Laguna Madre is a linear costa lagoon, developed on the pre-Holocene (FOOTNOTE 3) erosion surface by the buildup of a barrier island, resulting from the transgression brought about during the post-glacial rise in sea level. The Holocene deposits average 10-20 feet in thickness, although local depressions in the pre-Holocene surface may contain a considerably thicker section. The bulk of the Holocene deposits consists of subgraywacke and subarkosic sand. Local concentrations, however, include limestones, gravels, oolites, and relatively pure clays.

Source of the clay is unknown, but heavy and light mineral analyses of the sand fraction indicate locally reworked earlier deposits. The northern Laguna sediments are derived from the pre-Holocene sediments off the Nueces delta province and the southern Laguna sediments are derived from the Holocene and pre-Holocene deposits of the Rio Grande distributary province.

The immediate source of the lagoonal fill is the barrier island sand, which is believed to be derived from the earlier deposits of the nearshore gulf. During and after sea-level rise, the nearshore gulf floor was actively eroded by wave action to establish an equilibrium profile. The eroded material was sorted and the sand fraction was transported landward. Wind action piled the new beach sands into coastal dunes on the barrier. Where the earlier deposits were relatively unconsolidated, as offshore northern Padre Island, thick dune deposits could form because of large supply. Where the earlier deposits were more consolidated, and consequently less easily eroded, as in the vicinity of the Rio Grande delta, the dunes are lower and the barrier narrower.

Subsequent to the formation of the lagoon about 5,000 years ago, partial filling was accomplished by washover storm waves, wind transport, and tidal activity.