Delta ID    # 69.

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Contributed  by Professor George F. Hart, LSU.

The modern depositional environments are part of the highstand systems tract. The High Stand System Tract shows switching takes place about every 1,500 years.  Each of these deltaic complexes, formed by a river switch, covers an average area of 30,000 sq km and has an average thickness of some 35 meters. The St Bernard delta is one of four major delta complexes presently existing over coastal Louisiana.  Within this complex at least 12 individual sub-deltas have formed during the last 4,000 years.

The largest barrier island system of the detrital plain is associated with the reworking of the distributary channel and mouth bar sands of the Saint Bernard deltaic complex. The chain is over 75 km long and the individual islands more than 2 km wide. They have been moving landward for at least the last 100 years [Penland et al., 1985], retreating over a thick [up to 7 meters] sequence of lagoonal deposits. Because of the dominant south-east wave energy the sediments migrate northward.  Smaller islands are to the south [12-15 feet of sand] and larger to the north [15-30 feet of sand].  Submergence is causing a diminishing sediment supply and, especially to the south, the destruction of the barrier islands to leave remnant inner-shelf shoals.  The original source of the sand are buried distributary channels and river mouth bars and lie on the lower shore face and inner shelf and extend seaward under the thin and discontinuous central and southern Chandeleur Islands. Distributary channels form a veneer 50 feet thick and 1,300 feet wide [Suter et al., 1988] and extend up to 8 miles seaward of the central barrier islands. A broad sand sheet consisting of gently offshore dipping beds occurs under the revinement surface. Tidal channels, probably cut by hurricanes, are important associated environments. Offshore of the barrier islands are [assumed] submerged beach-ridge environments, about 20 feet thick and extending several miles offshore, identified as high angle clinoform reflectors. The thickness of barrier island sediments relative to the location of the revinement surface determines whether or not the sands will be preserved.  If the transgressed barrier shoreline sediment package lies above the advancing revinement surface, the entire sequence is truncated [Penland, Suter, and Boyd, 1985].  The mechanism of formation of the Barrier Island Chains from the deterioration of a delta was outlined by Penland and Boyd, [1981]; and  Suter et al, [1985] and is depicted in Text figures X-201 and X-206].