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Clay minerals in the shelf sediments of the northwestern part of the Bay of Bengal
Marine Geology, 33 (1979) M21--M32
M21
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Letter Section CLAY MINERALS IN THE SHELF SEDIMENTS OF THE NORTHWESTERN PART OF THE BAY OF BENGAL*
M. R A M A M U R T Y and P.C. S H R I V A S T A V A
Offshore Mineral Exploration and Marine Geology Division, Geological Survey of India, Calcutta (India) (Received December 15, 1978; accepted May 22, 1979)
ABSTRACT Rama Murty, M. and Shrivastava, P.C., 1979. Clay minerals in the shelf sediments of the northwestern part of the Bay of Bengal, Mar. Geol., 33: M21--M32. X-ray diffraction analysis of twenty-three clay fractions (< 2 u m) of grab samples from the northwestern part of the Bay of Bengal revealed the presence of major quantities of illite and quartz with minor amounts of montmoriUonite, kaolinite and felspar. The presence of relatively high percentages of the above minerals near the Hooghly river mouth and their decrease seaward into the open Bay of Bengal lead to the belief that the sediments have been contributed mainly through the Ganges--Brahmaputra river system. The high percentages of quartz in the fine fraction indicate a detrital nature. It is also inferred from the distribution patterns of the various clay minerals in the area that the sediments move predominantly towards southeast.
INTRODUCTION
The Bay of Bengal covers an area of about 2.2 • 106 km 2 (La Fond, 1966) bounded in the north by the deltaic regions of the Ganges and Brahmaputra rivers; in the east by the Burmese peninsula and its extension to the south, the Andaman and Nicobar ridges; in the west by the coast of India and in the south by a boundary line running from the southern tip of Sri lanka to the northern tip of Sumatra. As a part of the regional exploration programme of the continental margin of India, the Marine Geology Unit of the Geological Survey of India collected bottom samples from the northwestern part of the Bay of Bengal on board RV "Anusandhani" of the Calcutta Port Commissioners (Siddiquie et al., 1970). The area under study is covered by the British Admiralty chart No. 814. The locations of the samples were obtained by the Decca-Hi-Fix system and are shown in Fig. 1. The shelf is smooth without any major features and is 210 km wide off the Hooghly river mouth (Closs et al., 1974). This river, a distributary of the Ganges, debouches in the north and the River Dhamya, a distributary of the * Published with the kind permission of the Director General, Geological Survey of India.
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Mahanadi, joins in the west (Fig. 1). The rivers Bhagirathi, Jalangi and Churni, which take off from the Ganges, form the River Hooghly (Bhattacharya and Ghotankar, 1968), while the rivers Damodar, Rupnarayan and Haldia join the River Hooghly which ultimately debouches into the Bay of Bengal. The sediments in the area are carries by the Ganges and its tributaries. The northBrahmaputra River systems. The average discharge of the Ganges--Brahmaputra at the delta mouth is 31.5 • 103 m3/sec with a drainage area of 2048 • 103 km 2 (Curray and Moore, 1971). Siddiquie et al. (1970) have already described the distribution of sediments in the area. The northeastern part is an admixture of fine sand and grey clay which might have been carried by the Ganges and its tributaries. The northwestern part of the area is characterised by N--S tongues of fine to medium sand extending well into the clay zone. These sands further follow the Balasore coastline. It is clear from the sediment distribution map prepared by the Geological Survey of India (Siddiquie et al., 1975), part of which is shown Fig.2. Geology of a part of the drainage basin of the Ganges-Brahmaputra river system (after G.S.I. Map, 1962), bottom sediments and bathymetry of offshore region (after G.S.I. Map. 1975).
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M24 in Fig. 2, that sand is distributed in elongated patches in the form of sand bars. The sediments in the west are clays and to the south they are mixed with Kankar (calcareous concretions). The southwestern portion is occupied by coarse sand which is quite different from the sands reported from northern area, and this sand might have been brought in by the Mahanadi River. Knowledge of the clay mineralogy would not only aid in provenance studies, but would also give an important clue in unravelling the conditions under which the sediment was deposited. The main purpose of the present study is to understand the influence of source, weathering and environment of deposition on the clay mineralogy of Recent sediments o f f the Ganges delta. PREVIOUS WORK The Bay of Bengal has been covered during a number of expeditions and different investigators have contributed a wealth of information on various aspects of the oceanography. In these expeditions, mostly the southern part of the Bay of Bengal was studied. A considerable amount of work has been carried o u t on the sediments of the eastern continental shelf of India by the Andhra University (Anonymous 1954, 1958). La Fond (1957,1966) has presented a review of different aspects of oceanographic w o r k in the Bay of Bengal. However, not much has been done on the shelf sediments off the Ganges delta. Sastry et al (1958) were the first to study the clay minerals in the sediments of the Bay of Bengal. They studied a few samples near the confluences of the Godavari and Krishna rivers and found that montmorillonite, illite and kaolinite are present in the Godavari sediments, whereas montmorillonite forms an important constituent of the plastic clays of the Krishna River. Subba Rao (1963) carried out clay-mineral analysis of the shelf sediments off the east coast of India and found equal quantities of chlorite and illite with a small amount of montmorillonite and traces of kaolinite in the sediments contributed by the Ganges--Brahmaputra river system. Gorbunova (1966) studied the distribution of clay minerals in some samples of the Bay of Bengal, and observed significant concentrations of illite and chlorite. While describing the recent sediments of the Bay of Bengal, Siddiquie {1967) reported the presence of illite, kaolinite and montmorillonite as the major clay minerals. The studies carried o u t b y Goldberg and Griffin (1970) indicated that river-borne solids are the primary components of the deposits, and the eastern part receives its continental input through the Ganges--Brahmaputra river system. Venkatarathnam and Biscaye (1973), in their studies on clay mineralogy and sedimentation in the eastern Indian Ocean, state that the Ganges Cone and the Java Trench are characterized b y relatively more abundant illite and chlorite dispersed primarily by turbidity currents from the Ganges and Brahmaputra rivers. Studies by Rao and Rao (1977) revealed the presence of illite, chlorite and montmorillonite with traces of kaolinite in the sediments off the Ganges delta.
