Geomorphic and temporal evolution of a Mississippi delta flanking barrier island: Grand Isle, LA

Abstract

Barrier islands throughout the world are increasingly stressed due to accelerating sea-level rise (SLR), increasing storminess, and diminishing sediment supplies. This condition is particularly evident along the Louisiana coast because it has one of the highest rates of relative SLR in the world and very limited sand sources. One of the keys in predicting how barrier islands will change in the future is to understand how they have evolved since their formation. Historical shoreline changes are usually insufficient, because those records only provide a glimpse of a barrier's history. Moreover, the data needed to decipher barrier evolution at an intermediate scale (~101–103 years) is lacking for many coastal Louisiana barrier systems. One method for tracking barrier island development through time makes use of optically stimulated luminescence (OSL) dating of beach ridge sediments. This study combines OSL and the analysis of aerial imagery and previous core logs to document the timing of beach ridge formation, rates of progradation, and geologic framework of the only developed barrier island in Louisiana.Grand Isle is composed of prograding beach ridges organized in distinct, unconformable sets. Ridges increase in spacing from west to east, due to increased rates of shoreline progradation caused by decreased accommodation space. Proto-Grand Isle began forming approximately 750 years ago, prograded northeastward until approximately 620 years ago when sediment supply slowed and/or major storms impacted the island. During that time, much the seaward portion of the existing ridges were eroded. Following truncation of the first set of beach ridges (Group 1), the island once again began to prograde in a slightly more eastward direction (Group 2 ridge sets). OSL dates indicate the island's central ridges (Group 2, ridge sets 7–9) formed between 370 ± 30 and 170 ± 10 years ago at an average rate of one ridge every 14.8 years, or progradation rate of 14.9 m/yr.The island's sediment source comes from an eroding beach ridge plain of the Caminada headland to the west. The volume of Grand Isle's barrier lithosome is 9.26 × 107 m3, which computes to an average northeasterly longshore transport rate of 128,625 m3/yr. This is a high longshore transport rate for the relatively low wave energy of this coast (Hs = 0.6 m) but is explained by hurricane impacts that substantially increase average sand transport conditions. The substantial thickness of the Grand Isle barrier lithosome (~10 m) and its steady supply of sediment contribute to the island's stability in contrast to other barrier systems along the Louisiana coast.