Seismic Stratigraphy of the Central Bransfield Basin (NW Antarctic

ELSEVIER Marine Geology 157 (1999) 47±68

Seismic stratigraphy of the Central Brans®eld Basin (NW Antarctic Peninsula): interpretation of deposits and sedimentary processes in a glacio-marine environment

MarõÂa JoseÂPrietoa,Ł, Gemma Ercilla b, Miquel Canals a,MarcdeBatistc a GRC GeocieÁncies Marines, Dpt. Geologia DinaÁmica, GeofõÂsica i Paleontologia, Universitat de Barcelona, Campus de Pedralbes, E-08071 Barcelona, Spain b Dpt. de Geologia Marina i Oceanogra®a FõÂsica, Institut de CieÁncies del Mar (CSIC), Avda. Joan de BorboÂs=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 Brans®eld Basin is a deep narrow trough between the northern tip of the Antarctic Peninsula and the South Shetland Islands. Analyses of single-channel, high-resolution seismic re¯ection data are used to characterise the seismic stratigraphy of the Central Brans®eld Basin. The tectonised acoustic basement is overlain by a 1-s-thick sedimentary cover composed of two main sedimentary sequences. The Lower Sequence, which shows synsedimentary deformation, has only been identi®ed on the Antarctic Peninsula margin. The Upper Sequence is a complex sedimentary package composed of eight seismic units whose distribution, geometry and seismic facies allow two types of seismic units to be distinguished: slope and basinal units. The slope units, constituted by progradational strati®ed seismic facies, form a sedimentary wedge extending from the shelf edge. The basinal units ®ll the basin ¯oor showing chaotic and undulated seismic facies that change basinward into strati®ed seismic facies. Both types of seismic units display an inter®ngering pattern at the base of the slope, suggesting an alternating shift of the sedimentary depocentre, from the slope to the basin¯oor and vice versa. This alternate pattern indicates that the sedimentary processes responsible for the in®lling of the Central Brans®eld Basin followed a cyclic pattern, which has likely been associated with the advance and retreat of the ice sheets over the margins during glacial and interglacial episodes. During glacial periods, the ice sheets advanced, eroded the shallower sea ¯oor areas and deposited diamicton and debris ¯ow deposits along the moving grounding line, resulting in a progradational 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 to coastal locations and glacial troughs, where processes of meltwater formation might have been signi®cant. Sediment-laden under¯ows are generated within these troughs, from where they ¯ow and spread over the shelf and down the slope to the basin¯oor as sediment gravity ¯ow deposits. The combined effect of these processes is a progradational build up of the shelf and an aggradational in®lling of the basin ¯oor, together with the development of the inter®ngering pattern at the base of the slope.  1999 Elsevier Science B.V. All rights reserved.

Keywords: glacio-marine sedimentation; seismic stratigraphy; Central Brans®eld Basin; glaciated margins

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

1. Introduction the cessation of subduction, and from the extensional stress induced in the AP continental crust (Barker, The Central Brans®eld Basin (CBB) is a Ceno- 1982; Barker and Dalziel, 1983; Lawver et al., 1995). zoic marginal basin located between the Antarctic The present-day CBB is an asymmetric trough Peninsula (AP) and the South Shetland Islands (SSI), characterised by a steep rectilinear SSI margin, a Antarctica (Fig. 1). The CBB is separated from the gentle sinuous AP margin and an almost ¯at basin Western and Eastern Brans®eld Basins by two highs ¯oor disrupted by volcanic edi®ces. These volcanoes formed by Deception and Bridgeman Islands, respec- form a discontinuous lineament along the basin axis tively. During the past decades, the CBB has been the where they depict an incipient sea¯oor spreading object of various geological and geophysical studies. ridge (GraÁcia et al., 1996, 1997). Seismic and vol- These studies provided new data relevant to the canic activity (Pelayo and Wiens, 1989), high heat basin's formation and geodynamic evolution (Gam- ¯ow (Nagihara and Lawver, 1989), a positive mag- boa and Maldonado, 1990; Grad et al., 1992; Hen- netic anomaly (Roach, 1978; GraÁcia et al., 1996) and riet et al., 1992; Barker and Austin, 1994; Lawver a large negative gravity anomaly (Garret, 1990) have et al., 1995). Most of these surveys have focused been reported in the CBB, supporting the idea that on the deep structure of the basin and little atten- it is a young, active rift basin (Saunders and Tarney, tion has been paid to the detailed stratigraphy of the 1984; Fisk, 1990; Grad et al., 1992; Lawver et al., basin's sedimentary in®ll. Only Jeffers and Anderson 1995). (1990) and Ban®eld and Anderson (1995) studied the Seismic re¯ection data collected in the CBB in ®ne-scale seismic stratigraphy of the CBB using in- recent years have allowed the recognition of the termediate-resolution re¯ection seismic data. In the acoustic basement distinct from the sedimentary present study, we aim to contribute to this work cover (Gamboa and Maldonado, 1990; Jeffers and by interpreting the sedimentary architecture and the Anderson, 1990; Acosta et al., 1992; Henriet et al., stratal geometry patterns from seismic-stratigraphic 1992; Barker and Austin, 1994; Ban®eld and An- analyses of single-channel, high-resolution seismic derson, 1995; GraÁcia et al., 1996). The acoustic re¯ection pro®les. We also propose a depositional basement in the basin axis is generally believed to be model that relates the observed morphologic fea- composed of thinned continental crust intruded by tures, seismic facies, stratal and growth patterns to dykes (Ashcroft, 1972; Birkenmajer, 1992; Grad et the geodynamics of the CBB and to its glacio-marine al., 1992), and volcanic materials, all related to the sedimentary setting. Our study focuses on slope and extension along the basin axis (Barker and Austin, basin physiographic provinces, since it is here that 1995). In the AP margin, the acoustic basement prob- seismic-stratigraphic units reached their maximum ably consists of metasediments and metavolcanics thickness. For reasons of simplicity we refer to these that are laterally equivalent to the sedimentary series two provinces together as CBB. de®ned onshore (Smellie, 1984), and are believed to correspond to the in®lling of a previous marginal basin (Gamboa and Maldonado, 1990; Barker and 2. Geological setting Austin, 1995; Prieto et al., 1997). In fact, the Trinity Peninsula Group in the AP, and the Myers Bluff For- The Paci®c margin of the AP was a collision mation in the SSI show similar sedimentologic and margin during the Mesozoic and Cenozoic (Barker, structural characteristics which hint at a same origin 1982). Along the South Shetland Trench, subduc- in a backarc basin (Aitkenhead, 1975; Hyden and tion stopped or decreased to a slow rate about 4 Tanner, 1981; Smellie, 1984). The acoustic basement Ma ago, when spreading ceased at the segment of is arranged in blocks, rotated along normal faults cre- the Antarctic±Phoenix ridge between the Hero and ated by the stretching and break up of the former AP Shackleton Fracture Zones (Fig. 1a). The opening of and marginal basin (Gamboa and Maldonado, 1990; the Brans®eld Basin (BB) has been dated between 4 Jeffers and Anderson, 1990; Acosta et al., 1992). and 1.3 Ma (Barker and Dalziel, 1983). It probably The acoustic basement is overlain by a sedi- resulted from the mechanism of roll-back, following mentary cover with a thickness of 700±1000 ms M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 49 Ácia et al., 1996). Thick lines show : South Shetland Islands Microplate. (b) Bathymetric map of the Central Brans®eld Basin, with location of SSIMP Fig. 1. (a) Geodynamic setting of thethe Brans®eld location Basin. of the seismic lines in Figs. 3, 4, 6, 7 and 10. seismic lines recorded during the Gebra'93 cruise. Labels A to F indicate the location of the six large volcanic edi®ces (modi®ed from Gra 50 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68