M25
Grain-size variation and mineralogical studies of the same sediments have been carried out by Mallik (1975, 1976). As a part of his mineralogical studies, Mallik (1976) reported illite and kaolinite as the most important clay "minerals, as studied by the X-ray diffraction method. METHODS OF STUDY Twenty-three grab samples collected with a Petterson grab were selected for the study of clay minerals from the set of samples collected during the cruises of RV "Anusandhani". The sediments were dispersed in distilled water and clays (42 p m) were separated following the sedimentation method (Krumbein and Pettijohn, 1938). The clays, thus separated were dried and analysed by X-ray diffraction in the Mineral Physics Division of the Geological Survey of India, Calcutta. RESULTS The X-ray diffraction data of the clays (<2pm) from the shelf sediments of the northwestern part of the Bay of Bengal revealed the predominance of illite and quartz with minor amounts of montmorillonite, kaolinite and felspar. The clay minerals have a considerable range of concentrations. (Table I, Fig.3). The distribution patterns of iUite, quartz, montmorillonite and kaolinite (Fig. 3) were prepared using the above semi-quantitative data and taking the upper end of each zone limit as the value for the station. Illite increases from 10--25 to 50% from west to east up to central portion from where it gradually starts decreasing on all sides (Fig. 3A). The maximum concentration of illite is southward of the Hooghly river mouth. Most of the area is characterised by more than 50% quartz, except the central portion which has between 25 and 50% (Fig. 3B). The central portion forms a lensoid-shaped body aligned northwest--southeast. The montmorillonite is distributed throughout the area in percentages of 10-25 except in the central portion where its percentage is high (Fig. 3C). The major part of the central basin is covered with 25--50% of kaolinite, decreasing on all sides to 25--10% (Fig. 3D). DISCUSSION
The clay mineral data (Table I, Fig. 3) clearly show the predominance of illite and quartz with minor amounts of montmorillonite, kaolinite and felspar. The predominance of illite in the shelf sediments off the Hooghly river mouth is noteworthy and can be attributed to the source rocks and soils in the drainage basins of the Ganges and Brahmaputra rivers. Illite, which predominates in the soils of northwestern India (the Punjab) appears to be transported through these rivers to the Bay of Bengal. This is supported by
M26 TABLE I Clay m i n e r a l c o n s t i t u e n t s in e a c h sample Sample No.
Constituents
Percentage
123
illite a n d m o n t m o r i l l o n i t e kaolinite and quartz felspar
> 50 50--25 25--10
137
illite a n d q u a r t z kaolinite m o n t m o r i l l o n i t e a n d felspar
> 50 15--10 10--05
145
quartz illite kaolinite and montmorillonite felspar
> 50 25--15 15--10 10--05
148
quartz illite kaolinite and montmorillonite felspar
> 50 25--15 15--10 10--05
150
illite a n d q u a r t z kaolinite m o n t m o r i l l o n i t e a n d felspar
> 50 50--25 10--05
152
illite a n d q u a r t z m o n t m o r i l l o n i t e a n d kaolinite felspar hematite
> 50 50--25 10--05 < 05
158
illite a n d m o n t m o r i l l o n i t e quartz and kaolinite felspar hematite
> 50 50--25 15--10 < 05
163
illite a n d q u a r t z kaolinite montmorillonite felspar a n d h e m a t i t e
> 50 15--10 10--05 < 05
170
quartz k a o l i n i t e a n d illite montmorillonite felspar
> 50 25--15 25--15 10--05
173
quartz montmorillonite kaolinite a n d illite felspar
> 50 25--15 15--10 10--05
176
quartz montmorillonite k a o l i n i t e a n d illite felspar
> 50 25--15 15--10 10--05
179
quartz montmorillonite k a o l i n i t e a n d illite felspar
> 50 25--15 15--10 10--05
M27
Sample No.
