present sea level. Most of this variation may be attributed to eustatic sea-level 

 variations related to changing volumes of ice at the polar ice caps. Since eustatic sea- 

 level rise ceased about 3,000 to 6,000 years BP (Coleman and Smith 1964), ongoing 

 subsidence resulting from a compaction and sinking of Mississippi delta sediments is the 

 major cause of the relative sea-level rise, and hence land loss and coastal barrier erosion, 

 in Louisiana. 



The major factor offsetting subsidence-induced sea-level rise is sediment supplied 

 to the coast by the Mississippi River in a sequence of well-defined deltaic depocenters 

 (Fisk 1944; Kolb and Van Lopik 1958; Frazier 1967). During active sedimentation in each 

 depocenter, the shoreline progrades laterally as much as 120 km seaward, with the delta 

 plain vertically aggrading up to 5 m above mean sea level (Figure 2). Following delta 

 switching through upstream distributary diversion, sediment supply to the delta complex 

 quickly diminishes. Under these conditions, subsidence induced by substrate compaction 

 and dewatering becomes the dominant coastal process and deltaic transgression begins. 

 This period corresponds to Stage I, erosional headland and flanking barriers (Penland et 

 al. 1981; Penland and Boyd 1981, 1982), in which the reworking of distributary sand 

 bodies through shoreface retreat provides the only sand source for coastal barrier 

 generation (Figure 3). Shore-parallel transport distributes sand from the headland source 

 into downdrift marginal spits, tidal deltas, and flanking barrier islands. While sand is 

 being actively supplied from the erosional headland, the downdrift barrier systems in this 

 evolutionary stage exist in dynamic equilibrium. Subsidence gradually causes this 

 reworked distributary sand source to move below the reach of wave erosion and onshore 

 transport. With increased age and long-term subsidence, Stage 2 occurs; this barrier 

 system evolves into a transgressive barrier island arc, separated from the mainland by an 

 intra-deltaic lagoon. From this point on, sand sources no longer exist for barrier 

 nourishment, and the sediment dispersal pattern in this subsiding environment is the 

 destruction of the subaerial barrier and the formation of a subaqueous inner shelf shoal, 

 Stage 3. This occurs when sea-level transgression has overcome the ability of the barrier 

 to maintain its integrity through landward migration and vertical accretion. Geological 

 processes, therefore, interact in Louisiana to produce periods of rapid coastal 

 progradation, associated with delta building, and rapid coastal transgression, associated 

 with distributary abandonment and coastal barrier formation. Subsurface studies of the 

 Mississippi River in such areas have shown the existence of several major and minor 

 regressive-transgressive cycles in the past 8,000 years (Fisk 1944; Kolb and Van Lopik 

 1958; Frazier 1967). During the transgressive history of any one of the four abandoned 

 delta complexes, the following four mechanisms are identified as controlling coastal 

 barrier deterioration: (I) subsidence of deltaic sand source, (2) accumulation and 

 subsiding washover deposits, (3) infilling during migration of spit complexes and tidal 

 inlets, and (4) inequality in onshore-offshore sediment exchange. 



Subsidence of Deltaic Sand Source 



Following upstream diversion during the process of delta switching, the only source 

 of sand-size sediments for coastal barrier development comes from reworked distributary 

 sand bodies and flanking beach-ridge plains. During the evolution of an abandoned delta, 

 these sand sources continually subside and provide a diminishing sediment supply. The 

 maximum effective depth limit for erosion of deltaic sand sources is the base of the 

 advancing shoreface (Figure 4). Available bathymetric data locate the base of the 

 advancing shoreface seaward of the Bayou Lafourche headland and the Chandeleur 

 Islands, at a depth of around 6 to 8 m. Assuming that the estimated rates of relative 

 sea-level rise estimated between 0.6 and 1.5 cm/yr are correct (Kolb and Van Lopik 1958; 



17 