TWTT formed during two major sedimentary se- seismic units across the whole seismic grid. Further- quences separated by a regional unconformity. The more, the analysis of the main features (boundaries, Lower Sequence ®lls an older graben system and internal con®guration, geometry, thickness) of these is faulted and occasionally tilted towards the conti- seismic units has allowed us to interpret depositional nent (Gamboa and Maldonado, 1990; Prieto et al., processes and how the basin in®lling has evolved to 1997). The Upper Sequence is less tectonised and its present con®guration. controls the present morphology of the AP margin. Swath bathymetry data were acquired over an The Lower Sequence, or `rift sequence', has been area of 10 000 km2 using the combined SIMRAD attributed to the rift stage during which the AP mar- EM-12=EM-1000 system. The data set fully cov- gin was formed. The Upper Sequence, or `drift se- ers the AP continental margin from the upper slope quence', contains internal erosional unconformities down to the basin ¯oor and the slope of the SSI mar- and has been attributed to glacio-marine sedimentary gin, between Bridgeman and King George Islands processes induced by the advances and retreats of (GraÁcia et al., 1996) (Fig. 1b). the Antarctic ice sheet during the Plio±Quaternary glacial periods (Anderson et al., 1983; Jeffers and Anderson, 1990; Henriet et al., 1992; Ban®eld and 4. Morphology Anderson, 1995). The CBB is a NE±SW-trending basin, 230 km long, 130 km wide and 1950 m deep. The swath 3. Data base and methodology bathymetry map of the CBB (GraÁcia et al., 1996, 1997) provides a useful tool to recognise the physio- A grid, corresponding to 16 lines of single-chan- graphic elements and the main morphosedimentary nel, high-resolution seismic re¯ection data, with a features (Fig. 2). This map does not cover the AP and total length of about 1100 km, was collected in the SSI shelves, and to characterise these areas we used a CBB (Fig. 1b). The seismic source was a 2.9 l Bolt compilation of pre-existing single-beam bathymetry 1500 C airgun. The re¯ected signals were detected data (Lawver et al., 1995). A later map by Lawver with a SIG 120 three-channel streamer with an active et al. (1996) covers additional segments of the SSI section of 150 m. The data were recorded digitally slope. with the Elics Delph2 high-resolution acquisition The AP margin has an irregular shelf, about 60 system. Post-acquisition data processing, including km wide and 200 m deep. It is deeply incised by bandpass-®ltering, deco-®ltering and scaling, was four troughs, that run slightly oblique to nearly per- carried out on both the Delph2 and the Phoenix Vec- pendicular to the margin: the Orleans Trough in the tor processing systems. The vertical resolution of the southwest, Antarctic Sound in the northeast and two processed data was about 5 ms and penetration was unnamed troughs in between (Fig. 2). The troughs often in excess of 1 s TWTT. have U-shaped cross-sections, which suggests that Across-basin pro®les (trending approximately they are glacially scoured features (Jeffers and An- NNW±SSE) cross the slope of the AP continen- derson, 1990). The AP slope shows two platforms tal margin, the basin ¯oor, some of the central or terraces at different depths, giving the margin a seamounts and segments of the steep narrow SSI step-like morphology. A wide (10±15 km) wavy up- slope. These pro®les allow the comparison of both per platform (Jeffers and Anderson, 1990; Acosta et margins and their in¯uence in the sedimentary sup- al., 1992) is situated between 600 and 800 m. It is ply to the basin. Along-basin pro®les (trending ap- slightly tilted towards the basin (0.6±2.4%) and only proximately ENE±WSW) cross part of the AP slope slightly incised by the glacial troughs. The lower and the central basin ¯oor. These pro®les allow us platform lies at a depth between 1000 and 1400 m. to recognise the seismic sequences and seismic units It is a discontinuous feature, 20±35 km wide, with that built the AP margin and ®lled the basin ¯oor, a conspicuous convex shape that dips gently (3%) and to investigate their spatial and temporal vari- towards the basin. This platform connects basinward ations. We have correlated unconformity-bounded with a steep (8%) lower slope that abruptly termi- M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 51 data has been used (Lawver et al., 1995). Fig. 2. Morphosedimentary map of the Central Brans®eld Basin. To characterise the SSI margin, a compilation of pre-existing single-beam bathymetry 52 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 nates at the basin ¯oor. Where the lower platform is shaped volcanoes appear scattered between the axial not present, the slope is much steeper (up to 18%). lineament and the SSI margin (Figs. 1 and 2). The AP margin is indented by submarine valleys (Fig. 2). A large U-shaped valley, named Gebra Valley, was identi®ed by Canals et al. (1994) in the 5. Acoustic basement of the Central Brans®eld northeastern half of the AP margin (57ë45, 63ë30) Basin (Fig. 1b and Fig. 2). Gebra Valley is 30 km long, 6 km wide and extends from depths of 800 to 1900 The acoustic basement has been identi®ed at dif- m on the basin ¯oor. Its ¯anks are 200 m high and ferent depths in the CBB. It crops out at the sea¯oor the generally ¯at bottom is marked by irregularities at the AP and SSI margins, at a depth of around which might be interpreted as mass gravity ¯ow 600 m, whereas in the basin ¯oor it is covered by deposits. The head of the valley is formed by several a sedimentary cover up to 1 s TWTT, and has been semicircular and steep scarps (8%), the morphology identi®ed at a maximum depth of 3.2 s TWTT. The of which resembles that of active slide scars (Fig. 1b basement is everywhere tectonised. From both mar- and Fig. 2). On the southwestern half of the AP gins, normal faults deepen the basement basinward, margin, several 10±15-km long gullies incise the de®ning the axial trough where the basin is located. slope. They have a U-shaped cross-section, and reach The seismic character of the basement varies across widths of up to 2 km and depths of up to 40 m. the basin. At the margins the basement is composed The rectilinear SSI margin is characterised by a of strati®ed and largely continuous re¯ections, sug- narrow shelf (5 km) up to 200 m deep, where a gesting a sedimentary character (Fig. 3). In contrast, scarp extends down to the steep slope. The shelf on the basin ¯oor the basement shows a chaotic is incised by short troughs developed between the seismic character with its top formed by a strongly islands and at the mouth of the bays and fjords of re¯ective surface. This suggests a direct relation with the islands. They also cut the slope until reaching the axial volcanic edi®ces and volcanic intrusions a ¯at, 700 m deep platform, in front of Greenwich of the CBB. The top of this `crystalline' basement and King George Islands. This platform has