inorganic geochemical and clay mineralogical data, … · this type of dissolution is present in...

19. PALEOENVIRONMENTAL EVOLUTION OF THE WALVIS RIDGE DEDUCED FROMINORGANIC GEOCHEMICAL AND CLAY MINERALOGICAL DATA, DEEP SEA

DRILLING PROJECT LEG 74, SOUTHEAST ATLANTIC1

Henri Maillot, Sédimentologie et Géochimie, E.R.A. C.N.R.S. 764, Université de Lille I,59655 Villeneuve d'Ascq Cedex, France

andChristian Robert, Géologie Marine, L.A. C.N.R.S. 41, Faculté des Sciences de Luminy, Case 901,

13288 Marseille Cedex 9, France

ABSTRACT

Clay mineralogical and inorganic geochemical data from the Campanian to the Pleistocene provide informationbearing on the evolution of both continental and marine paleoenvironments in the Walvis Ridge area. (1) Alterationprocesses of basalts occurred under subaerial conditiohs during the Campanian and Maestrichtian and were virtuallyabsent in deeper marine environments. (2) Strong tectonic effects were present during the Campanian and persisted un-til the early Eocene. (3) Subsidence of this part of the Walvis Ridge became important during the late Maestrichtian andcontinued into the Paleocene and Eocene. (4) The influence of global climatic cooling was evident from the late Eoceneon. (5) Modification of oceanic circulation and the increasing influence of surface and deep water masses on the sedi-mentation characterized the Cenozoic.

INTRODUCTION

During Leg 74 of the Deep Sea Drilling Project, fivesites were drilled on the Walvis Ridge (Fig. 1). The sedi-ments recovered range from Pleistocene to Campanian.Site characteristics are summarized in the following:

Site

526

525

529

528

527

LatitudeLongitude

30°07.36'S03°08.28'E29°04.24'S02°52.12'E28°55.83'S02°46.08'E28°31.49'S02°19.44'E28°02.49'S01°45.80'E

WaterDepth

(m)

1064

2477

3035

3810

4428

Penetr.(m)

619

678

417

555

384

Number ofSamples

Min.

63

99

45

81

46

Chern.

47

13

45

29

OldestSedimentRecovered

Paleocene

Campanianon basement

Maestrichtian

Maestrichtianon basement

Maestrichtianon basement

The purpose of this study is to assist in the recon-struction of paleoenvironmental conditions in the re-gion by looking at the clay mineralogy and inorganicgeochemistry. Similar approaches have been used in thepast to determine the paleoenvironmental evolution ofthe South Atlantic: (1) Cretaceous formation of theSouth Atlantic basins (Robert et al., 1979; Maillot,1980; Maillot et Robert, 1980) and paleoenvironmentalconditions during deposition of black shales (Chamleyand Robert, in press); (2) Cretaceous/Tertiary bound-ary events on the Atlantic margins (Chamley and Robert,1979), on the Rio Grande Rise (Robert, 1981), and on the

20°

25C

30c

35°S

Angola Basin

0 5° 10

Figure 1. Site locations, Leg 74.

20° E

Moore, T. C , Jr., Rabinowitz, P. D., et al., Init. Repts. DSDP, 74: Washington (U.S.Govt. Printing Office).

Walvis Ridge (Chamley et al., this volume); (3) Cenozoicchanges in climates and currents in the South Atlantic(Maillot and Robert, 1980; Robert, 1980); and (4) paleo-environmental changes of the Falkland plateau area fromJurassic to Pleistocene (Robert and Maillot, in press).

METHODOLOGY

Clay MineralogyThe study involved the analysis of 334 samples. In each, the sedi-

ment fraction < 63 µm was decalcified in 0.2JV hydrochloric acid. Theexcess acid was removed by repeated centrifuging followed by homo-genization. The < 2-µm fraction was separated by decantation (settlingtime based on Stokes' law) and oriented aggregates made on glass slides.Three X-ray diffractograms were made using (1) an untreated sample,(2) a glycolated sample, and (3) a sample heated for 2 hr. at 490°C. A

663

H. MAILLOT, C. ROBERT

C.G.R. Thėta 60 diffractometer (copper Kα radiation focused by aquartz curved crystal monochromator) was used at scan speeds of 1 °20/min. with all instrument settings kept constant. A receiving slit of 1.25mm ensured precise resolution of poorly crystallized minerals.

The minerals recognized include chlorite, illite, irregular mixed lay-ers (chlorite-smectite and illite-smectite), kaolinite, smectite, and atta-pulgite (palygorskite and sepiolite). Associated minerals were quartz,feldspar, goethite, cristobalite, and clinoptilolite in variable abundance.

Semiquantitative evaluations were based on the peak heights andareas. The height of the 001 illite peak (glycolated sample) was taken asa reference. Compared to this value, smectite, attapulgite, and irregularmixed layers were corrected by multiplying their peak height by a factorof 1.5 to 2.5, depending on their crystallinity, whereas well-crystallizedkaolinite was corrected using a factor of 0.5. Final data are given inpercentages, the relative error being ± 5 % .

Geochemistry

The samples were dried at 105°C, then ground and homogenized.Sub-samples weighing 0.2 g were subjected to alkaline fusion, then dis-solved in HC1 and diluted to 100 m l . The treatment allowed gravimetricdetermination of SiO2 and spectrophotometric determination of CaO,MgO, A12O3, and Fe2θ3 (by atomic absorption). Another 2 g of eachsample were submitted to fluoroperchloric treatment, then dissolved inHC1 and diluted to 100 ml . The dilution was used for the colorimetricanalysis of TiO2 and the spectrophotometric analysis of Na2O and K2O(by emission) and of Mn, Zn, Li, Ni, Cr, Sr, Co, Cu, Pb, V, and Cd (byatomic absorption). The emission and atomic absorption apparatus wasa Type 503 Perkin-Elmer spectrophotometer using (1) base solution formajor elements and (2) complex synthetic solution for trace elements towhich 5% lanthane in a hydrochloric solution was added.

GENERAL RESULTS

Clay Mineralogy

In general, clay mineral assemblages from Leg 74sites show the absence of diagenesis with the depth ofburial. Although the influence of burial depth on thediagenesis of clay minerals has been observed by Dun-oyer (1969) for deeper boreholes, there is neither pro-gressive transition from smectite to mixed layers and il-lite nor reduction in illite crystallinity with increaseddepth at the sites under discussion.

Volcanogenic smectite is abundant at Site 525, in ashallow water environment, and results from subaerialalteration of basalts. Deeper Sites 527 and 528 probablycontain some volcanogenic smectite in the sediments in-tercalated in basalts. However, diversified clay assem-blages appear very soon after occurrences of basalt, in-dicating the dominant detrital character of clay particles(Chamley et al., this volume).

As there is no evidence for authigenesis of clay min-erals in sediments in this part of the South Atlantic, theyare considered to be essentially of detrital origin and tohave formed in soils and by continental weathering as isthe case in the North Atlantic (Chamley, 1979) and inother parts of the South Atlantic (Robert, 1981; Robertand Maillot, in press).

