Dingle , R. V.

Gentle, R. I.

Gerrard, I.

Simpson, E. S. W.

OceanRep

The continental shelf between Cape Town and Cape Agulhas R. V. Dingle, I. Gerrard, R. I. Gentle and E. S. W . Simpson Repriilted from ICSU/ SCOR Working Party 31 Symposium, Cambridge 1971: The Geology of the East Atlantic Conti-nental Margin. Edited by F. M. Delany, 1971. Institute of Geological Sciences Report No.?0/ 16, pp.l99-209 CONTENTS INTRODUCTION BATHYMETRY AND GENERAL REMARKS GEOLOGICAL RESULTS Basement Mesozoic Caenozoic CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES ILLUSTRATIONS Fig. 1. Bathymetry and survey tracks Fig. 2. Outline geological map Fig. 3. Sections across the Agulhas Arch Fig. 4. Sections across the continental shelf between Cape Town and Cape Agulhas 201 Page 203 203 203 203 206 206 206 209 209 204 205 207 208 SUMl'viARY Continuous seismic reflection profiles across the continental shelf between Cape Town and Cape Agulhas reveal large sub-bottom basement features formed of Lower Palaeozoic and older rocks partially over lain by Mes ozoic (Cretaceous) and Caenozoic sediments. One of these bas ement features, the Agulhas Arch, has been traced for over 70 miles (130 km) offshore and forms an elongated upwarp around which the younger sediments lap. During Cretaceous and Tertiary times the continental shelf between Cape Town and Cape Agulhas was built strongly upwards and outwards, although sedimentation was interrupted in late Cretaceous/ early Tertiary t imes by a period of tilting and, locally, folding and igneous intrusion. Quaternary sediments blanket most of the middle and outer shelf, but are only locally of moderate thickness. SOMMAIRE Des profile continua obtenus par la sismique reflexion a travers la plate-forme continentale entre Le Cap et le Cap des Aiguilles montrent !'existence de grands traits au sousbassement formes de roches du Paleozo1que Inferieur et de roches plus anciennes surmont6es en partie par des sediments Mesozo1ques (Cretace s) et cenozo1ques . Un de ces traits du socle, la voute des Aiguilles, a ete trace plus de 130 km au large et il forme un ploiement prolonge autour duquel se trouvent les sediments plus recents. Pendant le Cretace et le Tertiaire la plate-forme continentale entre Le Cap et le Cap des Aiguilles a developpe fortement en montant et vers l'exterieur. Cependant la s€dimentation 6tait interrompue pendant le Cretace posterieur et le Tertiare anterieur par une periode de rei"evement et localement, de plissement et d'intr usion ignee. Des sediments quaternaires couvrent la plupart de la plate-forme centrale et exterieure, mais ils n'atteignent que par endroits a une epaisseur moyenne. Z USAMMENFASSUNG Ununterbrochene seismische Reflexionsprofile queriiber dem Kontinentalschelf zwischen Kapstadt und dem Nadelkap zeigen grosse aus Unterpaliiozoischen und ii.lteren Gesteinen gebildeten unterbodenbasische Eigentiimlichkeiten, die teilweise von mesozoischen (kreidigen) und kiinozoischen Sedimenten iiberlagert sind. Man hat eine dieser basischen Eigentiimlichkeiten, die Nadelbeulung, in der Kiistenniihe, mehr als 130 km, entdeckt. Diese Beulung bildet eine verlangerte Beule, und urn dieser Beule legen die jiingeren Sedimente. Wahrend der Kreidezeit und des Tertiiirs wurde der Kontinentalschelf zwischen Kapstadt und dem Nadelkap kriiftig aufwarts und auswiirts gebildet, obgleich eine Kippungszeit und eine ortliche Faltungs- und Vulkanisch-intrusion in der Spiitkreidezeit/ dem FriihtertHir Sedimentation unterbrochen haben. Quartiirsedimente deckem den meisten Schelf, aber sind nur ortlich von mittler Dickheit. 