Glaciomarine turbidite and current controlled deposits in Prydz Bay, Antarctica

Marine Geology, 108 (1992) 365-381

365

Elsevier Science Publishers B.V., Amsterdam

Glaciomarine turbidite and current controlled deposits in Prydz Bay, Antarctica Berit Kuvaas a and German

Leitchenkov b

alnstitute of Solid Earth Physics, University of Bergen, AllOgaten 41, 5007 Bergen, Norway bSevmorgeologia, Maklina St. 1, 190121 St. Petersburg, Russian Federation (Received April 4, 1991; revision accepted June 3, 1992)

ABSTRACT Kuvaas, B. and Leitchenkov, G., 1992. Glaciomarine turbidite and current controlled deposits in Prydz Bay, Antarctica. Mar. Geol., 108: 365-381. A thick sequence (up to 2200 m) of presumed post late Eocene/early Oligocene glaciomarine sediments is inferred to be present on the Prydz Bay continental rise. In the absence of information from drillholes, we correlate to ODP Leg 119 drillsites on the shelf and compare with the seismic reflection pattern of glaciomarine sequences in the Weddell Sea. The inferred glaciomarine sediments in Prydz Bay appear to be deposited in a complex manner, suggesting interaction by both turbidity cur rents and strong bottom currents. Reflection seismic profiles from the lower continental slope and rise shows an abundance of current influenced deposits, such as sediment waves and large sediment ridges with similarities to contourite drifts. In addition, large channel-levee complexes are abundant, suggesting deposition by turbidity currents and other massflow processes. Large channels and sediment ridges trend oblique to the continental margin. The geometry and character of the seismic reflection pattern suggest that the ridges have been deposited under the combined influence of overflow from downslope channelized turbidity currents and strong bottom water flow. The observed sediment waves and the difference along eastern and western channel margins suggest that bottom currents are flowing towards the west. We suggest that the initiation of turbidite sedimentation occurred in the late Eocene-early Oligocene, when the Amery Ice Shelf reached the shelf edge for the first time. Onset of current controlled deposition may possibly be related to the opening of the Drake Passage at the Oligocene/Miocene boundary.

Introduction D e e p Sea Drilling Project, O c e a n Drilling Prog r a m a n d C I R O S drilling on the A n t a r c t i c continental m a r g i n have d e m o n s t r a t e d the presence o f g l a c i o m a r i n e sequences in the R o s s Sea, W e d d e l l Sea a n d P r y d z Bay. The p r o g r a d i n g g l a c i o m a r i n e sequences across the P r y d z Bay shelf are the best d o c u m e n t e d in terms o f s a m p l i n g by drilling a n d some seismic reflection studies (Stagg, 1985; M i z u koshi et al., 1986; C o o p e r et al., 1991b). H o w e v e r , few studies have c o n c e n t r a t e d on the slope a n d rise sequences off P r y d z Bay, a n d the extent o f g l a c i o m a r i n e sediments r e m a i n s u n c e r t a i n because

Correspondence to: B. Kuvaas, Institute of Solid Earth Physics, University of Bergen, Allrgaten 41, 5007 Bergen, Norway. 0025-3227/92/$05.00

o f severe sea bed multiples causing p r o b l e m s with c o r r e l a t i n g seismic sequences across the o u t e r shelf. Stagg (1985) defined five s e d i m e n t a r y sequences on the c o n t i n e n t a l slope a n d rise, a n d interpreted the u p p e r m o s t sequence (PD.1) as turbidites a n d hemipelagic sheets o f Pliocene a n d y o u n g e r age. M i z u k o s h i et al. (1986) defined six h o r i z o n s below the slope a n d rise a n d i n t e r p r e t e d the c o m p l e x stratification o f the shallowest sediments to be a result o f d e p o s i t i o n f r o m d o w n s l o p e t u r b i d i t y currents interacting with b o t t o m currents. The present s t u d y is based on a p p r o x i m a t e l y five t h o u s a n d k m o f m u l t i c h a n n e l seismic d a t a on the P r y d z Bay c o n t i n e n t a l margin. T h e d a t a were collected d u r i n g the 32nd a n d 33rd Soviet A n t a r c tic R e s e a r c h E x p e d i t i o n by Shelestov, G a n d j u h i n , B u t s e n k o a n d K u z n e t s o v a from 1986 to 1988

© 1992 - - Elsevier Science Publishers B.V. All rights reserved.

