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AAPG Bulletin

Abstract


Volume: 39 (1955)

Issue: 8. (August)

First Page: 1463

Last Page: 1593

Title: Central Texas Coast Sedimentation: Characteristics of Sedimentary Environment, Recent History, and Diagenesis: PART 1

Author(s): Francis P. Shepard, David G. Moore (2)

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.

1. 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.

2. 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 1/16 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 that in the upper bays.

3. 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.

4. 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.

5. 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 ¼-1/8-mm. and 1/8-1/16-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.

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6. 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 1/8 to 1/16-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.

7. 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.

8. 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 bout 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 (FOOTNOTE 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 diagnesis 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.

Text:

INTRODUCTION

The purpose of American Petroleum Institute Project 51 has been to establish criteria from the study of recent (FOOTNOTE 4) shallow-water marine and estuarine sediments which can be used as an aid in identifying the environment of deposition of ancient sediments. One of the two areas chosen to initiate this work is described in the present report. This area, around Rockport, Texas, 30 miles northeast of Corpus Christi (Fig. 1), includes a group of shallow connected bays separated from the open Gulf by exceptionally wide barrier islands and a broad relatively smooth continental shelf.

This particular location was chosen for the following reasons: (1) the bays have been relatively free from industrial contamination, particularly San Antonio Bay which lies somewhat to the northeast of Rockport; (2) sample collection grounds are rather easily accessible; and (3) two marine laboratories are located in the area, the Texas Game and Fish Commission at Rockport, and the

FOOTNOTE 3. These figures are based on shells and may be a little high, probably not more than a few hundred years.

FOOTNOTE 4. Referring in general to the top 20 cm. of the sediment.

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Fig. 1. Showing locations of samples in Rockport area (see also Fig. 2). General location in inset.

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Fig. 2. Location and number of continental shelf samples. Depths in feet Contour interval, 30 feet.

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Institute of Marine Science of the University of Texas at Port Aransas. The laboratory at Rockport offered the use of boat and dock facilities as well as office space. The bays in the area are well suited to the study because they show a range from very low salinity at the mouth of the Guadalupe River in upper San Antonio Bay to high salinity in the lower bays, particularly in Aransas Bay. There are several sources of sediment including rivers, oyster reefs, wind-blown and wave-eroded material from the sandy barrier islands, and wave-debris from low alluvial cliffs along mainland parts of the shore. The continental shelf in this area is favorable as a site of deposition since it has an even slope, broken only near the outer part where a few banks rise about ten fathoms above the gener l surface. The barrier islands are especially interesting as they contain a variety of depositional environments, including on the Gulf side broad beaches, "beach" ridges, and extensive dunes; and on the bay side overwash fans, low swampy areas, and channels. In addition these islands are among the largest barrier islands on the entire Gulf coast if not in the world. The Rockport area is climatically somewhat intermediate between the semi-arid climate of the Laguna Madre at the south and the humid area at the northeast. As a result of rainfall cycles, deposition representative of several climatic belts can be obtained in the same area. Finally, the fact that sediment studies had been made in areas both on the north and the south by petroleum company research laboratories made it possible through their generous cooperation, to make some comparative studies and thus to obtain a better general picture of the sedimentation along the Texas coast.

During the 3½ years which have been devoted to the study of the Rockport area, 1,200 sediment samples were collected largely from the bays and continental shelf but also from localities widely distributed over the area (Figs. 1 and 2). In addition, 26 borings were made in the area to an average depth of 45 feet, and many specific observations and collections have yielded data relative to currents, temperature, number and kinds of organisms, and the chemistry and bacteriology of both the waters and the sediments. A staff averaging about eight people has devoted a large part of its time during these years to making field collections and laboratory analyses of the samples. The present report contains only a summary of chemical and biological phases of the work. Several reports givin the results of special phases of these investigations have already been published and others are in press or in preparation.

ACKNOWLEDGMENTS

Many people have been of assistance in gathering and analyzing the data which are used in this report. Primary acknowledgment is extended to the members of Steering and Advisory Committees of API Project 51 for their many helpful suggestions. Two committee chairmen, A. Rodger Denison and Clarence L. Moody, have devoted much time and energy to the development of the project. Marcus A. Hanna, Harold N. Fisk, and Rufus J. LeBlanc have had extensive experience working on the recent sedimentation of the Gulf coast, and contributed from this experience. Particularly helpful suggestions have come from Parke A. Dickey, C. I. Alexander, and N. L. Thomas who have reviewed many

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manuscripts. The cooperation of the Texas Game and Fish Commission Laboratory, under the leadership of J. L. Baughman and its present director, Cecil Reid, is greatly appreciated. They provided space for equipment and office work during field seasons, and furnished boats for some of the offshore investigations and much of the bay work. One offshore trip was made on the M. V. Alaska of the U. S. Fish and Wildlife Service through the cooperation of Albert Collier. The Institute of Marine Sciences of the University of Texas provided its pier to obtain current measurements of Aransas Pass and cooperated in making these measurements. R. M. Norris was in charge of early field collections in the area. M. N. Bramlette, of the Scripps Institution, made field studies which led to the developmen of a general subaerial geological map of the Rockport area. Bramlette has also made a preliminary investigation of the general petrography of the sediments and has given extensive advice in connection with the development of the project. A committee of ten scientists at the Scripps Institution of Oceanography has met frequently under the chairmanship of Roger Revelle, and this group has been helpful in developing and interpreting various phases of the project.

The grain-size analyses of the cores have been under the efficient direction of D. M. Poole, and he has also made preliminary descriptions and photographs of most of the cores. Information received from special work in sub-projects is discussed briefly in this report. The authors of this information who have or are about to publish more extensive reports will be referred to in the appropriate places. They include: F. B Phleger, Frances L. Parker, and Jean Peirson for the Foraminifera; Fred M. Swain and Doris Malkin Curtis for the Ostracoda; R. H. Parker, E. L. Puffer, and W. K. Emerson for the macro-organisms; George Bien for the chemistry; R. E. Grim and W. D. Johns for the clay minerals; L. R. Wilson and A. E. LeBlanc for the microfossils; D. E. Contois and C. Oppenheimer for the ba teria; D. L. Inman for special-size parameters; D. M. Poole for heavy-mineral studies; and M. A. Beal for roundness studies. Others who have helped extensively include: J. R. Curray, J. W. Durham, J. W. Hedgpeth, R. J. Hurley, H. W. Lusk, T. E. Mahnken, J. R. Moriarty, E. V. Sanborn, D. B. Sayner, P. C. Scruton, and R. Young.

DESCRIPTION OF THE AREA

TOPOGRAPHY OF BAYS AND SHELF

The Rockport area includes four principal bays (Figs. 1 and 3). The largest, San Antonio Bay, is essentially an estuary located at the mouth of the Guadalupe River. This bay has an average width of about 7 miles, and extends into the land for a length of about 18 miles. The seaward boundary is Matagorda Island which forms a wide barrier separating the bay from the Gulf of Mexico. The delta of the Guadalupe River divides the head of San Antonio Bay into two smaller bays: Hynes Bay on the southwest, which is shallow, having few depths exceeding 3 feet, and the narrow Guadalupe Bay on the northeast which has equally small depths. A northern extension of Guadalupe Bay, called Mission Lake, averages about 2 feet deep and is actually not a lake as it is connected with the open bay.

Below the delta, San Antonio Bay deepens gradually except where irregularly shaped oyster reefs produce tracts of shoal water. These reefs are particularly well developed in the central section of the bay. In the lower part of San Antonio Bay there is only one long narrow reef, Panther Reef, which extends from Panther Point on Matagorda Island northward beyond the intracoastal waterway. On the southwest side of the bay, water close to 6 feet deep occurs over fairly large areas.

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The depth increases slightly south of the channel so that there is an area west of Panther Reef about 12 miles long and 2 miles wide which has water averaging 6½ feet deep, whereas east of the reef the water averages about 5 feet deep.

Southeast and northwest of San Antonio Bay there are lagoonal areas which vary considerably in width and extend parallel with the barrier islands. Mesquite Bay at the southwest represents a widening of the lagoon, and is connected with the open Gulf by a shallow inlet. This inlet, called Cedar Bayou, has depths of 3-4 feet. Mesquite Bay is shoaler than San Antonio Bay and is generally less than 4 feet in depth. Here, also, there are a few oyster reefs but they are not as pronounced as in San Antonio Bay.

Aransas Bay, another enlargement of the lagoon on the southwest, is the deepest bay of the area. Beyond Long Reef, which essentially divides the bay in two, the depths reach as much as 13 feet, and there is an area about 6 miles long by 4 miles across which is all deeper than 10 feet. Aransas Bay has a long narrow connection with the Gulf through Aransas Pass and Lydia Ann Channel, an inlet which has been established in its present position by jetties. Prior to this construction the inlet was migrating slowly southwest. Two other inlets, called Middle Pass and North Pass (Fig. 4), have been opened twice by hurricanes in recent years.

Fig. 3. Bottom topography of Rockport bays and nearshore area. Bay contour interval, 2 feet. Offshore contour interval, 10 feet.

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Copano Bay, which was only sparsely sampled, is intermediate in depth between San Antonio and Aransas bays, having rather large areas somewhat more than 6 feet in depth. It is marked by a series of living oyster reefs which extend almost at right angles to the elongation of the bay. Redfish Bay, a very shallow bay located southwest of Aransas Bay, has been investigated to some extent during the project. Here most of the depths are about one foot. A considerable part of this bay is laid bare by the lowering of the water level which occurs from time to time during strong northerly winds which push water out of Aransas Pass.

The continental shelf outside St. Joseph and Matagorda islands is notable for the regularity of its contours. As shown in Figure 2, the contours conform in general with the concave shape of the coastline. Near the shore there is a relatively steep zone (0.67 per cent) extending to 20-foot depths, beyond which the gentlest

Fig. 4. Two small inlets in St. Joseph Island opened by hurricane in 1919 and subsequently filled.

