seismic stratigraphy of the central bransﬁeld basin … · elsevier marine geology 157 (1999)...

ELSEVIER Marine Geology 157 (1999) 47–68

Seismic stratigraphy of the Central Bransfield Basin (NW AntarcticPeninsula): interpretation of deposits and sedimentary processes in a

glacio-marine environment

Marıa Jose Prieto a,Ł, Gemma Ercilla b, Miquel Canals a, Marc de Batist c

a GRC Geociencies Marines, Dpt. Geologia Dinamica, Geofısica i Paleontologia, Universitat de Barcelona, Campus de Pedralbes,E-08071 Barcelona, Spain

b Dpt. de Geologia Marina i Oceanografia Fısica, Institut de Ciencies del Mar (CSIC), Avda. Joan de Borbo s=n,E-08039 Barcelona, Spain

c Renard Centre of Marine Geology, University of Ghent, Krijgslaan 281 S8, B-9000 Ghent, Belgium

Received 24 October 1997; revised version received 11 September 1998; accepted 1 October 1998

Abstract

The Central Bransfield Basin is a deep narrow trough between the northern tip of the Antarctic Peninsula and the SouthShetland Islands. Analyses of single-channel, high-resolution seismic reflection data are used to characterise the seismicstratigraphy of the Central Bransfield Basin. The tectonised acoustic basement is overlain by a 1-s-thick sedimentarycover composed of two main sedimentary sequences. The Lower Sequence, which shows synsedimentary deformation,has only been identified on the Antarctic Peninsula margin. The Upper Sequence is a complex sedimentary packagecomposed of eight seismic units whose distribution, geometry and seismic facies allow two types of seismic units tobe distinguished: slope and basinal units. The slope units, constituted by progradational stratified seismic facies, form asedimentary wedge extending from the shelf edge. The basinal units fill the basin floor showing chaotic and undulatedseismic facies that change basinward into stratified seismic facies. Both types of seismic units display an interfingeringpattern at the base of the slope, suggesting an alternating shift of the sedimentary depocentre, from the slope to thebasinfloor and vice versa. This alternate pattern indicates that the sedimentary processes responsible for the infilling of theCentral Bransfield Basin followed a cyclic pattern, which has likely been associated with the advance and retreat of the icesheets over the margins during glacial and interglacial episodes. During glacial periods, the ice sheets advanced, eroded theshallower sea floor areas and deposited diamicton and debris flow deposits along the moving grounding line, resulting in aprogradational sedimentary wedge on the slope. At the end of glacial periods, coinciding with the retreat of the ice sheets,extensive sediment failures affected the continental margin. During interglacial periods the ice sheets remained restricted tocoastal locations and glacial troughs, where processes of meltwater formation might have been significant. Sediment-ladenunderflows are generated within these troughs, from where they flow and spread over the shelf and down the slope to thebasinfloor as sediment gravity flow deposits. The combined effect of these processes is a progradational build up of theshelf and an aggradational infilling of the basin floor, together with the development of the interfingering pattern at thebase of the slope. 1999 Elsevier Science B.V. All rights reserved.

Keywords: glacio-marine sedimentation; seismic stratigraphy; Central Bransfield Basin; glaciated margins

Ł Corresponding author. Tel: C34 3 4021375; Fax: C34 3 4021340; E-mail: [email protected]

0025-3227/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 9 8 ) 0 0 1 4 9 - 2

48 M.J. Prieto et al. / Marine Geology 157 (1999) 47–68

1. Introduction

The Central Bransfield Basin (CBB) is a Ceno-zoic marginal basin located between the AntarcticPeninsula (AP) and the South Shetland Islands (SSI),Antarctica (Fig. 1). The CBB is separated from theWestern and Eastern Bransfield Basins by two highsformed by Deception and Bridgeman Islands, respec-tively. During the past decades, the CBB has been theobject of various geological and geophysical studies.These studies provided new data relevant to thebasin’s formation and geodynamic evolution (Gam-boa and Maldonado, 1990; Grad et al., 1992; Hen-riet et al., 1992; Barker and Austin, 1994; Lawveret al., 1995). Most of these surveys have focusedon the deep structure of the basin and little atten-tion has been paid to the detailed stratigraphy of thebasin’s sedimentary infill. Only Jeffers and Anderson(1990) and Banfield and Anderson (1995) studied thefine-scale seismic stratigraphy of the CBB using in-termediate-resolution reflection seismic data. In thepresent study, we aim to contribute to this workby interpreting the sedimentary architecture and thestratal geometry patterns from seismic-stratigraphicanalyses of single-channel, high-resolution seismicreflection profiles. We also propose a depositionalmodel that relates the observed morphologic fea-tures, seismic facies, stratal and growth patterns tothe geodynamics of the CBB and to its glacio-marinesedimentary setting. Our study focuses on slope andbasin physiographic provinces, since it is here thatseismic-stratigraphic units reached their maximumthickness. For reasons of simplicity we refer to thesetwo provinces together as CBB.

2. Geological setting

The Pacific margin of the AP was a collisionmargin during the Mesozoic and Cenozoic (Barker,1982). Along the South Shetland Trench, subduc-tion stopped or decreased to a slow rate about 4Ma ago, when spreading ceased at the segment ofthe Antarctic–Phoenix ridge between the Hero andShackleton Fracture Zones (Fig. 1a). The opening ofthe Bransfield Basin (BB) has been dated between 4and 1.3 Ma (Barker and Dalziel, 1983). It probablyresulted from the mechanism of roll-back, following

the cessation of subduction, and from the extensionalstress induced in the AP continental crust (Barker,1982; Barker and Dalziel, 1983; Lawver et al., 1995).

