22. MESOZOIC SEDIMENTATION ON THE EASTERN FALKLAND PLATEAU

Robert W. Thompson, School of Natural Resources—Oceanography,Humboldt State University, Arcata, California

INTRODUCTION

Mesozoic sediments were encountered in two boringson the Falkland Plateau, Hole 327A and Site 330, bothof which are situated in water depths on the order of2500 meters on the southern flank of the slightlyelevated eastern end of the plateau (Figure 1). At Hole327A, calcareous ooze of Late Cretaceous(Maestrichtian) age was first encountered at a subbot-tom depth of 90 meters and the hole was terminated incarbonaceous claystones of Early Cretaceous(Neocomian-Aptian) age at a subbottom depth of 470meters. At Site 330, calcareous (nanno) clay of Albianage was first cored at a subbottom depth of 129 metersand thereafter Mesozoic sediments ranging back to atleast Upper Jurassic (Oxfordian) were drilled to a totaldepth of 550 meters. Below these a probable old soilprofile and underlying basement rocks of granuliticgneiss and granite pegmatite were encountered. TheMesozoic sequences penetrated at these two sitessuggest a depositional history as follows:

1) subaerial (paralic ?) sedimentation resulting in in-fill and aggradation of local bedrock basins;

2) Middle (?) to Upper Jurassic marine transgressionfollowed by accumulation of predominantlyterrigenous silts and clays in an open-shelf environ-ment;

3) euxinic conditions and deposition of dark car-bonaceous claystones which began in Upper Jurassicand persisted until near the end of the LowerCretaceous (late Aptian);

4) open marine deposition of pelagic carbonateoozes and zeolitic clays through the remainder of theMesozoic and most of the Tertiary.The following discussion summarizes thecharacteristics of the Jurassic and Lower Cretaceoussediments encountered at these two sites and their inter-pretation in terms of depositional environments andgeologic events. Detailed lithologic descriptions of thesedimentary column at these sites are presented in otherchapters, this volume.

MIDDLE (?) TO UPPER JURASSICSUBAERIAL SEDIMENTATION

The oldest sediments penetrated on the FalklandPlateau comprise a 2.7-meter section of silty sandstoneand sandy siltstone first encountered at a subbottomdepth of 547 meters at Site 330. Interbedded in this sec-tion are several thin (

R. W. THOMPSON

20° 40°

Figure 1. Location map showing Leg 36 drilling sites on the Falkland Plateau and sites around southern Africa whereLower Cretaceous carbonaceous sediments were encountered on other legs. Bathymetry from Chase, 1975. Contours inkm.

Cycle 3

Cyc

Cycle 1

•i . >:-'.

vV>

> .*,'.•

\\W\\\V

silty sand

sandy silt η c o r e

photo

silty sand

claystone (ash?)

I] coresilt photo A

sand-silt-clay

I M • lignite bed

^fc lignite clast

C> angular granule

^

MESOZOIC SEDIMENTATION, EASTERN FALKLAND PLATEAU

LIGNITE

Figure 4. Representative cores from basal subaerial sediments, lower Site 330. (A) Sample 330-15-2, 60-64 cm, mottledsilstone and lignite from upper part of "cycle"; (B) Sample 330-15-1, 64-87 cm, sandstone with angular quartz andfeldspar granules and lignite clasts from middle part of cycle. See Figure 2 for location in the section. Scale in cm.

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R. W. THOMPSON

QuartzQuartz Arenite

+ basal subaeri

deposits

• transgressi

marginal

deposits

Feldspar 3/1 bθ 3/1 Chert

and

Rock Fragments

Figure 5. Light mineral composition of sand fraction (0.062-2 mm) from Middle (?) -Upper Jurassic sediments, Site330. Classification after Folk, 1974.

and very poorly sorted; the nonopaque heavy mineralfraction is composed solely of angular grains ofyellowish-brown tourmaline (Table 1). Kaolinite makesup greater than 80% of the clay fraction (

MESOZOIC SEDIMENTATION, EASTERN FALKLAND PLATEAU

TABLE 1Petrographic Analyses of Sand Fraction from Middle(?)-Upper Jurassic Terrigenous Sediments, Site 330

Sample(Interval in cm)

13-4,73134, 8015-1, 1015-1,3015-2, 2615, CC

Mean (X)

Percent (by number) of NonopaqueHeavy Mineral Fraction S.G. > 2.9

Gar

net

4762798160

66

Tou

rmal

ine

1917169

31100

18Z

irco

n

76395

6

Ap

atit

e

72

2

Pyr

oxe

ne

31

1

1

OSIH

α>

O125

11

3

Alt

ered

572

2

4

Opa

que*

3

756441

84713

47

Lithology and Lithofacies

Sand-silt-clay; marginal depositsSandstone; marginal depositsSandstone; transgressive beachSilty sandstone; subaerial depositsSilty sandstone; subaerial depositsClayey sand; soil (?)