not been can be traced for 15 km from the volcanic edi®ces, identi®ed in front of Livingston and King George either forming the sea¯oor or as a highly re¯ective Islands, where the slope is a continuous feature from continuous surface below the sedimentary cover. the shelf break to the basin ¯oor (Lawver et al., 1995) (Fig. 2). The slope shows the highest dip, up to 26%, in front of King George Island. The foot of 6. The sedimentary cover in the Central the slope shows small depositional irregularities. Brans®eld Basin The basin ¯oor of the CBB is a relatively narrow (<30 km wide), ¯at trough. It shows four bathymet- The overall seismic stratigraphy of the CBB con- ric levels that progressively deepen the sea ¯oor from sists of a complex package of sedimentary sequences 1000 to 1950 m, from the southwest to the north- and units that overlies the acoustic basement. Two east. The deepest level constitutes the King George major sedimentary sequences have been identi®ed Basin, a very ¯at, 20 km-wide and 50-km long basin within this sedimentary cover: the Lower Sequence (Fig. 1b and Fig. 2). Six large volcanic edi®ces, and Upper Sequence (Fig. 3). with different morphologies (labelled from A to F in Fig. 2) rise above the sea¯oor (GraÁcia et al., 1996). 6.1. The Lower Sequence Altogether, they form a discontinuous lineament that extends from Deception to Bridgeman Islands. This The Lower Sequence (LS) has only been recog- lineament divides the basin ¯oor into two halves nised in the AP margin, where it directly overlies or subbasins: a northern subbasin, between the SSI the sedimentary basement. It extends from the upper margin and the volcanic lineament, and a southern platform (1.5 s TWTT deep) to the base of the slope subbasin, between the lineament and the AP margin. and the central basin ¯oor (3.2 s TWTT deep). The In addition to the large volcanic edi®ces, small cone- LS onlaps the basement onshore, and downlaps it M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 53 ails of this e is shown in Fig. 1b. 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 det seismic line show the prograding strati®ed facies and inter®ngering pattern at the foot of the Antarctic Peninsula slope. Location of the seismic lin 54 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 offshore (Fig. 3). The top of the LS is a widespread It overlies U1 on the upper platform and Unit 2 (U2) erosional surface identi®able throughout the AP mar- at the base of the slope (Fig. 3). The lower boundary gin (Fig. 3). The LS forms a faulted, slightly de- of U3 is a downlap surface along the whole margin, formed sedimentary wedge, composed of discontin- and the upper boundary is an erosional surface on the uous strati®ed facies with several con®gurations: par- upper platform, which becomes more conspicuous in allel, progradational, and divergent. The parallel con- the western sector of the AP margin (Fig. 4). This ®guration occurs on the upper platform, the progra- erosional surface changes basinward becoming con- dational facies on the slope and the divergent facies formable, locally outcropping on the slope (Fig. 3). at the base of slope, in the vicinity of normal faults, The seismic facies of this unit is progradational strat- suggesting synsedimentary deformation. Landward i®ed with very low-angle (1ë) dipping re¯ections on dipping (4ë) re¯ections have also been recognised at the upper platform which become steeper (10ë) to- the base of the slope, probably related to basement wards the slope. The main depocentres of U3 are blocks tilted along normal faults (Fig. 3). The main located within the upper platform, and their thick- depocentres of the LS are located at the base of the ness ranges from 0.15 to 0.2 s TWTT (Fig. 5b). AP margin and range from 0.15 to 0.25 s. This unit also seems to be present in the SSI margin, where a 0.1±0.2 s TWTT thick sedimentary wedge 6.2. The Upper Sequence overlies the basement (Fig. 6). It occupies the same stratigraphic level as U3 on the AP margin (i.e. be- The Upper Sequence (US) is composed of eight low Unit 4 on the basin¯oor and below Unit 5 on sedimentary units, labelled 1 to 8, from bottom to the slope). This would suggest that either the two are top. We have de®ned two types of sedimentary units equivalent or the one at the SSI margin is older. The based on their distribution: slope units (U1, U3, U5 U3 equivalent wedge at the SSI margin is composed and U7) and basin-¯oor units (U2, U4, U6 and U8). of the discontinuous progradational strati®ed seismic The four slope units are observed in the AP margin. facies, and is bounded at the bottom by a downlap Only units U3, U5 and U7 have been identi®ed in surface and at the top by an erosional surface. the SSI margin. The basin-¯oor units (U2, U4, U6 Unit 5 (U5) occurs within the AP and SSI mar- and U8), and their subunits, are located in the central gins (Figs. 4 and 6). On both margins U5 overlies basin ¯oor. Slope and basinal units inter®nger at the U3, and part of U4 at the foot of both slopes. The base of the slope. base of U5 is a downlap surface and the top is an erosional surface; both surfaces extend along the entire 6.2.1. Slope units margin (Fig. 4). U5 shows discontinuous prograda- Unit 1 (U1), the oldest sedimentary unit of the tional strati®ed seismic facies, with very low-angle Upper Sequence, has only been identi®ed on the (1ë) dipping re¯ections on the upper platform AP margin, where it extends from the upper plat- and higher-angle (6ë) dipping re¯ections downslope form to the basin ¯oor of the southern subbasin (Fig. 4). U5 constitutes an irregular wedge, partly (Figs. 3 and 4). Along the whole margin, U1 is com- eroded, on the platform. It thins progressively until it posed of progradational strati®ed seismic facies with pinches out both landward and basinward. The max- high-angle (7±10ë) dipping re¯ections that down- imum thickness of the unit is 0.15 s TWTT at the AP lap onto the LS basinward and onlap the basement margin and 0.1 s TWTT at the SSI margin (Fig. 5c). landward (Figs. 3 and 4). The top of the unit is an Unit 7 (U7) is the youngest unit on the slope at erosional surface in the upper platform, changing both margins. Along the AP margin, it constitutes to a conformable surface basinward (Fig. 3). The a sedimentary wedge on top of the platform edge, thickness of U1 ranges between 0.25 and 0.35 s which is continuous along the whole margin (Figs. 3, TWTT, and the main depocentres are located on the 4 and 6). This wedge is particularly well developed upper platform (Fig. 5a). U1 shows minor internal at the southwestern sector of the AP margin, fac- unconformities on the upper platform, adjacent to ing the mouth of Orleans Trough, where it mostly the Orleans Trough (Figs. 2 and 4). reaches the basin ¯oor (Fig. 4). It overlies U5 at the Unit 3 (U3) has been identi®ed in the AP margin. platform, and part of Unit 6 (U6) at the foot of the M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 55