The present-day CBB is an asymmetric troughcharacterised by a steep rectilinear SSI margin, agentle sinuous AP margin and an almost flat basinfloor disrupted by volcanic edifices. These volcanoesform a discontinuous lineament along the basin axiswhere they depict an incipient seafloor spreadingridge (Gracia et al., 1996, 1997). Seismic and vol-canic activity (Pelayo and Wiens, 1989), high heatflow (Nagihara and Lawver, 1989), a positive mag-netic anomaly (Roach, 1978; Gracia et al., 1996) anda large negative gravity anomaly (Garret, 1990) havebeen reported in the CBB, supporting the idea thatit is a young, active rift basin (Saunders and Tarney,1984; Fisk, 1990; Grad et al., 1992; Lawver et al.,1995).

Seismic reflection data collected in the CBB inrecent years have allowed the recognition of theacoustic basement distinct from the sedimentarycover (Gamboa and Maldonado, 1990; Jeffers andAnderson, 1990; Acosta et al., 1992; Henriet et al.,1992; Barker and Austin, 1994; Banfield and An-derson, 1995; Gracia et al., 1996). The acousticbasement in the basin axis is generally believed to becomposed of thinned continental crust intruded bydykes (Ashcroft, 1972; Birkenmajer, 1992; Grad etal., 1992), and volcanic materials, all related to theextension along the basin axis (Barker and Austin,1995). In the AP margin, the acoustic basement prob-ably consists of metasediments and metavolcanicsthat are laterally equivalent to the sedimentary seriesdefined onshore (Smellie, 1984), and are believedto correspond to the infilling of a previous marginalbasin (Gamboa and Maldonado, 1990; Barker andAustin, 1995; Prieto et al., 1997). In fact, the TrinityPeninsula Group in the AP, and the Myers Bluff For-mation in the SSI show similar sedimentologic andstructural characteristics which hint at a same originin a backarc basin (Aitkenhead, 1975; Hyden andTanner, 1981; Smellie, 1984). The acoustic basementis arranged in blocks, rotated along normal faults cre-ated by the stretching and break up of the former APand marginal basin (Gamboa and Maldonado, 1990;Jeffers and Anderson, 1990; Acosta et al., 1992).

The acoustic basement is overlain by a sedi-mentary cover with a thickness of 700–1000 ms

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Fig. 1. (a) Geodynamic setting of the Bransfield Basin. SSIMP: South Shetland Islands Microplate. (b) Bathymetric map of the Central Bransfield Basin, with location ofseismic lines recorded during the Gebra’93 cruise. Labels A to F indicate the location of the six large volcanic edifices (modified from Gracia et al., 1996). Thick lines showthe location of the seismic lines in Figs. 3, 4, 6, 7 and 10.


TWTT formed during two major sedimentary se-quences separated by a regional unconformity. TheLower Sequence fills an older graben system andis faulted and occasionally tilted towards the conti-nent (Gamboa and Maldonado, 1990; Prieto et al.,1997). The Upper Sequence is less tectonised andcontrols the present morphology of the AP margin.The Lower Sequence, or ‘rift sequence’, has beenattributed to the rift stage during which the AP mar-gin was formed. The Upper Sequence, or ‘drift se-quence’, contains internal erosional unconformitiesand has been attributed to glacio-marine sedimentaryprocesses induced by the advances and retreats ofthe Antarctic ice sheet during the Plio–Quaternaryglacial periods (Anderson et al., 1983; Jeffers andAnderson, 1990; Henriet et al., 1992; Banfield andAnderson, 1995).

3. Data base and methodology

A grid, corresponding to 16 lines of single-chan-nel, high-resolution seismic reflection data, with atotal length of about 1100 km, was collected in theCBB (Fig. 1b). The seismic source was a 2.9 l Bolt1500 C airgun. The reflected signals were detectedwith a SIG 120 three-channel streamer with an activesection of 150 m. The data were recorded digitallywith the Elics Delph2 high-resolution acquisitionsystem. Post-acquisition data processing, includingbandpass-filtering, deco-filtering and scaling, wascarried out on both the Delph2 and the Phoenix Vec-tor processing systems. The vertical resolution of theprocessed data was about 5 ms and penetration wasoften in excess of 1 s TWTT.

Across-basin profiles (trending approximatelyNNW–SSE) cross the slope of the AP continen-tal margin, the basin floor, some of the centralseamounts and segments of the steep narrow SSIslope. These profiles allow the comparison of bothmargins and their influence in the sedimentary sup-ply to the basin. Along-basin profiles (trending ap-proximately ENE–WSW) cross part of the AP slopeand the central basin floor. These profiles allow usto recognise the seismic sequences and seismic unitsthat built the AP margin and filled the basin floor,and to investigate their spatial and temporal vari-ations. We have correlated unconformity-bounded

seismic units across the whole seismic grid. Further-more, the analysis of the main features (boundaries,internal configuration, geometry, thickness) of theseseismic units has allowed us to interpret depositionalprocesses and how the basin infilling has evolved toits present configuration.

Swath bathymetry data were acquired over anarea of 10 000 km2 using the combined SIMRADEM-12=EM-1000 system. The data set fully cov-ers the AP continental margin from the upper slopedown to the basin floor and the slope of the SSI mar-gin, between Bridgeman and King George Islands(Gracia et al., 1996) (Fig. 1b).

4. Morphology

The CBB is a NE–SW-trending basin, 230 kmlong, 130 km wide and 1950 m deep. The swathbathymetry map of the CBB (Gracia et al., 1996,1997) provides a useful tool to recognise the physio-graphic elements and the main morphosedimentaryfeatures (Fig. 2). This map does not cover the AP andSSI shelves, and to characterise these areas we used acompilation of pre-existing single-beam bathymetrydata (Lawver et al., 1995). A later map by Lawveret al. (1996) covers additional segments of the SSIslope.