Excludes Sample 15, CC

Sample(Interval in cm)

12-3, 89134,73134,8015-1, 1015-1,3015-2, 2615, CC

Mean (X)

Percent (by number) of

a72.5758087868268

78

Light Fraction

ù1816159

116

30

15

α>

oO

If

85313

120.5

5

cα>

1

Q

OH

1423_—1

2

α>

δ0.5-—___

0.5

tr

QuartzFeldspar

2.83.64.48.26.14.52.2

4.5

Lithology and Lithofacies

Sand-silt-clay; marginal deposits

Seeabove

Note: Heavy minerals (S.G. > 2.9) in very fine-fine sand fraction (0.062-0.25 mm). Light minerals intotal sand fraction (0.062-2 mm). Determinations on grain mounts etched with HFI and stained withsodium cobalt-nitrate. Mica not counted.

Principally epidote, rutile, cassiterite.

"Core 13 - mainly pyrite; Core 15 - mainly magnetite, ilmenite (?), leucoxene.

lower section to a mixed organic fraction containingboth land-derived and marine (sapropelic) material inthe upper section; (4) an upward increase in the fre-quency of limestone interbeds in the section.

Little definitive evidence upon which to base an in-terpretation of depositional environment exists in thissequence. The subtle alternation of textural types intothin beds indicates an environment of moderate butfluctuating currents and terrigenous sediment supply.The occurrence of occasional sandstones, commonevidence of extensive bioturbation, and presence of thinlayers of fragmented pelecypod shells (assuming theywere benthonic types) imply periods of reworking andthe probability of reasonably well-aerated bottom con-ditions. The preponderance of silt and clay, generallylaminated and containing an abundance of land-derived detritus, imply that sediment supply, thoughapparently quite variable, was substantial and may in-dicate the presence of a river mouth situated in thegeneral vicinity. Finally, interpretation of the seismicreflection record suggests that the Upper Jurassicsediments at this site were deposited on a low gradient

surface in close proximity to a much steeper basin slopeto the southwest (Barker, this volume). The geometry issuggestive of a continental margin though probably theadjacent basin floor was of less than oceanic depth. Alllines of evidence seem consistent with the interpretationof a continental shelf as the depositional environmentfor this sequence.

The origin of the thin, olive-gray limestones inter-calated in this section remain an enigma which is dis-cussed in more detail below. In this part of the column,most of the limestones are partially to completelyrecrystallized into sparry or microsparry calcite, oftencontaining a significant admixture of terrigenous siltand sand. Several samples examined in thin sectionfrom the middle part of the section (Core 13, Site 330),however, contain recognizable allochems of gastro-pods, pelecypods, echinoid spines, and possible coralalong with about 10%-15% of angular, medium tocoarse quartz and feldspar. Assuming these sedimentshave not been redeposited, their presence implies thepersistence of local (?) bedrock shoals on which reefswere developed. Notably, seismic reflection records in-

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R. W. THOMPSON

Figure 6. Representative cores from Upper Jurassic marginal deposits, Site 330. (A) Sample 14-2, 50-70 cm, bioturbatedclayey silt; (B) Sample 12-5, 29-49 cm, laminated to bedded silty clay. Scale in cm.

dicate that Site 330 is situated over a protuberance inthe underlying bedrock. Whereas this basement hillappears to have been buried at this locality by LateJurassic time, similar features to either side of the line

of section may not have been. Thus, the possibility ofshelf-edge reef development is raised, and this, in turn,raises the possibility of conditions conducive to the for-mation of lime mud. Hence, limestones in the section

882

MESOZOIC SEDIMENTATION, EASTERN FALKLAND PLATEAU

Figure 7. Representative cores from Lower Cretaceous carbonaceous claystones, Hole 321 A. (A) Sample 26-1,130-144 cm, laminated sapropelic claystone; (B) Sample 26-2, 0-7 cm, laminated micritic limestone; (C)Sample 22-2, 60-70 cm, bioturbated carbonaceous claystone; (D) Sample 22-2, 23-32 cm, bioturbated micriticlimestone. Scale in cm.

883

R. W. THOMPSON

which lack evidence of coarser debris and generation byreworking may represent either recrystallized Coccolithooze as suggested for the overlying section (see below),or recrystallized lime mud of some other origin.

UPPER JURASSIC-LOWER CRETACEOUSEUXINIC CONDITIONS

Beginning in Upper Jurassic (Oxfordian) time andlasting into the Lower Cretaceous (Aptian), euxinicconditions prevailed on the eastern Falkland Plateau asevidenced by the accumulation of olive-black car-bonaceous claystones. These rocks are typically welllaminated, contain an average of 3%-4% organic car-bon, and are characterized by a very restrictedbenthonic fauna composed mainly of thin-shelledpelecypods and a few agglutinated foraminifera (Figure7). Such sediments were encountered at subbottomdepths of about 200 to 425 meters at Site 330, and from325 meters to the bottom of the hole at 470 meters atHole 327A (Figure 8). At Site 330, a jump fromprobable Oxfordian-Kimmeridgian to Neocomian-Aptian sediments over the unsampled depth intervalbetween 275 and 300 meters indicates a period of eithervery slow sedimentation rates or a significant hiatus.

