Theanatomy of a large submarine slump on a sheared continental margin (SE )

R.V. DINGLE

SUMMARY The morphology and structure of a large crust, whereas in the east, allochthonous submarine slump (Agulhas Slump) on the material has spread into the oceanic Transkei sheared continental margin off SE Africa are Basin. Characteristics related to a structural described from bathymetric and continuous setting on a sheared continental margin are seismic reflection records. It is a composite emphasized and discussed. The Agulhas Slump feature consisting of proximal and distal alloch- is probably the largest slumped mass so far thonous sediment masses separated by a large recognized from modem oceans (75o km long, glide plane scar. The locations of the various Io6 km wide, with a volume of over 2o,ooo structural elements of the slump are related to km 3) and is post- in age. A seismic underlying features: in the head region these triggering mechanism is tentatively proposed: are controlled by large-scale and the slump lies on two major fault zones whose Palaeogene depositional features, and in the extensions are known to be seismically active toe region by older basement ridges. In the (the and the Agulhas marginal western part of the slump, the basement ridges ). have dammed the slump over continental

SEDIMENTARY SEQUENCES on continental margins frequently include horizons at which anomalously rapid removal or accumulation of material appears to have taken place, and there is increasing evidence that long-term episodic, or 'catas- trophic' events play a major role in the construction and modification of outer continental margins (i.e. the continental slopes and rises) (e.g. Emery & Uchupi I972 , Ryan et al. i976 ). In recognizing that one such mechanism is the emplace- ment of large allochthonous masses of sediment by rapid downslope movement, marine geoscientists are complementing a large body of well-documented evidence from older terrains (e.g. Rupke 1976). Although it is generally not possible to make a very detailed in situ study of the structure and petrography of these relatively modern features, geophysical methods do allow a better appreciation of their overall anatomy than is often possible in their uplifted, metamorphosed and in- differently exposed ancient counterparts. An understanding of the nature of large-scale allochthonous masses and their tectonic settings on present-day con- tinental margins is important, therefore, not only for marine geoscientists, but also for stratigraphers and structural geologists. In his review of the distribution of large-scale submarine slumps on continental slopes, Moore (in press) distinguished two contrasting plate tectonic settings : (a) mid-plate, passive margins; (b) converging plate edge, active margins. Here attention is confined to the former category, within which descriptions of slump- ing on a regional scale (i.e. over hundreds of km) have so far been restricted to

aTlgeol. Soc. Lond. vol. z$4, i977, pp. 293-3io, 7 figs., x table. Printed in Great Britain.

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passive margins of the rifted type, e.g. NE America (Heezen & Drake 1964, Uchupi I967, Emery & Uchupi i97~); eastern Rockall Bank (Roberts I972);~and NW

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F I o. I. Bathymetry (in metres) and bathymetric profiles (in kilometres) on the SE Agulhas Bank. Insert shows location of area; ,~r~z, Agulhas marginal fracture zone. Arrows on profiles show limits of slump.

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Africa (Seibold & Hinz I974, Ryan et al. I976 ). The present paper will examine a different group within category (a) by describing the effects of large-scale slump- ing on the morphology and structure of near-surface sediments of a sheared continental margin, and will point out several possibly unique features which can be attributed to mass downslope movement in this particular tectonic setting. The area of study lies along the SE edge of the Agulhas Bank, , and has been surveyed extensively using a 12 kHz echosounder, and sparker and air- gun reflection seismic systems with a variety of navigational techniques (celestial, Decca Navigator and OMEGA) (Figs. I, 3). All conversions of travel time to thick- ness insediments are based on an assumed P-wave velocity of 1.8 km/s. Travel times are quoted in seconds double time (DT).

