Holocene Formation of Heald Sand Bank on the East Texas Inner Continental Shelf

Abstract:

Heald Bank is a Gulf of Mexico sand bank located ~50 km off the coast of east Texas on the inner continental shelf. The bank is proposed to be a remnant of barrier islands drowned and thus preserved during an episode of rapid sea level rise. For this hypothesis to be true, the transgressive ravinement that marks the erosion by the shoreline moving from shelf edge towards the modern location would by definition postdate and thus be stratigraphically above the bank. To test this hypothesis we present ~90 km of Compressed High Intensity Radar Pulse (CHIRP) and Multi-channel seismic (MCS) data collected from the University of Texas Geophysics marine field course during 2008, 2013, and 2014. In these data, there are several visible underlying channels beneath Heald Bank. These channels have channel fill which are truncated by an overlying erosional surface, which we interpret to be the transgressive ravinement from sea level rise. However, this interpretation places the ravinement below the sand bank, meaning it could not have been drowned and buried by a rapid sea level rise event. Thus, Heald Bank and potentially the related inner shelf banks (Thomas, Shepard, and Sabine) from eastern Texas to western Louisiana cannot be used as an example of coastal response to climate change and sea level rise. We examine alternate origins for the banks and their sand using published cores as well as age models and integrating them with our seismic data. One possibility is that the sand was sourced from the nearby Sabine River system immediately following local transgression or the sand was remobilized from sediment fill within underlying paleo-river channels imaged below the Heald Bank. In either case Heald Bank appears not to serve as an indicator for rapid sea-level rise, yet could be an important analog for sand transport mechanisms offshore.