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Provenance and distribution of clay minerals in the sediments of the western continental shelf and slope of India
Continental
Pergamon
Shelf Research, Vol. 15, No. 14, pp. 1757-1771.1995 Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0278-4343195 $9.50 + 0.00
027tM343(94)00092-1
Provenance and distribution of clay minerals in the sediments of the western continental shelf and slope of India V. PURNACHANDRA
RAO*
and B. RAMALINGESWARA
RAO*
(Received 6 September 1993; in revised form 24 August 1994; accepted 14 October 1994)
Abstract-The distribution of clay minerals from 156 surficial sediments of the western continental margin of India, ranging from 17 to 2000 m water depth, indicate that there are three principal sources of sediments. The illite and chlorite-rich assemblage derived from the Indus (Indus Province) is predominant in the continental margin sediments to the north of the Gulf of Kachchh. An assemblage of smectite with minor kaolinite, illite and chlorite, mostly derived from the Deccan Trap basalts (Deccan Trap Province), occurs all along the inner shelf from Saurashtra to Goa. Mite, however, dominates smectite in the outer shelf of Saurashtra and on the continental slope from Saurashtra to Goa. Some samples on the outer shelf of the Gulf of Cambay-Goa show trace contents of all clay minerals, while others from the same region show the dominance of smectitc over illite. A smectite and kaolinite-rich assemblage with minor ilhtc, chlorite and gibbsite derived from the Gneissic Province occurs both on the shelf and slope between Goa and Cochin. It appears that the Indus derived sediments are transported onto the continental slope and, to a lesser extent, the outer shelf of western India by a southerly surface current and admix with clays transported from the hinterland. The influence of the Indus borne sediments on the continental slope decreases from north to south and cross shelf transport processes dominate in the southwestern continental margin between Goa and Cochin.
1. INTRODUCTION Most fine-grained fluvially derived marine sediments are deposited in the nearshore and continental margins and only a small amount is transported to the deep sea (Chester, 1990). The fine-grained sediments are composed mainly of clay minerals and amorphous material. The composition and relative abundance of the clay minerals are controlled by their source rocks and weathering conditions. Their distribution on the continental shelf and slope is controlled by depositional processes, especially the current circulation patterns, and the settling of clay minerals in response to the salinity and energy conditions. A knowledge of these processes can also help to predict the transport pathways of pollutants (toxic metals and organic materials) which are preferentially concentrated in these fine-grained sediments. This paper reports the clay mineral composition of the sediments on the continental shelf and slope (up to 2000 m water depth) of the west coast of India (Fig. 1) and discusses the controlling factors for their distribution.
“National
Institute
of Oceanography,
Dona Paula-403004, 1757
Goa, India.
1758
V. Purnachandra
Rao and B. Ramalingeswara
66 ‘.6-l.
$4 66
Rao
63 . c
A
R A
B / A
N 142,
S
.I46
lil 144e
EA
Fig. 1.
Sample location
map-west
coast of India. Transects shown in Fig. 3.
labeled A-F from north to south are
Provenance
and distribution
1759
of clay minerals
1.1. Previous work
Clay mineral studies of the sediments from the west coast of India are few and have concentrated on localized areas of the shelf (Stewart ef al., 1965; Nair and Mutry, 1968; Nair et al., 1982a; Rao et al., 1983; Bhattacharya, 1984; Ramaswamy and Nair, 1989; Rao, 1991). Mattait et al. (1973) reported the clay mineralogy of samples from some widely spaced traverses of the continental shelf and Nair et al. (1982b) made a detailed study of the clayey sediments of the inner shelf. Subramanian (1980) and Naidu et al. (1985) reported the clay mineralogy of fluvial and bed load sediments from the west coast of India and Ramaswamy et al. (1991) the mineralogy of the particulate flux accumulated in traps moored at different locations in the Arabian Sea. Regional studies of the clay mineralogy of marine sediments from the Northern Indian Ocean (Griffin et al., 1968; Rateev et al., 1969; Goldberg and Griffin, 1970; Kolla and Biscaye, 1977; Kolla et al., 1976,198l) have shown the influence of the major rivers of the west coast of India on deep sea sedimentation. 1.2. Physiography and geology of the west coast of India The width of the continental shelf (Table 1) is widest (345 km) off Surat-Tarapur and narrows down to 60 km off Cochin. Off Saurashtra, it is about 7&110 km wide. The shelf break occurs at between 60 and 140 m water depth (Table 1). Two distinct sediment types occur on the continental shelf (Fig. 2): modern elastic clays on the inner shelf and relict sandy sediments on the outer shelf. The continental slope is covered by silty clays which are an admixture of dominant terrigenous and biogenic components. The outer shelf sediments are Holocene carbonate sands between Saurashtra and Mangalore, and terrigenous sands between Mangalore and Cochin (Nair and Pylee, 1968). The excess *r’Pb dating of core sediments has provided evidence that clays are currently accumulating on the continental slope (Borole, 1988). The Indus River is the largest source of sediments in the Arabian Sea and these sediments extend out to sea for distances of about 1000-1500 km. Recently the river has been dammed and channeled at several places. As a result, the annual river discharge has now been reduced to about 50 million tonnes (Milliman et al., 1984). It drains predominantly the Precambrian metamorphic rocks of the Himalayas and to a lesser extent semiarid and arid soils of west Pakistan and NW India (Krishnan, 1968). Deccan Trap basalts Table 1.
Approximate shelf width and shelf break at different locations of the west coast of India (M. Veerayya, personal communication) Width of the shelf
Width of the inner shelf
Water depth at shelf break
Location
(km)
(km)
(m)
Saurashtra Tarapur Bombay Ratnagiri Goa Bhatkal Cochin
70-l 10 345 280 128 94 110 60
40 175 80 40 45 45 25
80-140 95 go-125 110 120 120 60
1760
V. Purnachandra Rao and B. Ramalingeswara Rao
r
c
Surat
:--__
--__ .____ ___ -_ b-----
Tarapur 3ombay
\- Ratnoglrl
M
Clay
m
Carbonate Terrlgenous
-
Sands Sands
SW monsoon currents
---+
NE monsoon currents
_
Tidal
currents
70 Fig. 2.
73
General distribution of sediments and surface currents in the study region.
