STRATIGRAPHIC AND SEDIMENTOLOGICAL EVIDENCES FOR UNPRECEDENTED SHORELINE MIGRATION RATE IN HISTORICAL TIME: FOLLETS ISLAND, TEXAS
Follets Island is a transgressive island located on the upper Texas coast, an ideal location to study coastal response to accelerated sea-level rise. It is currently in a rollover phase, meaning the rate of bayline migration appears to be keeping pace with Gulf shoreline migration. In addition, the island has a limited sand supply, which makes it vulnerable to erosion during storms and relative sea-level rise. Five core transects that extend from the upper shoreface to the back barrier bay are used to constrain the thickness of washover, barrier and shoreface deposits and to estimate the sediment fluxes and the overall sediment budget for the island over centennial timescales.
Stratigraphic architecture reveals two prominent transgressive surfaces. The first is a flooding surface separating red fluvial clay from overlying bay mud and the second is a surface of erosion, ~ 2m deep, which separates back-barrier deposits from overlying shoreface/foreshore deposits. Radiocarbon ages are used to constrain the evolution of the barrier and its long-term rate of landward migration. Results from grain size analyses reveal that significant washover sands were deposited in the bay and about double this volume was deposited as sub-aerial washover deposits. Summing up the total overwash volume shows that overwash processes can account for the total volume of sand produced during shoreline erosion in historical time.
Results indicate that the current rate of the shoreline migration is unprecedented. With the current shoreline erosion and sea-level rise rate, Follets Island’s barrier is predicted to disappear in next century.