Fig. 4. Seismic line G1 and interpreted line drawing showing the distribution of the slope and basinal seismic units. Location of the seismic 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- 6.2.2. Basinal units cides with the sea ¯oor on the upper platform, where Unit 2 (U2) only occurs in the central and east- it is mainly conformable although locally erosional ernmost sectors of the southern subbasin, between relief of about 30 m deep and 1 km wide can be iden- the AP margin and the axial volcanic lineament, ti®ed. At the mouth of the Orleans Trough, where U7 here de®ned by volcanoes D and E (Fig. 1b). U2 reaches the base of the slope, its top is an erosional overlies U1 at the foot of the AP margin, while fur- surface with incisions about 50 m deep and 1 km ther down into the basin it directly overlies a rough wide. U7 is characterised by progradational strati- basement (Fig. 3). The top of U2 is a subhorizon- ®ed seismic facies of very low-angle (1.5ë) dipping tal conformable surface, although locally, there is re¯ections in the upper platform and more steeply evidence of strong erosion, i.e. in the vicinity of inclined re¯ectors (4±8ë) beneath the slope (Figs. 3 volcanic edi®ce D. A minor discontinuity within U2 and 6). Additionally, we have also observed within allows two subunits to be identi®ed: a lens-shape U7 semitransparent chaotic seismic facies at the head lower subunit and a subtabular upper subunit. Sev- of the Gebra Valley, and a 50 m high depositional eral seismic facies appear within U2. Seismically mound with landward-dipping re¯ections (2.6ë) in chaotic subtabular interstrati®ed depositional bodies the AP margin (Fig. 3). The thickness of the unit with erosional bases occur at the foot of the slope ranges from 0.15 to 0.2 s TWTT (Fig. 5d). and on the basin ¯oor in front of the Gebra Valley 56 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68

