The Pacific Margin

The Pacific margin includes the region southwest from the South Shetland Islands to 70°S and 80°W. This is a former active margin where ridge-crest segments were progressively subducted (Larter and Barker, 1991a; Henriet et al., 1992), followed by 1-4m.y. of uplift and then long-term subsidence (Larter and Barker, 1989, 1991b; Anderson et al., 1990; Gamboa and Maldonado, 1990; Bart and Anderson, 1995, 1996, 2000; Larter et al., 1997). Seismic profiles show that outer shelf sequences are separated from NW-SE trending mid-shelf basins by the Mid-shelf High (MSH) (Kimura, 1982; Anderson et al., 1990; Gamboa and Maldonado, 1990; Larter et al., 1997).

Several research groups have conducted seismic studies in the area (Kimura, 1982; Larter and Barker, 1989, 1991b; Anderson et al., 1990; Gamboa and Maldonado, 1990; Henriet et al., 1992; Bart and Anderson, 1995, 1996, 2000; McGinnis and Hayes, 1995; Rebesco et al., 1996, 1997, 2002, 2006; McGinnis et al., 1997; Larter et al., 1997; Jin et al., 2002; Jabaloy et al., 2003; Hernandez-Molina et al., 2006a). Cenozoic sequences have been drilled at DSDP Site 325 (Hollister et al., 1976) and at multiple sites during ODP Leg 178 (Barker et al., 1999).

Evidence of Oligocene glaciation exists on King George Island (Birkenmajer, 1991; Dingle and Lavelle, 1998; Troedson and Smellie, 2002), but offshore the first ice sheets on the Pacific margin are inferred from early Miocene IRD at DSDP Site 325 (Fig. AP-1). The oldest sediments from ODP Leg 178 are drift deposits at ODP Site 1095, dated at 9.6 Ma, where all cores show glacial influence and sedimentation rates decrease steadily from the late Miocene to the Quaternary. From seismic studies, Rebesco et al. (1996, 1997, 2002) suggest that sediment drift deposition began around the middle Miocene, with most growth in the late Miocene. However, Hernandez-Molina et al. (2004, 2006b) describe a buried sediment drift of early Miocene age (Fig. AP-1). Uenzelmann-Neben (2006) also interprets depositional patterns of early Miocene continental rise sediments as reflecting bottom current influence, but infers a different flow direction. A regular supply of both glacially derived terrigenous sediments and interglacial biogenic sediments has reached the continental rise since at least the middle Miocene.

The outer continental shelf is underlain by depositional sequences that thicken seaward (Larter and Barker, 1989, 1991b; Anderson et al., 1990; Gamboa and Maldonado, 1990; Larter and Cunningham, 1993; Bart and Anderson, 1995; Table AP-1). Aggrading sequences (S3) without a distinct paleo-shelf edge (PSE) are unconformably overlain by prograding sequences (S2, S1) with an abrupt PSE (Foldout AP-1). The change occurs at the S3/S2 boundary of Larter and Barker (1989, 1991b) and is observed all along the Pacific margin (Anderson et al., 1990; Bart and Anderson, 1995, Larter et al., 1997; Jin et al., 2002) and west along the margin of the Bellingshausen and Amundsen Seas (Nitsche et al., 1997). Drilling at ODP Sites 1097 and 1103, although with very poor recovery, suggests that the change occurred between 8 and 6 Ma (Iwai and Winter, 2002; Bart et al., 2005). Cores from S3 are diamictons with interbedded mudstones and graded sandstones, interpreted by Eyles et al. (2001) as continental slope deposits, although seismic profiles suggest a palaeo-shelf to slope transition further offshore. Cores from S1 and the upper part of S2 show abundant evidence for having been deposited subglacially (Eyles et al., 2001).

Larter and Barker (1989, 1991b) and Larter et al. (1997) interpreted the S3/S2 boundary as representing the onset of frequent advances of grounded ice to the palaeo-shelf edge. However, Bart and Anderson (1995) and Bart et al. (2005, 2007) suggest that palaeo-ice streams cut erosional troughs within S3 and hence existed earlier than the S3/S2 boundary. Although equivocal, the boundary could represent a change in the typical extent of glacial advances, in the dynamic behaviour of ice sheets that advanced onto the shelf, or in the way the ice transported sediments (Larter et al., 1997; Hernandez-Molina et al., 2006a).

Going up-section above the S3/S2 boundary, foreset stratal dips generally increase and PSE progradation in individual sequences decreases (Foldout AP-1). The regional S2/S1 boundary within the upper sedimentary section may have been produced by ice-sheet erosion during glacial periods and lower sea levels after the Late Pliocene increase in the volume of Northern Hemisphere ice sheets (Larter and Barker, 1989, 1991b). By seismic correlation across > 100 km to ODP Site 1101, Rebesco et al. (2006) estimate an age of ~3 Ma for a boundary that they identify as S2/S1. However, this boundary marks a change in stratal geometry that is generally characteristic of S3/S2, and Larter (2007) suggests that it probably corresponds to this earlier unconformity.

Table AP-1: Summary of stratigraphie schemes used in previous publications to describe the late Miocene to recent depositional sequences on the Pacific margin of the Antarctic Peninsula.

Continental rise

Outer continental shelf off Adelaide Island

Outer continental shelf off Anvers Island

Rebesco Sites 1095 et al. and 1101

Hernandez-Molina et al. (2006a)

Larter et al. (1997)

Site 1097

Bart and Anderson (1995)

Bart and Anderson (1995)

Site 1103

Larter and Barker (1989, 1991b)

SU 2

SU 3

Age constraints on sequence boundaries obtained from ODP Leg 178 drill sites are also shown. N.B. Larter et al. (1997) originally described the sequences offshore from Adelaide Island as A1-A3 because of uncertainty in correlations along the shelf, but as results from Leg 178 suggest that the boundaries are of similar age to the S1-S3 sequences described previously offshore from Anvers Island, the same sequence names are shown here for both areas.

Mid-shelf basins contain up to 2 km of sediment in a broad synform that is truncated at shallow depth beneath the seafloor (Kimura, 1982; Anderson et al., 1990; Gamboa and Maldonado, 1990; Larter et al., 1997), making the succession accessible to shallow drilling. However, these sediments have not been sampled, except for the thin Quaternary cover. Basin sediments are probably all Tertiary and may be as young as early Miocene off Adelaide Island, and middle Miocene off Anvers Island (Larter et al., 1997). The inner shelf is mostly shallower than 200 m, but has deep troughs, such as the Palmer Deep where ODP Sites 1098 and 1099 were drilled in 1,400 m of water. Holocene successions ~47 and ~ 108 m thick were recovered at ODP Sites 1098 and 1099, respectively (Shipboard Scientific Party, 1999; Domack et al., 2001; Ishman and Sperling, 2002; Leventer et al., 2002; Shevenell and Kennett, 2002).

Swath bathymetric data show seafloor features of subglacial origin on the shelf (O Cofaigh et al., 2002; Dowdeswell et al., 2004; Amblas et al., 2006), and confirm that a grounded ice sheet with ice streams extended to the shelf edge during the LGM (Pudsey et al., 1994; Larter and Vanneste, 1995). Seafloor core data indicate that retreat of the ice sheet from the outer and middle shelf after the LGM occurred between 18,500 and 13,000 cal. y. B.P. (Pudsey et al., 1994; Heroy and Anderson, 2005).

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