• ., I. Geological and bathymetric setting of the SE Agulhas Bank The continental margin off SE Africa consists of the wide, triangular-shaped Agulhas Bank in the S, and a narrow sector between Port Elizabeth and Durban (Fig, I;). Its remarkably straight outer edge is thought to have been formed by shearing motion along a large marginal offset during the separation of the Falkland Plateau from Africa (Francheteau & Le Pichon 1972, Scrutton 1973). On the SE side of the Agulhas Bank this offset truncates deep intra-continental sediment basins and basement ridges, and the continent/ocean boundary coincides with the outer edge of a large, non-magnetic basement ridge, the Agulhas Marginal Fracture Ridge (MFR) (Scrutton & du Plessis i972 ) (see Figs. 3, 7). This ridge has been identified on seismic profiles between 2203o'E and 26°E, N of which it disappears beneath sediment cover, although the large positive magnetic anomaly which lies over its outer edge continues at the foot of the continental slope as far north as 3o°S (in the vicinity of Durban). Behind the MFR, and between the two pre-Mesozoic basement highs which flank the Agulhas Bank (Agulhas and Port Alfred arches, Fig. 3), Mesozoic/Cenozoic sediments locally exceed 3 km in thick- ness. Seaward of the MFR, the sea floor falls precipitously to >4500 m in the Transkei Basin, where, because the ridge has acted as an efficient sediment dam, only a small continental rise prism has locally developed. N of 26°3o'E, the is narrow and shallow, with a well-defined shelf break at about I oo m, and a steep, locally rugged, continental slope (Fig. x). SW of East London, the shelf widens rapidly to form the Agulhas Bank, and W of Port Elizabeth, the abrupt shelf break, which lies between I2O m and I8O m, swings W and then S, forming a concave salient towards the coast. Between the shelf break and the top of the MFR (at approximately 2500 m), the middle and upper continental slope forms a seaward dipping platform with a maximum width of about I IO km (Fig. i, profile D). At both ends of the MFR, the continental slope drops directly from the shelf break to,the deep sea floor (> 4000 m), although locally there are smallrugged clefts and ridges that cut obliquely across the main bathymetric trends (Fig. I).~

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2. The Agulhas Slump (A) RECOGNITION OF THE SLUMP STRUCTURE Fig. 2 is a summary of the main components of submarine slumps that can be readily recognized on seismic and bathymetric records. It has been compiled from Lewis (t97t, fig. 2), Roberts (x972 , fig. 6) and Moore et al. (I976 , fig. 5) and is very similar to typical sections through landslips (e.g. Carrara & Merenda I976 ). The recognition of all the features illustrated has allowed us confidently to identify the structures discussed in this paper as having been formed by large-scale sub- marine slumping. The following features are considered particularly diagnostic: (i) Fissured z0ne--a zone of disturbed bedding (caused by numerous small tensional faults) which gives rise to a characteristically stepped or pitted sea floor upslope of the main glide plane. (ii) Glide plane scar (the 'head of the slump' of Lewis z 97 z)ma steep scarp which marks the outcrop of the major glide plane where the slumped mass has moved vertically and horizontally away from the under- lying strata. (iii) Tensional depression ('extension scar' of Roberts i972)--a prominent bathymetric notch which occurs at the upper end of the slumped mass immediately adjacent to the glide plane sear. (iv) Slumped mass--the detached body of sediment which is bounded on its under surface by the smooth, concave glide plane (d~ollement surface), and on its upper surface by the relatively smooth convex outline of the sea floor. Although the glide plane is often parallel or sub-parallel to the bedding of the stable underlying sediments in the

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F zo. 2. Main morphological and structural features of submarine slumps• Based on: Lewis (t97x), Roberts (I972), Moore et al. (x976), and Carrara & Merenda (z976). Schematic and not to scale.

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proximal part of the slumped mass, it characteristically truncates underlying bedding planes in its middle and lower sectors. The toe of the slumped mass occasionally shows evidence of compressional features (folding), although on the Agulhas Bank the phenomena are not as prominent or frequent as described by Lewis (1971) from the Kidnappers Slump, New Zealand. Within the slumped masses small, subsidiary rotational glide planes are commonly observed and bedding is often well preserved, although it is frequently tilted and dislocated, so that bedding planes outcrop on the sea floor. Small-scale folding is locally common.

The structures mentioned above can only be recognized where the slump has remained essentially intact. Where large-scale distension has taken place, the slumped mass becomes disjointed and in the toe region a mud flow facies may develop, which, under certain circumstances, becomes a turbidity current (Heezen & Drake 1964 .)

