Structure of the southern Falkland Islands continental shelf: initial results from new seismic data

Marine and Petroleum Geology, Vol. 12, No. 7. pp. 759-711, 1995 Copyright 0 1995 Else&r Science Ltd Printed in Great Britain. All rights reserved 0264~8172/95 $10.00 + 0.00

Structure of the southern Falkland Islands continental shelf: initial results from new seismic data Nigel H. Platt” Schlumberger Geco-Prakla, West Sussex RH6 ONZ, UK

Schlumberger

House, Buckingham

Gate, Gatwick Airport,

Peter R. Philip Schlumberger

Geco-Prakta,

Received 23 December

Bjergstedvn.

1, N-400 7 Stavanger,

7994; revised 29 March

1995; accepted

Norway 8 April

1995

Acquisition of a modern marine dataset has revealed the internal structure of sedimentary basins to the south of the Falkland Islands for the first time. The new data reveal a variety of structural styles and indicate the presence of a sedimentary section ranging from 2 to 10 km in thickness developed in three linked sub-basins. (1) The eastern part of the Malvinas Basin lies south-west of the islands. A prominent regional unconformity developed throughout the area marks the top of gently folded rocks of probable Palaeozoic age and their cover of Late Jurassic volcanics. To the north-west of the islands, the underlying strata approach the surface in a structural high which forms a prominent gravity low and may record the continuation of the Palaeozoic basin exposed on the islands themselves. Strata of probable Mesozoic and Tertiary age rest on the unconformity throughout the area and show a regional southward dip. These deposits increase in thickness south-westwards towards the depocentre of the Malvinas Basin midway between the islands and Argentina. (2) The South Falkland Basin and Burdwood Bank lie south of the islands. Thicknesses increase towards an area of southward-dipping extensional fault blocks within the South Falkland Basin. Downdip these are buried beneath a prograding and north-vergent stack of blind and emergent thrusts at the northern margin of the Burdwood Bank, which is a structural high lying in shallow water south of the islands. Thrust loading has resulted in the development of a foreland basin north of the thrust front. Complex antiformal structures are associated with the thrusting, whereas a shallow sedimentary basin developed on the crest of the Burdwood Bank is interpreted as a piggy-bank basin riding on, and sourced dominantly from within the thrust pile. (3) The Falkland Plateau Basin lies to the east and south-east of the islands and is characterized by a varied and extensive Mesozoic to Tertiary succession of submarine fan deposits thickening eastwards into deeper water. Igneous intrusives deep in the section may correlate with Jurassic volcanic rocks present within the Malvinas Basin to the west, or may record younger rifting associated with the opening of the South Atlantic. These initial results indicate considerable structural heterogeneity in this part of the Falkland Islands Continental Shelf. In the light of the hydrocarbon discoveries already made further west, significant economic potential is likely. Stratigraphic traps may occur at regional unconformities and within submarine fan deposits, whereas potential structural traps include Mesozoic extensional fault blocks and thrust and subthrust structures generated by Tertiary and Quaternary compression. Keywords:

Falkland Islands; seismic data; sedimentary

The presence of sedimentary basins south of the Falkland Islands (Figure 1) was identified from broadly spaced commercial and research seismic acquired in the 1970s. Modern seismic coverage exists offshore Argentina to the west, where significant hydrocarbon discoveries have been made (Pucci, 1987). However, seismic data around the Falkland Islands were sparse

* Correspondence to Dr N. H. Platt; present address: Ranger Oil (UK) Ltd., Walnut Tree Close, Guildford. Surrey GUI 4US, UK

wt

*2:7-c

Marine

and

basins

until the acquisition of new regional surveys in 1993 and no well has been drilled in this area to date. This paper is based on the initial evaluation of 8500 km of new seismic and potential field data from the area to the south of the islands. The new data reveal the regional structure of three linked sub-basins: (1) the eastern part of the Malvinas Basin (south-west); (2) the South Falkland Basin and Burdwood Bank (south); and (3) Falkland Plateau Basin (south-east). The area to the north of the islands is described by Lawrence and Johnson (1995).

