Sedimentary Processes Operative Along Western Louisiana Shoreline: ABSTRACT

Abstract

The western Louisiana shoreline represents the complex interaction of low-energy wave turbulence along a broad, shallow shelf, a small tidal range, and a varying of very fine sand and mud. These are in turn modified by shoreline orientation and complex offshore bathymetry, proximity to brackish estuary tidal efflux, and varying nearshore salinity. Three dominant types of strand-line sedimentation have been chosen as typical of the modern shoreline in the study area. Mud flats, both tidal and subaqueous, form one type. Another dominant type of strand-line deposit is described as a sand-rich, normal beach. Facies relations of the normal beach are similar to the shoreface sequence described for the tidal mud-flat setting. Intermediate between mud flats and normal beaches is a transitional type of strand line consisting of thin swash-zone and breaker-bar deposits resting on eroded, earlier-formed mud-flat and marsh sediments. Mud-flat progradation is initiated by establishment of a shallow-water breaker-bar system, providing a protected site for subaqueous deposition of mud. As sedimentation raises the level of the mud flat to an intertidal level, sand-rich breaker-bars of low amplitude slowly migrate shoreward across the flat. The flat ultimately becomes intertidal. Thin washover-fan sands associated with this setting constitute one form of strand-line ridge found in the Recent chenier plain. Shoreface sedimentation in the normal beach differs from that of the mud flat in having fewer but considerably larger breaker-bars and a rapid shoreward transition into swash-zone, berm, and thick washover-fan sands. The pronounced increase in thickness of washover-fan sands in the normal setting provides a second distinct type of strand-line ridge or chenier. The intermediate nature of the transitional beach setting is emphasized as a temporal, highly dynamic phase in strand-line development. Although apparently retrogradational, it nevertheless represents a dominantly progradational strand line which builds seaward at an intermediate rate compared with quickly prograding mud flats and relatively slowly prograding normal, sand-rich beaches. A classification of progradational shoreline types based on energy of coastal versus sediment supply (e.g. Bernard, 1965) can be used to typify rates of progradation in the study area. Energy of coastal processes include (1) deep-water wave period, (2) orientation of shoreline with respect to prevailing wave front, (3) width of inner shelf, and (4) tidal range. Sediment includes (1) fresh-water efflux and salinity variation, (2) suspended mud supply, and (3) sand supply. End_of_Article - Last_Page 454------------