Clay and slightly silty clay are the principal texturaltypes occurring in this part of the section (Figure 3),thus continuing the trend of decreasing particle sizewitnessed in the underlying units. The clay fraction isdominated by illite and montmorillonite with anoticeable and perhaps significant trend of decreasingillite/montmorillonite ratio upward in the section(Figure 8). The persistent though small (10%-25%) siltfraction is composed principally of quartz, K-feldspar,and mica along with probable biogenous remains.Coarse fractions (>62 µm) examined from theclaystones consist almost exclusively of carbonizedplant remains, Inoceramus prisms, and other pelecypodshell fragments. Only a trace of terrigenous sand is pre-sent, this being most conspicuous near the base.

The most obvious variations in composition andcharacter of the claystone section relate to type andabundance of organic carbon, percentage of limestoneinterbeds, and the CaCθ3 content. These are sum-marized graphically in Figure 8. In compiling theseplots, the entire section at Hole 327A has been shifteddownward by 50 meters from its actual position relativeto Site 330, as suggested by age correlation between thesites based on pollen and nannoflora.

Organic carbon content shows a gradual increasefrom 1% to 2% in the underlying terrigenous silts andclays (marginal deposits) to a value of about 4.5% in theUpper Jurassic carbonaceous claystones. Maximumvalues of nearly 6% are found in the early Aptianclaystones at both sites and the organic content dropsoff to less than 1% in the overlying Albian sediments.Most of the organic carbon is yellowish-brown,sapropelic (amorphous) material of probableplanktonic origin; however, a significant admixture ofstructured kerogen (land-derived) occurs in the lower100 meters of the claystone section and reappears againnear the top, in the late Aptian (Komer and Littlejohn,this volume).

The quantity of calcium carbonate in the claystonesand the percentage of limestone interbedded in the sec-

884

tion vary in a fashion much as the organic content.Both components increase upward in the section andpeaks in their abundance in late Oxfordian and earlyAptian correspond broadly to more subtle peaks in theorganic carbon content. The obvious exception to thiscorrespondence is in the early Albian where organiccontent drops abruptly while CaCθ3 continues to in-crease. Investigation of smear slides shows thatvariations in the carbonate content of the claystonesrelate primarly to the quantity of nannofossils present;thus the fluctuations shown in Figure 8 are thought toreflect mainly changes in the balance betweenterrigenous and biogenous supply. The nature of thelimestones examined from this part of the section sup-ports this view. Typically these contain the order of20%-40% allochems, principally Radiolaria, which inmost cases have been replaced with sparry-microsparrycalcite or authigenic silica. The allochems are set in amatrix composed of mixed clay and micrite ormicrospar which is laminated on a millimeter scalemuch as the encasing claystones (Figure 7). Traces ofnannofossils are present in many of the Cretaceouslimestones where micrite prevails in the matrix, but ab-sent in the Jurassic limestones where microspardominates due presumably to recrystallization. Thus,most of the limestones appear to have originated asclayey, radiolarian-rich, nanno oozes, and they are in-terpreted to represent intervals of increased biogenousinput relative to terrigenous rather than episodic bot-tom aeration or redeposition of lime mud generatedelsewhere.

Available evidence indicates that fluctuations inabundance of organic carbon, calcium carbonate, andlimestone interbeds in the sapropelic claystones reflectvariations in the relationship between terrigenous inputand plankton productivity combined with changingconditions of bottom aeration. Accordingly, one maypropose a steady decline in terrigenous input during theUpper Jurassic which was accompanied by the develop-ment of poorly oxygenated (euxinic) bottom con-ditions. Productivity may have increased during thissame interval in response to decreased water turbidityand onset of more open-marine surface conditions.This was still apparently a marginal marine environ-ment, however, as indicated by rather high sedimenta-tion rates of the predominantly terrigenous clays, theorder of 20-25 m/m.y. Very similar conditions prevailedduring late Neocomian-early Aptian time, but in lateAptian terrigenous supply apparently increased again.This is evidenced by the low CaC03 content of theclaystones, the decline in abundance of limestone, andthe reappearance of land-derived kerogen. As dis-cussed below, other changes are apparent in this part ofthe section which imply somewhat improved bottomcirculation and presumably herald the onset of more,typical pelagic sedimentation conditions which haveprevailed here since early Albian time.