366

(Fig. 1). The surveys have been carried out with an air gun source of 7.5 liter and a 24 channel streamer, 2300 m long. We concentrate on approximately 2000 km of seismic data on the continental rise and lower slope. From the results of ODP Leg 119 and comparison with an earlier study in the Weddell Sea (Kuvaas and Kristoffersen, 1991), we try to indicate the base of the glaciomarine sequence on the Prydz Bay continental slope and rise. When refering to the sediments as glaciomarine, we use the definition of Borns and Matsch (1988): "Glaciomarine sediment includes a mixture of glacial detritus and marine sediment deposited more-or-less contemporaneously". The purpose is to examine and interpret the seismic reflection pattern of the continental slope and rise deposits in terms of depositional processes on a glaciated continental margin. The bulk of the glaciomarine section appears to have been deposited under a complex interaction of downslope currents and bottom currents. We therefore describe the glaciomarine sediments as a complex mixture of turbidites and bottom current influenced sediment deposits. We avoid using the terms contourcurrents and contourites as very little is known about the bottom currents in the area. The problem of distinguishing deposits formed under the influence of bottom currents from turbidites on seismic reflection data has been discussed widely (McCave and Tucholke, 1986;Pickering et al., 1989) and Myers and Piper (1988) identified several seismic facies resulting from contour currents or bottom current circulation. However, even in areas characterized by widespread bottom current influenced sediments, these deposits are volumetrically much less important than mass-flow deposits (Pickering et al., 1989). In areas close to continental margins it should be expected that a complicated sedimentary pattern, as a result of material supplied by turbidity currents being reworked or redeposited by bottom currents, is the rule rather than the exception.

Environmental setting

Physiography Prydz Bay lies at the oceanward end of the graben occupied by the Lambert Glacier and

B. K U V A A S A N D G. L E I T C H E N K O V

Amery Ice Shelf (Fig. 1). On the shelf, there is a broad depression which is interpreted to be a result of glacial erosion (Anderson, 1989). The seafloor is smooth within the trough compared to the adjacent shelf. The continental slope off the glacially eroded trough is gentle and has a smooth morphology characterized by oceanward convex bathymetric contours whereas the slope to the east and west is steeper and incised by numerous canyons (Fig. 1). Immediately west of the oceanward convex contours (at 68°E), the bathymetry suggests a large canyon, trending NNW. To the east, a similar, but less distinct canyon (at 75°E) trending NNE is observed. Large sediment ridges adjacent to these channels are distinct features in the bathymetry (Fig. 1).

Geology The continental shelf of Prydz Bay has been built out at the extension of a major graben structure, the Lambert Graben, which extends inland for almost 700 km (Stagg, 1985). This graben structure is formed largely in Precambrian metamorphic basement, and is believed to have existed since early Cretaceous or even Permian time (Hambrey et al., 1991). A NE-SW trending basement horst-graben structure in the inner half of the Prydz Bay is covered by over 10km of sedimentary strata (Leitchenkov, 1990; Cooper et al., 1991b). The Amery Ice Shelf is the seaward extension of the Lambert glacier, and the entire glaciated area draining into Prydz Bay represents about a fifth of the East Antarctic Ice Sheet (Drewry, 1983). The Ice Shelf has probably advanced across the shelf edge repeatedly since late Eocene-early Oligocene times (Cooper et al., 1991a,b; Hambrey et al., 1991).

Oceanography The main features of the regional ocean circulation are the cold and fresh Antarctic Coastal Current, flowing westward along the shelf break, and the Circum Antarctic Current, which flows from west towards the east along the slope and rise (Foldvik and Gammelsrod, 1988). Guretski et al. (1987) have investigated the southern ocean

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circulation through observations during twenty years, and have modelled the circulation at depths of 100, 1000, 2000, 3000 and 4000 m. In the Prydz Bay area, very complicated circulation patterns are modelled at depths of 3000 and 4000 m because of the Kerguelen Plateau in the north (Fig. 1), which acts as a barrier for deep water currents. Shelf water, which circulates under the floating ice shelf, becomes cold and dense and contributes to the bottom water formation in the Weddell Sea (Foldvik, 1986). Whether a similar process takes place in Prydz Bay is not presently known.