Provenance and distribution of clay minerals
1761
are the predominant rock types cropping out in the Saurashtra and the drainage basins of the Narmada and Tapti Rivers, which annually discharge about 60 million tonnes of sediment through the Gulf of Cambay (Rao, 1975). A semi-arid climate prevails in both the regions. The humid coastal regions south of the Tapti Valley are occupied by the Western Ghats, drained by 16 minor rivers. The Ghats are composed of basalts between Bombay and Goa, and Precambrian granites, gneisses, schists and charnockites between Goa and Cochin (Krishnan, 1968). The Ghats are located on the coast between Goa and Bhatkal but are 50-80 km from the coast south of Bhatkal. Between Bhatkal and Cochin, recent alluvium occurs near the coast, further landward Miocene Warkala beds (ferruginized sandstones with clay intercalations) are present along with the Ghats composed of gneissic rocks, which are in places covered by thick laterites (Krishnan, 1968). 1.3. Oceanographic setting During the SW monsoon (June-September) the rainfall is high and the rivers carry maximum sediment load to the continental shelf. There is a southerly coastal surface current (Banse, 1968) about 150 km wide (Shetye et al., 1990) that is observed in water depths of 50 m on the continental shelf (Fig. 2). South of 15”N intense coastal upwelling occurs during the SW Monsoon. During the NE monsoon (November-February), the southerly surface current is replaced by a northerly surface current (Fig. 2). An undercurrent of about 40 km wide in the depth interval 100-250 m (Shetye et al., 1990), characteristic of Bay of Bengal waters, prevails during both the SW and NE monsoons (Sarma et al., 1986; Pankajakshan and Rama Raju, 1987). However, it grows progressively weaker from south to north, and is not detected north of about 20”N. 2. MATERIALS
AND METHODS
A total of 156 surficial sediment samples (Fig. 1) were analyzed for the present study collected during different cruises of R. V. Gaveshani, M. V. Nand Rachit and 0. R. V. Sagar Kanya. One hundred and twenty-one samples were selected from the outer continental shelf and slope sediments down to about 2000 m water depth (Fig. 1). The remaining 35 sediments are from the inner shelf and their analyses were reported by Nair et al. (1982b). The less than 2 pm fractions of all sediment samples were separated and rendered free of organic matter and calcium carbonate. Oriented slides were prepared by pipetting 1 ml of the concentrated clay suspensions onto glass slides and allowing them to dry in air. The air dried and ethylene glycol treated [for 1 h at lOO”C-Carrol(1970)] slides were scanned from 2”to 22”28 at 1.2” 20 min-’ on a Philips X-ray diffractometer (PW 1840 model) using Nickel filtered Cu-K, radiation, equipped with an automatic divergent slit. The minerals were identified using procedures outlined by Brindley and Brown (1980). Following the semi-quantitative method of Biscaye (1965), the weighted peak area percentages of the major clay minerals were calculated from the X-ray diffractograms of the glycolated samples. All the samples were scanned from 24”to 26”28 at l/2” 28 min-’ to differentiate kaolinite and chlorite (Biscaye, 1964). The weighted peak area percentage obtained from the 7 A reflection of kaolinite plus chlorite was divided proportionally by using the peak (3.5 A) areas of kaolinite and chlorite obtained from slow scan diffractograms. Eight representative samples were saturated with 1N KOH solution and four samples with 1N MgClz solution for 16 h each to confirm the type of smectite and the
1762
V. Purnachandra Rao and B. Ramalingeswara Rao
presence of vermiculite. Besides, six samples were heated to 500°C to identify the nature of mixed layer minerals (Wilson, 1987). Nair et al. (1982b) reported clay mineral compositions of the inner shelf sediments based on XRD analysis of only air dried oriented samples using a Philips X-ray diffractometer (PW1130) with a non-automatic divergent slit (l/4” slit up to 9”20 and 1”slit from 9”28). In order to utilize these samples in the current study, 35 samples were reanalyzed to assess the compatibility of the data reported by Nair et al. (1982b) with the present study. XRD analyses of six representative groups of samples from the inner shelf, outer shelf/shelf break and continental slope are presented in Fig. 3. Gibbs (1965) noted differential segregation of clay minerals in oriented clay slides and suggested the mineral percentages would mainly be a function of thickness of the slide. An attempt was made to overcome this by using 1 ml of concentrated clay solutions on each slide. Also variation in the relative abundances of clay minerals reported here are very large and marginal variations have been ignored in the interpretations of the data. Weighted peak area percentages of the clay minerals and their ratios (S/I and WCh) are presented in Table 2. 3. RESULTS Smectite, illite, kaolinite and chlorite are the major clay mineral groups present in all the sediments and their concentrations vary significantly from one region to the other (Table 2). Illite-smectite and chlorite-smectite are identified as mixed layer clay minerals in most of the slope sediments. Gibbsite is minor and only prominent in the sediments of the SW continental margin. (Fig. 4). The highest concentrations of illite (74%) and chlorite (27%) occur in the sediments close to the Indus River mouth, smectite (86%) in the vicinity of the Gulf of Cambay and kaolinite (54%) in the sediments off Cochin. In all samples, the smectite reflection at 15 A expands to about 17 8, upon glycolation (Fig. 3). During Ksaturation, the 15 A spacing shifted to about 13 A in the samples north of Gulf of Kachchh and to 12 A in the samples of Saurashtra and further south. However, during Mgsaturation, the intensity of the 15 A reflection increases significantly in all the samples without a change in the d-spacing. These results confirm that the smectite is a dioctohedral montmorillonite (Wilson, 1987) and the absence of vermiculite. 3.1. North of Gulf of Kachchh (14 samples) Illite, chlorite, smectite and kaolinite are present in decreasing order of abundance [Table 2; Fig. 3(a)]. The highest concentrations of illite and chlorite are located in the offshore sediments north of the Gulf of Kachchh and close to the Gulf. Smectite is more abundant in the nearshore than the offshore sediments (Table 2). Kaolinite is a minor component in these sediments. 3.2. Saurashtra-Goa
(93 samples)
For the convenience of description, the Saurashtra-Goa region has been divided into the Saurashtra, Gulf of Cambay-Ratnagiri and Ratnagiri-Goa sectors based on variations in clay mineral abundances. The boundaries between these sectors are approximate and variations in the mineral abundances tend to grade one to the other. Smectite is the most abundant mineral in the inner shelf sediments of all the sectors [Fig. 3(Ba), 3(Ca) and
Provenance
0
Sm*ctlt*
m
and distribution
IlllIe
1763
of clay minerals
m
Kaollnl:~
m
Chlalte
Fig. 3. Representative transects (six numbers-A-F) showing glycolated X-ray diffractograms (Cu-K, radiation) of the inner continental shelf (a), outer continental shelf/shelf break (b), and continental slope (c) from north to south (transect positions are shown in Fig. 1). A slow scan separation of kaolinite and chlorite is also inserted between illite and kaolinite plus chlorite reflections. Diffractogram “Ca” is reduced to 75% before redrawing. Pie diagram shows the percentage of major clay minerals calculated by the Biscaye (1965) method.
1764
V. Purnachandra
Rao and B. Ramalingeswara
Rao
3(Da)]. It is highest in the sediments of Saurashtra and close to the Gulf of Cambay but decreases southwards with a relative increase in kaolinite (KKh ratio-Table 2). The illite content is low in the inner shelf sediments of all the sectors (S/I varies from 7.7 to 14). In contrast to the inner shelf, illite is greater than smectite on the outer shelf off Saurashtra and on the slope from Saurashtra to Goa (Table 2). Within the Gulf of Cambay-Ratnagiri sector, some outer shelf and slope sediments show trace contents of all clay minerals [Table 2; Fig. 3(Cbii)], whilst the others from the same region show the dominance of smectite over illite in the outer shelf [Fig. 3(Cbi)] and illite and chlorite dominated sediments in the slope [Table 2; Fig. I]. Most of the outer shelf samples in the Ratnagiri-Goa sector are enriched with smectite and kaolinite [Fig. 3(Db)], while in the adjacent slope, illite dominates the other clay minerals (Table 2). Overall, the average Table 2.
