Previous seismic stratigraphic interpretations on the Antarctic Peninsula continental shelf are based on single-channel and multichannel seismic profiles (Larter and Barker, 1989, 1991b; Larter and Cunningham, 1993; Bart and Anderson, 1995; see "Background and Scientific Objectives"). During Leg 178, existing MCS profiles across Sites 1100, 1102, and 1103, collected by the Osservatorio Geofisico Sperimentale and the British Antarctic Survey (see Fig. F3; "Seismic Stratigraphy" in the "Explanatory Notes" chapter; and "Appendix" and Fig. AF1, both in the "Leg 178 Summary" chapter), were examined to establish the seismic stratigraphy described below. Sonic log velocities and individual laboratory (Hamilton Frame) measurements of P-wave velocity on cores recovered at Sites 1100 and 1103 have been used to estimate velocities and to assign depths to reflectors in all three sites along the shelf transect.
Bulk densities for Site 1103 have been derived from MST measurements (with a GRAPE, 2-cm spatial resolution), index property measurements (1.5-m spatial resolution), and lithodensity logging data (hostile environment lithodensity sonde, measurement separation ~15 cm; Fig. F33). Densities from downhole logging are available between 84 and 240 mbsf. The MST data used below 247 mbsf are discontinuous because of low recovery (34%). Also, the index properties measurements and the MST-density data are significantly offset. No density information is available within the depth interval 0 to 84 mbsf.
Two differently derived velocities are available. Discrete Hamilton Frame (PWS3) measurements provide high-quality data for the better recovered part of the hole (247 to 355 mbsf). Small matrix pieces occasionally recovered from 70 to 220 mbsf provide some velocity information (see "Physical Properties"). The sonic velocity data collected with the SDT at Site 1103 is of low quality because many of the recorded waveforms are often noisy; hence, picking the first arrival for the traveltime was difficult. However, several waveforms are recorded at each depth, and after removing noisy waveforms and changing tracking and window parameters for the velocity analysis, the data quality improved considerably. The final selection of sonic velocity data was based on six independent quality criteria (e.g., agreement of second and first run, agreement between values obtained by analogue picking, and coherence tracing). The velocity log constructed in this way is given in Figure F34 (see "Downhole Measurements" for a comparison with velocities derived from analogue picking). Near the seafloor, data points have been added from Site 1100. This seems to be appropriate because seismic tomography velocities show similar values near the seafloor for Site 1103 (Tinivella et al., 1996).
The gap in velocity data between 10 and 70 mbsf was closed by a linear interpolation (connecting the two last known data points with a straight line), and a two-way traveltime/depth function was calculated (Fig. F35). The curve has an approximately linear central part (80-240 mbsf, from logging data) and a sharp bend toward higher velocities below 250 mbsf.
Forty-two traces of the profile crossing Site 1103 (profile I95-152) have been filtered, statically corrected, gain-recovered, and displayed in Figure F36. The most important reflectors are marked and named. The seafloor (time = 0) has been placed at the maximum of the first positive reflection, instead of at the first onset of energy (the time difference would be ~6-7 ms), to facilitate comparison to traveltimes taken from the conventional printouts. Traveltimes to reflectors below are given from maximum to maximum. The proposed traveltime/depth relationship is necessarily speculative but ties reflectors well in depth to major changes in the lower part of the logging data (Reflectors a, b) and to observed sedimentological changes within the recovered part of Site 1103 (Reflectors c, d, and e).
We identify three seismic units, which correspond to Sequence Groups S1, S2, and S3 described by Larter and Barker (1989) (Fig. F3).
Sequence Group S1 is characterized by nearly flat-lying topset reflectors in the inner and middle shelf that seaward become steeply dipping foreset reflectors (Fig. F3). Topset reflectors are marked by lateral changes in reflector amplitude (i.e., from high to moderate), whereas foreset reflectors have lower amplitudes. S1 has an external wedge geometry. The lower boundary of S1 is a high-amplitude reflector that truncates reflectors from underlying Sequence Groups S2 and S3 (Fig. F3).
Sequence Group S1 at Site 1102 consists of an upper thin (~60 m) package of high-amplitude flat-lying topset reflectors and a lower thick sequence of steeply dipping (foreset) reflectors (Fig. F37). Truncation and downlap relationships within the foresets suggest that S1 comprises several individual prograding sequences (Larter and Barker, 1989).