Constituents
Percentage
180
quartz montmorillonite kaolinite and illite felspar
> 50 25--15 15--10 10--05
182
quartz montmorillonite kaolinite and illite felspar
> 50 25--15 15--10 10--05
183
quart~; montmorillonite and illite kaolinite and felspar
> 50 25--15 10---05
185
quartz illite kaolinite, felspar and montmorillonite
> 50 25--15
187
quartz montmorillonite and illite kaolinite and felspar
> 50 25--15 10--05
190
quartz illite felspar montmorillonite
> 50 25--15 15--10 10--05
193
quartz kaolinite, illite and felspar montmorillonite
> 50 25--15 10--05
197
quartz illite kaolinite, felspar and montmorillonite
> 50 25--15
quartz illite kaolinite, felspar and montmorillonite
> 50 25--15
200
quartz kaolinite and illite felspar montmorillonite
> 50 25--15 15--10 10--05
202
quartz kaolinite and illite felspar montmorillonite
> 50 25--15 15--10 10--05
199
15--10
15--10
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M29
the observation of Kanwar (1959, 1961) who reports that the Punjab softs, which are derived from the Himalayan rocks, are characterised by the predominance of illite. Further, Goldberg and Griffin (1970) pointed out that the river-borne solids control clay-mineral distributions in the Bay of Bengal, especially in the northwestern sector, where the weathering products of the Himalayan region through the Ganges--Brahmaputra river system join. The mineral assemblages derived from Himalayan region are characterised by abundant illite and chlorite (Gupta, 1961). Sen and Chatterjee (1960) reported illite as the predominant clay mineral in the Ganges monsoon silt which ultimately is derived from the Himalayan region where mechanical weathering plays a dominant role. The foregoing discussion clearly explains the presence of the highest percentage of illite immediately near the Hooghly river mouth and seaward decrease into the open Bay of Bengal (Fig. 3A). The high percentage of quartz in these sediments (Fig. 3B) suggests the continental source through the Ganges and Brahmaputra river system. Quartz, being resistant to weathering, has remained virtually unaltered during weathering and transportation processes. During the mineralogical studies, Mallik (1976) also found quartz and felspar minerals in the light fractions. From the above observation, it appears that quartz is the significant detrital component of these fine-grained sediments. This is further supported by the observations of Venkatarathnam and Biscaye (1977) who reported that the Ganges--Brahmaputra river sources are primarily responsible for high percentages of quartz in the Bay of Bengal, and the quartz in these sediments is mainly continent-derived. The Ganges derives its sediments load from a variety of rocks, especially from the upper Pre-cambrian rocks in its upper reaches and from the Rajmahal traps in its lower course before joining the Hooghly River. The Rajmahal traps are found as hills at the head of the Ganges delta near the border of Bihar and West Bengal (Fig. 2). They bear a great resemblance to the Deccan Traps in composition and range from dolerite to basalt, depending on their texture (Krishnan, 1968). These traps contain significant amount of Mg and Na and a low content of K. The rainfall map of India shows that the drainage basin of the Ganges River falls under the moderate rainfall classification. The weathering under conditions of moderate rainfall permits the magnesium to remain in the weathering zone, after it is released by the breakdown leading to the formation of montmorillonite (smectite). This is substantiated by the observation of Roy and Barde (1962) who reported the formation of montmoriUonite from the augite basalt on weathering under moderate precipitation. The soil map of India shows the presence of lateritic soils along the Orissa and Bengal coast. The clay minerals of red soils have been studied by several workers. An important investigation is that of Nagelschmidt et al. (1940), who found kaolinite in red softs from khondalites, granites and granite gneisses (Eastern Ghats). Sen et al. (1961) reported that the premonsoon Ganges silt is mostly kaolinitic.: Shanumugam (1964) suggested that the top
M30
blue layer of the Hooghly River silt contains muscovite, kaolinite and quartz and the yellow bottom layer contains palygorskite, muscovite, quartz, kaolinite and illite. It may be considered from the above data that the kaolinite might have been derived from the Ganges drainage areas as well as from Mahanadi (Dhanra drainage). A review of the literature shows that, at present, there are two schools of thought regarding the presence of clay minerals in marine sediments. Murray and Harrison (1956), Weaver (1958, 1960) and Taggart and Kaiser (1960) support the detrital theory by emphasizing the importance of source area in controlling the types of clay minerals. On the other hand, many investigators including Dietz (1942), Grim et al. (1949), Millot (1949), Grim (1953), Grim and Johns (1954), Powers (1954) and Nelson (1959) have offered explanations supporting environmental diagenesis. From the nature and composition of the clay minerals in the shelf sediment off the Ganges delta and also from the foregoing discussion, it is clear that the clay minerals in these sediments are derived from the adjacent land mass and are mainly influenced by factors such as the nature of the source rocks and the climatic conditions under which weathering of these rocks took place. Further, the high rate of sedimentation off the Ganges delta gives no time for the clay minerals to undergo any environmental diagenesis in the prevailing environment. Subba Rao (1963) concluded that the nature of the source rocks has a far greater influence in determining the clay-mineral composition of the self sediments off the east coast of India, particularly in areas of rapid sedimentation. The distribution of various clay minerals in the area under study (Fig. 3) indicates the maximum concentration of each clay mineral in the central basin aligning northwest--southeast. This suggests that the predominant direction of movement of sediments in the area is towards the southeast. MaUik (1975) also inferred that the probable transport direction in this area is south to southeast, based on the grain-size variations of the sediments. CONCLUSIONS
(1) The X-ray diffraction study of clays (<2 ~m) of the shelf sediments of the northwestern part of the Bay of Bengal reveals the predominance of illite and quartz, with minor amounts of montmorillonite, kaolinite and felspar. (2) The presence of significant proportions of quartz in the fine fractions suggests a detrital nature. (3) The relatively high percentages of the various clay minerals immediately near the Hooghly river mouth suggest major contribution through the Ganges-Brahmaputra river systems. (4) Based on the clay-mineral distribution patterns, it is inferred that the predominant direction of sediment transport is towards the southeast.