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 maximum thickness of depocentres and thick line marks the present day slope break. Contour interval 50 ms. M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 57

Fig. 5 (continued). 58 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68

Fig. 6. Seismic line G1 and interpreted line drawing showing the slope units (U3, U5 and U7) identi®ed at the South Shetland Island margin, and the basinal units (U4, U6 and U8) inter®ngering at the base of the slope. Location of the seismic line in Fig. 1b.

(Fig. 3). Basinwards, the dominant seismic facies is Both base and top of U4 are conformable surfaces. discontinuously strati®ed (Fig. 3). The thickness of However, where the top of U4 crops out at the U2 depocentres ranges between 0.2 and 0.3 s TWTT, sea¯oor it becomes an irregular surface, with mounds thinning towards the AP margin, until it disappears. and gentle erosional depressions as observed at the Unit 4 (U4) overlies U3 at the base of the AP foot of the AP slope (Fig. 7). An internal unconfor- slope, overlies U2 between the AP margin and the mity allows us to divide U4 into two subunits with axial volcanoes in the southern subbasin, and over- different seismic facies and spatial distribution. The lies the basement in the northern subbasin (Fig. 6). lower subunit is restricted to the southern subbasin, M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 59 e undulated and chaotic 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 th seismic facies at the foot of the Antarctic Peninsula slope. Location of the seismic line in Fig. 1b. 60 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 between the AP margin and the volcanic lineament, it overlies U6 (Figs. 3 and 6). The base of the unit where it downlaps the basement. The seismic facies is a conformable surface that onlaps U6. The top of this lower subunit is continuous, parallel and strat- of U8 forms the sea¯oor and locally has erosional i®ed with abundant interstrati®ed seismically chaotic truncations (Fig. 7). A continuous parallel strati®ed bodies (Fig. 7). The upper subunit occupies the seismic facies with high continuity and amplitude whole basin ¯oor and onlaps the underlying units. re¯ectors characterises this unit. In the vicinity of It mainly shows a continuous, planar strati®ed seis- the seamounts, this seismic facies gradually changes mic facies (Fig. 7) that changes to a discontinuous to a more chaotic facies, which could be attributed to subparallel strati®ed facies in the northern subbasin the presence of volcanic material (Fig. 3). Modi®ca- and King George Basin. The contact between these tions to the dominant seismic facies also occur in the subunits is a conformable surface with the excep- vicinity of the AP margin, where re¯ectors locally tion of King George Basin, where the upper subunit show undulating morphology (Fig. 7). The geome- onlaps onto the lower one. Lens-shaped bodies with try of U8 is lens-shaped, with the thickest section chaotic seismic facies appear locally within the U4 following the basin axis. The thickness is less than upper subunit and at its top in the northern sub- 0.1 s TWTT except for a 4 km wide graben at the basin (Fig. 6). From their seismic characteristics, foot of the SSI slope, in the central sector, where the we interpreted these as sills intruded through frac- thickness reaches 0.15 s TWTT (Figs. 3 and 4). tures into the sediments (Fig. 6). The thickness of U4 changes markedly along an across-basin section. The maximum thickness, which is around 0.35 s 7. Depositional growth patterns in Central TWTT, occurs at the base of the SSI margin and in Brans®eld Basin King George Basin. In the remaining basin the mean thickness is about 0.15 s TWTT, except at the Gebra The depositional growth pattern of the AP mar- Valley where U4 decreases to only 0.05 s TWTT gin is characterised by stacking of the LS and the (Fig. 7). slope units U1, U3, U5 and U7 of the Upper Se- Unit 6 (U6) covers the whole basin ¯oor lying quence. The SSI margin is characterised by stacking either over U5, as in the southernmost sector of the of slope units U3, U5 and U7. The LS is faulted and AP margin, or over U4, as in the rest of the basin. folded throughout the CBB. It ®lls and smooths the The base of the unit is a conformable subhorizontal irregular basement morphology and displays mainly surface. In King George Basin, sediments ®ll and an aggradational con®guration. Within the US, U1 onlap onto the irregular geometry of the depocentre. has a distinct progradational internal structure, in- The top of the unit coincides with the sea¯oor in dicating that the AP margin underwent signi®cant a large area at the base of the AP margin, where progradation during the time of deposition of U1 it often shows an erosional character (Fig. 7). The (Figs. 3 and 4). In contrast, U3, U5 and U7 in the unit has a characteristic continuous parallel strati®ed AP, essentially show an aggradational growth pattern seismic facies and contains interstrati®ed chaotic with only a subtle internal progradational con®gura- bodies near the foot of the AP slope (Fig. 3). In tion (Fig. 3). Nevertheless, facing of Orleans Trough, the central sector, mounded and tabular bodies with in the southwestern segment of the AP margin, the subparallel strati®ed seismic facies form the sea ¯oor growth pattern is locally progradational as suggested relief up to 150 m, which is highlighted in the swath by the basinward advance of the platform-break (3.5 bathymetry data (Fig. 7). The present-day Gebra km) during the deposition of U5 and U7 (Fig. 4). Valley is located over U6. The maximum thickness The basin ¯oor is composed of four vertically of U6 is 0.2 s TWTT in King George Basin, and it stacked, aggrading seismic units U2, U4, U6 and thins and pinches out towards the margins. U8, separated by erosional unconformities. These Unit 8 (U8) is the youngest basinal seismic unit units inter®nger with the slope units at the base of in the CBB and is uniformly distributed on the basin the AP and SSI slopes (Fig. 3). Locally, as in King ¯oor (Figs. 3 and 4). In the southernmost sector it George Basin, the basin-¯oor sedimentary package overlies U7 (Fig. 4) while in the rest of the basin can be subdivided into two sections. The lower sec- M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 61 tion (composed of U2 and part of U4) is attached to 8.2. Formation of the Upper Sequence the AP margin. The upper section (composed of the upper part of U4, U6 and U8) occupies the whole The identi®cation within the US of two types of basin ¯oor, onlaps the underlying units and has a seismic units with different physiographic locations clearly aggradational growth pattern. (slope and basinal units), and the inter®ngering pattern at the base of the AP and SSI margins (Fig. 3), suggest that the US responds to a cyclic depositional 8. Discussion pattern (Jeffers and Anderson, 1990). Their setting (offshore glaciated landmasses) and the observed The seismic-stratigraphic analysis of the CBB seismic facies (internal stratal pattern and sedimen- indicates that the sedimentary history of this basin tary architecture) suggest that these units have been comprises two clearly distinct stages corresponding deposited by glacio-marine sedimentary processes. to the formation of the LS and US. The regional Most glacio-marine depositional models based upon unconformity dividing the two sequences and their seismic stratigraphy (e.g. Larter and Barker, 1989; differentiated stratal patterns suggest a major change Jeffers and Anderson, 1990; Cooper et al., 1991; in the style in which the CBB was ®lled (Fig. 7). Larter and Cunningham, 1993; Bart and Anderson, We discuss two sedimentary scenarios. The ®rst 1996) do involve a strong cyclicity that is attributed explains the formation of the LS, which appears to to the alternation of periods of advance and retreat have been mainly conditioned by the initial structural over the shelf of the ice sheets covering the glaciated development of the CBB. The second scenario refers continents. to the formation of the US, whose deposition has Our interpretation is, that the CBB slope units been mainly governed by the growth and decay were deposited during glacial periods, when the of the ice sheets and therefore by glacio-marine ice sheets expanded and advanced over the margin sedimentation processes. (Fig. 8). Such an interpretation agrees with the generally accepted glacio-marine sedimentation models 8.1. Formation of the Lower Sequence (e.g. Larter and Barker, 1989; Jeffers and Anderson, 1990; Cooper et al., 1991; Larter and Cunningham, Deposition of the LS began with the initial frag- 1993; Larter and Vanneste, 1995; Bart and Ander- mentation, rotation, subsidence and extension of son, 1996). The development of the CBB basinal continental basement blocks, prior to the onset of units should, therefore, have taken place at the end sea¯oor spreading. In fact, the LS would correspond of glacial periods and during interglacial periods, to the `rift sequence' associated with the stretching when the ice sheets retreated (Fig. 9). During this and rifting stages of the Antarctic continent by Gam- latter phase, the shelf and the upper slope were boa and Maldonado (1990). The deposition of the mainly characterised by sediment starvation, depo- units forming the LS was induced by the creation sitional by-pass and erosion (Vorren et al., 1989; of accommodation space in response to extensional Henrich, 1990; Bart and Anderson, 1996). tectonic activity in the incipient CBB (Prieto et al., 1998). Tectonism and subsidence controlled the dis- 8.2.1. Slope units tribution of sediments in the newly formed basin, as The seismic con®guration of the slope units (U1, well as synsedimentary deformation of the deposits. U3, U5 and U7) suggests that they were formed Sedimentation of the LS took place in a narrow when the ice sheets expanded and advanced onto graben, presently situated beneath the AP margin the AP and SSI margins. An advancing ice sheet (Prieto et al., 1998) (Fig. 3). Signi®cant synsedi- produces severe erosion of the existing sea-¯oor mentary deformation is suggested by the divergent strata, and the eroded material is mainly transported strati®ed seismic facies occurring in association with subglacially at the base of the ice sheet (Fig. 8), normal faults (Fig. 7), and by locally landward-dip- before ®nally being deposited at the moving ice- ping con®gurations related to the rotation of faulted sheet grounding line. Eventually, the advance of basement blocks (Fig. 3). the grounding line to the platform break causes the 62 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68