Considerable interest from the tectonic viewpointcenters on the interval between Oxfordian-Kimmeridgian and earliest Aptian, the time duringwhich rifting between Africa and South America isthought to have occurred (Larson and Ladd, 1973).This 35-40 m.y. interval is represented in the uncoredsection between Cores 4 and 5 at Site 330 and probably

327A

100

HiatusKimmeridgian-Oxfordian

Middle Jurassid?)Basal Sandstones

Basement

Gneiss 575-

% Organic Carbon

1 3 5 7

% Calcium Carbonate

20 40 60

% Limestone

10 20

t-Site 327A Site 330

Il l i te/Montmoril lonite

1.0 2.0 3.0

glauconite

kaolinite

20 40 60

% Kaolinite

oo00 Figure 8. Graph showing compositional variations in the Jurassic-Lower Cretaceous sediments of Holes 321 A and 330. For data on organic carbon, calcium

carbonate and clay mineralogy, see Appendix. Limestone values represent aggregate thickness of limestones as percentage of recovered core length. Claymineral values are for combined

R. W. THOMPSON

lies a short distance below the bottom core of Hole327 A (Figure 8). The lowermost cores of Hole 327A(Neocomian-early Aptian) and youngest Jurassic coresof Site 330 (Oxfordian-Kimmeridgian) are allcharacterized by well-laminated sapropelic claystoneswith moderate to high organic contents, relatively lowCaCOi, and a virtual absence of limestone. Thoselimestones that do occur (Core 5, Site 330) consistmainly of coarse fibrous calcite which appears to havegrown by replacement in preexisting claystone. No allo-chems are present.

Using the reasoning outlined above, one might arguethat terrigenous supply increased in Oxfordian-Kimmeridgian time and was likewise relatively high inNeocomian-early Aptian, thus accounting for the lowCaCCh and absence of limestone. This would imply apronounced hiatus due to nondeposition or erosionsomewhere in the interval to account for the thin sec-tion. Such would allow for a proposal of uplift and ero-sion at or near the site in synchroneity with the separa-tion of Africa and South America. The presence ofreworked Jurassic palynomorphs in the Neocomian-Aptian claystones appears to support this view. Itseems rather fortuitous however, that virtually iden-tical, euxinic environments would precede and followsuch an event. An alternative argument, supported bythe apparent continuity of sedimentation conditionsacross this time interval, is that this area was part of anextremely starved basin in Late Jurassic, one to whichboth terrigenous and biogenous supply continued, butat a very slow rate. Age considerations allow a max-imum sedimentation rate during the interval of about 1-2 m/m.y. Development of an intervening sediment trapduring the rifting process might account for lowterrigenous supply. This possibility is discussed in thesubsequent section concerning tectonic events. Lowbiogenous supply would most likely reflect somechange of surface water conditions which theplanktonic coccolithophores could not tolerate.Substantial fresh water inflow with consequent reducedsalinity of the surface waters in this restricted basin isone obvious possibility. Salinity change is offered as theexplanation for a somewhat similar change from Coc-colith ooze to sapropelic clay in Holocene sediments ofthe Western Black Sea (Ross and Degens, 1974; Bukry,1974). Supporting evidence for this possibility on theFalkland Plateau is provided by the occurrence of thecoccolithophore, Braarudosphaera, including abraarudosphaerid limestone (Core 3, Site 330), in earlyAptian cores at both sites (see discussion by Wise andWind, this volume). Bukry (1974) points out that con-centrations of this Coccolith, both modern and ancientforms, are most common in coastal waters of low salini-ty. In the Black Sea, they are particularly common insediments marking the transition from earlier less salineconditions to those of the present with surface salinitiesthe order of 170/oo-18°/oo.

LOWER CRETACEOUS (ALBIAN) OPENMARINE SEDIMENTATION

By early to middle Albian time, conditions of openmarine (pelagic) sedimentation prevailed on the easternFalkland Plateau. This is evidenced by the occurrence

of light brown, yellowish-gray, and pink nannofossilclay, ooze, and chalk which were encountered at sub-bottom depths between about 120 and 200 meters atSite 330, and 155 and 323 meters at Hole 327A. Frag-mented pelecypod remains (including Inoceramus), insome cases reworked into thin coquina layers, evidencefor moderate to intense bioturbation, and a well-diversified benthonic foraminiferal fauna are foundthroughout this section. The organic carbon content isconsistently well below 1% (Figure 8). Clearly, well-oxygenated bottom conditions were established at thesesites by early-mid Albian time. Furthermore, theappearance of a significant planktonic foramassemblage, which becomes more profuse upward inthe section, and concomitant decrease in the claymineral content relative to biogenous constituents,reflect the onset of more normal marine conditionswhich probably relate to increased distance from theterrigenous source. Possible additional evidence of thewaning influence of continental runoff is the shift topredominance of montmorillonite over illite among theclay minerals and the common occurrence of clinop-tilolite. The persistence of braarudosphaerids, however,may indicate continued conditions of somewhat lowerthan normal salinity.