Fig. 5A. Echo-sounding profile of shelf seaward of St. Joseph Island. Inner part from U. S. Coast and Geodetic Survey data. 5B. Echo-sounding profile across three banks on outer shelf, showing ring depressions and small benches.

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slopes of the shelf are found out to about 100 feet. Seaward from the 100-foot contour the slope increases gradually, having an average grade of 0.16 per cent to to the edge of the shelf, which occurs at about 360 feet (60 fathoms) off Aransas Pass, but is nearer 270 feet on the northeast off Matagorda Bay. The nature of this slope is shown on the chart (Fig. 2) and from a profile taken from one of the fathograms made during offshore collections (Fig. 5; also Fig. 17).

The chief irregularity of the shelf is found in the group of banks which rise from 30 to 60 feet above a mean depth of 250 feet off Aransas Pass. Traverses of these banks with a fathometer have shown that most of them are surrounded by broad, ring-like depressions generally having a depth of about 6 feet (Fig. 5b). The tops of these banks are relatively flat and there are flat steps or terraces on the sides. It is difficult to say from available information whether these terraces occur at the same depth on the different banks although there is a slight indication that this may be the case.

GEOLOGY AND PHYSIOGRAPHY OF LAND AREA

For the Quaternary geology of the area the writers are dependent on the information provided by M. N. Bramlette, along with well data from engineering reports. Some data have also been provided by R. J. LeBlanc, Hugh Bernard, and W. Armstrong Price (1933).

The generalized compilation, shown in Figure 6, indicates two topographic trends. One of them runs northeast and southwest and is represented by the present-day barrier islands and by an old Pleistocene barrier called Live Oak Ridge, which extends through Aransas Pass and Rockport and northeastward except where it is interrupted by the bays. This ridge, which rises to heights of 30 feet, is similar in general character to St. Joseph and Matagorda islands which lie along the present outer coast. Northwest of these old barrier islands there are lowlands which represent old bays, some of them still below sea-level, such as Port Bay west of the Rockport area. The other significant trend is from north-northwest to south-southeast and is the result of the valleys which evidently were cut du ing the glacial stages of lowered sea-level and have been nearly filled with sediment during and following the post-glacial rise in sea-level. The combined Guadalupe and San Antonio rivers are the principal streams entering this area, although there are three small streams coming into Copano Bay. Along these old valleys there are fluvial and delta deposits. The Guadalupe River has built a delta at least 10 miles out into the head of San Antonio Bay, and Mission River has made a much smaller delta in Mission Bay, a tributary of Copano Bay.

The barrier islands are diversified physiographically. Inside of the long sand beach there are extensive dune tracts (Fig. 7), some extending more than a mile in from the beach. Beyond some of the beaches is a series of long so-called "beach" ridges parallel with the shore in a belt having a width of as much as a mile. The maximum widths of both St. Joseph and Matagorda islands are in part related to

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great overwash fans (Fig. 8). Some of these low areas are traversed from time to time by storm tides, and elsewhere the bifurcating of the channels is indicative of overwash.

The barrier flats on the northeast end of St. Joseph Island and along most of the bay side of Matagorda Island consist of a mass of more or less circular lakes and irregular channels with less actual land area than that covered by shallow water (Fig. 9). During times of low sea-level in the bays the land area becomes considerably enlarged. A similar swampy area is found along the east side of lower San Antonio Bay and along adjacent parts of Espiritu Santo Bay. The circular lakes are commonly rimmed by slight rises on which there are masses of small bushes. Comparison of airplane photographs made in 1929 with those made recently shows that some of the lakes have expanded so that rims formerly distinct have become intersecting (Fig. 10).

CLIMATE

The climate is of particular interest in the Rockport area because it is intermediate between two belts, one on the northeast called by Thornthwaite (1948)

Fig. 6. Compilation by M. N. Bramlette, showing Quaternary deposits of Rockport area. Arrows indicate general direction of channels in overwash fans.

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Click to view image in GIF format. Fig. 7. [Grey Scale] Aerial photograph of Matagorda Island, showing three environments of barrier islands. Small rip currents shown by cloudy water along shore. Shallow bars indicated by breakers in foreground and by lines parallel with coast in upper left. Aerial mosaic by courtesy of Edgar Tobin Surveys.

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"moist sub-humid," and the other on the southwest "dry sub-humid" changing in a short distance to "semi-arid." As a result of the drought which has affected most of Texas during the last 6 years, the climate of the Rockport area has become semi-arid. The rainfall at Victoria in the San Antonio River basin since 1948 has averaged 30 inches per year as compared with a previous average rainfall of about 39 inches for the years 1935-1947.

The temperature of the Rockport area ranges from about 100° in summer to about 32° in winter. In summer the temperatures are fairly constant around 85°, being considerably ameliorated during the day by the southeast winds along the the coast which blow in from the Gulf, where the water averages about 84°. In

Click to view image in GIF format. Fig. 8. [Grey Scale] Branching overwash on St. Joseph Island in vicinity of Jay Bird Point. Aerial mosaic by courtesy of Edgar Tobin Surveys.

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winter the temperature shows tremendous alternations. From a normal 60° it is frequently lowered to 40° or 50° by strong northers. These occasionally reduce the Rockport temperature to freezing or below freezing. The most severe northers may lower the temperature to such an extent that the fish are killed in large numbers (Gunter and Hildebrand, 1951).

Almost every year one or more hurricanes traverse the Gulf coast. These have crossed the Rockport area on three occasions since the Weather Bureau started keeping records (1919, 1941, and 1945). In each case extensive damage was done to one or more of the towns in the area, and undoubtedly the sedimentation in the bays was influenced. Even when hurricanes pass at some distance from the area, the effect is observed because the Gulf water level rises, causing the flooding of the outer beaches. Water moves rapidly up through the inlets and thus tends to flood the bay shores. Twice during the project field investigations, distant hurricanes produced this flooding (Fig. 11).

Long period variations in rainfall have a strong effect on the water of the bays. During a dry period when the principal rivers are low, evaporation exceeds run-off causing a great increase in salinity. Under these conditions the lower bays develop a salinity slightly higher than that of the open ocean. After the occasional floods the salinity is decreased very rapidly, especially near the river mouths, although the low salinity persists only for a relatively short time.

METHODS OF COLLECTION AND STUDY

FIELD PERIODS

Seven field periods have been spent in the Rockport area and these have lasted from 10 days to 3 months. The initial field work was conducted during the entire

Click to view image in GIF format. Fig. 9. [Grey Scale] Lakes and channels of marshy zone on bay side of Matagorda Island.

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summer of 1951. Other periods, all of shorter duration, have covered a part of each of the other three seasons.

During the field work the greater part of the time has been spent in collecting samples from the many environments of the bays. Several cruises have been made in the open Gulf, and on two occasions these have included sampling across the entire width of the continental shelf. Briefer trips have extended out only to

Click to view image in GIF format. Fig. 10. [Grey Scale] Comparison of circular lakes and photographs made in 1929 (upper) and 1951 (lower). Lake rims have intersected in several places. Upper is aerial mosaic by courtesy of Edgar Tobin Surveys.

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the 10-fathom curve. The open Gulf work has ranged somewhat beyond the general bounds of the Rockport area, as can be seen from the sample localities in Figures 1 and 2. The barrier islands were visited on many occasions during which collections were made in their various environments.

EQUIPMENT

Boats used in the work have varied from the 110-foot Alaska, belonging to the U. S. Fish and Wildlife Service and used in offshore collections, down to small motorboats for work in the bays.

The sampling equipment has been varied. Some heavy gear has been used in the offshore work, such as piston samplers which obtained cores up to 15 feet in length, and a Van Veen sampler which collected 2 cubic feet of material (Fig. 12). The piston samplers (Silverman and Whaley, 1952) are modifications of those developed by Kullenberg (1947) for the Swedish Deep Sea Expedition. The cores collected are not as long as those obtained by Kullenberg partly because of the shoaler water where hydrostatic pressure is less effective and partly because the vessels which have been used were not equipped to handle the heavy weights necessary for long cores. The Van Veen sampler, in addition to collecting a large amount of material, obtains an essentially undisturbed sample in fine sediments which is useful in determining the nature of minor sedimentary structures.

Many of the samples were taken by a short coring device known as a Phleger Bottom Sampler (Phleger, 1951, Pl. 2). Other larger-diameter corers of similar principle have also been employed. All of these coring devices have core liners which can be removed from the barrel in order to keep the sample intact. In many cases the cores have been frozen in dry ice to preserve them for special work on bacteriology and chemistry in the laboratory. An orange-peel sampler

Click to view image in GIF format. Fig. 11. [Grey Scale] Flooded beach at north end of Padre Island, during hurricane of September 26, 1953, which hit Gulf coast east of Mississippi Delta, 450 miles from position of photograph. These beaches are flooded only during hurricane tides so far as known.

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which holds about 0.3 cubic foot of material and a NEL sampler (LaFond and Dietz, 1948) have been used to obtain many samples in sandy bottoms as well as samples for biological purposes. Because of the shallow water in the bays, most cores were obtained by a very simple device, which consists of a metal container into which long tubes of plastic are inserted. This is pushed into the bottom from the deck of the boat.

Measurements of pH and Eh have been made in the field in order to determine hydrogen ion concentration and the oxidation-reduction potential of freshly collected samples. In addition, water samples have been collected and many water temperatures measured.

In order to obtain closely spaced cores, a "Bident" corer was used which consists of two core barrels mounted on the same frame. They are separeted by 33

Click to view image in GIF format. Fig. 12. [Grey Scale] Van Veen sampler in semi-open position. Shows release which drops when sampler hits bottom, allowing it to come up in closed position.

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inches making it possible to determine the continuity of any layering. During much of the work three cores were taken at each station, giving an opportunity for a comparison of closely spaced cores.