M31 ACKNOWLEDGEMENTS
T h e a u t h o r s are g r a t e f u l t o Shri V.R. V e n k o b a R a o , D i r e c t o r , O M E & MG Division, G S I f o r g u i d a n c e a n d going t h r o u g h t h e m a n u s c r i p t ; Dr. D.R. D a s g u p t a , mineralogist-in-charge, f o r his k e e n i n t e r e s t in t h e w o r k , Mrs. L. B h a t t a c h a r j e e , Mineralogist (Sr), a n d Dr. G.L. Dwivedi, Mineralogist (Jr), Mineral Physics Divn., G e o l o g i c a l S u r v e y o f I n d i a f o r analysing t h e samples. Assistance r e n d e r e d b y Prof. W.D. Keller, D e p t . o f G e o l o g y , U n i v e r s i t y o f Missouri, C o l u m b i a , U.S.A. in critically going t h r o u g h t h e m a n u s c r i p t a n d suggesting i m p r o v e m e n t s is highly a p p r e c i a t e d . REFERENCES Anonymous, 1954. Andhra Univ. Memoir. Oceanogr., 1. Anonymous, 1958. Andhra Univ. Memoir. Oceanogr., 2. Bhattacharya, S.K. and Ghotankar, S.T., 1968. The estuary of the Hooghly. Bull. Natl. Inst. Sci. India, 38 (1): 25--32. Closs, H., Narain, H. and Garde, S.C., 1974. Continental margins of India. In: C.A. Burk and C.L. Drake (Editors), The Geology of Continental Margins, Springer, Berlin, pp. 629---639. Curray, J.R. and Moore, D.G., 1971. Growth of the Bengal deep-sea fan and denudation of the Himalayas. Geol. Soc. Am. Bull., 82 (3): 563--572. Dietz, R.S., 1972. Clay minerals in recent marine sediment. Am. Mineral., 27: 219--220. Goldberg, E.D. and Griffin, J.J., 1970. The sediments of the northern Indian Ocean. Deep-Sea Res., 17: 513--537. Gorbunova, Z.N., 1966. Clay mineral distribution in the Indian Ocean sediments. Okeanologiya, 6 (2); 267--275 (in Russian). Grim, R.E., 1953. Clay Mineralogy. McGraw-Hill, New York, N.Y. Grim, R.E. and Johns, W.D., 1954. Clays and clay minerals. N.A.S.--N.R.C., Publ., 327: 81--103. Grim, R.E., Dietz, R.S. and Bradley, W.F., 1949. Clay mineral composition of some sediments from the Pacific Ocean off the California coast and the Gulf of California. Geol. Soc. Am. Bull., 60: 1785--1808. Gupta, R.N., 1961. Clay minerals in soils of the lower Gangetic Basin of Uttar Pradesh. J. Indian Soc. Soil. Sci., 9: 141--149. Krishnan, M.S., 1968. Geology of India and Burma. Higginbothams, Madras, 555 pp. Krumbrein, W.C. and Pettijohn, F.J., 1938. Manual of Sedimentary Petrography. Appleton, New York, N.Y., 549 pp. La Fond, E.C., 1957. Oceanographic studies in the Bay of Bengal. Proc. Indian Acad. Sci., 46: 1--46. La Fond, E.C., 1966. The Bay of Bengal. In: Encyclopedia of Oceanography, Reinhold, New York, N.Y., 110--118. Mallik, T.K., 1975. A note on grain size variation of sediments at the mouth of the Hooghly River, Bay of Bengal. Indian J. Earth Sci., 2(2): 142--153. Mallik, T.K., 1976. Shelf sediments of the Ganges delta with special emphasis on the mineralogy of the western part, Bay of Bengal, Indian Ocean. Mar. Geol., 22: 1--32. Millot, G., 1949. Geol. Appl. prospect Min., 2(1): 352 pp. Murray, H.N. and Harrison, J.L., 1956. Clay mineral composition of recent sediments from Sigsbee Deep. J. Sediment. Petrol., 26: 363--368. Nagelschmidt, G., Desai, A.D. and Alex-Muir, 1940. The minerals in the clay fractions of black cotton soils and red earths from Hyderabad, Deccan State, India. J. Agric, Sci., 30: 605--647.
M32 Nelson, B.W., 1959. Clays and clay minerals. N.A.S.--N.R.C., Publ., pp. 135--147. Powers, M.C., 1954. Clays and clay minerals. N.A.S.--N.R.C., Publ., 327: 68--80. Rao, N.V.N.D.P. and Rao, M.P., 1977. Clay mineral distribution in sediments of the eastern part of the Bay of Bengal. Indian J. Mar. Sci., 6(2): 166--168. Roy, B.B. and Barde, N.K., 1962. Some characteristics of the black soils of India. Soil Sci., 93: 142--147. Sastry, A.V.R., Poornachandra Rao, M. and Mahadevan, C., 1958. Differential thermal analysis of plastic clays from the deltaic regions of Godavari and Krishna rivers. Andhra Univ. Mere. Oceanogr., 2: 61--68. Sen, R.K. and Chatterjee, B., 1960. Properties of Ganges silt. J. Indian Soc. Soil Sci., 8: 139--142. Sen, R.K., Mukherjee, P.K. and Chatterjee, B., 1961. Clay minerals in Ganges silt. Proc. Indian Sci. Congr., part III, Abstract. 190. Sharumugam, A.L., 1964. A study of grain size parameters and heavy minerals -- Auckland Bar--River Hooghly. In: Int. Geol. Congr. Rep., 22nd session, India, 15: 233--247. Siddiquie, H.N., 1967. Recent sediments of the Bay of the Bengal. Mar. Geol., 5: 249-291. Siddiquie, H.N., Shrivastava, P.C. and Mallik, T.K., 1970. Cruise Report -- Marine geological cruise of RV "Anusandhani". Indian Mineral., 24(4): 383--386. Subbe Rao, M., 1963. Clay mineral composition of shelf sediments off the east coast of India. Proc. Indian Acad. Sci., 58: 6--15. Taggart, M.S. and Kaiser, A.D., 1960. Clay mineralogy of Mississippi River Deltaic sediments. Geol. Soc. Am. Bull., 71: 521--530. Venkatarathnam, K. and Biscaye, P.E., 1973. Clay mineralogy and sedimentation in the Eastern Indian Ocean. Deep-Sea Res., 20: 727--738. Venkatarathnam, K. and Biscaye, P.E., 1977. Distribution and origin of quartz in the sediments of the Indian Ocean. J. Sediment. Petrol., 47(2): 642--649. Weaver, C.E., 1958. Geological interpretation of argillaceous sediments. Part I. Origin and significance of clay minerals in sedimentary rocks. Bull. Am. Assoc. Pet. Geol., 42: 254--309. Weaver, C.E., 1960. Possible uses of clay minerals in search for oil. Clays Clay Miner. 8th Conf., pp. 191--224.