Reoxygenation of the bottom waters here could haveresulted from any one of a number of causes. A few ob-vious possibilities include: (1) subsidence below an ox-ygen minimum zone; (2) relative lowering of the bound-ary between well-oxygenated surface water and poorlyoxygenated deeper water; (3) improved bottom circula-tion and elimination of euxinic conditions in the entirebasin. The actual cause is unknown, although (3)appears most likely as discussed below in conjunctionwith tectonic events. Whatever the reason, initial stagesof this reoxygenation apparently began in late Aptiantime. In the uppermost part of the carbonaceous unit atHole 327A, many of the claystones and virtually all ofthe limestone interlayers show evidence of bioturbation(Figure 7). Even the occasional undisturbed claystonesections are characterized by thin bedding (occasionallygraded) with apparent textural variations rather thanthe paper-thin lamination of the sapropelic claystonesbeneath. This argues for increased current activity andpossibly some redeposition. By this point in time, then,the eastern Falkland Plateau appears to have been justthat, a submerged plateau or bank elevated above thesea floor to the north and south, and relatively isolatedfrom the influence of continental runoff from Africa.The breach between this part of South America andAfrica apparently was complete by early-middle Albiantime.

RELATION OF SEDIMENTATION TOTECTONIC EVENTS

Reconstruction of Gonwanaland and closing of theSouth Atlantic based on the tenets of sea-floorspreading and plate tectonics imply that the southerntip of Africa fit into the southwestern extremity of theArgentine Basin and that the Falkland Plateau wassituated along the southeast coast of Africa, extendingfrom Agulhas Bank to about the vicinity of Durban(Figure 9; Dingle and Scrutton, 1974). The finding on

886

MESOZOIC SEDIMENTATION, EASTERN FLAKLAND PLATEAU

r^j Basement high

i Sedimentary basin

+ Drill Sites 327A, 330

Figure 9. Relationship of Falkland Plateau to South Africain mid-late Jurassic, prior to breakup of westernGondwanaland. Modified from Dingle and Scrutton,1974.

Basement Highs

Figure 10. Bathymetric map around southern Africa show-ing structural elements of the South African continentalmargin. Contours in km. Compiled from Scrutton (1973);Dingle and Scrutton (1974).

the Falkland Plateau of continental basement rockswhich may well correlate with Precambrian meta-morphic rocks cropping out along the southeast coastof Africa in the vicinity of Durban offers support forsuch a reconstruction (Tarney, this volume). As dis-cussed below, the comparison of Mesozoic stratig-raphy on the plateau with that of southeast Africaprovides additional supporting evidence, and the tec-tonic events involved in the formation of the southeastAfrican continental margin offer clues to the interpreta-tion of the sedimentary record on the Falkland Plateau.

According to Dingle and Scrutton (1974), the lastmajor tectonic event to affect the southeast coast ofAfrica prior to the breakup of Gondwanaland was alate phase of the Cape Orogeny which occurred in latePaleozoic-early Mesozoic time, on the order of 200-235m.y.B.P. This resulted in the formation of northwest-southeast trending faulted folds (Cape Fold Belt) andintervening basins (Figure 10) in the area of the presentcoast and adjacent continental margin, and initiated acycle of erosion and intermontane basin filling. Seismicevidence indicates that these sedimentary basins aretruncated on their southeastern margins along the lineof the Agulhas Fracture Zone (Dingle, 1973;Francheteau and Le Pichon, 1972), hence presumablythey formerly extended onto the Falkland Plateau(Figure 9; Dingle and Scrutton, 1974). The Mesozoicstratigraphy in these basins is not well known; however,the initial phase of sedimentation, perhaps beginning asearly as late Triassic and lasting at least into Middle toLate Jurassic, was characterized by accumulation ofnonmarine conglomerates and sandstones. These sedi-ments, described from outcrops around the AlgoaBasin (Figure 10) as detrital fans and assigned to theEnon Formation (Dingle, 1973), were shed into thesteep intermontane basins of the Cape Fold Belt andtheir accumulation resulted in basin coalescence andburial of "the rugged early Mesozoic landscape." Thebasal sandy sediments encountered at Site 330 are con-sistent with this interpretation based on their lithologiccharacteristics. Furthermore, seismic reflectionevidence indicates that these sediments represent but awedge edge of a much thicker sequence (>500 m) whichhas "filled" a low in the basement topography im-mediately northeast of the site (see Barker, thisvolume).

