deep sea drilling project initial reports volume 71trast to the similar reducing facies of...

11. PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERALOGICAL ANDGEOCHEMICAL DATA, SOUTHWEST ATLANTIC, DEEP SEA

DRILLING PROJECT LEGS 36 AND 711

Christian Robert, Géologie Marine, L.A. CNRS 41, Faculté des Sciences de Luminy, Case 901,13288 Marseille Cedex 9, France

andHenri Maillot, Sédimentologie et Géochimie, E.R.A. CNRS 764, Université de Lille 1,

59655 Villeneuve d'Ascq Cedex, France

ABSTRACT

For Middle Jurassic to Pleistocene times, clay mineralogical and geochemical data provide information on the evo-lution of continental and marine paleoenvironments. They are a source of information on marginal instability, on thecontinental and shallow marine environments related to the development of the Southern Ocean during the Middle andLate Jurassic, and on tectonic relaxation of the continental margins at the end of the Late Jurassic. They also provideevidence for the influences of the South Atlantic opening and the movement of the Falkland Plateau in a reduced ma-rine environment until Aptian-Albian times, and the transition to an open marine environment during Albian time; theinfluences of the Albian-Turonian and Coniacian-Santonian Andean deformations in an open marine environment;the limited tectonic effects and strong influence of marine currents at the Cretaceous/Tertiary boundary; the influencesof the global climatic cooling and inferred bottom water circulation during the late Eocene and Oligocene; the wideningof the South Atlantic Ocean during Oligocene time, which was accompanied by an increased influence of the biogeniccomponents on sedimentation; increased carbonate dissolution from late Oligocene to early Miocene, related to thedeepening of the ocean; limited mineralogical and important geochemical modifications when the Drake Passage openedin the early Miocene; the influence of the late Miocene development of the Antarctic ice-sheet; the major Antarcticcooling and Patagonian glaciation during Pliocene time; and the change in the Antarctic Bottom Water circulation atthe Pliocene/Pleistocene boundary.

INTRODUCTION

During Legs 36 and 71 of the Deep Sea Drilling Proj-ect, holes were drilled at eight sites on the FalklandPlateau, in the Georgia Basin, and on the western flankof the Mid-Atlantic Ridge (Fig. 1). The sediments re-covered range from Pleistocene to Jurassic. Site char-acteristics are summarized in the list.

the South Atlantic basins (Maillot, 1979; Robert et al.,1979; Maillot, 1980; Maillot and Robert, 1980), paleo-environmental conditions during the deposition of theblack shales (Chamley and Robert, in press), Creta-ceous/Tertiary boundary events on the Atlantic margins(Chamley and Robert, 1979) and on the Rio Grande Rise(Robert, 1981), and Cenozoic changes in climates andcurrents in the South Atlantic (Maillot and Robert, 1980;Robert, 1980).

WaterPosition Depth Penetr.

(Latitude; Longitude) (m) (m)

No. Samples

Min. Geochem.

OldestSedimentRecovered

Leg 36

327, 327A328, 328A.B329330, 33OA

Leg 71

511512, 512A513, 513A

50-52.28'S; 46°47.02'W 2400 47049°48.67'S; 36°39.53W 5095 4715O°39.3rS;46°O5.73'W 1519 46550°55.19'S;46°53.00'W 2626 576

51°OO.28'S; 46°58.30'W49o52.18'S;40°50.71'W47°34.99'S; 24°38.40'W

258918464380

63290

380

1273084

46°02.77'S; 26°51.30'W 4318

PaleoceneJurassic

on basement

Jurassicmiddle Eoceneearly Oligocene

on basementearly Pliocene

The purpose of this study is to reconstruct the paleo-environmental conditions in the region from the claymineralogical and inorganic geochemical variations. Sim-ilar approaches have been used in the past to determinethe paleoenvironmental evolution of the South Atlantic.Previous studies include the Cretaceous development of

1 Ludwig, W. J., and Krasheninnikov, V. A., Init. Repts. DSDP, 71: Washington (U.S.Govt. Printing Office).

METHODOLOGY

Clay MineralogyThe study involved the analysis of 543 samples. In each, the sedi-

ment fraction <63 µm was decalcified in 0.2N hydrochloric acid. Theexcess acid was removed by repeated centrifugations followed byhomogenization. The <2 µm fraction was separated by decantation(settling time based on Stokes' law), and then oriented aggregates weremade on glass slides. Three X-ray diffractograms were made: (1) un-treated sample, (2) glycolated sample, (3) sample heated for 2 hr. at490°C. A C.G.R. Theta 60 diffractometer (copper Kα radiation fo-cused by a quartz curved crystal monochromator) was used at scanspeeds of 1° 20/min.; all instrument settings were kept constant. A re-ceiving slit of 1.25 mm allowed a more precise resolution of poorlycrystallized minerals.

The minerals recognized include chlorite, illite, irregular mixed-layer clays (chlorite-smectite and illite-smectite), kaolinite, smectite,and attapulgite (palygorskite). Associated minerals were quartz, feld-spar, cristobalite, and clinoptilolite in variable abundance. The sym-bols used in Figures 3-10, Results, are shown in Figure 2.

Semiquantitative evaluations were based on the peak heights andareas. The height of the 001 illite peak (glycolated sample) was takenas a reference. Compared to this value, smectite, attapulgite, and ir-regular mixed-layer clays were corrected by multiplying their peakheight by a factor of 1.5 to 2.5, depending on their crystallinity,

317

So

Figure 1. Site location map.

PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA

Lithology

>>Σ

Diatomaceous ooze

Nannofossil—

diatomaceous ooze

Muddy

diatomaceous ooze

Foraminiferal—

nannofossil ooze

Nannofossil chalk

Clay and claystone

Silty clay

Black shales

— i — i

— z — z —z- z-z-z-z-

Clay Mineralogy

Chlorite

Smectite

Sepiolite

Illite

Diatomaceous—

radiolarian ooze

Nannofossil—radiolarian

diatomaceous ooze

Muddy

radiolarian ooze

Nannofossil ooze

Marly nannofossil chalk

Zeolitic clay

Sand

Gneiss

Kaolinite

Mixed-layer clays

Attapulgite

(palygorskite)

Associated Minerals

• Very rare ° Rare

• Abundant O Very abundant

Figure 2. Symbols used in Figures 3-10.

whereas well-crystallized kaolinite was corrected using a factor of 0.5.Final data are given in percentages, the relative error being ± 5 % .

Illite crystalling was measured by the width, at half height, of the001 illite peak (10 Å), on the glycolated sample. The relative abun-dance of smectite versus illite (S/I index) was obtained from the ratioof the 001 smectite (18 Å) and illite (10 Å) peaks, on the glycolatedsample.

Geochemistry

The samples were dried at 105°C, then ground and homogenized.Samples of 0.2 g were subjected to alkaline fusion, then dissolved inHC1 and diluted to 100 ml. The treatment allowed gravimetric deter-mination of SiO2 and spectrophotometric determination of CaO,MgO, A12O3, and Fe2θ3 (by atomic absorption). Another 2 g of eachsample were subjected to fluoroperchloric treatment, then dissolved inHC1 and diluted to 100 ml. The dilution was used for the colorimetricanalysis of TiO2 and for the spectrophotometric analysis of Na2O andK2O (by emission) and of Mn, Zn, Li, Ni, Cr, Sr, Cu, Pb, V, and Cd(by atomic absorption). The emission and atomic absorption appa-ratus was a Perkin Elmer type 503 spectrophotometer using the fol-

lowing methods: (1) base solution for major elements, (2) complexsynthetic solution for trace elements to which a solution of 5% Ian-thane in hydrochloric acid was added.

GENERAL RESULTS

Clay Mineralogy

In general, clay mineral assemblages from these sitesshow an absence of diagenetic modifications. (1) We ob-served no evidence of burial diagenesis: there is neither aprogressive transition from smectite to mixed-layer claysand illite, nor a reduction in illite crystallinity with in-creased depth at these DSDP sites. (Compare Dunoyer'sfindings [1969] for the influence of burial diagenesis onclay minerals in deeper boreholes.) (2) Volcanic diagen-esis is absent also: smectite abundance does not increasesignificantly toward the bottom of the holes which werenearer the spreading ridge when the sediments were de-posited. Unfortunately, the base of the sedimentary sec-tion of Hole 513A, at the contact with the basalt, wasnot recovered. (3) Finally, there is no evidence of or-ganic diagenesis: the black-shale environment shows theabsence of in situ alteration of the clay minerals, in con-trast to the similar reducing facies of MediterraneanPlio-Pleistocene sapropels (Sigl et al., 1978).

As there is no evidence for authigenesis of clay min-erals in pelagic sediments in this part of the South At-lantic, they are considered to be essentially of detritalorigin, formed in soils and by continental weathering asin the North Atlantic (Chamley, 1979) and in other partsof the South Atlantic (Robert et al., 1979; Robert, 1981).

In the Jurassic and Cretaceous sediments, kaoliniteand large amounts of well-crystallized smectite, similarto the pedogenic smectite presently originating fromAfrican regions (Paquet, 1969), are present. These min-erals point to the existence of a globally warm contin-ental climate, in agreement with the data compiled byFrakes (1979). Clay mineral assemblages produced dur-ing long periods of constant, warm, climatic conditions(Millot, 1964; Chamley, 1979; Robert et al., 1979) arecharacterized by (1) chlorite, illite, and mixed-layer min-erals, resulting principally from the direct erosion ofrocks and from moderate continental weathering; (2)kaolinite, derived from soils developed in sloping andwell-drained upstream areas; (3) smectite, coming main-ly from deep soils in downstream areas of continentaldrainage basins, where relief is low and drainage poor;and (4) attapulgite (palygorskite), derived mainly fromsediments of confined coastal basins.