Seismic facies in Prydz Bay The seismic reflection pattern on the shelf in Prydz Bay is characterized by a seaward prograding sequence overlain by an upper unit up to 300 m thick consisting of chaotic and sometimes continuous flatlying reflectors (Fig. 2). Drilling results from ODP Leg 119 have confirmed that the prograding sequences on the outer shelf consist of glaciomarine sediments (Shipboard Scientific Party, Leg 119, 1989). Within the glaciomarine section at about 70 km landward of the present shelf edge, we observe an abrupt change from lowangle to high-angle foresets in the seaward direction of the prograding foresets (Fig. 2). This change can be observed on all studied profiles. On the lower slope and rise, we observe a distinct seismic reflector P1 which marks the base of a thick sequence of well-stratified reflectors that overlies a sequence of more irregular reflectors (Figs. 2 6). In the following, we concentrate on the seismic reflection pattern overlying this reflector. Severe sea bed multiples are a problem for correlation from the deep sea towards the shelf area, but upslope reflector PI appears to correlate with the observed change in dip on all profiles crossing the continental shelf (Fig. 2). We will later argue that reflector P1 represents the base of glaciomarine sediments in Prydz Bay. A second regional reflector P2 (Fig. 4) locally cuts across underlying layers and is observed in the continental rise sequences where it occurs about 0.2-1 s above reflector P1. Five seismic facies have been distinguished along the Prydz Bay continental slope and rise. These

B. KUVAAS AND G. LEITCHENKOV

facies have been defined on the basis of their morphology and internal seismic reflection pattern. The interpretation of different facies is based on the internal and external geometry, their extent and position in the slope and rise environment and comparisons with seismic patterns published in the literature. Facies 1 is characterized by well-stratified reflectors showing cut-and-fill features, interpreted as channel levee systems (Fig. 3a). This facies is characterized by well stratified reflectors that converge away from channel margins. The channel floors are in places observed as several reflectors with high amplitudes and good continuity but they may also be less distinctive. The channel infill is either well stratified/laminated or chaotic. This facies is best developed along the western and eastern margins of the oceanward-convex bathymetric contours. In the western area, on the continental rise, channels have widths of 70-80 km and typical levee deposits can only be observed along the western margins. In the east, on the continental slope, channels are less wide and levees are developed on both sides. Large scale mounded stratified facies constitute facies 2 (Fig. 3b). This facies forms large scale sediment ridges and is characterized by internal well-stratified gently to steeply dipping reflectors. The facies' surface and internal reflectors, which may be subparallel or divergent and often wavy (Facies 3), do not conform to deeper surfaces. The facies is best developed on the continental rise. The sediment ridges are in some cases located close to channel margins. Facies 2 is often mantled by deposits of facies 3. Facies 2 has similarities to well known contourite drifts, and we interpret the facies as a result of current influenced sedimentation. The wavy stratified facies, facies 3, is characterized by multiple symmetric and asymmetric undulating or wavy, normally continuous reflectors (Fig. 3c). The waves have wave-lengths of 2-4 km and amplitudes of less than 100 m. This facies is best developed on the continental rise and occurs at levels from 1500 ms sub-bottom depths to the present seabottom. It normally occurs within or in the vicinity of the large scale mounded stratified

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facies (Facies 2) and is interpreted as current controlled, migrating sediment waves. Facies 4 is characterized by well-stratified parallel to subparallel units (Fig. 3d). This facies consists of continuous reflectors that can be traced over large distances and have relatively uniform acoustic characteristics. This facies is observed on all profiles in the continental rise environment and is interpreted to be of predominantly turbidite and hemipelagic origin. A chaotic reflection pattern constitutes facies 5. This facies occurs as zones which normally have widths of 10 to 20 km, and disturb the seismic pattern at different levels throughout the sequence above reflector P1. The facies is observed on all profiles on the continental rise (Figs. 4-7), and is often associated with strong amplitude reflectors in the upper parts. Chaotic reflection patterns have been described from several areas. In some cases, the pattern clearly occurs within channels and has been interpreted as channel infill (Kolla and Coumes, 1985; Maldonado et al., 1985). In other cases, a chaotic reflection pattern can be interpreted as representing debris flows (Prior et al., 1984), slide deposits (Trincardi and Normark, 1989) or a result of multiple mass transport events, all reflecting sediment failure in the slope (Weimer, 1990). Hinz et al. (1979) interpreted a chaotic seismic reflection pattern in the Labrador Sea as a result of dewatering of primarily well-stratified sediments. Several authors have also attributed the chaotic seismic reflection pattern to current-influenced sedimentation. Mountain and Tucholke (1985) interpreted the hummocky and chaotic reflection pattern in sediments from the Blake Outer Ridge in this manner. Myers and Piper (1988) further described a mounded chaotic facies from the Labrador Sea and suggested a genesis in relation to bottomcurrent circulation. The chaotic zones observed on our seismic profiles occur as patches only on the continental rise, and we cannot observe any relation to channels. We have not observed slide scars or transport paths within the resolution of this data set, except for the previously described large channels, in the slope sequences. This, together with the absence of facies 5 on the continental slope, suggests that

