THE ROLE OF GEOLOGIC FRAMEWORK IN BARRIER ISLAND GEOMORPHOLOGY AT PADRE ISLAND NATIONAL SEASHORE, TEXAS

This study examines the relationship between modern barrier island morphology, framework geology, and offshore bathymetry to better understand the development history of Padre Island National Seashore (PAIS) in Texas. The widely accepted hypothesis for the development of PAIS is inconsistent with the development histories of other barrier islands throughout the Gulf of Mexico. This study uses electromagnetic inductance (EMI) surveys, bathymetric contours, island morphometrics, and sediment cores to support a new hypothesis of development for PAIS. A 100 km long EMI survey was conducted to identify infilled subsurface paleochannels that dissect the island. Alongshore bathymetric contours were extracted from National Geophysical Data Center coastal relief models to test for shore-oblique ridges and other bathymetric anomalies that would provide insight into the geomorphic history of PAIS. Island width and volume, beach width and volume, and dune height and volume were extracted from a 100 km LiDAR-derived digital elevation model using an automated approach. Apparent conductivity at all three frequencies (from the EMI survey), bathymetric contours, and island morphometrics were subsequently decomposed using a continuous wavelet transformation. Beach width, dune height, and dune width exhibit a very similar waveform that coincides spatially with an inflection point in the waveform for the offshore bathymetric contours. Portions of the island proximal to the infilled paleochannels tend to have a higher volume beach and taller/higher volume dunes. Variations in dune height and volume in the southern part of PAIS spatially coincide with repeating shore oblique ridge and swale bathymetry also present in southern PAIS. The high degree of similarity between the subsurface paleochannels, offshore bathymetry, and island morphometrics support a new theory of development for PAIS. Results suggest that the northern, central, and southern portions of the island may have different geomorphic histories. Based on these results, we propose a new theory of development along PAIS, where framework geology is an important factor affecting modern island morphology, which has the potential to improve our long-term coastal change predictions.