In Cenozoic sediments, and especially since the lateEocene, the abundance of chlorite, illite, and mixed-layer clays increases progressively, following the gener-al climatic cooling (Chamley, 1979) and the latitudinalzonation of climates and soils (Goldberg and Griffin,1964; Pedro, 1968). The distribution of the clay miner-als in the ocean is affected by several factors, the mostimportant being the influence of oceanic water massesand circulation (Robert, 1980): in the Plio-Pleistocenesediments of the South Atlantic, intermediate water mas-ses are enriched in illite and mixed-layer clays originat-ing from temperate areas, North Atlantic Deep Water is

319

C. ROBERT, H. MAILLOT

marked by kaolinite from equatorial regions, and Ant-arctic Bottom Water contains an important proportionof smectite (Chamley, 1975).

Geochemistry

In oceanic sediments, Fe and Mn accumulations orig-inate from materials that are detrital (fine or coarsefractions) hydrogenous, biogenic, and volcanogenic inorigin, or by diagenetic processes (Elderfield, 1977).Despite these various origins, however, it seems that therelative abundances of Fe and Mn are significant forpaleoenvironmental study (Turekian, 1965; Boström etal., 1972; Maillot, 1980; Maillot and Robert, 1980). ForKrishnaswami (1976), Fe-Mn values higher than con-centrations in a typical detrital shale are considered indi-cative of an authigenic influence in sediment formation.To show the influence of oceanic basalts and oxidizingcurrents, variations in the index

Mn* = log [Mn sample/Mn shale/Fe sample/Fe shale]

were studied, using the values of Mn shale and Fe shalegiven by Boström et al. (1976).

Al is principally related to the detritic minerals. Anindex proposed by Boström (Boström and Peterson,1969; Boström, 1970), D* = Al/Al + Fe + Mn, associ-ated with Si* = SiO2/Al2O3 permits recognition of de-trital particles. D* is close to 0.63 in typical terrigenousshales. A decrease of D* points to a less significant con-tinental influence on sedimentation. Very high values ofD* correspond sometimes to abundant kaolinite parti-cles. Si* expresses the excess Si (i.e., Si not associatedwith Al). This silica can be of volcanogenic (ashes), bio-genic (diatoms, radiolarians), or detrital (quartz) origin.

The indexes Mg* = MgO/Al2O3 and Fe* = Fe2O3/A12O3 permit interpretation of variations in MgO andFe2O3 with respect to clay mineral abundances.

The index Sr* = I03 Sr/CaO is generally related tothe processes of carbonate dissolution (Maillot and Rob-ert, 1980). When the variations related to burial diagen-esis or to the primary composition of the carbonates aremathematically determined, Sr* confirms the processesof carbonate dissolution as inferred by other methods(Maxwell et al., 1970; van Andel et al., 1977; Melguen etal., 1978a; Melguen et al., 1978b). Three types of disso-lution appear:

1) Very high Sr*: This type of dissolution is present insediments rich in organic matter and depends on the in-tensity of the reduced sedimentary environment (Maillotand Robert, 1980).

2) High Sr*: In open marine and oxidizing environ-ments, this type of dissolution appears in the vicinity ofthe carbonate compensation depth (CCD). High Sr* val-ues result from the low solubility of SrCO3 comparedwith CaCO3 (Nekrassov, 1966) or from the partial ad-sorption of Sr by clay minerals (Bausch, 1968; Maillot,in press).

3) Low Sr*: This type of dissolution occurs when thesediment has been deposited above the CCD (Maillotand Robert, 1980).

FALKLAND PLATEAU

Sites 330, 511, and 327

Lithology (Figs. 3, 4, and 5)The base of the sedimentary section in the vicinity of

Sites 330, 511, and 327 consists of Middle Jurassic sand-stones and silty clays overlying a Precambrian gneiss(Barker, Dalziel, et al., 1977). From the Upper Jurassicto the Aptian/Albian boundary, a black shale facies ispresent; it consists of claystones at Site 327, claystonesand limestones at Site 330, mudstones and nannofossilmudstones at Site 511. The Albian to Coniacian intervalis represented by nannofossil claystones at Sites 327 and330 and by alternating variegated claystones, nannofos-sil claystones, and muddy nannofossil chalks at Site511. Then, zeolitic clays and claystones are present fromthe Santonian to the early Maestrichtian. Foraminiferalnannofossil oozes and calcareous oozes occur duringMaestrichtian time and are replaced during Paleoceneand early Eocene time by pelagic clay (Site 511), zeoliticclay, and clayey siliceous ooze (Site 327). Above that, ahiatus occurs at all sites. Early Oligocene sediments (Site511) consist of muddy diatomaceous oozes and muddynannofossil diatomaceous oozes. At the top of the sedi-mentary column, Quaternary sediments consist princi-pally of gravelly sands, alternating with diatomaceousand foraminiferal oozes.

Clay Mineralogy

Several mineralogical units can be distinguished atthese sites (Figs. 3, 4, and 5). In the Middle Jurassic andthe lower section of the Upper Jurassic sediments ofHole 330 (Cores 15 to 11), kaolinite is the major claymineral (50 ± 35%), accompanied by mixed-layer clays(30 ± 20%), illite (25 ± 20%), smectite (30 ± 30%), andchlorite (5 ± 5%); associated quartz occurs in smallamounts. These clay mineral abundances are very simi-lar to those of the mineral associations characteristic ofthe early Aptian of the Cape Basin and of the late Ap-tian-early Albian of the Angola Basin (Robert et al.,1979), where they marked a tectonic instability relatedto the opening of the basins.

In uppermost Jurassic, Neocomian, and Aptian sedi-ments (Hole 330, Cores 10-3; Hole 511, Cores 70-60;Hole 327A, Cores 27-22), smectite increases (80 ±15%) relative to the underlying unit, whereas illite (15± 10%), mixed-layer clays (5 ± 5%), kaolinite (2.5± 2.5%), and chlorite (2.5 ± 2.5%) decrease. Smallamounts of quartz, feldspar, and (locally) clinoptiloliteand cristobalite occur. The transition from a clay min-eral association where illite, mixed-layer clays, and kao-linite dominate toward one with abundant smectite indi-cates a decrease in tectonic activity, resulting in a dimi-nution in erosional intensity and the development of flatcoastal lowlands.

In Albian to Campanian sediments (Hole 330, Cores2-1; Hole 511, Cores 58-25; Hole 327A, Cores 21-13),smectite (97.5 ±2.5%) is the major mineral, accom-panied by mixed-layer clays (2.5 ± 2.5%) and trace

320


amounts of illite and (locally) chlorite. Quartz, feldspar,cristobalite, and clinoptilolite are also present. The greatabundance of smectite suggests tectonic quiescence andgenerally subdued relief on the adjacent continentalland masses. At Site 511, in the Albian-Turonian andthe Coniacian-Santonian, two small increases in illite,mixed-layer clays, and kaolinite occur. These anomaliessuggest increased erosion of altered rocks and of moder-ately and deeply weathered soils, pointing to tne occur-rence of a local, moderate rejuvenation of the topog-raphy by tectonism.

The Maestrichtian-Paleocene sediments (Hole 511,Cores 25-22; Hole 327A, Cores 12-4) are rich in smec-tite (97.5 ± 2.5% of the clay fraction), accompanied bychlorite, illite, mixed-layer clays, kaolinite, and attapul-gite (2.5 ± 2.5%). A short hiatus near the Cretaceous/Tertiary boundary occurs during this interval in Hole511. Sediments corresponding in age to this hiatus wererecovered from Hole 327A, however, and are character-ized by a small increase in primary minerals (2.5 ±2.5%), mixed-layer clays (10 ± 10%), and kaolinite(trace amounts) both above and below the Cretaceous/Tertiary boundary that seems to result from a moderate,marginal tectonic rejuvenation (Chamley and Robert,1979).

In Eocene sediments (Hole 33OA, Core 1; Hole 511,Cores 21-17; Hole 327A, Core 2), smectite prevails inthe clay fraction (90 ± 5%) and is accompanied bymixed-layer clays (5 ± 5%), illite (2.5 ± 2.5%), chlo-rite, kaolinite, and attapulgite (trace amounts). This claymineral association indicates a period of tectonic quies-cence with a globally warm climate and major seasonalchanges in humidity. However, in Hole 33OA, Core 1displays a short period in which illite (7.5 ± 2.5%) andmixed-layer clays (15 ± 5%) increase.

Early Oligocene sediments (Hole 511, Cores 17-1) arecharacterized by a progressive increase in chlorite (7.5± 2.5%), illite (15 ± 10%), and mixed-layer clays (2.5± 2.5%) that is related to progressive climatic cooling(Chamley, 1979). Smectite decreases (65 ± 15%); kao-linite (2.5 ±2.5%) and attapulgite (trace amounts) arealso present.

In Pleistocene sediments (Hole 327A, Core 1), smec-tite remains the major mineral (85 ± 5%) and is accom-panied by chlorite and illite (2.5 ± 2.5%), mixed-layerclays (7.5 ± 2.5%), and trace amounts of kaolinite.

Geochemistry (Figs. 3, 4, and 5; Tables 1, 2, and 3)

In Middle Jurassic sediments (Hole 330, Core 15), D*is very high (0.81), pointing to a strong detrital influenceor to the presence of abundant kaolinite. Mn* has anegative value (-0.58) that indicates a reducing en-vironment. Organic acids completely dissolved the car-bonates: Sr* is very high (31.24) and marks the retentionof the strontium in the sediments at the time of the dis-solution. The low values of Mg* (0.03) and Fe* (0.19)are characteristic of a nearby continental influence andof a detrital clay mineral assemblage.

In Upper Jurassic sediments (Hole 330, Cores 14-8),a high value of D* (0.53-0.67) suggests a high detrital in-put from the continent. The slight decrease in D* in the

early Upper Jurassic (Fig. 4) occurs contemporaneouslywith the development of a shallow marine environmenton the Maurice Ewing Bank. Mn* remains negative, in-dicating that organic components are important andthat the environment is a reducing one. Sr* is related tothe dissolution induced by presence of organic matter.

