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Shell Development Company, Exploration and Production Research Division, GEORGE M. GRIFFIN Box 481, Houston 1, Texas

Regional Clay- —Products of Intensity and Current Distribution in the Northeastern Gulf of Mexico

Abstract: Three major rivers supply most of the tions in their clay-mineral suite which are of too clay-mineral detritus that the northeastern Gulf small a magnitude to affect significantly the gross of Mexico receives. The of the clay regional distribution pattern. supplied by each river is a product of the weather- Within the Gulf of Mexico, that portion of the ing versus parent- interplay in the drainage clay not flocculated by saline water at the river basins. In the western drainage basins, and mouths is distributed first by the wind-driven transportation of essentially unaltered mont- shallow water currents and then by the semi- mcrillonitic prevails. Eastward, weath- permanent oceanic currents. A gradational facies ering becomes more effective, and kaolinite gradu- pattern is developed in which the sources of supply, ally becomes more abundant m the soils and river their magnitudes, and the distributional directions clays. Consequently, the Mississippi River is are clearly evident. contributing a montmorillonitic clay-mineral suite, Clay-mineral distributional patterns m the and the Apalachicola River is contributing a modern Apalachicola River area are similar to kaolinitic suite. The Mobile River, between these those in the Texas lower Eocene (Wilcox) sedi- two rivers, is contributing an intermediate clay- ments. Similar weathering and current factors may- mineral suite. The river sediments pass through the have produced the analogous clay-mineral facies various bavs and estuaries with onlv minor altera- patterns.

CONTENTS

Introduction 738 Quantified offshore clay-mineral facies . . . 755 Acknowledgments 738 Rate of m the northeastern Gulf Preparation of samples 738 of Mexico and the rate of alteration of clay Standard preparation 738 757 Insoluble residues 740 Chlorite-containing 758 Supplementary treatments 740 Suggested applications of clay-mineral-distribution Identification of clay minerals 740 data to geologic problems 763 General statement 740 Limitations 763 Montmorillonite 741 Mapping of transgressive and regressive facies . 763 Vermiculite 741 Summary and conclusions 763 Chlorite 741 References cited 766 Kaolinite 741 Illite 741 Figure Gibbsite 742 1. Northeastern Gulf of Mexico showing the area Precision ot peak-height ratios 742 included in this study 739 Soils of major northeastern Gulf Coast drainage 2. Soil-clay analyses within major river basins . . 743 basins 747 3. Sample locations in northeastern Gulf of Mexico 744 Northeastern Gulf of Mexico 748 4. Sample locations in Mississippi Delta area . . 745 Introduction 748 5. Sample locations in Apalachicola Bay area . . 746 Clay minerals in rivers contributing to the north- 6. Derivation and distribution of Mississippi River eastern Gulf of Mexico 748 clay 747 Major rivers 748 7. Derivation and distribution of Mobile River Rivers of peninsular Florida 749 clay 748 Clay-mineral-facies relationships 750 8. Derivation and distribution of Apalachicola Offshore-distribution mechanism 750 River clay 748 General offshore clay-mineral-facies pattern . 755 9. Comparison of suspended sediment loads and

Geological Society of America Bulletin, v. 73, p. 737-768, 19 figs., 1 pi., June 1962 737

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clay mineralogy of Mississippi, Mobile, and 19. Distribution of clay minerals in early Eocene Apalachicola rivers 749 (Wilcox) time, Henderson to Sabine coun- 10. Approximate calcium carbonate content of ties, Texas 765 some northeastern Gulf sediments . . .751 Plate Facing 11. Surface currents in the Gulf of Mexico—June 753 1. Clay-mineral distribution, northeastern Gulf of 12. Working curve obtained by artificially mixing Mexico 764 Mississippi River and Apalachicola Bay Tables clays 754 1. Approximate clay-mineral composition of major 13. Clay-mineral facies map for northeastern Gulf rivers of the northeastern Gulf of Mexico 750 of Mexico 756 2. Discharge and load data for major rivers tribu- 14. Oscillation heating diagram for WH 1 .... 759 tary to the northeastern Gulf of Mexico . 750 15. Chlorite distribution—Group I 760 3. Transport of sediment by oceanic currents. . . 752 16. Chlorite distribution—Group II 761 4. Clay-mineral composition of rivers of the north- 17. Chlorite distribution—Group III 762 eastern Gulf of Mexico 755 18. Distribution of clay minerals in early Eocene 5. Postglacial sedimentation in the northeastern (Wilcox) time, Bastrop County, Texas . 764 Gulf of Mexico 757

T. Davidson, and A. R. Dahl of Iowa State INTRODUCTION Engineering Experiment Station allowed the This investigation is intended to clarify the writer to examine some of their Iowa Loess principal factors governing the regional distri- samples. J. J. Griffin of Scripps Institution of bution of clay minerals1 in a "mediterranean" Oceanography furnished a sample from the type of depositional basin. For this purpose, Ohio River. the northeastern Gulf of Mexico south to the Discussions with R. A. Rowland, J. F. Burst, latitude of Key West has been studied, along Hugo Steinfink, R. G. Stevenson, and C. E. with the principal rivers, bays, and beaches Weaver of Shell Development Company; W. fringing the area (Fig. 1). The writer believes F. Bradley, Consultant to Shell Development that the principal conclusions presented will be Company; and R. L. Ingram of the University applicable to many problems dealing with of North Carolina were of great help with re- ancient coastal-plain sediments; however, he gard to mineral problems. J. J. W. Rogers of cautions against a "blanket" extrapolation to Rice University and Gordon Rittenhouse of all types of sedimentary basins. It should be Shell Development Company assisted the pointed out that the sediments examined did writer with statistical phases of the report and not include the true "red clays" and other critically reviewed the manuscript. In addition, pelagic sediments of the deep ocean basins. R. J. LeBlanc and R. H. Nanz of Shell De- velopment Company and J. A. S. Adams of ACKNOWLEDGMENTS Rice University reviewed the report, as did a The writer acknowledges the assistance of number of others listed. the following organizations and persons in the D. B. Speights, W. D. Gregory, J. M. preparation of this report: Braunagel, and J. R. Guinn of Shell Develop- Shell Development Company allowed the ment Company assisted the writer in the use of material from a report prepared during sampling and analysis. J. F. Burst, B. S. Parrott, 1955-1958 for its Exploration and Production and H. B. Stenzel of Shell Development Com- Research Division. Rice University accepted a pany also aided in the sampling and contributed modified version of the same report as a to the writer's understanding of the Doctoral Thesis in the Department of Geology. and mineralogy of the Gulf Coast. Mrs. Jane Admiral E. H. Smith and Mrs. Nora G. Moore, Mrs. D. E. Groetsch, and Miss A. M. Fairbank of Woods Hole Oceanographic Insti- Gondolofo of Shell Development Company tution, and B. S. Parrott, C. F. Major, H. A. drafted the illustrations. Bernard, R. N. Ginsburg, and Reynolds Moody of Shell Development Company fur- PREPARATION OF SAMPLES nished many of the samples. R. L. Handy, D. Standard Preparation 1 The term "clay" refers to the size fraction composed Samples were received in several different of particles of less than 2 microns (0.002 mm) in equiva- forms. Those taken specifically for this project lent spherical diameter. "Clay" is usually composed of quartz, calcite, feldspar, and other common minerals were packed in 1-quart glass jars, with any together with a large percentage of distinctive minerals space remaining above the sediment filled at normally concentrated only in very fine-size fractions— the time of sampling with water pumped from the "clay minerals." The distribution of these distinctive the bottom of the gulf, bay, or river sampled. "clay minerals" is discussed here. Thus, these samples arrived in the laboratory

