A central objective of Leg 178 was to sample, date, and understand the glacially transported, unsorted sediments deposited in a generally progradational wedge on the outer continental shelf and continental slope. On the Antarctic Peninsula continental margin these sediments are focused into four lobes, in which both shelf topsets and progradational shelf foresets are maximal. The entire West Antarctic margin, however, to at least 105°W, shows the same seismic sequence groups. The main approach to this problem by Leg 178 was to drill a suite of sites along a transect through a well-developed progradational lobe off Anvers Island. Four sites were planned to describe the main changes in depositional geometry within the lobe (Fig. 16). An additional site (Site 1097) had already been drilled in an interlobe area to the southwest, where the deeper section, beneath the main glacial sequence groups, was more accessible.
We attempted three sites (1100, 1102, and 1103) along the shelf transect, with mixed success. One problem was the difficulty of recovering the sediment, which is unconsolidated in the upper section: it is ice-transported, so essentially unsorted, with hard clasts (some very large) within a soft, fine-grained matrix. Another, crucial problem was the 2-m limitation on vessel heave during shallow-water drilling, which severely slowed progress at all shelf sites but particularly at Sites 1100 and 1102. Site 1100 was chosen to sample and date the boundary between the two main glacial seismic sequence groups (S1 and S2), where S1 topsets overlie truncated S2 foresets at about 350 mbsf. The site lies at the edge of an area of hummocky, ice-scoured topography that has been interpreted as the top of a subglacial till, produced by the last ice-sheet advance. Four holes were drilled with very poor recovery in about 4 days. The deepest hole, 1100D, reached 110.5 mbsf but with only 4.8% recovery. By then, repeated pipe trips had so degraded the hole condition that the deep target (~400 mbsf) was judged impracticable.
The only core of Hole 1100C recovered 4.05 m of massive diamict, clast-rich in places and with a diatom-bearing silty clay matrix. In Hole 1100D, only the top three core catchers contained any trace of diatoms, with Section 178-1100D-3R-CC offering the most diverse assemblage. Traces of recrystallized and reworked radiolarians occurred in most core-catcher samples. Foraminifers were rare, and no age inferences can be drawn from the material examined because of the poor nature of preservation. In essence, faunas are open shelf, marine, but abraded. The sediments, poorly consolidated diamicts throughout, have been interpreted at the top of the section as tills and glaciomarine muds, reworked by iceberg grounding. They are useful analogs for deeper sediments at other shelf sites.
Site 1102, at the edge of the continental shelf in 442 m water depth, was selected to examine S1 foresets on the uppermost slope. Four holes were attempted in 1.5 days on site, but excessive heave limited penetration to 7 m at the first two. A camera survey of 50 m radius around the site bottom, undertaken while waiting on heave, showed massive rock boulders grading to a fine sediment cover, allowing Hole 1102C to be better located. Further heave curtailed 1102C at 6.5 m, but when this receded, very difficult conditions in Hole 1102D persuaded us that the rock carapace evident on the television images was thicker than anticipated and would prevent establishment of a stable hole. The rock field at the shelf edge may be the result of the sorting of tills by repeated iceberg scour, with bottom-current removal of suspended fines. Such a carapace may survive at paleoshelf breaks or may have been removed during a subsequent glacial advance.
Site 1103 was our final chance, at the inshore end of the shelf transect in 494 m water depth. Drilling might sample up to 250 m thickness of young S1 glacial topsets, with a deeper target, S3 glacial sediments, most likely slightly older than those reached in our other successful shelf hole at Site 1097. The long swell went away, and we were able to drill to 362.7 mbsf in about 3 days until drilling time expired. Recovery from the upper 247 m was only 2.3% but improved to 34% in the lower 115 m where the matrix became hard. We logged to 240 mbsf.