In Upper Jurassic to Aptian sediments, Mn* is alwaysnegative, indicating a reducing environment. Sr* de-creases (16.56-6.46) and is very close to the values re-corded in marine carbonates. The sediment is character-ized by indications of a continental detrital supply andby marine biogenic deposits.

At the end of the late Aptian (Hole 327) or at thebeginning of the Albian (Holes 330 and 511), an openmarine environment appeared. The values of Sr* (1.20-2.04) become typical of marine carbonates. D* (0.55-0.61) diminishes with decreasing continental influenceand Mn* becomes positive (0.56-1.12) in response to thepresence of an oxidizing environment which persisteduntil Coniacian-Santonian time.

In Coniacian-Santonian sediments, D* is again high(0.66), pointing to an increase in the detrital influence,whereas Mn* fluctuates between positive and negativevalues. This alternation could correspond to the periodduring which the site was near the boundary between anoxidized upper water mass and a reduced deep watermass.

During the Santonian, Campanian, and Maestrich-tian, D* decreases slightly (0.62) and Mn* remains nega-tive (-0.27), indicating the persistent presence of a re-ducing environment that is probably related to the loca-tion of the site in deep (or intermediate) water masses.During the Maestrichtian, Mn* becomes positive (0.61-0.78). The change is possibly related to variations in theintensity of ocean currents or to fluctuations in the sealevel.

In the Paleocene, D* decreases slightly (0.58) whereasMn* becomes negative again (-0.51). A reducing en-vironment was probably induced by the influence ofdeep or intermediate water masses. The same environ-ment persists until the early Oligocene.

Sites 329, 512

Lithology (Figs. 6, 7)

The base of the sedimentary section, from the upperPaleocene to the Oligocene/Miocene boundary (Site329) and during middle Eocene time (Site 512), consistsof nannofossil chalks and oozes. Above that, lower toupper Miocene (Site 329) and middle Miocene (Site 512)sediments consist of siliceous nannofossil oozes and up-per Miocene to Pleistocene sediments of diatomaceousoozes and sand.

Clay Mineralogy (Figs. 6-7)

In upper Paleocene and lower and middle Eocenesediments (Hole 329, Cores 33 and 32; Hole 512, Cores19-6 and Hole 512A, Core 2), smectite represents 95± 5 % of the clay fraction. Chlorite, illite, mixed-layerclays, kaolinite, and attapulgite are also present (2.5 ±2.5%). These clay mineral abundances are very close to

321


Pleistocene

Eocene

Paleocene

Maestrichtian

Campanian—Maestr.

Santonian

Cenomanian

early tomiddleAlbian

earlyAptian

lateAptian

Neocomian

100

200

13

14

15

17

"18

19

20

21

22

23

4 0 0 - 1 24

25

26

27

300

.«:«-. i

Clay Minerals

100%

z — z -

E

J T T I T T \ i i i i i i i i i i i i i

I I [ I 1 I I

l • i i l • l t T T π I I T

AssociatedMinerals

= log

-0.8

Mn

Mnsamplβ/FβsamplejM nshales/F βshalesj

0 0.8 1.6

Figure 3. Hole 327A results.

322


D

AlAl + Fβ + Mn

0.3 0.5 0.7

Si

SiO2

A.2O3

3 5 7 9 11

Mg

MgO

"A I2°30.10.20.3

Fe*

F e2°3

" A l2°30.2 0.6 1.0

Sr

SrCaO

2 3 4 5 678910 20 304050

0.9

1.76

Figure 3. (Continued).

323


Eocene

early tomiddleAlbian

Aptian

Neocomian

O×fordianKimmer-

idgian

LateJurassic

MiddleJurassic

Precambrian

100-

200

300

400

500

10

11

12

13

14

151617

Clay Minerals

100%

UJÅXlλ II

Y//A I I I I

rr77////λ

AssociatedMinerals

=log

Mn*

Mn . /Fesample / sample

Mn Fe

-0.8

shales / ' shales

0 0.8 1.6 2.4

PZ>λ

Figure 4. Holes 330 and 33OA results.

324


D*

AlAl+Fe+Mn

0.2 0.4 0.6 0.8_ i I I I I ' i

Si*

SiO2

3 5 7 9 11

Mg*

MgO= A I 2°3

0.2 0.4J I » '

0.9

D

3

1

Fβ*

F e 2°3A I 2°3

0.2 0.6i i i

-1.72

- 1 0

Sr*

3 SrCaO

2 3 4 567 9 20 3040 60 80J I ' i ' ' "i i i i i i i n

>


325


Figure 5. Hole 511 results.

326


D*

AlAl+Fe+Mn

0.5 0.7 0.9

/

Si*

SiO2

3 5 7 9 1113

/

Mg

MgO= A4°3

0.1 0.3 0.5

Fβ*Fe2°3

~A I2°3

0.4 0.8 1.2

V

Sr*

CaO

2 3 4 56 810 20 30 50


327


Table 1. Hole 511 geochemical results.

Core/Section(level in cm)

1-4, 152-3, 1034,84-2, 985-3,906-2, 509-5, 5011-3, 1012-2, 8015-1, 8016-1, 8017-2, 5018-1, 6020-2, 5821-1,5123-1, 3424-1, 4326.CC27-1, 4228-2, 5029-1, 1330-2, 8031-3, 13032-3, 7833-3, 13034-1, 3635-1, 5037-1, 7838-2, 6039-1, 12540-2, 1441-1, 8042-1, 9243-1, 3044-1, 5245-1, 9446-1, 5047-1,3848-1, 3849-2, 2350-1, 2351-1, 13052-2, 10053-1, 13054-2, 2555-2, 4256-2, 7157-2, 3558-1, 3860-3, 1861-2, 8062-2, 8063-1, 3064-2, 7565-2, 1466-2, 3267-1, 9068-1, 2369-2, 3470-2, 68

Siθ2(%)

58.1560.4055.6057.9562.6556.3549.2556.4557.3557.1065.0561.3059.9057.2056.0532.1021.7558.0559.5060.1554.8555.5553.9556.7553.2555.1553.3052.2052.5053.6057.3551.8047.9553.8556.2551.0055.5057.1054.6557.1044.5540.3037.0036.4542.5545.8047.4567.2563.2035.4054.1558.6054.7554.4052.8052.8051.5553.2057.6054.60

A12O

3

10.758.27

8.7711.1610.109.459.3310.2010.048.907.79

6.355.146.3511.047.44

5.5313.9914.6415.0014.2914.1713.6514.7614.2914.7613.5813.4614.0214.7816.5315.0015.0015.2315.7114.0516.4115.7114.4716.7712.2010.929.939.48

10.7211.9912.0712.0213.936.3110.7213.6213.6314.0514.0513.5813.4613.8215.1218.30

CaO(%)

0.84

1.26

6.054.44

2.593.644.27

3.811.75

4.200.42

5.25

8.15

9.581.05

23.6133.580.450.280.28

4.90

4.164.51

2.20

5.633.88

6.72

6.864.83

4.691.22

6.025.14

3.22

0.286.37

3.113.50

5.49

0.9812.1714.5217.8418.4312.9112.1710.770.52

0.5214.765.600.382.41

2.552.24

3.152.94

3.99

0.310.63

MgO N{%)

2.131.891.75

a2θ

%)

.61

.98

.40

1.96 2.732.02 4.70.77 .67

.38 2.70

.59'.08

2.59

J.39.62 2.93.56.04.10.31

3.221.461.172.51

2.56

2.562.95J.12

..31.82

.352.22

2.24 2.562.462.091.851.951.95.83.92.70.69.631.661.611.491.972.202.161.991.782.101.912.591.741.881.821.602.002.042.032.16

2.22

1.212.02

2.22

.85

.85

..12

.85

.99

.75

.97

.74

.75

.792.17

.982.00.97

2.16.87

2.18.87

.85

.56

.57

.79

.85

.72

.87

2.45 2.010.79.48.65.76.58.67.581.581.621.852.63

.08

.57

.60

.71

.48

.58

.57

.58

.57

.722.44

K2O(%)

2.672.11

2.532.67

2.54

2.47

2.20

2.452.532.11

1.93

1.31

1.081.202.321.841.17

2.792.732.672.17

2.54

2.362.182.02

2.152.02

2.062.41

2.532.82

2.95

2.602.24

2.99

2.452.71

2.602.62

2.623.302.17

1.79

2.11

1.99

2.11

1.511.76

1.261.72

2.39

3.253.46

4.19

3.73

3.46

3.46

3.303.461.78

TiO2

(%)

0.670.54

0.49

0.60

0.550.47

0.55

0.590.630.56

0.480.44

0.330.430.67

0.330.27

0.72

0.720.720.73

0.65

0.630.680.690.72

0.68

0.68

0.690.75

0.730.71

0.57

0.70

0.52

0.630.71

0.630.53

0.700.42

0.38

0.380.37

0.49

0.35

0.530.34

0.68

0.290.37

0.630.52

0.56

0.54

0.48

0.52

0.540.600.44

P2^5(%)

n.d.

n.d.n.d.

n.d.

n.d.n.d.

n.d.n.d.n.d.

n.d.

n.d.

n.d.n.d.

n.d.n.d.

n.d.n.d.n.d.

n.d.n.d.

n.d.

n.d.n.d.