In the Atlantic, Cretaceous sediments contain kao-linite and large amounts of well-crystallized smectite,similar to the pedogenic smectite presently originatingfrom African regions (Paquet, 1969). These mineralspoint to the existence of a globally hot continental cli-mate, in agreement with the data compiled by Frakes(1979). The characteristics of clay mineral assemblagesproduced during long periods of constant warm climaticconditions (Millot, 1964; Chamley, 1979; Robert et al.,

1979) include the following: (1) Chlorite, illite, andmixed layers result principally from the direct erosion ofrocks and from moderate continental weathering. (2)Kaolinite is derived from soils developed in sloping andwell-drained upstream areas. (3) Except in some UpperCretaceous and Paleogene levels where volcanogenicsmectite is abundant, this mineral comes mainly fromdeep soils in downstream areas of continental drainagebasins where relief is low and drainage poor. (4) Palygo-rskite (attapulgite) is derived mainly from sediments ofconfined coastal basins.

In the sediments from Leg 74, direct erosion of rocks,moderate continental weathering, and a sloping conti-nental morphology characterize the vicinity of the sitesin the Late Cretaceous. This influence is less evident atSite 529, where Cretaceous sediments are poorly repre-sented, and at Site 525, where abundant volcanogenicsmectite can dilute the detrital supply (Chamley et al.,this volume).

In Cenozoic sediments, especially since the late Eo-cene, the abundance of chlorite, illite, and mixed layersincreases progressively, following the general climatecooling (Chamley, 1979) and the latitudinal zonation ofclimates and soils (Goldberg and Griffin, 1964; Pedro,1968). The distribution of clay minerals in the ocean de-pends on several factors, the most important being theinfluence of oceanic water masses and circulation (Rob-ert, 1980). In the Plio-Pleistocene sediments of theSouth Atlantic, intermediate water masses are enrichedin illite and mixed-layers originating from temperateareas; the North Atlantic Deep Water is marked by kao-linite from equatorial regions, and Antarctic BottomWater contains a significant concentration of smectite(Chamley, 1975). The general climatic trend is evident influctuations in the abundance of the minerals in all Ce-nozoic sediments from Leg 74. Moreover, chlorite, il-lite, and mixed layers seem more important at shallowersites, suggesting an influence of the different watermasses on sedimentation.

Geochemistry

In oceanic sediments, Fe and Mn accumulationsoriginate from detrital (fine or coarse fraction), hydro-genous and biogenic, volcanogenic sources or from dia-genetic processes (Elderfield, 1977). Despite the varietyof origin, it seems that the relative abundances of Feand Mn are significant and may be used for paleoenvi-ronmental studies (Turekian, 1965; Boström et al.,1972; Maillot, 1980; Maillot and Robert, 1980). ForKrishnaswani (1976), Fe-Mn values higher than those intypical detrital shale indicate an authigenic influence inthe sediment formation. The index

M n * _ l o g Mn sample/Fe sampleMn shale / Fe shale

was used, the values of Mn shale and Fe shale beinggiven by Boström et al. (1976). The variations in this in-dex are considered to be due to the influence of oceanicbasalts and oxidizing currents.

664

CLAY MINERALOGY AND INORGANIC GEOCHEMISTRY

Al is principally related to the detrital minerals. Anindex proposed by Boström (1969, 1970) permits us toidentify detrital particles D* = Al/Al + Fe + Mn andis associated with Si* = SiO2/Al2O3 (siltstones, sand-stones). D* is close to 0.63 in typical terrigenous shales.A decrease in D* points to a less significant continentalinfluence on sedimentation. Very high values of D*sometimes correspond to abundant kaolinite particles.Si* expresses the excess Si—i.e., Si not associated withAl. It can derive from volcanogenic (ash) or be biogenic(diatoms, radiolarians) or detrital (quartz) in origin.

The ratios Mg* = MgO/Al2O3 and Fe* = Fe2O3/A12O3 permit the interpretation of variations in MgOand Fe2O3 with respect to clay mineral abundances andspecies (illite, smectites: nonferrous or Al-Fe clays, pa-lygorskites, etc.).

The index Sr* = 103(Sr/CaO) is generally related toprocesses of carbonate dissolution (Maillot and Robert,1980). When the variations related to burial diagenesisor to the primary composition of the carbonates aremathematically determined, Sr* confirms the processesof carbonate dissolution as inferred by other methods(Maxwell, Von Herzen, et al., 1970; van Andel et al.,1977; Melguen, 1978; Melguen, LePichon, et al., 1978).Three sorts of dissolution appear. (1) Very high Sr*:This type of dissolution is present in sediments rich inorganic matter and depends on the intensity of the re-duced sedimentary environment. (2) High Sr*: In openmarine and oxidizing environments, this type of dissolu-tion appears in the vicinity of the carbonate compensa-

tion depth (CCD). High Sr* values result from the lowsolubility of SrCO3 compared with CaCO3 (Nekrassov,1966) or from the partial adsorption of Sr by clay miner-als (Baush, 1968). (3) Low Sr*: This type of dissolutionoccurs when the sediment has been deposited above theCCD.

Site 526

WAL VIS RIDGE

Lithology

Sediments vary from calcareous oozes, often contain-ing abundant foraminifers, to shallow-water calcareoussands (Site 526 chapter). The sediments cored at Site 526range from late Paleocene to late Pleistocene. (See Fig.2 for lithologic symbols.)

The base of the sedimentary section (see Fig. 3) con-sists of upper Paleocene and upper Eocene fossiliferouscalcareous sands and sandstones; the noncalcareousfraction (20-40%) includes volcanic glass> palagonite,quartz, feldspar, and glauconite. This is overlain byrubbly, highly fossiliferous limestone faciës of late Eo-cene age. Through the lower Oligocene, the sedimentconsists of a foraminifer-nannofossil ooze with minor,nannofossil ooze and foraminifer-nannofossil chalk.The upper Oligocene to lower Miocene sediments con-sist of a nannofossil ooze. Middle Miocene, Pliocene,and Pleistocene sediments include'sand-sized foramin-ifer ooze, nannofossil-foraminifer ooze, foraminifer-

Lithology

J I

Nan no ooze

Nanno chalk

Limestone ~ • —^-T ' -

> Λ •» » «* Basalt

ForaminiferalNanno ooze

ForaminiferalManno chalk

MarlyLimestone

MarlyNanno ooze

Mudstone

Sand

Clay Minerals

Chlorite IlliteMixed-Layers

KaoliniteAttapulgite(Palygorskite)

Smectite

Sepiolite

Accessory Minerals

• Rare # Common O Abundant O Very abundant

Figure 2. Symbols used in graphic lithology columns (sediment), Figs. 3-9.

665


Figure 3. Site 526 results.

nannofossil ooze, and nannofossil ooze. A break in theaccumulation of pelagic sediments is found in the lower-most Oligocene.

Clay Mineralogy

In upper Paleocene and upper Eocene sediments(Hole 526C, Cores 8, 18, and 20), smectite is the mostabundant mineral (70-90%). It is accompanied by chlo-rite (5%), illite (5 to 15970), mixed-layers (5-15%), andkaolinite (0-5%). Quartz and feldspar are also present.

From upper Eocene to upper Pliocene (Hole 526C,Cores 5-7; Hole 526A, Cores 1-39; Hole 526B, Cores3-5) illite increases significantly (15-50%). Chlorite(5-10%), mixed-layers (5-20%), and kaolinite (traceamounts to 5%) also increase. Palygorskite is presentepisodically until middle Miocene (trace amounts to15%; Plate 1). Quartz and feldspar are also present.These results, which indicate the importance of illite

principally from the late Eocene on, point to the im-portance of climatic control on clay sedimentation.

