Assessing the Importance of Shoreward Vs. Alongshore Sand Transport during the Late Holocene Evolution of the Chandeleur Islands, LA

The Chandeleur Islands, a north-south trending barrier island chain east of the modern Mississippi Delta, overlie the abandoned St. Bernard delta complex of the Mississippi River. The islands started forming ~2,000 yr BP by headland erosion and spit elongation following deltaic abandonment. Deltaic-plain subsidence and expansion of bays behind the islands, contributed to isolation from their fluvio-deltaic source. High-resolution seismic data and vibracores, collected within a 5-km radius of the islands were used to map the distributary channel, interdistributary, delta-front and prodelta facies of the St. Bernard complex and the thickness and distribution of the overlying barrier-island facies. The northern end of the island chain straddles the transition from interdistributary to delta-front facies. A large spit, sourced from updrift erosion of the islands, progrades northward into deeper water over delta-front and prodelta deposits. The active sand body is 3-6 km wide and less than 4 m thick in the southern half of the study area but increases to 12 km wide and 9 m thick north of the islands. About 25% of the sand body's volume comprises the islands themselves, whereas 23 % is contained within an overwash zone behind the islands, and 52% is located north of the islands. This sediment distribution indicates that northerly alongshore transport, driven by waves that predominantly approach from the southeast, has been the dominant transport process of littoral sediment. Considering the limited regional availability of sandy sediment, the continued transport into deeper water brings the fate of these islands into question.