Facies and genesis of a hurricane washover fan, St. Joseph Island, Central Texas Coast




Andrews, Peter B. (Peter Bruce), 1935-

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Washover fans are subaerial coastal landforms, semicircular in map view, and wedge-shaped in cross-section. They form on the bayside of many barrier islands along the Texas Gulf Coast. Washover fans are composed largely of sediment that has been carried across a barrier island and dumped in the adjoining bay by catastrophic storms such as hurricanes. One such fan, measuring 4 1/2 by 4 miles, occurs at the north end of St. Joseph Island, on the central Texas coast. It grades northwestward into a subaerial tidal delta. The surface facies of these two features and the adjoining barrier island nucleus, as well as the geometry of the component sediment bodies, form the basis of this report. The data upon which the descriptions and interpretations are based were obtained by digging trenches and pits, up to 8 feet deep. The washover fan is fed via a low-level washover channel, Vincent Slough, which is cut through the barrier island nucleus. The fan is accreting laterally into the adjoining bay, Aransas Bay, through the deposition of shell debris and sand that is derived from the inner neritic zone of the continental shelf. Periodic hurricane surges, both flood and ebb, deposit tremendous volumes of sediment in Aransas Bay. The washover fan thus built up is composed dominantly of an imbricate set of sheet sands, 3-15 inches thick. Each washover sand rests on a sharp, scour base. Each unit consists of a shell placer at the base that grades upward into horizontally laminated, shell-free clean sand. The washover sand facies is separated into two subfacies; distributary channel center and distributary channel margin. Interbedded with them are several other distinctive facies; oyster bank, marsh, mudflat, pond, eolian flat, and intermittent pond over eolian flat. The fan rests on bay margin grassflat facies. Eolian mounds have formed on the surface of the fan, and they are composed of three facies; low mound, high mound - sand, and high mound - mud. The first two are also widespread in the barrier island nucleus, which commonly rests on backshore (beach) facies. The tidal delta is composed of extensive units of marsh, marsh creek, mudflat, pond, storm ridge, and high mound - mud facies. It rests on bay margin grassflat facies. All facies, except the washover sands, are dominated by sedimentary structures of biologic origin. In most facies, primary stratification and sedimentary structures are masked or destroyed by animal burrowing and/or root growth. Structures, and even textures, of secondary origin, are of paramount importance in marginal marine environments. Radiocarbon dates show that the tidal delta began to form at least 2400 years ago. The washover fan began to form about 1700 years ago. Progradation was rapid at first, but subsequently slowed considerably. Progradation has been minimal since about 450 years before present. The fan is 25-30 inches thick at its periphery, and thickens to more than 50 inches near its apex