Site 525

Lithology (Fig. 4)

The complete sedimentary section (Site 525, chapter)consists of Campanian to Pleistocene sediments rich tomoderately rich in biogenic calcareous components (Site525 chapter). The sediments interbedded between Cam-panian basalts include fine-grained, marly limestones,noncalcareous mudstones, cherts, Porcellanite, and vol-caniclastics. The upper Campanian to Maestrichtian in-terval is represented by cyclic sedimentation of alterna-ting beds of chalk, calcareous siltstone and sandstone,and marly limestones. The upper part of the sedimen-tary column (Paleocene-Quaternary) consists of nanno-fossil and foraminifer-nannofossil ooze and chalk.

Clay Mineralogy (Fig. 4)

In Campanian to upper Paleocene sediments (Hole525A, Cores 52-35), smectite dominates the clay as-semblages (40-100%). This mineral is accompanied bychlorite (0-5%), illite (0-25%), mixed-layers (0-10%)kaolinite (0 to 10%), palygorskite (0-10%), and sepio-lite (0-trace amounts). Quartz, feldspar, cristobalite,and clinoptilolite are also present. This clay mineral as-sociation, common during the Cretaceous/Tertiary tran-sition, reflects an influence of tectonics on sedimenta-tion (Chamley and Robert, 1979). The weakness of thisinfluence at Site 525 is probably due to the dilution ofthe detrital supply by smectite originating from subaer-ial alteration of basalts (Chamley et al., this volume).

From upper Paleocene to middle Eocene (Hole 525A,Cores 31-20) the abundance of smectite decreases slight-ly (50-80%), and palygorskite (0-20%), illite (5 to 25%),and mixed-layer clays (10-15%) increase. Chlorite, kao-linite, and sepiolite disappear. Quartz, feldspar, and cli-noptilolite are also present. Detrital particles originatingfrom both direct erosion of moderately weathered rocksand confined coastal basins are present contemporane-ously in the sediment, mixed with pedogenic smectitefrom flat coastal lowlands.

From the upper Oligocene up (Hole 525A, Cores 19and 17-9; Hole 525B, Core 50), the slight decrease in theabundance of smectite (40-75%) is balanced by thepresence of chlorite (trace amounts-5%), illite (10-20%),mixed-layers (5-20%), and kaolinite (trace amounts-5%). Quartz and feldspar are also present; clinoptiloliteoccurs in upper Oligocene and lower Miocene sedimentsonly. This relative importance of primary minerals andmixed-layer clays is related to progressive climatic cool-ing (Chamley, 1979).

Geochemistry (Fig. 4 and Table 1)

In Campanian to lower Maestrichtian sediments,average values of D* = 0.55 and Mn* = 0.36 suggestthe presence of an open marine environment or sub-aerial alteration of volcanic component (see Chamley etal., this volume). Relatively high values of Sr* arerelated to burial diagenesis of carbonates. The values of

666


Table 1. Site 525: geochemical results.

Sample(level in cm)

Hole 525A

3-3, 1006-2,408-4,5011-2,5513-2, 5015-2, 6317-1, 5520-1,5022-2, 5024-2, 5026-1,5028-3, 3031-2, 5035-2, 5536-3, 8438-4, 2839-2, 10539-5, 9940-3, 13540-5,8941-2, 10241-5, 9142-1,2742-2, 4242-3, 3143-6, 6944-2,4245A 5247-2, 8549-1, 10750-1, 5750-3, 4851-1,2051-3, 1352-3, 10952-4, 14252-5, 12953-1, 107

Hole 525B

3-1,845-1, 758-1, 9812-1, 4014-3, 4417-1, 2019-1, 6625-1,4427-1, 75

SiO2

( * )

.00

.10

.855.30.55

1.203.101.650.551.450.700.951.201.353.50

13.7011.2023.4027.4015.3016.7015.3010.9510.1514.8023.6538.1524.0523.0031.5564.2549.9021.4533.2521.3014.6532.5586.65

0.951.200.650.650.950.201.051.701.05

A1 2 O 3

(W)

0.140.140.380.870.350.250.710.310.120.270.150.180.370.520.783.423.006.507.754.254.594.143.142.834.065.919.815.674.967.495.026.054.338.965.594.594.713.13

0.330.320.250.170.170.170.260.430.26

CaO

W

50.6650.3549.8947.8847.8850.5249.2248.5544.1949.2250.1248.8849.2249.7247.5440.5441.1831.7627.8138.1334.7237.7841.6341.9836.6631.7617.7329.3031.2422.3610.0815.3331.3418.5530.0335.8226.780.07

48.3549.2248.5551.8950.3250.7251.0249.4249.89

MgO N a 2 θ°7o)

0.23 0.920.20 0.880.300.36

.08

.030.27 0.590.26 ().590.29 0.860.31 0.860.31 0.810.36 .08

0.32 0.780.27 0.790.29 0.820.29 0.760.31 0.790.86 0.940.701.351.450.850.931.04 (

.00

.35

.15

.04

.15).91

0.80 0.740.78 (1.04 (1.262.881.671.512.101.001.411.192.061.380.990.390.20

0.280.27 (0.27 (0.27 (0.240.22 (0.24 (0.310.24

).71).59.28.69.48.33.53.08.21.08.79.46.25.52.04

.08

).88).88).91X79

).71).841.04).79

K2O

w

0.090.090.160.280.160.120.300.150.060.130.070.120.160.210.300.800.961.402.111.161.480.950.630.561.201.541.261.291.052.021.361.661.342.291.400.521.660.84

0.140.120.100.080.080.070.100.150.09

TiO2

(%)

0.010.010.040.050.020.020.010.030.010.030.020.020.020.030.040.300.260.380.430.300.330.330.250.200.240.471.190.330.380.590.270.390.330.791.650.620.430.23

0.020.020.020.020.020.020.020.030.02

Fe 2 O 3

0.100.100.310.603.180.280.420.310.140.290.210.190.340.280.372.221.753.554.122.502.682.291.641.622.273.156.333.793.575.332.573.793.187.333.973.571.471.22

0.260.230.190.180.130.140.160.970.19

Mn(ppm)

12112624734746317930532626339550012179

263268411395526405516716547611684837884458

1,100974653247432905837758

1,2321,026

21

158153147111126111126258184

Zn(ppm)

39523228963217479514724715857979

1378484

10067

232111105

68116694850

15360

305379358247126168295374153279237147

1687953372632215826

Li(ppm)

1125234

233

107

2722141314109

121440242020

9119

1411443

211211232

Ni(ppm)

969995

12111111121114121517162315121297

13178

1664

12159

1617141053

121111

1007

114

Cr Sr(ppm) (ppm)

1317151418161714161414131716181921242615251714152423392021432828283429333333

16151687

13111412

,516,421,558,400,326,358,442795737758637789795

,000,168663684647600721705789821900742716505768711574274405689558658774668142

,332,421,453,421,474,474,442,200,158

Co(ppm)

779

121199879878798

12885768895

11455969

15764

66

788543343

Cu(ppm)

73155522122300465

1224444054

4243

27679

420003

883110100

Pb

(ppm)

32322951375341454747633047434221371822215336454548423146473927384839426245

23

263632

72327181918

V

(ppm)

000615884652356

162426

13716481111162921

2323236

101465955

11473

105—52

434023465

Cd

(ppm)

22222333333334323122211220

312310

333232322

Mg* (0.22) and Fe* (0.63) are very close to the meanvalue of the clay minerals. The detrital components,especially the clay minerals, are important.