The earliest evidence for marine transgression foundin Mesozoic sediments along the southeast coast ofAfrica is the occurrence of Upper Jurassic shallowmarine clays at Knysna (Figure 10) and in the AlgoaBasin. These mark temporary marine incursions, and inthe Algoa Basin they pass vertically and laterally intoparalic sediments of the Kirkwood Formation. Fullmarine conditions were established by Neocomian timewith deposition of shallow marine sediments of theSundays River Formation (Dingle, 1973). Both of theseformations are diachronous and become youngertoward the north. The earliest marine sediments at Site330 are Middle (?) to Late Jurassic in age, and these arefollowed upward by a sequence of silts and clays, thelithologic and faunal characteristics of which implydeposition on a fairly shallow shelf that becameprogressively further removed from the source area.Taken together, the sequences indicate a marinetransgression from southeast to northwest across theFalkland Plateau in Middle (?) to Late Jurassic time.Scrutton (1973) proposed that eastern Gondwanaland(Antarctica, India, Australia) separated from westernGondwanaland (Africa, South America) along thesouthern margin of the Falkland Plateau in MiddleJurassic time. This implies that the basement slopesouth of Site 330 represents part of a rifted continentalmargin and that the aforementioned transgressionacross the eastern Falkland Plateau and onto southeastAfrica reflects the extension of a narrow arm of the

887

R. W. THOMPSON

Trei(buried)

Figure 11. Diagram showing main tectonic elements ofsouthernmost South America in Early Cretaceous time.ATG - arc-trench gap; IA - island arc; MB - marginal basin;SC - stable continent. Adapted from Dalziel et al, 1975.

proto-Indian Ocean, the earlier history of which isdocumented in eastern Africa and Madagascar (Kent,1974). According to this supposition, the easternFalkland Plateau was part of the gradually subsidingsoutheast African continental margin during LateJurassic time, and the terrigenous silts and clays of thisage at Site 330 represent marginal deposits which, forthe most part, were supplied from the northwest, thatis, the African land mass.

Working from the South American side, a ratherdifferent picture of the situation evolves. Based on theirmapping in Patagonia and Tierra del Fuego, Dalziel etal. (1974a) have proposed the existence of a marginalbasin in southernmost South America which opened inLate Jurassic and persisted at least through Aptiantimes (Figure 11). This basin separated the stable con-tinental block (southeastern South America-southernAfrica) from an andesitic volcanic arc on the Pacificside, the roots of which are marked by the Patagonianbatholithic belt of southern Chile. Their reconstructionshows this marginal basin widening to the southeast,and they propose that the volcanic arc continued intothe Antarctic Peninsula. Recent work by Suarez (1976)substantiates this proposal and also divulges evidencefor the existence of a Late Jurassic-Early Cretaceousback arc (marginal) basin along the east side of the Ant-arctic Peninsula. Just how the subduction activity in-volved in opening this basin relates to rifting along EastAfrica is not clear; however, the proposal implies thatthe Falkland Plateau was not part of a true continentalmargin, but rather a shelf area flanked by less thanoceanic depths of a marginal basin (Dalziel et al., thisvolume). The early marine facies at Site 330 seem equal-ly consistent with this hypothesis. Furthermore, onemust speculate that Late Jurassic shallow marine sedi-ments found in the Magallanes Basin (Natland andGonzalez, 1974) reflect a continuation of this sameshelf seaway into southern Argentina and Chile.

The specific cause for the onset of euxinic conditionsand deposition of sapropelic claystones on the eastern

Falkland Plateau is unknown; however, the age of thesesediments, Upper Jurassic (Oxfordian) through Aptian,indicates they are related in some way to the initialfragmentation of Gondwanaland and opening of theSouth Atlantic. The fact that similar sediments of aboutthis same age are widespread in the South Atlantic sup-ports this view (Figure 1). Dark carbonaceous shaleshave been reported, for example, from the MagallanesBasin of southern South America (Natland and Gon-zalez, 1974; Dalziel et al., 1974b) where they are UpperJurassic (Kimmeridgian) to Early Cretaceous (Aptian)in age; from the Cape and Angola basins (Bolli, Ryan,et al., 1975) where respectively they are lower Aptianand upper Aptian to Coniacian; and from the Mozam-bique Ridge (Simpson, Schlich, et al., 1972) where theyare Neocomian-Aptian in age. Available evidencesuggests a progression in age of the South Atlantic eux-inic facies from oldest at the south to younger furthernorth; this also argues for association with the openingof the South Atlantic.

