CLAY MINERAL COMPOSITION OF SHELF SEDIMENTS OFF THE EAST COAST OF INDIA

BY M. SU•BA RAO

(Department of Geology, Andhra UniverMty, Waltair, India)

Received May 24, 1963

(Communicated by Prof. C. S. Pichamuthu, V.A.Se.)

ABSTRACT

The clay mineral composition of the sediments carpeting the conti- nental shelf off the east coast of India has been delermined by X-ray and Differential Thermal Analysis techniques. Sediments contributed by the Ganges-Brahmaputra system of rivers are composed of about equal quantities ofchlorite and illite with a small amount ofmontmorillonite a n d a trace of kaolinite. On the other hand, sediments from near the eonfluence of the Godavari river are composed of a mixture of very poorly ordered clay minerals; and those derived from the Krishna river comprise itlite, montmorillonite, and mixed-layers of these minerals. Sediments off Visakhapatnam, which ate accumulating a t a place relatively free from the influence of the terrigenous influx from the major rivers, are composed of illite, montmorillonite, mixed-layer montmorillonite and illite, with a trace of chlorite, and kaolinite. The contrasted clay mineral assemblages have been attributed primarily to the difference in the source rocks and their weathered products in the respeetive drainage basins of these rivers rather than to diagenesis in the marine environment.

INTRODUCT1ON

SASTRY et al. (1958) were the first to study the clay minerals in the sediments of the Bay of Bengal. They selected a few samples collected from near the Godavari and Krishna river confluences and determined their clay mineral nature by a portable Differential Thermal Analysis Apparatus. According to them, montmo¡ illite, and kaolinite are present in the Godavaxi sediments whereas montmorillonite forros ah important constituent of the plastic clays of the Krishna river.

EXPERIMENTAL

In the present study, a number of samples from near different river eonfluences, from the supposedly~ environs (Subba Rao,

6

Clay Mineral Composition of Shelf Sediments Off the East Coast 7

1958) as well as from the continental slopes were studied by the DTA method the clay fractions being obtained in accordance with the procedure outlined by Grito et al. (1949, p. 1788). A few representative samples were also analyzed by the X-ray diffraction method.

The thermai analysis data for material of different clay-size fractions of 23 samples are presented in Table I. The sample locations are shown in Fig. 1. The differential thermal analysis eurves flor material of rninus one

FIG. 1. Locaaon of the continental shelf and sample locations.

micron size) offour samples of the Gangetic sediments, five samples of Visakha- patnam sediments, and one each of the Godavari, Krishna, and Pennar sediments are presented in Figs. 2 A, 2 B a n d 2 C. Excluding the Gangetic sediments, almost aU the other samples have recorded ah intense endothermie reaction in the range of 130-190 ~ C., and another r on~

M . SUBBA RAO

TABLE I Thermal reactions in the sediments o f the shelf

Depth Size of Station in clay fraction

No. fathoms in microns

Peak temperatures in ~ C.

Endothermic Exothermi c

Sample~ofth› Gangeticsediment~: 113 32~ 117 100 121 63�89 123 75

SamplesofBaruva~ediments: 735 30 <0"1 737 74 <0.1 738 100 {2--1"01--0 1

1 --0".1 1--0.1 1--0"1 1- -0" 1

Samples of Visakhapatnam sedzments :

Fresh-water sample of Godavari

740 22 1 --0- 1 - - -I ~4, ~~ i ~ o l

742 41 1--0" 1 743 76 1 --0- 1 605 382 1--0" 1

Samples of the Godavari sediments : 99 2 1--0.1

112K ~ 1 - -0" 1 128K 2 1--0" 1 130K 3 l i --0" 1

t <0.1 134K 5 (1 --0' 1

<0"1 Untreated Treated

Samples of the Krishna sedŸ 745 11 J Untreated

Treated 745 (a) 100 ) Untreated

~Treated Samples of Pennar sediments"

