Clay supplies in the Central Indian Basin since the Late Miocene: climatic or tectonic control?

MARINE GEOLOaY INTERNATIONAl. JOURNAL OF MARINE GEOI.OG~ GEOCHEMISTRY AND GEOPHYSICS

ELSEVIER

Marine Geology 122 (1994) 151 172

Clay supplies in the Central Indian Basin since the Late Miocene: climatic or tectonic control? N. Fagel

a,1,

p. Debrabant

a,

L. Andr6 b

Sddimentologie, Universitb de Lille I, 59 655 Villeneuve d'Ascq cedex, France h MusOe Royal de l'Afrique Centrale, Steenweg of Leuven, 3080 Tervuren, Belgium

Received 7 February 1994; revision accepted 29 August 1994

Abstract

Mineralogical (X-ray diffraction, differential thermal analysis), geochemical [microprobe, inductively coupled plasma (ICP)-atomic emission spectrometry, ICP mass spectrometry] and Sr-Nd isotopic analyses have been carried out on the clay size fraction of Late Miocene to Pleistocene sediments from the Central Indian Basin. The samples were taken from five giant cores recovered between 1" and 10°S on a transect along 80°E. The clay assemblages are homogeneous and characterized by an alternation of illite- and smectite-rich levels. Most of the clays are detrital and were derived from a unique source: the weathering of the Indo-Gangetic Plain supplied most of the eroded material. Temporal clay mineralogical fluctuations in the depositional basin reflect environmental changes in the provenance. On the basis of spectral analyses of a mineralogical parameter (peak height ratios), the fluctuating smectite-illite clay sedimentation is controlled by periodic Late Miocene climatic changes. During the Late Pliocene, an irregular, probably tectonic, control appeared.

I. Introduction

The Central Indian Basin, located in the equatorial zone at the boundary between the influences of Asia and the Southern Ocean, displays a reference area where the Late Cenozoic global variations of both continental and marine influences on sedimentation were investigated (e.g. Nath et al., 1989; Bouquillon et al., 1990; Brass and Raman, 1990; Cochran, 1990; Stow et al., 1990; Aoki et al,, 1991; Bout-Roumazeilles, 1991; Debrabant et al., 1993). This basin spans several sedimentary transition zones: the sedimentation records the interplay of the distal detrital supply on the Bengal Fan in

1Present address: GEOTOP, Universit6 du Quebec a Montrdal, S 2025, case postale 8888, succursale centre ville, Montrdal, Qu6. H3C 3P8, Canada. 0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0025-3227(94)00093-X

the northern part, and the siliceous pelagic input associated with the equatorial divergence at 10°S (Fig. 1). In this particular oceanic area, the clay sedimentation is likely to record both the Himalayan tectonic events associated with the Indo-Asiatic collision (Gansser, 1966; Cochran et al., 1989a; Cochran, 1990) and the effects of interplate compression (Karner and Weissel, 1990; L6ger and Louden, 1990; Stow et al., 1990). Preliminary studies show mineralogical fluctuations in the clay size fraction of Neogene to Pleistocene sediments: smectite-rich levels alternate with illite-rich levels (Bouquillon et al., 1990; Debrabant et al., 1993). The origins and source areas of the clay minerals are numerous and much debated. First, although the mineralogical, sedimentological and isotopic investigations are in agreement with a Himalayan

N. Fagel et al./Marine Geology 122 (1994) 151 172

152

10° N

"I, C, S

?

~#"~,o.j / -

((.~ [/

i ~ )- /(

[]

•

"lg...tl, C,s

I f/~r"l/J

90-947

~7 x

I/O0-951 to %3~

I

J f~ ( ~ [

#2h81-375~

Oanges

o I

[((//2

~\<' L

MX~"

~

[]Bo~hol~ODP

~ O 90-943 f ~

I Boreholes DSDP

.~.~ ~'r/C-

~, • " ~ - ' ~

~'

• co~ sniv^ ~

J

70° E

"~1 5 0 0 0 ~ t -

80°

,:,.,,o

s:.,,~,,,o

C:chlo~ite

K:k~limte

~p~o~.~tof~o

.ollo,. ..o

" 75 °

l0o

O Cores SAFARI 11 1981

~U

I ~'(~?~Ri-s74 ~ ~ " 2 ~ - £ s

P"

( ,Iq fS (I, CoK)

~

/~

, '-~/ "i ; '~ t

~£~t~,o._~l~_=g~_~o_~l_

I U Dn-.~

///

8.5°

90 °

,s.s 95 °

Fig. 1. L o c a t i o n o f S H I V A 1990 c o r e s ( M D 90-942 to M D 9 0 - 9 4 7 ) a n d r e g i o n a l reference c o r e s ( m a p m o d i f i e d f r o m C o c h r a n et al., 1989a). T h e a r r o w s i n d i c a t e the p o t e n t i a l d e t r i t a l s o u r c e a r e a s (for references, see text).

supply (Bouquillon et al., 1990; Brass and Raman, 1990; Stow et al., 1990; Debrabant et al., 1993; France-Lanord et al., 1993a), there are numerous other potential detrital supplies, from west to east: the Chagos-Laccadive Ridge, southern India and Sri Lanka, the Deccan Traps, the Indonesian Arc, the 90°E Ridge and Western Australia (Aoki and Sudo, 1973; Bouquillon et al., 1990; Brass and Raman, 1990; Stow et al., 1990; Debrabant et al., 1993). Second, the genetic processes of the clays, especially the smectites, are still debated. An authigenic formation of these clays has often been proposed (Fr6hlich, 1981; Rao and Nath, 1988;

Bouquillon et al., 1989; Nath et al., 1989), but remains unquantified. Mineralogical [differential thermal analysis (DTA), microprobe analysis], geochemical [trace elements by inductively coupled plasma atomic emission spectrometry (ICP), atomic absorption spectrometry and ICP-mass spectrometry (MS)] and isotopic studies (Sr-Nd by MS) were combined to determine the origin of clays from Neogene to Quaternary sediments in the north Indian Ocean and to identify their terrigenous source areas. By using the clays as high resolution palaeoclimatic parameters (Fagel et al., 1992), spectral analyses of the clay content were used to estimate and explain the regularity of the mineralogical variations. This investigation is compared with the mineralogical and/or geochemical results obtained in adjacent areas (ODP Site 717 at 0.5°S (Bouquillon et al., 1990; France-Lanord et al., 1993a) and MD 81-374/375 at 12.5°S (BoutRoumazeilles, 1991; Fig. 1).