FIELD STUDIES

The early work consisted of taking a grid of samples over most of the Rockport bays in order to outline the various sedimentary environments. Most of these samples were taken in core tubes and were retained for analysis. In the work that followed, a series of closely spaced cores was obtained particularly at boundaries between sediment units. Some of these cores were removed from the liners in the field, described immediately, and then discarded. This method was used for one closely spaced sample traverse across San Antonio Bay and in several traverses out from the shore in Aransas and Mesquite bays. Because further study of many samples was desirable, this discarding of cores was soon abandoned.

During most of the field work extensive observations were made of environmental factors such as water temperature, currents, turbidity of the water, wind, and general weather conditions at the time of collection.

LABORATORY STUDIES AND TECHNIQUES

PARTICLE-SIZE ANALYSES

Most of the samples collected in the field work have been analyzed for particle-size distribution. All but the dune sands were disaggregated by using a 0.025 normal solution of sodium hexametaphosphate in which the samples were soaked 2-24 hours according to the clay content. This method breaks up all but the firmly cemented aggregates. Following the disaggregation, the sediments are washed through a sieve which retains all particles greater than 1/16 mm. (4 phi). This coarse fraction is subject to settling-tube analysis in the method developed by Emery (1938) and later modified by Poole, Butcher, and Fisher (1951), or is dried for sieving if the material is to be fractionated and studied under the microscope.

A working method for the rapid size analysis of the sediments finer than 1/16 mm. was needed. The hydrometer method was selected because it is faster than the pipette method and is comparable in accuracy. An objection to hydrometer size analysis has been that when the hydrometer is placed in a settling suspension after a known time interval, the density measured can not be correlated with any exact level in the suspension and, therefore, can not be related to a definite settling velocity. However, this objection was overcome by Day (1950) who developed a theoretical method of determining the depth in the suspension (called the "effective depth") where the density is equal to that shown on the hydrometer.

A practical method was needed for applying Day's equation for finding the effective depth at any time interval and, hence, the settling velocity at that depth in the suspended sediment being analyzed. A graphical solution to the equation

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was developed by D. L. Inman and J. D. Frautschy and from the effective depths and settling times the settling velocities can be determined. The final step was the construction of a nomograph relating settling velocities to diameters. This entire method was simplified by using a known hydrometer, and soil cylinders that had a standard inside diameter so that tables relating hydrometer readings to effective depth could be made, thus eliminating calculations for effective depth for each analysis.

BACTERIOLOGY

The studies carried on by D. E. Contois under the direction of Claude E. ZoBell have used methods indicated briefly as follows. The relative abundances of certain specific bacterial groups in sediment samples are determined by standard plate counts. These are physiological groups consisting of

(a) aerobic saccharolytic bacteria,
(b) aerobic proteolytic bacteria, and
(c) aerobic lipolytic bacteria,

and environmental groups consisting of

(d) aerobic marine bacteria,
(e) aerobic terrestrial bacteria, and
(f) anaerobic marine bacteria.

To ascertain the relative abundance or activity of any specific group, the number of bacteria of that group in a sediment sample is divided by the number of aerobic marine bacteria. For example, the number of aerobic terrestrial bacteria is divided by the number of aerobic marine bacteria to measure relative terrestrial influence. To measure relative proteolytic activity, the number of protein hydrolyzers is also divided by the number of aerobic marine bacteria. Thus in effect, the abundances of the various groups listed are expressed in terms of the number of common aerobic marine bacteria in the sample. The resulting ratios are coded by taking the logarithm of 1,000 times the ratio--i.e., X^prime = log (1,000xX). This better assures a normal distribution of the variables and allows he application of statistical tests of significance.

Analyses of initial data indicate that the distributions of these groups exhibit statistically significant differences between samples from various localities in the Rockport area and at times between samples from different depths within a locality. Also, determinations of total populations of aerobic marine bacteria in selected samples indicate viable populations of these organisms of from 11.5-14.5 × 104 bacteria per gram of oven-dried sample. Some of these samples were collected from depths within the sediments as great as 28½ feet. Thus these groups are regarded as an excellent means of characterizing sedimentary environments and processes as well as being agents of diagenetic processes.

TANK EXPERIMENT ON ORGANISMS

Laboratory experiments were conducted, involving the effects of burrowing organisms on the sediments in which they live. Collections were made in the San

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Diego area of burrowing organisms as nearly similar to those of the Rockport area as possible. These were placed in specially constructed aquaria containing sediment of varying textures and combinations of layering. The results of these

Click to view image in GIF format. Fig. 13. [Grey Scale] Tank experiments showing progressive effect of burrowing organisms on layered sediments. Photo shows depressions and the cutting-through of layers; also lenses of sand built on surface.

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experiments were gratifying in that they showed the destructive effects of the organisms on stratification. Figure 13 shows one of the tanks at successive time periods and the destruction of the layers where the organisms were active. It is notable that while destroying stratification of one kind, the animals (particularly the burrowing razor clams Tagelus) created other minor sedimentary structures by making mounds of coarser material about their burrows. When buried these would appear as small lenses.

CHEMICAL ANALYSIS

In order to facilitate the efficient chemical analysis of sediments and their interstitial and overlying waters, several new analytical procedures were developed and other standard techniques of processing the samples were improved by George Bien of Scripps Institution. The extraction of interstitial water from cores was made more efficient by the utilization of a sintered steel filter in a newly designed press. This device takes about ½ cubic inch of sample, and from this about ½-2½ milliliters of solution in equilibrium with the sediment can be squeezed. The exact amount depends on the grain size and water content of the sample.

A high-frequency titration procedure was developed for the determination of the sulphate and chloride contents of very small samples of water. The method was adopted for the analysis of interstitial water obtained in the press device described previously. Details of this technique can be found in a paper by Bien (1954).

The carbonate analyses were made by a standard procedure which was adapted for the recent sediment samples. Carbonate was determined manometrically by measuring the carbon dioxide evolved as a gas when the sample was digested in normal sulphuric acid. The volume of gas is corrected to standard temperature and pressure and then converted to carbonate as calcium carbonate by the formula,

[EQUATION]

where V is observed volume, P is pressure corrected for water vapor pressure at the room temperature, T is the room temperature in degrees Kelvin, and W is the weight of the sample used. The number 0.1605 is a conversion factor. This method is rapid and accurate. It is far more satisfactory than the dilute HCl treatment which removes more than the carbonates from the sample.

Standard analytical methods which were used in the analysis of the sediments and their interstitial and overlying waters are as follows.

1. The Kjeldahl method was used for the determination of total nitrogen in the sediments. The only modification made was: the ammonia was absorbed in standard potassium bi-iodate solution and the excess acid back titrated with sodium hydroxide solution which was standardized with the same standard KH(IO3)2 solution.
2. Krogh's method, with minor modifications, was used for the determinations of organic carbon in the sediment.
3. The chlorinity of overlying waters was determined by the regular Mohr method with Knudson burets.
4. Potassium and sodium of the overlying waters were determined with the flame photometer.

End_Page 1482------------------------------

GENERAL LITHOLOGY OF RECENT SEDIMENT UNITS

Description of sediment units and their general lithologies is important for two reasons. First, it is the only way to give an over-all picture of the sedimentary framework of the area. Second, it serves to illustrate that sediment units based only on textural terminology are usually insufficient to define environments of deposition. The general framework to be presented has a textural nomenclature developed by API Project 51 after systems in existence were found to be lacking in certain respects (Shepard, 1954). The system is based entirely on proportions of sand, silt, and clay as represented on a triangle (FOOTNOTE 5) (Fig. 14).

BAY SEDIMENTS

Figure 15 shows the distribution of bay sediment units. The sediments tend to have an orderly arrangement, consisting of silty clays in the deep central parts, a gradational mixed zone of sandy clay, clayey sand, or sand-silt-clay, and a generally narrow zone of fringing sands in the shallowest areas where active stirring by waves takes place. This ideal arrangement is complicated, however, by variations in source, topography, and water circulation so that exceptions to the order are common. The most prominent areas which do not conform with the general arrangement are northeast and southwest Aransas Bay, southeast San Antonio Bay, and Mesquite Bay. These broad areas of clayey sand and sand-silt-clay are

Fig. 14. Nomenclature based on sand-silt-clay percentages.

FOOTNOTE 5. This new system represents a compromise which was found to be acceptable to a large number of American sedimentationists who were consulted.

End_Page 1483------------------------------

the result of abundant sand supply and considerable mixing. The mixing is apparently the result of stronger current action in the narrows such as those at the juncture of Espiritu Santo and lower San Antonio bays, and in the narrow connection between Mesquite and Aransas bays. Mesquite Bay is also further affected by a small pass into the open Gulf which introduces sand from the barrier islands or from the Gulf. The clayey sand areas in lower Aransas Bay and behind Mud Island are also near an inlet to the open Gulf. The northern end of San Antonio Bay which is split by the Guadalupe River Delta lacks both the zone of fringing sands and the mixed sediments. This omission of apparently normal sediment units is evidently the result of the small sand supply introduced by the Guadalupe Riv r and the inaccessibility of the sands of the old and new barrier islands on the south.

In addition to the sediment units formed predominantly of mineral detritus, the bays have irregular areas of shell reefs and reef detritus. These are largest and most common in San Antonio and Copano bays, but occur to some extent in all the bays studied.

Fig. 15. Sediment distribution in Rockport area bays based on sand-silt-clay content.

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BARRIER ISLAND SEDIMENTS

The barrier island sediments consist predominantly of sand with little else on the Gulf side. These sands occur as a wide beach along the outside of the islands and as dunes which spread rather widely across the islands in a band in general parallel with the Gulf shores. Both the beaches and the dunes have clean, well sorted sand, as commonly observed in other areas. The so-called "beach" ridges which are found inside the first line of dunes along parts of St. Joseph and Matagorda islands have the same well sorted sand as the beaches and dunes. As explained later, the constituents and the roundness of the sands from these ridges are much more like dune sands than beach sands. This is despite the topographic contrast between the ridges and the active dunes of the area (Fig. 7).