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

Fe2O

3

4.50

3.36

4.86

5.084.72

3.93

4.005.366.29

4.905.042.82

2.252.72

12.154.682.54

7.656.68

5.935.04

6.15

7.366.79

5.68

6.085.83

5.866.83

6.08

6.00

6.33

7.515.40

7.485.11

5.044.04

4.86

4.794.97

3.57

4.11

3.82

5.113.79

4.47

3.15

3.908.94

3.684.86

5.154.90

5.15

4.93

5.614.25

5.75

3.65

Mn(ppm)

279779

268

321289289

289316

279279200

242300

353

300

10891495

579211

295258321

311

263

332

289

995353

326321284

4001884

284

2951042

500347

542

4002521

2.37%348448683447

5021

5190274137

384

226

316242

232

205

205

195221184

179

Zn(ppm)

142

105258

116

111105

105111147121121

105

158116

300168142

163

153168174

158

163163

200

163

211142

163

163211

226174

174

132179

195168

153158

168147

126105

142147

105379

300179

337

205

563

511379426532

253

200

121

Li(ppm)

6749

4643

3534

302941

38

3931

2530

8853

2961

4649

3632

3133

34

3632

34

40

4668

7684

42

45

3869

3949

49

455267

4953

68

2858

4824

4242

44

43

43

4041

41

49

45

Ni(ppm)

3918

23207

8817

312514

3

214

17842

3138

31

322521

28

1821

26

3226

3831

32

3237

39

38

25

295144

4835

7853

43

5347

209272

336331

8637

72

5399

130

12664

47

16

Cr(ppm)

584562

56

4844

48

4977

5163

383337

10140

29

6357

61514956

5552

6154

5358

59

6558

4537

47

42

48

4543

4319

1217

16

192018

35

2329

53152

62

68

63

6861

82

9985

Sr(ppm)

147168

300268

205

253284

242179274

132

295358

3792117111211

179211

216

263274

263221

268

253347

379274

263

189

263211

205

132247

179

226226

189

226342

295289

295

258

263168

184

226168

153

163

153158

168174

174137

247

Co

(ppm)

11

56

21

3

518

5

101

2

26812

1012

911

913

8

8

1215

1317

16

16

1915

20

2217

19

3817

39

283027

24

28

23

4128

22

314

54

67

9

93

34

Cu(ppm)

351928

23

1624

16

202022

25137232721

37

2811

91

303027

31

35

3834

3534

3946

18

513

25

1832

4131

46

2101

31

231321

3386

13

6436

42

29

3236

46

49343111

Pb(ppm)

1761

32

31

473539

404334

3329

3934

6539

48

45595555

50615247

4361

47494134

47

32

38

423535

48415148107

55

47

473732

58

25

3932

2038

38

42

40

47

49

4044

V(ppm)

9577

89

95

7184

74

92116

74

925142

53

103

66

75150116211137119

124

130118

141124

126128137157

104

118

103

108108117

9268137

68

5553

394245

53121

66

329668

208621

592

568

713

553

482

221234

Cd(ppm)

1122132322112222311231122122322211013

11122223211066155464311

Note: n.d. = no data.

those of the mineral assembly represented in Holes327A, 330, and 511.

In Oligocene sediments (Hole 329, Cores 31-28),smectite abundance decreases slightly (85 ± 10%), asobserved at Site 511. Associated minerals include chlo-rite (trace amounts), illite (5 ± 5%), mixed-layer clays(2.5 ± 2.5%), and kaolinite (trace amounts). This evo-lution corresponds to the mineralogic trend observedduring the first stages of Cenozoic glaciation (Chamley,1979).

During the Miocene (Hole 329, Cores 27-1; Hole512, Cores 5-1), smectite abundance decreases again (60± 20%) whereas chlorite (5 ± 5%), illite (15 ± 5%),mixed-layer clays (17.5 ± 7.5%), and (locally) kaolinite(5 ± 5%) increase, following the development of glacia-tion. Fluctuations in the relative abundance of smectiteand illite (S/I) during the Miocene are probably relatedto variations in climate and oceanic circulation.

Plio-Pleistocene sediments (Hole 512, Core 1) arecharacterized by a large increase in the abundance ofsmectite (90%); illite (5%), mixed-layer clays (5%), chlo-rite, and kaolinite (trace amounts) decrease. This strati-graphic interval was poorly represented at Site 512. Ifone considers the general trend of climate during lateCenozoic time, climatic determination of the clay min-eral content is improbable at this site, and the occur-rence of abundant smectite probably reflects detritalsupply by currents.

Geochemistry (Site 329, Fig. 6; Table 4)

In upper Paleocene sediments, D* (0.58) is very closeto the value characteristic of typical shales. Si* is low(4.11). Mn* is always positive (0.40), indicating thepresence of an oxidized environment.

Oligocene and Miocene sediments show a slight in-crease of D* (0.63), whereas Si* (13.95) increases sharp-

328



Core/Section

(level in cm)

1-1,46

1-2, 103

1-3, 148

1-5, 47

1-6,96

2-2, 139

3-2, 86

4-2, 142

5-2, 77

5-3, 117

6-1, 48

6-2, 95

6-*, 52

6-5, 102

6-6, 141

7-2, 51

7-3, 94

7-4, 145

7-6, 46

8-1, 91

8-3, 55

8-4, 104

9-1, 109

9-2, 139

10-1, 103

10-2, 147

11-1, 64

11-2, 96

11-4, 711-5, 47

11-6, 101

12-1, 147

12-3, 52

124, 103

12-6, 54

13-1, 97

13-2, 146

134, 46

14-1, 98

14-2, 133

14-4, 52

15-2, 35

15.CC

16-1, 31

Note: n.d. =

SiO2

(%)

26.90

33.35

30.75

22.80

28.50

43.75

4.45

58.80

53.50

50.50

54.65

53.75

54.25

53.10

52.80

54.75

53.60

52.80

47.80

51.05

55.60

57.05

56.75

57.40

57.35

58.50

53.95

56.65

58.40

59.45

58.00

57.85

58.10

57.55

59.80

59.15

64.90

62.60

61.70

58.35

55.85

75.30

70.95

79.70

no data.

A12O

3

6.22

7.77

6.92

5.35

6.25

10.04

0.83

14.60

14.64

14.05

14.64

14.29

14.50

14.42

14.32

14.81

14.61

14.69

13.11

14.15

14.64

15.08

14.86

15.53

15.48

15.46

16.89

16.73

16.70

15.41

16.64

16.95

15.93

15.46

15.35

14.24

13.18

13.93

13.93

15.69

16.79

8.08

15.21

7.91

CaO(*)

31.47

24.99

26.84

35.17

30.54

15.04

48.45

0.42

1.82

4.79

2.94

3.04

3.64

3.50

3.15

2.03

2.52

2.34

6.96

3.71

1.96

0.35

0.73

0.28

0.35

0.17

1.33

0.35

0.21

0.31

0.31

0.35

0.35

0.31

0.28

0.56

0.49

0.31

0.66

0.63

0.52

0.24

01.01

MgO(%)

1.25

1.55

(

.50

.20

.35

-.47

).75

.91

.79

.62

.81

.76

.78

.81

.80

.83

.72

.79

.61

.76

.90

.91

.91

1.94

2.03

2.01

1.29

1.26

1.33

1.21

.15

.37

.24

.16

.32

.32

0.99

0.99

0.88

0.99

1.00

0.36

0.41

0.61

Na2O

(%)

1.48

1.80

2.29

1.15

1.48

2.63

0.04

0.91

0.91

0.81

0.88

0.91

0.94

0.84

1.11

1.01

1.11

1.11

0.88

1.08

1.08

1.08

1.18

1.01

1.42

1.25

1.04

0.98

1.01

0.94

0.88

1.15

0.91

0.91

0.88

1.01

1.21

0.98

0.88

1.04

0.91

0.74

0.30

0.67

K2O

(%)

1.99

2.20

2.21

1.61

1.91

1.69

0.12

3.70

2.86

3.25

3.61

3.37

2.86

3.40

3.31

3.25

3.37

3.28

3.10

3.25

3.10

3.30

3.19

3.19

2.71

2.89

2.80

2.98

3.13

3.07

2.98

3.13

3.16

2.74

3.04

2.74

2.71

2.92

3.16

3.07

2.92

1.84

2.86

2.20

TiO2

0.22

0.18

0.25

0.15

0.23

0.41

0.03

0.52

0.52

0.48

0.49

0.48

0.48

0.46

0.43

0.54

0.56

0.48

0.48

0.48

0.35

0.50

0.54

0.52

0.52

0.55

0.63

0.69

0.73

0.65

0.74

0.71

0.69

0.65

0.75

0.69

0.71

0.79

0.82

0.85

0.79

0.35

0.36

0.63

P2θ5

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.56

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

Fe2O

3

2.97

3.22

2.64

2.19

2.86

3.50

1.43

5.33

5.65

5.26

5.33

5.43

5.12

5.65

5.86

6.58

5.76

6.43

5.62

5.86

5.26

5.12

5.72

6.19

5.93

6.76

6.22

6.79

5.76

6.22

5.72

5.98

6.33

6.36

5.83

6.22

4.65

6.55

7.83

7.72

8.48

1.93

2.07

3.57

Mn(ppm)

1968

1610

1568

2168

2221

2684

9250

379

247

232

237

221

226

232

232

258

237

221

237

221

226

200

195

232

211

200

284

258

247

221

237

258

237

295

289

374

216

221

626

484

289

74

32

111

Zn(ppm)

174

253

332

263

268

243

491

268

531

394

375

578

660

555

595

572

634

352

442

498

337

263

277

302

287

361

322

323

357

262

376

273267

248

207

447

232167

192

188

209

105

205

191

Li(ppm)

29

36

28

23

26

79

4

56

50

44

5047

47

45

49

53

53

58

44

52

51

48

47

54

46

56

89

96

91

82

94

101

91

9387

80

6354

5271

79

19

52

30

Ni(ppm)

36

35

26

25

29

49

23

23

90

83

80

105

87

83

97

93

91

85

86

80

5119

16

16

100

27

21

18

1611

15

21

11

15

1511

10

5065000

Cr(ppm)

26

26

26

25

26

31

19

97

74

6174

7567

75

79

83

91

79

74

75

113

126

111

102

118

111

104

113

108

105

113

100104

113104

93

111

94

103

113

118

39

55

79

Sr(ppm)