2. S a m p l i n g

and analytical methods

The studied samples were from five piston cores obtained during the oceanic cruise SHIVA 1990 of the Marion-Dufresne (Caulet, 1992). Boreholes recovered a N-S transect from 1 to 10°S at 80°E in the Central Indian Basin (Fig. 1; Table 1). The sedimentary column of SHIVA 1990 cores consists of brown, oxidized, siliceous clayey mud on top of a green to blackish, reduced, silty mud (Caulet, 1992). The sediment is enriched in biosiliceous and clayey components north to south and from the

Table 1 P r i n c i p a l c h a r a c t e r i s t i c s o f c o r e s r e c o v e r e d d u r i n g the M D 1990 S H I V A cruise ( C a u l e t , 1992)

Corvs

Penetration (m)

Waterdepth (m)

Latitude

Longitude

(S)

(E)

MD 90-947

1° 24'

76 ° 3T

34.37

4 781

MO 90-946

3 ° 16'

78008'

33.15

4 796

MD 90-944

6 ° 39'

80 ° 07'

40.17

4 902

MD 90-943

8 ° 19'

79 ° 49'

43.75

5 249

MD 90-942

10° 03'

79 ° 48'

48.83

5 356

Age o f oldest sedimcnm Late Pliocene Late Miocene > 6.7 Late Miocene > 5.7/6.3 Late Miocene > 6.3 Miocene Pliocene > 5

N. Fagel et al./Marine Geology 122 (1994) 151 172

base to the top of the cores. The radiolarian assemblages supply biostratigraphic information (e.g. Caulet, 1992). The clay mineral data were obtained by X-ray diffraction (XRD) of the < 2 lam size carbonatefree fraction (XRD methods after Holtzapffel, 1985). About 3 cm 3 of sample were split in distilled water and decalcified in 0.2 N HC1 pro analysis. The excess acid was removed by repeated washing (at least three times) with distilled water. The sample was mechanically homogenized and the < 2 gm fraction was separated by settling. Oriented mounts were made on glass slides and were scanned three times (Cu, Kc~ radiation): (1) from 2.5 to 28.5: 20 on untreated samples~ from 2.5 ° to 14.5° 20 on glycolated samples; and from 2.5 ° to 14.5~' 20 on samples heated for two hours at 490C. Semiquantitative evaluation (_+ 5%; Holtzapffel, 1985) was based on the peak heights, assuming that these weighted amounts added up to 100%. The height of the illite (001) peak was taken as the reference. The relative proportions of clay minerals were determined by multiplying their peak heights by a factor depending on the clay crystallinity: >1 for poorly crystallized minerals (chlorite = 1--1.5, smectite= 1.5-2) or < 1 for well crystallized minerals (kaolinite = 0.5-0.7; Holtzapffel, 1985). The high resolution mineralogical investigations were carried out on two cores: MD 90-946 in the northern part of the transect and MD 90-943 in the southern part. This choice is restricted by the available stratigraphic data (Martini, 1971; Sanfilippo et al., 1985; Johnson et al., 1989; Caulet, 1992; Nigrini and Caulet, 1992). According to the sedimentation rates estimated from biostratigraphic events, about 400 levels have been sampled with a constant sampling rate (1 sample/10 cm) in the two cores. Clay samples from about 100 levels from all the cores were analysed by ICP-MS (Reed, 1990; Jarvis and Jarvis, 1992) in the Geology Department of the Mus6e Royal de l'Afrique Centrale to determine V, Rb, St, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb, Lu, Hf, Ta and Th. The results are normalized to chondrites or post-Archean Australian shales using the

153

data from Bienvenu (1989) and Taylor and McLennan (1985), respectively. We retained the extended rare earth element (REE) diagram proposed by Bougault (1980) with the elementary sequence of Bienvenu (1989). The usual Ce anomaly corresponds to the equation: Ce/Ce*= CeN/(LaN x Pr N)1/2 where XN = shale-normalized REE concentration (McLennan, 1989). The Nd and Sr isotopic compositions were obtained by MS at the University of Brussels (Weis and Deutsch, 1984) on 15 smectite-rich levels. All the isotopic results are recalculated to correspond to the time of sediment deposition t (Faure, 1986). The Nd results are expressed by the conventional epsilon values gNdt according to the equation ~Ndt=104×[(143Nd/144Nd sample (t)/143Nd/144Nd C H U R ) - 1 ] , in which CHUR is the chondritic reservoir (143Nd/144Nd= 0.512636, 147Sm/144Nd =0.1966). Complementary (microprobe, Debrabant et al., 1985) or global analyses (DTA, Caillibre et al., 1982a) were also carried out on a few samples.

3. Clay mineralogybackground The SHIVA cores have a constant mineralogical composition in the clay fraction (Fig. 2; Debrabant et al., 1993). Two types of diffractogram are observed (Fig. 3): one shows a dominant smectite assemblage (Fig. 3B), whereas the other is enriched in illite (Fig. 3A). The stratigraphic evolution is homogeneous from the Late Miocene to Pleistocene for all the studied transect. For instance, comparing the mineralogical content of core MD 90-946 (3°S) with that of core MD 90-944 (6°S), three successive clay mineral zones are recognized (Fig. 2). From the Late Miocene to Early Pliocene (zone 1), strong alternations of smectite-rich and illite-rich layers occur. During the Late Pliocene, smectite-illite fluctuations only persist in the northern sites, but have been replaced in the southern sites by a fairly homogeneous and constant smectite-rich clay (zone 2). The amount of illite increases slightly during the Pleistocene (zone 3). This trend is more pronounced in the northern cores.

N. Fagel et al./Marine Geology 122 (1994) 151 172

154

MD 90-944

MD 90-946

Site 717- Leg ODP 116

i iIiLi i 20-

25-

30-

Lele~l

mix~-lay~r ~=~

Kaolinite

Fig. 2. Clay mineralogy of Late Miocene to Pleistocene formations of northern SH1VA core (MD 90-946, 3 S ) and southern core (MD 90-944, 6S; Debrabant et al., 1993). Zonation from clay mineral successions; comparison with ODP Site 717 (0.5 S: Bouquillon et al., 1990).

At the equator, site 717 displays a similar clay mineral evolution (Bouquillon et al., 1990) with higher illite peaks (Fig. 2). In particular, during the Late Pliocene, the number and amplitude of the primary mineral peaks decrease and the smectires become dominant. In both sites 717 and 944, this change simultaneously occurs with a lithological modification: the granulometry decreases in site 717 and the terrigenous sediment is replaced by a dominant siliceous component in MD 90-944. In summary, we note a homogeneous spatial evolution of the clay mineralogy between the equator and 10°S. Moreover, DTA analyses show the stability of the global major chemical composition of the clay fraction in the Central Indian Basin (Fagel, 1994). The curves of eight smectite-rich samples (80 90%)

present three endothermic peaks at about 130°-140 °, 490c'-520 ° and 770°-820°C, and an exothermic peak near 880 ° 900°C. Such a pattern characterizes alumino-ferriferous smectites or beidellites (Chantret et al., 1971 ). In the northern samples (947.8.76,947.23.6 and 946.23.115) a space occurs between the third endothermic peak and the exothermic peak: it reflects the increasing MgO/Fe203 ratio in the smectite chemical composition (Lucas and Trauth, 1965); smectites from the northern sites are close the "Cheto" type. Finally, microprobe investigations on the clay size fraction attested such global chemical trends. Based on analyses of about 30 particles, the structural formulas are reported in Table 2 and the results plotted in two triangular diagrams (Fig. 4).

155

N. Fagel et al./'Marine Geology 122 (1994) 151-172

A

lllite

Smectite

tt l

C+K

F

1

,/

\

fN I

I

!

