THE ROLE OF SHELL MATERIAL IN MAINLAND MARSH SHORELINE RESPONSE TO BARRIER ISLAND TRANSGRESSIVE SUBMERGENCE: CHANDELEUR SOUND, LOUISIANA, USA

Many recent studies have sought to understand the response of barrier islands and their attendant marshes to sea level rise. As barrier shorelines become less effective at protecting interior wetlands from wave attack and regulating estuarine processes, knowledge of the mode and rate of marsh response is critical for successful management and planning efforts. The Mississippi River delta plain (MRDP), specifically the Chandeleur Islands system in eastern Louisiana, serves as an excellent natural laboratory for studying these responses: this region is presently undergoing the highest rates of shoreline erosion (> 15 m/yr) in North America as wetlands are converted to open water in a regime of subsidence-driven rapid relative sea-level (RSL) rise (~1 cm/yr). The existing three-stage conceptual model for the evolution of MRDP barrier islands suggest that the disappearance of barrier islands could lead to the formation of a new barrier shoreline along the seaward periphery of mainland marshes.

Presently, sandy shorelines and islands in Chandeleur Sound are rare. Regional and shallow stratigraphy within the marshes show a lack of laterally widespread coarse-grained strata. Along some sections of fringing marsh, shell-lag shorelines and pocket beaches developed from winnowing of shell material. A conceptual model based on land-loss maps, field observations, and geomorphic relationships was developed to describe this process by which more resistant landmasses associated with natural levee deposits become isolated as marsh islands with shell-rimmed beaches and ultimately submerged to form shell mounds. It is likely that without adequate sand supply liberated during ravinement, this regional erosional behavior will continue and accelerate in the future under increased wave and tidal energy.