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in essentially the same state as found in nature. complete reaction with the carbonate present, Most samples borrowed from others were there would be no excess H+ to act on the clay dry when received; many had been in storage minerals. Consequently, acetic acid was selected for several years. This storage and drying did as the reagent and diluted 1:4 with de-ionized not appear to affect the clay-mineral properties water. This dilute acid was then allowed to investigated here. trickle at a. rate of about one drop every 5 A portion of each sample was placed in a seconds onto a sample of carbonate sediment 1-liter beaker with de-ionized water, and a weighing several grams. For convenience, the Selas Bacteriological Filter (#03) was in- sample was placed in a vacuum-filter funnel of serted. De-ionized water was periodically added ultrafine porosity, and reacted liquid was with- to the beaker and continuously extracted by drawn at about the same rate at which acid was the vacuum filter until the filtrate no longer added. Several days usually were required for gave a positive test for the chloride ion. This all carbonate to react and effervescence to cease. treatment generally was sufficient to disag- As soon as the reaction ceased, the acid supply gregate the samples, and chemical dispersing was stopped and the sample was flushed with agents were seldom needed. Particles larger de-ionized water. Thorough washing was neces- than approximately 2 microns were then re- sary to prevent the crystallization of extraneous moved by centrifugation. Part of the superna- acetates which otherwise appeared on subse- tant clay suspension was poured into a 100-ml quent X-ray patterns of the clay. A centrifuge beaker with a glass slide on the bottom. Slow size separation was then made, and the clay evaporation of the water at temperatures less was sedimented on a glass slide. Samples pre- than 35° C under a bank of infrared lights pared by this method gave results which did allowed an oriented film of clay to collect on not deviate in gross appearance or X-ray peak- the slide. Thickness of the clay film was not intensity ratios from unacidified samples. controlled precisely, but slides too thick to Changes in the exchange ion complex probably pass transmitted light or too thin to produce occurred but were not investigated. good X-ray patterns were rejected. As a by-product of the acid treatment, Slides were then mounted in a standard simple weighing before and after dissolution Norelco wide-angle diffractometer with geiger allowed an approximate value for "per cent counter and subjected to nickel-filtered copper carbonate in the total sediment" to be cal- radiation at 40 KV and 18 MA. Scanning speed culated. was normally 1° 26 per minute. Rate-meter settings were varied according to the diffraction Supplementary Treatments intensity; settings of 4-1-4, 8-1-8, and 16-1-8 Subsequent to the initial X-raying, several were commonly used. additional treatments commonly were made to Excellent X-ray patterns resulted from this better characterize the clay minerals. All technique, comparable with those from ancient samples were glycolated by the Brunton (1955) Cenozoic and Mesozoic rocks of the Gulf Coast. method, which was based on the fundamental Many of the earlier complaints of "poorly work of Bradley (1945). Many samples were crystalline Recent clays" may have resulted subjected to various heat treatments generally from incomplete sample preparation, particu- made on the same slides used for glycolation. larly with regard to salt removal. For static heatings, the slides were inserted into a pre-heated oven for 1 hour at temperatures Insoluble Residues ranging from 100°C to 800°C. A Foxboro Special problems arose in connection with Potentiometer Controller was used to main- samples which contained excess calcium car- tain the furnace at constant temperature. The bonate. It was necessary to prepare insoluble oscillation heating technique of Weiss and residues of these samples, and a variation of the Rowland (1956a; 1956b) was also employed usual techniques for preparing clay residues occasionally to good advantage, and differ- was used. It has long been considered that ential thermal analysis was sometimes used. severe treatment of this type, especially if HC1 is used, may cause undesirable alterations in IDENTIFICATION OF susceptible clay minerals. The agent supposed CLAY MINERALS responsible for these alterations is the H+ ion from acid in excess of that necessary to react General Statement with the carbonate. It was therefore considered Most clay minerals crystallize in the same that if only enough acid were added to allow general structural scheme so that overlapping

