these islands migrated 6.7 km, or 74 m/yr. The average width of these flanking barriers 

 is approximately 900 m. Soil borings indicate an average thickness for the Timbalier 

 Islands of around 5 m (Figure 8). Therefore, the total volume of sediments below the 

 subaerial barrier is about 3.1 x 10 m-^. This figure represents a loss of 3.9 x 10 rrrlyr 

 of sand from the sediment dispersal system to build the Timbalier Islands. This figure 

 only represents the sediment stored in recurved spit deposits. A stratigraphic dip section 

 through the central portion of Timbalier Island and extending 800 m offshore shows an 

 additional sediment sink at the infilled tidal channel of Cat Island Pass (Figure 8). The 

 thalweg of Cat Island Pass lies seaward of the Timbalier Island shoreline and was infilled 

 as this inlet migrated westward. The exact volume of sediment stored in this tidal 

 channel is unknown, but it must be emphasized that tidal inlets in Louisiana are 

 significant sediment sinks. At Quatre Bauyoux Pass, the volume of sediment stored in 

 the ebb-tidal delta has constantly grown, due to the tidal prism increasing in size 

 (Howard 1982). Tidal prism enlargement is caused when land loss in the backbarrier 

 areas increases the bay area, making a progressively greater volume of water available 

 for exchange during each tidal cycle. 



Another example of a migrating tidal inlet with significant sediment loss is located 



6.4 km southeast from Monkey Bayou, offshore from the southern Chandeleur Islands. 

 Seismic information reveals the presence of relict tidal inlets infilled by southward 

 migrating bgrrier complexes (Figure 9). Individual tidal channel fills contain as much as 



3.5 X 10 m of sand. These relict tidal inlets represent an earlier Holocene position of 

 the Chandeleur Island arc. The seismic sections in Figure 9 shows these sand bodies lying 

 in 8 to 10 m of water at the base of the advancing shoreface. 



Relict Infilled 

 Migrating Tidal Inlet 



30m 



500m 



Figure 9. An offshore seismic profile showing relict infilled tidal inlet 

 channel now bypassed by the shoreface and preserved in the inner continental 

 shelf. The black line indicates the transgressive contact between the over- 

 lying sandy barrier unit and the underlying St. Bernard delta. The seismic 

 line is shore parallel . 



23 