332337

316

300

300

316

437

105

142

158

126

158

179

168

137

132

142126

174

158

158

142

179

153

179

153

179

100

111

105

95

95

105

105

116

126

105

89

95

95

105

121

47

79

Co(ppm)

15

11

9

11

11

21

9

4

2

1

2

5

2

2

4

4

3

5

1

5

2

0

0

2

0

3

8

11

6

5

5

7

11

6

10

6

8

7

9

12

7

0

0

0

Cu(ppm)

16

28

19

13

12

6

15

14

29

26

25

37

22

24

36

27

25

23

28

31

26

26

22

21

31

19

20

22

19

17

20

26

23

24

23

17

7

3

0

5

11

0

9

232

Pb(ppm)

3333

32

28

36

55

42

27

31

38

33

35

54

37

35

34

32

31

35

34

32

33

32

36

35

34

40

33

33

11139

36

3234

33

34

44

31

3232

27

46

41

101

V(ppm)

21

26

21

1821

32

158

132

426374

311

458347

516

500

389

553

442

416

421

421

211

158

158

189

142

168

126

111

89105

89

142126

121

9595

95121

116

121

5

21

42

Cd

(ppm)

0

0

1

00

5

20

154

3

5

6

5

5

36

3

5

4

3

0

0

0

0

2

1

1

0

00

0

00

0

0

0

00

0

0

0

0

0

ly. Mn* values fluctuate above and below zero. The in-crease of Si* may be related to a detrital supply of silicaor to an increase in siliceous planktonic productivity.This increase occurred during the opening of the DrakePassage, which resulted in both increased detrital supplyby Pacific currents and greater siliceous productivity.

GEORGIA BASIN

Site 328

Lithology (Fig. 8)

The base of the sedimentary section, from upper Tu-ronian to Maestrichtian, consists of variegated clay-stones overlain by zeolitic clays and claystones from theMaestrichtian to the Eocene/Oligocene boundary. Olig-ocene and Miocene sediments consist of siliceous oozes,clayey oozes, and zeolitic clays. Pliocene and Pleisto-cene sediments are represented by diatom oozes withice-rafted detritus.

Clay Mineralogy

In upper Turonian to upper Eocene sediments (Hole328B, Cores 8-7; Hole 328, Cores 12-5), smectite rep-resents almost 100% of the clay fraction, accompaniedby trace amounts of illite, mixed-layer clays, and kaolin-ite. The persistently high smectite content through along stratigraphic interval points to the persistence of

both a hot climate with major seasonal changes in hu-midity and a low relief on the adjacent continents.

In upper Eocene, Oligocene, and Miocene sediments(Hole 328B, Cores 6-1), only a minor increase occurs inthe abundances of illite (5 ± 5%), mixed-layer clays(2.5 ± 2.5%), and kaolinite (5 ± 5%); smectite abun-dance diminishes (90 ± 5%). In the Georgia Basin, theCenozoic increase in primary minerals and mixed-layerclays related to the development of glaciation is less sig-nificant than it is on the Falkland Plateau (Sites 329 and512, principally).

In general, the abundance of smectite in the GeorgiaBasin is very significant when compared to the amountsfound on the Falkland Plateau or the Mid-AtlanticRidge, especially in Cenozoic sediments.

MID-ATLANTIC RIDGE

Sites 513, 514

Lithology (Figs. 9-10)

The base of the sedimentary section (Site 513) con-sists of a lower Oligocene chert fragment overlying ba-saltic basement. A nannofossil ooze is present from thelower to the upper Oligocene, and the uppermost Oligo-cene and lower Miocene are represented by interlayereddiatomaceous nannofossil oozes and nannofossil oozes.From the middle Miocene to the Pleistocene, the sedi-

329


Table 3. Hole 327A geochemical results.

Core/SectionGevel in cm)

1-2, 512-1, 882-2, 1482-4,512-5, 975-1,205-2, 985-3, 1485-5, 515-6,976-3, 51

6 A 9 76-5, 1487-2, 518-2, 1488-4,498-5, 979-1, 1489-3, 499-4, 1039-5, 14810-3, 4811-1,5811-2, 12212-2, 4812-3, 9712-4, 11514-1, 11014-2, 14714-4, 48a14-4, 48b14-5, 9614-6, 14815-2, 11316-1, 14316-3, 5216-4, 9416-6, 5118-1, 5118-2, 8818-3, 14718-5, 5018-6, 11019-1, 4919-2, 8820-1, 10220-2, 14421-1, 13921-3, 4721-4, 10022-2, 14422.CC23-1, 8523-2, 14224-1, 5924-2, 10325-1, 11325-2, 14625.CC26-1, 4826-2, 11027-1, 10327-2, 132

SiO2

(%)

72.0551.5551.3052.8052.4045.0545.4547.2049.0546.6053.0554.9550.9057.6053.5053.3559.9038.3550.3561.7553.7026.3027.8065.3027.0029.2016.8043.6055.2556.2555.7534.1515.8035.2035.0033.9031.2534.0526.8031.1539.3531.0527.7536.0039.7539.5539.1525.3033.6037.3569.4058.3567.0062.9560.2062.5526.6012.4023.8047.9058.1023.5058.15

A12O3

6.7310.1610.4511.6311.228.56

10.048.749.33

10.2210.8110.4510.759.09

13.1111.698.448.38

11.248.35

10.684.784.992.726.487.053.59

10.9214.1515.1014.388.033.977.487.016.596.697.295.276.676.255.615.787.038.698.628.485.376.446.739.378.269.698.806.377.803.421.964.14

12.6015.005.31

13.58

CaO(%)

1.412.201.761.481.53

12.008.50

10.008.229.515.004.076.651.280.870.650.760.630.920.480.73

31.3828.9312.2426.5823.8937.5013.68

1.511.261.15

21.7637.7821.4422.7024.5924.4922.7730.0825.6421.8326.5528.9322.0017.7018.5717.9830.7525.0821.79

1.227.243.531.787.985.11

33.2041.8728.89

7.700.70

33.930.77

MgO(%)

1.412.752.743.322.572.122.632.252.072.162.352.172.512.292.352.902.162.793.091.872.791.251.430.632.012.241.232.403.042.862.811.811.241.681.751.621.621.551.182.061.111.231.371.202.081.621.771.381.431.501.511.321.512.181.131.470.720.560.701.291.821.051.93

Na2θ(%)

1.753.342.803.123.253.844.724.084.263.574.113.494.004.843.543.622.883.834.082.293.741.492.021.252.603.881.692.092.872.972.971.631.492.132.091.621.722.021.491.752.461.521.361.692.021.791.751.251.371.721.891.691.351.291.041.090.670.470.741.151.290.551.35

K2O

2.113.012.952.893.181.601.931.691.701.871.901.791.971.852.622.672.212.291.871.512.800.790.760.600.950.760.451.691.931.982.401.720.571.470.910.851.031.150.880.571.101.000.791.361.121.361.270.760.980.991.451.361.721.721.251.280.780.511.082.773.251.203.19

Tiθ2(%)

0.450.330.230.490.210.330.380.310.400.470.330.560.450.530.730.650.500.550.580.430.720.220.230.100.220.350.130.470.600.550.580.320.150.330.520.270.330.270.180.180.200.220.250.300.270.330.350.250.270.270.420.470.430.400.230.320.170.120.220.500.670.250.58

P 2 O 5

n.d.0.600.580.711.09n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.0.340.430.58n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.0.320.550.28n.d.n.d.n.d.n.d.0.46n.d.

Fe2O3

7.2211.4711.0110.8710.124.224.253.863.434.225.335.154.434.655.976.045.61

14.736.584.937.512.623.191.363.863.052.224.558.296.127.584.501.223.152.792.544.073.572.042.191.472.362.142.643.153.723.861.192.542.972.862.933.008.343.363.502.121.763.694.765.902.264.86

Mn(ppm)

568453

143297414415845819515318427422115821113712610013215889

116663574279958721

2010306310471058895

1410863

18322874260520632037236827471842248923632116223721845050490030502900

2844800

6263000

416237321284263258232

1774179

Zn(ppm)

183240446356192114215145126116162217221114171149149189226242374205384216463284247263321358253258253589447247432263132168216237163147221147258132126132489679368379

1000921579147216221279195300

Li(ppm)

1848506150448888

1081159799

10999885845769237622433143984183754625746253433261517164315172120482833162526403844363839157

1240491441

Ni(ppm)

8014214284864

4525373433253828192318794243924337213341358384455842324152433438337230354134734549434432

233324292221

896195

105129189586858

Cr(ppm)

7251494444283738424437424443707168425372

128312822272322262726372316202521222723212523242523222729242735354134

10689582332629544

101

Sr

(ppm)

17922622122624241133236332634726324227915316852615317917495

168916874363847816

1079263174174184189216458432437489505542458526505505453411453368521405342284347137158168168295258274195105337137

Co(ppm)

3230271416

1119

131011997984774

2317148

15151822186

171719253532213119471419211745213220191423433742

52

141923

9191411

Cu(ppm)

2964848275

61316272614252121342813133256992322

2232620294325432415195419162412212315271922173741131495645689

12211569163468392168

Pb(ppm)

177534436513449414653333537373433253229193249403262415355404038555347484240354645

12939434232402646454434414437322955424537294539

V

(ppm)

53100896384165342796363683716746342327953

132132132

013215853

158211263211

79535353795353

1052679

132797926

026

1057979

15818415879

353358421126147111844779

Cd(ppm)

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

000773459300036314425405654336540002

333521

553131



Core/Section(level in cm)

20^t, 14820-6, 4621-3, 5022-3, 5523-2, 10225, CC26-3, 10431.CC33-1, 9733-2, 14833-4, 98

SiO2

(%)

37.4045.8559.6567.4024.1043.4069.9528.95

8.7011.9012.35

A12O3

2.342.274.025.013.083.893.673.192.322.323.59

CaO(%)

26.3421.13

9.764.27

31.8320.57

1.4730.0540.6136.0333.93

MgO(%)

0.600.700.901.051.100.850.900.750.750.751.05

Na2O(%)

2.022.902.942.931.491.923.880.660.650.680.92

K 2O(%)

0.540.480.811.050.630.780.780.730.430.430.41

TiO2

(%)

0.080.100.230.250.150.180.150.130.080.050.17

P2O5

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

Fe2O3

0.971.071.572.071.331.761.621.641.071.121.40

Mn(ppm)

13789

11112613712684

342316374295

Zn(ppm)

253289189195153295147163174211305

Li(ppm)

810182513181916121015

Ni(ppm)

5406

1090

12151214

Cr(ppm)

2121242425261925192223

Sr(ppm)

958811495300

1410947211895910979963

Co

(ppm)

00002002546

Cu

(ppm)

151121231822150937

Pb

(ppm)

1113117

20110

11172213

V

(ppm)

21163239323729188

1145

Cd

(ppm)

10001000222


330


ment consists of muddy diatomaceous oozes and diato-maceous muds.