Fig. 8. Sedimentary model showing the subglacial erosion and deposition during a glacial episode where the front of the ice sheet reaches the shelf edge. The result is the formation of a large sedimentary prograding wedge developed from the upper platform to the base of the slope. glacial debris to be deposited beyond the platform wedge with downlapping foresets similar to those edge and be transported further downslope by grav- geometries described by Bart and Anderson (1995) ity-driven processes (gravity and mass-¯ow) (Powell, and known as grounding zone wedge (Fig. 8). 1984), thereby contributing to the oversteepening of The strong erosion associated with the advancing the slope (Larter and Barker, 1989; Larter and Cun- ice sheets may have created the widespread trun- ningham, 1993) and to the development of down- cation surfaces that bound the top of each seismic lapping strati®ed slope units with a predominance unit. The size and volume of each slope unit in the of proximal debris ¯ow deposits. These processes AP margin, as well as their internal con®guration, end with the formation of a prograding sedimentary are probably controlled by the duration and inten-

Fig. 9. Model showing the sedimentary processes during a glacial retreat. At the upper platform the erosion and sediment by-pass predominate over sedimentation. Slope instabilities and subglacially-generated marine currents rework and redistribute sediments initially deposited on the platform. M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 63 sity of the glacial period (Bart and Anderson, 1995; mentary record of the glacial advances is contained Vanneste et al., 1995; Kuvaas and Kristoffersen, (Bart and Anderson, 1996). 1996). The velocity of the advance, the maximum The mounded landward-dipping body within U7 extension and the permanence of the ice sheets over (Fig. 3) was previously observed by Ban®eld and An- the margin are, hence, the key factors. The glacial derson (1995), who interpreted it as diamicton de- period with the largest magnitude and duration pre- posits which had developed beneath the ice sheet sumably corresponds to the deposition of U1, as close to the grounding line. Such a grounding zone suggested by the great extent of this unit (thickness moraine would indicate the maximum extent reached of 0.25±0.35 s TWTT), and also by its relatively by the ice sheet during the last glacial advance high-angle (10ë) progradational foresets on the upper recorded in this segment of the AP upper platform. platform (Figs. 3 and 4). In contrast, the glacial peri- In this setting, the prograding sedimentary wedge of ods that led to the deposition of U3, U5 and U7 must U7 would have been deposited seaward from that have been of much smaller magnitudes and duration, grounding zone (Fig. 3), and would consist mainly as suggested by their reduced dimensions (thickness of glacio-marine and sediment gravity ¯ow deposits, from 0.15 to 0.2 ms), and by only subtle evidence reworked by marine currents and probably by melt- of progradation on the upper platform (Figs. 3 and water streams in a proglacial environment. The iso- 4). Comparable changes in the internal con®gura- lated landward- dipping mounded body of U7 is, how- tion of slope strata observed along other segments ever, a local feature. During the time of its deposition of the Antarctic margins have been attributed to the the grounding line possibly advanced to the platform high-frequency sea-level ¯uctuations of the Plio± edge in most of the AP margin segments, such as the Pleistocene which would have limited the duration Orleans Trough segment, where ice feed was greatest. of grounding events on the shelf (Anderson et al., The preservation and lateral continuity of the 1991; Bartek et al., 1991; Alonso et al., 1992). slope units in the CBB were probably strongly con- The variable seaward extension and basinward trolled by subsidence and isostatic depression pro- progradation of each individual slope unit along the duced by extensional tectonism and ice-sheet load- AP margin can be attributed to the uneven volumes ing, respectively. There are no data about subsidence of sediment supplied to the margin by the ice sheet. values in the CBB, although in a recent study on The strongest progradation occurs in an area fac- the geodynamic evolution of the CBB GraÁcia et al. ing the Orleans Trough, indicating that this trough (1996) claimed a signi®cant rate of subsidence dur- is a dominant sediment source for the AP margin. ing the opening of the basin and the formation of the The highest progradational advances (3.5 km, from LS, which probably continued during the sedimenta- U5 to U7) have been identi®ed offshore from that tion of the US. through (Fig. 4). This probably was a thicker and faster-moving ice stream with enhanced transport ef- 8.2.2. Basinal units ®ciency with respect to adjacent segments of the ice The seismic features of the basinal units (U2, sheet. Similar point-source transport and deposition U4, U6 and U8), with chaotic interstrati®ed and un- has been suggested to explain the variable extent of dulating strati®ed facies at the foot of the slopes, seismic units in the NW Antarctica Peninsula con- changing to a planar continuous facies on the central tinental shelf (Bart and Anderson, 1996). Variations basin ¯oor (Figs. 3 and 7), suggest that these units in the seismic con®guration within the slope units, are mainly generated by downslope depositional pro- such as minor internal unconformities, have been cesses, such as mass gravity ¯ows, slumps, turbidity identi®ed in front of Orleans Trough (Fig. 4). We currents and gravitational instabilities (Drewy and attribute them to short-lived stabilisations or retreats Cooper, 1981; Wright and Anderson, 1982; Ku- of the grounding line during a main glacial episode vaas and Kristoffersen, 1996). The result of these which might have been combined with local shifts of processes are the Gebra Valley, small gullies and the transport and depositional pathways. This would deposits, such as the channels and levees forming the con®rm the idea that it is the mouth of a glacial poorly developed depositional fanlobe at the foot of trough where the most complete and detailed sedi- the AP slope (Figs. 7 and 10). 