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Structure of the southern Falkland Islands continental shelf: N. H. Platt and P. R. Philip

65'W

bO'W F*LIUND

lSLANDS DESIGNATED *REA

SOUTH ATLANTIC OCEAN

%

Figure 1 Location map showing regional geology, Biddle et al., 1986; Richards and Farbin, 1$94) -.

seismic

lines illustrated

Geographical location The Falkland Islands lie in the South Atlantic Ocean, about 700 km off the Argentinian coast (Figure I). The islands lie on the South American continental shelf, where water depths reach up to 200 m. South of the islands there is a rapid deepening north of the Burdwood Bank, which is a large, flat-topped bathymetric high 180 km to the south of the islands. Water depths increase to 2000-3000 m over the Falkland Plateau, which extends almost 2000 km east of the islands, and reach 4000-6000 m in the Argentine Basin and the Scotia Sea to the north and south of the Falkland Plateau.

Tectonic setting Falkland Islands and Gondwana The Falkland Islands and Falkland Plateau lie near the south-eastern edge of the South American plate (Figure 2). The Burdwood Bank to the south (Figure I) forms part of the North Scotia Ridge, a complex plate boundary extending for 2000 km from South Georgia in the east to Tierra de1 Fuego in the west (Ludwig, 1983) and linking with the foreland fold and thrust belts of southern South America (Winslow, 1981; Biddle et al., 1986). A pre-drift reconstruction of Gondwana (Figure 2) shows that the Falkland Islands were probably located close to the south-western coast of Africa before opening of the South Atlantic during the Late Jurassic and Cretaceous (Le Pichon and Hayes, 1971). Thus although the shelf around the islands forms a morphological part of the Argentine continental margin, the onshore outcrops of Palaeozoic strata on the Falkland 1972; Marshall, 1994a) show Islands (Greenway,

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and Falkland

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Designated

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striking stratigraphic similarities with the Ecca and Dwyka Groups of the Karoo succession in southern Africa (Visser , 1993) and with the Palaeozoic of northern Argentina (Buenos Aires province) and southern Uruguay (Urien and Zambrano, 1973). The results of palaeomagnetic studies on Jurassic dykes on West Falkland (Mitchell et al., 1986; Taylor and Shaw, 1989) have suggested that the Falklands are a displaced micro-craton produced during the break-up of Gondwana (Dalziel and Grunow, 1992; Marshall, 1994a), rotated by 180” and moved to their current position off Argentina as a result of complex movements associated with the opening of the South Atlantic. Both the current study and parallel studies on new seismic data to the north of the islands (Lawrence and Johnson, 1995) record a Mesozoic-Tertiary rifting and compressional history consistent with that of southern South America, but without direct evidence of complex rotational movements. South American sedimentary basins The offshore areas of southern South America include several important Mesozoic-Tertiary basins (see Figure l), which have been the focus of exploration activity for some years (Urien and Zambrano, 1973; Pucci, 1987; 1994; Bellosi and Jalfin, 1989; Uliana et al., 1989). Linked to the west with the Magallanes Basin of onshore Argentina and Tierra de1 Fuego, the Malvinas Basin (Yrigoyen, 1989) covers an area of 115 000 km’, extending from Argentine waters in the west into the study area south of the Falklands (Figure 1). Sediment thicknesses reach 9 km in the western part of the basin. As in the Magallanes Basin (Winslow, 1982; Biddle et al., 1986), sedimentation may have been contemporaneous with and related to Tertiary

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Structure of the southern Falkland islands continental

Figure 2 Pre-drift reconstruction of Gondwana, showing the Falkland Islands in relation to southern Africa and Antarctica before South Atlantic opening (after Norton and Sclater, 1979). AFZ = Agulhas Fracture Zone

deformation of the Andean Chain. To the east, the Malvinas Basin is linked to the South Falkland Basin and the Falkland Plateau Basin (Figure I) to the south and south-east of the islands, respectively (Richards and Fannin, 1994).

Stratigraphy Precambrian and Palaeozoic In the absence of well penetrations, the Precambrian and Palaeozoic stratigraphy (Figure 3) of the offshore area is inferred from comparison with outcrops on the