The AP margin has an irregular shelf, about 60km wide and 200 m deep. It is deeply incised byfour troughs, that run slightly oblique to nearly per-pendicular to the margin: the Orleans Trough in thesouthwest, Antarctic Sound in the northeast and twounnamed troughs in between (Fig. 2). The troughshave U-shaped cross-sections, which suggests thatthey are glacially scoured features (Jeffers and An-derson, 1990). The AP slope shows two platformsor terraces at different depths, giving the margin astep-like morphology. A wide (10–15 km) wavy up-per platform (Jeffers and Anderson, 1990; Acosta etal., 1992) is situated between 600 and 800 m. It isslightly tilted towards the basin (0.6–2.4%) and onlyslightly incised by the glacial troughs. The lowerplatform lies at a depth between 1000 and 1400 m.It is a discontinuous feature, 20–35 km wide, witha conspicuous convex shape that dips gently (3%)towards the basin. This platform connects basinwardwith a steep (8%) lower slope that abruptly termi-

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Fig. 2. Morphosedimentary map of the Central Bransfield Basin. To characterise the SSI margin, a compilation of pre-existing single-beam bathymetry data has been used(Lawver et al., 1995).


nates at the basin floor. Where the lower platform isnot present, the slope is much steeper (up to 18%).

The AP margin is indented by submarine valleys(Fig. 2). A large U-shaped valley, named GebraValley, was identified by Canals et al. (1994) in thenortheastern half of the AP margin (57º45, 63º30)(Fig. 1b and Fig. 2). Gebra Valley is 30 km long,6 km wide and extends from depths of 800 to 1900m on the basin floor. Its flanks are 200 m high andthe generally flat bottom is marked by irregularitieswhich might be interpreted as mass gravity flowdeposits. The head of the valley is formed by severalsemicircular and steep scarps (8%), the morphologyof which resembles that of active slide scars (Fig. 1band Fig. 2). On the southwestern half of the APmargin, several 10–15-km long gullies incise theslope. They have a U-shaped cross-section, and reachwidths of up to 2 km and depths of up to 40 m.

The rectilinear SSI margin is characterised by anarrow shelf (5 km) up to 200 m deep, where ascarp extends down to the steep slope. The shelfis incised by short troughs developed between theislands and at the mouth of the bays and fjords ofthe islands. They also cut the slope until reachinga flat, 700 m deep platform, in front of Greenwichand King George Islands. This platform has not beenidentified in front of Livingston and King GeorgeIslands, where the slope is a continuous feature fromthe shelf break to the basin floor (Lawver et al.,1995) (Fig. 2). The slope shows the highest dip, upto 26%, in front of King George Island. The foot ofthe slope shows small depositional irregularities.

The basin floor of the CBB is a relatively narrow(<30 km wide), flat trough. It shows four bathymet-ric levels that progressively deepen the sea floor from1000 to 1950 m, from the southwest to the north-east. The deepest level constitutes the King GeorgeBasin, a very flat, 20 km-wide and 50-km long basin(Fig. 1b and Fig. 2). Six large volcanic edifices,with different morphologies (labelled from A to F inFig. 2) rise above the seafloor (Gracia et al., 1996).Altogether, they form a discontinuous lineament thatextends from Deception to Bridgeman Islands. Thislineament divides the basin floor into two halvesor subbasins: a northern subbasin, between the SSImargin and the volcanic lineament, and a southernsubbasin, between the lineament and the AP margin.In addition to the large volcanic edifices, small cone-

shaped volcanoes appear scattered between the axiallineament and the SSI margin (Figs. 1 and 2).

5. Acoustic basement of the Central BransfieldBasin

The acoustic basement has been identified at dif-ferent depths in the CBB. It crops out at the seafloorat the AP and SSI margins, at a depth of around600 m, whereas in the basin floor it is covered bya sedimentary cover up to 1 s TWTT, and has beenidentified at a maximum depth of 3.2 s TWTT. Thebasement is everywhere tectonised. From both mar-gins, normal faults deepen the basement basinward,defining the axial trough where the basin is located.The seismic character of the basement varies acrossthe basin. At the margins the basement is composedof stratified and largely continuous reflections, sug-gesting a sedimentary character (Fig. 3). In contrast,on the basin floor the basement shows a chaoticseismic character with its top formed by a stronglyreflective surface. This suggests a direct relation withthe axial volcanic edifices and volcanic intrusionsof the CBB. The top of this ‘crystalline’ basementcan be traced for 15 km from the volcanic edifices,either forming the seafloor or as a highly reflectivecontinuous surface below the sedimentary cover.

6. The sedimentary cover in the CentralBransfield Basin

The overall seismic stratigraphy of the CBB con-sists of a complex package of sedimentary sequencesand units that overlies the acoustic basement. Twomajor sedimentary sequences have been identifiedwithin this sedimentary cover: the Lower Sequenceand Upper Sequence (Fig. 3).

6.1. The Lower Sequence

The Lower Sequence (LS) has only been recog-nised in the AP margin, where it directly overliesthe sedimentary basement. It extends from the upperplatform (1.5 s TWTT deep) to the base of the slopeand the central basin floor (3.2 s TWTT deep). TheLS onlaps the basement onshore, and downlaps it

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Fig. 3. Interpreted line drawing of seismic line G3 showing the distribution of the slope (U1, U3, U5 and U7) and basinal units (U2, U4, U6 and U8). Two details of thisseismic line show the prograding stratified facies and interfingering pattern at the foot of the Antarctic Peninsula slope. Location of the seismic line is shown in Fig. 1b.


offshore (Fig. 3). The top of the LS is a widespreaderosional surface identifiable throughout the AP mar-gin (Fig. 3). The LS forms a faulted, slightly de-formed sedimentary wedge, composed of discontin-uous stratified facies with several configurations: par-allel, progradational, and divergent. The parallel con-figuration occurs on the upper platform, the progra-dational facies on the slope and the divergent faciesat the base of slope, in the vicinity of normal faults,suggesting synsedimentary deformation. Landwarddipping (4º) reflections have also been recognised atthe base of the slope, probably related to basementblocks tilted along normal faults (Fig. 3). The maindepocentres of the LS are located at the base of theAP margin and range from 0.15 to 0.25 s.

