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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 (
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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
^
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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.
879
-
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
(
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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-
881
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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
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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
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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
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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
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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
889
-
R. W. THOMPSON
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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