Molvinos Basin

f

S.Folkland Basin

shelf: N. H. Platt and P. R. Philip

Falkland Islands. The islands form part of a structural high, which extends into the shallow offshore and is referred to as the Falkland Platform (Figure I). The stratigraphy of the pre-Mesozoic sequence onshore was reviewed by Greenway (1972) and re-evaluated by Marshall (1994a). Metamorphic rocks of the Precambrian Cape Meredith Complex are comparable with the Pan-African ‘Namaqualand’ suite of southern and south-western Africa. These rocks are overlain by quartzitic and micaceous sandstones and shales of the Gran Malvina Group, which is probably of OrdovicianSilurian to Devonian age. These deposits find equivalents in southern Africa, including the quartzites of the Table Mountain Group (Hobday and Von Brunn, 1979). The clean quartzites may have reservoir potential only where diagenesis and compaction have not resulted in porosity occlusion. Tillites, sandstones and mudstones of the Permian Lafonian Diamictite Formation (Frakes and Crowell, 1967) may also be correlated with southern African and South American equivalents; the tillites are similar to the Dwyka tillites of southern Africa (Visser, 1987) and to tillites in the Pampa de Tepuel Formation of Patagonia (Uliana and Biddle, 1987). Similar and approximately coeval glacial deposits are also found in Antarctica as well as other parts of Gondwana (St John, 1986). Glacial organic-rich black shales of Permo-Carboniferous age form regional source rocks in onshore South America (Russell, 1990) and may offer potential for hydrocarbon generation in the study area (Marshall, 1994b). The Upper Lafonian Group of south-east Falkland consists of around 3000 m of banded siltstones and alternating sandstones and shales bearing the characteristic Gondwanan Glossopteris flora. This elastic succession is lithologically and palaeontologically comparable with parts of the Karoo Supergroup of southern Africa (Adie, 1952).

Falkland Plateau Easin

Falkland PlOtfO~lTl

:~~~~~-~~~~~~~

_-_-_-_-_-_-_-_-_-_-_-: )I-I-_-_-_*galjaqo_no_ fomytio~-_-_-_-_-_--------_---_---_---_----_-----*-----_--------_-_-_--_--

Figure 3 Stratigraphy Pz = Palaeozoic

of

the

Falkland

Islands

and

surrounding

Marine

offshore

and Petroleum

areas

B

Mudstone

t-z-?J

Sandstone

m

Volcanics

lzl

Quartzitrr, Shales etc

m

Basement

(redrawn

Geology

from

Richards

1995 Volume

and

Fannin,

12 Number

1994).

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761

Structure of the southern Falkland Islands continental shelf: N. H. Platt and P. R. Philip Mesozoic The only Mesozoic rocks exposed on the Falkland Islands are the Lower Jurassic dolerite dykes exposed on West Falkland (Taylor and Shaw, 1989). These are thought to be equivalent in age to the widespread and voluminous tholeiitic flood basalts, dolerite sills and layered intrusions of East Antarctica, southern Africa, Tasmania and South America (Mitchell et al., 1986; Dalziel ef al., 1987; Uliana and Biddle, 1987). The Mesozoic succession is inferred by comparison with: (1) Ocean Drilling Program boreholes drilled to the east of the study area in deep water of the Falkland Plateau Basin (Harris and Sliter, 1977; Parker et al., 1983); and (2) with outcrops and exploration wells in the Magallanes Basin and the western Malvinas Basin to the west of the study area in southern Argentina. Jurassic. Results of DSDP drilling in the Falkland Plateau Basin indicate a Mesozoic sequence ranging back to at least the Oxfordian. The base of the rift succession is marked on seismic traverses of the area by a prominent unconformity (Lorenzo and Mutter, 1988). The unconformity shows significant surface topography (Barker, 1977) and the presence at site 330 (Figure I) of lignite, calcrete and other pedogenetic indicators (Thompson, 1977) indicates that the metamorphic and granitic basement was locally subject to subaerial weathering and erosion before Late Jurassic transgression. The overlying strata at site 330 begin with a thin succession of ?fluvial to marginal marine Middle to Upper Jurassic sandstones, siltstone and claystones (Thompson, 1977). These in turn are overlain by transgressive marginal marine sandstones and Oxfordian-Kimmeridgian dark marine shales with thin intercalated limestones. The shales have total organic carbon (TOC) values of up to 4% and form potential hydrocarbon source rocks. Both onshore and offshore in southern Argentina, at outcrop in the Magallanes Basin (Winslow, 1982; Figure I) and in wells drilled in the western Malvinas Basin, the eroded basement surface is unconformably overlain by Upper Jurassic rhyolitic lavas and volcanielastic rocks of the Serie Tobifera. These rocks form reservoir targets in Argentinian waters. Similar silicic volcanics are reported from the south-western African margin and the Antarctic Peninsula (St John, 1986). This phase of volcanism is associated with regional uplift and erosion and appears to be strongly linked with Late Jurassic faulting and graben formation. Late Jurassic volcanic activity throughout the South Atlantic region was linked by Dalziel ef al. (1987) with the extensional fragmentation of Gondwana. In the Falkland Plateau Basin Cretaceous. 1977; Ludwig, 1983), the Lower (Thompson, Cretaceous consists of dark carbonaceous claystones. The Upper Cretaceous succession comprises pelagic clays, nannofossil ooze and chalk (Thompson, 1977). Lorenzo and Mutter (1988) report major intra- and these unconformities; erosional end-Cretaceous probably reflect changes in the rate of South Atlantic opening and convergence on the North Scotia Ridge. The Lower Cretaceous shales are of particular interest as these include prospective source rock horizons. Aptian-Albian organic-rich shale horizons