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properties and reactions result; as in any field Intensification of the 14-A peak accompanie^s of science under active development, differ- dehydration, with a slight shift of the 14-A ences of opinion have arisen as to classification peak toward lower d spacings. Thus, if a peak and identification. To avoid misunderstanding, at about 14 A was observed after heating to it seems necessary to specify the methods used 600° or 700°C for 1 hour, it was attributed to for the identification of the various clay chlorite. Concomitant with the increase in in- minerals. It is also necessary to keep in mind tensity of the 14-A reflection, the 7-A peak of that in geological clay studies some rather com- chlorite migrates slightly toward higher d plex mixtures of clay minerals are encountered, spacings and vanishes. and identification tests may fail to yield unique solutions. Kaolinite A regular basal sequence of 7.2-A, 3.56-A, Montmorillonite and 2.38-A peaks plus a collapse to a non- Montmorillonite is used here as a group term diffracting state after heating to 550°-600°C for clay minerals which swell to approximately for 1 hour (Brindley, 1951; Richardson, 1951) 17 A upon being subjected to ethylene glycol generally suffice for the determination ot vapor at 60° C for 1 hour (Bradley, 1945; kaolinite. Unfortunately, this basal sequence Brunton, 1955). It is also considered that the and temperature of collapse is very similar to montmorillonite lattice will collapse irreversibly that of many chlorites, so that difficulties are if heated above approximately 300° C (Mac- encountered in chlorite-kaolinite mixtures. Ewan, 1951). After being heated at 300° C, it The kaolinite (003) peak at 2.38 A is, how- will not rehydrate with water alone but can ever, considerably more intense than the peak still be made to swell to 17 A with ethylene of the same spacing from chlorite and often glycol vapor, as mentioned by Bradley (1945). may be used to show the presence of kaolinite. No attempt has been made to distinguish be- Splitting or broadening of the 3.56-3.6-A peak tween individual minerals in the montmoril- normally suggests the presence of both chlorite lonite group, as their distinction by X-ray dif- and kaolinite. Also, the distinctive chlorite fraction is nearly impossible in complex mix- (001) peak at about 14 ^A and the chlorite tures. (003) peak at about 4.75 A may often establish the presence of chlorite in mixtures. There are Vermiculite many samples, however, in which it is not pos- Considerable difficulty was encountered in sible to prove directly the presence of kaolinite trying to isolate properties of vermiculite that if chlorite is admixed. would allow its positive identification in the In the present paper, kaolinite has been presence of both chlorite and montmorillonite. identified by a combination of what might be The only suitable test when vermiculite is a called "default and inference". By default, if minor constituent seems to be based on thermal a 14-A peak is not developed after heating the reactions, as monitored by X-ray diffraction. samples to 600°-700°C, chlorite is considered Vermiculite is known to regain part of its inter- to^be an unimportant constituent, and the layer water after being heated to 400° or 500°C 7-A peak is attributed principally to kaolinite. (Walker, 1951) whereas montmorillonite and By inference, the Apalachicola and Mobile chlorite do not. Therefore, any evidence of rivers are contributing clays sufficiently rich rehydration indicated by movements of the in kaolinite that identification of the mineral basal peaks after heating to 400° or 500°C is rather certain. The 7-A component can be should be attributable to vermiculite. To seen to diminish gradually away from the check this phenomenon, samples were heated mouths of these rivers. Thus in the absence of to 400°-500 for 1 hour, quickly removed to contrary evidence, the 7-A peak is assigned the X-ray diffractometer while hot, and mainly to kaolinite in the zone of gradual de- scanned rapidly several times while air of 100 crease. per cent relative humidity was introduced into It is not possible to differentiate kaolinite the sample chamber. If basal-peak shifting was from several other members of the kaolinite noted, it was attributed to vermiculite. group unless the clay is nearly monomineralic. Therefore, kaolinite, as used here, refers to the Chlorite kaolinite group of clay minerals. Weiss and Rowland (1956a) have shown that chloritic clay minerals dehydrate at variable Illite temperatures beginning at about 550°-650°C. The group name illite is used here for clay

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minerals exhibiting an integral sequence of 15-A (montmorillonite) /7-A (kaolinite) peak- basal reflections at approximately 10, 5, and height ratio was used. 3.3 A. Ethylene glycol produces no lattice ex- Commonly it was necessary to use peaks not pansion, and heat treatment up to 700°C (the entirely free from the effects of other minerals. highest generally used here) produces only a For example, clay-size chlorite flakes probably sharpening of the basal reflections. contributed somewhat to the 7-A peak used as In some instances it was difficult to decide a kaolinite indicator. Difficulties of this type whether "illite" or "mica" was the more ap- are inherent in all work dealing with natural propriate term. Generally, illite is character- clay-mineral mixtures and are caused by over- ized on the X-ray pattern by somewhat diffuse lapping X-ray diffraction peaks from materials reflections, that is, by broad, ragged basal with generally similar structural schemes. peaks. Howe ver, o in the Apalachicola River In the measurement of peak heights for com- samples, the 10-A and other basal peaks be- putation of ratios, some variability was noted come more intense and sharpen as progressively between runs, even though the mounted coarser fractions of the same sample are ex- sample was left undisturbed in the goniometer amined. The increase culminates in very well- holder since the previous runs. Such variation defined muscovite flakes several millimeters in is due to a composite of (1) lack of electronic diameter that produce an X-ray pattern similar precision in the X-ray circuits, (2) human fail- to that of muscovite from Southern Appal- ings in measuring the peak heights, and (3) achian pegmatites. This 10-A material was slight changes in external conditions such as quite arbitrarily called "illite" in the clay relative humidity. Collectively they may be fraction (< 2 microns) and "muscovite" when referred to as "rerun" errors, as they become present as coarse flakes in the sands. apparent when a sample slide is rerun several successive times without disturbing its position. Gibbsite A series of reruns was made of typical Gulf Gibbsite was identified by a peak at 4.83 A Coast sediments to evaluate the magnitude of on the X-ray patterns. The identification was this type of error. Computation at the 95 per spot-checked by differential thermal analysis cent confidence level indicates than an un- which shows a diagnostic endothermic reaction certainty of approximately ± 10 per cent may at 320°-330°C when gibbsite is present. As be introduced into measured 15-A/7-A ratios with kaolinite, it was sufficiently abundant in by these "rerun" errors. Apalachicola and Mobile river samples that its Uncertainty also arises from the small sample presence there is almost certain. In many off- size ordinarily used. Typically, from a 1-quart shore samples, weak to very weak 4.83-A peaks sample of mud, a subsample of 50-100 cc was are also tentatively assigned to gibbsite. The extracted. When the slide from this subsample peak is at a rather unique location in the X-ray was X-rayed several successive times, an av- diffraction spectrum, although it must be erage value was determined which was nearly measured rather carefully to be distinguished free of "rerun" errors. When the average value from adjacent 4.75-A chlorite and 5.00-A illite from this subsample was compared with the peaks. Also, it is possible for some mixed-layer average value from other similarly treated sub_- combinations to give peaks at 4.83 A. samples from the same sample, the 15-A/7-A ratio usually varied slightly. Comparison of PRECISION OF average subsample values for a number of PEAK-HEIGHT RATIOS typical Gulf Coast sediments allowed compu- In several sections of this paper, peak-height tation of the "subsample" error which at the ratios of the various clay minerals have been 95 per cent confidence level amounted to an used as semiquantitative indicators of relative approximate variation of + 10 per cent in the abundance. These ratios were obtained by 15-A/7-A ratio between subsample means. measuring the heights of basal peaks character- As the "rerun" and "subsample" errors can istic of the common clay minerals. For kaolinite either cancel each other or be additive, a total the 7-A peak=was used, for montmorillonite uncertainty from these two causes of as much either the 15-A or 17-A peak, and for illite the as + 20 per cent of the 15-A/7-A ratio re- 10-A peak (for glycol-expanded specimens). sults. It is, of course, desirable to reduce this To compare, for example, the relative abun- uncertainty as much as possible; therefore, in dance of montmorillonite to kaolinite, the future studies, it is strongly recommended that

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NAUTICAL MILES Figure 5. Sample locations in Apaiachicola Bay area

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several reruns be made of each slide and that weathering in the southern and southeastern the resulting peak-height ratios from the re- states where it is the dominant clay mineral. runs be averaged. This procedure will eliminate (2) Montmorillonite is the dominant clay most "rerun" errors and thereby reduce the mineral in the parent "rocks" (mainly Pleisto- total error to about half of what it is in this cene loess) of the western Mississippi River study. basin soils. It is passed into the soils and pre-

MISSOURI RIVER (NEAR ST. CHARLES, MO.)

