Sediment Zones Bordering the Barrier Islands of Central Texas Coast<xref ref-type="fn" rid="ch05fn1"><sup>1</sup></xref>

Abstract

An investigation of the Recent sediments along the Texas coast has been centered around Rockport and Aransas Pass. Fifteen different environments of deposition have been defined ID the bays, on the barrier islands and on the open continental shelf. Methods have been devised by which investigators can ordinarily classify into the correct environment of deposition test samples from the Texas coastal area. A variety of techniques is used in distinguishing the environments, a particularly effective system being based on estimation with a binocular microscope of the general composition of sieved sizes of the sand fraction of sediments. The grain-size characteristics of the sediments offer important supplementary information on the environments. The contained organisms, especially the Foraminifera, provide valuable independent evidence in finding the nature of the site of deposition. It seems likely that environments in ancient sediments like those now existing along the Texas coast could be distinguished by similar methods, making it possible to find the direction of the sand masses constituting ancient barrier islands by determining the environment of deposition of sediment from core samples and outcrops. Among the diagnostic characteristics are the following : (1) approaches to the barrier islands are indicated by rapid increases in the sand content ; (2) the muddy sediments of the bays differ from the muddy sediments of the open shelf by the appreciable glauconite and echinoid content of the latter ; (3) the nearness to an entering river can often be determined by the high plant fiber content, by lamination, and by high ratios of Ostracoda to Foraminifera ; (4) the inner shelf deposits (inside about 20 fathoms) lack the abundant planktonic Foraminifera which characterize the outer shelf and both the total Foraminifera and shells increase in the outer shelf deposits. All of these characteristics apply to the central Texas coast area, probably apply also to most of the western Gulf coast, and possibly have a general application to all similar environments.