Sedimentology of the outer Texas coast

textThe Recent as represented on the Texas coast may be separated chronologically into the Ingleside, Alazan, and Post-Alazan or Present times. Each subdivision is associated with a major climatic fluctuation, the Ingleside and Post-Alazan with the “first and second climate optimums”, respectively, and the Alazan with the “Little Ice Age”. Characteristic of the Ingleside and Present times, and possibly of Alazan time, are the associated coastal barrier and lagoon systems. The physiographic features of the Present coastal barrier system forming the outer Texas coast, manifest a direct relation to the climatic characteristics of the Present, favoring onshore movement of sediments. The sediments carried by mainland streams to the bay systems, then moved through the barrier passes by tidal currents, and contributed to a southwestward moving longshore current. Wave transport and eddy action transfer the sediments to an inshore drift current, which is reversible in direction with the longshore component of the local wind. The direction of the inshore drift current affects directly the orientation of foreshore features: spits, bays between cusps, and the associated backwash channels and ripple marks. Three basic profiles representing calm, normal, and storm wind conditions result from secretion or removal of beach sediments. Hurricanes, occurring at any one locality on the Texas coast about once every 25 years, produce great waves that flood the barrier system in the region where the storm track crosses the coastline. A hurricane flood flattens low foredunes, breaks through gaps in higher foredunes, forms washover fans and temporary passes, and carries the material across the island channels, depositing the sediments in front of back dune fields and in the lagoon. The dry portion of the present wind regime (that free of precipitation) transports the Gulf beach sands inland over the barrier system. Precipitation, occurring more often with northerly winds than with southerly winds, reduces the effect of the former on sand movement. SE is the prevailing wind direction and also the resultant effective direction causing Aeolian movement of sand over the barrier system, producing an over-all resultant NW movement. Blowout tongues, barchans, and seif dunes of the barrier system, also the dune fields and wind-cut ridges and furrows of the mainland, have developed trends in agreement with the NW direction of Aeolian sand movement. The NW trends are superimposed on a sand sheet that centers at 27°N latitude on the mainland and extends W from Laguna Madre. The sand sheet was probably the product of a different wind regime because of its non-agreement with the present direction of sand movement. A gradual change of physiographic features southwestward along the coast is the direct result of the different degrees of sand movement corresponding with a climatic range from humid in the vicinity of Sabine Pass to semi-arid on northern Padre Island. Desert-type dunes, the seifs and barchans typical of the great ergs of the world, are found in the semi-arid portion of the Texas barrier system. Of the entire continental shores of the United States, only along the Texas shores do such dunes develop for no other shore in the United States approaches the same degree of aridity.Geological Science