Sediment Zones Bordering the Barrier Islands of Central Texas Coast1
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 in 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 dis-tinguished 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 genera! application to all similar environments.
Figures & Tables
Finding Ancient Shorelines
For many years the primary function of the Research Committee of the Society of Economic Paleontologists and Minerologists was to organize a symposium concerned with the most pertinent problems in sedimentary geology. This year (1955) the main theme centered on the characteristics of near shore deposition, particularly aimed toward recognition of ancient shorelines. The chairman that year was Dr. Henry W. Menard of the Navy Electronics Laboratory. Committee members selected the topic for the symposium, aided in selection of the speakers, and often participated in the program. The plan was to ask each speaker to present his paper in twenty minutes. Following the formal presentation a discussant, who has had opportunity to examine certain parts of the paper, was asked to comment. This approach led to some lively and focussed comments which benefited the entire presentation and is very interesting to read in that context. Papers included: Continental terrace sediments in the northeastern Gulf of Mexico, Sediments of the eastern Mississippi Delta, Sandless coastal terrain of the Atchafalaya Bay area, Louisiana, Sediment zones bordering the barrier islands of central Texas coast, Dynamic geology of the modern coastal region, northwest Gulf of Mexico, Particle size distribution in nearshore sediments