6.2. The Upper Sequence

The Upper Sequence (US) is composed of eightsedimentary units, labelled 1 to 8, from bottom totop. We have defined two types of sedimentary unitsbased on their distribution: slope units (U1, U3, U5and U7) and basin-floor units (U2, U4, U6 and U8).The four slope units are observed in the AP margin.Only units U3, U5 and U7 have been identified inthe SSI margin. The basin-floor units (U2, U4, U6and U8), and their subunits, are located in the centralbasin floor. Slope and basinal units interfinger at thebase of the slope.

6.2.1. Slope unitsUnit 1 (U1), the oldest sedimentary unit of the

Upper Sequence, has only been identified on theAP margin, where it extends from the upper plat-form to the basin floor of the southern subbasin(Figs. 3 and 4). Along the whole margin, U1 is com-posed of progradational stratified seismic facies withhigh-angle (7–10º) dipping reflections that down-lap onto the LS basinward and onlap the basementlandward (Figs. 3 and 4). The top of the unit is anerosional surface in the upper platform, changingto a conformable surface basinward (Fig. 3). Thethickness of U1 ranges between 0.25 and 0.35 sTWTT, and the main depocentres are located on theupper platform (Fig. 5a). U1 shows minor internalunconformities on the upper platform, adjacent tothe Orleans Trough (Figs. 2 and 4).

Unit 3 (U3) has been identified in the AP margin.

It overlies U1 on the upper platform and Unit 2 (U2)at the base of the slope (Fig. 3). The lower boundaryof U3 is a downlap surface along the whole margin,and the upper boundary is an erosional surface on theupper platform, which becomes more conspicuous inthe western sector of the AP margin (Fig. 4). Thiserosional surface changes basinward becoming con-formable, locally outcropping on the slope (Fig. 3).The seismic facies of this unit is progradational strat-ified with very low-angle (1º) dipping reflections onthe upper platform which become steeper (10º) to-wards the slope. The main depocentres of U3 arelocated within the upper platform, and their thick-ness ranges from 0.15 to 0.2 s TWTT (Fig. 5b).This unit also seems to be present in the SSI margin,where a 0.1–0.2 s TWTT thick sedimentary wedgeoverlies the basement (Fig. 6). It occupies the samestratigraphic level as U3 on the AP margin (i.e. be-low Unit 4 on the basinfloor and below Unit 5 onthe slope). This would suggest that either the two areequivalent or the one at the SSI margin is older. TheU3 equivalent wedge at the SSI margin is composedof the discontinuous progradational stratified seismicfacies, and is bounded at the bottom by a downlapsurface and at the top by an erosional surface.

Unit 5 (U5) occurs within the AP and SSI mar-gins (Figs. 4 and 6). On both margins U5 overliesU3, and part of U4 at the foot of both slopes. Thebase of U5 is a downlap surface and the top is an ero-sional surface; both surfaces extend along the entiremargin (Fig. 4). U5 shows discontinuous prograda-tional stratified seismic facies, with very low-angle(1º) dipping reflections on the upper platformand higher-angle (6º) dipping reflections downslope(Fig. 4). U5 constitutes an irregular wedge, partlyeroded, on the platform. It thins progressively until itpinches out both landward and basinward. The max-imum thickness of the unit is 0.15 s TWTT at the APmargin and 0.1 s TWTT at the SSI margin (Fig. 5c).

Unit 7 (U7) is the youngest unit on the slope atboth margins. Along the AP margin, it constitutesa sedimentary wedge on top of the platform edge,which is continuous along the whole margin (Figs. 3,4 and 6). This wedge is particularly well developedat the southwestern sector of the AP margin, fac-ing the mouth of Orleans Trough, where it mostlyreaches the basin floor (Fig. 4). It overlies U5 at theplatform, and part of Unit 6 (U6) at the foot of the

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Fig. 4. Seismic line G1 and interpreted line drawing showing the distribution of the slope and basinal seismic units. Location of theseismic line is shown in Fig. 1b. Dots indicate the position of the platform edge at the end of sedimentation of each slope unit.

slope. Its base is a downlap surface, and its top coin-cides with the sea floor on the upper platform, whereit is mainly conformable although locally erosionalrelief of about 30 m deep and 1 km wide can be iden-tified. At the mouth of the Orleans Trough, where U7reaches the base of the slope, its top is an erosionalsurface with incisions about 50 m deep and 1 kmwide. U7 is characterised by progradational strati-fied seismic facies of very low-angle (1.5º) dippingreflections in the upper platform and more steeplyinclined reflectors (4–8º) beneath the slope (Figs. 3and 6). Additionally, we have also observed withinU7 semitransparent chaotic seismic facies at the headof the Gebra Valley, and a 50 m high depositionalmound with landward-dipping reflections (2.6º) inthe AP margin (Fig. 3). The thickness of the unitranges from 0.15 to 0.2 s TWTT (Fig. 5d).