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are documented over much of the South Atlantic region (Katz and Pheifer, 1983), including southern Africa, and are interpreted as the deposits of a restricted, euxinic basin formed in association with the fragmentation of Gondwana and early opening of the South Atlantic. Sampling of black shales on the Falkland Plateau to the east of the study area at DSDP sites 330 and 511 (Comer and Littlejohn, 1977; Parker ef al., 1983) showed TOC values of up to 6%, a dominance of type II (oil-prone) kerogen and maturities corresponding to the early stages of oil generation. The presence of these facies in deeper water offers excellent potential for the updip migration of hydrocarbons into the study area. Although Cenomanian-Turonian source horizons were penetrated by DSDP drilling offshore West Africa and were inferred from the area north of the islands by Lawrence and Johnson (1995), these were absent at sites 330 and 511 (Katz and Pheifer, 1983). Throughout onshore and offshore southern Argentina, the Serie Tobifera and presumed basement are unconformably overlain by a thick pile of sedimentary strata of late Mesozoic and Cenozoic age. Basal Cretaceous marginal marine (?to fluvial) sandstones of the Valanginian Springhill Formation form the most important hydrocarbon reservoir in the Malvinas Basin to the west, as well as in the Magallanes Basin (Biddle er al., 1986). The Springhill Formation may locally include interbedded source rock horizons. In wells drilled in the western Malvinas Basin, the remainder of the Mesozoic and Tertiary sequence comprises open marine pelagic carbonate oozes and zeolitic clays. As in the Falkland Plateau Basin, these may include source rock horizons; Pucci (1987) reports that organic-rich shales of Cretaceous age form important source rocks throughout the region. In southern Argentina, the fine-grained Cretaceous rocks also form regional hydrocarbon seals above the Springhill Formation and Serie Tobifera. The marine transgressive to fluvial-deltaic mudstones, siltstones and sandstones of the Hauterivian Rincon and the Barremian Nueva Argentina formations are overlain by predominantly marine facies of the Aptian to Cenomanian Arroyo Alfa Formation (shales and interbedded minor sandstones) and Turonian Cabeza de Leon Formation (mainly shales). These shales are widespread in both the Magallanes and Malvinas basins and, as on the Falkland Plateau, may include AptianAlbian source rock horizons. Tertiary The Tertiary stratigraphy offshore can also only be inferred from comparison with DSDP drilling and with outcrops and wells in the Magallanes and western Malvinas basins. In the Falkland Plateau Basin, the Ocean Drilling Program penetrated a succession consisting mainly of diatomaceous nannofossil, calcareous and zeolitic oozes (Barker, 1977; Ludwig, 1983). Tertiary deposits of southern Argentina (Biddle et al., 1986) are subdivided into the marginal marine Lower Magallaniano Formation of PalaeoceneEocene age, the fluvio-deltaic facies of the MioceneOligocene Upper Magallaniano Formation and the

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Plio-Miocene Santa Cruz Formation, comprising fluvial elastic deposits with local marine carbonate interbeds.

Structure The study area is divisible into three structural zones: (1) the eastern part of the Malvinas Basin to the west and south-west of the islands; (2) the South Falkland Basin and Burdwood Bank to the south; and (3) the Falkland Plateau Basin to the east and south-east. Each area displays different structural styles (Figures 4-6 and Plates I-.?). Gravity and magnetic transform maps reveal ENE-WSW and NNE-SSW regional structural trends across most of the study area (ARK Geophysics, unpublished data). Onshore, the NNE-SSW trend coincides with the orientation of Falkland Sound between East and West Falkland and of a major monocline adjacent to it on West Falkland. A similar fault trend is evident on Beauchene and the Jason Islands (Marshall, 1994a). Eastern Malvinas