(B) GENERAL FEATURES A large submarine slump has been recognized and mapped in detail between 22°E and 27°E, whereas between 27°E and 3o°E structures which possess many of the characteristics of slumped masses, but for which conclusive evidence for confident identification of the various components is lacking, have been mapped in outline (mostly from bathymetric records). The whole structure (22°E to 3o°E) is referred to as the Agulhas Slump (Fig. 3), but for descriptive purposes it is subdivided into western and eastern sectors (about 26°E). Statistical details (Table I) suggest that the Agulhas Slump is one of the largest submarine slumps identified from modern oceans. The Agulhas Slump is a composite feature. It consists of a narrow proximal part which comprises the outer continental shelf, shelf break, and upper slope, and a wide distal part. W of 26°E the distal part, which constitutes the main slumped mass, is confined behind the MFR, where it forms the middle slope, but E of this point it spills over the continental edge into the Transkei Basin, where it forms the middle and lower slope and continental rise (Figs. 3, 7). BathymetricaUy, the proximal part is characterized by short, steep slopes and relatively rugged sea floor micro-relief, whereas the distal part is typically smooth or hummocky. At its W end, the Agulhas Slump lies along the E flank of the Agulhas Arch, where, because of intense fissuring, and the presence of numerous large glide plane scars, it is characterized by particularly rugged sea floor micro-relief. The Agulhas Arch is truncated by the marginal fracture zone and only a thin veneer of Mesozoic/ Cenozoic sediments lies on its crest. It has not been possible to define the E limits of the Agulhas Slump. E of 26°3o'E the distal part extends into the Transkei Basin and at least three 'facies' have been recognized, but their relationships are not clear. It is possible that one of them (apparently the youngest) is a separate slump feature which originated on the continental slope near East London (shown as 'undifferentiated' on Fig. 3).

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'Fzo. 3. Geological map of the Agulhas Slump. Insert Shows location of geophysical data; thick lines are illustrated profiles. ~, Agulhas Arch; P.~, Port Alfred Arch.

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(C) PROXIMAL PART OF THE AGULHAS SLUMP The proximal part generally lies between the 19 ° m and 700 m isobaths, and can be traced as a continuous feature between 2i°3o'E and 27°3o'E (600 km). It consists of a fissured zone and a slumped unit (Figs. 3, 4). The main glide plane beneath the slumped mass is usually prominent in any particular cross-section, but along the length of the Agulhas Slump there are numerous glide planes in the proximal part which are obviously not continuous features. They probably form an en echelon series, similar to that suggested by Roberts (1972) for the RockaU Slump. The fissured zone is widest (27 km) and most prominently developed W of 24°E (Fig. 4). It occurs at the outer edge of the continental shelf and principally affects the uppermost, horizontal or subhorizontal sediments that cap the Agulhas Bank (predominently lithified and semi-lithified Neogene limestones). Within the zone, the strata are cut by numerous normal faults which have small throws to the S. The larger faults have curved fault planes. Usually, there has been sufficient move- ment along individual faults to produce small topographic depressions, and the

