Previous seismic stratigraphic interpretations exist for the vicinity of Site 1097, based on single-channel seismic (Bart and Anderson, 1995) and MCS profiles (Larter and Barker, 1989, 1991b; Larter et al., 1997) (see "Background and Scientific Objectives").

The existing multichannel and single-channel seismic profiles across Site 1097, collected by CSIC-Universidad de Granada and Rice University, respectively (Fig. F2; also see "Appendix" and Fig. AF1, both in the "Leg 178 Summary" chapter, and "Seismic Stratigraphy" in the "Explanatory Notes" chapter), have been re-examined to establish the seismic stratigraphy described below. Individual laboratory (Hamilton Frame) measurements of P-wave velocity on cores recovered at Site 1097 have been used to estimate velocities and thus to assign depths to reflectors.

Acoustic stratigraphy at Site 1097 is characterized by a sequence of reflectors that dip gently seaward from the mid-shelf high (MSH), a structural high located ~27 km landward of Site 1097 (Larter and Barker, 1991b; Barker and Camerlenghi, Chap. 2, this volume). We differentiate three seismic units (Figs. F28, F29) that coincide with Sequence Groups S1, S2, and S3 of Larter and Barker (1991b) and Larter et al. (1997).

The upper 30-40 mbsf of seismic Unit I is obscured by the gun reverberation, accentuated in the single-channel seismic profile (see Fig. F1). The lower part of Unit I at the site location is characterized by nearly flat-lying moderate- to low-amplitude discontinuous reflectors, which change toward the shelf edge to low-angle dipping reflectors of weak amplitude. Landward, these reflectors pinch out ~20 km from Site 1097 before reaching the MSH. The lower boundary of seismic Unit I is marked by a high-amplitude reflector that farther offshore truncates reflectors from the underlying seismic Unit II.

Seismic Unit II has a wedge geometry. At Site 1097, it is characterized by gently dipping high- to moderate-amplitude semicontinuous topset reflectors. The varied acoustic character of Unit II is best explained by variable P-wave velocities and bulk densities from sediments cored at Site 1097 (i.e., 2450 m/s at the top and 1950 m/s toward the bottom) (Fig. F30). Thickness of seismic Unit II increases toward the outer shelf, where gently dipping topset reflectors grade into weak amplitude, steeply dipping foresets. Seismic Unit II thins gradually landward until it pinches out at the MSH. The lower boundary of Unit II is a high-amplitude reflector against which outer shelf reflectors from Unit II appear to downlap seaward of Site 1097, indicating a prograding sequence.

Seismic Unit III (150-410 mbsf and continuing to beneath the cored section) is marked by gently dipping reflectors that become more steeply dipping toward the outer shelf. The seaward vertical stacking of reflectors indicates an aggrading to moderately prograding sequence. Reflectors from seismic Unit III are characterized by lateral changes in amplitude and continuity. Alterations in acoustic character can be explained by variable P-wave velocities and bulk densities from sediments cored at Site 1097 (Fig. F30). The upper 50 m of seismic Unit III has low velocities averaging 1900 m/s. From 200 to 230 mbsf the velocities range from 2100 to 2700 m/s. Below this high-velocity zone, the velocity drops to ~1900 m/s and then slowly increases downhole to ~2100 m/s at 350 mbsf. Thickness of seismic Unit III decreases gradually landward until it pinches out at the MSH and increases seaward toward the paleoslope. At Site 1097, the Unit III/Unit II boundary (the S3/S2 boundary) appears conformable.

All the seismic units drilled at Site 1097 are the result of glacial sedimentation on the Antarctic Peninsula continental shelf. Although no sediments were recovered from seismic Unit I, the truncation of reflectors from the underlying unit suggests erosion at its base, possibly by ice-sheet advance. Sediments recovered from seismic Unit II (80 to 150 mbsf) are characterized by massive diamicts that are interpreted as deforming tills (see "Lithostratigraphy"). This lithologic interpretation is compatible with biostratigraphic, reworked assemblages that indicate subglacial till deposition (Zone I; see "Biostratigraphy"). These sediments suggest that during the development of seismic Unit II, a grounded ice sheet regularly extended at least to where Site 1097 is located and probably to the shelf edge. These results confirm the interpretations of Larter and Barker (1989, 1991) and Larter et al. (1997).

In the upper part of seismic Unit III (198-235 mbsf), core recovery improved. This coincides with a change downhole to lower P-wave velocities and moderate amplitude reflectors, a lithologic change from the massive diamict of Unit II to muddy diamict (see "Lithostratigraphy"), and the beginning of biostratigraphic Zone II (see "Biostratigraphy"), all of which indicate an ice-proximal marine environment.

Drilling results from Site 1097 show that all three seismic units were deposited under a glacial regime and differentiate two stages of margin growth. During the time of deposition of seismic Unit III, margin growth was characterized by vertical aggradation to moderate progradation of the continental margin (Fig. F28). During deposition of seismic Units I and II, glacial sequences show more pronounced progradation (3 km), expressed by low-angle topsets and steep foresets (Fig. F28). The differences between seismic Units II and III in the stacking patterns of glacial sequences seaward of the drill site suggest an important change in the style of sedimentation, linked to a change in the glacial regime in this part of the Antarctic margin.