Clay mineral distribution (average) in the clay fraction of the sediments from the western continental margin of India
Weighted S
Area (A)
(B)
North of Gulf of Kachchh (i) Nearshore (3)* (ii) Offshore (11)
S/I
WCh
22 9
47 63
7 6
23 19
0.5 0.2
0.3 0.3
71 30 19
13 40 51
5 10 10
11 20 21
7.7 0.8 0.4
0.5 0.5 0.5
73 7 All minerals 45 24 All minerals 12 52
10 10 occur in traces 15 15 occur in traces 14 22
12.0
1.0
2.7
1.0
0.2
0.6
55 8 All minerals 42 13 All minerals 16 49
21 16 occur in traces 25 19 occur in traces 16 18
14.0
1.5
3.4
1.4
0.4
1.0
Off Saurashtra-Goa (a) Saurashtra (i) Inner continental shelf (5) (ii) Outer continental shelf (7) (iii) Continental slope (11) (b) Gulf of Cambay-Ratnagiri (i) Inner continental shelf (6) (ii) Outer continental shelf (9) (iii) Continental
(iii) Continental
(6) (10) (14)
slope
(c) Ratnagiri-Goa (i) Inner continental (ii) Outer continental
(C)
peak area percentages of I K Ch
shelf (5) shelf (3)
slope
(9) (3) (5)
Off Goa-Cochin (a) Goa-Bhatkal (i) Inner continental shelf (4) (ii) Outer continental shelf (5) (iii) Continental slope (20)
44 9 13
10 52 45
31 21 16
16 18 20
7.2 0.3 0.5
1.5 1.0 0.8
(b) Bhatkal-Cochin (i) Inner continental shelf (7) (ii) Proximity to Shelf Break (5) (iii) Continental slope (8)
42 16 14
12 35 33
29 33 34
I6 16 19
7.1 0.9 0.6
2.1 2.3 2.1
S-smectite,
I-illite,
K-kaolinite,
Ch-chlorite;
* number
of samples
studied.
Provenance
and distribution
of clay minerals
1765
illite and chlorite contents in the outer shelf and slope sediments decrease from Saurashtra to Goa, with a corresponding increase in kaolinite (Table 2).
3.3. Goa-Cochin
(49 samples)
A smectite and kaolinite-rich assemblage with low illite (S/I = 7.2) and chlorite (WCh = 1.5-2.0) occurs on the inner shelf from Goa to Cochin [Fig. 3(Ea), 3(Fa); Table 21. In the Goa-Bhatkal sector, illite relatively increases in the outer shelf/shelf break (S/I = 0.3) and
Bombay (54)
Ratnagiri (70)
28 Fig. 4. Seven representative sample diffractograms from the continental slope, showing variations in the intensities of illite (I), chlorite (Ch), quartz (Q) and gibbsite (G) reflections.
1766
V. Purnachandra Rao and B. Ramalingeswara Rao
on the slope (S/I = 0.5). [Fig. 3(Eb-c); Bhatkal-Cochin sector show that there higher than in the preceding sector [S/I Gibbsite reflections became prominent Bhatkal-Cochin sector (Fig. 5).
Table 21. The corresponding sediments in the is a lower net increase of illite and kaolinite is = 0.6-0.9; K/Ch = 2.0; Fig. 3(Fb-c); Table 21. only in the shelf and slope sediments of the
4. DISCUSSION 4.1. North of Gulf of Kachchh The Indus is located in this region and brings sediments mainly from the glacial Himalayas and semi-arid to arid soils. Therefore, the physical/mechanical weathering products (illite and chlorite) dominate the chemical weathering products (smectite and kaolinite) in the river discharge. The illite and chlorite-rich sediments thus characterize the Indus derived products. The analyses of the suspended load of the Indus river (Konta, 1985) and the sediments of the northern Arabian Sea (Kolla et al., 1981) also show enhanced illite and chlorite abundances.
4.2. Saurashtra-Goa Smectite is the predominant weathering product of basalts in arid to semi-arid climates and smectite and kaolinite in humid tropical climates (Millot, 1970; Charnley, 1989). The mineralogy of the black cotton soils derived from weathered basalts (Karkare, 1965), and of the suspended and bed loads of the Narmada and Tapti rivers (Subramanian, 1980; Naidu et al., 1985) showed high smectite content. High values of smectite in the inner shelf of Saurashtra [Fig. 3(Ba)] and in the vicinity of the Gulf of Cambay [Fig. 3(Ca)], together with decreased abundances towards the south and a concomitant increase in kaolinite [Fig. 3(Da); Table 21 reflect the weathering products of plateau basalts in semi-arid climates and basalts associated with Ghats in humid climates, respectively. In contrast to the north of the Gulf of Kachchh [Fig. 3(A)], illite abruptly decreases in the shelf sediments off Saurashtra [Table 2; Fig. 3(B)]. This suggests that the macro-tidal currents operating in the Gulf act as barriers to the longshore sediment transport from the north (Hashimi et al., 1978; Nair et al., 1982a). The higher illite and chlorite values in the outer shelf (Table 2) and much higher values on slope of Saurashtra [Fig. I] indicate a greater influence from the Indus source. These values suggest that the effect of the tidal barrier is perhaps greatest on the inner shelf, moderate on the outer shelf and insignificant on the continental slope. The Indus borne clay mineral assemblage is thus abundant on the slope. Smectite followed by kaolinite are dominant clay minerals in some samples of the outer shelf (mid-shelf) off the Gulf of Cambay-Goa (Table 2). This implies that they are transported from the hinterland. Samples showing trace contents of all clay minerals (Table 2) are either associated with relict carbonate sands [Halimeda and fecal pelletdominated sands-Rao et al., (1994)] on the outer shelf or located close to the shelf break and/or on the upper continental slope. Hydrographic and tracer sediment studies indicate the prevalence of high energy environment on the outer shelf (Anonymous, 1976; Veerayya and Nath, 1994), and high tidal and internal wave generated currents at the shelf
Provenance
and distribution
1767
of clay minerals
3ombay
m
Indus
y//k
Basaltic
source 1 B I
b\\p
Gnoissic
source
-
Influence of the lndus Longshore current
-_-_ em_.