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sea floor characteristically has a stepped or pitted appearance. Subsequent erosion has probably enhanced these lines of weakness and two possible agents are sug- gestedmscouring by the SE-flowing , and wave and current action during Pleistocene low sea level stands. It has not been possible to measure throws on individual faults, but there is a distinct (if somewhat erratic) increase in the maximum topographic depth of fissures (up to 25 m) on approaching the main glide plane. The greatest density of fissuring lies up to 21 km upslope from the glide plane. E of about 24°E, the fissured zone narrows (to < I o km) and the fissures are generally small (Fig. 3). Throughout most of the area, the main slumped unit in the proximal part lies immediately seaward of the shelf break (i.e. at between 3o0 m and 5o0 m), but E of 26°E it lies at progressively greater depths ( > I ooo m). A prominent tensional depression usually marks the upper edge of the proximal slump unit, and is accompanied by a steep, narrow glide plane scar (Fig. 4). In combination with the en echelon arrangement of the glide planes, this imparts a ragged outline to the outer shelf edge and shelf break. In contrast, the toe of the proximal slump unit, and the accompanying outcrop of the glide plane, are probably continuous and fairly regular. Between 22°E and 24°E, where it has a relatively simple structure, the proximal slumped unit has a maximum thickness of 0"33 s to o-36 s DT (298 m to 324 m). It is cut by numerous small glide planes and fissures, and usually has an irregu- lar, convex upper surface. Internal bedding is often well preserved, although many of the reflectors may represent subsidiary glide planes parallel to the basal glide plane. In this area, the slump has developed along bedding surfaces in a seaward prograding Lower Tertiary unit of clays, marls and siltstones, whereas the sub-horizontal capping of Neogene limestones is truncated by the structure (Fig. 4, profile 3). In two areas the proximal part of the Agulhas Slump is particularly complex (Figs. 3, 5). (i) Due south of Cape St. Francis, an elongate (80 kin), closed depression (St Francis depression) has been formed on the upper continental slope by subsidiary faulting within the proximal slump mass (Fig. 5a). The depression, which has a maximum width of 15 km and a maximum depth of Ioo m beneath the adjacent sea floor, is bounded along its southern edge by a rocky, horst-like ridge. The main glide plane scars, as well as subsidiary rotational faults, strike approximately E-W (I5 ° to the general shelf edge trend) and have developed in two seaward prograding Neogene sediment units that are only present on this part of the eastern Agulhas Bank (see Dingle I973). This probably accounts for the unusual trend and faulting patterns that were responsible for the formation of the St. Francis depression. A maximum thickness of 0.55 s DT (495 m) has been recorded in the proximal slump in this area. (ii) Between 26°E and 27°E, a narrow, deep (maximum 600 m below the adjacent sea floor) valley (Recife depression) is thought to mark the position of a wide glide plane scar that formed where a large, ridge-shaped sediment mass moved away from and down the upper continental slope (Fig. 5b). The scar can be traced eastward for about I OO km from the outer continental shelf at its western

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end, where it forms a narrow (2"2 km), deep (300 m) cleft cutting obliquely across the shelf trend, into water depths > 2ooo m. It then strikes parallel to the margin and dies out. Subsidiary faulting probably accounts for the irregular surface of the detached mass. Lateral movement has clearly been greatest (about z o km) in the central portion of the structure and to account for differential displacement the actual fault patterns must be more complicated than those shown in Fig. 5 b, which is based on only 5 crossings.

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A similar, apparently detached block occurs on the upper continental slope at about 33°S 28°E. Here a 20o m deep valley crosses the slope at about 3 °0 to the shelf trend, and is bounded on its southern side by a massive rocky ridge. There are no seismic profiles from this area, which cannot confidently be related to the general picture farther S. Between 21°3o'E and 25°3o'E (275 km), the main glide plane at the base of the proximal slump is deflected upwards, or horizontally, by a monoclinal or weakly anticlinal structure in the underlying Cretaceous strata (e.g. Fig. 4, profile 3)- The strike of this structure therefore controls the location of the proximal slump toe and accompanying lower glide plane outcrop, and its presence was responsible for the separation of the proximal and distal slumps, and for the formation of the extensive glide plane scar at the head of the distal slump.