M21
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Letter Section CLAY MINERALS IN THE SHELF SEDIMENTS OF THE NORTHWESTERN PART OF THE BAY OF BENGAL*
M. R A M A M U R T Y and P.C. S H R I V A S T A V A
Offshore Mineral Exploration and Marine Geology Division, Geological Survey of India, Calcutta (India) (Received December 15, 1978; accepted May 22, 1979)
ABSTRACT Rama Murty, M. and Shrivastava, P.C., 1979. Clay minerals in the shelf sediments of the northwestern part of the Bay of Bengal, Mar. Geol., 33: M21--M32. X-ray diffraction analysis of twenty-three clay fractions (< 2 u m) of grab samples from the northwestern part of the Bay of Bengal revealed the presence of major quantities of illite and quartz with minor amounts of montmoriUonite, kaolinite and felspar. The presence of relatively high percentages of the above minerals near the Hooghly river mouth and their decrease seaward into the open Bay of Bengal lead to the belief that the sediments have been contributed mainly through the Ganges--Brahmaputra river system. The high percentages of quartz in the fine fraction indicate a detrital nature. It is also inferred from the distribution patterns of the various clay minerals in the area that the sediments move predominantly towards southeast.
INTRODUCTION
The Bay of Bengal covers an area of about 2.2 • 106 km 2 (La Fond, 1966) bounded in the north by the deltaic regions of the Ganges and Brahmaputra rivers; in the east by the Burmese peninsula and its extension to the south, the Andaman and Nicobar ridges; in the west by the coast of India and in the south by a boundary line running from the southern tip of Sri lanka to the northern tip of Sumatra. As a part of the regional exploration programme of the continental margin of India, the Marine Geology Unit of the Geological Survey of India collected bottom samples from the northwestern part of the Bay of Bengal on board RV "Anusandhani" of the Calcutta Port Commissioners (Siddiquie et al., 1970). The area under study is covered by the British Admiralty chart No. 814. The locations of the samples were obtained by the Decca-Hi-Fix system and are shown in Fig. 1. The shelf is smooth without any major features and is 210 km wide off the Hooghly river mouth (Closs et al., 1974). This river, a distributary of the Ganges, debouches in the north and the River Dhamya, a distributary of the * Published with the kind permission of the Director General, Geological Survey of India.
M22 SCALE 20
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k85 •
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BB°t50t -
Fig.1. Map showing the location of the area and sample stations.
Mahanadi, joins in the west (Fig. 1). The rivers Bhagirathi, Jalangi and Churni, which take off from the Ganges, form the River Hooghly (Bhattacharya and Ghotankar, 1968), while the rivers Damodar, Rupnarayan and Haldia join the River Hooghly which ultimately debouches into the Bay of Bengal. The sediments in the area are carries by the Ganges and its tributaries. The northBrahmaputra River systems. The average discharge of the Ganges--Brahmaputra at the delta mouth is 31.5 • 103 m3/sec with a drainage area of 2048 • 103 km 2 (Curray and Moore, 1971). Siddiquie et al. (1970) have already described the distribution of sediments in the area. The northeastern part is an admixture of fine sand and grey clay which might have been carried by the Ganges and its tributaries. The northwestern part of the area is characterised by N--S tongues of fine to medium sand extending well into the clay zone. These sands further follow the Balasore coastline. It is clear from the sediment distribution map prepared by the Geological Survey of India (Siddiquie et al., 1975), part of which is shown Fig.2. Geology of a part of the drainage basin of the Ganges-Brahmaputra river system (after G.S.I. Map, 1962), bottom sediments and bathymetry of offshore region (after G.S.I. Map. 1975).
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M24 in Fig. 2, that sand is distributed in elongated patches in the form of sand bars. The sediments in the west are clays and to the south they are mixed with Kankar (calcareous concretions). The southwestern portion is occupied by coarse sand which is quite different from the sands reported from northern area, and this sand might have been brought in by the Mahanadi River. Knowledge of the clay mineralogy would not only aid in provenance studies, but would also give an important clue in unravelling the conditions under which the sediment was deposited. The main purpose of the present study is to understand the influence of source, weathering and environment of deposition on the clay mineralogy of Recent sediments o f f the Ganges delta. PREVIOUS WORK The Bay of Bengal has been covered during a number of expeditions and different investigators have contributed a wealth of information on various aspects of the oceanography. In these expeditions, mostly the southern part of the Bay of Bengal was studied. A considerable amount of work has been carried o u t on the sediments of the eastern continental shelf of India by the Andhra University (Anonymous 1954, 1958). La Fond (1957,1966) has presented a review of different aspects of oceanographic w o r k in the Bay of Bengal. However, not much has been done on the shelf sediments off the Ganges delta. Sastry et al (1958) were the first to study the clay minerals in the sediments of the Bay of Bengal. They studied a few samples near the confluences of the Godavari and Krishna rivers and found that montmorillonite, illite and kaolinite are present in the Godavari sediments, whereas montmorillonite forms an important constituent of the plastic clays of the Krishna River. Subba Rao (1963) carried out clay-mineral analysis of the shelf sediments off the east coast of India and found equal quantities of chlorite and illite with a small amount of montmorillonite and traces of kaolinite in the sediments contributed by the Ganges--Brahmaputra river system. Gorbunova (1966) studied the distribution of clay minerals in some samples of the Bay of Bengal, and observed significant concentrations of illite and chlorite. While describing the recent sediments of the Bay of Bengal, Siddiquie {1967) reported the presence of illite, kaolinite and montmorillonite as the major clay minerals. The studies carried o u t b y Goldberg and Griffin (1970) indicated that river-borne solids are the primary components of the deposits, and the eastern part receives its continental input through the Ganges--Brahmaputra river system. Venkatarathnam and Biscaye (1973), in their studies on clay mineralogy and sedimentation in the eastern Indian Ocean, state that the Ganges Cone and the Java Trench are characterized b y relatively more abundant illite and chlorite dispersed primarily by turbidity currents from the Ganges and Brahmaputra rivers. Studies by Rao and Rao (1977) revealed the presence of illite, chlorite and montmorillonite with traces of kaolinite in the sediments off the Ganges delta.