3

5

7

C+K

I

I

10 15

40 A

I

I

I

3

5

7

I

I

10

15

I

40A

Fig. 3. Diffractograms of clay size fraction from core MD 90-947, showing two distinct clay assemblages: (A) illite-rich and chloriterich sample (947.15.76). (B) smectite-rich sample (947.15.96); N = Natural unheated test, GL =glycolated test, H = heated test. l = illite, C = chlorite, K = kaolinite, S = smectite, Q = quartz and F = feldspar. Table 2 Chemical composition (structural formulae) of smectites determined by particular microprobe analyses. Values are given in atomic number per 1,/2 mesh Sample

Depth Lat. Age

6.1 946.4.116

SlxucturalFormula

Nb. anal.

(Si 3,21A10,79) (AI 0,57 Mg 0,55 Fe 0,97 Ti 0,07) K 0,67 Na 0,08 Ca 0,02 O 10 OH2

8

6.11 946.13.137

18.2

3

5

(Si3,03 A1 0,97) (Al 0,38 M g 1,27Fe 0,72 Ti 0,04)K 0,83 Na 0,05 Ca 0,I0 O10 O H 2

8

4.1 944.4.46

4.1

6

1,6

(Si 3,38 AI 0,62) (AI 0,88 M g 0,28 Fe 0,88Ti 0,04)K 0,4t Na 0,07 Ca 0,02 M g 0,05 Ot0 O H 2

13

4.8 944.24.46

32.7

6

5

(Si3,45 AI 0,55) (AI 0,70 M g 0,21Fe 1,12Ti 0,03)K 0,29 Na 0,08 Ca 0,02 M g 0,07 O10 OH2

18

3.17 943.30.96

43.1

8

6,3

(Si3,45 A10,55) (AI 0,84 M g 0,32Fe 0,88Ti 0,05)K 0,39 Na 0,12 Ca 0,02 O I0 O H 2

23

2.15 942.27.136

38.4

10

•5

(Si3,31AI 0,69) (At 0,85 M g 0,35 Fe 0,77Ti 0,II) K 0,49 Na 0,08 Ca 0,03 M g 0,03 O10 O H 2

5

in m o s t o f the samples, o c t a h e d r a l a n d t e t r a h e d r a l s u b s t i t u t i o n rates c h a r a c t e r i z e d d i o c t a h e d r a l a l u m ino-ferriferous smectites (Caillibre et al., 1982b; N e w m a n , 1987). S a m p l e s 6.1 (946.4.116) a n d 6.11 (946.13.137) have a p a r t i c u l a r clay c o m p o s i t i o n , with an i m p o r t a n t t e t r a h e d r a l s u b s t i t u t i o n rate ( > 0 . 6 ) , a high i n t e r l a y e r p o t a s s i u m c o n t e n t close

to illite c o m p o s i t i o n a n d a m a g n e s i u m - e n r i c h e d o c t a h e d r a level. Except for s a m p l e 6.11, C e n t r a l I n d i a n Basin clays present similar substitution rates to the regional smectites, i.e. the m e d i a n a n d distal p a r t o f the Bengal F a n , the 90°E R i d g e a n d C e y l o n ( F i g . 4a a n d b, r e g i o n a l results f r o m B o u q u i l l o n et al., 1989).

156

N. Fagel et al./Marine Geology 122 (1994) 151 - 172

.4+

51

//

K++Na++Ca2+intefoliar

tettahedral

°5"11

2

/6[]

/

\\~ ~'

°.•

.I

\

\

15

,\ v

v

v

A13++Fe3+octahedral Legend

v

v

(a)

v

v

v

v

~

Mg2+octahedral

v

A13++Fe3+octahedral

v

Co)

v

v~

v

~

\

Mg2+octahedral

• CentralIndianBasinsmectites [] Regionalsmectitas (Bouquillonet al., 1989)

Fig. 4. Distribution of smectite species in the triangular diagrams: octahedral Al3*+Fe3+-octahedral Mg2+ tetrahedral Si4~; octahedral AI3+ -t-Fe3+ octahedral Mg2+ dnterlayered N a++K + +Ca 2+. Comparison with regional smectites (Bouquillon et al., 1989). 4. Results 4.1. Trace element contents

As for the element content, the trace element clay composition is almost constant (Fagel, 1994). The smectites and illites display similar extended REE chondrite- or shale-normalized patterns ( Fig. 5 ). The chondrite-normalized pattern is characterized by a light (L)REE relative to heavy ( H ) R E E fractionation (8 < LaN/YbN < 14) and a negative N b ; F a anomaly. The shale-normalized patterns are fiat, not fractionated (no depletion and no enrichment) and without any significant negative Ce anomaly (0.81 < Ce/Ce* < 3.31 ). 4.2. S r - N d isotopic results

The Sr and Nd isotopes were measured on smectite-rich samples analysed for trace elements (Table3; Fig. 6). A wide range of variations characterizes the measured ratios (0.713450< aTSr/S6Srt __0.000020 < 0.740650), whereas 143Nd/

144Nd ratios are homogeneous (0.511880 < 143Nd/ 144Ndt +0.000020<0.512230). In a diagram eNat versus 8VSr/S6Srt, the Central Indian Basin fine fraction is located on a mixing curve between two poles (Fig. 6): (1) with high Nd radiogenic and low Sr radiogenic signatures (juvenile pole), which could represent either a sea water derivation (Piepgras et al., 1979) or basaltic magmatic rock supplies (Deccan Traps, mid-ocean ridge and 90°E Ridge basalts (Subbarao et al., 1979; Dupr6 and All6gre, 1983; Hamelin, 1986; Hamelin et al., 1986; Dosso et al., 1988; Hofmann, 1988; Mahoney, 1988); and (2) with low Nd radiogenic and high Sr radiogenic signatures (mature pole) close to the High Himalayan leucogranites and paragneiss (Vidal et al., 1982; Deniel et al., 1987). To define the first pole, we plotted the fine fraction in a Wood diagram (Th T ~ H f / 3 , modified by Bienvenu, 1989; Fig. 7). All the clays are located in a narrow area in the orogenic magma field. This location does not fit the range of the Deccan basalt samples (Mahoney, 1988). Our

N. Fagel et al,/Marine Geology 122 (1994) 151 172

157

(aK~hondrites normalized tmttern

1000

he.end • Clay-rich level Smectite [] Biog~ic level Sme~tite

10~

10

i

f

i

|

i

i

~

i

i

i

i

i

i

i

i

|

Rb Th Ta Nb La Ce Pr Nd Zr Hf Sm Eu Gd Dy Ho Er y

i

i

(b~Shales normalized pattern

I0

0,1

~

Yb Lu V

|

i

i

i

i

i

i

i

i

i

i

Rb Th Ta Nb La Ce Pr Nd Zr Hf Sm Eu

i

v

i

i

i

Gd Dy Ho Er y

i

i

i

Yb Lu V

Fig. 5. Extended R E E spectra normalized to chondrites (a) and to shales (b) on the clay size fraction of the Central Indian Basin. Example of (a) a detrital smectite-rich level (944.4.46, smectite = 90%, silt and clay fraction = 90%); (b) a biogenic smectite-rich level (944.7.86, smectite = 75%, biogenic fraction = 35%); and (c) a detrital illite-rich level (944.16.106, illite = 30%, silt + clay fraction = 85%).

results support a dominant, single source area. In the Sr-Nd diagram the SHIVA samples are comprised between the illite-rich or smectite-rich levels from site 717 (Bouquillon et al., 1990; France-Lanord et al., 1993b), which are close to the Himalayan rock signature and the Indian Ocean sea water composition (Piepgras et al., 1979). Indeed, a detailed discussion of all the geochemical data (including leaching experiments) clearly indicates a sea water alteration of the measured Sr and Nd isotopic compositions of the clay size fraction (Fagel, 1994). To summarize, the SHIVA clays are thus generated from Himalayanderived material, i.e. from some protolith of average calc-alkaline composition.

