Follets Island: A Case Study of a Dying Barrier Island

Abstract

Parts of the Texas coast appear to have experienced unprecedented landward migration in historical time, which are in the range of 1–4 m/yr. A likely cause of this change is accelerated sea-level rise, which is currently about 2.5 mm/yr in the northern Gulf of Mexico.

This is five times the rate (average 0.5 mm/yr) for the previous 4000 yr. Follets Island, a pristine transgressive barrier on the upper Texas coast, is an ideal location to study coastal response to accelerated sea level rise. The island 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. The barrier is a thin veneer of sand, nowhere thicker than 2 m. Stratigraphic architecture reveals two prominent transgressive surfaces: The first is a flooding surface separating red fluvial clay from overlying bay mud, and the second flooding surface separates bay mud and washover 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 in high resolution. The results indicate that the current rate of landward migration is unprecedented. The cause of this increased rate of landward migration is assessed using numerical modeling, allowing more realistic modeling of future barrier change.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015