Basin

Water depths in this area increase south-westwards away from the islands, reaching 500 m near the edge of the Falkland Islands Designated Area. An interpreted section and a seismic data panel from the area southwest of the islands are shown as Plate I and Figure 4. This line lies to the east of the Malvinas Basin depocentre and illustrates the south-westward regional dip and increasing thickness towards the edge of the Falkland Islands Designated Area. The occurrence of a gravity low to the north of the Malvinas Basin initially suggested the presence of a sedimentary basin or igneous intrusion to the west and north-west of the islands. However, these interpretations are not supported by the seismic data, which indicate indurated rocks at the surface, nor by the magnetic data, which show no large anomaly here. In contrast, the magnetic anomaly map does indicate some high amplitude features south-west of the islands. In the eastern Malvinas Basin, Figure 4 shows the development of two principal seismic sequences, separated by a major regional unconformity: (1) a south-westward dipping succession of ?Palaeozoic and older Mesozoic rocks characterized by block faulting and open anticlinal folds; and (2) a series of ?younger Mesozoic and Tertiary strata thickening markedly towards the south-west. A depression beneath the unconformity in the central part of this line is infilled with complex onlap by a sedimentary succession of variable thickness which displays low amplitude reflections. The unconformity has a very irregular surface, recording onlap onto significant relief. The overlying units have a generally simple, dipping structure and thicken progressively towards the south-west. Several seismic sequences may be recognized within this package and, although dating of these deposits awaits reliable correlation from wells, the stratigraphy is likely to be similar to that encountered by exploration drilling in the western Malvinas Basin. Discussion.

The platform

around

the islands is

Marine

shelf: N. H. Platt and P. R, Philip

thought to show a geology similar to that exposed onshore, with Palaeozic rocks present at or near the seafloor. The north-westerly trend of the gravity low west and north-west of the islands is parallel with coastal outcrops and promontories of northern West Falkland and may record the presence of a Palaeozoic basin extending offshore. Further south, thicknesses increase south-westwards away from the islands and towards the edge of the study area. This area represents the north-eastern feather edge of the Malvinas Basin; substantially greater thicknesses are present in the main basin depocentres further south and west. The irregular surface of the regionally traceable unconformity could be fault-controlled, erosional, volcanic or karstic in origin. In the absence of well ties, the lithologies beneath the unconformity remain uncertain, but as Mesozoic carbonate strata are scarce in the region it is more likely that the topography records significant relief developed at the top of the volcanic/volcaniclastic Serie Tobifera of Late Jurassic age. The style and orientation of magnetic anomalies south-west of the islands likewise suggests the presence of dyke swarms. This interpretation is consistent with the occurrence both of volcanic rocks offshore and Jurassic dykes on West Falkland. South Falkland Basin and Burdwood Bank Water depths reach a maximum of 2000 m in this area, but decrease rapidly southwards to 200 m or less over the Burdwood Bank. Thicknesses increase southwards away from the islands towards the depocentre of the South Falkland Basin (Richards and Fannin’, 1994). This asymmetrical basin forms a prominent gravity low. The southerly regional dip is associated with prominent tilted blocks, defined by extensional faults downthrowing to the NNW. The basin depocentre lies just north of the Burdwood Bank. An interpreted section (Plate 2) and a seismic data panel (Figure 5) across the area demonstrate three principal unconformity bounded seismic sequences: 1.

2.

3.

A southward-dipping, poorly layered succession of high velocity rocks. These strata are thought to be of probable Palaeozoic or older Mesozoic age and are broken into rotated blocks defined by northward dipping faults. A gently southward-dipping, strongly layered sedimentary succession onlapping unconformably onto the underlying rocks. Although showing a smoother surface topography, the unconformity at the base of this succession may correlate with the prominent unconformity recognized on Plate I and Figure 4. The layered succession is fragmented into fault blocks by a complex pattern of northward dipping extensional faults showing evidence of reactivation from lines of deeper faulting (Figure 5). The time thickness of these rocks exceeds 3 s in the axis of the basin. Younger rocks in the basin axis which form an asymmetrical wedge unconformably onlapping the southward dipping strata beneath. These rocks lie subhorizontally and appear unfolded to the north, but further to the south are deformed in a series of

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-_-----

EASTERN

1

-_...

zkm

----

RAEIN

FALKLAND

Basin

section for the South Falkland Basin and Burdwood

SOUTH

section for the eastern part of the Malvinas

MALVINAE

GFl93-166

--

0

Plate 2 Interpreted

N

Plate 1 Interpreted

SW

Bank

BASIN

4,

r

FALKLAND

-- --

SOUTH

PLATFORM

-

------

mdlmentay

BASIN

I I I

N’ s

Jumulc/CrcMceous?