4.3. Spectral analyses of the smectite/illite fluctuations To investigate the ability of the clay fraction to record short-term periodicities, we sampled two SHIVA cores (MD 90-946 and 90-943) at high resolution over well known biostratigraphic intervals. The smectite/illite peak ratio, directly measured on the X-ray diffractograms (glycolated sample) was generally measured as a representative parameter. The fluctuations of this parameterdefined time series were treated by basic spectral analysis. Three mathematical transformations were applied on the mineralogical parameter: discrete Fourier transform (DFT) (Jenkins and Watts,

MD 86375

0-45-55

2.8

2.9 65-15-20

0-55-45

2.5

5.2

35-25-40 35-25-40

2.1

5-35.60

3.8

MD 90-942

0-55'.45 35-30-35

3.1 3.5

MD 90-943

35-30-35 30-25-45 20-30-50

6.9

4.2 4.4 4.10

0-25-75

6.5

MD 90-944

15-45--40

6.2

0-50-50 15-45-40

7.3

MD 90-946

65

85

85

78

70

90

80 80

75 65 75

75

65

65

85

Sample Lithology (B-D-C') ( ~ )

MD 9~-947

Cores

1,87-2,48

>5

4,7-5

1,5-2,3

0,3

5-5,1

4,3-4,4

0,9-1,5

2,4-2,6 3,5-3,7 5,7-6,3

>6,5

3,8-4,7

2,4

Age (Ma) 0,9-2

88

95

75

89

95

92

114

36

114

I I0

181

48

38 75

50

73 115 83

171

55

74

119

87

98

113

120

2,23

7,65

1,90

2,34

1,52

5,56

8,78 3,20

4,23

1,47

5,16

4,42

2,92

Rb Sr ~Rb/~/Sr (ppm) (ppm) 131 31 12,27

0,716023

0,733749

0,714204

0,715986

0,713516

0,733957

0,739387 0,717531

0,724968

0,713499

0,724939

0,724233

0,718755

37

19

17

13

17

15

14 9

13

14

15

16

36

0,~ 15960

0,733210

0,714073

0,715923

0,713520

0,733560

0,717350

0,739200

0,724610

0,713450

0,724430

0,723950

0,718670

5

25

54

15

2,1 12

19 31

30

3,5 6,4

6,2

26

30

6,1 5,8

17

28

42

54

5,1

6,5

9,2

12

Nd Error 8 fSr/~OSrt Sm x 10.6 (ppm) (ppm) 0,740914 13 0,740650 4,8 24

~/S~t~)Sr0

0,12

0,13

0,08

0,11 0,12

0,12

0,13

0,12

0,18

0,14

0,13

0,13

0,12

14/Sn~144,Nd

0,512094

0,512201

0,512128

0,512071 0,512063

0,511956

0,512111

0,511887

0,512231

0,512033

0,512100

0,512093

0,512035

143Nd/~llq4Nd 0

14

12

14

18 36

45

37

20

12

28

8

13

Error x 10`-6 35

0,512085

0,512200

0,512130

0,512070 0,512060

0,511950

0,512110

0,511880

0,512230

0,512021

0,512093

0,512088

0,512030

143Nd/]44Nd t

10,56

8,48

9,95

11,02 11,17

13,28

10,.23

14,61

7,93

11,79

10,46

10,61

11,75

-~Ndl

Table 3 Rb Sr and S m - N d results on the clay size fraction of Central Indian Basin sediments. The following parameters are reported tbr each sample: the bulk sediment lithology expressed as a percentage ( B = b i o g e n o u s fraction, D = d e t r i t a l fraction, C - c l a y fraction); the smectite-rich percentage in the clay content (S); and the age (in Ma) based on radiolarian zonation (Caulet, 1992). 87Sr/S6Sr normalized to 86Sr/88Sr=0.1194. Average value of NBS987 standard during the period analysis was 0.710245 (0.71015
&

txa

¢5

159

N. Fagelet al./Marine Geology 122 (1994) 151 172

eNdt 10

0,720

0,7

0,740

I

I

0,780

0,760

I

I

I 87Sr186Srt

-2 -4

-8 10

2"10 3 " ~ 6"5 ~'10,8~2'9 ~--6"26. ~

12

.

7.3 •

~+++++~÷++++++.[..v.~-~ ÷÷÷+ ÷÷+÷÷+++ ÷+÷÷ ~+++÷+++

14 16 18

20

Legend O Biogenic-richlevel Clay-rich level [] [~

Clay fraction Legll6ODP Indi~l !Ocean Seawater

~

Himalayangneiss

~2~

90° E Ridge sediments

~-~

Himalayangranite

~-]

Indonesian Arc sediments

[ I /

t DeccanTraps basalts

•

~ IndianPeaxingul~ulit~

MORB Indiau Oeelm lxtsldts

Fig. 6. Sr and Nd isotopic results on the clay size fraction of Central Indian Basin sediments compared with regional source areas. Data from Bouquillon et al. (1990) and France-Lanord et al. (1993a) for Site 717; Cox and Hawkesworth (1985) and Mahoney (1988) for the Deccan Traps; Deniel et al. (1987) and Vidal et al. (1982a) for the Himalayan rocks; Spooner and Fairbairn (1970) for granulites; Dosso et al. (1988), Hamelin (1986), Hamelin et al. (1986), Hofmann (1988), Subbarao et al. (1979) for Indian mid-ocean ridge basalts; and Dupr6 and All6gre (1983), Subbarao et al. (1979) and Piepgras et al. (1979) for Indian Ocean sea water. 1968; Bringham, 1974); autocorrelation (Jenkins and Watts, 1968; Davis and Kidd, 1977); and D F T o f the autocorrelation function ( D F T A ) ( B l a c k m a n and Tukey, 1958; Bloomfield, 1976). We retain as significant the periods simultaneously occurring in several independent functions. The application o f spectral analysis (method and preliminary conditions) on geological parameters is discussed in detail by Fagel et al. (1992).