64 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 ic line in Fig. 1b. 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 seism M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 65

These processes probably began at the end of The basinal units also contain sediments from glacial periods, when large volumes of sediment sources in the SSI margin. During interglacial peri- deposited on the slope during ice-sheet advance be- ods, terrigenous materials, mainly from the fjords in came unstable (Fig. 9). Several studies (Aksu and the SSI, were transported basinward by melt water Hiscott, 1989; Henrich, 1990; Syvitski et al., 1996) plumes and turbidity currents (Jeffers and Ander- have demonstrated that sediments that are initially son, 1990). Despite the relatively smaller size of deposited on a steep slope are subsequently removed the SSI drainage area (Domack and Ishman, 1993) and redeposited on the basin ¯oor via turbidity cur- and, consequently, the faster decay of ice sheets, at rents. Moreover, the glacio-isostatic rebound pro- this point the SSI margin seems to have delivered duced by the retreat of the ice sheets would probably a comparatively large amount of sediment to the also have produced instabilities big enough to gen- CBB (Jeffers and Anderson, 1990). This observation erate mass movements of varying magnitudes, as is also supported by our data set, where a rather suggested by Bart et al. (1998) (Fig. 9). The role of thick (0.7 s TWTT) sedimentary cover is trapped in gravity ¯ows in the formation of the CBB basinal the northern subbasin, behind the sedimentary bar- units during interglacials has also been demonstrated rier formed there by the axial volcanic lineament. by means of detailed sediment core studies, which The large volume of sediment coming from the SSI identi®ed small-scale turbidite sequences (FabreÂs et margin has been attributed to the warmer conditions al., 1997). We, therefore, propose that the basinal of the ice sheet covering the SSI, to high rates of units of the CBB were deposited after the glacial meltwater production (Grif®th and Anderson, 1989), maxima and during periods of ice-sheet retreat. to oceanographic and physiographic characteristics After the main phases of glacial advance in the AP of the margin and bays and to the more easily eroded margin, the transfer of sediments to the basin ¯oor volcaniclastic source (Domack and Ishman, 1993). seems to have occurred through the Orleans Trough, Ice-rafted debris derived from calved icebergs the Antarctic Sound and the two smaller glacial (Henrich, 1990), and pelagic and hemipelagic par- troughs which lie in between (Figs. 2 and 9). In ticles from gravitational settling (Ban®eld and An- this set of troughs, ice sheets and ice streams clearly derson, 1995) are also produced by the set of pro- reached their maximum thickness during glaciation, cesses that contributed to the formation of the basinal but ice tongues probably lasted longer than in the units. Nevertheless, pelagic or hemipelagic compo- open unconstrained shelf and continued to occupy nents probably represent only a small proportion of the troughs long after the start of the ice retreat. each basinal unit (Yoon et al., 1994), as suggested Some glacial ice might even have lasted until the by the irregular distribution of these units and their total disintegration of the ice sheet. In such a set- variable thickness along and across the basin. ting, ice-tongues would have continued to promote The irregular distribution of the basinal units was active sediment transport and release to the margin also in¯uenced by volcanic and tectonic factors, such and deep basin (Fig. 9). A particularly signi®cant as the growth of volcanic edi®ces and the presence of role might have been played by turbidity currents basement highs during their deposition. This could generated by sediment-laden subglacial meltwater explain the differential distribution displayed by U2 (Henrich, 1990). These currents would have been and part of U4, which are restricted to the southern able to excavate gullies downslope (Vorren et al., subbasin, compared to that of U6 and U8 basinal 1989; Kuvaas and Kristoffersen, 1991), such as those units that occupy the whole basin ¯oor. identi®ed in front of the Orleans Trough and the AP margin (Figs. 2 and 10). Due to the longer exposure of glacial troughs and channels to subglacial condi- 9. Conclusions tions, turbidity currents would have remained active for a longer time in these areas. Such conditions The seismic-stratigraphic analysis of high-resolu- would have concluded with the complete disintegration seismic records from the CBB allows us to iden- tion of the ice tongues and, eventually, the in®lling tify two sedimentary sequences: a faulted, folded of the less active troughs. Lower Sequence, and an almost undeformed Upper 66 M.J. Prieto et al. / Marine Geology 157 (1999) 47±68