the chaotic zones do not represent debris flows or slide deposits. Also, the limited extent of this facies, occurring as local zones, does not favour an interpretation in terms of bottom currents. Anomalous seismic reflection patterns, in the form of chaotic zones, have been described from several areas and may be caused by gas in the sediments (Geyer, 1983). The high-amplitude reflectors (bright spots) which often can be observed in the upper parts of these zones, might be caused by a high negative reflection coefficient at the top of gas layers. Other indicators of gas in the sediments are velocity variations, observed on Fig. 5 as doming upward of reflectors over the chaotic zones. Boulton et al. (1981) ascribed this velocity effect to prolonged elevated pore pressure caused by trapped gas. In other cases, we can observe zones with acoustic turbidity (Fig. 7). This is also believed to be caused by gas in the sediments, and a common feature of adjacent reflectors is a downward deflection close to the actual zone (Hovland and Judd, 1988), as can also be observed in Fig. 7. The disturbance in the layers above the acoustic turbidity, observed as polarity reversals, might be caused by gas seepage to the seabottom. In view of the arguments presented above, we interpret the chaotic zones on the continental rise to be a result of gas-charged sediments. However, considering the limited seismic coverage in the area, and also the fact that we are studying an area with a very high sediment-input, we cannot exclude the possibility that some of these zones represent slumps and/or debris flows. Also, the close association with current controlled deposits may indicate that some of the chaotic zones should be interpreted in a similar manner. The sedimentary record in Prydz Bay is also characterized by several erosional reflectors that cannot be traced over large distances. In some cases, they can be interpreted as channel margins, whereas others appear to have no relation to channel structures.

Seismic stratigraphy and depositional processes Figure 4 (two upper panels) serves to illustrate the complexity of the sedimentary pattern on the western continental rise. Both channel-levee facies

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(Facies 1), mounded stratified facies (Facies 2) and sediment waves (Facies 3) are observed. Two generations of channels can be discerned. The lowermost channel (channel a) is characterized by a well-defined western channel margin which defines the boundary between levee deposits in the west and channel infill to the east (Fig. 4). Reflector P2 truncates the eastern part of the channel, and this area is also heavily disturbed by a chaotic reflection pattern (Facies 5). Therefore, levee

deposits cannot be observed along the eastern channel margin. Reflector P2 appears to form the basis of a large mound with a maximum thickness of 1500 m and with an internal seismic reflection pattern corresponding to facies 2. This feature (Figs. 3b and 4) has similarities to well described current controlled deposits, such as the Eirik Ridge on the southwestern margin of Greenland (Arthur et al., 1989). Further to the west, well-developed sediment waves

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(Facies 3) are observed above reflector P2 (Fig. 4). The erosional character of reflector P2, together with the occurrence of facies 2 and 3 in the overlying sequence suggest that reflector P2 marks the transition to a period with increased bottom current intensity and the onset of current influenced sedimentation. The uppermost channel (channel b) is difficult to trace, as both channel margins and bottom are difficult to observe (Fig. 4). However, the bathymetry suggests that this channel extends from the upper slope to the continental rise (Fig. 1). On the adjacent seismic profile (profile 33007, Fig. 1), the western channel margin is clearly recognized (Fig. 3a). The channel has well-developed levees on the western margin, whereas the eastern margin is characterized by the large mounded sediment ridge (Fig. 4), rather than typical levee deposits. The sediment ridge appears to parallel the channel for a long distance, as observed from the bathymetric map (Fig. 1). The occurrence of well-developed levee deposits