Fig. 5.
source (1)
T
Cross shelf
Synthesis
of sediment
(G 1 by
transport
sources in the study region. (Dashed of clay minerals on the outer shelf.)
double
arrows show bypassing
1768
V. Purnachandra Rao and B. Ramalingeswara Rao
break (Shenoi et al., 1988). Aragonitic muds and coarse grained shallow water carbonate sediments slumped from the outer shelf were reported in some surficial sediments of the continental slope (Hussain and Guptha, 1985). Trace contents of all clay minerals in these samples indicate that the clay fraction of these sediments is also biogenic, probably derived from Halimeda plates which produce abundant aragonitic muds upon disintegration (see Rao et al., 1994). Unlike other sediments, the clay slides prepared from the sediments of this region are whitish supporting that the clay fraction is an acid-insoluble residue of the carbonate muds. Trace contents of detrital clay minerals suggest that they bypassed the outer shelf due to high energy conditions and deposited on the deeper slope. Kolla et al. (1976) reported smectite-rich sediments, transported from the Deccan Trap basalts, on the deeper continental slope of the west coast of India. The dominance of Indus sourced clays in some continental slope samples [Fig. 3(Cc) and (DC); Table 21 indicate that they are carried by a southerly surface current (Fig. 5) and mixed with the clays transported by the Narmada and Tapti Rivers. 4.3. Goa-Cochin Smectite and kaolinite-rich assemblages on the inner shelf reflect a source from intense chemical weathering products. As the Ghats with steeper western gradients are located on the coast between Goa and Bhatkal (Krishnan, 1968) and receive maximum rainfall (3000 mm) during the SW monsoon, one would expect both mechanically derived residual clay minerals (illite and chlorite) and abundant kaolinite and smectite. The low illite and chlorite contents in the inner shelf clays [Fig. 4(Ea)] and their increased abundance in the outer shelf/shelf break sandy sediments and slope clays [Fig. 4(Eb) and (EC)] are therefore anomalous. The 001 reflections of illite are broad and asymmetric in the Goa-Cochin samples in contrast to the symmetrical reflections of illite close to the Indus. The less crystalline illites and chlorites are associated with an increase in kaolinite and gibbsite, the characteristic clays of this region [Fig. 3(Eb); Fig. 41. It is therefore likely that some illite and chlorite are also sourced from Western Ghats but during weathering in the humid tropical climate and are less crystalline than those derived from the Indus. Their higher proportions on the outer shelf/shelf break (sandy) and slope (silty clays) compared to the inner shelf (clays) sediments may largely be due to the combined influence of depositional environment and effective transport processes and to a lesser extent mixing of clays from the Indus. Maldonado and Stanley (1981) and Stanley and Liyanage (1986) reported high smectite content in clayey sediments and high illite in sandy sediments and suggested that the clay minerals’ abundance vary with grain size and thus reflect the depositional environment. High surface runoff and offshore transport of the surface water during the SW monsoons together with the narrow shelf (Table 1) in this region are probably responsible for the effective transport of illite and chlorite farther from the coast and their accumulation in the outer shelf and slope. The highest kaolinite occurs between Bhatkal and Cochin (Table 2). Some kaolinite together with gibbsite probably derived from Ghats which are covered by laterites and weather in humid climates. Kaolin deposits occur in Warkala deposits which outcrop near the coast. So some kaolinite might have derived directly from these sources. High contents of kaolinite and gibbsite indicate intense chemical weathering on land and their presence on the continental slope [Figs 3(Fc) and 51 indicate cross shelf transport of these minerals
Provenance
and distribution
of clay minerals
1769
from the coastal region. It appears that the Indus derived clay minerals are less significant than the clays derived from the hinterland.
5. CONCLUSIONS There are three principal sediment sources: The Indus Province-rich in illite and chlorite, the Deccan Trap Province-rich in smectite and the Gneissic Province-rich in smectite and kaolinite (Fig. 5). The inner shelf sediments generally reflect these sediment provinces, indicating that they are not affected by longshore currents, as also suggested by Nair et al. (1982b). However, the sediments of the outer shelf of Saurashtra and the continental slope of Saurashtra-Goa, have a clay mineral assemblage that reflects admixing of Indus Province clays with those from the hinterland (Fig. 5). The clay mineralrich sediments mostly bypass the outer shelf between the Gulf of Cambay and Goa because of the high energy environment. Between Goa and Cochin the shelf and slope sediments represent mostly the influence of Gneissic Province (Fig. 5) indicating cross shelf transport in this region. The effect of the Indus decreases from north to south and is insignificant on the SW continental margin. The northerly current associated with NE monsoon has little effect on clay mineral distribution due to limited sediment discharge from the rivers during this season. Acknowledgements--We thank Dr E. Desa, the Director of the National Institute of Oceanography, Mr R. R. Nair, the Head of the Geological Oceanography Division and Dr M. Veerayya for their encouragement. We thank Dr V. S. N. Murthy for helping us in understanding the current pattern and Mr Girish Prabhu for scanning clay slides with X-ray diffractometry. We thank the three anonymous reviewers for their critical comments which improved the quality of the manuscript.
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distribution
Rao
clay minerals
in the World Ocean.
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of the Deccan
Traps.
Ph.D.
Thesis (unpublished),
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tide
of India. Hindu
Kolla V. and P. E. Biscaye (1977) Distribution and origin of quartz in the sediments of the Indian Ocean. Journal of Sedimentary Petrology, 47,642-649. Kolla V., L. Henderson and P. E. Biscaye (1976) Clay mineralogy and sedimentation in the western Indian Ocean. Deep Sea Research, 23,949-961. Kolla V., J. E. Kostecki, F. Robinson and P. E. Biscaye (1981) Distribution and origin of clay minerals and quartz in surface sediments of the Arabian Sea. Journal of Sedimentary Petrology, 51,563-569. Konta
J. (1985) Mineralogy and chemical maturity of suspended matter in major rivers sampled under the SCOPEiUNEP project. Milleiungen aus dem geologisch-paleontologischen Institute. University of Hamburg, Heft 58,557-568. Krishnan M. S. (1968) Geology of India and Burma, Higginbothoms Ltd, Madras, 536 pp. Mattait B., J. Peters and F. J. Eckhardt (1973) Results of petrographic analysis on sediments from the Indian Pakistan continental margin (Arabian Sea) (German) “Meteor” Forsch Ergebrisse Reiche no. 14, Suite l50. Maldonado A. and D. J. Stanley (1981) Clay mineral distribution patterns as influenced by depositional processes in the Southeastern Levantine Sea. Sedimentology, 28,21-32. Milliman J. D., G. S. Quraishee and M. A. A. Beg (1984) Sedimentdischargefrom the Indus River to the ocean. Marine Geology and Oceanography of the Arabian Sea and coastal Pakistan. Scientific and Academic Editions, New York, pp. 65-70. Millot G. (1970) Geology of clays, Springer-Verlag, New York, 375 pp. Naidu A. S., T. C. Mowatt, B. L. K. Somayajulu and K. S. Rao (1985) Characteristics of clay minerals in the bed loads of major rivers of India. Milleiungen Aus dem geologisch-Paleontologischen, Institute. University of Hamburg, Heft 2, 559-568. Nair R. R., N. H. Hashimi and V. P. Rao (1982a) On the possibility of high velocity tidal streams as dynamic barriers to longshore sediment transport: Evidence from the continental shelf off the Gulf of Kutch, India. Marine Geology, 47,77-86. Nair R. R., N. H. Hashimi and V. P. Rao (1982b) Distribution and dispersal of clay minerals on the western continental shelf of India. Marine Geology, 50, Ml-M9. Nair R. R. and P. S. N. Mutry (1968) Clay mineralogy of mud banks off Cochin. Current Science, 37,589-590. Nair R. R. and A. Pylee (1968) Size distribution and carbonate content of the sediments of the western continental shelf of India. Bulletin National Institute of Science India, 38, 411420. Pankajakshan T. and D. V. Rama Raju (1987) Intrusion of Bay of Bengal water into Arabian Sea along the west coast of India during Northeast monsoon. S. Z. Qasim 60th birthday felicitation volume 1987, pp. 237-244. Rao K. L. (1975) India’s water wealth, Orient Longman, New Delhi, 255 pp. Rao V. P. (1991) Clay mineral distribution in the continental shelf and slope off Saurashtra, west coast of India. Indian Journal of Marine Sciences, 20, l-6. Rao V. P., R. R. Nair and N. H. Hashimi (1983) Clay mineral distribution on the Kerala continental shelf and slope. Journal of Geological Society of India, 24, 540-546. Rao V. P., M. Veerayya, R. R. Nair, P. A. Dupeuble and M. Lamboy (1994) Late Quaternary Halimeda bioherms and aragonitic faecal pellet-dominated sediments on the carbonate platform of the western continental shelf of India. Marine Geology, 121 (in press). Ramaswamy V. and R. R. Nair (1989) Lack of cross shelf transport of sediments on the western margin of India: evidence from clay mineralogy. Journal of Coastal Research, 5, 541-546. Ramaswamy V., R. R. Nair, S. Manganini, B. Haake and V. Ittekkot (1991) Lithogenic fluxes to the deep Arabian Sea measured by sediment traps. Deep Sea Research, 38, 169-184. Rateev M. A., Z. N. Gorbunova, A. P. Lisitzin and G. L. Nosov (1969) The distribution of clay minerals on the ocean. Sedimentology, 13,21&43. Sarma Y. V. B., V. S. N. Murthy, D. P. Rao and J. S. Sastry (1986) Circulation and watermass structure in the Central Arabian Sea during December 1982. Indian Journal of Marine Sciences, 15.234-240.
