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
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
C. ROBERT, H. MAILLOT
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
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
C. ROBERT, H. MAILLOT
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
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
>
Figure 4. (Continued).
325
C. ROBERT, H. MAILLOT
Figure 5. Hole 511 results.
326
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
Figure 5. (Continued).
327
C. ROBERT, H. MAILLOT
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
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
Table 2. Hole 330 geochemical results.
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
C. ROBERT, H. MAILLOT
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
Note: n.d. = no data.
Table 4. Hole 329 geochemical results.
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
Note: n.d. = no data.
330
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
C. ROBERT, H. MAILLOT
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
Figure 6. Hole 329 results.
332
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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 • '
Figure 6. (Continued).
333
C. ROBERT, H. MAILLOT
Figure 7. Holes 512 and 512A results.
334
lateMiocene
m. Miocene
I. Oligocene
e. Oligocene
lateEocene
middleEocene
earlyEocene
latePaleocene
earlyCampanian—Maestrichtian
lateTuronian—Senonian
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
C. ROBERT, H. MAILLOT
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
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
Figure 9. (Continued).
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
C. ROBERT, H. MAILLOT
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
•
•
IlliteCristal unity
{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>)(>\\
Figure 10. Hole 514 results.
338
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
C. ROBERT, H. MAILLOT
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
PALEOENVIRONMENTAL SIGNIFICANCE OF CLAY MINERAL DATA
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
C. ROBERT, H. MAILLOT
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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