202 The continental shelf between Cape Town and Cape Agulhas R. V. DINGLE, I. GERRARD, R. I. GENTIE 2nd E. S. W. SIMPSON I TRODUCTIO. The seismic survey upon which this paper is based was carried out in June 1969 as part of the current investigation of the continental margin around southern Africa . The equip-ment used was an E. G. & G. Sparkarray sys-tem with a power output of 1000 joules. Seismic traverses shown in Fig. 1 are superimposed upon a bathymetric chart of the western Agulhas Bank compiled during the course of earlier bathymetric and bottom-sampling work. Near-shore, bottom samples have provided invaluable checks on the interpretation of the seismic records, particularly with the identification of Cretaceous and Tertiary horizons which have largely been extrapolated from records obtained east of Cape Agulhas (see Dingle, in press, a). Isochrons measured in milliseconds one-way-time from a sea level datum are used to repre-sent basement structures in Fig. 2. BATIIY\IETRY A~D GENERAL RE\IARKS Although the general form of the continental margin around South Africa has been discussed by Simpson (1966). no detailed bathymetric studies of the shelf between Cape Town and Cape Agulhas have been previously published. The bathymetry of the eastern part of the upper Agulhas Bank has been discussed by Dingle (in press, a). The most striking morphological variation that is apparent on moving from the eastern to the western part of the Agulhas Bank is an increase in the depth of the shelf break from 110-160 m east of longitude 20 ° 30'E to about 270 m in the south-east of the area dis-cussed in this paper, on the western side of the Bank (Fig.1). This change occurs over a structural divide, called here the Agulhas /lrch, which is discussed in more detail below. Moving north-westwards, the depth of the shelf break remains in the region 270-280 m to the vicinity of Cape Point, where it is anomalously shallow (200 m), poorly defined, and the shelf itself narrows from between 25-30 miles (46-55 km) to about 12 miles (22 km). North-west of this area a further deepening of the shelf takes place and west of the Cape Peninsula it occurs at about 350- 380 m. Although not fully understood, these variations in depth of shelf break, which are accompanied by fluctuations in the width of the continental shelf, can be correlated with distinct changes- in the gross sub-bottom geological structure, and in par-203 ticular with the form of the pre-Mesozoic base-ment morphology. The morphology of the shelf itself is relatively featureless, due to a fairly extensive Quater-nary sediment cover, except for a continuous nearshore rocky zone and a large shoal area extending south- south- east of Cape Agulhas over the Agulhas Arch. GEOLOGICAL RESULTS On the seismic records three main rock sequences can be distinguished (Figs. 3 and 4). which are separated by major unconformities. These are the basement, Mesozoic, and Caeno-zoic sequences, and the distribution and struc-tures of these will be dealt with in turn. Basement 'Basement' is used here to denote rocks of pre-Mesozoic age. Onshore, the basement rocks consist of Palaeozoic Cape Supergroup quartzites (Table Mountain Group) and shales (Bokkeveld Group), with areally less important outcrops of pre-Cape granites and low-grade metamorphics (Malmesbury Group). These rocks are strongly folded and faulted in the Cape Fold Belt, which displays a virgation zone just east of Danger Point; north-east trends north-west of Danger Point and east to south-east trends to the east. Offshore continuations of individual structures are poorly known, but off Danger Point and Struys Point prominent bathy-metric and sub-bottom basement ridges appear to indicate the seaward extension of onshore fold structures. By plotting isochrons to the base-ment, three major features have been delineated in the sub-Mesozoic topography between Cape Town and Cape Agulhas (Fig. 2): 1. the Cape Peninsula basement feature (Cape Town to Rocky Bank); 2. Rocky Bank to Cape Agulhas basement zone; and 3. the Agulhas Arch. Of these, the latter is the most clearly defined and consists of a north-west trending elongated upwarp of basement rocks lying to the south-south-east of Cape Agulhas . The exact configu-ration is not known, but data suggest that in the south-east it is a roughly symmetrical feature on either side of which the sub-Mesozoic base-ment plunges relatively steeply beneath t he overlying sediments. A median bathymet ric (and possibly also structural) low lies along the axis of the Arch between two nanking ridges. Towards the north-western end the cross section (\) 0 ~ 0 Continent II .,.u ,. ••• •• 'j' , .. r I n~utlcal ~~~~~ I ~0 IGil st 1 .... n· If lt' -- - Extent of baaement outcrop Survey linea A • Section deptlla In met rea ,. 