SOUTHWEST PASS

Figure 6. Derivation and distribution of Mississippi River clay (salt-free, <2 micron fraction)

served there because of ineffective weathering. SOILS OF MAJOR NORTHEASTERN (3) Illite is present in subordinate amounts GULF COAST DRAINAGE BASINS throughout the Mississippi basin but is the The clay minerals being contributed to the dominant clay mineral in soils east of the Mis- northeastern Gulf of Mexico are derived prin- sissippi River and north of the Ohio River. It cipally from soils within the major river basins. is derived principally from illitic Pleistocene A survey of available literature was made and tills. a map prepared (Fig. 2) which shows that three It is important to note the gradual increase main contrasting soil types have to be con- in kaolinite in an easterly direction in the soil sidered: kaolinitic, montmorillonitic, and illitic. clays and the reciprocal decrease in montmoril- Conditions producing these soil types may be lonite. These features are a product of the summarized as follows: (1) Kaolinite is the gradually intensifying chemical weathering product of intense, present-day lareriric conditions eastward.

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NORTHEASTERN GULF OF MEXICO with the regional distribution of clay minerals, only factors affecting the gross distribution Introduction will be discussed. Some very interesting but The area studied is shown on Figure 1, and small-scale effects, such as size sorting and locations of most of the 600 samples are indi- near the fresh-water-salt-water in- cated on Figures 3, 4, and 5. All bottom terface, will be discussed in another paper. samples were taken from the upper 10 cm of

TOMBIGBEE RIVERIVEIR W .AL.ABAMA RIVER (JACKSON ALA.) \ ' / (C(CLAIBORNEl , ALA.) ^MOBILE RIVER' CHATTAHOOCHEE RIVER FLINT RIVER (STEAM MILL, CA.) (BAINBRIDGE, GA.)

iLACHICOLA RIVER (NEAR BEVERLY, FLA.)

MOBILE BAY LOWER PART

ST. GEORGE ISLAND BEACH

SHELF. (115 FT) "BASE OF SLOPED (5976 FT.I wHiei UPPER SLOPE Figure 7. Derivation and distribution of (610 FT.) Mobile River clay (salt-free, <2 micron fraction)

Figure 8. Derivation and distribution of sediment to eliminate complications of varying Apalachicola River clay (salt-free, <2 time horizons; however, in areas of very slow micron fraction) deposition, or scour, pre-Recent sediment may occasionally have been penetrated. This depth generally should not have included layers that Clay Minerals in Rivers Contributing Ewing and others (1958) have shown to be to the Northeastern Gulf of Mexico Pleistocene turbidity-current deposits. Major rivers. The type of clay minerals con- Three major rivers contribute most of the tributed by each major river is controlled quite clastic sediment to the eastern Gulf—the Mis- definitely by the type of clay minerals avail- sissippi, Mobile, and Apalachicola. To under- able in the soils of its watershed. Figures 6, 7, stand the distribution of clay minerals in the and 8 show portions of X-ray patterns (3°—14° Gulf, the type and magnitude of clay minerals 26, copper Ka radiation) of random clay being introduced by these rivers was studied. samples from the main tributary streams of the As the present paper is concerned principally Mississippi, Mobile, and Apalachicola rivers;

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they also indicate the downstream distribution in amount and type of clay supplied by these of these clays. rivers. The drainage systems are compared in In general, there is an easterly increase in the Table 2, and in Figure 9 the suspended loads proportion of kaolinite (black peaks on fig- are dissected according to approximate clay- ures) relative to montmorillonite (striped peaks mineral composition. Only the 30 per cent of on figures). The westerly, montmorillonitic end the Mississippi's load believed to enter the member sampled was the Missouri River (Fig. eastern Gulf (Scruton, 1956) is considered in 6), and the easterly kaolinitic end member Figure 9. It is apparent that the richly mont- sampled was the Flint River (Fig. 8). morillonitic clay from the Mississippi River

SUSPENDED COMPOSITE CLAY SEDIMENT LOAD OF MAJOR RIVERS APALACHICO KAOLINITE MOBILE CHLORITE

MOBIIE RIVER

APAIACHICOIA RIVER

Figure 9. Comparison of suspended loads and clay mineralogy of Mississippi, Mobile, and Apalachicola rivers. Relative amounts of suspended sediment contributed by the three major rivers is shown on the left considering only that part of the Mississippi's load believed to enter the northeastern Gulf of Mexico. The clay-mineral composition of the suspended load is shown in the center; the area of the circles is proportional to the suspended load contributed. If clay from the three rivers is combined in proportion to the suspended load, a clay composition similar to that on the right will be obtained.

An important departure from this regional should dominate the clay mineralogy of the trend will be noted in the Ohio River sample eastern Gulf, whereas the Mobile and Apa- (Fig. 6). There the clay-mineral suite is com- lachicola rivers can act only as local modifiers. posed principally of illite and chlorite, a re- Rivers of peninsular Florida. The clay- flection of illitic and chloritic soils in its basin. mineral contribution from rivers of the Florida Small departures from the regional trend in Peninsula is small and becomes rapidly ob- the ratio of montmorillonite to kaolinite were scured within the richly calcareous offshore noted in clays from a few small Gulf Coast sediments (Fig. 10). No suspended clay could rivers (Pearl, Pascagoula, Chipola rivers). The be recovered from the Suwanee and Withla- character of the clay minerals in these smaller coochee rivers, and only a small amount of streams may be relatively more influenced by clean sand could be found on the river bottoms local bank slumping of unindurated Cenozoic in scattered locations. These rivers derive much sediments and other "accidents" than is the of their water from giant clear springs (Fergu- case with the major streams. These departures, son and others, 1947), and little clay-mineral because of their limited quantity, have little matter is furnished by the predominantly car- influence on the type of clay minerals being bonate sediments in the drainage basins. introduced into the Gulf of Mexico. Farther south, the estuaries of the Peace and The approximate percentage composition of Caloosahatchee rivers contain bottom muds the < 2 micron clay-mineral fraction being in- rich in montmorillonite and similar to the Mis- troduced into the Gulf by the three major sissippi River-type clay but with less illite. Off- rivers is given in Table 1. shore samples in this area are very rich in car- It is very important to note the inequality bonate, necessitating that insoluble residues be