Sequence Group S1 at Site 1100 is characterized by a thick (i.e., 350 ms/402 m) unit of flat-lying parallel and subparallel topset reflectors (Fig. F38). High amplitudes are common in the uppermost topset reflectors, whereas the lower section has alternating high- and moderate-amplitude reflectors. The topset section was drilled with very low recovery at Site 1100, to a depth of 110.5 mbsf.
Sequence Group S1 at Site 1103 (0-255 mbsf) consists of horizontally stratified topset reflectors (Figs. F3, F39). The entire topset section was drilled, with very low recovery. From this site and toward the mid-shelf high (MSH), Sequence Group S1 decreases in thickness to about 57 m. The estimated 255 m thickness of Sequence Group S1 at Site 1103 is based on a velocity model, but it is clear from Figure F34 and from the recovered core that a change in sediment properties, most probably coincident with the sequence boundary, lies above 255 m, perhaps at 242-247 mbsf, where logging was disrupted and recovery improved (see "Operations").
Sequence Group S2 is typified by low-angle dipping reflectors that become more steeply dipping seaward (Fig. F3). Strata from S2 are truncated at the top by the erosional unconformity that marks the boundary between S1 and S2. At Site 1100, Sequence Group S2 consists of steeply dipping foreset reflectors that downlap onto a lower deeper reflector, indicating a prograding sequence (see "Appendix" and Fig. AF1, both in the "Leg 178 Summary" chapter). Internal truncations and downlap relationships in the foreset section are common, which implies the existence of many individual prograding sequences. Sequence Group S2 is not found at Site 1103, where an unconformity separates reflectors from Sequence Group S1 above from reflectors of Sequence Group S3 below (Fig. F39).
Sequence Group S3 (255 mbsf and continuing below the cored section) is characterized by a series of reflectors that dip gently seaward from the MSH, a structural high located landward of Site 1103 (Larter and Barker, 1989, 1991b) (see "Background and Scientific Objectives"). Reflectors of Sequence Group S3 at Site 1103 are parallel or subparallel throughout the section. In tracing the reflectors away from the site, however, we can distinguish two reflector packages separated by a group of three high-amplitude reflectors (Reflectors c, d, and e in Fig. F36). Above these three reflectors, S3 is characterized by parallel and subparallel gently dipping reflectors that are truncated landward by the unconformity that bounds Sequence Group S1 and Sequence Group S3 (Fig. F39 and Reflectors a and b in Fig. F36). Below the three high-amplitude reflectors, S3 reflectors diverge seaward and pinch out landward (Fig. F36). The three strong reflectors (c, d, and e) separating the two reflector packages correspond to changes in the density and variations in the velocity of sedimentary formations (i.e., from 3300 to 2100 m/s between 270 and 285 mbsf) (Figs. F33, F34). All reflector packages in S3 have lower amplitudes landward and increase in amplitude seaward of the site location.
Sediment recovered from Sequence Group S1 at Sites 1100 and 1103 (see "Lithostratigraphy") indicates that during the development of S1, a grounded ice sheet regularly extended across the continental shelf. Individual prograding sequences recognized by truncation and downlap of reflectors in Sequence Group S1 also suggest repeated episodes of ice advance and retreat.
Sequence Group S3 was recovered at Site 1103. The top of this sequence corresponds roughly to an improvement in sediment core recovery at 247 mbsf. The upper part of S3, above the three strong Reflectors c, d, and e (Fig. F36), consists of massive diamict (see "Lithostratigraphy"). Below these reflectors, sediments are poorly sorted sandstones and mudstones. S3 has been interpreted as sediment gravity flows and turbidites deposited on a very active, glacially influenced second-order slope (see "Lithostratigraphy").
Drilling results from Sites 1100, 1102, and 1103 indicate that the three Sequence Groups S1, S2, and S3 were deposited under a glacial regime. The glacial section drilled in the shelf transect of the Antarctic Peninsula is interpreted as changing upward from deposits representing a proximal glaciomarine environment (Sequence Group S3) to principally subglacial strata deposited beneath the base of a grounded ice sheet on the shelf (Sequence Group S1). During deposition of S2 and S1, glacial sequences are characterized by low-angle topsets and steep foresets that prograde the margin by 20 km (along the line of Fig. F3). The differences in acoustic character and geometry of glacial sequences between S3 and S2 indicate an important change in the style of sedimentation, which is probably linked to a change in the glacial regime of this part of the Antarctic margin.