Abstract

Padre Island is the widest and longest barrier island in the world. It is 200 km long, up to 3 km wide, and 10–15 m thick. This 2,000-year-old barrier island was formed predominantly from northward longshore-drifted sediments from the Rio Grande River Delta.

A 3-km-long, shore-normal, 50 MHz ground-penetrating radar (GPR) profile was collected across Padre Island from the Gulf of Mexico to Laguna Madre at the northern end of the Padre Island National Seashore. A 1-km-long, shore-parallel GPR profile was also collected and intersects the shore-parallel profile approximately 1 km landward of the Gulf of Mexico. Vibracores taken along the shore-normal profile, at the beach foreshore, in the center of the island, at the lagoon margin, and at a recent washover fan were used, in conjunction with the description of a deep rotary core taken through the northern part of the island, to constrain the GPR interpretations.

In the shore-normal profile, GPR reflectors indicate both seaward- and landward-dipping clinoforms, as well as concave-upward scour-and-fill features. Seaward-dipping reflectors indicate approximately 2 km of progradation of the barrier island, suggesting an average progradation rate of 1 m/year. Landward-dipping reflectors, in the landward 1 km of the GPR profile, indicate approximately 1 km of landward growth, suggesting an average landward growth rate of 0.5 m/year. Nested concave-upward scour-and-fill features in the central part of the island are interpreted as tidal and washover channels 3–8 m deep. In strike cross section these scours are channel-form in outline and contain nearly horizontal, to slightly inclined, radar reflectors. The presence of channel-form scours supports the interpretation that landward growth was dominantly due to deposition by short-lived tidal deltas and storm washovers. Even though there was periodic landward island growth, North Padre Island has been progradational throughout its history due to a high sediment supply.

The understanding of the architecture and growth history of Padre Island provides much needed information about the internal geometry and the evolution of modern barrier-island systems. In addition, systematic variations in scour depths across the island suggest that storm frequency and intensity varied during the island’s history and provides a proxy for estimating sea-level fluctuations.