Site 1097 is located on the outer continental shelf of the Antarctic Peninsula in 552 m water depth, some 14 km from the continental shelf edge (Fig. 2). It lies in a topographic low between two of the lobate depocenters where the uppermost, fully glacial progradational sediment Sequence Groups S1 and S2, recognized in seismic data on the continental shelf, are best developed (Fig. 14). It was selected to examine the underlying Sequence S3 where that sequence would be more easily accessible, being less deeply buried. The nature of S3 was unknown, and the site was intended to examine Antarctic Peninsula paleoclimate before the fully glacial sediment sequence groups were deposited.
Drilling at Site 1097 was slow and difficult for three main reasons. First, the nature of the sediments (unsorted, including large clasts) confined us to rotary-core-barrel (RCB) drilling, not the best way of recovering the soft till matrix, and dictated a slow start. Second, icebergs interrupted drilling on two occasions; and third, limitations on ship heave in water depths less than 650 m also halted drilling twice. In rather more than 5 days, Site 1097A was drilled to a depth of 436.6 mbsf. The sediment shows glacial influence in all of the recovered intervals, including S3. Recovery varied from about 2.3% in the uppermost 80 m of Sequence Group S1 to 18% in Sequence Group S2 down to 150 m, and to 16% in S3.
Recovery was too poor for S1 to be described or dated. Material recovered from the underlying S2 is massive diamict, defined as poorly sorted sediment, with clasts supported by a muddy matrix. It appears to reflect subglacial deposition, in keeping with seismic reflection interpretations of the nature of S2. The age of S2 at this site is constrained, mainly by diatoms (Fragilariopsis barronii and upper part of Thalassiosira inura Zones) supported by a radiolarian assemblage (Upsilon Zone) with a broader age range, to younger than 4.6 Ma and older than about 3.8 Ma. The boundary between S2 and the underlying S3 (seen as conformable in presite seismic survey) lies within the upper T. inura and Upsilon Zones, at about 4.5 to 4.6 Ma.
S3 sediments are more variable but include similar massive diamicts, clast-rich with sizes ranging up to boulders (a 0.5-m granite boulder was drilled within S3), and containing reworked diatoms and foraminifers (as do S2 diamicts). Diamicts are interpreted as deformation tills formed by subglacial reworking and transport of marine muds because of a grounded ice sheet that periodically occupied the continental shelf (Fig. 15). Stratified diamicts in one core, with erosive bases that grade upward to clayey silt with dropstones, are interpreted as debris flows that occurred near the ice-sheet grounding line. Laminated and massive bioturbated muds contain variable amounts of iceberg-rafted debris and are considered glaciomarine. They show evidence of soft sediment deformation that could be the result of iceberg turbation.
Biofacies variation throws light on the changing depositional environments sampled within S3. Three biofacies were distinguishedbased on the degree of reworking of benthic foraminifers and other biogenic matterthat reflect subglacial deposition, ice-proximal deposition, and open shallow-marine shelf conditions (Fig. 15). On this basis, the topmost sediments of S3 are subglacial, as is S2. However, they are marine from 180 to 210 mbsf, subglacial from 220 to 290 mbsf, and vary between more marine and ice proximal beneath. This variation reflects periodic migration of the ice-grounding line across the continental shelf.
Preliminary biostratigraphic age determination of Sequence S3 was difficult because of reworked forms, but two possible diatom age constraints are (1) older than 4.85 Ma below 218 mbsf and (2) younger than 5.6 Ma below 289 mbsf. Radiolarians suggest an age older than 4.6 Ma below 294 mbsf. Regional similarities in benthic foraminifers suggest an early Pliocene age for much of S3 at this site. Paleomagnetic measurements show some stable high-inclination directions, others stable but low inclination (possible debris flows), and still others (diamicts) unstable. At this time, in the absence of clear biostratigraphic age constraints, a magnetostratigraphic interpretation is unjustified.
Downhole logs were not run because of hole instability, and many shipboard measurements on Site 1097 cores were of limited value because of poor recovery. The physical properties data are sparse, but porosity in Unit II as low as 30% may be attributed to subglacial shear consolidation. Also, natural gamma activity drops toward the base of the hole, matching a general decrease in clay content. Limited velocity and density measurements prevent an accurate correlation between hole and seismic profiles but serve to clearly distinguish the seismic sequences.
Drilling at the site was sufficient to confirm the nature of S1 and S2 and to establish the upper part of S3 as glacial (although perhaps less fully glacial than S1 and S2). The age and nature of the S3/S2 boundary were established.
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