(D) DISTAL PART OF THE AGULHAS SLUMP The distal glide plane scar is wide (up to I2 km), and apparently simple in outline (Figs. 3, 6). It is usually less steep than that at the head of the proximal slump and it has a less prominent bathymetric expression. On the flank of the Agulhas Arch it widens rapidly from a series of narrow, prominent scars into a gash which cuts across the bathymetric contours. Eastwards the scar becomes narrower (between 5 and 8 km), although locally it widens and steepens where it is deflected around the St. Francis and Recife depressions. At the E end of the latter structure the distal glide plane scar loses its identity, and the relationship between the proxi- mal and distal slump masses is not clear. They may not be separated E of 27°E. At two localities (centred at 24°Io'E and 25°3o'E) there appear to be narrow slivers of strata which did not become detached during the slumping episode. The toe of the proximal slump and the glide plane of the lower slump are, therefore, not in juxtaposition along these sectors. The distal slumped mass contains the bulk of the allochthonous material in the Agulhas Slump, and exhibits several changes in character about longitude 26°E (see Fig. 6). The W sector is confined behind the MFR and at its W end abruptly dies out where it is bounded by the ridge and the Agulhas Arch flank. It attains a maximum width of about 7 ° km at 23°3o'E, and narrows steadily eastward to about 3 ° km S of the St. Francis depression, where the marginal fracture ridge shifts N by about 25 km. The glide plane at the base of the slump is strongly con- cave, and throughout much of the W sector, is deflected upwards by an anticline in the underlying strata which lies about 20 km N of the MFR (e.g. Fig. 6, profiles 6, 7, 8). The strata forming the anticline are truncated by the glide plane. Flexur- ing and deformation in the slumped mass caused by movement over this structure inhibits the recognition of compressional features specifically associated with the toe of the Agulhas Slump. Although the upper surface of the slump is generally convex, it is locally hum- mocky and irregular. These irregularities are associated with zones of faulting, small-scale folding, and the presence of large subsidiary glide planes. Between h e Agulhas Arch flank and 23°E, much of the slump is distorted internally, and a s a broken surface. This is interpreted as the result of compression of the slumped ass as material attempted to move S and SE into a confined space behind the

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MFR. Bedding within the slump is generally well preserved, but is frequently strongly tilted and discontinuous. Towards the base of the slump, reflecting hori- zons are often parallel to the glide plane and probably represent tectonically in- duced surfaces. The average maximum sectional thickness is about o-6 s DT (54o m). E of 26°E, the distal slump overrides the MFR, which here lies at about 3ooo m and is about 5o km from the shelf break. Between this point and 27°E, the distal slump comprises two 'facies' (Fig. 3; Fig. 6, profiles 9, I o, I I). That on the con- tinental slope consists of large, broken masses which produce an irregular, asymme- trically 'blocky' sea floor (elevations of about 16o m). Bedding is often well preserved in these masses and it seems that the slump has broken up by distension; a process that was not possible W of 26°E. On the lower continental slope, the irregular sea floor topography becomes subdued and grades (over about 2o km) into a 'facies' which has no internal structure ('structureless facies') and gives rise to a very smooth sea floor. This facies is tentatively equated with the mud flow regime des- cribed by Moore et al. (I976) from the distal and lower sections of the Bassein Slump. The edge of the 'broken facies' lies at the base of the continental slope, and S of the Recife depression it bulges into the Transkei Basin. A small area of 'broken facies' has been located E of 29°E, but it is not known if it is continuous with the main area farther W. The 'structureless facies' has spread laterally over the sea floor in the northern Transkei Basin, and its southern limit extends roughly E--W for about 4oo kin. Its outcrop widens from about 14 km in the W, to an estimated 4 ° km in the E. Despite good seismic records across the nose of this facies (e.g. Fig. 6, profile I I), no distortion has been recognized in the densely laminated basinal sediments over which the slumped material has moved. This strengthens the view that the material comprising the 'structureless facies' was relatively in- cohesive, and that it flowed easily outwards. Against this hypothesis is the fact that the nose of the slump is abrupt and forms a small but quite prominent hump on the sea floor. A third, poorly defined 'facies' has also been mapped in the E Agulhas Slump (shown as 'undifferentiated' on Fig. 3). It occupies a large area of the lower slope/ continental rise between 27°E and 3o°E, and gives rise to a relatively level, slightly hummocky sea floor, bounded on its distal margins by a small, but pro- minent slope increase. No seismic profiles are available from the area, but its location suggests that it may be a slumped mass, and if so, is younger than the main Agulhas Slump, over which it extends, locally truncating facies boundaries.

