Stratigraphic and Sedimentological Evidence for Accelerated Washover Rates: Case Study of Follets Island, Texas

Abstract

Follets Island is a Late-Quaternary transgressive island located on the upper Texas coast, an ideal location to study coastal response to accelerated sea-level rise, storms, and sediment supply. However, the island has a limited and diminishing sand supply, which makes it even more vulnerable to erosion during storms and relative sea-level rise. Four core transects that extend from the upper shoreface to the back-barrier bay are used to constrain the thickness of washover, barrier, and upper shoreface deposits and to estimate sediment fluxes in the context of the overall sand budget for the island over centennial timescales.

Follets Island has both the Late Pleistocene and Holocene sediments. The Holocene sediments are part of the Transgressive System Tract (TST) sitting on top of the Late Pleistocene sediments (part of the preceding Highstand System Tract, HST). The sequence boundary (SB) separates the two system tracts. Stratigraphic architecture reveals two prominent transgressive surfaces. A lower surface is a flooding surface separating red fluvial-deltaic clay from overlying bay mud and an upper surface, a surface of erosion that separates back-barrier deposits from overlying shoreface and foreshore deposits.

Radiocarbon ages are used to constrain the evolution of the barrier and its long-term rate of landward migration while ²¹⁰Pb dates are used to constrain the modern sand overwash flux. Results show that significant washover sands are deposited in the bay, and about twice this volume is deposited as subaerial washover deposits. The total sand washover volume shows that overwash processes account for about half of the sand produced by shoreline erosion in historical time. Our results also indicate that the historical rate of shoreline migration is about an order of magnitude faster than the geologic rate.

The study is unique because most published data on washover sediment budgets are derived from field measurements and aerial photographs to quantify washover while there is limited information on unit thickness and age. This work provides good core coverage and improved chronostratigraphic resolution, to provide sufficient detail to relate pre-historic and historic overwash rates. It also points to the fact that most sands eroded from coasts are not necessarily transported offshore during storms. Numerical modeling will be used to quantify the individual contributions of sea level, storm and sediment supply to Follets Island.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018