One possible circumstance which might havecaused the stagnation and onset of euxinic conditions inthese basins is the combination of density stratificationin the water column and restricted deep water circula-tion due to the existence of relatively shallow bathy-metric ridges or sills. Many Quaternary examples ofsapropelic mud deposition apparently are (or were)caused by such a situation, for example, in the Adriatic(van Straaten, 1972), the eastern Mediterranean (Ryan,1972), and the Black Sea (Ross and Degens, 1974). Inthe South Atlantic region, density stratification mighthave been the product of runoff from South Americaand Africa into the initial semiisolated basins; this wasperhaps enhanced by warming of the surface waters.The Falkland Plateau could well have provided therestricting sill for the Cape Basin in Early Cretaceoustime much as the Walvis Ridge did for the AngolaBasin (Bolli, Ryan, et al., 1975), since the eastern end ofthe plateau is not thought to have cleared the tip ofAfrica until Albian (Dingle and Scrutton, 1974). Whatfeature, then, restricted circulation in the earlier basinsituated south of the Falkland Plateau? The AgulhasPlateau (Figure 10), thought by Scrutton (1973) torepresent an abandoned segment of the early Mid-Atlantic Ridge, is one possibility, however several fac-tors argue against this. First and most obvious is thedepth. The crest of the Agulhas Plateau is presently atdepths of about 2500-3000 meters. Whereas the featuremay have subsided since initial formation, it is doubtfulthat the crest was ever shallow enough to restrict cir-culation in the 200-500 meter depth range, the es-timated depositional depth of the euxinic facies on theFalkland Plateau and in the Magallanes Basin. Secondis the age. The oldest rocks thus far dredged fromAgulhas Plateau are Coniacian (Scrutton, 1973). Whilethe oceanic basement rocks of the plateau are likely tobe older, still one must question whether in fact thefeature even existed prior to the spreading episodewhich separated Africa from South America, anepisode which apparently postdates inception of eux-inic conditions. Finally, supposition of the AgulhasPlateau as the restricting feature for areas to the west

888

MESOZOIC SEDIMENTATION, EASTERN FALKLAND PLATEAU

would still necessitate looking for an additional restric-tion to the northeast to account for the EarlyCretaceous euxinic sediments on the MozambiqueRidge.

A second, and seemingly more likely, possibility is ashallow restriction in the Late Jurassic seaway betweenEast Africa and East Antarctica. A tempting prospecthere is the vicinity of Mozambique where many authors(e.g., Dietz and Sproll, 1970; Smith and Hallam, 1970)place the Princess Martha coast of Antarctica inGondwana reconstructions. The euxinic sediments ofthe Mozambique Ridge (Site 249) would thus haveoriginated in the same restricted basin as those to thewest. Possible support for this contention is theevidence of widespread basalt extrusion in Mozam-bique and Madagascar beginning in Neocomian-Aptian time and lasting to Albian-Coniacian (Kent,1974; Valuer, 1974). This may reflect the completebreakthrough of this seaway and the beginning of sub-sidence to oceanic depths. Recent investigation quotedby Kent suggests seismic continuity between the EarlyCretaceous (?) basaltic basement drilled at Site 249 onthe Mozambique Ridge and continental basalt ex-trusions in Mozambique; this argues for possibleshallow water origin of the former as suggested byValuer (1974).

Another aspect of the euxinic sediments on theFalkland Plateau which deserves further inquiryregards their very fine grained nature and, particularly,any reason for possible conditions of very lowterrigenous supply and slow sedimentation rates nearthe Jurassic-Cretaceous boundary as suggested above.These are in marked contrast to euxinic sediments inthe Cape Basin, for example, which occur interbeddedwith rapidly deposited coarse terrigenous material(mudstones and sandstones) presumably suppliedlargely by the ancestral Orange River drainage (Bolli,Ryan, et al., 1975).

Southeast Africa and the Falkland Plateau arethought to have separated by strike-slip motion alongwhat are now the Agulhas and Falkland fracture zones(Francheteau and Le Pichon, 1972; Scrutton, 1973).Estimates of the time of inception of this event andrates of subsequent spreading motion involved in theinitial opening of the southernmost Atlantic vary con-siderably. Larson and Ladd (1973) estimate initialopening to have occurred in the Neocomian (125-130m.y.B.P.) based on their interpretation of magneticlineations in the Cape Basin. Recently, however, theseanomalies were reinterpreted by Emery et al. (1975) toindicate initial opening in the Middle to Late Jurassic(165 m.y.B.P.). In view of the uncertainty, it does notseem unreasonable to suppose that initial fragmenta-tion, prior to the actual spreading and separation ofAfrica and South America, commenced in the UpperJurassic. Then, using the spreading history of Larsonand Ladd (1973) and assuming the eastern end of theFalkland Plateau originally lay near Durban, the areaof the drill sites would have been situated about adja-cent to Agulhas Bank in middle to late Aptian, andwould have become progressively further removedfrom continental influence during Albian time as the

eastern tip of the plateau cleared the corner of Africa. Itis of some interest to review the stratigraphic record atSite 330 and Hole 327A in this light.