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TABLE 1. APPROXIMATE CLAY-MINERAL COMPOSITION OF MAJOR RIVERS OF THE NORTHEASTERN GULF OF MEXICO

No. of Per cent Per cent Per cent River samples montmorillonite kaolinite illite Other clay minerals

Mississippi 68 60-80 10-20 20-30 Mixed layer montmorillonite/chlorite (minor) Mobile 5 40-50 40-50 0-5 Gibbsite (minor) Vermiculite ? (minor) Chlorite (trace) Apalachicola 77 0-20 60-80 0-5 Gibbsite (minor) Vermiculite (0-10) Chlorite (trace)

used for clay-mineral study. In the only sample quartz spheres can be transported by current available from which enough insoluble material velocities of less than 0.1 cm/sec. Clay-mineral could be recovered for clay-mineral determina- particles may be suspended even more easily tion (S 6339, 84 per cent carbonate), from 8 because of their flaky shapes. Kuenen (1942) miles off the mouth of the Caloosahatchee computed the time needed for a particle to River, the clay was generally similar to that in settle 1000 m and the horizontal transport the Caloosahatchee Estuary. This river has been effected while the particle is settling to that connected to Lake Okeechobee by canal for depth (Table 3). more than 50 years, and the influence of this He concluded (p. 44), on the clay suite is not known. "... As for flakes of clay of IM and less there is no Clay-Mineral-Facies Relationships limit to the distance of transport, for they could be carried three times around the earth while drop- Offshore-distribution mechanism. Once the ping the first 1000 meters. It is of the greatest im- clays have been delivered to the Gulf, their portance when considering problems of sedimen- further distribution is governed by the off- tation ... to bear in mind the great distance smaller shore-current regime; thus, a brief review of particles may travel on their way to the bottom. the transporting ability of currents is pertinent. According to Hjulstrom (1939), clay-sized Other workers, in particular Murray and

TABLE 2. DISCHARGE AND LOAD DATA FOR MAJOR RIVERS TRIBUTARY TO THE NORTHEASTERN GULF OF MEXICO Figures in parentheses are approximations computed from published data.

Average Average River system Drainage area discharge suspended Suspended (sq mi) (mill gal/day) sediment cone. sediment load (ppm) (mill tons/yr)

Mississippi (total) 1, 243,600 1 309,000** 250** 213** Directed to east- ern Gulf* (92,700) (64) Mobile 43,600tt 39,300 tt 48tt (5) Apalachicola 18,800ttt 14,000*** (80) 3ttt

* Approximately 25-35 per cent of the Mississippi's flow is deflected toward the eastern Gulf (Scruton, 1956, p. 2915 t Gatewood (1956) Edwards and others (1956) tt Robinson and others (1956) *** U.S. (1944) ttt U.S. Geological Survey (1909)

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Irvine (in Murray and Renard, 1891), Correns tribution of Mississippi River sediment in de- (1937), and Revelle (1944), came to similar tail and measured oceanographic factors for conclusions. several years. He determined (p. 2915) that SHALLOW-WATER CURRENTS IN THE GULF OF "... as much as 70-75 per cent of the annual MEXICO: In the inshore coastal waters of the sediment load of the Mississippi initially moves northeastern Gulf, water movement is affected westward after it is delivered to the Gulf of more by wind, tide, and river discharge than Mexico." Most of the remainder of the Mis- by oceanic currents (Lipsey, 1919; Scruton and sissippi's load is carried eastward and con- Moore, 1953; Hela and others, 1955; Scruton, tributes to the sediment of the northeastern 1956). As sediment is introduced by the rivers Gulf. as turbid, fresh-water plumes, its initial direc- OCEANIC CURRENTS IN THE GULF OF MEXICO: tion of transport is determined by near-shore As the suspended sediment moves farther off- currents generated by the complex interplay shore, it gradually becomes influenced by the

TABLE 3. TRANSPORT OF SEDIMENT BY OCEANIC CURRENTS After Kuenen (1942)

Time per Horizontal distance Particle Settling 1000m transported t while diameter velocity * depth settling 1000 m (mm) (mm/sec) (km)

1 50 6 hours 2 0.1 4 2 days 25 0.01 0.07 6 months 1500 0.001 (In) 0.0007 40 years 150,000

* Assuming a settling velocity one-half that of quartz spheres in pure water at 15°C Assuming an average current velocity of 10 cm/sec

of these factors; and in general the wind ap- more permanent oceanic currents, and its pears to have more influence on current direc- further transport is governed by them. Surface- tions than the other contributors, especially in water circulation in the Gulf is known from the surface-water layer. ships' observations of drift, but sub-surface Lipsey (1919) compiled extensive informa- water movements are much less well known. tion on the resultant wind components for Hydrographic Office current charts, based high and low Mississippi River stages, and he on snips' observations of drift, show the surface showed conclusively that wind direction di- effects of the Yucatan Current diverging into rectly controls the distribution of sediment several branches in the southern part of the about Southwest Pass. He found that the re- Gulf. Part of the flow turns sharply eastward sultant wind during seasons of high river flow and issues directly through the Florida Straits is from the southeast, and the resultant wind as the Florida Current. This part of the flow is during seasons of low river flow is from the of little concern to sedimentation in the north- northeast. Although these resultant directions eastern Gulf except that it may help drive the are quite prominent, there are also many days more local currents. The amount of suspended on which the wind blows from westerly quand- Gulf sediment removed through the Florida rants and directs the water eastward. Straits is probably not large, as the Florida Scruton and Moore (1953) observed the dis- Current does not mix with Gulf water except tribution of turbid water offshore from the Mis- in a thin-surface layer less than 200 m thick, sissippi Delta and noted the coincidence be- and this mixed layer may be a transient tween wind direction and turbid-water move- phenomenon (Parr, 1935, p. 66-72). ment. For 2 days prior their observations the Another portion of the Yucatan Current is wind had blown up to 11 knots from the south- shown on the Hydrographic Office charts to west, and the turbid water extended 80 miles proceed northward, diverging gradually into eastward from Pass-a-Loutre. northeasterly and northwesterly moving drifts Scruton (1956) considered the seaward dis- (Fig. H). The northwestern drift will not be