748 8 1--0" 1 ~2-1-o

750 100 ~Ÿ 1--0" 1 I <0.1

Samples of Madras sediments: ~2-1.o

751 105 ~1--0-1 t <o.1

135, 560 135, 560 140, 565 160, 575, 810

140, 390, 565 155, 575 140, 550,830 150, 575,810

160, 580 130, 560 165, 580 165, 570, 830 160,565 170,570,650

170,565,820 165,570 160, 570 155,560, 650 170,565 150,550,670 145,555 175, 590, 745,800 180, 570, 610

170, 550, 650,805 185, 555,630, 820 180~550,595 180, 510,750

165, 410,570 155,500,570 155, 545, 660, 800 170.575

145, 530, 810 145,550 150, 400, 560, 830

�9 ~

370 310, 870

280,870 280, 865 335,700,870 310,870

400,845 340, 75 390,450,875 375,770,855 390,860 320,390,610,850

370,855 320,860 290,85O 310,430,870 290, 855 350,480, 860 350,500,650,865 380,720,350 385, 850

39O, 59O, 855 39O, 59O, 845 370,770,840 450,590.

350, 450,850 350,810 330,835 850

310,855 300,850 310,460,780,850

Clay Mineral Composition of Shelf Sediments Off the East Coast 9

S A M P L E N O . I I `Jl I

5 A k 4 P L E N o . l i ' . : i

�91 FMS.)

S A M P L Ir N o , 1 2 1 I I I ~ " ~ - ' ~ - I I 1 ~

C6~�89 FMS.)

S A M P L E N o . 1 2 3

(75 F M $ . )

(•, J J J l i * �9 �9 �9 o o ,£ +£ , o o ,ooo

T E M P E R A T U R E - -~ - . . .

F]G. 2 A. Thermal analysis curves--Gangetic samples.

in the range of 550-580 ~ C., which is, however, less intense than the former. At about 820 ~ C. a very small endothemaic reaction occurs in many samples. N o conspicuous exothermic reactions have been recorded, except the one tha t occurs at about 850 ~ C. In some samples a broad exothermic hump with a minor endothermic peak occurs between 200 ~ and 400 ~ C. and a small one immediately following the endothermic reaction at about 550 ~ C. The differential thermal analysis curves obtained of a few samples before and after treatment with hydrogen peroxide (Fig. 2 C - - sample 745, for example) showed that the hump was reduced in the samples given the hydrogen peroxide treatment, which indicates that the hump is due to the presence of organic matter (Grim et al., 1949; p. 1794).

The curves for the Gangetic samples are characterised by two almost equally intense endothermic reactions at 135-160 ~ and at 560-575 ~ C. It is very significant that in no sample was the presence of kaolinite in appreciable amounts suggested by the thermal data.

On the whole, the curves ate of the type that one frequently obtains with complex rnixtures of poorly ordered clay miJaerals in wkich the three

10 M . SUBBA RAO

iAMPLE N e,TaO ('~2 F MS.')

SA~PLe u..74i ~ ~ 03 F~I.)

'r, A I,/~L ~" Ne.742

|

SAMPLE N~. 6051

�91 ' ~&o ' ,6o '--~&, ' ,£ ' , o£ TEidl I I [ RA?UI~ E - -~ C.

FIG. 2B. Therrnal analysis curves--Vimkhapatnam samples.

layered clay minerals are the dominant constituents, and also in which there is a considerable quantity of expandable material either presentas distinct montmorillonite or mixed-layer assemblages.

In complex mixtures of the type described above, differential thermal analysis cannot be applied successfully in diagnosing the clay minerals. The more prominent thermal features indicated by this technique relate merely to the expulsion of water from hydration states of various levels of stability (Grim et al., 1949; p. 1791). Seven representative samples were, therefore, analyzed by powder X-ray diffraction method, and the following results were obtained:

C/~~r, Mineral Composition of Shelf Sediments Off the East Coast I[

I ~ M P L E No.134 I 0~ VMS0

SAMPLF. Ne,745 (11 F'M $,~

UNTRE ATED I

SAMK, E I~o.745 t O I FMS.) TgtEATs

IIAMI~,,E No.741 i (e ~MS,)

FIG. 2 C.

g ' 8o ' � 9 1 ' ' ' a 600 %00 ~OC~~ TEk4PERAT~RE -- ~C.