21° 2 Fig. 1. Bathymetry and survey tracks. Depths in metres. Sections A-B to 0-P are shown in Figs. 3 and 4. Inset shows location of area studied. u: , .. Sf. "' 0 (]l Tertiary (depth of outer boundary in metres) Tertiary volcan ics Cretaceous Basement, mainly sediments Basement: granite ~ .......... Outcrop or suboutcrop of Cretaceous inner margin _1., _ Isochrons to basement in milliseconds one-way-time - ·-·- Shelf break, depths in metres ' 0 naut ical m iles ~0 I I I I I Km SO .· Fig. 2 . Outline geologic al map. Datum for isochrons to the basement is sea level. is markedly asymmetrical with a steeply plung-ing northern flank and a more gently shelving south-western flank (Fig. 3). Between Cape Agulhas and Rocky Bank the basement forms a wide ledge (Fig. 4, sections I-J and K-L), with local, small, high features such as the extension of the Danger Point folds . Beyond about the 100 msec isochron the base-ment plunges steeply beneath the younger sedi-ments, this 'basement shelf break' being characteristically accompanied, on the landward side, by a small high feature. North-west of Rocky Bank (which is a large basement boss detached from the Cape Penin-sula by a steep depression), along the western side of the Cape Peninsula feature, the base-ment ledge is narrow and it plunges to seaward very steeply beneath the overlying sediments (Fig. 4, sections M-N and 0-P). Topographi-cally, it is quite irregular, with a steep east-west valley-like depression crossing it about 10 miles (6 km) north-west of Cape Point. The steepness of the western edge of what is essen-tially an unfolded basement feature suggests the possibility of north-west trending fault control. \lesozoic The Mesozoic and Caenozoic sediments over-lie the basement with a strong unconformity, and except on the northern side of the Agulhas Arch and for two small patches west of the Cape Peninsula, the Caenozoic oversteps the Mesozoic rocks and rests directly on basement. No Mesozoic samples have been collected from the area under discussion, but east of the Agulhas Arch bottom samples suggesting a relatively complete Neocomian to Maastrichtian succession have been recovered (Dingle, in press, b). No post-basement rocks older than Cretaceous have so far been found and none are known to occur on land in the adjacent coastal area. In view of this, the Mesozoic succession is assumed to be Cretaceous in age although the possible occurrence of a Jurassic or even Triassic marine succession cannot be ruled out. In all places west of the Agulhas Arch the Cre-taceous rocks come in below the 'basement shelf break' and dip relatively steeply seawards (at about 2°). Maximum thicknesses for the Cre-taceous succession are not known, as the basement reflector has not been traced far beyond the landward edge of the Cretaceous, but about 200 msec (one-way-time) have been measured west of the Cape Peninsula and much greater thicknesses can be assumed by extra-polation (Ludwig, 1968). The Cretaceous rocks outcrop beneath the Caenozoic sediments on the upper continental slope where the junction is accompanied by a small bathymetric nick. The depth of this junction appears to increase some-what north-westwards (see Fig. 2). Lithologi-cally, offshore Cretaceous rocks east of the 206 Agulhas Arch consist of marine clays with varying amounts of glauconite, pyrite, and carbonaceous