M25
Grain-size variation and mineralogical studies of the same sediments have been carried out by Mallik (1975, 1976). As a part of his mineralogical studies, Mallik (1976) reported illite and kaolinite as the most important clay "minerals, as studied by the X-ray diffraction method. METHODS OF STUDY Twenty-three grab samples collected with a Petterson grab were selected for the study of clay minerals from the set of samples collected during the cruises of RV "Anusandhani". The sediments were dispersed in distilled water and clays (42 p m) were separated following the sedimentation method (Krumbein and Pettijohn, 1938). The clays, thus separated were dried and analysed by X-ray diffraction in the Mineral Physics Division of the Geological Survey of India, Calcutta. RESULTS The X-ray diffraction data of the clays (<2pm) from the shelf sediments of the northwestern part of the Bay of Bengal revealed the predominance of illite and quartz with minor amounts of montmorillonite, kaolinite and felspar. The clay minerals have a considerable range of concentrations. (Table I, Fig.3). The distribution patterns of iUite, quartz, montmorillonite and kaolinite (Fig. 3) were prepared using the above semi-quantitative data and taking the upper end of each zone limit as the value for the station. Illite increases from 10--25 to 50% from west to east up to central portion from where it gradually starts decreasing on all sides (Fig. 3A). The maximum concentration of illite is southward of the Hooghly river mouth. Most of the area is characterised by more than 50% quartz, except the central portion which has between 25 and 50% (Fig. 3B). The central portion forms a lensoid-shaped body aligned northwest--southeast. The montmorillonite is distributed throughout the area in percentages of 10-25 except in the central portion where its percentage is high (Fig. 3C). The major part of the central basin is covered with 25--50% of kaolinite, decreasing on all sides to 25--10% (Fig. 3D). DISCUSSION
The clay mineral data (Table I, Fig. 3) clearly show the predominance of illite and quartz with minor amounts of montmorillonite, kaolinite and felspar. The predominance of illite in the shelf sediments off the Hooghly river mouth is noteworthy and can be attributed to the source rocks and soils in the drainage basins of the Ganges and Brahmaputra rivers. Illite, which predominates in the soils of northwestern India (the Punjab) appears to be transported through these rivers to the Bay of Bengal. This is supported by
M26 TABLE I Clay m i n e r a l c o n s t i t u e n t s in e a c h sample Sample No.
Constituents
Percentage
123
illite a n d m o n t m o r i l l o n i t e kaolinite and quartz felspar
> 50 50--25 25--10
137
illite a n d q u a r t z kaolinite m o n t m o r i l l o n i t e a n d felspar
> 50 15--10 10--05
145
quartz illite kaolinite and montmorillonite felspar
> 50 25--15 15--10 10--05
148
quartz illite kaolinite and montmorillonite felspar
> 50 25--15 15--10 10--05
150
illite a n d q u a r t z kaolinite m o n t m o r i l l o n i t e a n d felspar
> 50 50--25 10--05
152
illite a n d q u a r t z m o n t m o r i l l o n i t e a n d kaolinite felspar hematite
> 50 50--25 10--05 < 05
158
illite a n d m o n t m o r i l l o n i t e quartz and kaolinite felspar hematite
> 50 50--25 15--10 < 05
163
illite a n d q u a r t z kaolinite montmorillonite felspar a n d h e m a t i t e
> 50 15--10 10--05 < 05
170
quartz k a o l i n i t e a n d illite montmorillonite felspar
> 50 25--15 25--15 10--05
173
quartz montmorillonite kaolinite a n d illite felspar
> 50 25--15 15--10 10--05
176
quartz montmorillonite k a o l i n i t e a n d illite felspar
> 50 25--15 15--10 10--05
179
quartz montmorillonite k a o l i n i t e a n d illite felspar
> 50 25--15 15--10 10--05
M27
Sample No.
Constituents
Percentage
180
quartz montmorillonite kaolinite and illite felspar
> 50 25--15 15--10 10--05
182
quartz montmorillonite kaolinite and illite felspar
> 50 25--15 15--10 10--05
183
quart~; montmorillonite and illite kaolinite and felspar
> 50 25--15 10---05
185
quartz illite kaolinite, felspar and montmorillonite
> 50 25--15
187
quartz montmorillonite and illite kaolinite and felspar
> 50 25--15 10--05
190
quartz illite felspar montmorillonite
> 50 25--15 15--10 10--05
193
quartz kaolinite, illite and felspar montmorillonite
> 50 25--15 10--05
197
quartz illite kaolinite, felspar and montmorillonite
> 50 25--15
quartz illite kaolinite, felspar and montmorillonite
> 50 25--15
200
quartz kaolinite and illite felspar montmorillonite
> 50 25--15 15--10 10--05
202
quartz kaolinite and illite felspar montmorillonite
> 50 25--15 15--10 10--05
199
15--10
15--10
15--10
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Fig.3, Percent distribution o f clay minerals in shelf sediments o f the Bengal--Orissa coast, A. Tllite. B. Quartz. C. Montmorillonite. D. Kaolinite.