Thermal analysis curves--Godavari, Krishna and Pennar samples.

Samples 123 (Gangetic atea) is composed of about equal quantities of chlorite and iUite and these clay minerals are fairly well crystallized. Small amounts of montmorillonite, and kaolinite are also present.

Sample 133 (near the confluence of the Godavari) is composed of a mixture of very poorly ordered clay minerals. The ordering is so poor that it is not possible to identify them with any certainty.

Sample 745 (near the Krishna river confluence) is composed of illite, montmorillonite, and mixed-layers of these minerals with possibly some ehlorite.

Sample 740 (22 fathoms off Visakhapatnam) is composed of illite and montmorillonite, mixed-layer montmoriUonite and illite, and probably also ehlorite and kaolinite,

12 M. SUBaA RAO

Sample 741 (33 fathoms off Visakhapatnam) is composed of mont- moriUonite, illite, chlorite, and mixed-layers of these minerals with some kaolinite.

Sample 604 (70 fathoms off Visakhapatnam) is a complex mixture of illite, montmorillonite, and chlorite, possibly with some kaolinite. Mixed-layers are also present and the clay minerals are poorly ordered.

Sample 605 (381 fathoms off Visakhapatnam) has about the same compo- sition as 604.

DISCUSSION

The nature and composition of clay minerals in the continental shelf sediments, which have apparently been contributed by the adjacent land mass, are largely influenced by factors such as the nature of the source rocks, the climatic conditions under which weathering of these rocks takes place, the rate of sedimentation at the sampling site, and the environment of deposition (Weaver, 1958).

The Godavari river derives its sediment load from a variety of rocks, especially from the Deccan Traps in its upper reaches and from the Khondalitic suite of rocks in its lower course. The clay minerals from the black and red soils associated with these rocks have been investigated by a few soil scientists. Working on the soils from near Hyderabad, Nagelschmidt et al.

(1940) found that the red clay is predominantly composed of kaolinite or halloysite whereas the black clay contains mainly beidellite, although both types of soils have resulted from the weathering of the same coarse crystalline granite of gneiss. From a study of the dehydration curves of the clay frac- tions, Ray Chaudhuri et al. (1943) have also come to the conclusion that the beidellitic group of minerals characterize the black soils, while the red clay fractions contain more of the kaolinitic group of minerals. According to Sen (1954), the clay minerals of the black soils are mixtures of nontronite and montmorillonite, and the earliest alteration products of the basalts of the Deccan Traps are the amorphous palagonite-chlorophaeite. At the end of the hot summers, the torrential rains wash these alteration products into the rivers where the material is probably further degraded. In the Neuse river estuary, Griffin and lngram (1955) found 50-75 per cent. by weight of material to be amorphous to very poorly crystallised to X-rays and they could not decide whether this was amorphous material brought in by the river itself, or whether it was formed in the estuary from the clay minerals that are anstable under marine conditions. It has generaUy been found, however,

Clay Mineral Composition of Shelf Sediments Off the East Coast 13

that the clay mineral assemblages in the area of active deposition are dependent largely on the character of the source area (Milney and Earley, 1958). From a study o f the Mississippi delta and off-shore sediments, Johns and Grito (1958), and Taggart, Jr., and Kaiser, Jr. (1960) have concluded that the most significant aspect of the clay mineral distribution in the Mississippi delta atea is not so much the change noted as the diagenesis being of such a low order of magnitude. The amorphous material found in the Godavari sedi- ments is, therefore, detrital in origin and reflects essentially the character of their source area, namely the weathered products primarily of the Deccan Traps and secondarily of the Precambrian formations. The presence of kaolinite in the drainage basin soils and its reduction to insignificant propor- tions in the marine environment is suggestive of its breakdown probably in the brackish water environment and even earlier and its subsequent re- arrangement into chlorite or illite. The detailed studies which are under way are expected to throw additional light on the clay mineral diagenesis in the Godavari deltaic sediments.