on the western channel margin as opposed to contourite-like deposits on the eastern margin suggests that downslope mass-flows interact with strong bottom currents. The large sediment ridge associated with the eastern channel margin is probably a result of fine grained material deposited from overbank flow which interacts with bottom currents. We therefore suggest that neither levee nor current-controlled deposits is the appropriate description, but rather a combination of the two. The seismic reflection profiles on the central continental rise are characterized by reflectors truncating underlying layers together with both migrating sediment waves and large sediment ridges (Figs. 5 and 8). We observe a buried sediment ridge (ridge a, Fig. 5) characterized by internal wavy subparallel reflectors possibly with some thinning towards the west. Some of the reflectors appear to be eroded at the eastern margin of the ridge. The sediment ridge can possibly be interpreted as a levee along the western margin of a channel, as migrating sediment waves also have

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been observed migrating upslope on levee deposits (Damuth, 1979; Normark et al., 1985). However, no eastern channel margin can be observed from the seismic data. We therefore suggest, on the basis of the migrating sediment waves, together with the morphology of the sediment ridge, that this deposit was formed under the influence of bottom currents. A similar, but smaller scale sediment ridge (ridge b, Fig. 5) occurs to the east of, and in a stratigraphically higher level than the lowermost sediment ridge (ridge a). The depression between these two sediment ridges is characterized by an erosional surface which can be interpreted as a channel together with levee deposits showing onlap towards the sediment ridges. The uppermost seismic reflection pattern is characterized by layers that drape over the lower relief. Over the axes of the sediment ridges, reflectors from both sides meet and cannot always be traced across (Fig. 5). The observed seismic pattern in the sediments overlying the ridges can be a result of interaction of bottom currents and a relatively static water mass, where the topography of the sediment ridges created preferential paths for the currents. These currents in turn result in continuing deposition on the ridges, perpetuating the process. The deposits can possibly be interpreted as some form of double depositional ridges, formed as a response of sediment deposition on either side of the bottom water flow. Depositional ridges formed in this manner have been studied in the North Atlantic by Davies and Laughton (1972), and deposits with similar appearance have also been described by McCave and Tucholke (1986) as "detached" drift deposits, formed as a result of oppositely directed currents. The uppermost reflectors over the sediment ridges are truncated at the seabottom. This pattern appears to reflect fluctuating periods with both erosion and deposition on either sides of the ridge axes. The strongest currents and greatest erosion were concentrated along the western flanks of the underlying sediment ridges, and layers truncated at the seafloor are interpreted as a result of periods with intensified bottom currents. Truncation of layers at the seafloor is well-known from contourites from the Blake Outer Ridge (Markl and Bryan, 1983), and has also been described from the Fram Strait by Eiken and Hinz (in press).

Indications of current influenced sedimentation are also observed in our easternmost study area. A mound with widths from 20 to 30 km is observed in the present morphology (Fig. 6). Reflectors from both west and east dip upwards towards an axis located at the western margin of the present seaftoor mound, in a similar manner to that described from the central continental rise. Again, reflectors meet but cannot be traced across the feature. The uppermost reflectors are truncated at the seabottom on the eastern side of the axis, whereas on the western side, sediment waves occur. The eastern margin of the mound coincides with a depression in the seafloor, and evidence of truncation of reflectors and the occurrence of sediment waves suggest that it was formed under the influence of currents. The mound itself may correspond to a "mounded stratified facies" (e.g. Myers and Piper, 1988), which results from bottom current moulding of sediment. Discussion

The apparent correlation of P1 on the continental rise with the change in dip on the continental shelf in Prydz Bay, leads us to suggest that the change in dip is a result of some major change in the sediment input, rather than downfaulting of underlying basement rocks (Cooper et al., 1991b). Also, detailed interpretation of basement rocks suggests no tectonic activity in Cenozoic time (Leitchenkov, 1990). According to the results from Site 739, reflector P1 has an age of late Eoceneearly Oligocene. On the continental rise profiles, we note a change in the characteristics of the seismic reflection pattern at reflector P1, similar to the transition from preglacial to glacial sediments at the base of the Crary Trough Mouth Fan (reflector W4) in the Weddell Sea (Fig. 4) as described by Kuvaas and Kristoffersen (1991). Consequently, given similarity in reflection pattern in Prydz Bay and the Weddell Sea, together with results from Site 693 (Weddell Sea) and 739 (Prydz Bay), we suggest that reflector P1 corresponds to the base of glaciomarine sediments on the continental slope and rise in Prydz Bay. Even though parts of the seismic reflection pattern (e.g. Fig. 5) possibly could be interpreted