Sequence. Tectonic activity, related to the ®rst stage members who participated in the Gebra 93 cruise. of extension and opening of CBB, was responsi- We thank M. FarraÂn and J. Sorribas for processing ble for the observed stratal patterns, synsedimentary the swath-bathymetry data. We would also like to deformation and spatial distribution of the Lower thank E.W. Domack, J.B. Anderson and the edi- Sequence. This sequence was deposited in a graben tor, whose comments and suggestions have greatly presently situated below the AP margin. In contrast, improved the article. This work was supported by tectonism played only a minor role in the formation the Spanish `Programa Nacional de InvestigacioÂn en of the Upper Sequence. Stratal pattern within this la AntaÂrtida', project ANT-93-1008-C03-01, funded sequence were mainly controlled by the advance and by the CICYT. The GRC GeocieÁncies Marines at the retreat of ice sheets during glacial cycles. University of Barcelona has been supported by `Gen- The successive advance and retreat of the ice eralitat de Catalunya' grant GRC 94=95-1026. M.J. sheet over the margin, produced a cyclical sedimen- Prieto bene®ted from a fellowship from the `Uni- tation pattern. The formation of slope units (U1, U3, versitat de Barcelona' (CPI-16), and M. de Batist U5 and U7) in the AP and SSI margins occurred dur- is senior research assistant of the FWO Vlaanderen. ing glacial periods, and the development of basinal The participation of the University of Gent (Bel- units (U2, U4, U6 and U8) occurred during ice-sheet gium) in the Gebra 93 cruise was supported by the retreats at the end of glacial periods and during Belgian Program of Antarctic Research Programme, interglacial periods. This cyclical pattern did not project A3-02-002. change signi®cantly during the time of sedimentation of the Upper Sequence. The sedimentary growth pattern of the slope units was progradational during References U1 and mainly aggradational during the deposition of U3 to U7. This change from progradational to Acosta, J., Herranz, P., Sanz, J.L., Uchupi, E., 1992. Antarctic aggradational growth of the margin was mainly con- continental margin: Geologic image of the Brans®eld Trough, trolled by the duration of glacial periods. Variations an incipient ocean basin. In: Poag, C.W., Graciansky, P.C. in sediment supply related to the presence of ice (Eds.), Geologic Evolution of Atlantic Continental Rises. Van streams controlled the non-uniform progradation of Nostrand Reinhold, New York, pp. 49±61. Aitkenhead, N., 1975. The geology of Duse Bay-Larsen inlet each slope unit during the associated glacial period area, North-East Graham Land (with particular reference to in the AP margin. The sedimentary growth pattern of the Trinity Peninsula Series). BAS Sci. Rep. 51, 62 pp. the basinal units is aggradational. Aksu, A.E., Hiscott, R.N., 1989. Slides and debris ¯ows on the Geodynamic and tectonic evolution of the CBB high-latitude continental slopes of Baf®n Bay. Geology 17, partially conditioned the deposition of the Lower Se- 885±888. quence and Upper Sequence in various ways (Prieto Alonso, B., Anderson, J.B., DõÂaz, J.I., Bartek, L.R., 1992. Pliocene±Pleistocene seismic stratigraphy of the Ross Sea: et al., 1998). First, the absence of the Lower Sequence Evidence for multiple ice sheet grounding episodes. Antarct. and U1 in the SSI margin was probably due to a later Res. Ser. (Contrib. Antarct. Res. III) 57, 93±103. development of this margin at the time of deposition. Anderson, J.B., Kurtz, D.D., Domack, E.W., Balshaw, K.M., Consequently, there was no accommodation space 1983. Development of a polar glacial-marine sedimentation to deposit these units. Second, tectonic subsidence model from Antarctic Quaternary deposits and glaciological favoured the preservation of slope units in the AP and information. In: Molnia, B.F. (Ed.), Glacial-Marine Sedimen- tation. Plennum Press, New York, pp. 629±666. SSI margins. Third, and ®nally, tectonism determined Anderson, J.B., Bartek, L.R., Thomas, M.A., 1991. Seismic and the structural framework of the basin and therefore sedimentological record of glacial events on the Antarctic the distribution of basinal units (Prieto et al., 1998). Peninsula shelf. In: Thomson, M.R.A., Crame, J.A., Thomson, J.W. (Eds.), Evolution of Antarctica. Cambridge University Press, Cambridge, pp. 687±691. Ashcroft, W.A., 1972. Crustal structure of the South Shetland Acknowledgements Islands and Brans®eld Strait. BAS Sci. Rep. 66, 43 pp. Ban®eld, L., Anderson, J.B., 1995. Seismic facies investigation We cordially thank the captain of the R=V Hes- of the Late Quaternary glacial history of Brans®eld Basin, peÂrides, Commandant V. Quiroga, and the crew Antarctica. In: Cooper, A.K., Barker, P.F., Brancolini, G. M.J. Prieto et al. / Marine Geology 157 (1999) 47±68 67

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