rocks In baam.nt/ophldih In

RANK

- Upper

FALKLAND

I_ tJmhd&d

BURDWOOD

Inlrudvec

UpperJumsslc Cnlaceous

s

4

3

.I

NE

Structure of the southern Falkland Islands continental

shelf: N. H. Platt and P. R. Philip

stacked, southward dipping thrust slices with prominent hangingwall anticlines (Figure 5). An area of complex structure at the northern edge of the Burdwood Bank shows repeated thrust units, but little evidence of sedimentary layering within the slices. An asymmetrical, shallow basin developed on the crest of the Burdwood Bank shows a fill onlapping to the north and south. This area is also notable for the presence of a prominent near-surface reflector approximately 400 ms beneath the water bottom. This reflector cross-cuts the sedimentary layering, and although nearly parallel to the seafloor, may not be attributed to multiple energy. This area marks the boundary Discussion. between the Falklands micro-craton, part of the South American plate, to the north, and the Scotia plate to the south (Ludwig, 1983; Biddle et al., 1986). Seismic refraction experiments by Ludwig et al. (1968) suggested that the northern part of the Burdwood Bank was underlain by a sedimentary basin up to 8 km deep. Later interpretation of sparse seismic reflection profiles from this area (Bianchi and Barbitta, 1982) and from the North Scotia Ridge to the east (Ludwig and Rabinowitz, 1980) pointed to the development of a thick sedimentary sequence in an asymmetrical basin, with extensive folding and thrusting at the northern margin of the ridge. The Burdwood Bank was interpreted by these workers as a collision complex or accretionary prism formed as a result of oblique convergence of the North Scotia Ridge towards the Falkland Plateau. Further west, an asymmetrical half-graben geometry similar to that of the South Falkland Basin was likewise inferred for the eastern part of the Magallanes Basin by Lesta et al. (1980). Structural complexities in the south were attributed by these workers to diapiric movements. Complex, north-vergent thrust geometries similar to those observed from the northern margin of the Burdwood Bank were described by AlvarezMarron et al. (1993) and Klepeis (1994) from the Magallanes foreland thrust and fold belt of Tierra de1 Fuego, about 800 km to the west along the same plate boundary (Figure I). Field studies in that area that northward directed contractional indicate deformation during the Oligo-Miocene resulted in the thrusting of Jurassic and Cretaceous volcaniclastic and elastic rocks over Tertiary siliciclastic deposits of the Magallanes Basin to the north. The new data suggest that the timing and style of Mesozoic to Tertiary deposition and tectonic events are similar south of the islands in the study area. The section shown in Plate 2 reveals a compressive boundary between the South Falkland Basin and the Burdwood Bank to the south. Strong subsidence and Tertiary sedimentation in the basin axis may be correlated with loading of stacked thrusts onto an extensional foreland to the north of the Burdwood Bank. The prominent unconformity at the base of the Mesozoic shows a much more regular surface south of the islands than to the west, suggesting that the Serie Tobifera volcanic rocks may be absent here. The structure and tectonic evolution of South Falkland Basin/Burdwood Bank area may be compared

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Structure

of the southern

Falkland

Islands

continental

shelf:

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S 0

1

2

ST

T3

Sl-WTT

4

5

6

Figure 4 Seismic

data panel for the eastern

part of the Malvinas

with other ancient and recent compressional plate boundaries. Thrust loading at the onset of orogenesis causes thickening of the crust and strong isostatic subsidence is thought to control foreland basin formation (Watts et al., 1982). During this process, thick piles of submarine fan deposits accumulate in the basin axis and onlap progressively and with regional unconformity onto the deposits of the extended continental margin (Ricci Luchi, 1986), as well as onto deformed strata at the prograding thrust front (Hinz et al., 1989). The current morphology of the South Falkland Basin and Burdwood Bank may be compared with that reached in the Alps during the PalaeoceneEocene, when thick turbidite deposits (‘flysch’) were laid down in a deep foreland trough developed on top of an extended Mesozoic shelf margin (Trtimpy, 1980). Progressive onlap towards the basin margins and erogenic uplift led eventually to the more widespread shallow marine and continental sedimentation of the Molasse (Homewood et al., 1986). Analogy with the Alps and other fold belts suggests that further south, away from the foreland, the thrust slices are likely to be composed of older rocks originating from deeper levels in the crust. We may also expect to find transported basement massifs and fragments of oceanic crust (ophiolites) caught up in the collision; these may correlate with the complex, unstratified thrust sheets recognized in the south (Figure 5). In the thrust belt of southern Tierra de1 Fuego to the west (Figure I), Cordillera Darwin is notable for the presence of well-developed compres-