6.2.2. Basinal unitsUnit 2 (U2) only occurs in the central and east-

ernmost sectors of the southern subbasin, betweenthe AP margin and the axial volcanic lineament,here defined by volcanoes D and E (Fig. 1b). U2overlies U1 at the foot of the AP margin, while fur-ther down into the basin it directly overlies a roughbasement (Fig. 3). The top of U2 is a subhorizon-tal conformable surface, although locally, there isevidence of strong erosion, i.e. in the vicinity ofvolcanic edifice D. A minor discontinuity within U2allows two subunits to be identified: a lens-shapelower subunit and a subtabular upper subunit. Sev-eral seismic facies appear within U2. Seismicallychaotic subtabular interstratified depositional bodieswith erosional bases occur at the foot of the slopeand on the basin floor in front of the Gebra Valley


Fig. 5. (a,b,c,d) Isopach maps of the slope units U1, U3, U5 and U7 at the Antarctic Peninsula margin. Dashed line depicts the maximumthickness of depocentres and thick line marks the present day slope break. Contour interval 50 ms.


Fig. 5 (continued).


Fig. 6. Seismic line G1 and interpreted line drawing showing the slope units (U3, U5 and U7) identified at the South Shetland Islandmargin, and the basinal units (U4, U6 and U8) interfingering at the base of the slope. Location of the seismic line in Fig. 1b.

(Fig. 3). Basinwards, the dominant seismic facies isdiscontinuously stratified (Fig. 3). The thickness ofU2 depocentres ranges between 0.2 and 0.3 s TWTT,thinning towards the AP margin, until it disappears.

Unit 4 (U4) overlies U3 at the base of the APslope, overlies U2 between the AP margin and theaxial volcanoes in the southern subbasin, and over-lies the basement in the northern subbasin (Fig. 6).

Both base and top of U4 are conformable surfaces.However, where the top of U4 crops out at theseafloor it becomes an irregular surface, with moundsand gentle erosional depressions as observed at thefoot of the AP slope (Fig. 7). An internal unconfor-mity allows us to divide U4 into two subunits withdifferent seismic facies and spatial distribution. Thelower subunit is restricted to the southern subbasin,

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Fig. 7. Interpreted line drawing of seismic line G8 showing the geometry and the distribution of basinal units. Two details of this seismic line show the undulated and chaoticseismic facies at the foot of the Antarctic Peninsula slope. Location of the seismic line in Fig. 1b.


between the AP margin and the volcanic lineament,where it downlaps the basement. The seismic faciesof this lower subunit is continuous, parallel and strat-ified with abundant interstratified seismically chaoticbodies (Fig. 7). The upper subunit occupies thewhole basin floor and onlaps the underlying units.It mainly shows a continuous, planar stratified seis-mic facies (Fig. 7) that changes to a discontinuoussubparallel stratified facies in the northern subbasinand King George Basin. The contact between thesesubunits is a conformable surface with the excep-tion of King George Basin, where the upper subunitonlaps onto the lower one. Lens-shaped bodies withchaotic seismic facies appear locally within the U4upper subunit and at its top in the northern sub-basin (Fig. 6). From their seismic characteristics,we interpreted these as sills intruded through frac-tures into the sediments (Fig. 6). The thickness ofU4 changes markedly along an across-basin section.The maximum thickness, which is around 0.35 sTWTT, occurs at the base of the SSI margin and inKing George Basin. In the remaining basin the meanthickness is about 0.15 s TWTT, except at the GebraValley where U4 decreases to only 0.05 s TWTT(Fig. 7).

Unit 6 (U6) covers the whole basin floor lyingeither over U5, as in the southernmost sector of theAP margin, or over U4, as in the rest of the basin.The base of the unit is a conformable subhorizontalsurface. In King George Basin, sediments fill andonlap onto the irregular geometry of the depocentre.The top of the unit coincides with the seafloor ina large area at the base of the AP margin, whereit often shows an erosional character (Fig. 7). Theunit has a characteristic continuous parallel stratifiedseismic facies and contains interstratified chaoticbodies near the foot of the AP slope (Fig. 3). Inthe central sector, mounded and tabular bodies withsubparallel stratified seismic facies form the sea floorrelief up to 150 m, which is highlighted in the swathbathymetry data (Fig. 7). The present-day GebraValley is located over U6. The maximum thicknessof U6 is 0.2 s TWTT in King George Basin, and itthins and pinches out towards the margins.

Unit 8 (U8) is the youngest basinal seismic unitin the CBB and is uniformly distributed on the basinfloor (Figs. 3 and 4). In the southernmost sector itoverlies U7 (Fig. 4) while in the rest of the basin

it overlies U6 (Figs. 3 and 6). The base of the unitis a conformable surface that onlaps U6. The topof U8 forms the seafloor and locally has erosionaltruncations (Fig. 7). A continuous parallel stratifiedseismic facies with high continuity and amplitudereflectors characterises this unit. In the vicinity ofthe seamounts, this seismic facies gradually changesto a more chaotic facies, which could be attributed tothe presence of volcanic material (Fig. 3). Modifica-tions to the dominant seismic facies also occur in thevicinity of the AP margin, where reflectors locallyshow undulating morphology (Fig. 7). The geome-try of U8 is lens-shaped, with the thickest sectionfollowing the basin axis. The thickness is less than0.1 s TWTT except for a 4 km wide graben at thefoot of the SSI slope, in the central sector, where thethickness reaches 0.15 s TWTT (Figs. 3 and 4).

7. Depositional growth patterns in CentralBransfield Basin

The depositional growth pattern of the AP mar-gin is characterised by stacking of the LS and theslope units U1, U3, U5 and U7 of the Upper Se-quence. The SSI margin is characterised by stackingof slope units U3, U5 and U7. The LS is faulted andfolded throughout the CBB. It fills and smooths theirregular basement morphology and displays mainlyan aggradational configuration. Within the US, U1has a distinct progradational internal structure, in-dicating that the AP margin underwent significantprogradation during the time of deposition of U1(Figs. 3 and 4). In contrast, U3, U5 and U7 in theAP, essentially show an aggradational growth patternwith only a subtle internal progradational configura-tion (Fig. 3). Nevertheless, facing of Orleans Trough,in the southwestern segment of the AP margin, thegrowth pattern is locally progradational as suggestedby the basinward advance of the platform-break (3.5km) during the deposition of U5 and U7 (Fig. 4).