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considered further as it does not affect eastern seasonally, but at its maximum in early sum- Gulf sedimentation; the northeastern drift, mer, it extends nearly the whole length of the however, appears to have a deciding effect on Florida peninsula. Chew (1955) pointed out the distribution of fine-grained sediment in the the possible effectiveness of this loop current northeastern Gulf. in the transportation and concentration of red- To the south of the Mississippi Delta, the tide-producing dinoflagellates along the Florida northwesterly and northeasterly drifts are west coast; it is probable that it also distributes

f'igure 11. Surface currents in the Gulf of Mexico—June. Redrawn after Leipper (1954) with additions near shore. Depths in fathoms

about equal in velocity, on the order of 10-15 Apalachicola River clay over part of the Florida cm/sec, and are capable of suspending and west coast shelf. Because of insufficient samples, transporting river-clay particles brought into the landward extent of the Apalachicola-type them by the ephemeral wind-driven currents. suite has not been determined; however, the The northeasterly component swings gradually seaward edge of measurable Apalachicola-type east and then south, paralleling the Florida clay contribution is near the continental slope coast seaward of the continental slope. This off the Florida west coast. current pattern affords opportunity for the On the shelf off the Mississippi-Alabama- distribution of Mississippi River clay through northwest Florida coast, the currents ap- the deep areas of the northeastern Gulf. parently change direction seasonally (U. S. At approximately 25° N. lat., this south- Navy Hydrographic Office Miscellaneous 19690 ward-moving current swings eastward and in series). Charts for the spring and summer show part joins a large counterclockwise eddy on the easterly currents, and charts for the fall and Florida shelf. The extent of this eddy varies winter show westerly currents. At the beach,

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the predominant littoral drift is westward weight, and X-rayed with copper Ka radia- (Beach Erosion Board, 1956). tion at 50 per cent relative humidity. The most rapid influx of suspended sediment The two end-member samples were chosen into the Gulf occurs during the spring runoff because they represent average clay-mineral period in the contributory streams, so that the compositions poised in locations from which spring current pattern would probably control their direct contribution to the Gulf of Mexico the initial suspended sediment distribution by the two "extreme" rivers seemed very (Scruton, 1956, p. 2915). However, hurricanes, likely. The Mississippi sample (G2137) is from common in autumn, create intense turbulence, the lower part of Pass-a-Loutre, and the change the locations of channels and bars, and Apalachicola sample (G3055) is from the generate temporary water movements which western part of Apalachicola Bay, very near redistribute enormous amounts of sediment. West Pass (Figs. 4, 5). The clay-mineral distributional pattern prob- ably indicates the resultant effect of water movements over a very long period and in- TABLE 4. CLAY-MINERAL COMPOSITION OF RIVKRS OF THE NORTHEASTERN GULF OF MEXICO cludes the occasional effects of seasonal storms. General offshore clay-mineral-facies pattern. Figures in parentheses are for relatively minor rivers. The facies distribution of the clay minerals was first noted by arranging the 3°—14° 26 (copper Per cent Rivers Apalachicola- Ka radiation) portions of X-ray patterns on a type clay chart of the area. Plate 1 illustrates the general northeastward increase in kaolinite (solid peaks) Missouri 0 1 1 > L. Mississippi o and the reciprocal northwestward increase in n U. Mississippi 4 / the montmorillonite component (dotted A Pearl (55) 2 Pascagoula (24) peaks). Generally, the clay-mineral suites £ change toward the principal sources of supply Tombigee 12 \ C/5 } Mobile 51 Alabama 51 J —very gradually in the deeper parts of the IT' Gulf and more rapidly near the major rivers. i Chattahoochee > Apalachicola 91 \ 100 Quantified offshore clav-mineral facies. So Flint >100 / that the facies distribution might be quantified, a series of standard mixtures of Mississippi River and Apalachicola River clay was pre- DISTRIBUTION: After construction of the pared and a working curve developed. Figure working curve, the 15-A/7-A ratio of each 12 illustrates the curve and shows X-ray pat- natural sample was measured, and each sample terns of the artificial mixtures at 20-per cent was assigned a value indicating the apparent intervals. percentage of Apalachicola-type clay contained Three subsamples of each mixture were used, in a "matrix" of Mississippi-type clay. This is, and each subsample was run three times. Al- of course, an artificial value in the case of the though the subsamples were taken from various rivers and bays in locations with no homogenized mixtures some uncertainty still access to Apalachicola River sediment. How- lies in subsampling error. The total uncertainty ever, if the two end members are thought of appears to be near + 15 per cent, in terms of as representing clay-mineral weathering ex- the percentage of Apalachicola-type clay dilut- tremes in the area, the values are very useful. ing Mississippi-type clay. All values are based For example, the clay-mineral compositions on height measurements of the peaks near 7 A of the various rivers may now be compared in and 15 A, taken from X-ray patterns of the terms of "per cent Apalachicola-type clay" salt-free, < 2 micron clay fractions mixed by (Table 4). An eastward increase in Apalachi- cola-type (kaolinitic) clay is evident in all 2 In this discussion it should be kept in mind that principal rivers which drain extensive areas in- although the 7-A peak is taken as representing mainly land from the Cenozoic coastal plain. The the mineral faolinitc, and the 14-A to 15-A peak is con- smaller rivers deviate somewhat from the re- sidered to be mainly indicative of montmorillonite, other minerals may contribute to these peaks. For example, gional pattern, probably because they receive chlorite probably increases both the 7-A and 14-A to more locally derived material from Cenozoic 15-A peaks a slight amount; vermiculite has a similar coastal-plain sediments. This Cenozoic ma- effect. terial is largely uncemented and subject to

