BARRIER ISLAND GROWTH RATES FROM AN OPTICALLY STIMULATED LUMINESCENCE-DATED BEACH RIDGE SEQUENCE IN LOUISIANA

The fundamental processes driving long-term (millennial) and short-term (seasonal to annual) morphological changes within barrier island systems are well developed and understood. However, the details necessary to understand barrier evolution at decadal to centennial time scales are still lacking for most barrier systems, including those of Louisiana. One method for resolving barrier island growth over time makes use of optically stimulated luminescence (OSL) dating of beach ridge sediments. In Louisiana, while this method was used to document deltaic progradation, it has not been used extensively to better understand barrier island systems. Here, we use OSL to document the timing of beach ridge formation, rates of sediment transport, and the fundamental geologic framework of Grand Isle, the only inhabited barrier island in Louisiana.

Grand Isle is composed of beach ridges organized in distinct, unconformable sets. Ridges increase in spacing from west to east, likely due to decreasing accommodation space. Grand Isle began forming by accretion of beach ridges approximately 720 years ago. Progradation occurred northeastward until approximately 505 years ago when deposition ceased, the seaward portion of the ridges was eroded, and deposition resumed, albeit in a slightly more eastward direction. The island’s central ridges formed between 370±30 and 170±10 years ago at an average rate of 13 yr/ridge or progradation rate of 14 m/yr. The sediment source for Grand Isle is the eroding beach ridge plain of the Caminada headland that, along with flanking barriers on either side, forms the Bayou Lafourche transgressive depositional system. We calculated the Grand Isle barrier lithosome at ~9.26x10⁷ m³, which requires an average longshore transport rate of 128,625 m³/yr available for deposition during the period of the barrier formation. This relatively large volume of sediment can be reconciled by the high rates of erosion and eastward transport of Caminada beach ridge sand.

These results demonstrate the utility of the OSL method for providing insights into the temporal evolution of beach ridge plain formation along the central Gulf Coast and expanding the current body of knowledge regarding the spatial extent and age of the Grand Isle barrier lithosome.