In upper Maestrichtian sediments, D* (0.57) is veryclose to the value characteristic of typical shales. Si*(2.07) is typical of the values recorded in marine car-bonates. Mn* is always positive, indicative of an openmarine environment and perhaps of metal enrichmentassociated with the spreading center (see Chamley et al.,this volume).

In the lower to upper Paleocene, D* (0.56) is close tothe values recorded in typical shales. The values of Si*,Fe*, and Mg* are homogeneous with those of the upperMaestrichtian sediments, which is also indicative of anopen marine environment. Sr* is typical of the values re-corded in marine carbonates, though slightly lower inthe upper Paleocene.

From upper Paleocene to Eocene, D* (0.37) decreas-es with the progressive diminution of the continental in-fluence, and Mn* (1.04) increases in response to thepresence of an oxidizing environment (oxidizing watermasses or volcanic events along the oceanic spreading

center). Sr* is typical of marine carbonates. These val-ues decrease slowly, suggesting a burial diagenesis ofcarbonates. The position of the site has always beenabove the CCD and perhaps above the lysocline.

From Oligocene to Pleistocene, D* (0.44) increasesslightly as Mn* decreases (0.87). These values suggest aslight input of detrital materials. During early Mioceneand late Miocene times, Fe* and Mg* values were veryhigh but decreased thereafter. The occurrence of astrong volcanogenic input of ferromagnesian compo-nents is suggested. Sr* (2.89) increases slightly in theMiocene section; this value is very close to those re-corded in marine carbonates. The effects of burial dia-genesis of carbonates are slight. The site has alwaysbeen located above the CCD, preventing any strong dis-solution effect.

Site 529

Lithology (Fig. 5)

The sediments consist of carbonate oozes and chalks(Site 529 chapter), with cherts present in the lower Eo-

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Cri

O

lopt

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0••000

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80

0

O

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= log Mn samf> e /Fe sample]Mn shales/ Fe shales 1

-1.2-0.8-0.4 0 0.4 0.8 1.21 j | 1 1 1

V///λ

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AlAl + Fe + Mn

0.3 0.4 0.5 0.6 0.71 1 1 1 1

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668


2 3 4 5 6 7 8 9 1 0 2 0 3 0 4 0 5 0 6 0 8 0 1 0 0 2 0 0I I I I I I I I I I I I I I 1 I I I I

Figure 1. (Continued).

669



cene and Paleocene. Volcanic materials are particularlycommon in the basal Paleocene-Mastrichtian sequence.Erosional hiatuses and slump deposits occur frequently atthis site.


In upper Maestrichtian to upper Paleocene sediments(Cores 43-35), chlorite (0-trace amounts), illite (traceamounts-25%), mixed-layer clays (0-10%), smectite(60-95%), kaolinite (0-5%), and palygorskite (0-5%) arepresent, accompanied by quartz and clinoptilolite. Thisclay assemblage reflects the influence of rock erosion andmoderate weathering, as well as of sediment suppliedfrom confined basins. The importance of flat coastallowlands where smectite forms is very apparent.

In upper Paleocene and lower Eocene sediments(Cores 33-26), palygorskite (0-20%) and mixed-layerclays (trace amounts-15%) increase, whereas the abun-

dance of smectite diminishes (50-90%). Illite (traceamounts-25%) is also present. Chlorite and kaolinitedisappear. Clay minerals are accompanied by quartz,feldspar, and clinoptilolite. The importance of detritalsupplies both from moderately weathered rocks and fromconfined coastal basins increases during this period.

From the upper Eocene up (Cores 24-1), chlorite(0-5%), illite (5-25%), mixed-layers (0-15%), smectite(45-85%), kaolinite (0-10%), and palygorskite (0-traceamounts) are present, accompanied by quartz, feldspar,and clinoptilolite.


We studied only the base of the sedimentary section(upper Maestrichtian-middle Eocene). In upper Mae-strichtian sediments, D* (0.59) is very close to the valuecharacteristic of typical shales. The values of Si* (3.40)suggest the presence of only clay minerals in the sedi-

670


D*

Al" Al + Fe + Mn

0.3 0.4 0.5 0.6 0.7

S i *

SiO2

2 4 6 8 10



Mg*

MgO"SüoT

0.2 0.4 0.6 0.8 1.0 1.2

Fe>

0.4 0.8 1.2 1.6

= 103

S r *

Sr

"üSo~3 4 5 67 8910

Core/Section(level in cm)

24-5,6027-4, 5029-3, 4433-1, 8436-1, 5838-1, 2440-1, 3541-1, 11041-1, 12042-2, 7342-5, 7343-1, 7000-0,00

SiO2

(%)

2.350.552.006.308.057.70

14.4020.0517.9512.3012.0510.0000.00

A12O3

(<%)

0.570.170.531.471.951.933.254.841.473.653.402.950.00

CaO

w48.2149.2848.2144.1342.8543.8339.5334.6334.8239.9140.4042.6800.00

MgO

w0.360.300.320.360.470.470.701.051.070.760.730.700.00

Na2O

w0.920.560.670.710.670.720.981.260.890.780.740.610.00

K2O

w0.180.100.200.420.430.450.861.190.981.050.870.860.00

TiO2

W0.070.010.030.100.100.120.270.370.330.230.230.190.00

Fe2O3

0.360.070.290.540.610.861.762.832.041.641.691.790.00

Mn(ppm)

237253158363389521642663968679605505605

Zn(ppm)

4258535374589291

147555956

000

Li(ppm)

2136768

1514896

00

Ni(ppm)

7657

1510142216111787

Cr(ppm)

119

1114151517222719181416

Sr(ppm)

816632874

1,0791,037

779574605458705726758711

Co(ppm)

810879779

128996

Cu(ppm)

645

1117108

10144726

Pb(ppm)

26323529343247453947475243

V(ppm)

555

12141863634763798424

Cd(ppm)

3332221121112

671


ment. Fe* and Mg* increase and decrease simultane-ously—owing, almost certainly, to volcanic events in therift area.

Sr* (1.77) is typical of the values recorded in marinecarbonates and is probably related to burial diagenesis.Marine carbonates are not subjected to dissolution.Mn* (0.62) is always positive, which is indicative of anopen marine environment and, perhaps, of metal en-richment related to the rift (see Chamley et al., thisvolume). At the Cretaceous/Tertiary boundary, the geo-chemical record of the sediment reflects the variationsof the clay minerals and the influence of volcanogenicmaterials. During the Paleocene, the geochemical pa-rameters of the sediments are constant (D* = 0.61; Si*= 4.13). Sr* is typical of marine carbonates. They in-crease in the upper Paleocene and then decrease up-ward. This trend was apparently induced by a slight dis-solution of carbonates during this time.

From lower to middle Eocene, the highly positivevalue of Mn* (1.23) is characteristic of an oxidizing en-vironment. D* decreases slightly (0.53), suggesting aweaker influence of the terrigenous components. Si*(3.68) is very close to the composition of typical detritalshales, without any biogenic or volcanogenic siliceousinfluence. High positive Mg* (1.20) and Fe* (0.52) maybe caused by abundant ferromagnesian components.Sr* (1.49) indicates the presence of relatively well pre-served marine carbonates.