M D 90-946

Based on the sedimentation rate estimations (Caulet, 1992), two intervals were selected: (1) a Late Miocene interval (5.7-5.8 to 6.3-6.5 Ma); and (2) a Late Pliocene interval (1.2-1.5 to 4.3-4.7 Ma). Late Miocene interval." Sixty-six samples were col-

lected

between

21.96

and

28.36

m

(samples

N. Fagel et aL/Marine Geology 122 (1994) 151 172

160

Hfl3 <) Smectites from clay-rich levels 0 Smeetites from the biogenic levels

/,

/

.~

/ \ '~

\

/~ Rid2es and interolst¢ volcanism A : N-MORB "

/

B:E-MORB

C : Alkalin interplate magmas Subduction zones

/" /

Th

\

,"\'k

z/

/I

/

/~

',

--

/l / / v" / ~

\

\ . '\

B ) '~ ] /

\ \

Ta

Fig. 7. Distribution of the clays in a Wood diagram (modified by Bienvenu, 1989). The Central Indian Basin fits the range of orogenic magma, far from the Deccan Traps basalt signature (Mahoney, 1988).

946.16.66 to 946.20.106). In agreement with the biostratigraphic limits, they cover an interval of 500,000 to 800,000 years. Dating is based on the following events (Johnson et al., 1989; Nigrini and Caulet, 1992): top Stichocorysjonhnsoni defined at 5.7-5.8 Ma, observed between 21.96 and 22.16 m (section 946.20.76 to 946.20.106); bottom Solenosphaera omnitibus dated at 6.3-6.5 Ma, observed between 28.06 and 28.36 m (section 946.20.76 to 946.20.106). The evolution of the clay mineralogy obtained with either low resolution sampling (i.e. one sample each 30-40 cm; Debrabant et al., 1993) or high resolution sampling, (i.e. one sample each 10 cm; this study) is showed in Fig. 8. According to Shannon's law (Bringham, 1974), the regular sampling rate (Te=10cm) allows earth orbital periodicities to be observed (eccentricity, obliquity and probably precession): the lower period available is the double of Te, i.e. 2x(8000-13,000 years)= 16,000-26,000 years. There is an opposition between the abundances of smectite and illite measured by XRD (Fig. 8). We have therefore calculated the ratio between the illite peak height at 10A, and the smectite peak height at 17A on the glycolated diagram. The under peak area ratio has also been determined. These two parameters,

height or area ratio, offer a good correlation in spite of the higher amplitude variations of the height ratio (Fig. 8). We built a temporal signal composed of 128 points (27) from the natural I/S curves versus time to apply the mathematical transformations. Before any treatment, the parameter is divided by its mean value to reduce its influence. Fig. 9 displays the results of the spectral analyses obtained on the surface ratio I/S parameter. The height ratio gives almost the same patterns. According to the doubtful age limits, we analysed the spectra with different potential time intervals, To: 500,000, 600,000, 700,000 and 800,000 years (Table 4). Whatever the mean sedimentation rate, DFT spectra (Fig. 9c) reveal some periodicities T close to the earth's orbital parameters. Some peaks increased in the autospectra (the result of the autocorrelation function application, Fig. 9b), especially peak x = 7 , which is close to 100,000 years (T=To/x). Moreover, the autocorrelation function or "correlogram" presents some maxima; the signal is thus periodic and the distance between the maxima is situated between 90,000 and 120,000 years. Such results suggest an influence of a periodic process characterized by a 100,000 years periodicity: Late Miocene illite-smectite fluctuations are controlled by a periodic orbital forcing.

N. Fagel et al./Marine Geology 122 (1994) 151 172

161

M D 90-946 C h v miacmlo m, Hi2h resolution samolin2 C ~ v mineralo t,v low ~solution samvlin~

Clay Parameter s e l ~ l ~ l for spectral analvmis

Zone2

0

20

40

60

80

100

20

40

60

% 80 100 0

1

I/S 2

4 20

40

60

8O 100

I/ [ ~ [ I ] [ H I [ ] [ ] ] H ] [ ] ] ~

0

1/1,21,2/1,5

tc

5

1,5t2,4-

10

3,5/3,9-

15

4,3/4,7 0,6

5/5.1 5.3/5,4-

0,12

1,8 I/S

2O nttio

5,7/5,8

6,2/6,7-

25

6,3/6,530

Age (Ida)

Dept. (m)

Fig. 8. Clay mineralogy of SHIVA core MD 90-946. Comparison of the clay stratigraphic evolutions obtained with a low (30-40 cm; Debrabant et al., 1993) or a high (10 cm) resolution sampling spacing. Trends of clay parameters selected for spectral analyses are present. According to Imbrie et al. (1992), the strong climatic response around 100,000 years does not reflect the eccentricity, but corresponds to a nonlinear influence of the precession. In the text, the term eccentricity is used to mean this and is written within inverted commas.

Late Pliocene interval This interval (between 4.46 and 16.75 m) covers 2.8 to 3.5 Ma (1.2-1.5 to 4.3-4.7 Ma). The clay assemblage evolution is

based on 125 samples (section 946.4.116 to 946.12.145) taken each 10 cm. It is in agreement with the global (low resolution) mineralogical trend (Fig. 8). Illites and smectites vary inversely, from 5 to 40% and 40 to 90%, respectively, whereas the other clays remain constant. This opp~)sition is characterized by the peak height illite/smectite (10/17A) ratio. Stationarity is low; 0 . 1 4 < I/S< 1.72, with a mean of 0.52, but the high value (0.7) at the base becomes lower (0.4) at the top.

162

N. Fagelet al./Marine Geology 122 (1994) 151 172

Am~m~

Am~litude

010 ]

0.100

90 000 to 120000

(a)

(b)

) 0

o.,o

I

5.715.8

6.3 6.5

,

-

0.I0

Am~m~ "E"

0.5-

l

looooo

yea~s__~ At

~litude

i•"

1.51-

r

13 T

T 11 15 1

~

1~

,rV 18

v

Spectralindex x = Toll"

-P

(el "

64 Spectralindex x = To/T

Fig. 9. MD 90-946: spectral analysis of the illite/smectite signal (10/17 ,~ peak area ratio, glycolated X-ray diffractogram) between 5.7 5.8 and 6.3-6.5 Ma. (a) Vertical distribution of the I/S signal deduced from the mean values; (b) "correlogram" or autocorrelation of the US signal; (c) "spectrogram" or discrete Fourier transform (DFT); (d) "autospectra" or discrete Fourier transform of the autocorrelation function (DFTA). According to the different time interval To, each peak in DFT and DFTA spectra (Fig. 9c and d) is characterized by its spectral index x. For each To, the corresponding period is reported in Table 4.

The sedimentation rate (3-4 m/Myr) only allows discrimination between the obliquity and eccentricity periods (Te = ___22,000 years). The results, autocorrelation function or "correlogram", spectrogram and autospectra, are presented in Fig. 10. We cannot interpret the "correlogram" (Fig. 10b) and there is no peak in the spectrograms (Fig. 10c and d). Such spectral analyses show that the Late Pliocene clay fluctuations in core MD 90-946 are not controlled by low frequency periodic processes, i.e. eccentricity and obliquity. Because the sampling rate was too low, a potential precessional control cannot be seen.

