Effect of Hurricanes on Nearshore Sedimentary Environments of Coastal Bend Area of South Texas: ABSTRACT

Abstract

Tropical storms of strength (winds > 75 mph) cross the coastline between Tampico, Mexico, and Port Arthur, Texas, with a frequency of approximately one storm in two years. Greatest geological effects are produced by waves driven by winds that sometimes gust to 200 mph and by the tremendous elevations of water level that occur during storm tides (maximum of 22 ft. recorded at Port Lavaca, Texas, during Hurricane Carla, Sept., 1961). The comparison of a part of the nearshore environmental complex of the coastal bend area of south Texas before and after Hurricane Carla shows the effects of the storm. The inner continental shelf bottom was both a contributor and a receiver of deposits. As the storm moved landward, it picked up mollusk shells indicative of depths up to 12 fathoms (for example, Dinocardium, Semele, Strombus, Murex, Atrina) and removed Pleistocene (?) rock fragments, coral blocks, and other materials from the sea floor, and deposited all these on the barrier island. After the storm passed inland, water rushed from the lagoon back to sea through numerous channels cut into the island by the storm. These currents deposited a thin layer (0.5-1.5 in.) of sand over what was previously sandy mud bottom out to depths of about 60 ft. and a graded layer of fine sand, silt, and clay farther out on the shelf. The seaward side of the barrier island (Padre Island) was severely altered. The storm removed a belt of foredunes 20-50 yards wide from the seaward side of the island and left a belt of larger dunes, partly stabilized by vegetation, with wave-cut cliffs up to 10 ft. high. Waves breaching inland deeply excavated many dunes along their flanks. The formation of a broad, flat hurricane beach, consisting of poorly sorted sand and coarse shell, drastically altered the beach profile. The landward side of the barrier island (wind-tidal flats) received much washover material containing placers of surf zone and beach mollusks (especially Donax sp.). The storm also submered high-level mud flats along the landward side of Laguna Madre and covered them with a fresh layer of mud. A much milder storm (Cindy) passed through the area in September, 1963, and a small bar was deposited over the seaward edge of the pre-existing hurricane beach. End_of_Article - Last_Page 530------------