ABSTRACT
The central Texas coast in the Rockport area has been used as a test area for investigating the characteristics of sediments of protected bays, barrier islands, and of the open Gulf continental shelf. By using a team of sedimentationists, biologists, and chemists, a considerable number of differences have been determined in these three major environments and in subdivisions of each. Influences such as salinity, waves and currents, depth of water, and source material are indicated as the primary controls of the sediment types. In addition to the assemblages of organisms, statistical studies of the constituents of the coarse fraction have been particularly helpful in characterizing the environments.
Among the characteristics which differentiate the sediments of the depositional environments in this area are the following.
Bays near river mouths can be identified by their relatively high content of plant fibers, ferruginous aggregates, and ostracods which may predominate over Foraminifera (a relation unique to this environment). Stratification is better preserved than in the other environments in the area. Montmorillonite is the dominant clay mineral and is much more common than in other environments. The CaCO3 content is especially high, being derived as particulate matter from the river.
The central and deeper parts of bays are characterized by their lack of stratification, their high clay content, and especially by the abundance of Foraminifera in the coarse fraction (greater than mm.). Parts of these central bay areas are also characterized by the presence of oyster reefs or by the abundance of oyster shells. Montmorillonite and CaCO3 are less abundant than in the upper bays.
The lower bays near inlets have sediments with high sand contents (commonly more than 50 per cent) along with a higher content of clay than silt. The faunas show varying admixtures of open Gulf forms with those typical of the bays.
The barrier island flats and inlets are characterized by sediments with high sand content but they differ from the lower bays in having more plant fibers. The sediments from the flats have calcareous aggregates and the inlets commonly contain an appreciable proportion of echinoid plates and spines whereas these are rare in bay sediments.
The Gulf beaches and dunes consist predominantly of sand, rarely having more than 3 or 4 per cent of silt and clay. This sand is well sorted but almost equally divided between the --mm. and –-mm. sizes. The two environments are distinguished from each other by the consistently greater roundness of the dune sands and by the greater content of shells and Foraminifera in the beach sands.
The nearshore Gulf sediments extending from the barrier island beaches down to depths of about 30 feet are also characterized by a predominance of sand but they differ from the barrier island sands in being concentrated in the to -mm. grain size. Glauconite is found in these sediments, whereas it is rare on the barrier islands and very rare in the bays. Shells are rather scarce in this turbulent area.
The inner shelf, extending from depths of about 30 feet out to 120 feet, is characterized by sediments with rather considerable amounts of glauconite and echinoids in the coarse fraction. In general shells are scarcer than in the bay sediments. Silt is the dominant size grade, unlike the bay sediments, where clay ordinarily is far more abundant. Lenses of sandy sediment of variable thickness and continuity are common. CaCO3 is distinctly lower than in the bay environments.
The outer shelf sediments differ from those of the inner shelf in having a considerable abundance of the easily recognized pelagic Foraminifera, and a higher content of CaCO3. In the area off St. Joseph and Matagorda islands the outer shelf has a high clay content and very little sand but sand is more abundant on the northeast.
By using these environment indicators and assemblages of fossils, it has been possible to differentiate the depositional environments of deposits found in a series of borings made to maximum depths of 85 feet in the bays, 65 feet on the barrier islands, and 60 feet on the delta of the Guadalupe River. It was found that bay deposits extend to as much as 80 feet under the axis of lower San Antonio Bay before encountering sediments deposited during the time of a late glacially lowered sea-level. Barrier island deposits of the Recent cycle continue to depths of 60 feet under the islands. In contrast, bay deposits comparable with those of the present bays are found under the Guadalupe Delta below depths of 5–8 feet and under the lagoon side of Matagorda Island bay deposits are found below about 30 feet. Sediments from an offshore boring in 42 feet of water furnished by the Humble Oil and Refining Company show about 12 feet of shelf deposits underlain by about 60 feet of barrier island sediments. Carbon-14 determinations by the Magnolia Petroleum Company Field Research Laboratories show that the deeper deposits under the bays are contemporaneous in part with the rise of sea-level following the glacial period and that the sands of the barrier islands began to grow upward at least 6,500 years 3 ago as islands or shoals. Carbon-14 dates also indicated that the protected bay sediments were first deposited about 9,500 years before the present.
Studies of early diagenesis of these central Texas coast sediments have shown that the clays have lost far less water by compaction than have sediments in the deep basins off California, probably because of more rapid deposition in the Texas area. The stiffening of the clays found at depths of 20 feet or more under San Antonio Bay is attributed more to the relative increase of effective porosity than to loss of water. At the greatest depths in the borings the clays show some shaly partings. Chemically there is an increase in the chloride ion content between the overlying water and the surface sediment but only an irregular increase with depth. Sulphate ion shows no net decrease to depths of at least 7 or 8 feet, which is in contrast to the conditions in the California basins.