•
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M29
the observation of Kanwar (1959, 1961) who reports that the Punjab softs, which are derived from the Himalayan rocks, are characterised by the predominance of illite. Further, Goldberg and Griffin (1970) pointed out that the river-borne solids control clay-mineral distributions in the Bay of Bengal, especially in the northwestern sector, where the weathering products of the Himalayan region through the Ganges--Brahmaputra river system join. The mineral assemblages derived from Himalayan region are characterised by abundant illite and chlorite (Gupta, 1961). Sen and Chatterjee (1960) reported illite as the predominant clay mineral in the Ganges monsoon silt which ultimately is derived from the Himalayan region where mechanical weathering plays a dominant role. The foregoing discussion clearly explains the presence of the highest percentage of illite immediately near the Hooghly river mouth and seaward decrease into the open Bay of Bengal (Fig. 3A). The high percentage of quartz in these sediments (Fig. 3B) suggests the continental source through the Ganges and Brahmaputra river system. Quartz, being resistant to weathering, has remained virtually unaltered during weathering and transportation processes. During the mineralogical studies, Mallik (1976) also found quartz and felspar minerals in the light fractions. From the above observation, it appears that quartz is the significant detrital component of these fine-grained sediments. This is further supported by the observations of Venkatarathnam and Biscaye (1977) who reported that the Ganges--Brahmaputra river sources are primarily responsible for high percentages of quartz in the Bay of Bengal, and the quartz in these sediments is mainly continent-derived. The Ganges derives its sediments load from a variety of rocks, especially from the upper Pre-cambrian rocks in its upper reaches and from the Rajmahal traps in its lower course before joining the Hooghly River. The Rajmahal traps are found as hills at the head of the Ganges delta near the border of Bihar and West Bengal (Fig. 2). They bear a great resemblance to the Deccan Traps in composition and range from dolerite to basalt, depending on their texture (Krishnan, 1968). These traps contain significant amount of Mg and Na and a low content of K. The rainfall map of India shows that the drainage basin of the Ganges River falls under the moderate rainfall classification. The weathering under conditions of moderate rainfall permits the magnesium to remain in the weathering zone, after it is released by the breakdown leading to the formation of montmorillonite (smectite). This is substantiated by the observation of Roy and Barde (1962) who reported the formation of montmoriUonite from the augite basalt on weathering under moderate precipitation. The soil map of India shows the presence of lateritic soils along the Orissa and Bengal coast. The clay minerals of red soils have been studied by several workers. An important investigation is that of Nagelschmidt et al. (1940), who found kaolinite in red softs from khondalites, granites and granite gneisses (Eastern Ghats). Sen et al. (1961) reported that the premonsoon Ganges silt is mostly kaolinitic.: Shanumugam (1964) suggested that the top
M30
blue layer of the Hooghly River silt contains muscovite, kaolinite and quartz and the yellow bottom layer contains palygorskite, muscovite, quartz, kaolinite and illite. It may be considered from the above data that the kaolinite might have been derived from the Ganges drainage areas as well as from Mahanadi (Dhanra drainage). A review of the literature shows that, at present, there are two schools of thought regarding the presence of clay minerals in marine sediments. Murray and Harrison (1956), Weaver (1958, 1960) and Taggart and Kaiser (1960) support the detrital theory by emphasizing the importance of source area in controlling the types of clay minerals. On the other hand, many investigators including Dietz (1942), Grim et al. (1949), Millot (1949), Grim (1953), Grim and Johns (1954), Powers (1954) and Nelson (1959) have offered explanations supporting environmental diagenesis. From the nature and composition of the clay minerals in the shelf sediment off the Ganges delta and also from the foregoing discussion, it is clear that the clay minerals in these sediments are derived from the adjacent land mass and are mainly influenced by factors such as the nature of the source rocks and the climatic conditions under which weathering of these rocks took place. Further, the high rate of sedimentation off the Ganges delta gives no time for the clay minerals to undergo any environmental diagenesis in the prevailing environment. Subba Rao (1963) concluded that the nature of the source rocks has a far greater influence in determining the clay-mineral composition of the self sediments off the east coast of India, particularly in areas of rapid sedimentation. The distribution of various clay minerals in the area under study (Fig. 3) indicates the maximum concentration of each clay mineral in the central basin aligning northwest--southeast. This suggests that the predominant direction of movement of sediments in the area is towards the southeast. MaUik (1975) also inferred that the probable transport direction in this area is south to southeast, based on the grain-size variations of the sediments. CONCLUSIONS
(1) The X-ray diffraction study of clays (<2 ~m) of the shelf sediments of the northwestern part of the Bay of Bengal reveals the predominance of illite and quartz, with minor amounts of montmorillonite, kaolinite and felspar. (2) The presence of significant proportions of quartz in the fine fractions suggests a detrital nature. (3) The relatively high percentages of the various clay minerals immediately near the Hooghly river mouth suggest major contribution through the Ganges-Brahmaputra river systems. (4) Based on the clay-mineral distribution patterns, it is inferred that the predominant direction of sediment transport is towards the southeast.