MD90-943 Two intervals were selected: (1) the first part of the Early Pliocene (5-5.6 Ma) and (2) the

second part of the Early Pliocene (4.3-4.4 to 5.0-5.1 Ma).

The first part of the Early Pliocene." One hundred and two samples were taken along 10 m of the core (section 943.21.140 to 943.28.96). High resolution mineralogical evolution (To = 10 cm) is compared with global evolution obtained with a sampling rate three times lower (Fig. 11). Some new peaks appear next to variations lower than the resolution of the diffractometric analyses. The sedimentation velocity is stabilized around 17 m/Myr. Palaeomagnetic measurements confirm these values (D. Schneider, pers. commun.). Investigated interval covers approximately 600,000 years. The sampling rate (Te = 6000 years) allows

N. Fagel et al./Marine Geology 122 (1994) 151-172

163

Table 4 Index of frequency peaks observed in spectrogram ( D F T ) and autospectra ( D F T A ) of the height H or area ratio S of the parameter I/S measured at 10/17 A, on glycolated diagram. The interpretation changes according to the duration of the studied interval (500,000-800,000 years). For each peak, the length of the interval To divided by the spectral index x gives the corresponding period T in 103 years. Periods close to the Milankovitch periodicity are given in bold Spectral Index 3 5 7 10 11 13 15 18 19 21 22 23 24 25 26 28 30 31 33 37

DFT

DFTA S

SH SH S

SH

SH SH SH

S SH SH H S

S H SH S H H S

H S SH SH S

H S

Periods 1"o=800000 267 000 160000 114 000 80 000 72 000 61 500 53 000 44 000 42 000 38 000 36 000 35 000 33 000 32 000 31 000 29 000 27000 26000 24 000 22 000

all periods higher than 12,000 years to be detected (Shannon's law). Quantification of the illite/smectite variations is based on the curve of height ratio I/S. As this parameter presents a moderate stationarity (Fig. 11), we have also performed spectral analyses on the abundance variations of the illites (parameter L 4-34%, mean 15%) and smectites (parameter S, 67-81%, mean 70%). The autocorrelation function has been calculated for the natural curve composed of 100 points. For the Fourier transform treatments, we have extended the window to 750,000 years to obtain 128 points. This artefact does not alter the spectral results (B. Demoulin, pers. commun.). For the three parameters (I/S, I, S), the spectra are fairly close (Table 5). For instance, the orbital forcing result occurs essentially around 100,000 years. The "correlogram" presents maxima at 120,000 years. We note an evolution from DFT to DFTA; although present in DFT, the 125,000 year peak increases in DFTA; on the contrary, we observe the decrease or even the disappearance of the 95,000 years peak in the DFTA. The obliquity is not expressed in the "correlogram". Although

Periods To = 700 000 233 000 140000 100 000 70 000 64 000 54 000 47 000 39 000 37000 33 000 32 000 30 000 29 000 28 000 27 000 25 000 23 000 23 000 21 000 19 000

Pexkxls "I"o=600000 200 000 120 000 86000 60 000 54 000 46 000 40 000 33 000 31 000 29 000 27 000 26 000 25 000 24 000 23 000 21 000 20 000 19 000 18 000 16000

Pexiods To = 500 000 167 000 100 000 71000 50 000 45 000 38 000 33 000 28 000 26000 24 000 23 000 22 000 21 000 20 000 19 000 19 000 17000 16000 15000 13 000

present in the DFT, 42,000 and 54,000 year peaks are reduced in DFTA for the parameters US, I and gather together for S. In the same way, the "correlogram" does not confirm the occurrence of some precessional periodicities revealed in DFT at 20,000 and 18,000 years. To summarize, only periodic climatic forcing results around 100,000 years are strongly expressed in the lower Pliocene clay sediments of core MD 90-943. Obliquity and precessional influences are doubtful; however, the important 75,000 year peak should be interpreted as a non-linear combination such as 1/42,0001/95,000 = 1/75,000.

Second part of the Early Pliocene: Ten metres have been investigated with a 10 cm sampling spacing (section 943.15.75 to 943.13.130). A systematic correlation occurs between the clay mineralogy and the bulk sediment lithology: the illite-rich peaks correspond to coarse detrital levels. This suggests turbiditic supplies. In this instance, the hypothesis of a constant sedimentary velocity is not available. We cannot apply the spectral treatments.

N. Fagel et al./Marine Geology 122 (1994) 151-172

164

Amplitude

AmpUtude

I,

0-

(a) -I-2-

-3

1.2/1.5

4.314.7 Age (Ma)

A~ plitude

Amplitude J

100_

50-

(d)

(c)

25-

0

'

2

64 2 Spectral index x = To/T

64 Spectral index x = To/T

Fig. 10. M D 90-946: spectral analysis of the illite/smectite signal (10/17 ,A, peak height ratio, glycolated X-ray diffractogram) between 1.2-1.5 and 4.3-4.7 Ma. (a) Vertical distribution of the I/S signal deduced from the mean values; (b) "correlogram" or autocorrelation of the I/S signal; (c) "spectrogram" or discrete Fourier transform; (d) "autospectra" or discrete Fourier transform of the autocorrelation function.

5. Discussion

5.1. Origin of the clays." detrital or authigenic minerals?

The mineralogical, geochemical and isotopic compositions consistently suggest a dominant detrital origin. There is no continuous and irreversible trend of the clay assemblages with depth. Smectites have the alumino-ferriferous composition of beidellites, the classical pedogenetic clays. Extended REE chondrite-normalized patterns in smectite-rich levels (weathering minerals) are similar to those in illite-rich levels (primary mineral). The LREE/HREE ratio (8
upper crust (10-15, Taylor and McLennan, 1985). The negative Nb-Ta anomalies also characterize a detrital component. Fluviatile suspended material presents the same shale-normalized pattern (Martin and Whithfield, 1983): ubiquitous flat with a reduced Nb-Ta anomaly. Moreover, Sr and Nd isotopic compositions are close to those of a mature crustal pole. 5.2. Source areas." multiple or unique?

Using both the isotopic and geochemical results, we were able to add information about the source areas of the fine-grained material in the Central Indian Basin. The homogeneous major, trace and isotopic compositions of the fine fraction, the same

20

40

60

80

100

~ne lb

D

:~

0,4

0,6

0,8

0,12

1,2

f

Height peak ratio ItS 2

Height peak ratio 1,8 I/S

1,6

20

Cb.lori~

60

80

_7,__

Kaolinite

[ ' ~

40

Smeetite

I][m]

[----7 ~m~ed-layer

C'lav parameter for selected spectral analysis

%

?ig. i I. Clay mineralogy of SH1VA core M D 90-943. Comparison of the clay stratigraphic evolutions obtained with a low (30 40 cm; Debrabant et al., 1993) or a ligh (10 cm) resolution sampling spacing. Trends of clay parameters selected for spectral analyses are shown.