The basin floor is composed of four verticallystacked, aggrading seismic units U2, U4, U6 andU8, separated by erosional unconformities. Theseunits interfinger with the slope units at the base ofthe AP and SSI slopes (Fig. 3). Locally, as in KingGeorge Basin, the basin-floor sedimentary packagecan be subdivided into two sections. The lower sec-


tion (composed of U2 and part of U4) is attached tothe AP margin. The upper section (composed of theupper part of U4, U6 and U8) occupies the wholebasin floor, onlaps the underlying units and has aclearly aggradational growth pattern.

8. Discussion

The seismic-stratigraphic analysis of the CBBindicates that the sedimentary history of this basincomprises two clearly distinct stages correspondingto the formation of the LS and US. The regionalunconformity dividing the two sequences and theirdifferentiated stratal patterns suggest a major changein the style in which the CBB was filled (Fig. 7).We discuss two sedimentary scenarios. The firstexplains the formation of the LS, which appears tohave been mainly conditioned by the initial structuraldevelopment of the CBB. The second scenario refersto the formation of the US, whose deposition hasbeen mainly governed by the growth and decayof the ice sheets and therefore by glacio-marinesedimentation processes.

8.1. Formation of the Lower Sequence

Deposition of the LS began with the initial frag-mentation, rotation, subsidence and extension ofcontinental basement blocks, prior to the onset ofseafloor spreading. In fact, the LS would correspondto the ‘rift sequence’ associated with the stretchingand rifting stages of the Antarctic continent by Gam-boa and Maldonado (1990). The deposition of theunits forming the LS was induced by the creationof accommodation space in response to extensionaltectonic activity in the incipient CBB (Prieto et al.,1998). Tectonism and subsidence controlled the dis-tribution of sediments in the newly formed basin, aswell as synsedimentary deformation of the deposits.Sedimentation of the LS took place in a narrowgraben, presently situated beneath the AP margin(Prieto et al., 1998) (Fig. 3). Significant synsedi-mentary deformation is suggested by the divergentstratified seismic facies occurring in association withnormal faults (Fig. 7), and by locally landward-dip-ping configurations related to the rotation of faultedbasement blocks (Fig. 3).

8.2. Formation of the Upper Sequence

The identification within the US of two types ofseismic units with different physiographic locations(slope and basinal units), and the interfingering pat-tern at the base of the AP and SSI margins (Fig. 3),suggest that the US responds to a cyclic depositionalpattern (Jeffers and Anderson, 1990). Their setting(offshore glaciated landmasses) and the observedseismic facies (internal stratal pattern and sedimen-tary architecture) suggest that these units have beendeposited by glacio-marine sedimentary processes.Most glacio-marine depositional models based uponseismic stratigraphy (e.g. Larter and Barker, 1989;Jeffers and Anderson, 1990; Cooper et al., 1991;Larter and Cunningham, 1993; Bart and Anderson,1996) do involve a strong cyclicity that is attributedto the alternation of periods of advance and retreatover the shelf of the ice sheets covering the glaciatedcontinents.

Our interpretation is, that the CBB slope unitswere deposited during glacial periods, when theice sheets expanded and advanced over the margin(Fig. 8). Such an interpretation agrees with the gen-erally accepted glacio-marine sedimentation models(e.g. Larter and Barker, 1989; Jeffers and Anderson,1990; Cooper et al., 1991; Larter and Cunningham,1993; Larter and Vanneste, 1995; Bart and Ander-son, 1996). The development of the CBB basinalunits should, therefore, have taken place at the endof glacial periods and during interglacial periods,when the ice sheets retreated (Fig. 9). During thislatter phase, the shelf and the upper slope weremainly characterised by sediment starvation, depo-sitional by-pass and erosion (Vorren et al., 1989;Henrich, 1990; Bart and Anderson, 1996).

8.2.1. Slope unitsThe seismic configuration of the slope units (U1,

U3, U5 and U7) suggests that they were formedwhen the ice sheets expanded and advanced ontothe AP and SSI margins. An advancing ice sheetproduces severe erosion of the existing sea-floorstrata, and the eroded material is mainly transportedsubglacially at the base of the ice sheet (Fig. 8),before finally being deposited at the moving ice-sheet grounding line. Eventually, the advance ofthe grounding line to the platform break causes the


Fig. 8. Sedimentary model showing the subglacial erosion and deposition during a glacial episode where the front of the ice sheetreaches the shelf edge. The result is the formation of a large sedimentary prograding wedge developed from the upper platform to thebase of the slope.

glacial debris to be deposited beyond the platformedge and be transported further downslope by grav-ity-driven processes (gravity and mass-flow) (Powell,1984), thereby contributing to the oversteepening ofthe slope (Larter and Barker, 1989; Larter and Cun-ningham, 1993) and to the development of down-lapping stratified slope units with a predominanceof proximal debris flow deposits. These processesend with the formation of a prograding sedimentary

Fig. 9. Model showing the sedimentary processes during a glacial retreat. At the upper platform the erosion and sediment by-passpredominate over sedimentation. Slope instabilities and subglacially-generated marine currents rework and redistribute sediments initiallydeposited on the platform.

wedge with downlapping foresets similar to thosegeometries described by Bart and Anderson (1995)and known as grounding zone wedge (Fig. 8).