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Basin

sional structures and metamorphic rocks of possible ophiolitic origin (Winslow, 1982; Klepeis, 1994). The small basin developed on top of the Burdwood Bank rides on a thick allochthonous thrust pile and is interpreted as a piggy-back basin (Ori and Friend, 1984). The age of the sedimentary fill is uncertain, but the strata1 geometries probably record relatively recent tilting, uplift and erosion of the basin margins, suggesting that this succession is of Tertiary to Quaternary age. The prominent reflector present beneath the water bottom over parts of the Burdwood Bank shows typical characteristics of a bottom simulating reflector (BSR), thought to result from the presence of natural gas hydrates solidified beneath the seafloor under low temperature, high pressure conditions (Kvenvolden and Bernard, 1983). The reflector is caused by changes in seismic phase across the base of the gas hydrate zone. Falkland Plateau Basin Water depths in this area increase progressively from less than 200 m at the edge of the Falkland Platform to over 2000 m at the offshore limit of the study area. The thick sedimentary packages present to the south of the islands may be traced eastwards into the Falkland Plateau Basin. The succession thickens away from the islands, reaching time thicknesses of more than 5 s in the deep water. An intepreted section (Plate 3) and a seismic data

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iin 0

h

t

N

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Structure

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N 1

2

3

sTwTr4

5

6

7

Figure 6 Seismic

data panel for the Falkland

Plateau Basin

panel (Figure 6) across this area reveal the presence in deep water of a complex, probably late Mesozoic to Tertiary, succession of onlapping sedimentary sequences interpreted as submarine fan and prograding lowstand wedge deposits. High amplitude events recognised at TW’IT of 5-6 s pass laterally into complex ‘winged’ structures. The entire succession thins and onlaps westwards onto a platform area around the islands. The margin of this platform is defined by a series of oblique faults or complex fault terraces. Nearshore, the sedimentary succession rests unconformably on a faulted unconformity surface underlain by older rocks of probable Palaeozoic or older Mesozoic age. Discussion. Thick sedimentary packages were reported from the Falkland Plateau Basin by Ludwig et al, (1978) on the basis of sparse seismic data. The new data allow a more accurate assessment of the basin revealing the accumulation of great geometry, sedimentary thicknesses downdip of a complex faulted platform margin. The structure of this area shows similarities with offshore West Africa (Teisserenc and Villemin, 1990) and Brazil (Mohriak et al., 1989), but shows less evidence of carbonate rocks in the Mesozoic succession and only localized indications of salt. This suggests closer analogy with the continental margin south of the Walvis Ridge in offshore southern Africa (Gerrard and Smith, 1983; Maslanyj et al., 1992; Malan, 1993). The high amplitude events may record the presence

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of intra-sedimentary volcanic rocks, whereas the winged features probably represent intrusive sills. The presence of intrusive and extrusive rocks within the sedimentary section may record larger scale rifting processes associated with the break-up of Gondwana and the opening of the South Atlantic (Taylor and Shaw, 1989). Although it is tempting to correlate these features with the Late Jurassic Serie Tobifera volcanic rocks present in the Malvinas Basin to the west, Jurassic igneous rocks are absent from East Falkland (Taylor and Shaw, 1989) and from the Falkland Plateau (Thompson, 1977). Thus the sills may be associated with younger Cretaceous or Tertiary rifting in the South Atlantic region. Strata1 geometries indicate that the thick sedimentary pile developed in the offshore areas comprises a complex series of submarine fan deposits. A prominent lowstand prograding wedge complex and possible lowstand basin floor fans are marked on Figure 6.

Maturity Although the absence of wells in the study area hinders accurate thickness determinations, it appears likely from the seismic data that burial of JurassicCretaceous source horizons was sufficient for hydrocarbon generation throughout much of the South Falkland and Falkland Plateau basins. Nevertheless, this is in contrast with the results from DSDP drilling on the Falkland Plateau, where the sequence is thinner and Cretaceous source horizons were reported to be

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Structure of the southern Falkland Islands continental shelf: N. H. Platt and P. R. Philip thermally immature (Comer and Littlejohn, 1977). To the south-west of the islands, the progressive thickness increase offshore suggests that Mesozoic source rock lithologies would also be mature for hydrocarbon generation in a ‘kitchen’ area in the deeper areas of the Malvinas Basin towards and beyond the margins of the Falkland Islands Designated Area. There may be significant potential for updip migration into the stratigraphic and structural traps outlined here. The reported interbedding of source and reservoir horizons within the Springhill Formation also provides a mechanism for charging unconformity associated plays in this area. Palaeozoic source rocks are likely to be thermally overmature offshore, reflecting both the depth of burial and the anticipated high Mesozoic thermal gradients associated with widespread Late Jurassic volcanism. Nevertheless, Permian source rocks onshore on West Falkland show low maturities, as discussed by Marshall (1994b).

