Sites 1100, 1102, and 1103 (Fig. F1) are part of a transect of the continental shelf, off Anvers Island. They are positioned along multichannel seismic (MCS) reflection profiles on the depositional axis of Lobe 1, one of the four shallow progradational lobes of the continental shelf on the Pacific margin of the Antarctic Peninsula (Larter et al., 1994, 1997). With Site 1097 drilled some 180 nmi (330 km) to the southwest in the trough between Lobes 3 and 4, the continental shelf transect was planned to sample, understand, and date the major changes in geometry of the glacial prograding wedge. These changes are assumed to mark major stages in the development of the ice sheet.
Glacial deposition on the continental shelf and slope is largely confined to the lobes and is described by the internal configuration and geometry of seismic Sequence Groups S1 and S2 (Fig. F2; "Continental Shelf" in Barker and Camerlenghi [Chap. 2, this volume]; "Appendix" and Fig. AF1, both in the "Leg 178 Summary" chapter). These sequence groups are considered to have been produced by ice-stream sediment transport during periodic ice-sheet sediment grounding to the shelf edge over the past 5 m.y. or so (Larter and Barker, 1989, 1991b). Sequence Group S1 is aggradational and moderately progradational. Sequence Group S2 is mainly progradational, with a marked upper boundary where topsets have been eroded and foresets truncated. The two seismic sequence groups are not always continuous between lobes, so strict correlation along the continental shelf cannot be made. However, sequence group geometries are virtually identical in the four lobes of the Antarctic Peninsula continental shelf, and westward to at least 105°W in the Amundsen Sea.
The continental shelf transect as originally planned was composed of five sites (APSHE-01A, -02A, -03A, -04A, and -10A [Fig. F2]). Four of these (APSHE-01A through -04A) were considered primary sites, and APSHE-10A was the alternate for site APSHE-05A (Site 1097), considered less valuable because of later interaction with ridge-crest subduction (Larter and Barker, 1991a). The strategy on the transect was to drill (1) APSHE-01A at the shelf edge to identify and date glacial-interglacial transitions in the youngest S1 foresets, (2) APSHE-03A and APSHE-02A to identify and date the main change in wedge geometry during progradation of the glacial wedge (the onset and completion of S2 topset erosion, respectively), and (3) APSHE-04A to examine the "preglacial"/glacial transition (the conformable boundary between the oldest part of Sequence Group S2 and the youngest Sequence Group S3).
All four sites were to be compared, if possible, to the continental rise sequences.
Operations at Site 1097 revealed that drilling progress in continental shelf glacial sediments would be slower than estimated from previous Ocean Drilling Program experience in similar environments (Leg 119 in Prydz Bay, Antarctica, and Leg 152 on the East Greenland margin). In addition, time spent waiting on ice (icebergs approaching the vessel while drilling) and weather (a 2-m limit on vessel heave when drilling in water depths shallower than 650 m) suggested that the original drilling objectives for the continental shelf transect could not be achieved within the allocated time. We decided to begin the transect by drilling proposed site APSHE-03A (Site 1100) to gain information on the sharp, most easily identifiable erosional truncation at the S1/S2 boundary, estimated to lie at ~400 mbsf. Site 1100 was chosen in 458.6 m of water on the seaward termination of a till body 40 m thick. The till body was presumably deposited during the latest advance of the ice sheet, which apparently did not reach the continental shelf edge at this location (Vanneste and Larter, 1995; Fig. F3). The ship was positioned with a 300-m southeast offset from site APSHE-03A along seismic profile I95-152 (to shotpoint 1010). No penetration was obtained with the 3.5-kHz shipboard sub-bottom profiler. The seafloor topography close to the site is rough because of iceberg grounding.
Drilling progress at Site 1100 was extremely slow because of excessive vessel heave (see "Operations"). When it was clear that no significant progress was being made at this site (100.9 m of penetration and more than 84 hr spent on site, mainly waiting on weather, excluding the digression to drill Site 1101 on the continental rise), we decided to drill site APSHE-01A (Site 1102). We hoped that softer, less consolidated, recent foreset sediment would allow safer drilling with critical vessel heave. Site 1102 was located according to the original plan at a water depth of 430.5 m, less than 1 km from the sharp edge between the continental shelf and the steep (over 17º) continental slope. The modified objective of the site was to obtain recovery of recent foreset sediments, presumably debris flow deposits, for comparison with older, buried foreset deposits to be drilled at other sites of the continental shelf transect. However, a combination of excessive heave and the presence at the surface of an unstable carapace of loose gravel and boulders (imaged with a camera survey conducted while waiting for heave to subside; see Movie M1, a QuickTime video, and "Seabed Observations, Site 1102" in "Lithostratigraphy") prevented the creation of a stable hole. The drill collar penetrated no more than 14 mbsf.
We then decided to drill alternate site APSHE-10A (Site 1103), hoping that, vessel heave permitting, drilling in older formations would result in better recovery because of the expected higher degree of lithification of the sediments. At this site, seismic Sequence Group S1 unconformably overlies Sequence Group S3 at relatively shallow depth (~210 m), and the S3-S4 transition was expected at ~430 m.
Site 1103 was located in 493.5 m of water, on a rough seafloor with no 3.5-kHz profiler penetration. The unexpected improvement in sea conditions allowed us, finally, to drill as planned. Operations at Site 1103 ended before completion of the drilling program because the Leg 178 drilling time expired at 1130 hr on 26 March.
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Ms 178IR-110