The strong erosion associated with the advancingice sheets may have created the widespread trun-cation surfaces that bound the top of each seismicunit. The size and volume of each slope unit in theAP margin, as well as their internal configuration,are probably controlled by the duration and inten-


sity of the glacial period (Bart and Anderson, 1995;Vanneste et al., 1995; Kuvaas and Kristoffersen,1996). The velocity of the advance, the maximumextension and the permanence of the ice sheets overthe margin are, hence, the key factors. The glacialperiod with the largest magnitude and duration pre-sumably corresponds to the deposition of U1, assuggested by the great extent of this unit (thicknessof 0.25–0.35 s TWTT), and also by its relativelyhigh-angle (10º) progradational foresets on the upperplatform (Figs. 3 and 4). In contrast, the glacial peri-ods that led to the deposition of U3, U5 and U7 musthave been of much smaller magnitudes and duration,as suggested by their reduced dimensions (thicknessfrom 0.15 to 0.2 ms), and by only subtle evidenceof progradation on the upper platform (Figs. 3 and4). Comparable changes in the internal configura-tion of slope strata observed along other segmentsof the Antarctic margins have been attributed to thehigh-frequency sea-level fluctuations of the Plio–Pleistocene which would have limited the durationof grounding events on the shelf (Anderson et al.,1991; Bartek et al., 1991; Alonso et al., 1992).

The variable seaward extension and basinwardprogradation of each individual slope unit along theAP margin can be attributed to the uneven volumesof sediment supplied to the margin by the ice sheet.The strongest progradation occurs in an area fac-ing the Orleans Trough, indicating that this troughis a dominant sediment source for the AP margin.The highest progradational advances (3.5 km, fromU5 to U7) have been identified offshore from thatthrough (Fig. 4). This probably was a thicker andfaster-moving ice stream with enhanced transport ef-ficiency with respect to adjacent segments of the icesheet. Similar point-source transport and depositionhas been suggested to explain the variable extent ofseismic units in the NW Antarctica Peninsula con-tinental shelf (Bart and Anderson, 1996). Variationsin the seismic configuration within the slope units,such as minor internal unconformities, have beenidentified in front of Orleans Trough (Fig. 4). Weattribute them to short-lived stabilisations or retreatsof the grounding line during a main glacial episodewhich might have been combined with local shifts ofthe transport and depositional pathways. This wouldconfirm the idea that it is the mouth of a glacialtrough where the most complete and detailed sedi-

mentary record of the glacial advances is contained(Bart and Anderson, 1996).

The mounded landward-dipping body within U7(Fig. 3) was previously observed by Banfield and An-derson (1995), who interpreted it as diamicton de-posits which had developed beneath the ice sheetclose to the grounding line. Such a grounding zonemoraine would indicate the maximum extent reachedby the ice sheet during the last glacial advancerecorded in this segment of the AP upper platform.In this setting, the prograding sedimentary wedge ofU7 would have been deposited seaward from thatgrounding zone (Fig. 3), and would consist mainlyof glacio-marine and sediment gravity flow deposits,reworked by marine currents and probably by melt-water streams in a proglacial environment. The iso-lated landward- dipping mounded body of U7 is, how-ever, a local feature. During the time of its depositionthe grounding line possibly advanced to the platformedge in most of the AP margin segments, such as theOrleans Trough segment, where ice feed was greatest.

The preservation and lateral continuity of theslope units in the CBB were probably strongly con-trolled by subsidence and isostatic depression pro-duced by extensional tectonism and ice-sheet load-ing, respectively. There are no data about subsidencevalues in the CBB, although in a recent study onthe geodynamic evolution of the CBB Gracia et al.(1996) claimed a significant rate of subsidence dur-ing the opening of the basin and the formation of theLS, which probably continued during the sedimenta-tion of the US.

8.2.2. Basinal unitsThe seismic features of the basinal units (U2,

U4, U6 and U8), with chaotic interstratified and un-dulating stratified facies at the foot of the slopes,changing to a planar continuous facies on the centralbasin floor (Figs. 3 and 7), suggest that these unitsare mainly generated by downslope depositional pro-cesses, such as mass gravity flows, slumps, turbiditycurrents and gravitational instabilities (Drewy andCooper, 1981; Wright and Anderson, 1982; Ku-vaas and Kristoffersen, 1996). The result of theseprocesses are the Gebra Valley, small gullies anddeposits, such as the channels and levees forming thepoorly developed depositional fanlobe at the foot ofthe AP slope (Figs. 7 and 10).

64M

.J.Prieto

etal./M

arineG

eology157

(1999)47–68

Fig. 10. Seismic line G13 and interpreted line drawing showing the geometry of U7 and the channels and gullies eroded in the slope. Location of the seismic line in Fig. 1b.


These processes probably began at the end ofglacial periods, when large volumes of sedimentdeposited on the slope during ice-sheet advance be-came unstable (Fig. 9). Several studies (Aksu andHiscott, 1989; Henrich, 1990; Syvitski et al., 1996)have demonstrated that sediments that are initiallydeposited on a steep slope are subsequently removedand redeposited on the basin floor via turbidity cur-rents. Moreover, the glacio-isostatic rebound pro-duced by the retreat of the ice sheets would probablyalso have produced instabilities big enough to gen-erate mass movements of varying magnitudes, assuggested by Bart et al. (1998) (Fig. 9). The role ofgravity flows in the formation of the CBB basinalunits during interglacials has also been demonstratedby means of detailed sediment core studies, whichidentified small-scale turbidite sequences (Fabres etal., 1997). We, therefore, propose that the basinalunits of the CBB were deposited after the glacialmaxima and during periods of ice-sheet retreat.

