Swanson and Thurlow 1973), the period of sediment supply from distributary sand bodies 

 is effectively limited to between 400 and 1,200 years, depending on the variation in the 

 subsidence rate over time. The rate of subsidence varies proportionally, according to the 

 thickness of the delta; thin deltas subside slowly and thick deltas subside rapidly. 

 Seismic data offshore from the Chandeleur Islands indicate that the St. Bernard delta 

 complex, which was abandoned about 1,800 years BP (Frazier 1967), is no longer 

 receiving an adequate sediment supply from St. Bernard distributary sand bodies through 

 the processes of shoreface retreat and sediment entrainment. The upper surface of the 

 St. Bernard delta, offshore from the Chandeleur Islands, lies two or more meters below 

 the base of the advancing Chandeleur Island shoreface. An offshore seismic profile 

 (Figure 5) reveals the base of an ancestral flanking barrier associated with the earlier 

 stages of coastal barrier evolution of the Chandeleur Islands. The basal portion of the 

 barrier has been bypassed by the shoreface and is now preserved on the inner portion of 

 the Louisiana continental shelf. 



Accumulation in Subsiding Washover Deposits 



In Louisiana, storm overwash is a major process of sediment transport during 

 barrier transgression (Boyd and Penland 1981; Ritchie and Penland 1982). Sea level is 

 subject to frequent and dramatic elevation changes on the northern gulf coast In response 

 to hurricane and winter frontal storms and the waves associated with them. Overwash 

 elevations exceeding 1.7 m may be expected to occur on the Louisiana barrier coast 10- 

 20 times/yr, causing washover sedimentation throughout more than 75% of most barrier 

 systems (Figure 6). This sediment then is stored until again reworked by the advancing 

 shoreface. During transgression, the site of overwash deposition in the backbarrier 

 lagoons, such as Chandeleur Sound or Terrebonne Bay, is continually subsiding. 

 Therefore, progressively greater quantities of sediment are required during transgression 

 for the barrier system to remain subaerial (Figure 7). For the region of active overwash 

 in the northern Chandeleur Islands, Penland and Boyd (1981) calculated an average 

 shoreline retreat rate of 5 m/yr, or 500 rnVlOO yr. A landward advance of 500 m in the 

 coalescing washover fans requires 2,500 m of sand per meter of fan front, assuming the 

 average water depth behind the Chandeleur Islands platform is 5 m. Using Kolb and Van 

 Lopik's average subsidence rate of 60 cm/100 yr for the St. Bernard Delta, this washover 

 volume will be required to increase by 300 m-^, or 12% per 100 yr. Washover sediments 

 become permanently lost from the barrier sediment dispersal system after the depth of 

 water into which the washover accumulation is advancing exceeds the nearshore depth of 

 the advancing shoreface. 



The Infilling of Migrating Spit and Tidal Inlet Complexes 



Wave-induced longshore sediment transport is a significant factor in the 

 development of any shoreline of an abandoned Mississippi River delta. Flanking barrier 

 spits and islands are supplied with sand transported alongshore by waves from erosional 

 headland sources. The configuration of the Bayou Lafourche headland (Caminada-Moreau 

 coast) indicates that sediment is transported alongshore, both to the northeast by waves 

 from the southwesterly quadrant, and to the west by waves from the southeasterly and 

 northeasterly quadrants. This has resulted in the growth of a symmetrical set of flanking 

 barriers, Caminada Spit and Grand Isle to the northeast, and the Timbalier Islands to the 

 west. Similarly, the transgressive barrier island arc configuration of the Isles Dernieres 

 results in bidirectional longshore transport, west toward Raccoon Point and east toward 

 Wine Island Pass. In contrast, the north-south orientation of the Chandeleur Island arc 

 results in an asymmetrical net transport pattern towards the north in response to the 



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