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rapid erosion; thus, it may not have adjusted to regional weathering influences. Rate of Sedimentation in the Northeastern Gulf Bays and sound receiving sediment from of Mexico and the Rate of Alteration of Clay adjacent northeastern Gulf rivers reflect the Minerals type of material being supplied by the river. Phleger (1955) estimated the amount of Some small-scale changes in clay type between sedimentation in the northeastern Gulf of the rivers and their adjacent salt-water en- Mexico since the last glacial interval by cor- vironments were noted, but as they are not of relating the base of a zone of warm-water sufficient magnitude to affect the regional in- pelagic fauna through a number of the same terpretation, they will be the subject of a cores used in this study (Table 5). Maximum separate paper. If values for "Apalachicola-type" clay are TABLE 5. POSTGLACIAL SEDIMENTATION" IN THE applied for each offshore sample, a most inter- NORTHEASTERN GULF OF MEXICO esting diagram results. On Figure 13 the dis- After Phleger (1955). Locations of cores shown on tribution of essentially "uncontaminated" Figure 3. Mississippi River-type clay throughout most of the deep basin is clearly evident. This area, F.stimated with less than 20 per cent of Apalachicola-type postglacial admixture, might be referred to as the "Mis- Core no. Water depth deposition sissippi fades." Toward the northeast, the (m) (cm) clay-mineral suite gradually assumes the char- WH 3 3017 65 acter of the Apalachicola-type clay as that WH 4 2972 40 river is approached. WH 5 2788 170 The facies pattern is logical and correlatable WH 6 2468 90 with the sources of supply and the current pat- WH7 1875 120 WH 8 1417 115 terns of the area (Figs. 11, 13). Apparently, WH 9 1372 150* Mississippi-type clay was borne to this part of WH 10 1298 150* the northeastern Gulf of Mexico basin by the WH 11 914 210* general easterly drift of the offshore currents. WH 12 732 205* WH 13 631 180* The westerly longshore drift along the northern WH 14 471 155 coast line is clearly effective in skewing the WH 15 298 195* clay-mineral facies westward along the north- west Florida, Alabama, and Mississippi coasts. * Core did not penetrate through postglacial sediment. In occasional anomalous samples, the clay- mineral suite contains too much or too little and minimum depositional rates have been Apalachicola-type clay. Along the Florida calculated from Phleger's data, based on dif- west coast submarine escarpment between ferent time premises. If the 65-cm thick warm- 25° 30' and 26° 20' N. lat., a "swell" of a clay water faunal zone at station WH 3 was de- type normally expected to occur on the upper posited during the 5000-year interval encom- slope and shelf extends outward into the deep passing the present stillstand of the sea, the Gulf. It is in this precise area that Phleger depositional rate at Station WH 3 in the deep- (1960, p. 294-295) has reported "displaced est part of the eastern Gulf basin has averaged faunas," apparently transported down-slope by 0.013 cm/yr. However, if the warm-water slumping or turbidity currents. The same faunal zone represents the whole 30,000-year mechanism was suspected on the basis of the period since the late Wisconsin began clay-mineral anomalies. to wane (dates from Fisk and McFarlan, 1955), Farther north, on the lower slope at about the depositional rate would be about 0.002 28° 30' N. lat., 87° W. long., two samples cm/yr. (WH 38, 41) contain more Apalachicola-type We can then estimate a minimum required clay than surrounding samples. This feature time period for river-derived clay minerals to might also be explainable by submarine slump- be appreciably altered by sea water. The 10-cm ing. The slope here is less steep than in the section examined from the top of the deepest previous example, but the contribution of water core (WH 3) represents from 769 to land-derived detritus poised for slumping is 5000 years, depending on the time premise probably greater. used. As the clay-mineral assemblage at WTH 3

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apparently was derived from the Mississippi irregularly toward the north and west. It is River and is still nearly identical in composi- noteworthy that several of the most chloritic tion to the river clay, the time period required samples (WH 1, 111, 120) contain a fauna sug- for any gross alteration of the clay mineral gested by Phleger (1960, p. 294-295) to have suite must be greater than the 769-5000-year been "displaced" by turbidity currents from period which has elapsed since its deposition. higher positions on the west coast escarpment. Much of this chlorite, especially in the south- Chlorite-Containing Sediment eastern part of the Eastern Gulf, may be de- Diagenetic formation of clay-size chlorite in rived from the Atlantic Coastal Province, some Recent marine sediments has been pro- which is receiving sediment from the meta- posed by several investigators, most notably morphic rocks of the Appalachian Province. Powers (1954; 1959) and Grim and Johns Along most of the Atlantic Coast, a relatively (1954). Chlorite has also been reported to be weak and intermittent, southerly moving, more abundant in Louisiana lower Eocene littoral drift operates. This current, which marine and beach sediments than in continental moves very close to the shore in a direction strata of the same age (Burst, 1959). counter to that of the Gulf Stream, is the The proposed mechanism for chlorite forma- primary agent responsible for the southward tion is the lateral growth of brucite [Mg (OH)2] movement of sand from the Carolinas and between the basal surfaces of montmorillonite Georgia onto the Florida east coast beaches and of micas stripped by weathering of part of (Shaler, 1893; 1895; Gulliver, 1896; Martens, their potassium. Calculation shows magnesium 1931). It is known that the Atlantic Coastal to be 172 times more abundant per unit vol- river and offshore sediments are in general ume in average sea water than in Mississippi relatively rich in chlorite (Powers, 1954; River water. Consequently, conditions are Brown and Ingram, 1954; Griffin and Ingram, supposed to be more favorable in sea water for 1955; Murray and Sayyab, 1955). Insoluble the growth of chlorite than in fresh-water residues from several lime-mud samples at Key bodies. Considerable effort has been made in Largo Dry Rocks and Rodriguez Bank at the the present project in heat-treating samples at northern end of the Florida Keys were found various temperatures from 100° to 800°C to to be comparatively rich in clay-size chlorite ascertain whether or not clay-size chlorite is and talc; this places detrital chlorite in a loca- being developed in the marine waters of the tion fairly accessible to the southeastern part northeastern Gulf of Mexico. of the area under discussion. A number of samples, principally from the Several other lines of evidence appear to con- southern and southeastern part of the area, verge here. Parker (1954), who studied the dis- exhibited a moderate 14-A line on heating to tribution of microfauna in many of the same temperatures between 500° and 700°C. The samples used for this study, made the following 14-A line was generally most intense at 600°C. pertinent comments (p. 478): Oscillation over the 14-A and 7-A peaks of clay "It is easy to explain the introduction of plank- from Sample WH 1 while heating (the method tonic species which occur in low latitudes into the of Weiss and Rowland, 1956a) showed reactions Gulf of Mexico by the surface current flowing typical of the clinochlore (magnesian) type of north from the Caribbean Sea. The presence of 3 chlorite (Fig. 14) . Globigerina inflata and G. pachyderma (and possibly The maximum chlorite content lies in Globorotalia hirsuta, G. punctulata, and G. scitula) Sample WH 1, with Samples WH 4, 7, 108, are more difficult to explain since they apparently 111, 112, 120, and 127 having almost as great do not occur in the Caribbean Sea nor are they a chlorite content (Fig. 15). Somewhat lesser found in the western part of the Gulf of Mexico. amounts of chlorite were found in quite a few It is postulated that these species may be introduced from the Atlantic by a shallow coastal current other samples (Fig. 16). Traces of chlorite, or flowing from east to west along the Florida coast. questionable chlorite, are present in many R. C. Reid (personal communication) says that it samples from the three major rivers and in is very possible that such a current exists and that many of the shelf and slope samples (Fig. 17). the conformation of the Florida Keys strongly In general, chlorite is most abundant in the suggests it. The circulation in the Gulf itself being southeastern portion of the area in water divided into two main eddies in the eastern and depths greater than 10,000 feet and decreases western parts probably would explain why Globi- gerina inflata and G. pachyderma, which are rare, appear only in the eastern Gulf of Mexico area." 8 Copper Ka radiation was used in Figures 14, 15, 16, and 17. Fairbank (1956) made a microscopic study

