ALONG-STRIKE VARIABILITY OF HOLOCENE COASTAL RETREAT RATES IN EAST TEXAS

The low gradient east Texas coast and inner-continental shelf, from Sabine Lake at the Texas Louisiana border to the western end of Galveston Island, experienced extreme along-strike variations in coastal retreat rates during the Holocene. Around 7.7 ka. a barrier shoreline was located approximately 55 km offshore. Towards the west the shoreline retreated landward 55 km occupying a position on the lagoon side of Galveston Island by about 5.3 ka. Towards the east the shoreline retreated more gradually, occupying a position on the lagoon side of Bolivar Peninsula and 30 km offshore Sabine Lake by about 1.5 ka. Shortly after 1.5 ka. the shoreline offshore Sabine Lake retreated 30 km to its present position. Heald and Sabine banks, located on the inner continental shelf above fluvial terrace deposits of the Trinity/Sabine incised valley, are the only preserved remnants of these former shoreline positions.

Fluctuating rates of sea-level rise were not the forcing mechanism behind the episodes of rapid shoreline retreat because these events were localized; rather, along-strike variations in coastal retreat rates were caused by the variable inner shelf gradients, that increase towards the west, and the orientation of the Sabine incised valley and associated fluvial terrace deposits, that trend northeast/southwest (parallel to shore). As shorelines retreated over fluvial terrace deposits, these deposits served as local shoreline sand sources that enabled barrrier islands to persist offshore, out of equilibrium with sea level. Once the localized sand sources became depleted, and/or sea level reached some critical threshold, the barrier shorelines were not able to maintain themselves. They became stranded offshore as banks, and new shorelines formed