M31 ACKNOWLEDGEMENTS
T h e a u t h o r s are g r a t e f u l t o Shri V.R. V e n k o b a R a o , D i r e c t o r , O M E & MG Division, G S I f o r g u i d a n c e a n d going t h r o u g h t h e m a n u s c r i p t ; Dr. D.R. D a s g u p t a , mineralogist-in-charge, f o r his k e e n i n t e r e s t in t h e w o r k , Mrs. L. B h a t t a c h a r j e e , Mineralogist (Sr), a n d Dr. G.L. Dwivedi, Mineralogist (Jr), Mineral Physics Divn., G e o l o g i c a l S u r v e y o f I n d i a f o r analysing t h e samples. Assistance r e n d e r e d b y Prof. W.D. Keller, D e p t . o f G e o l o g y , U n i v e r s i t y o f Missouri, C o l u m b i a , U.S.A. in critically going t h r o u g h t h e m a n u s c r i p t a n d suggesting i m p r o v e m e n t s is highly a p p r e c i a t e d . REFERENCES Anonymous, 1954. Andhra Univ. Memoir. Oceanogr., 1. Anonymous, 1958. Andhra Univ. Memoir. Oceanogr., 2. Bhattacharya, S.K. and Ghotankar, S.T., 1968. The estuary of the Hooghly. Bull. Natl. Inst. Sci. India, 38 (1): 25--32. Closs, H., Narain, H. and Garde, S.C., 1974. Continental margins of India. In: C.A. Burk and C.L. Drake (Editors), The Geology of Continental Margins, Springer, Berlin, pp. 629---639. Curray, J.R. and Moore, D.G., 1971. Growth of the Bengal deep-sea fan and denudation of the Himalayas. Geol. Soc. Am. Bull., 82 (3): 563--572. Dietz, R.S., 1972. Clay minerals in recent marine sediment. Am. Mineral., 27: 219--220. Goldberg, E.D. and Griffin, J.J., 1970. The sediments of the northern Indian Ocean. Deep-Sea Res., 17: 513--537. Gorbunova, Z.N., 1966. Clay mineral distribution in the Indian Ocean sediments. Okeanologiya, 6 (2); 267--275 (in Russian). Grim, R.E., 1953. Clay Mineralogy. McGraw-Hill, New York, N.Y. Grim, R.E. and Johns, W.D., 1954. Clays and clay minerals. N.A.S.--N.R.C., Publ., 327: 81--103. Grim, R.E., Dietz, R.S. and Bradley, W.F., 1949. Clay mineral composition of some sediments from the Pacific Ocean off the California coast and the Gulf of California. Geol. Soc. Am. Bull., 60: 1785--1808. Gupta, R.N., 1961. Clay minerals in soils of the lower Gangetic Basin of Uttar Pradesh. J. Indian Soc. Soil. Sci., 9: 141--149. Krishnan, M.S., 1968. Geology of India and Burma. Higginbothams, Madras, 555 pp. Krumbrein, W.C. and Pettijohn, F.J., 1938. Manual of Sedimentary Petrography. Appleton, New York, N.Y., 549 pp. La Fond, E.C., 1957. Oceanographic studies in the Bay of Bengal. Proc. Indian Acad. Sci., 46: 1--46. La Fond, E.C., 1966. The Bay of Bengal. In: Encyclopedia of Oceanography, Reinhold, New York, N.Y., 110--118. Mallik, T.K., 1975. A note on grain size variation of sediments at the mouth of the Hooghly River, Bay of Bengal. Indian J. Earth Sci., 2(2): 142--153. Mallik, T.K., 1976. Shelf sediments of the Ganges delta with special emphasis on the mineralogy of the western part, Bay of Bengal, Indian Ocean. Mar. Geol., 22: 1--32. Millot, G., 1949. Geol. Appl. prospect Min., 2(1): 352 pp. Murray, H.N. and Harrison, J.L., 1956. Clay mineral composition of recent sediments from Sigsbee Deep. J. Sediment. Petrol., 26: 363--368. Nagelschmidt, G., Desai, A.D. and Alex-Muir, 1940. The minerals in the clay fractions of black cotton soils and red earths from Hyderabad, Deccan State, India. J. Agric, Sci., 30: 605--647.
M32 Nelson, B.W., 1959. Clays and clay minerals. N.A.S.--N.R.C., Publ., pp. 135--147. Powers, M.C., 1954. Clays and clay minerals. N.A.S.--N.R.C., Publ., 327: 68--80. Rao, N.V.N.D.P. and Rao, M.P., 1977. Clay mineral distribution in sediments of the eastern part of the Bay of Bengal. Indian J. Mar. Sci., 6(2): 166--168. Roy, B.B. and Barde, N.K., 1962. Some characteristics of the black soils of India. Soil Sci., 93: 142--147. Sastry, A.V.R., Poornachandra Rao, M. and Mahadevan, C., 1958. Differential thermal analysis of plastic clays from the deltaic regions of Godavari and Krishna rivers. Andhra Univ. Mere. Oceanogr., 2: 61--68. Sen, R.K. and Chatterjee, B., 1960. Properties of Ganges silt. J. Indian Soc. Soil Sci., 8: 139--142. Sen, R.K., Mukherjee, P.K. and Chatterjee, B., 1961. Clay minerals in Ganges silt. Proc. Indian Sci. Congr., part III, Abstract. 190. Sharumugam, A.L., 1964. A study of grain size parameters and heavy minerals -- Auckland Bar--River Hooghly. In: Int. Geol. Congr. Rep., 22nd session, India, 15: 233--247. Siddiquie, H.N., 1967. Recent sediments of the Bay of the Bengal. Mar. Geol., 5: 249-291. Siddiquie, H.N., Shrivastava, P.C. and Mallik, T.K., 1970. Cruise Report -- Marine geological cruise of RV "Anusandhani". Indian Mineral., 24(4): 383--386. Subbe Rao, M., 1963. Clay mineral composition of shelf sediments off the east coast of India. Proc. Indian Acad. Sci., 58: 6--15. Taggart, M.S. and Kaiser, A.D., 1960. Clay mineralogy of Mississippi River Deltaic sediments. Geol. Soc. Am. Bull., 71: 521--530. Venkatarathnam, K. and Biscaye, P.E., 1973. Clay mineralogy and sedimentation in the Eastern Indian Ocean. Deep-Sea Res., 20: 727--738. Venkatarathnam, K. and Biscaye, P.E., 1977. Distribution and origin of quartz in the sediments of the Indian Ocean. J. Sediment. Petrol., 47(2): 642--649. Weaver, C.E., 1958. Geological interpretation of argillaceous sediments. Part I. Origin and significance of clay minerals in sedimentary rocks. Bull. Am. Assoc. Pet. Geol., 42: 254--309. Weaver, C.E., 1960. Possible uses of clay minerals in search for oil. Clays Clay Miner. 8th Conf., pp. 191--224.