The low parts of the barrier islands inside the dunes and "beach" ridges are variable in texture but are predominantly sand with appreciable amounts of silt and clay in most samples. On St. Joseph Island these flat areas consist largely of overwash fans and commonly have 80 per cent or more of sand. On Matagorda Island, where overwash fans are less extensive, there is more silt and clay. This is particularly true of the bay fringes in the channel and swamp area which borders San Antonio and Espiritu Santo bays (Fig. 7). In a few of these areas the sediments have a high percentage of clay, but this is rare and most of the "mud" flats and channels have sediment consisting of at least 50 per cent sand. Inlets which connect the open Gulf and the bays are also predominantly sand with varyi g but small amounts of silt and clay.

CONTINENTAL SHELF SEDIMENTS

The shelf sediments show more variation in texture than do the barrier island sediments (Figs. 16 and 17). This is also indicated by some samples obtained in the same area by Stetson (1953). Along the island shores in the shallow open Gulf, the sediments are predominantly sand. These shallow sands extend seaward to approximately the 30-foot contour. Seaward of this sand zone the sediments change abruptly to a sand-silt-clay with only a narrow band of silty sand intervening. At the south off Corpus Christi Bay the sand-silt-clay band is very narrow but it widens continuously northward until it comprises almost the entire shelf off the Colorado and Brazos rivers. In the southern section either a silty clay or a clayey silt lies seaward of the narrow band of sand-silt-clay. Off St. Josep and Matagorda islands the shelf is covered with fine sediment, very largely a silty clay. For the most part, the sand content decreases seaward and the clay increases. The sand content is very small on the outer shelf in this southern area, but north of Pass Cavallo, the outer shelf runs high in sand, much of it 50 per cent or more.

A high content of silt is a feature which seems to be common to almost the entire continental shelf, except very near the shore. Outside the 5-fathom curve there are very few samples where the silt is less than 30 per cent, the chief exception being found in the sample line off the Brazos River where the silt content

End_Page 1485------------------------------

Fig. 16. Generalized sediment distribution on continental shelf off central Texas coast based on sand-silt-clay content.

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varies around 20 per cent (Fig. 17). The addition of the sand to the sediment in the northeast may be in some way connected with the Brazos and Colorado rivers which enter the coast here. At the mouths of both these rivers the sediments are much more sandy than those at the mouth of the Guadalupe.

Fig. 17. Variation in sand-silt-clay in series of lines extending out across continental shelf in central Texas coast area (see also Fig. 2).

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FACTORS INFLUENCING THE ENVIRONMENTS

SEDIMENTARY ENVIRONMENT DEFINED

Since much of the present report concerns "sedimentary environments," it seems advisable to define the term as it is used here. A sedimentary environment is a spatial unit in which external physical, chemical, and biological conditions and influences affecting the development of a sediment are sufficiently constant to form a characteristic deposit. The influences include depth of water, degree of protection from wave attack, salinity and temperature of the water (as they influence the biological activities), availability of streams carrying sediment into the area under discussion, the nature of currents such as are necessary to transport sediment in the area, and any other influence which may affect sedimentation. These factors are discussed as they apply to the Rockport area.

SOURCES OF SEDIMENT

Directly or indirectly the rivers can be considered as the principal source of sediments. The combined drainage of the San Antonio and Guadalupe rivers enters at the head of San Antonio Bay, although some of it has spilled over into Green Lake and thus enters the bay indirectly (Fig. 18). In 1935 a cut was made in the river bank, diverting the major flow from the two channels which enter Guadalupe Bay to a channel coming directly into the southwest side of Mission Lake. The outlet from Green Lake comes into the northeast side of Mission Lake. Hence, active delta building is occurring now at both ends of this small restricted body of water (Fig. 18) and little sediment is carried out of the old channels. The flow into Green Lake may account for the very minor changes which have occurre in Guadalupe Delta since the survey of 1874. The San Antonio-Guadalupe system has a flow of a little less than a million acre-feet per year, and these rivers transport about 775 acre-feet per year of sediment in suspension, as measured at stations about 25 miles above the juncture (Texas, 1952). The other streams which enter directly into the Rockport bays contribute much less sediment.

Indirect sources of sediment come to the offshore area from the Colorado of Texas and the Brazos rivers which enter directly into the Gulf at distances of 40 and 90 miles, respectively, northeast of the Rockport area. Judged from stations at a distance of about 40 miles from their mouths, both rivers transport much larger loads of sediment than does the Guadalupe-San Antonio system; the Colorado carries 29,200 acre-feet per year and the Brazos 21,984 acre-feet. The water discharged through these rivers is respectively about 1,000,000 and 5,500,000 acre-feet per year. The Brazos River has been flowing into the Gulf since the time of the earliest published charts. The Colorado, however, has only recently been given a direct connection with the Gulf as the result of a ditch dug by the Ar y Engineers across Matagorda Peninsula in 1936. Prior to that time the Colorado probably contributed sediment to the open Gulf through Pass Cavallo, which is a natural inlet at the mouth of Matagorda Bay. In addition to

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Click to view image in GIF format. Fig. 18. [Grey Scale] Comparative photographs showing development of small delta (lower photograph) in 1951, which was not present in 1929 photograph at time of aerial mosaic (latter by courtesy of Edgar Tobin Surveys). This delta has resulted from cutting of new outlet for Guadalupe River in 1935.

End_Page 1489------------------------------

terrigenous minerals, these rivers also introduce significant amounts of wood fragments which are prominent in deposits near the river mouths.

A small source of bay sediment comes from the cliffs situated along the shores of San Antonio Bay and locally in Aransas Bay, along Live Oak Peninsula. No studies have been made of the retreat of these bay cliffs, but similar cliffs were studied in both Galveston and Corpus Christi bays. A retreat of as much as 100 feet in about a century was found in both places. According to W. Armstrong Price (personal communication) a large supply of sediment for the barriers may come from the erosion of cliffs north of Port O'Connor near the outlet of Matagorda Bay. Therefore, it seems likely that the cliffs in the Rockport area form a local source of sediment which may be of some importance.

Biological sources of sediment were particularly important in the bays during the past when the oyster reefs were developing more actively. Intensive oyster raking removes most of the oysters now growing on the reefs but they still form a small source of sediment. The importance of oyster reefs in the past is indicated in the descriptions of the borings which were made along the center of San Antonio Bay (pp. 1555-62); these results are supported by many probings in the various bays (Norris, 1953). Biological sources of sediment are also of some importance in Redfish Bay where an abundance of gastropods and pelecypods are found, and in some bay areas the Foraminifera constitute as much as 2-3 per cent of the sediment. On the outer shelf the Foraminifera are abundant, and locally mollu k shell and echinoid fragments form a significant percentage of the sediment samples.

A small amount of sediment is developed through authigenic processes. In the offshore area, glauconite is found as discrete grains as well as in the tests of Foraminifera, having formed in part as the result of alteration of clays. Pyrite is found rarely in the shelf sediments. In the bays, pyrite forms to a considerable extent in the foraminiferal tests and in other small shells and develops to some extent from an impregnation or replacement of wood fibers. Except in the delta, pyrite is rarely present at the surface in large enough aggregates to be visible in the binocular study of the samples,(FOOTNOTE 6) but it has been found commonly in bay cores at depths more than a few feet below the surface. Glauconite, on the other hand, was observed both near the surface and below in the sh lf cores.

Wind-blown dune sand forms a small contribution to the bay deposits and a significant contribution to the barrier flats on the bay sides of the islands. Another small sediment source on the barrier islands comes from evaporation when the sea-level is lowered by changes in wind direction which result in water being blown to the other side of the bay or out of the inlets. Evaporation produces a calcareous evaporite deposit which surrounds the sand grains and thus constitutes a small source of sediment. Salt is deposited also as a result of this evaporation, although this ordinarily does not persist as an important material in the sediments

FOOTNOTE 6. According to M. N. Bramlette, pyrite does occur as minute grains.

End_Page 1490------------------------------

after they become slightly buried. In Redfish Bay the marine grasses constitute an appreciable element in the sediments, but these are of minor consequence in the other parts of the area studied.(FOOTNOTE 7)

DISTRIBUTIVE AGENTS

In the shallow bays, waves produced by moderate to strong winds, which are common in this area, stir the sediment from the bottom so that the water becomes very turbid. Surface currents driven before the wind may be accompanied by a return flow underneath. Where surface eddies develop along the sides of the bays there tends to be a surface return flow. Northerly winds in the winter months are generally strongest and most effective in developing currents. In general, wind-induced currents are more effective in transporting sediments in the bays than are currents coming from the entry of water from the stream mouths. The chief exception to this is found in Mission Lake and Guadalupe Bay, where during times of flood a considerable amount of fresh water enters and moves south towards the pper part of San Antonio Bay. Transportation by residual river flow, combined with currents induced by the northers in San Antonio Bay, evidently carries the bulk of the sediment down the west side of the bay with the result that there may be some counter current coming up the east side with water from Espiritu Santo Bay. The predominant flow down the west side of central San Antonio Bay is in part the result of bottom configuration produced by oyster reefs.

Currents generated by celestial tides are of minor consequence in most parts of the bays although they are significant in and near large inlets like Aransas Pass. Currents in Aransas Pass observed over a period of a year were found to have a greater relation to changing wind conditions than to the tides of the moon and sun. An analysis of wind direction and velocity versus direction of current flow is shown in Figure 19. An excellent correlation is evident between northerly winds and outflowing currents, whereas only a fair correlation exists between southerly winds and inflowing currents. This better correlation of current direction with the north winds is as would be expected since longshore and oceanic currents in the open Gulf limit the amount of the hydraulic head built up in the offshore area during southerly winds. The bays, on the other hand, are shallow and relatively restricted so that a strong and continuous northerly wind is capable of piling up a considerable amount of water on the downwind side, leaving the windward bay shores with a tide of one or more feet below normal. The entering rivers further accentuate the outflow through Aransas Pass. During northers strong southerly flows are also observed in the narrows between bays. These are due to concentration of flow at the narrows. A striking inflow through Aransas Pass develops at the time of Gulf hurricanes which cause a general rise of sea-level even at distances of several hundred miles from the storm center. At

FOOTNOTE 7. Copano Bay has extensive grass flats especially on the west side.