Clay Mineralogy (Figs. 9-10)In lower Oligocene to upper Miocene sediments (Hole

513A, Cores 33-11), smectite is the major mineral (80± 10%), accompanied by illite (10 ± 5%), mixed-layerclays (7.5 ± 2.5%), attapulgite (2.5 ± 2.5%), chlorite,and kaolinite (trace amounts). Locally, some traceamounts of sepiolite occur. The presence of primaryminerals and mixed-layer clays in appreciable amountsseems to result from a climatic change—the initiation ofCenozoic cooling during late Eocene time (Shackletonand Kennett, 1975; Chamley, 1979).

During the late Miocene and a part of the Pliocene(Hole 513, Core 9, and Hole 513A, Cores 10-1; Hole514, Cores 35-13), smectite remains the major mineral(55 ± 25%). Illite (17.5 ± 7.5%), chlorite (10 ± 10%),mixed-layer clays (10 ± 5%), kaolinite (5 ± 5%), at-tapulgite (5 ± 5%), and sepiolite (trace amounts) arealso present. These data point to a deterioration in theclimatic conditions during and after the late Miocene.

In upper Pliocene to Pleistocene sediments at Site 513(Hole 513, Cores 6-1), the abundance of chlorite (12.5± 7.5%), illite (17.5 ± 7.5%), and mixed-layer clays(12.5 ±2.5%) continues to increase; smectite abun-dance decreases (50 ± 20%). Kaolinite (7.5 ± 7.5%)and attapulgite (trace amounts) are also present. Thistrend of the clay mineralogy follows the development ofCenozoic glaciation. In Hole 514 (Cores 13-1) smectiteabundance increases slightly (55 ±25%). Chlorite (12.5± 7.5%), illite (17.5 ± 7.5%), mixed-layer clays (7.5± 2.5%), kaolinite (5 ± 5%), and attapulgite (2.5 ±2.5%) are also present. The conflicting evolution of theclay mineral associations at Sites 513 and 514 is alsopresent in the S/I. Whereas the climate is dominated bythe development of the late Cenozoic glaciation, themineralogical changes observed certainly indicate a sig-nificant influence of oceanic currents (Robert, 1980).

Geochemistry (Site 513, Fig. 9; Table 5)

During Oligocene time, D* is high (0.60). Si* is veryhigh in the lower Oligocene (16.54) and progressivelydecreases to 6.73 in the upper Oligocene. These valuessuggest a strong detrital input. High positive Mn* values(0.82) in the lower Oligocene decrease slightly towardthe upper Oligocene (0.52); these high values are almostcertainly related to volcanic events along the rift, whichwere very influential during the early Oligocene butwhich later decreased when Site 513 edged away fromthe rift area. Mg*, very high during early Oligocene time(0.48), decreased afterward (0.26); this trend can be ex-plained by the chemical composition of calcite minerals.In the lower Oligocene, Sr* (2.85) is typical of marinecarbonates. Values increase in the upper Oligocene, sug-gesting a slow dissolution of the carbonate fraction.

In Miocene sediments, D* increases slightly (0.62),suggesting that terrigenous components had greater in-fluence. In the lower Miocene, Mn* is very close to itsOligocene values, but a major break occurs betweenCores 14 and 13 (0.12). Simultaneously, Mg* decreases

upward through the Miocene sediments. Sr* is relativelylow in the lowermost Miocene and increases consider-ably in Core 12 and above. It is suggested that Site 513passed through the CCD by seafloor subsidence or byelevation of the CCD itself. These modifications in thechemical characteristics of the sediments occurred at thetime that the Drake Passage opened, inducing an in-creased supply of detrital material from southern re-gions that diluted the volcanic components originatingfrom the rift.

In Plio-Pleistocene sediments, D* increases slightlyagain (0.64). High values are clearly evident at the topof the Pleistocene, in agreement with the data of Lisitzin(1962) and Angino (1964) concerning pelagic circum-Antarctic sediments. Mn* becomes slightly negative( - 0.07). The lower sedimentation rates would normallyfavor an increased contribution from volcanogenic com-ponents, but the increasing distance of Site 513 from therift meant that such material had little apparent in-fluence. The increase in Sr* (10.77-14.36) points to astrong dissolution of carbonates—Site 513 is located be-low the CCD.

PALEOGEOGRAPHIC AND PALEOCEANO-GRAPHIC EVOLUTION OF THE SOUTHERN

OCEAN AND FALKLAND PLATEAUThe variations in clay mineral assemblages and geo-

chemical indices permit us to reconstruct several stagesin the evolution of the South Atlantic.

In the Early Jurassic reconstruction of Gondwana-land, the present-day Falkland Plateau was located alongthe southeastern coast of South Africa, near the Mo-zambique Plateau (Craddock, 1970; Thompson, 1976;De Wit, 1977). The first marine incursion reached theMaurice Ewing Bank during Middle to Late Jurassictime (Barker, Dalziel, et al., 1977).

Sediments of this age are characterized by a prepon-derance of detrital components; the continental envi-ronment exercised major influence. The organic matter,continental in origin (Herbin and Deroo, 1979), favorsthe dissolution of the carbonates, but the strontium re-leased by dissolution is trapped in the sediment.

The clay fraction from Middle to Upper Jurassic sed-iments contains major amounts of chlorite, illite, mixed-layer clays, and kaolinite (Fig. 11). This mineral assem-blage indicates intense erosion of altered rocks and ofmoderately to deeply weathered soils from areas of highrelief. It suggests the presence of a youthful topog-raphy, rejuvenated by tectonism. Similar mineral asso-ciations occur in lower Aptian sediments in the CapeBasin and in upper Aptian-lower Albian sediments ofthe Angola Basin (Robert et al., 1979), where they indi-cate marginal instability and tectonic activity, related tothe initial opening of the oceanic basins. In the FalklandPlateau area, the Middle to Late Jurassic mineral as-semblage marks the first stage of the opening of theSouthern Ocean.

At the end of the Middle Jurassic, a reducing, shal-low, marine environment appears, which partially washesthe strontium released during the dissolution of the car-bonates by organic acids.

331


Pleistocene

lateMiocene

middleMiocene

earlyMiocene

Oligocene

lateEocene

latePaleocene

α

5

1 0 0 -

200

300

4 0 0 -

AssociatedMinerals

ò Cl

log

Mn*

Mn /Fe .samples/ samples

M n s F e sshales / samples

-0.8-0.4 0 0.4 0.81.2 1.6


332


Al

Al + Fβ + Mn

0.40.5 0.6 0.7' • • i

Si

S i O 2AI2O3

9 11 13 15 17 19

Mg

MgQ

~ AI2O3

0.2 0.3 0.4

Fβ

F a 2°3

0.4 0.6 0.81.0' ' ' I

Sr*

1 0 'CaO

2 3 4 5 678910' i • •

20 30 4050i • '


333


Figure 7. Holes 512 and 512A results.

334

lateMiocene

m. Miocene

I. Oligocene

e. Oligocene

lateEocene

middleEocene

earlyEocene

latePaleocene

earlyCampanian—Maestrichtian

lateTuronian—Senonian


αSJ

Q

B1

~B2

B3

~B4

B5

~B6

100

200

300

400

10

11

12

B7

:B8

C l a y M i n e r a l s

1 0 0 %

! I ! I I I ' I I : I I I T T F

'J I j I I I I I I I I I I I I I I I I I I-

••α i i i i i i i i T

••» I I I I I I I I I I I I I I -

i i i r π i i i i i i

AssociatedMinerals

Figure 8. Holes 328 and 328B results. Cores B1-B8 are from Hole 328B.

335


Relative Abundance1 2 3 4 5 6 7 8

Figure 9. Holes 513 and 513A results. Cores A1-A33 are from Hole 513A.

At the end of the Jurassic, smectite abundance in-creased markedly. This mineral is principally derivedfrom deep soils in downstream, flat, continental low-lands (Paquet, 1969). Its increase suggests the end or animportant diminution of the earlier tectonic activity,with peneplanation of continental areas and develop-

ment of smectite in the newly formed coastal plains. Asin the South Atlantic basins, the greater importance ofsmectite corresponds to a period of stabilization of thecontinental margins, following the first stages of oceanbasin opening. The influence of the continental environ-ment decreases and that of shallow-water environments

336


logMn

Mn

sampleMπ

shales

F βsample

Feshales

-0.4 0 0.4 0.8 1.2

DAl

Al + Fe + Mn

0.5 0.6 0.7 0.8

Sisio2

5 7 9 11 13

Mg

MgO= A I 2°3

0.2 0.3 0.4 0.5 0.6

FeF a2°3A I 2°3

0.4 0.6 0.8

Sr

3 Sr= 1 0 Cao

2 4 6 8 10 12 14 16


increases; reducing conditions slightly decrease. Theoverall importance of the detrital components remainsconstant. By this epoch, the separation of Africa andAntarctica was probably complete, or nearly so. Thus,tectonic quiescence appears to be characteristic of the

Falkland Plateau and Georgia Basin areas during theLate Jurassic.