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and distribution
of clay minerals
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Shenoi S. C., M. K. Antony and D. Sunder (lY88) Nature of the observed oscillatory flows in shelf waters of the western continental shelf of India. Journal of Coasful Research, 4,617-626. Shetye S. R., A. D. Gouveia, S. S. C. Shenoi, D. Sunder, G. S. Michael, A. M. Almeida and K. Santanam (1990) Hydrography and circulation off the west coast of India during the southwest monsoon 1987. Journal of Marine Research, 48,359-378. Stanley D. J. and A. N. Liyanage (1986) Clay mineral variations in the northeastern Nile delta as influenced by depositional processes. Marine Geology, 73,263-283. Stewart R. A.. 0. H. Pilkey and B. W. Nelson (1965) Sediments of the northern Arabian Sea. Marine Geology. 3,4131127. Subramanian V. (1980) Mineralogical input of suspended matter by Indian Rivers into the adjacent areas of the Indian Ocean. Marine Geology, 36, M29-M34. Veerayya M. and A. S. M. Nath (1994) Sediment movement on the continental shelf of Bombay, India. Continental Shelf Research, 14,1689-1700. Wilson M. J. (1987) A handbook of determinative methods in clay mineralogy, Chapman and Hall. New York. 308 pp.
Pergamon
Shelf Research, Vol. 15, No. 14, pp. 1757-1771.1995 Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0278-4343195 $9.50 + 0.00
027tM343(94)00092-1
Provenance and distribution of clay minerals in the sediments of the western continental shelf and slope of India V. PURNACHANDRA
RAO*
and B. RAMALINGESWARA
RAO*
(Received 6 September 1993; in revised form 24 August 1994; accepted 14 October 1994)
Abstract-The distribution of clay minerals from 156 surficial sediments of the western continental margin of India, ranging from 17 to 2000 m water depth, indicate that there are three principal sources of sediments. The illite and chlorite-rich assemblage derived from the Indus (Indus Province) is predominant in the continental margin sediments to the north of the Gulf of Kachchh. An assemblage of smectite with minor kaolinite, illite and chlorite, mostly derived from the Deccan Trap basalts (Deccan Trap Province), occurs all along the inner shelf from Saurashtra to Goa. Mite, however, dominates smectite in the outer shelf of Saurashtra and on the continental slope from Saurashtra to Goa. Some samples on the outer shelf of the Gulf of Cambay-Goa show trace contents of all clay minerals, while others from the same region show the dominance of smectitc over illite. A smectite and kaolinite-rich assemblage with minor ilhtc, chlorite and gibbsite derived from the Gneissic Province occurs both on the shelf and slope between Goa and Cochin. It appears that the Indus derived sediments are transported onto the continental slope and, to a lesser extent, the outer shelf of western India by a southerly surface current and admix with clays transported from the hinterland. The influence of the Indus borne sediments on the continental slope decreases from north to south and cross shelf transport processes dominate in the southwestern continental margin between Goa and Cochin.
1. INTRODUCTION Most fine-grained fluvially derived marine sediments are deposited in the nearshore and continental margins and only a small amount is transported to the deep sea (Chester, 1990). The fine-grained sediments are composed mainly of clay minerals and amorphous material. The composition and relative abundance of the clay minerals are controlled by their source rocks and weathering conditions. Their distribution on the continental shelf and slope is controlled by depositional processes, especially the current circulation patterns, and the settling of clay minerals in response to the salinity and energy conditions. A knowledge of these processes can also help to predict the transport pathways of pollutants (toxic metals and organic materials) which are preferentially concentrated in these fine-grained sediments. This paper reports the clay mineral composition of the sediments on the continental shelf and slope (up to 2000 m water depth) of the west coast of India (Fig. 1) and discusses the controlling factors for their distribution.
“National
Institute
of Oceanography,
Dona Paula-403004, 1757
Goa, India.
1758
V. Purnachandra
Rao and B. Ramalingeswara
66 ‘.6-l.
$4 66
Rao
63 . c
A
R A
B / A
N 142,
S
.I46
lil 144e
EA
Fig. 1.
Sample location
map-west
coast of India. Transects shown in Fig. 3.
labeled A-F from north to south are
Provenance
and distribution
1759
of clay minerals
1.1. Previous work
Clay mineral studies of the sediments from the west coast of India are few and have concentrated on localized areas of the shelf (Stewart ef al., 1965; Nair and Mutry, 1968; Nair et al., 1982a; Rao et al., 1983; Bhattacharya, 1984; Ramaswamy and Nair, 1989; Rao, 1991). Mattait et al. (1973) reported the clay mineralogy of samples from some widely spaced traverses of the continental shelf and Nair et al. (1982b) made a detailed study of the clayey sediments of the inner shelf. Subramanian (1980) and Naidu et al. (1985) reported the clay mineralogy of fluvial and bed load sediments from the west coast of India and Ramaswamy et al. (1991) the mineralogy of the particulate flux accumulated in traps moored at different locations in the Arabian Sea. Regional studies of the clay mineralogy of marine sediments from the Northern Indian Ocean (Griffin et al., 1968; Rateev et al., 1969; Goldberg and Griffin, 1970; Kolla and Biscaye, 1977; Kolla et al., 1976,198l) have shown the influence of the major rivers of the west coast of India on deep sea sedimentation. 1.2. Physiography and geology of the west coast of India The width of the continental shelf (Table 1) is widest (345 km) off Surat-Tarapur and narrows down to 60 km off Cochin. Off Saurashtra, it is about 7&110 km wide. The shelf break occurs at between 60 and 140 m water depth (Table 1). Two distinct sediment types occur on the continental shelf (Fig. 2): modern elastic clays on the inner shelf and relict sandy sediments on the outer shelf. The continental slope is covered by silty clays which are an admixture of dominant terrigenous and biogenic components. The outer shelf sediments are Holocene carbonate sands between Saurashtra and Mangalore, and terrigenous sands between Mangalore and Cochin (Nair and Pylee, 1968). The excess *r’Pb dating of core sediments has provided evidence that clays are currently accumulating on the continental slope (Borole, 1988). The Indus River is the largest source of sediments in the Arabian Sea and these sediments extend out to sea for distances of about 1000-1500 km. Recently the river has been dammed and channeled at several places. As a result, the annual river discharge has now been reduced to about 50 million tonnes (Milliman et al., 1984). It drains predominantly the Precambrian metamorphic rocks of the Himalayas and to a lesser extent semiarid and arid soils of west Pakistan and NW India (Krishnan, 1968). Deccan Trap basalts Table 1.