The Krishna river, except for a small part in its upper reaches, and the Thungabhadra, its important tributary, drain the Dharwar rocks, a considerable part of which is believed to be of sedimentary origin, and the sedimentaries of the Cuddapah and Kurnool systems which comprise quartzites, slates, and limestones. The change in the source rocks even under the same climatic conditions has resulted in a different set of clay minerals contributed to the sea by the Krishna river.

As against the South lndian river-derived sediments, the presence of only small quantities of montmorillonite in the Gangetic sediments is noteworthy. If montmorillonite is being contributed by the Ganges, its reduction to suela low proportions in the marine environment cannot be ascribed to diagenetic processes because the ilIitization and chloritization of monlmorillonite in the oceanic milieu is an extremely slow process which goes to completion only in areas of slow deposition (Dietz, 1942; and Johns and Grim, 1958). Kanwar (1959, 1961) reports that the Punjab soils which are derived from the Himalayan rocks are characterized by the predominance of illite with some chlorite. The chloritic schists in the drainage basin of the Damuda river, a tributary that joins the Ganges in its lower course, are composed of about 90 per cent. illite which has resulted from the hydrothermal alteration of chlorite (Kameswara Rao, 1963). From studies on the Ganges silt collected at Palta (Barrackpore, Calcutta), Sen and Chatterjee (1955) reported that illite and montmorillonite contents, as calculated from the potassium and magnesium contents of H-clays, are respectively higher and lower in the

14 M. StmBA RAO

monsoon silt than those in the relevant premonsoon sample. By measure- ment of refractive indices, and briefringence by immersion methods, Sen et al. (1961) reported that the premonsoon silt is mostly kaolinitic while there is a slight increase in the percentage of illite in the monsoon silts. While these few observations are not conclusive, it appears that montmorillonite is not being contributed to the Gangetic sediments in appreciable quantities. Montmorillonite which might have been originally contributed to the sedi- ments that were laid down in the Tethys was probably destroyed during the lithification of the sediments, and by the metamorphism which the Himalayan region suffered during its uplift.

CONCLUSION

The present study, though very limited in its scope, seems to indicate that certain modification of some of the clay minerals takes place in marine environments, but the nature of the source rocks has a far greater influence in determining the clay mineral composition of the sediments, particularly in areas of rapid sedimentation.

The relict sediments of the outer shelf, mainly oolitic in character, have been encroached upon by modern detrital sediments brought down to the sea by the rivers from both the southern and northern sides (Subba Rao, 1958). It is quite possible that with samples obtained on a closely controlled grid pattern, the contrasted clay mineral assemblages may be used as a tracer to evaluate the directions ,of sediment transport from these river-mouths and also the relative influence and spread of the sediments on the shelf and in particular on the calcareous sediments of the outer shelf, which belong to an earlier sedimentation cycle of the Pleistocene period.

ACKNOWLEDGEMENTS

The author wishes to express his indebtedness to Prof. Ralph E. Grito of lllinois University, U.S.A., for kindly analyzing the samples by X-ray diffraction method. He is grateful to the late Prof. C. Mahadevan for his keen interest in the work during its progress. He is thankful to Prof. C. S. Pichamuthu, Head of the Department of" Geology, Andhra Uni- versity, for his critical reading of the manuscript.

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1. Dietz, R .S . . . "Clay minerals in recent marine sediments," Amer. Mineral., 1942, 27, 219-20.

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Clay Mineral Composition of Shelf Sediments Off the East Coast

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15

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