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Victoria University Antarctic Research Expedition Science and Logistics Reports 1982-83: VUWAE 27

PART II - SHIPBOARD PROGRAMME - P.J. Barrett

page 15

PART II - SHIPBOARD PROGRAMME - P.J. Barrett

Work on Cruise IV of USCGC GLACIER (February 12-19, 1983) for the McMurdo Sound Sediment Studies project produced 1200km of bathymetry, much of it in areas previously uncharted, and seafloor samples from 26 sites. Several preliminary conclusions are drawn:
1.Sediment in the deep water (about 850m) around Ross Island is mud deposited from suspension with a small ice-rafted component, mainly basaltic. Its variable firmness a few centimetres below the seafloor may be due to differences in sedimentation rate or composition.
2.Granite Harbour is a deep basin (890m) cut or cleaned out by an enlarged Mackay Glacier and little modified by sedimentation since that event.
3.Tillite pebbles from seafloor sediment off Granite Harbour suggest that the strata forming the south-trending ridge about 20km offshore include the Cenozoic glacial sequence cored at MSSTS.
4.The Western Shelf and Slope in McMurdo Sound have not been crossed by similar extensions of Taylor or Ferrar Glaciers, at least since the Early Cenozoic. The Slope is cut by at least two narrow valleys, but their origin is not yet understood.
5.The McMurdo Volcanics extend as scattered cones or cone complexes along the edge and possibly beneath the McMurdo Ice Shelf and for some distance offshore from Ross Island.
The shipboard programme for the McMurdo Sediment Studies project had three main aims:
1.To take several deep water samples not accessible from the sea ice.
2.To sample off (and if possible in) Granite Harbour.
3.To obtain continuous depth records from previously uncharted areas in western McMurdo Sound.

The ship's track and stations for bottom sampling are shown in Figure 4. Station data is given in Table 2.

Four deep water grab samples (Stations 3-6, 854-889m) were taken, all mud with a little sand, and rare mainly angular basaltic pebbles up to 8cm long. Biota are sparse with only a few worms and fine sponge spicules. The grab sample was normally sufficiently undisturbed to see a section from the seafloor down 10 to 15cm, which showed the upper 2cm to be soft and slightly oxidised, in contrast to a firmer layer beneath which had dark grey mottles and page 16
FIG. 4. Map of McMurdo Sound area, showing the track and bottom sample locations from USCGC GLACIER Cruise IV. shallow areas thought to be of volcanic origin are marked by a star. Depths in metres.

FIG. 4. Map of McMurdo Sound area, showing the track and bottom sample locations from USCGC GLACIER Cruise IV. shallow areas thought to be of volcanic origin are marked by a star. Depths in metres.

page 17 sand lenses. The sediment has been deposited largely from suspension with a small coarse component rafted by floating ice, probably some of it glacial and some shore ice. The sediment below 2cm is softest at Station 6 in the northern part of Bird Basin, becoming firmer to the south, though the floor of the basin remains at about the same depth (Fig. 5A). The variation in firmness may be due to differences in sedimentation rate or composition.

Samples from slightly shallower depths (Stations 1, 2, 3A, 7, 8, 26; 547-755m) were also taken for foraminiferal studies by B.L. Ward to cover the range that includes the carbonate compensation depth. Sample 1, a few miles north of Scott Base, had pebbles encrusted with bryozoans, indicating their exposure above the seafloor and again a very low sedimentation rate.

With Granite Harbour virtually ice-free we first carried out a bathymetric survey, 250km of track in an area of about 400km2. The work in Granite Harbour also involved the setting of a sediment trap in 800m of water at the tip of the Mackay Glacier Tongue (to be retrieved in November 1983) and the recovery of 13 bottom samples. The latter were mainly grab samples, but included 2 cores 60cm long and 20cm across for detailed sedimentological study. A grab sample near the middle of the Harbour (Station 16, 849m) contained black mud, but a piston core attempt nearby (Station 16A, 880m) yielded only 25cm of core because the corer encountered stony till, showing that the pattern of sedimentation in Granite Harbour is not simple. The bathymetry is also quite complex for although the Harbour can be described in gross terms as a broad basin almost 900m deep, the floor is hummocky with a relief of 200-300m (Fig. 5B).