Approximate shelf width and shelf break at different locations of the west coast of India (M. Veerayya, personal communication) Width of the shelf
Width of the inner shelf
Water depth at shelf break
Location
(km)
(km)
(m)
Saurashtra Tarapur Bombay Ratnagiri Goa Bhatkal Cochin
70-l 10 345 280 128 94 110 60
40 175 80 40 45 45 25
80-140 95 go-125 110 120 120 60
1760
V. Purnachandra Rao and B. Ramalingeswara Rao
r
c
Surat
:--__
--__ .____ ___ -_ b-----
Tarapur 3ombay
\- Ratnoglrl
M
Clay
m
Carbonate Terrlgenous
-
Sands Sands
SW monsoon currents
---+
NE monsoon currents
_
Tidal
currents
70 Fig. 2.
73
General distribution of sediments and surface currents in the study region.
Provenance and distribution of clay minerals
1761
are the predominant rock types cropping out in the Saurashtra and the drainage basins of the Narmada and Tapti Rivers, which annually discharge about 60 million tonnes of sediment through the Gulf of Cambay (Rao, 1975). A semi-arid climate prevails in both the regions. The humid coastal regions south of the Tapti Valley are occupied by the Western Ghats, drained by 16 minor rivers. The Ghats are composed of basalts between Bombay and Goa, and Precambrian granites, gneisses, schists and charnockites between Goa and Cochin (Krishnan, 1968). The Ghats are located on the coast between Goa and Bhatkal but are 50-80 km from the coast south of Bhatkal. Between Bhatkal and Cochin, recent alluvium occurs near the coast, further landward Miocene Warkala beds (ferruginized sandstones with clay intercalations) are present along with the Ghats composed of gneissic rocks, which are in places covered by thick laterites (Krishnan, 1968). 1.3. Oceanographic setting During the SW monsoon (June-September) the rainfall is high and the rivers carry maximum sediment load to the continental shelf. There is a southerly coastal surface current (Banse, 1968) about 150 km wide (Shetye et al., 1990) that is observed in water depths of 50 m on the continental shelf (Fig. 2). South of 15”N intense coastal upwelling occurs during the SW Monsoon. During the NE monsoon (November-February), the southerly surface current is replaced by a northerly surface current (Fig. 2). An undercurrent of about 40 km wide in the depth interval 100-250 m (Shetye et al., 1990), characteristic of Bay of Bengal waters, prevails during both the SW and NE monsoons (Sarma et al., 1986; Pankajakshan and Rama Raju, 1987). However, it grows progressively weaker from south to north, and is not detected north of about 20”N. 2. MATERIALS
AND METHODS
A total of 156 surficial sediment samples (Fig. 1) were analyzed for the present study collected during different cruises of R. V. Gaveshani, M. V. Nand Rachit and 0. R. V. Sagar Kanya. One hundred and twenty-one samples were selected from the outer continental shelf and slope sediments down to about 2000 m water depth (Fig. 1). The remaining 35 sediments are from the inner shelf and their analyses were reported by Nair et al. (1982b). The less than 2 pm fractions of all sediment samples were separated and rendered free of organic matter and calcium carbonate. Oriented slides were prepared by pipetting 1 ml of the concentrated clay suspensions onto glass slides and allowing them to dry in air. The air dried and ethylene glycol treated [for 1 h at lOO”C-Carrol(1970)] slides were scanned from 2”to 22”28 at 1.2” 20 min-’ on a Philips X-ray diffractometer (PW 1840 model) using Nickel filtered Cu-K, radiation, equipped with an automatic divergent slit. The minerals were identified using procedures outlined by Brindley and Brown (1980). Following the semi-quantitative method of Biscaye (1965), the weighted peak area percentages of the major clay minerals were calculated from the X-ray diffractograms of the glycolated samples. All the samples were scanned from 24”to 26”28 at l/2” 28 min-’ to differentiate kaolinite and chlorite (Biscaye, 1964). The weighted peak area percentage obtained from the 7 A reflection of kaolinite plus chlorite was divided proportionally by using the peak (3.5 A) areas of kaolinite and chlorite obtained from slow scan diffractograms. Eight representative samples were saturated with 1N KOH solution and four samples with 1N MgClz solution for 16 h each to confirm the type of smectite and the
1762
V. Purnachandra Rao and B. Ramalingeswara Rao
presence of vermiculite. Besides, six samples were heated to 500°C to identify the nature of mixed layer minerals (Wilson, 1987). Nair et al. (1982b) reported clay mineral compositions of the inner shelf sediments based on XRD analysis of only air dried oriented samples using a Philips X-ray diffractometer (PW1130) with a non-automatic divergent slit (l/4” slit up to 9”20 and 1”slit from 9”28). In order to utilize these samples in the current study, 35 samples were reanalyzed to assess the compatibility of the data reported by Nair et al. (1982b) with the present study. XRD analyses of six representative groups of samples from the inner shelf, outer shelf/shelf break and continental slope are presented in Fig. 3. Gibbs (1965) noted differential segregation of clay minerals in oriented clay slides and suggested the mineral percentages would mainly be a function of thickness of the slide. An attempt was made to overcome this by using 1 ml of concentrated clay solutions on each slide. Also variation in the relative abundances of clay minerals reported here are very large and marginal variations have been ignored in the interpretations of the data. Weighted peak area percentages of the clay minerals and their ratios (S/I and WCh) are presented in Table 2. 3. RESULTS Smectite, illite, kaolinite and chlorite are the major clay mineral groups present in all the sediments and their concentrations vary significantly from one region to the other (Table 2). Illite-smectite and chlorite-smectite are identified as mixed layer clay minerals in most of the slope sediments. Gibbsite is minor and only prominent in the sediments of the SW continental margin. (Fig. 4). The highest concentrations of illite (74%) and chlorite (27%) occur in the sediments close to the Indus River mouth, smectite (86%) in the vicinity of the Gulf of Cambay and kaolinite (54%) in the sediments off Cochin. In all samples, the smectite reflection at 15 A expands to about 17 8, upon glycolation (Fig. 3). During Ksaturation, the 15 A spacing shifted to about 13 A in the samples north of Gulf of Kachchh and to 12 A in the samples of Saurashtra and further south. However, during Mgsaturation, the intensity of the 15 A reflection increases significantly in all the samples without a change in the d-spacing. These results confirm that the smectite is a dioctohedral montmorillonite (Wilson, 1987) and the absence of vermiculite. 3.1. North of Gulf of Kachchh (14 samples) Illite, chlorite, smectite and kaolinite are present in decreasing order of abundance [Table 2; Fig. 3(a)]. The highest concentrations of illite and chlorite are located in the offshore sediments north of the Gulf of Kachchh and close to the Gulf. Smectite is more abundant in the nearshore than the offshore sediments (Table 2). Kaolinite is a minor component in these sediments. 3.2. Saurashtra-Goa
(93 samples)
For the convenience of description, the Saurashtra-Goa region has been divided into the Saurashtra, Gulf of Cambay-Ratnagiri and Ratnagiri-Goa sectors based on variations in clay mineral abundances. The boundaries between these sectors are approximate and variations in the mineral abundances tend to grade one to the other. Smectite is the most abundant mineral in the inner shelf sediments of all the sectors [Fig. 3(Ba), 3(Ca) and
Provenance
0
Sm*ctlt*
m
and distribution
IlllIe
1763
of clay minerals
m
Kaollnl:~
m
Chlalte
Fig. 3. Representative transects (six numbers-A-F) showing glycolated X-ray diffractograms (Cu-K, radiation) of the inner continental shelf (a), outer continental shelf/shelf break (b), and continental slope (c) from north to south (transect positions are shown in Fig. 1). A slow scan separation of kaolinite and chlorite is also inserted between illite and kaolinite plus chlorite reflections. Diffractogram “Ca” is reduced to 75% before redrawing. Pie diagram shows the percentage of major clay minerals calculated by the Biscaye (1965) method.