~.gefMa) Depth (m)

)

Cltv..mwala~

--

rq

g~

N. Fagel et al./Marine Geology 122 (1994) 151 172

166

Table 5 Index of frequency peaks observed in spectrogram ( D F T ) and autospectra ( D F T A ) of the illitic percentage (I), the smectiterich percentage (S) and the height ratio I/S measured at 10/17 A ( H ) on glycolated diagram. The interpretation changes according to the duration of the studied interval (500,000800,000 years). For each peak, the length of the interval To divided by the spectral index x gives the corresponding period T in 103 years. Periods close to the Milankovitch periodicity are given in bold Spectral Index 6 8 9 10 11 12 14 15 16 17 18 19 21 22 23 25 28 29 30 31 33 34 35 36 37 38 40 41 42

DFT HIS HIS

DFrA HIS I SH

HIS

H HIS HIS

H S I

HIS

HIS IS HIS

[] S IH S IS S IH

HIS S IH HIS

IS I H S I SH

H HIS

IH S

Perk~ls 125 000 94 000 84 000 75 000 68 000 63 000 54 000 50 000 47 000 44000 42 000 40 000 36 000 34 000 33 000 30 000 27 000 26000 25 000 24 000 23 000 22 000 21 000 21 000 20 000 20 000 19 000 18 000 18 000

trace element distribution of the illite- and smectite-rich levels, the elementary ratios such as Th/Ta, Th/Hf and Ta/Hf all agree with a single, dominant, mature source area. The clays derive from a protolith of an average calc-alkaline composition. The SHIVA Sr-Nd isotopic compositions close to those of the site 717 illites or smectites, the important potential supplies from Ganges-Brahmaputra drainage basin, are in favour of an Himalayan source area. According to the Wood diagram, there is no significant contribution from basaltic areas such as the Deccan Traps or the 90°E Ridge. To summarize, most of the SHIVA clays are generated by erosional and weathering processes in the Indo-Gangetic Plain.

5.3. Interpretation of the Neogene to Quaternary clay sedimentation in the Central Indian Basin Spectral investigations on the clay size fraction allow the interpretation of the Late Miocene to Pleistocene illite/smectite fluctuations based on the palaeoenvironmental variations of climatic and/or tectonic factors (Fig. 12). We divide the stratigraphic clay asssemblages into two intervals, each with a specific control: (1) Late Miocene to the first part of the Early Pliocene; and (2) the second part of the Early Pliocene to Pleistocene.

From the Late Miocene to the first part of the Earl), Pliocene (Fig. 13a) From 1° to 10°S, the Late Miocene/Early Pliocene clay assemblages (mineralogical zone 1) have a similar content. The clay fraction is made up of alternate smectite-rich and illite-rich levels. Geochemical and Sr-Nd isotopic measurements on clays agree with a detrital origin and imply a major contribution from the weathering products of the Indo-Gangetic Plain. Spectral analyses were performed on the mineralogical parameters of two cores from the northern (MD 90-946) and the southern part (MD 90-943) of the studied transect. In both cores, mineralogical alternations are cyclic with a period of around 100,000 years. Such rhythmicity involves a climatic orbital forcing on the clay sedimentation. Indeed, we assign it to changing weathering and erosional conditions in the Indo-Gangetic Plain. Numerous climatic influences affect the weathering and erosional activities (i.e. sea-level fluctuations, changes in vegetation cover, extension of the glacial ice). Whatever the exact process, the variations of the weathering rate, in particular changes in hydrolysis, control the transformation of the Himalayan illites into smectites. Such a hypothesis is in agreement with the hydrogen and oxygen isotopic compositions measured on the clay size fraction at site 717 (Bouquillon et al., 1990; France-Lanord, 1993a). 6D and 6180 values of smectite-rich samples indicate the occurrence of a substantial proportion of soil clays. Moreover, the difference between stable isotope measurements in the illiteand smectite-rich clay fraction represents different alteration histories of the same source material.

N. Fagel et al./Marine Geology 122 (1994) 151 172

167

Aim dltude

Amplitude

120 000 I000-

(a) (b) o -

I

30 5

h.

v

0 Depth (m) 5.6 Age (Ma)

- I0OO

I

I00 000 years I

At

Amplitude

Amplitude

,,][~,,

3 0 3 0 0 - "E" 75

.T. 200-

20300-

(d) 36

10 300-

3OO

i

2 16 2.7 10.7 21.3

p

i

24 32

i

.

~

32 40 48 42.7 53.3 64

(c) 100-

''/64 Spectral Index w

56 74.7

FrequencyX x/To (10-6)

0

~ltll

I I I 8 24 16 2.7 10.7 21.3 32

2725 2 0 ~

I l I 32 40 48 42.7 53.3 64

I 56 64 Spectral Index 74.7 Frequencyx x/To (10-6)

Fig. 12. M D 90-943: spectral analysis of the smectite-rich percentage S between 5 and 5.6 Ma. (a) Vertical distribution of the S signal deduced from the m e a n values; (b) "correlogram" or autocorrelation of the S signal; (c) "spectrogram" or discrete Fourier transform ( D F T ) ; (d) "autospectra" or discrete Fourier transform of the autocorrelation function ( D F T A ) . In D F T and D F T A spectra (Fig. 12c and d), each peak is characterized by its period, expressed in 103 years. Along the x-axis, we show the spectral index x and the associated frequency x/To.

Indeed, both hydrogen and oxygen isotope values suggest that illites and chlorites preserve their isotopic compositions from the metamorphic precursors with no detectable effects of alteration. In contrast, smectites appear to have been generated at low temperatures in the Indo-Gangetic Plain.

From the second part of the Early Pliocene to Pleistocene (Fig. 13b) During the second part of the Early Pliocene, illite/smectite alternations still occur at both sites. However, a systematic correlation appears between the lithology and the clay mineralogy in the southern sites (cores MD 90-944, 90-943 and 90-942, Debrabant et al., 1993). The high abundance of

primary minerals belongs to coarser detrital levels. They reflect irregular turbiditic supplies, which could be a sedimentary record of some tectonic Himalayan rejuvenations. Indeed, there is no evidence for a global tectonic event: the Pliocene is often characterized by a relaxation stage in the Himalayan tectonic evolution between two main orogenic movements (Gansser, 1964, 1966, 1981; Powell, 1986; Le Fort, 1989). Although Copeland and Harrison (1990) and Copeland et al. (1990) suggest an episodic uplift occurring by small local pulses. Their data, based on sedimentation rates, show that the intensity of erosional processes in the Himalayan area are irregular in both time and space. Such local increasing activity could be

N. Fagelet al./Marine Geology 122 (1994) 151-172

168

I0 ° N

I~iiiiii~ll III

s. liT lll ~lTililllllll

5° PeriodicaJ fluctuations (100 000 years) 0° ~)-947/(

~)J.

