6. Site 10971

Shipboard Scientific Party2


Site 1097 (Fig. F1; also see "Appendix" and Fig. AF1,  both in the "Leg 178 Summary" chapter) was drilled on the outer continental shelf of the Pacific margin of the Antarctic Peninsula to gain a better understanding of the major changes in the geometry of the glacial prograding wedge.

Site 1097 lies at a water depth of 552 m on the outer shelf, between glacial progradational Lobes 3 and 4 described by Larter et al. (1994, 1997). The site lies 14 km inshore of the continental shelf edge, within the broad trough that connects the shelf edge to the glacially overdeepened inner shelf in Marguerite Bay (Fig. F2A, F2B) and George VI Sound. These areas were conduits for ice draining from a large area of Palmer Land and Alexander Island.

Single-channel and multichannel seismic (MCS) profiles (Bart and Anderson, 1995; Larter et al., 1997) show that the thickness of topset beds of glacial seismic Sequence Groups S1 and S2 of Larter and Barker (1989, 1991a) is reduced to <200 m in the trough (compared to ~ 400 m within the progradational lobes). Sequence Groups S1 and S2 are considered to have been produced by ice-stream transport during ice-sheet grounding to the shelf edge. They are the units whose deposition has determined the main progradation and aggradation of the margin during the Pliocene-Pleistocene (Barker, 1995). The sampling of these units within a progradational lobe was among the drilling objectives on the continental shelf drilling transect.

Beneath Sequence Group S2 is Sequence Group S3, which is more continuous along the margin and parallel-bedded downdip (lacking a clear paleoshelf break). S3 either pinches out or is truncated at its downdip end (on the evidence of seismic reflection profiles). It was therefore considered to record an earlier or transitional stage of glacial deposition on the outer shelf or upper slope, before ice sheets regularly extended to the shelf break. Sequence Group S3 is more accessible to drilling in an interlobe area than on a lobe, and at this site S3 is clearly separated from collisional tectonics because the age of the ridge crest-trench collision on this sector of the margin is ~17 Ma (Barker and Camerlenghi, Chap. 2, this volume). Below S3 lies Sequence Group S4, across the "uplift unconformity." The upper boundary of Sequence Group S4 shows erosional unconformity associated with syn- to postcollisional uplift. S3 onlaps this unconformity, marking postcollisional subsidence. The collision and the S3/S4 unconformity were considered here to predate the onset of Antarctic Peninsula glaciation.

The site was spudded in on a highly reflective and irregular seafloor that showed no 3.5-kHz penetration (Fig. F3) and probably was scoured by iceberg grounding during the Holocene. Drilling at Site 1097 was intended to examine the preglacial and early glacial history of the Antarctic Peninsula margin and to test the hypotheses of the "preglacial" nature of Sequence Group S3 and the uplift/subsidence origin of the S4/S3 boundary by (1) sampling "preglacial" (S3) lithologies and helping test the synchroneity of the conformable S3/S2 boundary along the margin and (2) sampling the (probably tectonic) S3/S4 boundary on the upper continental slope where it should be near-conformable.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Leg 178 Participants" in the preliminary pages of the volume.

Ms 178IR-107