Site 528 (Holes 528 and 528A)

Lithology (Fig. 6)

The sediments recovered comprise nannofossil chalkand ooze, foraminiferal-nannofossil ooze, and chalk(Site 528 chapter) from upper Paleocene to Pleistocene.This sedimentation is interrupted by volcanogenic sand-stones and claystones, interpreted as turbidity currentdeposits during early Paleocene and middle Maestrich-tian time.


The middle Maestrichtian sediments intercalated inbasalts show the presence of chlorite (up to traceamounts), illite (10-25%), mixed-layers (5-20%), smec-tite (45-80%), kaolinite (0-5%), and palygorskite (up totrace amounts). There is no evidence of the influence ofvolcanism on the clay mineral assemblage (Chamley etal., this volume).

From middle Maestrichtian to upper Paleocene (Cores528-38 to 528-26), the clay assemblage includes chlorite(0-5%), illite (10-35%), mixed layers (5-15%), smectite(35-70%), kaolinite (0-10%), palygorskite (0-10%).Quartz, feldspar, and goethite are also present. Thisclay mineral assemblage contains particles from the di-rect erosion of rocks and from moderate weathering.However, in Cores 528-30 and 528-31, the sediment con-sists of almost 100% well-crystallized smectite. Consid-ering the abruptness of mineralogical change, the smec-tite is probably volcanogenic in origin; the emergentparts of the Walvis Ridge (Chamley et al., this volume)are the most likely source.

From upper Paleocene to upper Eocene (Cores 528-25to 528-15) illite (5-15%), mixed-layers (trace amounts-10%), smectite (35-75%), and palygorskite (0-35%) arepresent (Plate 1), accompanied by feldspar and clino-ptilolite. As at previous sites, this unit reflects the in-fluence of moderate continental weathering and con-fined coastal basins on sedimentation.

From upper Eocene upward (Cores 528-14-528-2 and528-A, Cores 28-1), chlorite (0-5%), illite (10-25%),mixed-layer clays (trace amounts-10%), and kaolinite(trace amounts-5%) increase, smectite (55-70%) di-minishes, and palygorskite occurs episodically in traceamounts. Quartz and feldspar are also present. The in-fluence of the Cenozoic climate cooling on clay particlesis very comparable with the record at Site 529.


During middle Maestrichtian time, Mn* values (0.68)are characteristic of oxidizing sedimentation. However,this value is low compared to those of the sedimentsoverlying the basalts (Maillot and Robert, 1980). D*(0.48) suggests a slight influence by terrigenous compo-nents. Si* (3.94) is close to the mean value for clay min-erals. Fe*, very high during middle Maestrichtian, de-creases thereafter. Sr* (2.06), typical <of the values re-corded in marine carbonates, includes the effects ofburial diagenesis of carbonates and perhaps of some dis-solution of carbonates. During the late Maestrichtian,Mn* (0.90) is highly positive and Fe* (0.49) decreases;Mn is more oxidizing than Fe. These values are almostcertainly related to volcanic events in the rift area,which were very influential during the Maestrichtianand became less so when Site 528 edged away from therift area. D* (0.58) increases slightly, suggesting thegreater influence of terrigenous components. Sr* (2.01)is typical of the values recorded in marine carbonatesand reveals what may be the effects of burial diagenesisand a slight dissolution of carbonates.

The geochemical record of the Cretaceous/Tertiaryboundary suggests only major modifications in the clayassemblage (Chamley et al., this volume).

In the lower Paleocene, D* (0.54), Si* (3.44), Mg*(0.20), and Fe* (0.62) suggest homogeneous depositsof sediment in oxidizing conditions (Mn* = 0.69). Sr*decreases, a trend which can be explained by the lackof dissolution, probably related to the occurrence ofslumps.

At the beginning of the upper Paleocene, D* in-creases slightly (0.66), suggesting a greater influence ofterrigenous components. Si*, Mg*, and Fe* variationscan be explained by the chemical composition of clayminerals. Mn* is strongly positive (1.11) in response tothe presence of an oxidizing environment, related per-haps to volcanic events along the mid-oceanic ridge orto the presence of oxidized water masses. Sr* (2.30) in-creases indicating dissolution of calcareous sediments;it has been suggested that Site 528 came near the CCDby subsidence of the seafloor or by a rise in the CCDitself. From the uppermost Paleocene to the lower Oli-gocene, Mn* (1.02) is highly positive in response to thepresence of an oxidizing environment. D* (0.50) dimin-

672




Hole 528

1-2, 701-5, 702-1, 204-1, 205-4,537-1, 508-4,6010-1,6012-5, 9115-2, 8016-4, 10218-1, 7020-1, 7022-1, 8723-4, 7825-1, 10128-1,2130-2, 8330-4, 4531-2, 12031-5, 13832-1, 10132-5, 6733-1, 10733-5, 7134-1, 5735-1, 5336-1, 13236-3, 3437-1, 14037-5, 5438-2, 6239-1,2742-3, 10343-1, 9844-1, 6446-2,4646-5, 54

Hole 528A

2-1,285-3, 498-1,6011-1,6016-1, 10021-1, 8825-1,98

SiO2(%)

2.552.406.354.557.454.254.454.352.854.501.851.401.301.452.803.453.85

11.5517.2019.7019.5511.9012.3022.5513.8012.107.055.956.20

19.6014.2520.5018.2525.1519.2022.3524.2520.75

2.651.201.351.100.701.001.50

A12O3

0.640.602.141.472.121.381.491.340.571.530.470.320.420.530.741.151.173.365.035.445.973.783.676.504.143.722.421.932.025.544.185.675.066.734.845.785.614.67

0.870.340.260.300.250.400.53

CaOW

47.5448.2144.5346.7044.7046.8445.8746.8151.1944.8649.5549.3848.8848.8847.3746.6746.2040.5833.4833.1332.0138.8038.8330.1538.3038.8042.8544.3643.8631.7436.3330.3032.7426.2531.6427.9628.4631.47

47.8448.6848.5548.2148.8547.8848.45

MgO<%)

0.310.310.520.400.480.320.360.370.270.500.330.320.260.300.300.350.340.931.241.351.420.830.801.450.980.840.580.530.52

.49

.04

.61

.311.12.87.79.91.49

0.400.270.270.290.270.270.29

Na2O<*)

1.081.100.770.810.820.780.710.860.650.810.730.730.650.730.630.710.610.781.111.111.150.690.810.940.820.710.660.600.620.890.710.910.860.980.910.811.130.91

1.180.910.980.911.010.780.71

K2O<*)

0.150.160.540.370.600.390.420.390.210.450.190.150.160.180.190.300.340.961.141.641.691.171.371.961.291.100.780.630.661.811.521.931.702.051.421.711.641.64

0.220.120.110.150.150.150.17

TiO2

W

0.030.040.110.080.100.070.080.070.040.080.030.030.020.020.030.050.060.251.170.300.430.540.230.360.230.220.130.120.130.250.250.350.350.480.320.370.400.37

0.030.020.010.020.010.020.02

Fe2O3

0.400.361 070.791.040.660.720.660.341.000.430.360.210.330.260.400.332.332.933.363.501.642.143.572.361.860.790.900.832.792.193.433.334.793.724.504.654.43

0.540.190.070.070.290.260.33

Mn(ppm)

321337789747

1,179442579458200816532563358242221284921

1,1051,3161,5531,379

795684916

1,2211,010

7421,7052,3262,7051,9472,3842,9263,0002,2322,2371,8681,884

416284221211268300405

Zn(ppm)

