Internal sand bank seismic stratigraphy provides insight into paleo-barrier island preservation

Abstract

Barrier islands are rarely preserved on continental shelves following sea-level rise. Proxies like overwash deposits, tidal inlets, and wave ravinements identify the location of paleo-barrier islands through time. Barrier island remnants are potential sand resources for beach nourishment to combat shoreline erosion from increasing rates of sea-level rise. Additionally, understanding the conditions that lead to barrier island drowning can be used to advise coastal policy makers. This study aims to identify barrier island signatures and deposits to understand the coastal processes that maximize preservation of paleo-barrier island remnants. We employed high resolution chirp sub-bottom data coupled with legacy sediment cores collected over Heald and Sabine Banks, on the east Texas shelf, which have been identified as possible preserved barrier island associated facies. Heald Bank exhibits a predominantly homogenous, low-amplitude facies with few low-amplitude internal horizons overlying the transgressive ravinement, whereas Sabine Bank consists of high-amplitude, landward-dipping reflectors beneath this surface, likely indicative of preserved subaqueous overwash deposits. This stratigraphy suggests Sabine Bank includes barrier island associated facies, whereas Heald Bank is mostly a marine sand bank. The overwash unit of Sabine Bank displays landward-thinning and landward-dipping deposits with reflections increasing in amplitude and displaying lower slopes to the NW. We hypothesize that higher slopes to the SE indicate proximity to the former barrier island. The Sabine River paleo-valley is mostly filled with estuarine sediment, leaving only ∼4 m of antecedent accommodation in a limited area of the NE portion of the paleo-valley. The low shelf gradient, which increases accommodation, and initially high sediment supply that decreased during the drowning of Sabine Bank are the major factors controlling partial preservation of the subaqueous portion of the paleo-barrier island.