Trap types

4.

5.

6.

Several key exploration targets are evident from the seismic data. 1. Rotated fault blocks and open anticlinal folds beneath the regional unconformity in the eastern part of the Malvinas Basin. 2. Fault block and pinch-out plays in the Cretaceous sequence throughout the area. 3. Thrust plays south of the islands in the deformed area to the north of the Burdwood Bank. Subthrust plays may also be developed, although the great depth of burial beneath the thrust stack may result in overmaturity of source rock horizons. 4. Stratigraphic traps where submarine fan sandstones pass laterally into interbedded shales. These traps are most common within the thick continental margin sequence of the Falkland Plateau Basin, but submarine fan deposits are also anticipated in the deep foreland basin north of the Burdwood Bank and downlapping fan-like features are locally present draping the ?top basement unconformity in the eastern Malvinas Basin.

7.

Conclusion 1.

2. 3.

Newly acquired seismic, gravity and magnetic data reveal the regional structure of the continental shelf south of the Falkland Islands and document the geometry of three linked sub-basins: the eastern part of the Malvinas Basin; the South Falkland Basin; and the Falkland Plateau Basin. Thicknesses increase offshore from the islands, particularly to the south and east. The eastern part of the Malvinas Basin lies in the south-western part of the study area and is linked to the western Malvinas and Magallanes basins of onshore and offshore Argentina, already the targets of significant hydrocarbon exploration. Sedimentary packages are thought to be similar in character and facies to those in the depocentre of the Malvinas Basin to the south-west of the study area. Well penetrations of the Mesozoic succession in the Malvinas Basin and on the Falkland Plateau

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east of the islands indicate the presence of regionally extensive reservoirs and mature hydrocarbon source horizons overlying a folded and faulted sequence of Palaeozoic sedimentary rocks and Late Jurassic volcanic rocks. The South Falkland Basin lies to the south of the islands and is characterized by a thick sedimentary sequence thickening to the south. Southwarddipping extensional fault blocks are well developed in this area. The South Falkland Basin is over-ridden from the south by a prograding thrust pile at the northern margin of the Burdwood Bank. This marks a major compressional plate boundary linked to the west with the southern Andes foreland fold and thrust belt in the Magallanes Basin of southern South America and Tierra de1 Fuego and to the east with the North Scotia Ridge. In the study area, thrust loading of the South Falkland Basin on the extensional foreland has led to the development of a Tertiary-Quaternary foreland basin in advance of the thrust front. Complex faulting at the margin of the Falkland Platform is associated with a rapid increase in thickness into the Falkland Plateau Basin to the east and south-east of the islands. The thick sedimentary succession of the Falkland Plateau Basin appears to be dominated by prograding submarine fan systems derived from the west. High amplitude events deep within the succession record the presence of igneous sills and dykes, and volcanic rocks may also be developed. The range of structural styles indicates a variety of hydrocarbon trap types: (a) stratigraphic traps at major unconformities (e.g. Upper Jurassic-Lower Cretaceous in the eastern part of the Malvinas Basin) and within Tertiary submarine fan deposits (e.g. Falkland Plateau Basin and South Falkland Basin); (b) structural traps associated with extensional fault blocks (pre-Late Jurassic in the eastern Malvinas Basin; Jurassic-Cretaceous in the South Falkland Basin and at the faulted western margin of the Falkland Plateau Basin); and (c) structural traps associated with compressional structures (thrust plays at the nothern margin of the Burdwood Bank; possible sub-thrust plays in the South Falkland Basin succession beneath). The structural heterogeneity of this area underlines many similarities with the basins of southern Argentina, an established hydrocarbon province, and also serves to indicate significant potential for the development of new exploration plays in this vast and currently underexplored area of continental shelf.

Acknowledgements We thank Schlumberger Geco-Prakla for permission to publish, and are grateful to the Falkland Islands Government and the British Geological Survey for their continued co-operation. Thanks also go to the referees, to A. McGrandle (ARK Geophysics), to G. Grant, J. Chatterton, R. Grice and T. McBrown, and to all our colleagues involved in survey design, acquisition and processing.

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