(E) AGE AND STRUCTURAL OONTROLS OF SLUMPING Cretaceous and Palaeogene clays and calcareous siltstones have been collected from the main distal glide plane scar, whereas the sub-horizontal limestones which cap the Agulhas Bank, and which are cut by the proximal slump and fissures, have been dated as Neogene (Dingle i973). A Lamont-Doherty core (VI9-226) from the distal slump (i I km N of the MFR) contains a mixture of Eocene and Miocene sediments. These samples indicate that the slump has involved strata as young as Pliocene. A relatively young age is independently suggested by the fact that,

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except for a small patch of sand waves in the St. Francis depression, the sea floor micro-relief has not been infilled or draped by post-slumping sediments and the topography has a 'fresh' appearance. To some extent this effect will have been enhanced by a combination of low terrigenous sediment input onto the Agulhas Bank and the probability of vigorous scouring by the SE-flowing Agulhas current during Pleistocene low sea level stands. The location of slumps was controlled by structures beneath the plane of ddcollcment. According to Scrutton & du Plessis (I972), the MFR was emplaced in Lower Cretaceous times during the separation of the Agulhas Bank from the Falkland Plateau. The former patterns of sedimentation on the Agulhas Bank were disrupted by this event (Dingle I976 ) and during Upper Cretaceous times the sediments of the Alphard Formation prograded southwards towards the MFR in the form of a wide, delta-like body (Fig. 7a). The monoclinally shaped leading edge of this sediment wedge marked the position of the Upper Cretaceous con- tinental shelf edge. Irregular basement features, immediately behind and probably related to the MFR, were draped with sediment during this time. They are now deeply buried, but differential compaction has given rise to elongate anticlinal ridges parallel to and about 20 km behind the main MFR (Fig. 7)- A strong un- conformity separates the Maastrichtian and Palaeogene sediments. The latter were deposited as a seaward dipping sequence of calcareous clays and siltstones proxkmal d~stal

Neogene Palaeogene Cretaceous

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FIG. 7- Diagrammatic cross-sections illustrating structural control across the western (a) and eastern (b) Agulhas Slump. MFR, marginal fracture ridge; AA, Agulhas Arch; PAA, Port Alfred Arch.

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which eventually buried the Cretaceous shelf break, and beyond it lay at a high angle on the former upper continental slope (i.e. upon the foreset beds of the Upper Cretaceous wedge) (Fig. 7a). On the continental shelf, Neogene sediments consist of lithified and semi-lithified limestones which dip seaward at very low angles. In deeper water they were probably conformable with the Palaeogene sequence. E of about 24 °, where the continental shelf is (and was) narrow, Neogene sediments dip seawards at angles similar to those of the lower Tertiary strata. Because of its angles of rest, this Cenozoic sequence of clays, silts and limestones was potentially unstable, and when slumping occurred, two main points of detach- ment developed: (i) along the outer face of the Upper Cretaceous sediment wedge, with a glide plane that approximately followed the Mesozoic/Palaeogene boundary (distal part) ; (ii) within the dipping Palaeogene sediments behind the former Upper Cretaceous shelf break (proximal part). It is likely that the slide was a single unit, which subdivided on the 'weak' point above the Upper Cretaceous shelf break where movement was diverted horizontally. Tensional forces affecting a narrow belt behind the main glide plane gave rise to the fissured zone. In the western part of the Agulhas Slump, movement down slope was restricted by the MFR and the associated anticlines in Cretaceous strata. Where the monoclinally shaped Upper Cretaceous sediment wedge dies out against the Agulhas Arch a convergence of detached material (i.e. a zone of compression) resulted in strong folding and faulting in the distal slump. E of 26 ° down slope movement was not confined and the slumped mass distended, producing the broken and structureless 'facies' (Fig. 7b). The Recife depression occurs where the structural control at the head of the proximal slump changes from faulted base- ment (NE) to steeply dipping sediments (SW). It also lies upslope of the area where the distal slump spills over the MFR, and its unusual structure probably results from an interaction of these factors. The trigger mechanism for the Agulhas Slump is not known. Whilst it may not be necessary to invoke a trigger which dislodged material simultaneously along 700 km of the outer continental shelf (one large sector may have shifted, inducing laterally progressive downslope movement), it does seem necessary to invoke a large tectonic trigger in preference to a meteorological or sedimentary event. The great length of the head region, and the lithified state of much of the sedi- ment involved, indicates that a locally unstable accumulation of unlithified sedi- ment, such as produced by abnormal river discharge, or large storm-induced movement was not the cause of the slumping. In addition, the continuous nature of the associated structures indicates that the phenomenon was 'geologically in- stantaneous' along the length of the slump head region. Earthquakes have been shown to cause massive submarine slumping, e.g. on the Grand Banks (Heezen & Drake 1964) ; off northern Algeria (Heezen & Ewing 1955) ; Alaska (Coulter & Migliaccio 1966). The continental margin of SE Africa lies on two major fault zones: the faults associated with the Cape Fold Belt, and the Agulhas marginal fracture zone. W of about 24°E the Cape Fold Belt is known