From the end of the Jurassic to the Aptian/Albianboundary, chlorite, illite, and mixed-layer clays remainrelatively abundant, perhaps in response to local in-

337


Φ

<

Pleistocene

latePliocene

earlyPliocene

(UJ

*•>αΦQ

10-

20-

3 0 -

4 0 -

5 0 -

6 0 -

7 0 -

8 0 -

9 0 -

100-

110-

120-

130-

140-

2o

U

J2

3

4

5

6

8

9

10

"11

12

-

13

14

15

16

17

18

19

20

21-22

23

24

25•

26

27

28

29

30

31

32

33

34

35

>Q

jsoXJ

5 u

"I"

n

ü"z

is is

Θ ° Θ

s 5 s

G 5 5

III]

13 I0

3 13

IID

IS

5_S.

G "

ü_GH

"-°-_-]

>HGÖG ]

Clay Mine

0

K : : : : |

Meimß: |B ••• • • • 1 ^

IMrM•^'•:•'w

I 1• M

mm1 i

wßM

• w• wmm

m•fm

IE,

m''•:'•yú.

Mmm>•

IMWe1:1

IM

AssociatedMinerals

rals

N

CB

100% ^

IllllllllTTTTTTT

fe

I

n1 .Ij .I .I '1 <iI '= *

' 1j ;I1 *j

jj ;^ *I 'J ;i .ajJsjj

j *E

ΠΠ

TΠTTII

I

J "J *

(β

αWITJΦ

u.•

•

:

.

•

•

•

•

•

*

•

#

•

•

•••

•

•

•

•

•

*

αoc

Cli

•

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{1 /10 °fl \\ 1 1 \J (7/

2 3 4 5 6• i i i i

\

I

I/

1

)

<

Smβctite/lllite

Relative Abundance

1 2 3 4 5 6 7 8 9

\

\

)

jL7

r\?I\

J/

>\)>(y

iJ>\\></L>)(>\\


338


Table 5. Site 513 geochemical results.

Sample(level in cm)

513-1-4, 1103-3, 804-1, 1254-5, 1255-3, 146-2, 146-6, 149-2, 110

513A-1-1.13O2-3, 343-1, 744-1, 244-5, 245-3, 256-1, 246-5, 247-6, 248-3,9010-2, 2211-2,2012-2, 413-1, 814-1, 5015-2, 2016-1, 2116-3, 2116-5, 2116-7, 2617-1, 4717-5, 4718-3, 1920-1,2321-1, 14321-5, 14324-2, 8028-1,4029-2, 9030-4, 2031-3, 1531-7, 1533-1, 533-5, 533-7, 5

Note: n.d. = no

SiO2

66.2063.4562.5566.3066.1063.6564.9560.1060.6063.2059.9560.0560.3060.9058.8058.5559.9562.8566.0563.5554.6545.4029.6030.8048.5561.0063.4058.3535.1026.7516.9516.6036.7511.708.85

26.2023.2041.8031.9518.2521.1029.8530.25

data.

A12O3

10.088.80

10.987.797.26

10.6310.2112.1613.4610.6312.4011.9310.2711.4513.3412.0412.0410.478.50

10.0413.116.934.274.855.797.147.977.727.862.952.953.146.492.691.903.452.723.371.422.021.561.541.35

CaO

0.940.911.080.870.770.981.121.081.150.981.611.571.221.471.851.751.611.331.151.121.85

13.7428.6826.7615.677.632.556.54

10.2831.0338.1337.9520.9241.6345.1231.8036.7321.6931.1339.5336.3832.0131.83

MgO Nc

.54 3.75

.68 5.09

.67 4.45

.52 4.85

.49 4.871.63 3.711.63 4.041.91 4.451.88 3.981.58 4.721.98 4.212.00 4.311.78 : .081.93 3.712.11 3.941.92 4.482.02 3.811.82 3.711.51 4.081.87 2.972.61 4.152.710.981.04 ;1.261.401.711.701.74

.10-.53..78.40

>.19.88

1.27.44

0.80 2.090.690.851.660.680.600.950.871.180.600.750.780.900.86

.55

.60..29.11.25.58.28

>.48.60.06.52.21.45

K2O

1.962.022.321.781.782.172.122.442.592.242.412.412.262.422.562.422.432.292.202.292.62

.54

.04

.20

.48

.702.001.902.020.780.690.781.580.640.510.900.720.920.400.550.450.450.42

TiO2

0.440.460.610.410.400.570.550.630.700.480.600.580.540.580.680.630.580.510.430.500.600.320.200.230.250.310.400.350.400.140.120.150.280.110.100.110.130.110.070.100.080.080.07

P2O5

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

Fe2O3

3.683.544.043.863.325.294.684.905.004.544.724.687.155.004.755.005.184.653.865.006.086.431.972.182.573.293.683.723.571.251.321.543.321.110.821.541.251.570.500.890.750.640.64

Mn(ppm)

347326384316268432347353437289342400358637432384732358379332784647

10891116600458400526568868874

1100653774563453542347547816658484500

Zn(ppm)

105116121899595

10511113212113289

12112417512613615082

10313279548283

12111314192797163

11347547059813943

1065842

Li(ppm)

34333427263330374031373834383936353624388034192023323534361412163512102020392022294838

Ni(ppm)

912411115

24100

460009007

2819149

17121215256

101631118

186

2058

34172

Cr(ppm)

29293323192826323128353531363637343425333314101114202519219

111116119

111296977

12

Sr(ppm)

137126163132132147158174195174179179147158179174179147126142184611984

1011632358200368484

10001142m i637

10001326916989695784

103210371000916

Co(ppm)

32200436

18357

20106

13840393557546847989884425

1072

Cu(ppm)

495341402743293678

10738515748576724531582

120132301311141414242143

150

2724

3890

2530

Pb(ppm)

2519262622332633573320291414141514131215319

1517131312161121222217232621231918131497

V(Ppm)

951039784688784

11313492

11110010392

113105798958799245212466688458741624295047581818392429346350

Cd(ppm)

1121111211111111110110111311022202322012211

stability caused by the progressive sliding of the Falk-land Plateau along the South African margin and by thedistant tectonic activity associated with the opening ofthe Cape Basin during the Early Cretaceous (Robert etal., 1979). This abundance ended near the Aptian/Al-bian boundary, by which time the South Atlantic basinshad opened and their margins had stabilized. A diminu-tion in the importance of the continental environmentoccurred at the same time, and open marine conditionsbegan to influence the chemical composition of the sedi-ment. Slow subsidence of the ocean basin affected thedetrital index (D*): from the Coniacian to the Campan-ian, the importance of deeper water masses becamegreater, and strong, oxygen-rich currents were presentby early Maestrichtian time.

During the Late Cretaceous, two small increases inchlorite, illite, mixed-layer clays, and kaolinite occurred.The first was from Albian to Turonian time, the secondduring Coniacian-Santonian times. These mineralogicalevents suggest the occurrence of minor tectonic activitynearby or of major, distant, tectonic events. Uplift oremergence of the Maurice Ewing Bank are not evidentfrom clay mineralogical and geochemical data, despitethe presence of Inoceramus fragments, indicators ofshallow-water environment, at Site 511. It appears thatthe mineralogical increases are contemporaneous with

the first Andean deformational phases—a collision eventbetween southernmost South America and the AntarcticPeninsula—and migration of a Pacific island arc towardSouth America (Dalziel et al., 1974; De Wit, 1977).

In the early Paleocene, a minor increase in chlorite,illite, and mixed-layer clays occurred at Site 327, sug-gesting a minor rejuvenation of continental relief. Thisevent is missing at Site 511, where a hiatus occurred,and in the Georgia Basin, where only smectite is present.The mineralogical change that occurred at the Creta-ceous/Tertiary boundary results mainly from a margin-al tectonic rejuvenation and secondarily from increasedoceanic circulation; both probably originated from amajor phase of ocean basin opening (Chamley andRobert, 1979). These mineralogical changes were locallyimportant—as they were, for instance, on the RioGrande Rise (Robert, 1981)—but were of very minorimportance in the Argentine Basin and were practicallyabsent in the Cape Basin and on the Falkland Plateau.An orogenic episode located in the Antarctic Peninsulaand Southern Andes (Termier and Termier, 1957) couldexplain the minor mineralogical changes at Site 327.

Paleocene and Eocene sediments are characterized bysmectite, which indicates a period of tectonic quiescenceon the continental land masses adjacent to the FalklandPlateau region.

339


Stratigraphym.y. Age Clay Minerals

100%Main Events

60

70

80

90

100

110

120

130

140

150

160

170 L

Paleocene

Maestrichtian

Campanian

Santonian

Coniacian

Turonian

Cenomanian

Albian

Aptian

Barremian

Neocomian

LateJurassic

MiddleJurassic

Restricted Cretaceous/Tertiary boundary event

Andean deformation

Andean deformation;Magallanes Basinformation

South Atlanticinitiation andFalkland Plateausliding

Stabilization ofcontinental margins

Marginal instabilitySouthern Oceaninitiation

Legend Chlorite, I MiteMixed-layer clays

| Kaolinite \\\\\ Smectite

Figure 11. Evolution and significance of Mesozoic clay sediments onthe Maurice Ewing Bank, Holes 327A, 330, and 511.

MARINE PALEOENVIRONMENTAL ANDCLIMATIC EVOLUTION DURING

CENOZOIC TIME

Influence of Antarctic Bottom Water(AABW) on the Clay Sedimentation

In the South Atlantic realm from the late Eocenecooling onward, the influences of climate and currentson the clay mineral distribution became very important(Robert, 1980) and progressively masked the tectonicand morphologic influences. Continuously less intensecontinental weathering resulted in a minor developmentof soils and in an increase of chlorite, illite, and mixed-layer clays in detrital sediments (Chamley, 1979). Thecomposition of the clay fraction became increasinglysimilar to that of present-day sediments.

