ABSTRACT

Overwash processes play an important role in determining the stratigraphy of microtidal barrier islands. Along the microtidal coast of south Texas, regional variability in barrier island geometry produces a spectrum of washover types. South Padre Island is a low-profile, transgressive feature with a discontinuous to nonexistent foredune ridge. It displays sheet overwash, coalescing washover terraces, and washover fans fed by large hurricane channels. North Padre and Mustang Islands are high-profile barrier islands with continuous foredune ridges, and thus only small, discrete interdune washovers occur. The relict tidal inlets between Mustang and Padre Islands are the sites of the largest washovers in the system, termed reactivated tidal deltas. This last type, although relatively rare in modern washover deposits, is probably similar in mode of origin to the large lobate backbarrier features found on many high-profile barriers on the Texas coast.

Internal structures of the washovers were studied by trenching, box coring, and vibracoring. The dominant stratification type is plane beds with extensive heavy mineral laminae. Washover margins display some landward dipping foreset bedding. Hurricane channels show cut-and-fill structures with shell lags, plane beds, and some trough crossbedding. Directional features indicate that the washovers are formed by storm surge flood, and modified by surge ebb. This is supported by the existence of separate flanking ebb channels on some washovers, as well as current velocity simulations for the storm surge of Hurricane Allen, which show that flood currents are both stronger and longer lived than the ebb.