End_Page 1491------------------------------

the times of these sea-level rises old vestigial passes which are ordinarily closed may be opened, carrying water into the bays (Fig. 4).

Currents are considerably different in the open Gulf than in the bays. The waves commonly approach the coast diagonally and generate longshore currents (Shepard and Inman, 1951) which are responsible for carrying sediments for great distances along the coast (Bullard, 1942). A local piling-up of water along the shore causes a seaward return flow in the form of rip currents (Shepard et al., 1941) which can be observed frequently from the air (Fig. 7). These currents transport sediment out from the shore and are responsible for carrying the finer sands out from the beaches. Waves, which during exceptional storms break out to depths of at least 30 feet of water, have an important effect on sedimentation. Wave action stirs up the fine sediment from the sea floor and currents transport it oth in and out. The part carried shoreward, however, has little chance of deposition under the turbulent conditions near shore so that the net result is offshore transport.

Fig. 19. Relationship between wind directions and currents in Aransas Pass during year's survey from May, 1952, to April, 1953. Currents measured twice daily at Institute of Marine Science pier, and wind data from U. S. Weather Bureau at Corpus Christi. Top and bottom sections show current velocity and direction for winds over 15 miles an hour. Figures in small squares are totals for each quadrant.

End_Page 1492------------------------------

Other currents affecting the area are not as well known as those previously discussed but they are considered briefly. The currents associated with "surf-beat," which is a phenomenon related to the periodic building-up of waves and the development of currents as reflection of this build-up (Munk, 1949), may transport the sediments for considerable distances out from the coast. Sediments carried out either by surf-beat or rip currents may be deposited locally or may be incorporated into the semi-permanent currents which flow predominantly southwest along this coast.

The importance of currents, whatever their cause, is indicated even on the outer part of the shell. Thus the finding of a well sorted sand with Foraminifera suggestive of present conditions in a core from 30 fathoms of water off the Colorado River seems to prove that the currents are strong enough to transport sand. This strong current was confirmed by the difficulty in getting the large Van Veen sampler to the bottom on the outer shelf in the vicinity of the small banks.

TEMPERATURE AND SALINITY

Rather extensive information is available on the temperature and salinity of the bays in the Rockport area as a result of the work by Collier and Hedgpeth (1950), Hedgpeth (1953), and a compilation by Parker (1955). These factors are

Fig. 20. Diagram from Hedgpeth (1953, Fig. 23) showing relation of maximum, minimum, and mean averages of temperature and salinity in Aransas Bay. Numbers refer to months of year; Roman numerals refer to the mean.

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important in sedimentation principally because they control the growth of organisms in the various parts of the bays. A more direct influence comes from the electrolytic effect of the varying degrees of salinity on colloids introduced by the rivers. This problem is not yet solved despite extensive work by petroleum company laboratories and studies such as those of Whitehouse (Whitehouse and Jeffrey, 1953) related to API Project 51.

The bays of the Rockport area are so shallow that the water temperatures change rapidly especially during "northers" when the air is cooled as much as

Fig. 21. Figure from Parker (1955) showing relationships of precipitation in San Antonio-Guadalupe drainage basin and total monthly stream discharge with salinity and water temperature in Aransas Bay.

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10°C. in an hour and the wind is strong enough to stir the water to the bottom of the bay. Mean water temperatures for the cold months of January and February lie between 10° and 15°C., and the high water temperatures during July and August are close to 30°C. These temperature extremes are about the same as those encountered in the bays from Laguna Madre to Louisiana (Hedgpeth, 1953, Fig. 23). The water temperature along the open Gulf beach and in Aransas Pass falls as low in winter as it does in the bays but ranges about 2°C. cooler in summer.

The monthly relation of salinity to temperatures is indicated in Figure 20. It may be seen that salinity does not vary seasonally as much as temperature. For more details of the salinity in recent years the reader is referred to the compilation by Parker (1955). These observations indicate a considerable increase in salinity during the summer with rather sudden drops in May and September.

Fig. 22. From Collier and Hedgpeth (1950, Fig. 20), showing salinity in Copano Bay as related to rainfall in near-by Beeville station and rainfall minus evaporation.

End_Page 1495------------------------------

These drops are the result of the development of flood conditions. Parker shows the average rainfall in the San Antonio-Guadalupe river drainage basin (Fig. 21) as a partial explanation of this variation.

An important factor in the increase of salinity in the bays is the relation of precipitation to evaporation (Fig. 22). Although the rainfall at Beeville (50 miles northwest of Rockport) averages 30 inches, the evaporation is considerably higher, leaving a deficit. As a result the salinity in the lower bays, far from the stream mouths, is often higher in summer than the salinity in the open ocean. This condition becomes very important south of the area in Laguna Madre where the salinity is almost constantly above that of the ocean and reaches 70 ^pmil in the summer months, compared with the 30-40 ^pmil typical of the marine environment. Studies in the Rockport area by Ladd (1951) were conducted in 1940 following a period of relatively heavy rainfall which placed the bays in a different salinity range from what has existed since 1948. Thus lower Aransas Bay, which had been called "polyhaline" by Ladd, is now "marine."

During both dry and humid periods salinities are higher on the east side of San Antonio Bay than on the west side. A continuous increase in salinity is found down the west side of the bay and on into Mesquite and Aransas bays. However, in Aransas Bay the recent surveys under dry spring and summer conditions show a maximum in the lower bay which decreases seaward in Lydia Ann Channel, whereas in the older surveys there was a continuous increase from upper Aransas Bay to the open Gulf.

In the bays there is little difference in salinity and temperature from top to bottom, especially during periods of relatively strong wind which produce complete mixing. A real difference often exists in the lower part of Aransas Bay and in the relatively deep waters of Lydia Ann Channel and Aransas Pass. In Aransas Pass during a time of low salinities (1936-1937), Collier and Hedgpeth (1950) found a subsurface intrusion of high-salinity water coming into the channel (Fig. 23). Presumably under present conditions of higher salinity in Aransas Bay this intrusion no longer occurs. Variations in salinity and temperature in relation to several directions of wind and "tide"(FOOTNOTE 8) are indicated in Figure 23. This diagram supports the current measurements (Fig. 19) as it shows that wat r moves out of the bay with the north wind and into the bay with the southeast wind. Thus the bays in general can be considered as acting as a unit with reference to temperature and salinities.

Salinity and temperature differences between top and bottom in the Rockport bays are very minor compared with those found, for example, at the mouth of the Mississippi River where during low river stages a great salt wedge is intruded under the fresh water which flows out from the river, and where similarly great variations in salinity and temperature are found with depth. Also, the vertical temperature difference is very minor compared with those found in general in the

FOOTNOTE 8. Note that "tide" in this case refers to the current rather than to a celestial tide.

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Fig. 23. After Collier and Hedgpeth (1950, Figs. 15 and 16) redrawn by J. R. Curray, showing intrusion into Aransas Bay of high-salinity water from Gulf, and relation of several directions of wind and current to salinity and temperature in bay.

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open ocean where the surface waters become very much heated in summer, and underneath the warm water there is a sharp thermocline to waters of low temperature.

METHODS OF CHARACTERIZING SEDIMENTARY ENVIRONMENTS

In the investigations of API Project 51 the attempt has been made to use nearly all of the standard analysis methods, along with some new methods, of finding differences in sediment characteristics in the various environments of the bays, the barrier islands, and the continental shelf. This work has been aimed at providing stratigraphers more tools for determining the depositional environments of ancient sediments. Methods of studying environment may be considered under: (1) physical characteristics, (2) biological characteristics, (3) chemical, and (4) bacteriological characteristics. Under each of these, many techniques have been used to help define the environment.

PHYSICAL METHODS

Size analysis:
Many methods have been suggested for showing size analyses in graphic form (see especially Krumbein, 1936, and Inman, 1952). Of these

Fig. 24. From Inman and Chamberlain (1955), showing the distribution of sediment types based principally on three measures of particle size distribution: median diameter, sorting, and skewness.

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methods, contours of median diameters, called isopleths, are best known and in some sediment studies this has been the chief method used for sediment description. Isopleths are of importance where the sediments are predominantly well sorted terrigenous materials, but they may be of little value where sediments of distinctly different sizes come from two or more different sources.

A method of depicting sediments, which has the advantage of being based on three measures of particle-size distribution, has been used by Inman and Chamberlain (1955). The sediments are classified according to their median diameter, sorting (deviation), and skewness. These three measures are obtained from the cumulative size frequency distribution of the sediment with the following definitions quoted directly from Inman and Chamberlain:

(1) the phi median diameter Md^phgr = ^phgr50
(2) the phi deviation measure ^sgr^phgr = ½(^phgr84 - ^phgr16)
(3) the phi skewness measure ^agr^phgr = <fr>M^phgr - Md^phgr</>^sgr^phgr</fr>

where M^phgr = ½(^phgr16 + 84) is the phi mean diameter, and ^phgr16, ^phgr50, and ^phgr84 are the diameters in phi units corresponding with the 16th, 50th, and 84th percentiles, respectively, of the cumulative weight-per cent (coarser) curve. The results of

Fig. 25. Environments of deposition in Rockport area based largely on coarse-fraction analysis.

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applying this means of interpreting sediments in the Rockport area are shown (Fig. 24) from the article by Inman and Chamberlain (1955).

Another method of characterizing the size of grains is with sand-silt-clay ratios, discussed previously. Triangle diagrams of the sand-silt-clay ratios for the environments of the Rockport area (FOOTNOTE 9) show that the points have distinct groupings in each environment (Fig. 26). The method is of special value because the two major sources of sediment for the area are: (1) sand introduced by the longshore currents and tides coming in through the inlets, and (2) clay introduced by the streams coming into the upper bays.