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as a result of sedimentary deformation, the overall picture, as seen on the seismic profiles, has been interpreted in terms of turbidity- and bottom currents. The presumed glaciomarine sediments in Prydz Bay show depositional geometries and reflection characteristics suggesting both channellevee deposits and current influenced sediments. Sediment supplied to the margin by downslope movement has been reworked or redistributed by bottom currents. The variability of the current controlled deposits in the area is probably a result of large variability in source locations at the shelf edge. Piper and Brisco (1975) also interpreted sediments from the continental rise of Wilkes Land as turbidites and contourites and Colwell et al. (1988) described late Cenozoic sediments with indications of vigorous bottom currents from the Raggatt Basin, southern Kerguelen Plateau. From the time of initial glaciomarine input to the Prydz Bay continental slope, sediments were transported downslope by turbidity currents forming large channel-levee complexes as observed above reflector P1. We think that the change occurring at this level, which also appears to correlate with the change in dip in the prograding sequences on the continental shelf, is related to the initial Lambert Ice Shelf advance out to the shelf edge, depositing large amounts of sediments directly on the continental slope. At reflector P2, a major intensification in bottom currents probably occurred, as observed from the erosion and initiation of current controlled deposits. This oceanographic change may have been related to the initiation of the Antarctic Circumpolar Current, occurring at 23.5 Ma (Barker and Burrell, 1977), at the Oligocene/Miocene boundary. In the overlying sequence, both channel-levee deposits and current influenced sediments characterize the glaciomarine sediments. In the Weddell Sea, glaciomarine sediments are believed to be supplied to the continental slope and rise during glacial periods (Kuvaas and Kristoffersen, 1991) and the Crary Trough Mouth Fan was essentially sediment starved during interglacials. The main effect of the bottom water formation is erosion, by cold and dense Ice Shelf Water, in the slope sediments. By analogy to the Weddell Sea, we suggest that the glaciomarine sediments in

B. K U V A A S A N D G. L E I T C H E N K O V

Prydz Bay were deposited mainly during glacial periods. Disconformities in deep-sea sediments caused by intensification of bottom circulation have been related to periods with climatic deterioration (Ciesielski et al., 1982). It is therefore reasonable to suggest that the strongest effects of bottom currents also occurred during glacial periods. However, if bottom water formation during interglacials also occurs in Prydz Bay (not presently known) some of the erosional surfaces observed could be explained by dense and cold water running downslope from the shelf area. It is also possible that the formation of bottom water is of major importance in the building of current influenced deposits. Our seismic reflection profiles are, however, not of sufficient resolution to distinguish the seismic stratigraphy of glacial and interglacial periods. The sediments above reflector P1 on the continental rise in Prydz Bay have a maximum thickness of 2100-2200 m, which exceeds thicknesses in the Weddell Sea by almost a factor of two. However, as the Lambert glacier has the larger drainage area, a higher sediment input is reasonable in the Prydz Bay area. It is also possible that parts of the sediments deposited on the slope in Prydz Bay may have been supplied from adjacent areas by bottom currents. Our seismic data does not allow any definitive statements about the orientation of the deposits, but if the observed sediment ridges correlate as suggested in Fig. 8, they are elongated oblique to the continental margin. The orientation of the large sediment ridge in our westernmost study area can be explained by its close association to the eastern margin of a large channel. In the bathymetry, this channel can be followed from the shelf edge to the continental rise (Fig. 1). The combination of well-developed levees on the western margin of the uppermost channel (channel b, Fig. 4) and contourite-like deposits on the eastern margin, leads us to suggest that westerly flowing bottom currents have been active in the area. We propose the following model for the formation of the observed sediment accumulations. The westerly flowing bottom currents acted constructively with the overbank flow on the western side of the channel and created a deposit that extends far