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to be seismically active (see Theron 1974) and since i 963, four shocks of magnitude greater than 5"o (Richter scale) have been recorded, but although the structures continue E and SE under the Agulhas Bank, there are no records of epicentres on the Agulhas Bank. Similarly, only one record > 5.o (Richter scale) can be associa- ted with the northerly extension of the marginal fracture zone (31 °2o'S, 3o°3o'E, 0liver i956 ). The SE part of the Agulhas Bank is, therefore, not known to be seismically active, although the extension of structures which underlie it have a record of intermittent activity. Although this evidence is weak, in view of the absence of any obvious alternative mechanisms, it is tentatively suggested that the Agulhas Slump was initiated by a large seismic shock in post-Pliocene time.

3" Discussion and conclusions All the diagnostic, large-scale features of submarine slumps and terrestrial land- slips have been recognized in the Agulhas Slump: a wide fissured zone, prominent glide plane scar, and tensional depression in the head region; and a well developed concave glide plane under the main slump mass, which has a convex, locally hummocky upper surface. Toe structures range from compressed (in the W) to distended/mud flow (in the E). The main characteristics of the Agulhas Slump are: great length and straight- ness of the head region; large length/width ratio; and strong structural control of slump head geometry, and extent and direction of downslope movement. These features, particularly the first, contrast strongly with those of most large submarine slumps so far recognized (see Table I for comparative data and for abbreviations used below), where mass movement has proceeded in a radial fashion from relatively short, irregular head regions, and has resulted in fan-shaped allochthonous bodies of sediment: the Bassein (At), Grand Banks (PR), and an unnamed slump offthe Cayar seamount, NW Africa (PR), are typical examples (Moore et al. 1976, Heezen & Drake 1964, Seibold & Hinz 1974)- An exception is the long, relatively straight head region of the Rockall Slump (PR, Roberts 1972), although here, the linearity is not as well- developed as in the Agulhas Slump, and its length is much less. On the SE Agulhas Bank, the degree of basement control over the extent and direction of downslope movement appears to have been greater than in any previously described sub- marine slump. For over 35 ° km a combination of folds in underlying sediments and an adjacent MFR restricted the slump to the continental crust, and downslope movement was probably no more than about I2 km. NE of 26°E, the MFR is buried, and the slump was free to move over the continent/ocean boundary into the Transkei Basin. Here downslope movement increased from about 60 km S of the Recife depression, to over 200 km SE of Cape Padrone. Much of this increase was due to the development of a highly mobile 'structureless facies' (? mud flow regime). The result of this basement control has been an asymmetric dispersion of allochthonous sediment into the ocean basins adjacent to the Agulhas Bank, and contrasts strongly with the situation adjacent to the large Grand Banks (PR) and Bassein (Ac) slumps, where turbidity and mud flow facies occur in approxi- mately concentric zones around the distal part of the slumps. A radial outline is also recorded for the Ranger Slump (As, Normark 1974), despite the fact that the

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toe region is dammed. The Ranger Slump occurs on an active continental margin plate boundary which is structurally complex, consisting of numerous sheared and rifted segments. Details of the distal Rockall Slump are not known. The length/ width ratios shown in Table I illustrate the contrasts in amount of downslope movement between the largest submarine slumps described to date, and the Agul- has Slump. The combination of unusual features which characterizes the Agulhas Slump is thought to be a result of its structural setting--on a sheared continental margin where the straightness and close relationship of the slump to basement features can be readily related to linear basement structures, which are typically well- developed on such margins (e.g. Le Pichon & Hayes I97I, Dingle I976 ). No similarly detailed studies are available from other sheared continental margins, but profiles taken on the Cape Palmas and Ivory Coast escarpments (Emery et al. i974, Delteil et al. I974) suggest that large-scale slumping has locally played an important role in the development of the W African margin. However, because