matter (Dingle, in press, b). Caenoz01c Caenozoic rocks overlie the Cretaceous with strong unconformity. Differentiation into Ter-tiary and Quaternary series is not always clear cut, particularly in the region between Rocky Bank and Cape Agulhas where the two sequences have little or no angular discordance. The Tertiary rocks dip seaward at low angles (1 o or less), dips in the north of the area (west of the Cape Peninsula) tending to be somewhat greater than those obtaining in the south. The maximum thickness of Tertiary rocks found so far in the area under discussion is about 130 msec (one-way-time) west of Danger Point, and although the average thickness of the succession is not much less than this value, the Tertiary rocks south-west of Rocky Bank are anomalously thin (less than 50 msec). West of the Cape Peninsula an irregular, thick prograding lens of Quaternary sediment discordantly overlies the Tertiary section at the foot of the basement outcrop (Fig. 4, sec-tions M-N and 0 - P), and south-west of Cape Agulhas a flat-lying wedge of Quaternary sedi-ment progrades to within 6-8 miles (11-14 km) of the shelf break (Fig. 3, section E-F, and Fig. 4, section 1-J). Between these two areas the Tertiary rocks are overlain by Quaternary sediments but because of their flat attitude and lack of bathymetric expression they cannot be resolved on the seismic records. Only Upper Miocene bryozoan/ foraminiferal limestone has been collected so far from the Tertiary west of the Agulhas Arch (Siesser, in press), but east of it Palaeocene to Miocene marls and limestones have been recovered (Dingle, in press, a). On the eastern side of the Agulhas Arch, roughly between the Alphard Banks and the basement outcrop , a large number of small early Tertiary intrusive plugs of aegirine-trachyte pierce the folded Cretaceous rocks, which are here devoid of Tertiary sediment cover (Dingle, in press, a). These intrusions, dated at 58 t 2 million years generally do not reach the sea floor except as insignificant mounds, but in the immediate vicinity of the Alphard Banks they reach to .within 20 m of the sea surface as a cluster of slender pin-nacles. CONCLUSIONS Until ages can be ascribed to the various horizons within the major rock sequences identified, the geological history of this part of the continental shelf can be discussed in general terms only. Comparison with the tl.) 0 ...:I D C ~I ... 0'""-•t ,. ..:!-- - .=--- -~~-7-_:~f~=::~=~--=.;..~;~!'~~~ ----- ---- -;::::::-~ ... ;;:::> ~..-~,.·_;.../"' ~--- .... ~ .. :-::,..;"' ,... -· - l l rl.; ' _.,......,.. ....tt:llll • & , (c) .... -.......---=::::. ........__ -:::::: ~:_ "-' ''---·---=-.._ .... ~,---~ . ~- -»9 Fig. 3. Sections across the Agulhas Arch. Vertical scale in seconds (one-way-time). (a). southern edge (note Tertiary overlying Cretaceous on the extreme left). (b). northern edge. (c. d), profiles E-F. G-H are essentially a continuous south-west to north-east section across the western part of the Agulhas Arch. A prograding Recent sediment wedge reaches to within 8 nautical miles of the shelf break. It produces a small but distinctive rise in the sea fioor landward of the slightly uneven Tertiary exposure. but the boundary with the underlying Tertiary cannot be confidently resolved. Note folding and faulting (with intrusives?) of Cretaceous on north-eastern flank and the north-south asymmetry of the Arch. (\.) 0 00 p,.. a.c ~~ 'I ._o .,J -·-.:;; --~ .. ., .z , --., lt -· r~ .... depth ..1 / -(a) •sJ •2.i I 1,1 s ' autlc&J ,;u., n 10 0 basement K o,c L I j I ~ L •., ---- -.:::---_. ... "'1:: .. -·-... -:::---- ---·------- ---- ~--... ;1"' 1o-- :-::::...