Coarse-fraction analysis:
A method which is one of the most valuable for characterizing environments in the Rockport area is called "coarse-fraction analysis" (Shepard and Moore, 1954). In this method the binocular microscope is used to estimate the percentages of significant constituents in different size fractions, using the standard Wentworth size grades. The constituents considered important in the Rockport area include echinoids, Foraminifera, glauconite, ostracods, shells (mollusks et cetera), plants including fibers, and terrigenous minerals (land-derived minerals such as quartz, feldspar, and ferro-magnesians). In addition, aggregates, formed where various minerals or organic debris have been cemented together mostly by calcium carbonate or iron oxide, were included in the classification.

Various diagrams for presenting this information have been developed. Some of these are illustrated in Figures 27 and 28. The relation of the constituents to environments, both as they vary in different sieve sizes and in relation to the total coarse fraction, has been clearly indicated by the studies. In characterizing the environments of the Rockport area, coarse-fraction analysis is superior to grain-size analysis, because size is important largely as it is related to transportation of material of relatively uniform shape and density. A considerable percentage of the grains in the Rockport area consists of mollusk shells, Foraminifera, echinoids, and ostracods, many of which have grown in or near their place of deposition so that their size has little to do with currents or wave ac ion. As a result, one finds coarse-grained sediment in some parts of the bays despite the fact that the area is protected from large waves and the currents are of small velocity. Similarly, plant fragments are of nearly the same specific gravity as water when introduced and therefore do not follow the laws of settling velocities. Therefore, plants can not be used in relation to sizes. The same applies to minerals, such as glauconite and pyrite, which may form in place as authigenic substances. All of these constituents have significance in relation to the environments.

The coarse-fraction method can be made more valuable by a small amount of differentiation of organisms. For example, oysters can be separated from the other mollusk shells, and Foraminifera can be classified as pelagic or benthonic, giving more usefulness to the observations. Still further differentiation must, of course, depend on the training of the observer.

FOOTNOTE 9. See Figure 25 for the environments, and for discussion of these environments see pp. 1528-45.

End_Page 1500------------------------------

Fig. 26. Triangle diagrams showing sand-silt-clay contents of samples in various environments indicated in Figure 25. Four subdivisions of delta environment are indicated, and two subdivisions of "bay marginal."

End_Page 1501------------------------------

Fig. 27. Coarse fraction of bay environments showing average percentage of material in different sizes of coarse fraction, percentages of constituents in each sieve size, and percentage of materials in total sample (shown in pie diagram along with percentage of silt and clay). Shells constitute most of part greater than 1 mm. (> 0^phgr).

End_Page 1502------------------------------

Fig. 28. Coarse fraction of barrier island and shelf environments. Shells constitute most of part greater than 1 mm. (>0^phgr).

End_Page 1503------------------------------

Minor internal structures:
In describing and photographing the hundreds of cores from the Rockport and Mississippi Delta areas it was noted that certain minor internal structures were characteristic of the sediments of particular environments, whereas in other environments the sediments were commonly structureless. Moore and Scruton (ms., '55) made a study of these minor features and found that they could be divided essentially into two types, "layered" and "mottled" (Fig. 29). Layered structures were subdivided into "regular" and "irregular," according to the uniformity of the banding. These were found to be typical of areas where few organisms live, such as near the mouths of rivers. Mottled structures, which consist of lumps and pockets of irregular shape in a matrix of sediment of contrasting size, were found o be characteristic of zones of mixed sediments in areas with abundant mud-dwellers. These mottled structures are common in areas which have two sources of sediment. The lower bays of the Rockport area are a good example of this situation, having a silty clay source from the river and a sand source from the barrier islands. Homogeneous, structureless sediments were found in areas of only one source of a predominant size such as the deep central parts of bays and the outer shelf.

The principal methods of formation of minor internal structures are believed to be variations in competence of a transporting agent from a single source in the case of layered structures, and the work of organisms in destroying layering in the case of the mottled type. Burrowing animals are capable of creating, as well as of destroying, minor structures (Fig. 13). Irregular layering and mottled structures are examples of types which may be formed by local current action induced by organic activity such as syphoning.

Mineral analysis:
The sand-size minerals of the Rockport area consist

Fig. 29. Minor internal structures in cores of Rockport area. In part related to effect of organisms indicated in Figure 13.

End_Page 1504------------------------------

principally of quartz with lesser amounts of feldspar. Samples from various parts of the area from the rivers contributing to the area were counted for quartz, plagioclase, and potash feldspar by Harry Lusk, using a staining technique (Gabriel and Cox, 1929). The results were very uniform and showed no appreciable trend. Quartz averaged 65.2 per cent, plagioclase 22.5 per cent, and potash feldspar 12.3 per cent. By way of contrast, the light minerals of the shelf of the east-central Gulf are much higher in quartz, whereas typical counts on the west coast of the United States show feldspars more abundant than quartz.

The heavy minerals are scarce, generally less than one per cent of the sand fraction. Heavy-mineral studies of the sediments of the bays, barrier islands, and open shelf of the area have been made by M. N. Bramlette and D. M. Poole. They found that the bays are characterized by a suite of stable minerals consisting of black opaques (including leucoxene), tourmaline, garnet, zircon, and rutile, with the addition of staurolite near the Guadalupe Delta. Epidote is fairly abundant in the bays but is a non-diagnostic mineral. Only a very small amount of hornblende is present in the upper bays, but it increases nearer the barrier islands and reaches a maximum on the Gulf beaches. The southern part of Aransas Bay does not follow this general pattern, having very few heavy minerals, of which iotite mica is the most abundant.

The major source of the stable minerals in the Rockport area is the Guadalupe River. Hornblende was probably introduced by the Colorado River on the north of the area and was carried south along the beaches or even along the continental shelf by longshore currents (Bullard, 1942).

The studies by Bramlette and Poole show that hornblende is an important mineral on the continental shelf, being widely distributed but most abundant near shore. Hornblende is probably swept into the bays hurricane storm tides and by strong winds. Other abundant minerals on the shelf are black opaques (including leucoxene), epidote, tourmaline, garnet, and zircon.

Roundness studies:
The roundness of grains is dependent on the cleavage which they possess on the amount of abrasion due to transportation. However, the average roundness of a deposit may be strongly influenced by selective transportation of rounded grains. Since quartz is the dominant mineral in most of the sands of the Rockport area, a comparison of the rounding of quartz grains has been used in differentiation of environments. Because rounding is also a function of size, it was decided that grains of only the 1/8-1/16-mm size, which are very abundant in all of the area, would be used for this purpose. The roundness was determined by two methods. One, proposed by Powers (1953), determines an average roundness by comparing grains with photographs and obtaining a geometric mean of the numbers of various gr des of roundness in each sample. The other method devised in part by Beal (Beal and Shepard--in press) consists of photographing the grains and comparing the pictures with the roundness chart of Krumbein (1941), thus removing the difficulty of third-dimensional aspects which comes from looking at the grains under a microscope. Both methods have

End_Page 1505------------------------------

produced similar results, and show a consistent difference in the rounding of grains of the same size in dunes and in other environments (Fig. 30). The dune sands are distinctly more rounded as has been shown in previous studies (MacCarthy,

Fig. 30. Index of rounding in several environments. Dune sands are much more rounded than those of other environments.

Fig. 31. From data supplied by R. E. Grim and Wm. D. Johns, showing relationship of relative abundance of three clay minerals to environments of Rockport area. Montmorillonite, which decreases going down the bays, increases in crossing continental shelf.

End_Page 1506------------------------------

1935). The method also indicates that the sand in the so-called "beach" ridges on both islands has a roundness of dune type even at as much as 7 feet below the surface.

Clay-mineral analysis:
The use of X-ray diffraction analysis and differential thermal analysis serves to show the nature of the clay minerals that are found in various parts of the Rockport area (Grim and Johns, 1955). The ratios of the principal minerals, montmorillonite, illite, and chlorite, are computed in probable parts of ten of the total clay minerals (Fig. 31). When these analyses have been averaged for different environments they show some striking contrasts although they also give a certain amount of overlap, particularly in different parts of the bays. These contrasts are probably related to the changing salinity from the bay heads to marine conditions in the open Gulf. Grim and Johns conclude that these differences are chiefly the result of early diagenesis rather than differential sedimentation. S nce the clay minerals are relatively stable after this period of early diagenesis, these variations can be used in helping diagnose environments of depositions.

Fig. 32. Facies of Foraminifera in Rockport area. Other organisms show similar facies relation.

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Fig. 33. Representatives of assemblages of Foraminifera in various facies of Rockport area. Localities for these facies are indicated in Figure 32, except for outer shelf. Prepared by F. B. Phleger and Frances L. Parker.

End_Page 1508------------------------------

See corresponding image.

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BIOLOGICAL METHODS

One of the most important factors in identifying sedimentary environment is the study of the fauna and flora which inhabit these environments. Many animals are sensitive to temperature, salinity, and water turbulence. Others may be controlled by the type of food material, predation, the bottom sediment, and other physical and chemical variables. Therefore, the shells or tests of the invertebrates, which are commonly found in sediments, constitute valuable clues to conditions during their life.

Foraminifera:
A study of 320 equal-size samples from the bays, the marshes,

Fig. 34. Abundance of Foraminifera in two sizes (combined) as related to environments of Rockport area. Foraminifera of outer shelf are largely pelagic and should, therefore, not be confused by their abundance with Foraminifera in "bay no special influence" environment.

End_Page 1510------------------------------

and the open Gulf (including beaches) has resulted a publication entitled "Ecology of Foraminifera from San Antonio Bay and Environs, Southwest Texas," by Parker, Phleger, and Peirson (1953). Their studies that there are four principal bio-facies consisting of river, marsh, bay, and open Gulf,(FOOTNOTE 10) and two sub-facies, including beaches and the upper portion of San Antonio Bay (Figs. 32 and 33). They also recognize the occurrence of mixed faunas in the lower bays, particularly near the inlets. The relation of floods and of marine invasions during periods of low run-off are considered in connection with these faunas.

