shore. This steeper profile is related to the coarser average grain size of barrier island shoreface sediment compared to chenier plain shoreface detritus. The greater coarseness is in part the result of tidal jet winnowing of clay from longshore-moving sediment as it sweeps past estuary mouths. As a consequence of the steep barrier Island profile, a higher rate of wave energy issipation takes place in shallow offshore water. This higher rate of energy transfer contributes to the further winnowing of barrier island sand that is initiated by longshore current-tidal flow interaction. The textual differences between barrier island interridge sand and sandy, interridge chenier plain mud appear to mask similarities of origin.
A close geographic association invariably exists between chenier deposits and river or estuary mouths. Although major rivers are the ultimate source of sediment composing chenier plains, cheniers are not uncommonly located adjacent to estuaries receiving little or no sediment from the rivers passing through them to the sea. This suggests that sand build-up is related primarily to dynamic interaction between sediment-bearing longshore currents and tidal flow from estuary months, and is not necessarily dependent upon the rivers as sediment sources. The interaction, here termed the "dynamic diversion" effect, is particularly important to chenier plain growth because of its influence on deposition of in-transit sediment that approaches an estuary during ebb tide. Both surf suppression by he issuing jet and decrease in velocity of nearshore transport associated with current deflection cause deposition of sediment on the updrift shoreface of mainland or developing spit. Some sediment stranding may also take place during flood tide. A second important site of ebb flow deposition is offshore, and downdrift, from the estuary mouth. Here longshore flow reasserts its dominance over the tidal jet, and energy dissipating gyral flow occurs as the seaward-displaced longshore drift moves back toward the shoreline. Deposition at this site results in barrier island plain development. Exploration for subsurface chenier and barrier plains sands might well be pursued in the vicinity of provable intersection between paleoshorelines and ancient river systems.
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