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of the heavy and light minerals in the north- terials and crystallization of new minerals have eastern Gulf of Mexico. She found an anoma- altered clay-mineral depositional patterns. The lously high augite content in many of the same changes likely to have occurred are not yet samples now found to contain chlorite. This fully predictable. augite content does not, according to Mrs. Fairbank (Personal communication, 1958), Mapping of Transgressive and Regressive Facies match the heavy-mineral suites found in the Two groups of samples from lower Eocene more northerly samples of the northeastern Wilcox illustrate a small-scale applica- Gulf of Mexico. Augite is a common constituent tion of the method to a paleogeographic prob- of mafic igneous rocks such as the lem.4 The first group (Fig. 18) includes typical diabase intrusive masses common in parts of the samples from west to east across Bastrop Atlantic coastal states. County, Texas, and the second (Fig. 19) was Another possible source for chlorite is the taken from a broader section 300 miles to the metamorphic rocks of Cuba, particularly those north, from Henderson to Sabine counties, near the western end of the island in the Organ Texas. Mountains of Pinar del Rio Province (geology The dominant feature of the clay-mineral described in Schuchert, 1935). The strong analyses is a gross variation in the relative northerly moving current issuing through the abundance of kaolinite and montmorillonite. Yucatan Channel could transport some ma- In all fluviatile Wilcox samples, kaolinite is the terial from this source into the Gulf of Mexico; predominant clay mineral, whereas samples however, the amount of sediment introduced taken from paleoenvironments exposed to would probably not be great, as much of the marine waters (lagoonal, beach, shallow marine) water entering through the Yucatan Channel contain abundant montmorillonite and much is sharply deflected through the Florida Straits less kaolinite. to the Atlantic Ocean (Parr, 1935). It is postulated that in Wilcox time a condi- Although chlorite is an interesting, varietal tion existed similar to that in the Apalachicola clay mineral in these sediments, it is a minor Bay area in the northeastern Gulf of Mexico component, and therefore has been treated (Figs. 8, 18, and 19). Thorough weathering separately from the general clay-mineral facies produced kaolinitic soils in ancestral Texas distribution. With the present data, it is not which were eroded and carried toward the possible to locate a definite chlorite source, but Gulf by small- or moderate-size rivers. At the possible source areas are not lacking. ancient shore line, this kaolinitic debris was blanketed by montmorillonitic clay being SUGGESTED APPLICATIONS OF transported by longshore currents from a major CLAY-MINERAL-DISTRIBUTION DATA clastic-source area which probably lay to the TO GEOLOGICAL PROBLEMS north, as montmorillonite becomes slightly more abundant in that direction. Thus, clay- Limitations mineral variations could be used in this area to Several conditions must be met to make distinguish fluviatile from brackish and marine geologic use of the model developed in the sediments and to outline transgressive and re- northeastern Gulf of Mexico: gressive parts of the Texas Wilcox section. (1) The major rivers must have been con- tributing differing clay-mineral suites. The SUMMARY AND CONCLUSIONS greater the contract between clay types con- Three major rivers supply the bulk of clay- tributed, the better should be the clay-mineral- mineral detritus to the northeastern Gulf of facies development. Mexico. The type of clay supplied is a direct (2) Longshore and oceanic currents must function of the clav minerals in the soils of the have been competent to transport clay-sized

detritus. The velocity necessary to transport 4 clay particles is very low (about 0.1 cm/sec), The exact stratigraphic relationship of the samples and ancient currents probably were able to was not invariably known with certainty, as is indicated by the question marks on the figures; precisely correla- transport clay-sized particles and influence tive marine and nonmarine samples were not obtainable their distribution, as they have in the north- here. These relationships arc consistent in about 100 eastern Gulf of Mexico. Texas Wilcox samples collected from the same (3) Postdepositional changes must not have general area. H. B. Stenzel furnished the environmental reached the intensity where migration of ma- estimates shown on the figures.

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river basins and is controlled by the parent comes more abundant in the soils and rivers of rock-weathering complex. The westernmost the Mobile River basin; the Apalachicola River major river, the Mississippi, is supplying a very basin, at the eastern extremity of the area, is large amount of richly montmorillonitic clay. contributing a clay-mineral suite very rich in In an easterly direction, kaolinite gradually be- kaolinite with much less montmorillonite.

55-19 FLUVIATILE MIDDLE WILCOX

55-31 LAGOONAL? BASAL WILCOX

LONSSHORE CURRENTS 55-2 BEACH SAND? CARRYING MONTMORILLO- BASAL WILCOX NITIC CLAY FROM A MOKE EASTERLY MAJOR RIVER ? 55-3 SHALLOW MARINE OR LAGOONAL BASAL WILCOX

55-35 SHALLOW MARINE BASAL WILCOX

Figure 18. Distribution of clay minerals in early Eocene (Wilcox) time. Samples collected from outcrops in Bastrop County, Texas

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Minor alterations in the clay-mineral suite supply, their magnitudes, and the distribu- occur as the river clays enter saline waters, but tional directions are evident. these changes are of too small a magnitude to When viewed in regional aspect, the study alter the regional distribution patterns. of clay-mineral distributions in ancient un- Once the clays have been delivered to the metamorphosed sediments should form a valu- Gulf of Mexico, distribution of the residual- able adjunct to other paleogeographic methods. suspended sediment is governed first by wind- The similarity in distributional patterns of driven, shallow-water currents and later by clay minerals from the modern Apalachicola semipermanent oceanic currents. The eventual River area and from the Texas lower Eocene distribution of clay minerals follows a grada- Wilcox sediments is attributed to similar tional facies pattern in which the sources of weathering and distributional factors.

m^ff^i

FLUVIATILE FLUVIATILE MIDDLE WILCOX ? MIDDLE WILCOX? (Henderson Co., Texas) (Cherokee Co.,Texas)

PB-7 LAGOONAL ? MIDDLE WILCOX PENDLETON BLUFF (Sabine Co., Texas! LONGSHORE CURRENTS CARRYING MONTMORILLO- NITIC CLAY FROM A MORE E A S TERL Y MAJOR RI VER ?

STB- SHALLOW MARINE ? UPPER WILCOX SABINETOWN BLUFF (Sabine Co, Texas)

Figure 19. Distribution of clay minerals m early Eocene (Wilcox) time, Henderson to Sabine counties, Texas

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MANUSCRIPT RECEIVED BY THE SECRETARY OF THE SOCIETY, JULY 7, 1960

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