63686858535853584742324226475837428484

1081295866

104865847375363587468

10579797963

32212121212626

Li(ppm)

33869656352113665

122322251110231513765

161219141718161914

4211223

Ni(ppm)

812231628129

196

171098

171778

212226291513151714101194

12132820

9241214

14133446

10

Cr(ppm)

1451201417171317161389

1116989

1517191818162620161513—272435263423332627

16151112111217

Sr(ppm)

1,526,574,558,584,421,410,495,500,179,126774689737858947

1,1371,132

537484495600753768663774821874832879611716689647500684505642642

1,6001,4581,4681,3371,4321,3371,000

Co(ppm)

3698

157885

1312111112855

121913127

108975569

11101413118

1111

766788

12

Cu(ppm)

87

108

206870

1888

1189

199

2742404713578

1094

1009

19133127322528

131075338

Pb(ppm)

4739252429263531275545584438375142452153456045434735356150193447374246242439

42314639404228

V(ppm)

89

1615181513138

139

127459

183234534742587474372918—522351377452474953

14346894

Cd(ppm)

44343333323332233130021012232232322222

3323322

ishes with decreasing continental influence, and Si*(3.62) is typical of the composition of detrital shales.Sr* increases (1.56) from the uppermost Paleocene tothe lower Eocene and is 2.67 from the middle Eocene tothe lower Oligocene. During the latter interval, the car-bonates are dissolved, as demonstrated by LePichon etal. (1978) and van Andel et al. (1977).

From the upper Oligocene to the Pleistocene, sedi-mentation is very homogeneous (D* = 0.55; Si* = 3.38;Mg* = 0.56; and Fe* = 0.56). Mn* is high (1.08) in re-sponse to the presence of an oxidizing environment. Up-per Oligocene and lower-middle Miocene sedimentsshow a slight increase of Sr* (3.05) resulting from agreater dissolution of the carbonates.

Site 527

Lithology (Fig. 7)

The dominant lithology consists of nannofossil oozeand chalk, clayey nannofossil ooze and chalk, and nan-nofossil clay (Site 527 chapter). At the beginning of theupper Maestrichtian, the sequence is composed of inter-bedded basalts and sediments.


Sediments interbedded in basalts contain a clay as-semblage composed of chlorite (trace amounts) illite(40%), mixed-layers (15%), smectite (45%), and traceamounts of chlorite, kaolinite, and palygorskite. Clayparticles do not show any influence of volcanism (Cham-ley et al., this volume).

From upper Maestrichtian to upper Paleocene (Cores38-27), chlorite (0-10%), illite (5-55%), mixed-layerclays (trace amounts-25%), smectite (10-70%), kaolin-ite (trace amounts-10%), palygorskite (0-15%), and se-piolite (0-trace amounts) are present, accompanied byquartz, feldspar, and goethite. This clay assemblagereflects principally a major influence of the direct ero-sion of rocks and of moderate weathering on the clayfraction.

From upper Paleocene to upper Eocene (Cores 26-16),clay minerals (Plate 1) include chlorite (0-5%), illite(0-35%), mixed-layer clays (0-10%), smectite (45-100%), kaolinite (0-trace amounts), and palygorskite(0-10%). Feldspar and clinoptilolite are also present. Asat previous sites, the influence of moderate continental

673


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weathering and of confined coastal basins are recogniz-able in this unit.

From the upper Eocene up (Cores 16-1), the diminu-tion of smectite (45-70%), the disappearance of paly-gorskite, and the increased importance of chlorite (traceamounts-10%), illite (10%-30%), mixed-layer clays(trace amounts-10%), and kaolinite (trace amounts-5%) are all related to the progressive deterioration ofclimate.


In upper Maestrichtian sediments, the geochemicalcriteria indicate the presence of an oxidized environ-ment (Mn* = 0.89, D* = 0.54, Fe* = 0.58, and Mg* =0.23). Mn is higher than Fe. The variation of chemicalindices and clay minerals is not greatly influenced byvariation of the oxides (Chamley et al., this volume).

Sr* (2.01) is typical of marine carbonates; its values in-crease between Section 527-37-3 and 527-35-4, suggest-ing an increase in the dissolution of the carbonates.

During the Cretaceous/Tertiary transition, chemicalvariations are almost certainly related to those of theclay minerals (Chamley et al., this volume).

From lower Paleocene to upper Paleocene, Mn* (0.84)and D* (0.49) become typical of an oxidizing environ-ment, and the influence of detrital components de-creases.

Si* (3.42) and Mg* (0.29) reflect the mean compo-sition of clay minerals. Sr* decreases (1.80) and marks anondissolved carbonates fraction showing a strong ef-fect of burial diagenesis on the carbonates. Dissolutionof carbonates began only during the late Paleocene.From the top of the upper Paleocene to the lower Eo-cene, Mn* (1.08), Mg* (0.66), and Fe* (0.66) increase;

676


A I + Fe + Mn

0.3 0.4 0.5 0.6 0.7

S i *

SiO 2

A i 2 σ 3

2 3 4 5 6 7

Mg*

MgO

0.2 0.4 0.6 0.8 1.0 1.2

Fe*

0.4 0.8 1.2 1.6

Sr*

- _§!~ CaO

2 3 4 5 6


these high values are almost certainly related to volcanicevents (possibly on the mid-oceanic ridge).

From middle Eocene to upper Miocene D* (0.61) in-creases with increasing continental influence. All thechemical criteria become typical of the chemical compo-sition of the clay minerals: Si* = 3.17, Mg* = 0.29,Fe* = 0.45. Sr* is always high (3.02) in response to astrong dissolution of the carbonate fraction. The sedi-ments deposited between the end of the middle Eoceneand the upper Eocene, as well as during the middle Mio-cene, are more dissolved (van Andel et al., 1977; and LePichon et al., 1978). During the Plio-Pleistocene, highpositive Mn* values (1.10) and slightly decreased D*(0.51) can be explained by the presence of an oxidizingenvironment. Si* (3.32) reflects only the composition ofclay minerals. Sr* (2.87) decreases slightly and only thesediments from the upper Pliocene are dissolved.

TECTONICS AND SUBSIDENCE IN THEWALVIS RIDGE AREA

In the region studied during Leg 74, basaltic base-ment, dated as Campanian, was recovered at Site 525.Volcanic emissions were produced, then altered, in asubaerial environment. As a consequence, the Campan-ian and Maestrichtian sediments overlying the basalt arecharacterized by abundant well-crystallized smectite andminor precipitation of transition elements. At Site 527,where basaltic material was produced in a submarine en-vironment, the influence of volcanism is not recogniz-able in the mineralogical and geochemical data. At allsites, the sediment located above the basaltic basementcontain abundant detrital clay particles; the assemblageis characterized by significant amounts of illite andmixed-layer clays principally, and geochemical indices

677




1-2,603-1,604-1,606-1, 308-2,912-1, 7013-3, 6015-1, 8817-2, 0518-3, 7020-1,5222-1, 5024-4, 1626-1, 10228-4, 8029-2, 830-1,9930-3, 9931-1, 8831-3, 9732-1, 2632-3, 2633-3, 13135A 2737-137-338-1, 10138-4, 4942-1, 54

SiO2

w2.101.551.850.751.358.30

22.352.90

13.902.401.703.053.453.856.555.057.759.40

13.5013.7530.0047.709.80

18.059.50

15.5518.6015.3034.10

A12O3

(*)