In addition to the counts made by the Marine Foraminifera Laboratory at Scripps Institution, the coarse-fraction studies by the present writers have shown some of the gross relationships of the abundance of Foraminifera to other sediment

Fig. 35. Ratio between Foraminifera in ½-¼-mm. to Foraminifera in ¼-1/8 mm. sieve sizes as related to environments of Rockport area. ½-¼ mm. Foraminifera predominate in environments related to barrier islands, whereas smaller Foraminifera are more abundant in bay environments.

FOOTNOTE 10. Various divisions occur also on the shelf but these are still being studied by the Scripps Foraminifera Laboratory.

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constituents in the different environments. These are shown in Figures 34-36. Figure 34 shows the relation of environments to the sum of percentages of Foraminifera in the ½-¼-and ¼-1/8-mm. size fractions. One of the bay environments and the outer shelf have particularly high counts. Figures 35 and 36 show the relationships of the ratios of Foraminifera percentages in different size fractions to the environments. These ratios are also clearly related to environments. An exceptional abundance of Miliolidae was observed in ½-¼-mm. fractions in the barrier island environments. These results are mentioned again discussing environmental characteristics.

Fig. 36. Relationships similar to those of Figure 35 for next smaller sieve. Trend in this comparison is same as previous. Right-hand part of figure indicates that Foraminifera obtained in samples from bay borings are related to bay types with respect to size distribution.

End_Page 1512------------------------------

There can be little doubt that the Foraminifera are one of the most sensitive indicators of environments. This has been found in the Rockport area and in most other areas where marine sediments have been studied.

Ostracoda:
The Ostracoda have been studied by F. M. Swain who has worked largely with collections from San Antonio Bay, the barrier island marshes, and the shallow open Gulf in the vicinity. The report by Swain (1955) gives a sequence of bio-facies which is similar to those described for the Foraminifera. It includes (1) a fluvial and pro-delta facies which is particularly well developed in the low-salinity Mission Lake as well as along the river channels and in some parts of the delta; (2) a bay facies which has the following sub-facies: (a) mid-bay, (b) lower-bay and marginal, and (c) barrier island marsh; and (3) open Gulf facies. Swain shows that the development of these facies is largely dependent on the salinity differences but also on available food supply and perhaps on the type of bottom. wain recognized 47 species, one variety, and one sub-species, and notes that 15 of these are known to occur as fossils, eight of which are present as far back as Miocene. Some of the diagnostic Ostracoda are illustrated in Figure 37. Doris Malkin Curtis has also studied the divisions of the Ostracoda on the deeper shelf. She found that significant breaks in the fauna occur at approximately 20- and 50-fathom depths.

Macro-organisms:
The study of the macro-organisms during the first 1½ years of the project was conducted under the direction of J. Wyatt Durham and for the next 2 years under Joel W. Hedgpeth. Most of the collecting was done by R. H. Parker and E. L. Puffer with some help from W. K. Emerson. A brief paper concerning the mollusks of the oyster-reef biotope of the bays has been published by Puffer and Emerson (1953). A series of subdivisions of the faunal communities was made by Puffer which largely corresponds with that reported earlier by Harry S. Ladd (1951). This has subsequently been expanded and modified by Parker (Figs. 38, 39).

The macro-organisms show the same general relationship to salinity revealed by the Foraminifera and Ostracoda facies. The effect of the recent change of salinity has been pronounced on the macro-organisms causing open Gulf forms, such as echinoderms, corals, and certain Gulf mollusks, to move into the lower bays (Parker, 1955). In turn, a species of pelecypod, Rangia cuneata, and a gastropod Littoridina sphinctostoma, have almost disappeared from the low-salinity waters of San Antonio Bay coincident with the increasing salinity. The Rangias that remain have a smaller than average size which may indicate either that this mollusk can not attain maturity or that its adults are dwarfed in the higher-salinity waters now present in San Antonio Bay (Parker, 1955). During the years 1951-1953, the oyster reefs in Aransas Bay, normally composed of the common bay oyster, Crassostrea virginica, were dominated by the high-salinity Gulf oyster, Ostrea equestris (Puffer and Emerson, 1953, and Parker, 1955). In general, the bay macro-organism assemblages have approximately the same faunal boundaries as those of the Ostracoda and Foraminifera, with the exception of the

End_Page 1513------------------------------

Click to view image in GIF format. Fig. 37. [Grey Scale] From Swain (1955), showing characteristic ostracods in several environments of Rockport area.

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Click to view image in GIF format. Fig. 38 and 39. [Grey Scale] Macro-organisms representative of Rockport environments. Although many names are included in diagram, complete list of organisms by number is appended. Some of the characteristic mollusks of Texas sedimentary environments prepared by R. H. Parker. Scale variable, but paper in preparation will give correct scale for all species shown here.

End_Page 1515------------------------------

Click to view image in GIF format. Fig. 38 and 39. [Grey Scale] Macro-organisms representative of Rockport environments. Although many names are included in diagram, complete list of organisms by number is appended. Some of the characteristic mollusks of Texas sedimentary environments prepared by R. H. Parker. Scale variable, but paper in preparation will give correct scale for all species shown here.

End_Page 1516------------------------------

Fig. 38 and 39. See caption on pages 1515 and 1516.

End_Page 1517------------------------------

oyster-reef assemblage with its characteristic substrate composed of shell, predominantly Crassostrea virginica. The shallow, high-salinity bays near inlets, such as Redfish Bay, also appear to have a distinctive molluscan fauna (mostly small gastropods) which are not reflected in the Foraminifera and Ostracoda bio-facies.

The mollusks on the shallow part of the continental shelf (out to 10 fathoms) in the Gulf of Mexico are similar to those found at present in lower Aransas Bay and Lydia Ann Channel, except for about five to ten species characteristic of the open Gulf. However, the surf zone of the Gulf beach has a distinct population composed of the pelecypod, Donax sp., and two gastropods, Terebra cinerea and Olivella mutica, all of which are capable of boring rapidly into the hard sand close to the beach. On the other hand, the macro-organisms found on the shelf in 10-11 fathoms to at least 60 fathoms are quite distinct and seldom found on the beaches, even after heavy storms (Fig. 39). Echinoderms are also very characteristic of the open Gulf from the surf zone to the shelf edge, although some spec es are found in the inlets where salinity conditions are similar to those of the open Gulf.

Microfossils:
The microfossils were studied from 59 samples obtained in the Rockport area. This study was made by Arthur E. LeBlanc as a master thesis problem under the direction of L. R. Wilson at the University of Massachusetts. These samples covered most of the fifteen depositional environments of the Rockport area and thus give interesting information concerning the environmental patterns of the microfossils. Counts were made of the micro-foraminifera, hystrichosphaerids, fungus spores, non-arborescent spores and pollen, and arborescent pollen in each of these samples. Also a considerable break-down of the species and genera was made. This material is discussed in the thesis by LeBlanc.

In summarizing the pertinent points of the investigation, the following significant correlations appear (Table I).

Table I. AVERAGE NUMBER OF MICRO-FOSSILS PER UNIT VOLUME FOR ENVIRONMENT

End_Page 1518------------------------------

1. The fungus spores are most abundant in and around the Gaudalupe Delta and on the barrier flats.
2. The micro-foraminifera are common in many environments especially in the middle and lower bays where they increase away from the river mouths and on the outer shelf. In the deeper parts of Aransas Bay (Bays, no special influence), however, the micro-foraminifera are distinctly less abundant than in the shoaler bays.
3. On the barrier flats of the islands the non-arborescent spores and non-arborescent pollens reach their highest abundance, and the arborescent pollen is most abundant in the inlets and in the near-shore Gulf.
4. The beach sands are almost devoid of microfossils.
5. The fungus spores and non-arborescent spores decrease outward across the shelf in contrast to the micro-foraminifera, which decrease only out to about 40 feet, beyond which they increase enormously.

The changes in abundance of micro-foraminifera with depth in core are interesting since they show some of the same variations that are indicated by the Foraminifera. Thus the general increase in micro-foraminifera which is indicated a short distance below the bottom in the bay samples is duplicated by the increase of the Foraminifera (p. 1571).

CHEMICAL METHODS

Surface variations:
In using the environmental boundaries established by the coarse-fraction and other methods (Fig. 25), the chemical data in the top 20 cm. of sediment were averaged and compiled into Figure 40. For convenience, water content and grain size are shown in the same figure. This figure indicates the importance of clay-size particles in regulating chemical characteristics of the sediments as shown by the relationships of the organic carbon, pH, Eh (the apparent EMF of the sediment determined with a platinum calomel electrode combination), and water-content curves to the percentage of clay in each of the environments. An increase in clay content is associated with similar increases in water content and organic carbon content which in turn result in lower Eh values and higher pH. An exception to his pattern is caused by the specialized conditions in the "bay near barrier island" environment (FOOTNOTE 11) where a lowering of clay content is not accompanied by a corresponding decrease in organic carbon. The Eh value, on the other hand, does rise considerably. This condition is probably an effect of the shallow, aerated waters surrounding the islands.

The CaCO3 contents, determined manometrically by CO2 assay, are also influenced by clay content, but another variable, the distance from the Guadalupe River, is important causing a steady decrease in CaCO3 with distance from the river (Figs. 41, 42). The reason for the high content in upper Guadalupe Bay and near the delta is that particulate calcium carbonate material is being introduced into the Guadalupe and San Antonio River basis, probably from the Austin chalk and various Lower Cretaceous and Paleozoic formations. This particulate matter was observed in the petrographic microscope examination made by M. N. Bramlette of the samples from San Antonio Bay. The influence of the Guadalupe

FOOTNOTE 11. "Bay near barrier" and "bay near mainland" are subdivision of the "bay marginal" environment of Figure 25.

