Regional Variability of Washover Deposits on South Texas Coast: ABSTRACT

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 re atively rare in modern washover deposits, is probably similar in mode of origin to the large lobate back barrier 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 cross-beds. 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. In the ancient record, low profile barriers should be relatively thin deposits consisting of numerous coalescing washovers. Thicker sand bodies corresponding to high profile barriers should be found along strike. Since back-barrier facies have a higher preservation potential than other barrier environments, such a deposit might be dominated by reactivated tidal deltas. End_of_Article - Last_Page 1473------------