0.550.450.510.300.422.957.090.834.310.530.300.570.680.771.871.532.142.894.144.029.45

14.892.955.462.724.555.444.509.69

CaO

w48.6148.0147.6849.9549.7242.1528.9348.8838.0047.7150.2249.8947.7149.4246.9446.1744.2842.1836.9438.3723.106.82

40.1834.1541.8536.4934.1535.8220.19

MgO(<R>)

0.290.270.310.270.270.751.430.321.330.360.270.420.380.330.540.580.630.630.990.921.652.460.691.160.680.981.190.992.19

Na2OW1.091.040.940.810.791.061.310.671.260.770.710.840.770.810.790.780.740.820.910.941.291.600.780.860.670.770.780.771.15

K2O(*)

0.130.130.150.100.120.641.550.270.600.220.150.240.270.280.540.540.930.931.391.512.263.760.931.610.961.511.811.582.63

TiO2

<*)

0.030.030.030.020.020.110.320.050.130.080.020.040.050.050.090.100.160.200.300.300.480.690.090.290.160.260.250.280.50

Fe 2 O 3

w0.310.290.320.190.271.453.820.431.090.570.190.310.360.411.071.291.641.973.152.864.585.791.562.791.472.893.073.324.93

Mn(ppm)

284332337263358

1,6894,000

326732721300247284558389

1,684816811

2,0371,8421,0372,553

9531,1213,0002,1501,7101,7844,000

Zn(ppm)

2611684

1116353

126100111684768587437453647767189

182687984

13784

100121

Li(ppm)

33312

1332

432213549869

1413275312191014141433

Ni(ppm)

45553

49879

151364

127

132115183126284412131824263134

Cr Sr(ppm) (ppm)

10119

118

182812131210111217151112201614343616231921242537

1,442,495,358

1,2631,3681,2531,0741,4741,000

800737826

1,0161,1051,042

984795632532537389242784705900753674668405

Co(ppm)

35645

2449

76

1173666

1896

161214145455557

Cu(ppm)

688

110

51666

5971553

12202021544838621598

17231054

Pb(ppm)

4243363431363222252630252227265249245146221824251923111923

V(ppm)

79733

24674

10830138

112627473261

18413475544446062

Cd

(ppm)

23324213243434322311005222120

point to the importance of detrital components. Thepresence of a youthful continental morphology, re-juvenated by tectonism, is suggested (Robert et al.,1979). The detrital influence was more important atSites 527 and 528, located in deeper water and with amore extensive alluvial source area than Sites 525 and529, located in a shallow-water environment. Moreover,at these sites the occurrence of volcanogenic smectitehas diluted the detrital supply. Tectonism and volcan-ism seem to have been directly involved in the evolutionof the Walvis Ridge during the Campanian and theMaestrichtian. Volcanism disappeared subsequently,and the importance of tectonism decreased from lateMaestrichtian to late Paleocene.

Changes in the geochemical index Sr* shows that dis-solution effects were very weak during the Late Cre-taceous and increased progressively during the Paleo-cene. This geochemical trend parallels the decrease in il-lite and mixed-layer clays in the clay fraction and mayhave been caused by the deepening of the sites. As ob-served on the Rio Grande Rise (Robert, 1981), subsi-dence accompanied tectonic activity on the Walvis Ridgefrom Campanian to Paleocene.

Fluctuations appear in the general trend of Sr* (Fig.8). During the late Maestrichtian, a short increase in dis-solution which appeared at Site 527 is also present at theshallower Site 528, where it is relatively weak. It is ab-sent at the other sites. During the early Paleocene, theoccurrence of slumps diminished the effect of dissolu-tion on the sediment. Later, in the late Paleocene, thereis a slight dissolution effect at all the sites. Generally, itis most marked at the deepest, Site 527, and decreasestoward the southeast; the slightest influence is recordedat Site 525 (the shallowest site for which geochemicaldata are available). This depth pattern is similar to thepresent one, suggesting that there has been no important

Site 527 528 529 525

A B C 0 A B C 0 A B C 0 A Bi • i__i I i i 1—L

SE

microfauna

4428 m

Dissolution: 0 Absent

3035 m

B Medium

Strong dissolution

(van Andel, 1977;Melguen, 1978)

Strong dissolution

(van Andel,1977;Melguen. 1978)Recrystallization

Slumps and breccias

C Strong

Figure 8. Dissolution of carbonates.

change in the relative depth position of the site since thelate Paleocene.

From upper Paleocene to upper Eocene, the abun-dance of chlorite, illite, and mixed-layer clays points toa renewal of the tectonic activity on the Walvis Ridge.The paroxysm seems to have occurred at the end of the

678


late Paleocene and in the early Eocene. It marks the endof the tectonic activity in this region which began duringthe Campanian, contemporaneously with a change inthe direction and speed of ocean opening (Goslin et al.,1974).

At the same time, the deposition of abundant paly-gorskite in the sediment suggests an important contribu-tion from enclosed or semi-enclosed confined basins. Alocal phase of subsidence could have induced submer-gence of flat areas on the Walvis Ridge where palygor-skite developed, as was the case on the Rio Grande Riseduring the same period (Robert, 1981). Palygorskitevery often occurs in long, flexible fibers, indicating thatit formatted close to the sites (Plate 1). Moreover, thepresence of fibers growing at the periphery of well-shaped particles suggests that palygorskite probably de-veloped by transformation processes, as shown by Trauth(1974).

During the late Eocene, palygorskite disappeared.Clay mineral assemblages are uniform at all sites wheresmectite dominates the clay fraction, and point to simi-lar sources of detrital supply. The Walvis Ridge, as wellas Rio Grande Rise, was probably mostly submerged bythis time.

From the late Paleocene on, the subsidence of theWalvis Ridge was continual and can be seen in the geo-chemical indices (Fig. 8). From middle Eocene on, thedepth gradient in dissolution observed in late Paleocenesediments is also present, with some differences: no dis-solution occurred at Sites 525 and 529, whereas thedeeper Site 528 and especially Site 527 were influencedby dissolution effects. The persistence of increased dis-solution since the late Paleocene was probably related tothe progressive deepening of the site toward more chem-ically active deep waters as the ridge subsided.

CLIMATIC AND OCEANOGRAPHIC EVOLUTIONOF THE WALVIS RIDGE AREA

From the upper Eocene upward, all the sedimentarysequences recovered show an irregular increase in chlo-rite, illite, and mixed-layer clays. This trend is a conse-quence of the Cenozoic climatic cooling which began inthe middle to late Eocene (Shackleton and Kennett,1975). Climatic cooling was responsible for the progres-sive diminution in the intensity of weathering on thecontinents, which prevented the complete developmentof soils. As a consequence, the abundance of smectite orkaolinite decrease, whereas chlorite, illite, and mixed-layers become abundant (Chamley, 1979). A peak in theabundance of these minerals occurred in late Miocene toearly Pliocene sediments, at a time when the Antarcticice sheet developed (Shackleton and Kennett, 1975) andreached its greatest extent (Mercer, 1978).