End_Page 1519------------------------------

River is again shown by low pH values in the "bay near river" environment. This is the only point on the graph at which pH is not a reflection of Eh. Evidently the water introduced by the river (pH 6.62) has a stronger influence on pH than have the relatively low Eh and high carbonate values.

Calcium carbonate is expressed in two ways in Figure 40. One is "visual

Fig. 40. Chemical data, water content, and sand-silt-clay relations of environments in Rockport area. Visual CaCO3 is based on estimates from coarse-fraction analyses. Asterisk indicates subdivisions of bay marginal environments.

End_Page 1520------------------------------

Fig. 41. Percentage of carbonate of samples in Rockport area based on gasometric analysis. High percentages found in and around Guadalupe delta and low percentages in near-shore environments.

End_Page 1521------------------------------

CaCO3" which was obtained from coarse-fraction (greater than 0.062 mm. in size) estimates of percentage of mollusk shells, Foraminifera, Ostracoda, and echinoid fragments in the various environments. The other is the gasometric chemical analysis of the carbonate constituents of the sediments. In general the larger shells were picked out as much as possible before making the chemical analyses so that the result only partly reflect the importance of the macro-organisms in the different samples, and show largely the calcium of carbonate of the finer material.

It is notable that the two curves more nearly coincide in the environments where river-borne clay constitutes a small percentage of the sediments ("bays near mainland" and "bays near barriers"), and in the open Gulf environments far from a river source. The quantities of carbonate shown by chemical analysis in these environments must be partly the result of "visual carbonate." A rough estimate of what may be primarily inorganic CaCO3 can therefore be obtained by subtracting the visual from the gasometric CaCO3 values. In order to eliminate the effect of such organisms as Foraminifera, Ostracoda, echinoids, and the smaller mollusks, the coarse fraction was entirely removed from a series of samples so that the carbonate analysis was made of the fine fraction alone. The results of the two methods seem to indicate approximately the same thing, as can be

Fig. 42. Percentages of CaCO3 in silt and clay fraction related to environments. Slight increase shown in samples of outer shelf (see also Fig. 43).

End_Page 1522------------------------------

Fig. 43. CaCO3 content in parts per thousand on shelf off central Texas area. Underlined figures are from Trask (1953). General increase in CaCO3 content with increasing distance from shore.

End_Page 1523------------------------------

seen by comparison of Figure 41, which shows the percentage of carbonate from the entire sample in all but the offshore area, with Figure 42 showing percentage of carbonate from the silt and clay fraction only. Both figures show that the calcium carbonate is very high near the Guadalupe Delta, decreases down San Antonio Bay, and remains fairly constant in Mesquite and Aransas bays except near the two inlets where it is very low. Similarly, the calcium carbonate is low in most of the continenntal shelf sediments, largely less than 5 per cent (Fig. 43). The values show a decided increase on the outer shelf, reaching up to 16 per cent in the analyses by the writers and 31 per cent in one of the values obtained in the same area by Trask (1953).

It may be noted that the lowest values of calcium carbonate are found in some of the inlets and on the "barrier flats" of the islands. Furthermore, most of the open Gulf samples have a lower percentage than the bay samples. The exceptions to this rule are found in some samples in the "bays near inlets" or on the bay slopes near the barrier islands. The calcium carbonate is distinctly low on the southeast side of San Antonio Bay, suggesting the influence of an inlet, although in this case the inlet has an indirect connection to the open Gulf through Espiritu Santo Bay.

Continuity of pH with depth:
In addition to the surface measurement of pH, readings were made at various depths in cores from four of the major environments. Figure 44 shows that there is a distinct differences between the river sediment and the group from the bays and open Gulf. It is notable that all core depths the readings from bays are less than those of the open Gulf. There is only a small separation between bay and open Gulf environments. The gradation from river to open Gulf is reasonable inasmuch as the river water has a very much lower pH than has the Gulf water, and therefore an intermediate pH should be the average of the bays.

Spectrochemical analysis:
One of the most successful chemical methods of differentiating sedimentary environments has been the analysis of spectro-chemical data. Four long cores from the Guadalupe River, San Antonio Bay, Aransas Bay, and the open Gulf of Mexico, 20 miles offshore (Fig. 44), have been subjected to these analyses at representative depths in the core. The oxides, K2O, Na2O, TiO2, Fe2O3, and Al2O3, have been plotted against depth in core and compared with curves of clay content from these same cores.(FOOTNOTE 12) It was found that the oxide curves generally were of the same shape as that of the clay content and must therefore be related to the clay minerals in the sediments.

A small increase in K2O and a significant decrease in Al2O3 are notable as the stations increase in distance from the river source. The ratio of Al2O3/K2O, plotted against depth at the four localities (Fig. 44), is apparently a valuable criterion for determining relative distance from the source. It is of significance that the values of this ratio are relatively constant with depth at any one of the localities.

FOOTNOTE 12. The relation to depth in core is discussed mostly under Diagenesis pp. 1578-91; see also Figures 70-72.

End_Page 1524------------------------------

Fig. 44A. Locations of samples for chemical and spectrochemical analyses. Variation with depth in core indicated in lower diagrams.

Fig. 44B. Variations of Al2O3/K2O with environment and continuity with depth.

Fig. 44C. Variations of pH with environment and continuity with depth.

End_Page 1525------------------------------

Evidently the spectrochemical analysis of these sediments gives good empirical evidence of the relative distance from their source. This may be related to rates of deposition of other physical factors.

BACTERIOLOGICAL METHODS

The main emphasis in the bacteriology study by Contois has been placed upon an attempt to establish relationship between environmental characteristics, as defined by the bacterial groups and geochemical parameters of these sediments. It is hoped that some of the latter may be established thereby as meaningful criteria of environmental characteristics and processes.

A statistically significant correlation (r ^ne 0.78, P < 0.01) exists between terrestrial influence in environments as measured by the ratio of terrestrial bacteria to marine bacteria (log(1000×TB/MB)) and the ratio of potassium to sodium (log(10×K/Na)) of total sediment samples. This relationship, shown in Figure

Fig. 45. A. Correlation between terrestrial influence and sodium-potassium ratio. B. Partial regression of potassium-sodium ratio upon montmorillonite-illite ratio. C. Partial regression of depth in sediment upon montmorillonite-illite ratio.

End_Page 1526------------------------------

45A, was established from the study of cores respectively from the Guadalupe Delta (G15), middle San Antonio Bay (S334), Aransas Bay (A190), and the offshore Gulf (J72) (Fig. 44). Samples were taken from the 0, 15, 30, 60, 120, and 240-cm. depths within the cores. These parameters apparently are not correlated with depth or time of burial of the sediment. Hence, the possibility is presented of employing ratios of potassium to sodium as a means of determining terrestrial influence in older sediments at the time of their deposition.

Further analyses of these and other data provided by Grim and Johns indicate that the ratio of montmorillonite to illite (log(10×M/I)) in these sediments can be expressed as a function of terrestrial influence and depth or (more probably) of time of burial of the sediment. These results indicate that the relative abundance of the two clay minerals at time of deposition in one locality is controlled by terrestrial influence. However, once deposited, the montmorillonite undergoes a slow transformation into illite. Figure 45B shows the regression of potassium/sodium ratios upon montmorillonite/illite ratios, independent of depth. Figure 45C depicts montmorillonite/illite ratios, independent of terrestrial influence, as a function of depth. In each instance, the variable of depth is mployed in the form e-D where e is the base of natural logarithms and D is the depth in decimeters. If it can be shown that the transformation of montmorillonite to illite is a function of time, it may be possible to utilize montmorillonite/illite ratios as a means of determining deposition rates.

Fig. 46. Relations between bacterial activities and Rockport environments.

End_Page 1527------------------------------

An inverse correlation has been found between lipolytic activity and proteolytic activity of samples. This relationship also apparently holds in various localities in the Rockport area as is illustrated in Figure 46. In this figure, the relative activity was obtained by averaging the results of all analyses of samples from the 0-10-foot depths in the various localities.

An inverse correlation between lipolytic activity and saccharolytic activity as well as a positive correlation between accharolytic activity and proteolytic activity apparently is indicated also (Fig. 46). In the latter two cases, however, no statistically significant relationship was found between these parameters in individual samples.

The full significance of these relationships is not clear at this time but the data indicate fundamental differences between early diagenesis of the organic matter in certain of the localities.

DISTINCTIVE MODERN ENVIRONMENTS

ENVIRONMENTS OF ROCKPORT AREA

The sedimentary environments which have been recognized in the Rockport area, largely through the coarse-fraction studies, are indicated in Figure 25. In addition to the delta, eight distinct subdivisions have made of the bay environment, four of the barrier island environment, and three of the continental shelf. It is not yet known whether all of these sixteen environments can be recognized elsewhere along the Texas coast, but there are indications that some of the subdivisions are general, not only along the Texas coast but in many similar areas including most of the Gulf from Texas to Florida.

Some of the environments in the Rockport area have gradational rather than exact boundaries. The characteristics of each sediment sample within any one of the subdivisions are not exactly the same but in most respects they are remarkably similar. Although the subdivisions have been based to a very large extent on the relative abundance of the constituents of the coarse fractions (Figs. 27, 28), some use also has been made of physical and biological characteristics of the sediments in making these subdivisions. It may be noted that the subdivision are not the same as those suggested for different classes of organisms (Fig. 32), nor do they conform exactly with those suggested by the size parameters (Fig. 24), although actually the various systems of subdividing the Rockport area have m ch in common. In the following description of the sixteen sedimentary environments, the characteristics which are related to size analyses, and to mineral and organic composition, are included where they appear to be of significance. It is suggested that in reading the following description of the environments frequent comparison be made to Figures 26-28.

DELTA ENVIRONMENT

The Guadalupe Delta sediments have not been studied as much as those of the other environments but the size analysis of twenty-one samples and the

End_Page 1528------------------------------

Acknowledgments:

(2) Scripps Institution of Oceanography.

Copyright 1997 American Association of Petroleum Geologists

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