Shoreface Sedimentation along a Transgressive Barrier Island: Understanding the Interplay between Short-Term Events and Long-Term Sea Level Rise in Shaping these Dynamic Coastal Environments

Abstract

Barrier Islands are dominant features along the Texas coast. Most of these islands along the Texas coast today formed in the late Holocene (<5000 years ago) as sea level rise slowed (Anderson et al., 2014, and references therein), and may be classified as one of three types: (1) regressive, (2) aggradational, and (3) transgressive (Galloway and Hobday, 1983). Transgressive barrier islands are those where the sediment supplied to the island is exceeded by erosional processes on the shoreface. As a result, these islands are particularly vulnerable to storms and increases in sea level rise.

To understand better the response of a transgressive barrier island to long-term events such as increases in sea level rise, and short-term events such as hurricanes; this study investigated the shoreface of Follets Island, a transgressive barrier island located along the upper Texas coast. Follets Island has a consistent low-elevation profile, with a maximum elevation of ~2 m, and is less than 1 km in width in most places making it highly vulnerable to sea level rise, and storm surge from hurricanes and tropical storms. The island itself is a relatively thin (1–4 m) accumulation of transgressive sands overlying fluvial deposits from the nearby Brazos River (Bernard et al., 1970; Morton, 1994; Wallace et al., 2010), and is actively eroding with some of the highest shoreline erosion rates observed along the Texas coast over the past several decades (McGowen et al., 1977; Morton and Pieper, 1975; Wilkinson and Basse, 1978).

The goal of this study was to compare shoreface modifications from a short-term event (e.g., Hurricane Ike, 2008) to those expected to result from longer-term processes (e.g., increases in sea level rise). Processes affecting the barrier island shoreface are critical in shaping the sequence because this is where the sequence is modified, truncated or destroyed (Rodriguez et al., 2001). Additionally, Folletts Island proved an ideal study location because it was severely impacted by Hurricane Ike in 2008, the storm breached the island in over 75 places and caused dramatic morphological changes to the subaerial portions of the island; and over the past 40 years sea level rise in the area has been ~4 mm/yr (National Oceanic and Atmospheric Administration, 2015), an order of magnitude greater than the rate for the late Holocene (Milliken et al., 2008).

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