Generally, the cooling expressed by the clay particlesappears more marked toward high latitudes. In the sedi-ments recovered during Legs 36 and 71, smectite re-mains abundant compared to the Cape Basin, WalvisRidge, or Rio Grande Rise (Robert, 1980). Moreover,chlorite, illite, and mixed-layer clays are more abundanton the Maurice Ewing Bank, an elevated feature, thanin the Georgia Basin (Site 328) or on the Mid-AtlanticRidge (Sites 513, 514), where deeper sites were drilled.These sites are located in oceanic regions subjected toAABW currents (Le Pichon et al., 1971). In the VemaChannel, the abundance of smectite in the sedimentsunderlying AABW has been demonstrated (Chamley,1975; Melguen et al., 1978a).

The mineralogical studies carried out on marine sedi-ments in deglaciated Antarctic coastal areas show prin-cipally the presence of chlorite and illite (Kagami, 1964;Michel, 1964; Chamley, 1965); at present, weatheringdoes not permit the formation of smectite, althoughsmall amounts of the mineral were found in some sedi-ments studied by Kagami (1964). Smectite has beenfound, however, in the sediments recovered duringDSDP Legs 28 (Ross Sea) and 35 (Bellingshausen Sea),where it sometimes represents more than 50% of theclay fraction (Cook et al., 1975; Zemmels and Cook,1976). The Ross Sea is an important source of theAABW which flows through the Bellingshausen Sea.Thus, it seems that the Antarctic continent is directly in-volved in the detrital supply of smectite to regions sub-jected to AABW.

Smectite is not derived from weathering on the glaci-ated Antarctic land mass; it may result from erosion ofMesozoic or early Cenozoic sediments outcropping onor around Antarctica. Such a mechanism has been pro-posed by Kemp (1972) to explain the occurrence of Mes-ozoic and early Cenozoic palynomorphs in bottom sedi-ments from the continental shelf around Antarctica.Unfortunately, mineralogical studies of the Mesozoicand early Cenozoic sediments, as well as of the volcanic-lastics which have been described by Cailleux (1963) andWarren (1965), have not yet been carried out. In theNorthern Hemisphere, clay mineralogical studies of bot-tom sediments from the Arctic Ocean indicated the pres-ence of kaolinite derived from ancient shales of NorthernAlaska and Canada (Naidu et al., 1971; Darby, 1975).

In the region studied on Legs 36 and 71, smectite isprobably derived mainly from the Weddell Sea area, amajor source of AABW for the Atlantic Ocean (Gor-don, 1971; Ledbetter and Ciesielski, 1982). AABW isproduced beneath an extensive ice-shelf formed by thecoalescence of glaciers that drain and erode a large areaof Antarctica. During Plio-Pleistocene time, clay par-ticles may also have originated in the Weddell Basin,where erosion by AABW has removed all sedimentyounger than early Pliocene, in many areas exposingolder sediments (Ledbetter and Ciesielski, 1982).

Another possibility for the origin of detrital smectiteis the Falkland Plateau itself, where Mesozoic andCenozoic sediments crop out. Intense erosion startedduring the Miocene with the initiation of the Antarctic

340


Circumpolar Current (ACC). Differences in the smec-tite abundance between elevated and deeper sites be-came significant near the Eocene/Oligocene boundary,when a pre-AABW probably developed (Ciesielski etal., 1982). On the Maurice Ewing Bank, Plio-Pleisto-cene sediment shows an increase in the abundance ofsmectite at a time when the ACC, which had previouslytruncated the sedimentary sequence on the Bank, dim-inished in intensity, permitting another brief period ofsedimentary deposition (Ciesielski and Wise, 1977). Thus,beginning with the late Miocene erosion, the FalklandPlateau may have supplied sediments rich in smectitefor distant redeposition, as shown by Ciesielski et al.(1982.)

Volcanic Influence on the Rift, Carbonate Dissolution,and the Opening of the Drake Passage

In the Oligocene sediments at Site 513, above the ba-saltic basement, a strong increase in the concentrationof transitional elements occurs (Table 5). These elementsremain abundant until the early Miocene, when theiramounts decrease. By the same time, however, a strongincrease in the sedimentation rate masks the diminutionin the real abundance of the transitional elements thatoccurred when the site edged away from the rift area. Asimilar effect has been shown at Site 19 on the Mid-Atlantic Ridge (Maillot and Robert, 1980).

From lower Oligocene to lower upper Oligocene sedi-ments a low Sr*, derived from marine carbonates is typi-cal; it suggests that Site 513 was located above the paleo-lysocline. At the top of the upper Oligocene and in thelower Miocene, a small increase in Sr* is probably re-lated to a slight dissolution of carbonates; by this time,the site was probably situated below the lysocline andabove the CCD. From the top of the lower Miocene up-ward, Sr* strongly increases, in response to the passageof Site 513 through the CCD.

During early Miocene time, a break occurs in the geo-chemical indexes (Fig. 9). Below this break, as duringOligocene time, the sediments are characterized by bothdetrital and volcanogenic influences. Above the break,several changes occur in the sedimentation. In additionto the increase in Sr* related to the deepening of Site513, a strong decrease of Mn* and a slight increase of D*correspond to a dilution of the volcanic influences ofthe rift by terrigenous elements. These geochemicalevents occur contemporaneously with the opening of theDrake Passage (Barker and Burrell, 1977), which is notclearly recognizable in the clay mineralogy. However,fluctuations appear in the S/I of Site 513 at this time,and persist until the beginning of the late Miocene.These variations could result from changes in currentvelocity, since the opening of the Drake Passage wasmarked by intense scouring and removal of sediments,as evidenced on the Maurice Ewing Bank (Ciesielski andWise, 1977). In Core 11 of Hole 513A, the reworking ofEocene sediments revealed by a study of silicoflagellatesis also recognizable by a sharp increase of the S/I index.At Site 329, a large decrease in the S/I index occurredfrom the Oligocene to the early middle Miocene, con-temporaneously with the opening of Drake Passage

(Fig. 6); the significance of this break is not evident atthe present time.

Influence of Climate on the Clay SedimentationThe first influence of Cenozoic climatic cooling on

the clay minerals was recorded during the late Eocene atSite 511, consistent with the data of Kennett (1977) andMercer (1978). The general evolution of the Cenozoicglaciation is well expressed by the S/I index, shown inFigures 6, 9, and 10 for Sites 329, 513, and 514 only.

In the lower Oligocene, there is a progressive decreasein the S/I index upwards from the base of the sedimen-tary section. By the early Oligocene, the deterioration ofclimate had resulted in the development of moderndeep-water masses and a more intense oceanic circula-tion (van Andel et al., 1977; Moore et al., 1978).

During the late Miocene, a decrease occurred in theS/I index, corresponding to the time during which theAntarctic ice-sheet developed (Shackleton and Kennett,1975). This decrease is present at both Sites 329 and 513.Another S/I ratio change occurred during the Pliocene,at the beginning of the Gauss Magnetic Epoch, at a timewhen a second strong cooling induced an increase in theAntarctic ice-sheet (Watkins and Kennett, 1971; Mercer,1978) and initiated extensive glaciation over Patagonia(Mercer, 1976).

Near the Pliocene/Pleistocene boundary, a major de-crease in the S/I occurred at Site 513 and a slight in-crease at Site 514. During Pliocene times, the abundanceof smectite in the clay fraction demonstrates that bothsites were under the influence of AABW or circumpolardeep water; this influence was probably felt more strong-ly at Site 513. At the end of the Pliocene, smectite abun-dance decreased at Site 513, and the mineralogical com-position of the clay fraction became more similar to thatof Recent, high-latitude, detrital sediments (Biscaye,1965; Griffin et al., 1968; Froget, in press). At Site 514,the increase in the abundance of smectite probably ex-presses an increased AABW flow favoring the transportand redeposition of smectite particles. These data sug-gest a change in AABW circulation at a time when a de-crease in AABW production has been noted by Ledbet-ter and Ciesielski (1982).

Some paleoclimatic features, such as the early-mid-dle Miocene warming reported by Margolis et al. (1977),are not recorded by the clay minerals, probably becausecurrents exercised an important influence in this region.

CONCLUSIONSIn the South Atlantic, clay mineralogical data are

chiefly influenced by climate. During Mesozoic times,the relative persistence of a globally warm climate, withcontrasts in humidity, permits an interpretation of themorphologic and tectonic evolution of the continentalland masses: the initiation of the ocean basins, stabiliza-tion of the continental margins, major phases of oceanwidening, and inferred tectonic events. During Ceno-zoic time, the importance of these influences decreasedas the climate began to cool. By this time, clay mineral-ogical data express different phases in the degradationof climate; from them we can also infer variations in

341


currents the initiation of the bottom-water masses andchanges in their circulation, and the development ofAntarctic and Patagonian glaciations. Geochemical dataexpress principally the modification of the oceanic envi-ronment: the evolution from a continental to a reducedmarine and then to an open marine environment, the in-fluences of marine currents, volcanogenic influences ofthe rift, carbonate dissolution, and the deepening of theocean basins. Correlated mineralogical and geochemicalstudies can thus help to reconstruct paleoenvironments.

ACKNOWLEDGMENTS

Thanks are due to the U.S. National Science Foundation forsamples from Legs 36 and 71. Financial support was provided byCNEXO (France) for geochemical studies. Technical assistance wasprovided by M. Acquaviva and C H . Froget for clay mineral analysisand illustrations, C. Maillot for geochemical analysis; F. Dujardintyped the manuscript. B. Bornhold, H. Chamley, P. Debrabant, G.Jones, and S. Wise reviewed and improved the manuscript.

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