In the absence of a continuous sedimentary record, stratigraphic subdivision was not attempted. Three lithofacies, however, could be distinguished within the lower 115 m: (1) diamictites, (2) poorly sorted sandstones, and (3) muddy siltstones, interpreted as reflecting deposition on a glacially influenced slope (Fig. 17). Massive diamictites lack internal structure, but there are composite successions where thinner, massive diamictites are interbedded with thin zones of crudely stratified facies defined by deformed mudstone stringers. Because these stringers may demarcate bed contacts if muds are deposited on bed tops between gravity flow events, they were subsequently reworked. Textural grading in deformed mudstones might indicate they were emplaced as thin turbidites. Consequently, composite diamictite successions can be interpreted as the result of alternating debrite and turbidite deposition. The scale of downslope movement is uncertain, and therefore so is the environment of deposition. Protracted downslope motion results in the generation of mature, well-sorted, graded sandstones, but sandstones here are compositionally immature and poorly sorted. Diatoms are very rare and poorly preserved, and siltstone clasts are a source of reworked diatoms. A setting is indicated on a glacial continental shelf or slope, close to a source of poorly sorted debris. Biofacies are insufficient to constrain age, water depths, or position relative to the shelf break, but the open marine influence is less evident than at Site 1097.
Age-diagnostic biostratigraphic material is sparse in the poorly recovered upper part and is partly reworked in the lower part. Diatom valves were often fragmented beyond identification, yet these fragments could be abundant in the material recovered. From the top of the hole down to 210 m, several biostratigraphically important species (Actinocyclus ingens, F. barronii, T. insigna, T. inura, T. oestrupii, and T. torokina) were periodically present, allowing dates younger than their first appearances to be established. Below, identifiable diatoms were more sparse. Samples from ~290 m had some diatoms of low abundance and diversity (Denticulopsis spp., Nitzschia januaria, and Rouxia californica), possibly of late Miocene age. (Because of the preservation state of the assemblage, however, the sediment may not be of this age.) Radiolarians were sparse in core-catcher samples, precluding any biostratigraphic determination. Reworked Cretaceous radiolarians were present in four out of five of these. Sample 178-1103A- 24R-CC (228.0 mbsf) contained radiolarians from the Upsilon Zone, but marker species were not encountered in any other sample.
The foraminiferal fauna in core-catcher samples down to 218 mbsf contain rare, well preserved, and white-colored N. pachyderma sinistral and older, reworked, darker colored benthics and planktonics, together with Inoceramus prisms and sponge spicules. Below 250 mbsf foraminifers are rarer with more evidence of reworking, but Samples 178-1103A-31R-CC and 33R-CC include a well-preserved assemblage of the shelf species Cassidulinoides parkerianus. The lowest three samples, 178-1103A-35R-CC to 38R-CC (338.77 to 355.34 mbsf), are barren. Several of the upper cores (178-1103A-13R, 14R, 15R, 17R, and 18R) contain partly dissolved calcareous nannofossils. No identifications were possible, but coccolith production is indicated.
At Site 1103 we measured the magnetization of cores spanning 250-350 mbsf. As with previous shelf sites, diamicts yielded unstable directions, and finer grained lithologies were stable. The low recovery and sparse occurrence of these lithologies make construction of a magnetostratigraphy difficult at this time. It may be possible to combine GHMT and split-core paleomagnetic data.
Hole 1103A was logged with the TC, GHMT, and FMS-sonic tool strings over an upper 150-m interval where there was no significant core recovery, but a blockage at 242 mbsf prevented penetration lower into the hole. The narrow hole diameter made for good-quality logs. Porosity varied between 30% and 50% (lower than at the rise sites), showing a change to higher porosity only at the base of the log below 225 mbsf, close to the lithified diamictite seen below 250 mbsf in the core. Resistivity and susceptibility display broad peaks and troughs, and individual clasts can be seen in FMS images, which are all of high quality. Distinct zones of spikes in the GHMT magnetic field log probably indicate clast-rich intervals. The magnetic induction of the sediment causes most of the remaining field anomaly, and it remains to be seen whether magnetic polarities can be determined for the clast-free intervals. Log sonic velocities, combined with Hamilton frame P-wave velocities from the lower hole, should allow the seismic record to be tied to drilling results. Poor core recovery from Site 1103 also means that there is no continuous MST record; physical properties data interpretation is therefore limited. Initial investigation suggests that the laminated beds below 250 mbsf are derived from the diamicts, or the same material source. Interestingly, this source differs from that at Site 1097, also on the shelf.
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