1764
V. Purnachandra
Rao and B. Ramalingeswara
Rao
3(Da)]. It is highest in the sediments of Saurashtra and close to the Gulf of Cambay but decreases southwards with a relative increase in kaolinite (KKh ratio-Table 2). The illite content is low in the inner shelf sediments of all the sectors (S/I varies from 7.7 to 14). In contrast to the inner shelf, illite is greater than smectite on the outer shelf off Saurashtra and on the slope from Saurashtra to Goa (Table 2). Within the Gulf of Cambay-Ratnagiri sector, some outer shelf and slope sediments show trace contents of all clay minerals [Table 2; Fig. 3(Cbii)], whilst the others from the same region show the dominance of smectite over illite in the outer shelf [Fig. 3(Cbi)] and illite and chlorite dominated sediments in the slope [Table 2; Fig. I]. Most of the outer shelf samples in the Ratnagiri-Goa sector are enriched with smectite and kaolinite [Fig. 3(Db)], while in the adjacent slope, illite dominates the other clay minerals (Table 2). Overall, the average Table 2.
Clay mineral distribution (average) in the clay fraction of the sediments from the western continental margin of India
Weighted S
Area (A)
(B)
North of Gulf of Kachchh (i) Nearshore (3)* (ii) Offshore (11)
S/I
WCh
22 9
47 63
7 6
23 19
0.5 0.2
0.3 0.3
71 30 19
13 40 51
5 10 10
11 20 21
7.7 0.8 0.4
0.5 0.5 0.5
73 7 All minerals 45 24 All minerals 12 52
10 10 occur in traces 15 15 occur in traces 14 22
12.0
1.0
2.7
1.0
0.2
0.6
55 8 All minerals 42 13 All minerals 16 49
21 16 occur in traces 25 19 occur in traces 16 18
14.0
1.5
3.4
1.4
0.4
1.0
Off Saurashtra-Goa (a) Saurashtra (i) Inner continental shelf (5) (ii) Outer continental shelf (7) (iii) Continental slope (11) (b) Gulf of Cambay-Ratnagiri (i) Inner continental shelf (6) (ii) Outer continental shelf (9) (iii) Continental
(iii) Continental
(6) (10) (14)
slope
(c) Ratnagiri-Goa (i) Inner continental (ii) Outer continental
(C)
peak area percentages of I K Ch
shelf (5) shelf (3)
slope
(9) (3) (5)
Off Goa-Cochin (a) Goa-Bhatkal (i) Inner continental shelf (4) (ii) Outer continental shelf (5) (iii) Continental slope (20)
44 9 13
10 52 45
31 21 16
16 18 20
7.2 0.3 0.5
1.5 1.0 0.8
(b) Bhatkal-Cochin (i) Inner continental shelf (7) (ii) Proximity to Shelf Break (5) (iii) Continental slope (8)
42 16 14
12 35 33
29 33 34
I6 16 19
7.1 0.9 0.6
2.1 2.3 2.1
S-smectite,
I-illite,
K-kaolinite,
Ch-chlorite;
* number
of samples
studied.
Provenance
and distribution
of clay minerals
1765
illite and chlorite contents in the outer shelf and slope sediments decrease from Saurashtra to Goa, with a corresponding increase in kaolinite (Table 2).
3.3. Goa-Cochin
(49 samples)
A smectite and kaolinite-rich assemblage with low illite (S/I = 7.2) and chlorite (WCh = 1.5-2.0) occurs on the inner shelf from Goa to Cochin [Fig. 3(Ea), 3(Fa); Table 21. In the Goa-Bhatkal sector, illite relatively increases in the outer shelf/shelf break (S/I = 0.3) and
Bombay (54)
Ratnagiri (70)
28 Fig. 4. Seven representative sample diffractograms from the continental slope, showing variations in the intensities of illite (I), chlorite (Ch), quartz (Q) and gibbsite (G) reflections.
1766
V. Purnachandra Rao and B. Ramalingeswara Rao
on the slope (S/I = 0.5). [Fig. 3(Eb-c); Bhatkal-Cochin sector show that there higher than in the preceding sector [S/I Gibbsite reflections became prominent Bhatkal-Cochin sector (Fig. 5).
Table 21. The corresponding sediments in the is a lower net increase of illite and kaolinite is = 0.6-0.9; K/Ch = 2.0; Fig. 3(Fb-c); Table 21. only in the shelf and slope sediments of the
4. DISCUSSION 4.1. North of Gulf of Kachchh The Indus is located in this region and brings sediments mainly from the glacial Himalayas and semi-arid to arid soils. Therefore, the physical/mechanical weathering products (illite and chlorite) dominate the chemical weathering products (smectite and kaolinite) in the river discharge. The illite and chlorite-rich sediments thus characterize the Indus derived products. The analyses of the suspended load of the Indus river (Konta, 1985) and the sediments of the northern Arabian Sea (Kolla et al., 1981) also show enhanced illite and chlorite abundances.