In Late Jurassic time, the sedimentary environmentat Site 330 changed from one dominated by ter-rigenous silts and clays with occasional coarser beds toone characterized by accumulation of very fine grainedcarbonaceous claystones. The change was gradual andprobably reflects a combination of reduced relief in thesource area, increased remoteness of the depositionalsite due to the transgression, and slow subsidence of theAfrican margin into deeper, poorly oxygenated watersof the marginal basin to the south. The rather highsedimentation rates (20-25 m/m.y.) and the persistentadmixture of terrestrial palynomorphs in these earlyclaystones indicate that sediment supply from landremained substantial during this time. Beginning in theOxfordian-Kimmeridgian (Core 7, Site 330) and last-ing into the late Neocomian-early Aptian, the possibili-ty of exceptionally low terrigenous supply wassuggested above to account for the very thin (or mis-sing) section. The reason for this change may well relateto the development of an intervening sediment trapalong the Agulhas-Falkland transform fault zone.Perhaps enlightening in this regard is the observationby Scrutton and du Plessis (1973) of a basement ridgerunning beneath and parallel to the slope northeastfrom Agulhas Bank (Figure 10). The crest of this"marginal fracture ridge," the origin of which theyrelate to strike-slip faulting, is now at a depth of about2000 meters, but data from Dingle (1973) imply thepossibility of at least 1500 meters of marginal sub-sidence since Early Cretaceous time. The ridge, then,could have isolated the plateau sites from significantterrigenous input in Late Jurassic-Early Cretaceoustime and thus account for very slow sedimentationrates. Indeed Dingle states: "If Scrutton and duPlessis's (1973) dating of the formation of the Agulhasmarginal fracture ridge is correct (Late Jurassic-EarlyCretaceous) then the late Uitenhage group sedimentswere dammed behind a high ridge—earlier representa-tives of the group were deposited before southernAfrica separated from the Falkland Plateau." The earlyrepresentatives of the Uitenhage Group probably cor-respond to the terrigenous silts and clays (marginaldeposits) and underlying subaerial sands of lower Site330, as discussed above. Furthermore, uplift of such aridge could provide a situation which allowed rework-ing of Late Jurassic sediments as indicated by thePalynology of the early Aptian claystones. Going onestep further, one must suspect that increased ter-rigenous supply in middle-late Aptian, as suggested bythe lithology and Palynology of the uppermost car-bonaceous claystone section, may relate to movementof the site into the proximity of Agulhas Bank wherethe marginal fracture ridge disappears and wheresoutherly transport of sediment from rivers emptyingalong the African west coast became a distinct possibili-ty.

In Albian time, influx of terrigenous sediment ta-pered off significantly and pelagic conditions with well-oxygenated bottom waters became established at the

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Falkland Plateau sites. This change is roughly syn-chronous with the disappearance of euxinic sedimentsin the Cape Basin (Site 361) and on the MozambiqueRidge (Site 249). Also at about this time, the tip of theFalkland Plateau was clearing Africa and volcanism,probably accompanied by subsidence, was occurringaround Mozambique and Madagascar. Thus, thechange in conditions apparently reflects the tectonicremoval of the restrictive sills which separated thesebasins and the onset of free exchange of deep andshallow water masses. Available ages, though tenuous,suggest that reoxygenation progressed from deep water(Cape Basin) to shallow (Falkland Plateau) with Hole327A, the shallowest plateau site, being last. This im-plies that termination of the euxinic conditions in-volved inflow of denser Indian Ocean water along thebottom accompanied by surface removal of basinwaters. The apparent early Aptian to late Albian hiatusat Site 249 on the Mozambique Ridge (Simpson,Schlich, et al., 1974) may be a manifestation of thisprocess.

One final comment concerning the occurrence ofMesozoic euxinic facies in the Atlantic seems ap-propriate. Euxinic sedimentation was apparentlywidespread in the North Atlantic as well as the SouthAtlantic in Late Jurassic-Early Cretaceous time(Saunders et al., 1973). To explain this by analogy tomodern euxinic environments such as the Black Seanecessitates a great deal of speculation regarding possi-ble restrictive sills and tectonic movements to initiateand terminate the euxinic conditions. This may give onereason to question the analogy, and to wonder aboutpossible alternative interpretations. One possibility isthe lack of very cold waters at high latitudes in theMesozoic Atlantic so that bottom circulation wasgenerally more sluggish than at present. A second is thedevelopment of an oxygen minimum layer in the watercolumn and accumulation of at least some of theorganic-rich sediments in the limited depth range wherethis layer intersected the bottom. The fact that carbona-ceous sediments of both the Falkland Plateau andMagallenes Basin were deposited in depths well abovethe adjacent basin floor makes this a tempting inter-pretation. Still, the association of euxinic facies withinitial fragmentation of Gondwanaland, and the ap-parent correspondence in time between barrier removaland disappearance of restricted conditions seems morethan fortuitous. This is particularly true in the SouthAtlantic where the start and stop of euxinic sedi-mentation is not everywhere synchronous, but appearsto have progressed northward as the new basin opened.

ACKNOWLEDGMENTSI extend thanks to appropriate personnel at Humboldt

State University and the Deep Sea Drilling Project who madepossible my participation on Leg 36. Appreciation is extendedto T.L. Thompson and others at the Amoco Production Co.Research Laboratory who arranged for and carried outanalyses of organic carbon and palynomorphs. I.W.D.Dalziel, D.H. Elliot, and C.C. von der Borch read earlier ver-sions of this paper and made many helpful suggestions forwhich I am grateful. Finally, I wish to acknowledge the con-structive criticism of J.R. Curray who reviewed the finalmanuscript.

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