~j \ ~ _ 1" I~

~ v#

~,~illllllll

Jllllllllll a

~.c~

I

I

Xx~

/(~/1¢

<~

Gangesl)elta

i

Ill _

70° E

75°

,~oJ

-x~

" ~"X,i~- 90,942 --

--

80°

--

/

/,,,Y

t

I /

{(~

--

Approx. limit

JL_._. t

/

--

85°

5°

10° S

90°

95 ° I0 ° N

Aperiodic fluctuations

,,,.-~.o:~." \t~t~L / / /

,+ KZT v/h

5°

Seismic

/ " /

//

/Smectites j Ganges Delta Approx. limit of Bdeltaic sediments 5o

Intefplate deformation

.-0

90-943

Lo-90.~4zI I I

I 70° E

75 °

10o

15° S

I 80 °

85°

90 °

95 °

Fig. 13. Interpretative model of clay mineral assemblages in the Central Indian Basin. (a) Late Miocene to the first part of the Early Pliocene interval: periodic climatic control; ( b ) second part of the Early Miocene to Pleistocene interval: aperiodic tectonic control.

N. Fagel et al./Marine Geology 122 (1994) 151 172

recorded in the SHIVA boreholes, in agreement with their particular location along the same transect. Since the Late Pliocene, the clay mineralogy displays distinct patterns along the studied transect (mineralogical zone 2). Although the fluctuating clay mineralogy persists in the northern cores (MD 90-947 and MD 90-946), the illite-chlorite clay assemblages disappear in the southern cores (MD 90-944, MD 90-943 and MD 90-942) and are replaced by a dominant smectite-rich content. Spectral analyses performed on core MD 90-946 do not show any periodicity in the clay mineralogy. We could not perform spectral analysis on core MD 90-947 as no continuous biostratigraphic data is available (the sediment is characterized by an important terrigenous component). It is also difficult to define a constant sedimentation rate for turbiditic carbonate supplies (Caulet, 1992; Debrabant et al., 1993). The Neogene tectonic evolution of the Central Indian Basin could explain the divergence between the northern and southern sites. Increased resistance to subduction, and shortening across the Himalayas since the continent-continent collision combined with continued spreading on the southeast Indian Ridge, have placed the Central Indian Basin under a large N-S compressive stress regime (Stein and Okal, 1978). In this area, onset of the interplate deformation occurs during the Late Miocene (Cochran et al., 1989c). Northern and southern sites are separated by a fracture zone, the Indrani fracture zone, characterized by bathymetric expression (Sclater and Fischer, 1974). Deformation has been relatively constant since 8 Ma (Cochran et al., 1989c). However, in site 717, the occurrence of reworked foraminifers in the upper section, Late Pliocene Pleistocene, argues that an accelerated deformation took place until 2 Ma (Scott and Ldger, 1990). For Scott and L6ger (1990), such carbonate-rich material came from the sediments accumulated on the 90°East Ridge or the Chaggos Laccadive Ridge. This source might be related to the deformation history. They propose the next interpretation: as long as the deformation took place slowly, the sediments from Bengal Fan could continue to override any gradual uplift, but if ridge uplift accelerated it might have

169

blocked the sediment supply from the north and allowed a contribution from other sediment supplies. We propound the following interpretation. Since the Late Pliocene, the southern sites have been supplied by material from the Bengal Fan and the smectite-rich dominance involves stability in the weathering and erosional conditions in the IndoGangetic Plain. In contrast, the influence of the Bengal Fan supply is restricted to the west by the deformation occurring along the fracture zone or by the long wavelength undulations (Weissel et al., 1980; Geller et al., 1983; Cochran et al., 1989b). The western supplies were able to reach the northern boreholes. Seismic instabilities on the Chaggos-Laccadive Ridge could favour some turbiditic supply. Such a depositional mode could explain why there is no periodic control of the clay sedimentation. Moreover, the influence of the Chaggos-Laccadive material has already been suggested by other studies (Caulet, 1992; Debrabant et al., 1993). Both the lithology (bioclastic calcareous levels) and the clay mineralogy (illites and chlorites) of the northern cores are in agreement with such an origin. To conclude, we argue that tectonic rejuvenation could explain why there has been no orbital forcing in the clay mineralogy fluctuations since the Late Pliocene. Other aperiodic climatic change could have occurred but is obliterated by tectonic influences.

6. Conclusions

By using complementary geochemical and isotopic investigations on the fine fraction of Neogene to Quaternary sediments from the Central Indian Basin, we are now able to (1) show the detrital origin for all the clays, in particular the smectites; (2) to identify the dominant clay source area, i.e. the Himalaya Complex and the weathering of the Indo-Gangetic Plain products; (3) to interpret Late Miocene to Pleistocene illite/smectite clay fluctuations according to palaeoenvironmental variations of climatic and/or tectonic factors and to establish a shift of the dominant control with time. (1) Since the Late Miocene, the Central Indian Basin clay sedimentation has been dominated by

170

N. Fagelet al./Marine Geology 122 (1994) 151-172

detrital supplies. This is supported by both the major chemical composition, the extended REE patterns and the Sr and Nd isotopic content. (2) The geochemical and isotopic results imply a single, dominant, mature source area derived from a calc-alkaline protolith. A Himalayan origin for the clays agrees with such an observation. The same origin has already been shown by several workers for chlorite and illite: they are abundant in the suspended matter of the Ganges (Konta, 1985), in the Indo-Gangetic Plain soils (Sidhu and Gilkes, 1977; France-Lanord et al., 1993a) and in the Quaternary sediments of the Bengal Fan (Bouquillon and Debrabant, 1987; Bouquillon et al., 1989). Moreover, our results involve an important southern extension of the GangesBrahmaputra inputs until 10 °S. The occurrence of illitic peaks until such a latitude has already allowed the same conclusion to be drawn by Debrabant et al. (1993). The gradual latitudinal evolution of the clay assemblages, i.e. the relative decrease in the primary minerals and the increase in smectite abundance, could be explained by the main sources remaining constant, with only the differential sedimentation process changing (Gibbs, 1977). (3) The multidisciplinary approach has allowed an interpretation to be advanced for the evolution of the Late Miocene to Pleistocene clay assemblages in the Central Indian Basin under climatic and/or tectonic control. From the Late Miocene to the first part of the Early Pliocene, spectral analyses performed on mineralogical parameters from two cores (MD 90-946 and MD 90-943) show periodic alternations. This periodicity is in agreement with orbital fluctuations and is assigned to a climatic control, i.e. variable weathering and erosional conditions in the Indo-Gangetic Plain. During the second part of the Early Pliocene, in core MD 90-943, the irregular increase in illites (linked with lithological variations) could record successive random tectonic events. From the Late Pliocene to Pleistocene, in the southern cores, the homogeneous clay content reflects stabilization of the weathering conditions during a stage of weak tectonic activity. In contrast, the northern cores, MD 90-947 and MD 90-946, always display mineralogical alternations without any periodicity.

According to the geodynamic constraints, and in particular the intraplate deformation, we suggest a partial (at least) tectonic control. The extension of the Ganges supplies is limited to the west by the tectonic movements occurring along the Indrani fracture zone. This explains the western turbiditic supplies produced by the seismic instabilities on the Chaggos-Laccadive Ridge.

Acknowledgements We are grateful to J.P. Caulet for providing the samples. We thank E. Gilles, C. Gilson, P. Hermand, J.L. Li6geois, J. Navez and P. R6court for their technical support. B. Demoulin and M. Demoulin are sincerely acknowledged for their mathematical input. Andr6 thanks the Lotto for financial support for the ICP-MS equipment.

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