TAB LE I. Quantitative data for the Agulhas Slump and comparative data for other large submarine slumps. W of 26°E E of 26°E Total Length: head 480 km 220 km 700 km toe 420 km 380 km 8oo km Width: (mean)* 64 km x68 km (maximum) 95 km 220 km Area 29 232 km 2 5 ° 256 kmt 79 488 kmS Cross-sectional area (mean) 32"o km s t 7"6 km ~ (lines 6, 7, 8) (line 9) Thickness: (mean)t o.374 km o.2ao km (line 9) o'x87 km~ (maximum) 0.8oo km 0.400 km Volume: xo 933 kmS 9 398 km s 2o 33I km s

Comparisons Margin type Slump length width* area(km 2) volume(kins) length~width PS Agulhas 750 km Io6 km 79 488 20 331 7"0 AC Bassein 108 km 37 km 4 ooo 900 2"9 PR Rockall x60 km 13"8 km 2 200 300 x x.6 AS Ranger 35 km 8.6 km 3oo 2o 4"o AS Kidnappers 45 km 5"6 km 25 ° 8 8.o PR Grand Banks§ 240 km i4o km 27 500 760 1.7 * calculated as area/mean length of slump. + calculated as cross-sectional area/width for lines indicated. :1: because only one complete profile was available, a nominal value of half the thickness of the western part of the slump has been assumed for volume calculations. § excluding areas affected solely by turbidity flows (maximum length and width from Heezen & Drake t964, fig. 2). Moore (x976) considered this slump to consist of several small slumps and turbidity channel routes.

Abbreviations: P, passive, A, active, S, sheared, C, compressional, R, rifted.

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the Agulhas fracture zone is one of the longest continuous marginal offsets known, the Agulhas Slump could well be unique because other sheared margins (both active and passive) are mostly composed of several short marginal offsets, and may not offer the along-slope continuity necessary for the generation of such a massive feature. If they exist, similar massive slumps may be expected on other notably long marginal offsets such as the Falkland, Senja, and Newfoundland fracture zones.

ACKNOWLEDGEMENTS.This study was funded by the South African National Committee for Oceano- graphic Research. Data were collected over several by the Joint / Geological Survey Marine Geology Unit, and the Bernard Price Institute (Geophysics) at the University of Witwatersrand, Johannesburg. Professor L. O. Nicolaysen is thanked for making the B.P.I. data available. Dr R. A. Scrutton (Edinburgh) has suggested improvements to the manuscript.

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RYAN, W. B. F., von RAP, U., ARTHUR, M., McCoY, F., SARNTHEIN, M., WESER, O., LOPATIN, B. G., CrrA, M. B., LiJTZE, G. F., CEPEK, P., WIND, F., HAMILTON, N., MOUNTAIN, G., WHELAN,J., CORNFORD,C. & BANYRA, L. x976. Passive continental margin. Geotimes October, 2I-4. SCRUTTON, R. A. x973. Structure and evolution of the sea floor south of South Africa. Earth Planet. Sd. Lett. x9, 25o-6. & DU PLESSXS,A. 197a. Possible marginal fracture ridge south of South Africa. Nature, Lond., 242, I8O-2. S~mOLD, E. & HINZ, K, z974. Continental slope construction and destruction, west Africa. In: Burk, C. A. & Drake, C. L. (eds.), The Geology of Continental Margins. Springer-Verlag, New York, 179-96. THERON, J. N. I974. Die seismiese geskiedenis van die suidwestlike Kaap provincie. Seismol. Bull. Geol. Surv. S. Aft. 4, 9-I6. UOHUPI, E. I967. Slumping on the continental margin southeast of Long Island, New York. Deep-Sea Res. x4, 635-9.

Received 28January I977; revised typescript received 9 May x977.

RICHARD VERNON DINGLE, Marine Geoscicnce Group, Department of Geology, University of Cape Town, South Africa.

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