---_ _I / -· - (c) - -· ·-· Fig. 4. Sections across the continental shelf between Cape Town and Cape Agulhas. Vertical scale in seconds (one-way-time). (a), section west of the Cape Peninsula. The basement outcrop is narrow and descends steeply beneath younger sediments. A thick Recent sediment wedge progrades sea wards from the foot of the basement outcrop. (b), section west of Rocky Bank. The shelf is anomalously narrow with steeply dipping Cretaceous and Tertiary sediments and a thick Recent wedge at the foot of the basement outcrop. (c), section west of Danger Point. The basement is a wide ledge beneath the thin Caenozoic cover. Recent sediments are not readily differentiated. (d), section west of Cape Agulhas. This is similar to (c). The prograding Recent sediments reach to within 6. 5 nautical miles of shelf break, and are accompanied by a slight bathymetric feature (cf. profile E-F, Fig. 3). area east of Cape Agulhas {described by Dingle, in press, a) is helpful. The Cape Supergroup sedimentary succession was folded during the early Mesozoic and then subjected to a prolonged period of erosion. Although marine Jurassic sediments occur in Madagascar the earliest apparent post-orogenic sediments were laid down along the southern Cape coastal area as a transgressive sequence in late Mesozoic {Lower Cretaceous) times and rapidly filled deep coastal intermontane basins with terrestrial, estuarine and marine deposits . During the Cretaceous the continental shelf was built strongly upwards and outwards, but sedi-mentation must have ceased in late Cretaceous or earliest Tertiary times and the sediment pile tilted seawards with folding in some areas. This was possibly accompanied by uplift and erosion, and at least in the area north-east of the Agulhas Arch, tectonic activity, followed or accompanied by igneous intrusion, took place. These events were possibly contem-poraneous with the Laramide movements in other parts of the world. Following these earth movements, early Tertiary seas transgressed over the western Agulhas Bank, which slowly subsided. During this phase of sedimentation the Agulhas Arch was tilted downwards on its western flank, where at least 100 msec of sediment accumu-lated, whilst remaining stable on its north-eastern flank where no thickness of Caenozoic sediments can be detected. Sample evidence suggests that little or no sedimentation occurred between Upper Miocene and Quaternary times and that during the Pleistocene sedimentation recommenced with the vigorous erosion of soft sediments exposed on the upper shelf. ACK~Ol\'LF.OGBIE, TS This work was undertaken as part of the South African national programme of oceanographic research with financial support from the National Committee {SANCOR). The Director, Geological Survey of South Africa is thanked for allowing use to be made of the seismic equipment and the officers and crew of the University of Cape Town research vessel 'Thomas B. Davie' are thanked for their co-operation during the work at sea. REFERENCES DINGLE, R. V. [In press, a.] Preliminary geological map of part of the eastern Agulhas Bank, South African continental margin. Proc. geol. Soc. [In press, b.] Some Cretaceous ostracoda! assemblages from the Agulhas Bank, South African continental margin. Trans. R. Soc. S. Afr. Dd 5006ll 10f71 209 LUDWIG, W. J., NAFE, J. E., SIMPSON, E. S. W. and SACKS, S. 1968. Seismic refraction measurements on the southeast African continental margin. Jnl geophys. Res., Vol. 73, pp. 3707-3719. SIESSER, W. G. [In press.] Carbonate: com-ponents and mineralogy of the South African coastal limestones and limestones of the Agulhas Bank. Trans. geol. Soc. S. Afr. SIMPSON, E. S . W. 1966. Die geologie van die vastelandsplat. Tegnikon, Vol. 15, pp. 168-176. R. V. Dingle, R.I. Gentle and E. S. W. Simpson, SANCOR Marine Geology Group, Department of Geology, University of Cape Town, Rondebosch, South Africa. I. Gerrard, Department of Mines {Geological Survey), Marine Geophysical Unit. Dd. ~02193 K 12 Printed In Encland for Her M•I-Y'• StotloneryOIII.-by Commercial Colour Pr-

The continental shelf between Cape Town and Cape Agulhas

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