A bathymetry line was run east from Cape Roberts and two bottom samples were taken to help with CIROS site selection. The seafloor east of Cape Roberts is sandy or gravelly, even in 489m of water at Station 19, where a boulder of granite 30cm long was taken by the grab. Worm trails of cemented sand over part of the surface of the boulder show that it was partly buried in sand. Grabs from near Cape Roberts and the 108m deep shoal 18km offshore represented a gravel pavement, judging from the encrusting bryozoans and other organisms. Bathymetric data obtained off Granite Harbour can now be used to chart the North-South-trending ridge and West-facing scarp that is cut in the East-dipping sequence to be sampled by CIROS 3 and 4. Some idea of the sequence was obtained from pebbles in the till recovered at Station 8 which is on a broad, flat "delta" surface at 550m. Although most pebbles were granite and presumably from Granite Harbour, a number were sandy mudstone with scattered coarse sand grains, like the glacial mudstones in the Early-Mid Cenozoic MSSTS core. The west-facing scarp was also sampled by grab (Station 11) but recovery was poor and no new lithologies were seen. Nevertheless, we hope that processing the sand fraction may yield micro-fossils that indicate the age range of strata forming the scarp (Pig. 5C).

The bathymetric data obtained on this cruise is significant not only because of new areas charted but also because it confirms our view of a fundamentally different character of the Western Shelf and Slope than shown on previous bathymetric maps, such as that of McGinnis (1973). Earlier maps and page 18
FIG. 5. Topographic features in McMurdo Sound and Granite Harbour. Depths in metres. Horizontal scale bars 5km long.

FIG. 5. Topographic features in McMurdo Sound and Granite Harbour. Depths in metres. Horizontal scale bars 5km long.

A.Southern part of Bird Basin, contrasting the smooth flat floor with the rough topography beyond the margin.
B.Cross-section near the head of Granite Harbour, with the glacially moulded topography still evident in the deepest parts.
C.Bathymetry across the "scarp" off Granite Harbour.
D.Submarine valleys glacially carved by the Debenham Glacier 10km south of Station 19.
E.Narrow valley of unknown origin off New Harbour.
F.Submarine peaks along the edge of the McMurdo Ice Shelf, and inferred to be volcanic cones.
page 19 reports (including Barrett et al., 1974) show submarine valleys extending from the Dry Valleys into the "moat" around Ross Island. However, the Western Shelf is in fact a rather flat feature (average depth 180m) with two very narrow (about 1km) valleys running from its edge to near the foot of the slope. These features seem far too small and well-defined to be glacial in origin, especially when compared with the broad submarine extension of the valley now containing the Debenham Glacier (5km wide and 500m deep) or the 800m deep basin carved by the Mackay Glacier off Granite Harbour (Figs. 5D, E). The lack of broad glacial valleys in the Western Shelf east of the Dry Valleys, in contrast to those cut by the outlet glaciers north of 77 10′S, suggest that none of the Dry Valleys have served as outlet glaciers since the deposition of the near surface strata of the Western Shelf, which at MSSTS-1 are as old as Early Cenozoic (Webb, Leckie and Ward, in press).

The bathymetric data also revealed another feature no doubt seen before, but not to our knowledge taken into account in discussions of McMurdo Sound geology. Tracks along the edge of the McMurdo Ice Shelf showed a hilly topography superimposed on the gentle slope of the Western Shelf (Fig. 5F). Because of the nearby Dailey Islands, scattered basaltic cones that rise to 132m above sea level, these hilly features are also thought to be of volcanic origin. Defense Mapping Agency Hydrographic Center Chart 29321 shows a number of unusually shallow soundings in deep water around Ross Island, and one of these is crossed by Track XXX of Northey et al. (1976), showing it to have a conical section and an apparent height of 165m above the floor of Bird Basin. These are also judged to be of volcanic origin, and may present a significant extension of the Erebus volcanic Province (Figure 1).

Results of the cruise will appear in several different places. Selected samples will be used by Ms B. Ward in her study of the distribution of modern foraminifera in McMurdo Sound, and the Granite Harbour samples and data will be used as part of a thesis on the Mackay Glacier-Granite Harbour sedimentary system by Mr. Macpherson. The bathymetry will be added to an earlier compilation to complete a bathymetric map of McMurdo Sound with the assistance of Geophysics Division and the Institute of Marine and Freshwater Science, DSIR. The samples and bathymetry will also provide the basis for papers on recent sedimentation in McMurdo Sound.

A cruise plan is being prepared for work in February 1984 mainly to carry out further site investigation for CIROS drilling. A shipboard survey has become necessary because sea ice conditions off Cape Roberts prevented the planned "land" based seismic survey off Cape Roberts. The survey will link with previous shipboard surveys in the area. The cruise plan also includes some time for gravity cores from Bird Basin (not collected in 1983 because of equipment problems and [unclear: lack] of time), and seafloor photography.

page 20
TABLE 2. Grab/Core Stations from USCGC GLACIER in McMurdo Sound, 12-19 February, 1983.

TABLE 2. Grab/Core Stations from USCGC GLACIER in McMurdo Sound, 12-19 February, 1983.