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from the channel margin. The influence of the Coriolis force, acting towards the left of the flow direction on the Southern Hemisphere, would also enhance this process. Along the eastern margin, the westward flowing bottom current interacted with overbank flow from turbidity currents flowing within the channel. This interaction decreased the velocity and hence increased the rate of deposition close to the channel margin, forming a mounded sediment ridge. We also see evidence of both downslope and westerly currents in our central and eastern study area. The sediment waves (Facies 3) on the central continental rise (Fig. 5), with steep west-facing slopes and crests migrating eastward, indicate a component of westerly flowing currents. Because of the limited data available, the current direction could be towards anywhere between the NNW and the SSW. However, sediment waves are often found to be oriented obliquely to regional bathymetric contours, and they generally migrate upslope and upcurrent (Flood and Shor, 1988). The large sediment ridges on the central continental rise cannot be interpreted as reworked or redistributed overbank material as no relation to large channels has been observed. However, the possible correlation between our seismic profiles suggests an elongation oblique to the margin. This, together with the occurrence of large channellevee deposits in the area, suggests a strong influence of downslope currents during their formation. The same arguments can be applied to the deposits on the eastern continental rise. Normally the position and morphology of drift deposits are controlled by the pre-existing seafloor, which controls and directs the various branches of the bottom currents (McCave and Tucholke, 1986). In Prydz Bay, the formation of the sediment ridges appears to have been initiated over an even surface. Also, the continental slope has a relatively uniform dip. It is therefore reasonable to suggest that the Prydz Bay area has been characterized by great irregularity of current directions. As different currents interacted, they were slowed and possibly diverted, causing deposition of sediments. The Kerguelen Plateau acts as a barrier to the regional deep-sea currents, creating a complex circulation pattern with local westward currents at 4000 m

water depth (Guretski et al., 1987), even though the overall circulation pattern is towards the east (Foldvik and Gammelsrod, 1988). Eittreim et al. (1972) described clockwise circulation in the South Indian Basin, and hence westward-flowing currents along the Wilkes Land margin. Knowing that the formation of sediment drifts normally requires millions of years at typical accumulation rates (tens of meters per million years; McCave and Tucholke, 1986), the large sediment ridges observed indicate that currents must have been stable for long periods. As no information about sedimentation rates are available, and since ridge formation was disrupted by downslope sediment supply varying in response to climatic fluctuations, we consider it premature to speculate on the time necessary for the formation of the ridges. Even though the sediment ridges in Prydz Bay have similarities with well-known contourite drifts described from other oceans, we are reluctant to use the term "contourites". The glaciomarine sediments in Prydz Bay have been deposited under the combined influence of downslope processes and strong bottom currents. However, very little is known about the current directions in the area, and the path of the current may not always follow the bathymetric contours. Conclusions Thick sequences (over 2000 m) of presumed glaciomarine sediments are present on the continental rise in Prydz Bay, Antarctica. These sediments appear to have been deposited by a complex interaction of turbidity currents forming large channel-levee systems and strong bottom currents. Several indicators of current-influenced sedimentation are observed, such as the frequent occurrence of sediment waves, large sediment ridges with similarities to well-known contourites, and truncation of reflectors at the seafloor. Initiation of turbidite sedimentation and the formation of large channel-levee deposits probably occurred when the Amery Ice Shelf reached the shelf edge for the first time, possibly in the late Eocene-early Oligocene. During the opening of the Drake Passage at the Oligocene/Miocene boundary, the formation of the Circumpolar Current possibly initiated the

380

f o r m a t i o n o f large s e d i m e n t ridges with indications o f c u r r e n t c o n t r o l l e d deposition. As b o t h large and smaller scale c h a n n e l - l e v e e systems are present, t o g e t h e r with sediment ridges e l o n g a t e d o b l i q u e to the c o n t i n e n t a l m a r g i n , we suggest that d o w n s l o p e processes h a v e been the p r e d o m i n a n t process d u r i n g d e p o s i t i o n o f the g l a c i o m a r i n e sequence. H o w e v e r , the o b s e r v e d sediment waves t o g e t h e r with the a s y m m e t r i c d e p o s i t i o n o f sedim en t s on the flanks o f d o w n s l o p e channels, suggest that b o t t o m currents were flowing westwards.

Acknowledgments

Sevmorgeologia, L e n i n g r a d kindly p r o v i d e d the m u l t i c h a n n e l seismic d a t a used in this study. We wish to a c k n o w l e d g e helpful discussions with Y n g v e Kristoffersen, G a r r i k G r i k u r o v , O la Eiken, Alexander Shor and Marcus Gorini. Yngve Kristoffersen and D a v i d Piper kindly read the m a n u s c r i p t a n d helped to i m p r o v e it. B. K u v a a s was s u p p o r t e d by a g r a n t f r o m V I S T A .

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