4.2. Saurashtra-Goa Smectite is the predominant weathering product of basalts in arid to semi-arid climates and smectite and kaolinite in humid tropical climates (Millot, 1970; Charnley, 1989). The mineralogy of the black cotton soils derived from weathered basalts (Karkare, 1965), and of the suspended and bed loads of the Narmada and Tapti rivers (Subramanian, 1980; Naidu et al., 1985) showed high smectite content. High values of smectite in the inner shelf of Saurashtra [Fig. 3(Ba)] and in the vicinity of the Gulf of Cambay [Fig. 3(Ca)], together with decreased abundances towards the south and a concomitant increase in kaolinite [Fig. 3(Da); Table 21 reflect the weathering products of plateau basalts in semi-arid climates and basalts associated with Ghats in humid climates, respectively. In contrast to the north of the Gulf of Kachchh [Fig. 3(A)], illite abruptly decreases in the shelf sediments off Saurashtra [Table 2; Fig. 3(B)]. This suggests that the macro-tidal currents operating in the Gulf act as barriers to the longshore sediment transport from the north (Hashimi et al., 1978; Nair et al., 1982a). The higher illite and chlorite values in the outer shelf (Table 2) and much higher values on slope of Saurashtra [Fig. I] indicate a greater influence from the Indus source. These values suggest that the effect of the tidal barrier is perhaps greatest on the inner shelf, moderate on the outer shelf and insignificant on the continental slope. The Indus borne clay mineral assemblage is thus abundant on the slope. Smectite followed by kaolinite are dominant clay minerals in some samples of the outer shelf (mid-shelf) off the Gulf of Cambay-Goa (Table 2). This implies that they are transported from the hinterland. Samples showing trace contents of all clay minerals (Table 2) are either associated with relict carbonate sands [Halimeda and fecal pelletdominated sands-Rao et al., (1994)] on the outer shelf or located close to the shelf break and/or on the upper continental slope. Hydrographic and tracer sediment studies indicate the prevalence of high energy environment on the outer shelf (Anonymous, 1976; Veerayya and Nath, 1994), and high tidal and internal wave generated currents at the shelf
Provenance
and distribution
1767
of clay minerals
3ombay
m
Indus
y//k
Basaltic
source 1 B I
b\\p
Gnoissic
source
-
Influence of the lndus Longshore current
-_-_ em_.
Fig. 5.
source (1)
T
Cross shelf
Synthesis
of sediment
(G 1 by
transport
sources in the study region. (Dashed of clay minerals on the outer shelf.)
double
arrows show bypassing
1768
V. Purnachandra Rao and B. Ramalingeswara Rao
break (Shenoi et al., 1988). Aragonitic muds and coarse grained shallow water carbonate sediments slumped from the outer shelf were reported in some surficial sediments of the continental slope (Hussain and Guptha, 1985). Trace contents of all clay minerals in these samples indicate that the clay fraction of these sediments is also biogenic, probably derived from Halimeda plates which produce abundant aragonitic muds upon disintegration (see Rao et al., 1994). Unlike other sediments, the clay slides prepared from the sediments of this region are whitish supporting that the clay fraction is an acid-insoluble residue of the carbonate muds. Trace contents of detrital clay minerals suggest that they bypassed the outer shelf due to high energy conditions and deposited on the deeper slope. Kolla et al. (1976) reported smectite-rich sediments, transported from the Deccan Trap basalts, on the deeper continental slope of the west coast of India. The dominance of Indus sourced clays in some continental slope samples [Fig. 3(Cc) and (DC); Table 21 indicate that they are carried by a southerly surface current (Fig. 5) and mixed with the clays transported by the Narmada and Tapti Rivers. 4.3. Goa-Cochin Smectite and kaolinite-rich assemblages on the inner shelf reflect a source from intense chemical weathering products. As the Ghats with steeper western gradients are located on the coast between Goa and Bhatkal (Krishnan, 1968) and receive maximum rainfall (3000 mm) during the SW monsoon, one would expect both mechanically derived residual clay minerals (illite and chlorite) and abundant kaolinite and smectite. The low illite and chlorite contents in the inner shelf clays [Fig. 4(Ea)] and their increased abundance in the outer shelf/shelf break sandy sediments and slope clays [Fig. 4(Eb) and (EC)] are therefore anomalous. The 001 reflections of illite are broad and asymmetric in the Goa-Cochin samples in contrast to the symmetrical reflections of illite close to the Indus. The less crystalline illites and chlorites are associated with an increase in kaolinite and gibbsite, the characteristic clays of this region [Fig. 3(Eb); Fig. 41. It is therefore likely that some illite and chlorite are also sourced from Western Ghats but during weathering in the humid tropical climate and are less crystalline than those derived from the Indus. Their higher proportions on the outer shelf/shelf break (sandy) and slope (silty clays) compared to the inner shelf (clays) sediments may largely be due to the combined influence of depositional environment and effective transport processes and to a lesser extent mixing of clays from the Indus. Maldonado and Stanley (1981) and Stanley and Liyanage (1986) reported high smectite content in clayey sediments and high illite in sandy sediments and suggested that the clay minerals’ abundance vary with grain size and thus reflect the depositional environment. High surface runoff and offshore transport of the surface water during the SW monsoons together with the narrow shelf (Table 1) in this region are probably responsible for the effective transport of illite and chlorite farther from the coast and their accumulation in the outer shelf and slope. The highest kaolinite occurs between Bhatkal and Cochin (Table 2). Some kaolinite together with gibbsite probably derived from Ghats which are covered by laterites and weather in humid climates. Kaolin deposits occur in Warkala deposits which outcrop near the coast. So some kaolinite might have derived directly from these sources. High contents of kaolinite and gibbsite indicate intense chemical weathering on land and their presence on the continental slope [Figs 3(Fc) and 51 indicate cross shelf transport of these minerals
Provenance
and distribution
of clay minerals
1769
from the coastal region. It appears that the Indus derived clay minerals are less significant than the clays derived from the hinterland.
5. CONCLUSIONS There are three principal sediment sources: The Indus Province-rich in illite and chlorite, the Deccan Trap Province-rich in smectite and the Gneissic Province-rich in smectite and kaolinite (Fig. 5). The inner shelf sediments generally reflect these sediment provinces, indicating that they are not affected by longshore currents, as also suggested by Nair et al. (1982b). However, the sediments of the outer shelf of Saurashtra and the continental slope of Saurashtra-Goa, have a clay mineral assemblage that reflects admixing of Indus Province clays with those from the hinterland (Fig. 5). The clay mineralrich sediments mostly bypass the outer shelf between the Gulf of Cambay and Goa because of the high energy environment. Between Goa and Cochin the shelf and slope sediments represent mostly the influence of Gneissic Province (Fig. 5) indicating cross shelf transport in this region. The effect of the Indus decreases from north to south and is insignificant on the SW continental margin. The northerly current associated with NE monsoon has little effect on clay mineral distribution due to limited sediment discharge from the rivers during this season. Acknowledgements--We thank Dr E. Desa, the Director of the National Institute of Oceanography, Mr R. R. Nair, the Head of the Geological Oceanography Division and Dr M. Veerayya for their encouragement. We thank Dr V. S. N. Murthy for helping us in understanding the current pattern and Mr Girish Prabhu for scanning clay slides with X-ray diffractometry. We thank the three anonymous reviewers for their critical comments which improved the quality of the manuscript.
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