Compared to clay mineral assemblages recovered inother regions of the South Atlantic, such as the Mid-Atlantic Ridge (Maillot and Robert, 1980), the FalklandPlateau (Robert and Maillot, in press) or the other sitesfrom the Walvis Ridge, the relative abundance of clayspecies at Site 526 appears more homogeneous; fluctu-ations are present, but one cannot discern any impor-tant break, probably because the currents present on the

flank of the Walvis Ridge locally removed sediments (asshown by Bornhold and Summerhayes, 1977) and ho-mogenized the composition of the clay fraction. Thesecurrents may have been efficient since the early Oligo-cene, when the importance of global circulation in-creased and when the Benguela upwelling system started(Peypouquet, personal communication, 1981).

From the upper Eocene on, the geochemical index ofdissolution (Sr*) also shows a progressive and irregularincrease (Fig. 8). The first stage occurred during the lateEocene, in agreement with the major phase of carbonatedissolution evidenced by investigations of the calcareousbiogenic components in the southeast Atlantic (vanAndel et al., 1977, Melguen, LePichon, et al., 1978).Another comparable stage occurred during the middleMiocene. During the late Miocene and early Pliocene, asmooth increase in the dissolution occurred and reachedits maximum at the early/late Pliocene boundary. Themain stages of carbonate dissolution recorded corre-spond to the phases of shoaling of the CCD observed byvan Andel et al. (1977) and Melguen, LePichon, et al.(1978) and attributed to a cooling of bottom waters. ThePliocene phase of dissolution had not previously beennoted and may be related to an increase in the cooling orin the circulation of bottom waters.

Cenozoic clay assemblages show also geographicaldifferences in the abundance of chlorite, illite, and mixedlayers (Fig. 9). These minerals are more abundant atSites 525 and 526, located in a shallower environment,than at Sites 527 and 528, drilled in deeper waters wheresmectite dominates. The difference is noted first in theEocene, and its importance increased with time until thePlio-Pleistocene.

These depth differences can be explained by com-parison with data on suspended matter and recent sedi-ments, taking into account the vertical distribution ofthe different water masses and oceanic currents (Sver-drup et al., 1942; Dietrich, 1963; Reid et al., 1977).Elevated sites, such as 525 and 526, are more subject tothe Benguela, which flows from the south and south-east. This current favors the transport of detrital com-ponents from arid regions of South Africa which havebeen carried to the ocean by rivers and winds. In addi-tion to smectite, the detrital supply probably containsmore abundant chlorite, illite, and mixed-layers, as isthe case along the arid coast of northwest Africa (Born-hold, 1973; Diester-Haass and Chamley, 1980; Robert,1980).

Deeper sites, such as 527 and 528, which containmore abundant smectite, are subjected to intermediateand deep waters formed around Antarctica. Previousstudies have shown that smectite increases in the sedi-ments under the influence of Antarctic Bottom Water(Chamley, 1975; Melguen, Debrabant et al., 1978), andthat smectite probably comes from erosional processeson Antarctica or on the seafloor of the Southern Ocean(Robert and Maillot, in press). Thus, deep and bot-tom waters enriched in smectite probably enter theAngola Basin through the Romanche Fracture Zone andthrough the Walvis Passage, located close to the areastudied during Leg 74 (Connary and Ewing, 1974; Mc-

679


Site 5 2 7 528 525 5261 0 0 % NW

5 2 7 5 2 8 5 2 9 525

O A B C O A B C O A B C O A B C

Figure 9. Cenozoic evolution of clay minerals, Walvis Ridge.

Coy and Zimmerman, 1977). These trends became ap-parent by late Eocene, contemporaneous with the begin-ning of the Cenozoic climatic cooling.

During the Late Cretaceous, variations of the geo-chemical index Mn* depended on volcanic influences(Chamley et al., this volume). During Cenozoic time,the influence of metalliferous accumulations associatedwith volcanism or hydrothermal activity cannot be ex-cluded. However, the coincidence of carbonate dissolu-tion with metalliferous accumulation events (Fig. 10)leads us to interpret the Mn* data as being greatly af-fected by oceanic currents (Maillot, 1980). Schematical-ly, if one considers the Paleobathymetric position of thesites, (1) an increase in Mn* may be related to an in-creased influence of oxidized surficial or bottom watersor to a decrease in the temperature of bottom waters,and (2) a decrease in Mn* may be due to either the lackof oceanic circulation leading to the presence of euxinicenvironments or the increased influence of poorly ox-idized intermediate water masses.

In the area under consideration, Mn* retains positivevalues, the oceanic environment being always oxidized.Different results were obtained at Sites 362 (Leg 40) and

P ^PresentDepth

Yλ

4426 m

Oxidation: 0 Absent

SE

Cooling of thebottom waters

End of thebarrier effectof the ridge

Subsidence ofthe ridge

3810 m 3035 m 2477 m

A Slight B Medium C Strong

Figure 10. Oxidation of the marine paleoenvironment.

532 (Leg 75), where oxygen minimum layers were pres-ent from the middle Miocene to the Pleistocene (Maillotand Robert, 1980; Maillot and Robert, in press). Twomajor increases in metalliferous components occurredsimultaneously at the four sites of Leg 74, during thelate Paleocene and the middle Eocene. This points to theexistence of two main stages in the development of thecirculation of deep water masses. These are probablyrelated to the subsidence of the Walvis Ridge, whichprogressively favored the exchanges of water massesbetween the Angola and Cape basins, and to changes inthe vertical structure of the water masses in the SouthAtlantic.

A brief increase in Mn* during late Pliocene time isevident, especially at Site 528. This increase in metallif-erous concentrations is probably related to an increasein the velocity or in the cooling of deep waters followingthe development of the Arctic ice sheet.

CONCLUSION

Combined inorganic geochemical and clay mineral-ogical studies on sediments recovered during Leg 74 aidin the reconstruction of both continental and oce-anic paleoenvironments since the Late Cretaceous. Thus,information obtained on volcanics, tectonics, and onmarine and continental influences during the Late Cre-taceous enable us to determine the geologic evolution ofthe Walvis Ridge during the period concerned. On theother hand, inorganic geochemical and clay mineralogi-cal data permit us to follow the changes which occurredin climate and oceanic circulation during Cenozoic time.

680


The proximity of the sites permits us to examine thehistory of sedimentation both in its essential featuresand in detail.

ACKNOWLEDGMENTS

Thanks are due to the National Science Foundation for samplesfrom Leg 74. Financial support was provided by the CNEXO (France)for geochemical studies. B. Bornhold, H. Chamley, T. Moore, and P.Borella reviewed and improved the original manuscript, we also thankM. Acquaviva and C H . Froget for technical assistance in clay miner-als analysis, E. Hanton and C. Maillot for assistance in geochemicalanalysis, M. Bocquet for the illustrations, J. Carpentier for the photo-graphs, and F. Dujardin for typing the manuscript.

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1 µm iµm

1µm

1µm

Plate 1. Electromicrographs (scale bar = 1 µm) 1. Sample 526A-17-2, 70 cm (× 12,500); middle Miocene palygorskite in short, straight fibers;smectite in fleecy particles; well-shaped illite. 2. Sample 527-24-4,16 cm (× 8000); late Paleocene; palygorskite fibers; well-shaped illite; smec-tite with blurred contours. 3. Sample 528-21-1, 96 cm (× 10,000); early Eocene; palygorskite in long, straight, or flexuous fibers; smectite withblurred contours; well-shaped illite. 4. Detail of Fig. 3 (× 20,000); early Eocene; fibers of palygorskite, probably growing at the periphery of adense particle (illite?).

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inorganic geochemical and clay mineralogical data, … · this type of dissolution is present in...

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