After the main phases of glacial advance in the APmargin, the transfer of sediments to the basin floorseems to have occurred through the Orleans Trough,the Antarctic Sound and the two smaller glacialtroughs which lie in between (Figs. 2 and 9). Inthis set of troughs, ice sheets and ice streams clearlyreached their maximum thickness during glaciation,but ice tongues probably lasted longer than in theopen unconstrained shelf and continued to occupythe troughs long after the start of the ice retreat.Some glacial ice might even have lasted until thetotal disintegration of the ice sheet. In such a set-ting, ice-tongues would have continued to promoteactive sediment transport and release to the marginand deep basin (Fig. 9). A particularly significantrole might have been played by turbidity currentsgenerated by sediment-laden subglacial meltwater(Henrich, 1990). These currents would have beenable to excavate gullies downslope (Vorren et al.,1989; Kuvaas and Kristoffersen, 1991), such as thoseidentified in front of the Orleans Trough and the APmargin (Figs. 2 and 10). Due to the longer exposureof glacial troughs and channels to subglacial condi-tions, turbidity currents would have remained activefor a longer time in these areas. Such conditionswould have concluded with the complete disintegra-tion of the ice tongues and, eventually, the infillingof the less active troughs.

The basinal units also contain sediments fromsources in the SSI margin. During interglacial peri-ods, terrigenous materials, mainly from the fjords inthe SSI, were transported basinward by melt waterplumes and turbidity currents (Jeffers and Ander-son, 1990). Despite the relatively smaller size ofthe SSI drainage area (Domack and Ishman, 1993)and, consequently, the faster decay of ice sheets, atthis point the SSI margin seems to have delivereda comparatively large amount of sediment to theCBB (Jeffers and Anderson, 1990). This observationis also supported by our data set, where a ratherthick (0.7 s TWTT) sedimentary cover is trapped inthe northern subbasin, behind the sedimentary bar-rier formed there by the axial volcanic lineament.The large volume of sediment coming from the SSImargin has been attributed to the warmer conditionsof the ice sheet covering the SSI, to high rates ofmeltwater production (Griffith and Anderson, 1989),to oceanographic and physiographic characteristicsof the margin and bays and to the more easily erodedvolcaniclastic source (Domack and Ishman, 1993).

Ice-rafted debris derived from calved icebergs(Henrich, 1990), and pelagic and hemipelagic par-ticles from gravitational settling (Banfield and An-derson, 1995) are also produced by the set of pro-cesses that contributed to the formation of the basinalunits. Nevertheless, pelagic or hemipelagic compo-nents probably represent only a small proportion ofeach basinal unit (Yoon et al., 1994), as suggestedby the irregular distribution of these units and theirvariable thickness along and across the basin.

The irregular distribution of the basinal units wasalso influenced by volcanic and tectonic factors, suchas the growth of volcanic edifices and the presence ofbasement highs during their deposition. This couldexplain the differential distribution displayed by U2and part of U4, which are restricted to the southernsubbasin, compared to that of U6 and U8 basinalunits that occupy the whole basin floor.

9. Conclusions

The seismic-stratigraphic analysis of high-resolu-tion seismic records from the CBB allows us to iden-tify two sedimentary sequences: a faulted, foldedLower Sequence, and an almost undeformed Upper


Sequence. Tectonic activity, related to the first stageof extension and opening of CBB, was responsi-ble for the observed stratal patterns, synsedimentarydeformation and spatial distribution of the LowerSequence. This sequence was deposited in a grabenpresently situated below the AP margin. In contrast,tectonism played only a minor role in the formationof the Upper Sequence. Stratal pattern within thissequence were mainly controlled by the advance andretreat of ice sheets during glacial cycles.

The successive advance and retreat of the icesheet over the margin, produced a cyclical sedimen-tation pattern. The formation of slope units (U1, U3,U5 and U7) in the AP and SSI margins occurred dur-ing glacial periods, and the development of basinalunits (U2, U4, U6 and U8) occurred during ice-sheetretreats at the end of glacial periods and duringinterglacial periods. This cyclical pattern did notchange significantly during the time of sedimenta-tion of the Upper Sequence. The sedimentary growthpattern of the slope units was progradational duringU1 and mainly aggradational during the depositionof U3 to U7. This change from progradational toaggradational growth of the margin was mainly con-trolled by the duration of glacial periods. Variationsin sediment supply related to the presence of icestreams controlled the non-uniform progradation ofeach slope unit during the associated glacial periodin the AP margin. The sedimentary growth pattern ofthe basinal units is aggradational.

Geodynamic and tectonic evolution of the CBBpartially conditioned the deposition of the Lower Se-quence and Upper Sequence in various ways (Prietoet al., 1998). First, the absence of the Lower Sequenceand U1 in the SSI margin was probably due to a laterdevelopment of this margin at the time of deposition.Consequently, there was no accommodation spaceto deposit these units. Second, tectonic subsidencefavoured the preservation of slope units in the AP andSSI margins. Third, and finally, tectonism determinedthe structural framework of the basin and thereforethe distribution of basinal units (Prieto et al., 1998).

Acknowledgements

We cordially thank the captain of the R=V Hes-perides, Commandant V. Quiroga, and the crew

members who participated in the Gebra 93 cruise.We thank M. Farran and J. Sorribas for processingthe swath-bathymetry data. We would also like tothank E.W. Domack, J.B. Anderson and the edi-tor, whose comments and suggestions have greatlyimproved the article. This work was supported bythe Spanish ‘Programa Nacional de Investigacion enla Antartida’, project ANT-93-1008-C03-01, fundedby the CICYT. The GRC Geociencies Marines at theUniversity of Barcelona has been supported by ‘Gen-eralitat de Catalunya’ grant GRC 94=95-1026. M.J.Prieto benefited from a fellowship from the ‘Uni-versitat de Barcelona’ (CPI-16), and M. de Batistis senior research assistant of the FWO Vlaanderen.The participation of the University of Gent (Bel-gium) in the Gebra 93 cruise was supported by theBelgian Program of Antarctic Research Programme,project A3-02-002.

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