Six lines approximately normal to the axis of McMurdo Sound were set out with flags on wooden beacons at 2 1/2 km spacing to facilitate surveys across the sound (Fig. 10). Ice breakout prevented those lines north of the Strand Moraines from extending more than 16 km east from the Victoria Land coast. It was intended to make extensive piston coring, bottom photography, bathymetric and gravity surveys. However, electronic problems with the triggering of the camera prevented any underwater photography and the piston coring was only partially successful.

Piston Coring

Piston cores were obtained from 9 sites (Table 8). The corer, 1 m long by 47 mm in diameter, was loaded with 40 kg of lead. It was lowered on a 4 mm wire rope through a 25 cm diameter hole drilled in the sea ice by mechanical auger and triggered to free fall the last 5 m to the sea floor. Core was recovered in a plastic liner and held with a phosphorbronze cone core catcher, extended by plastic strips. Most cores obtained were less than 60 mm long and only one attempt in three produced any core at all. The reasons for non-recovery of core were not ascertained but it appears likely that when lowered rapidly the corer descends on a helical path induced by spinning due to unwinding of the hawser laid rope. In addition, there may have been core loss due to inadequate functioning of the core catcher.

Eleven piston core samples were examined. Most of the cores, (PC1-8) contain dominantly calcareous foraminiferal faunas, with several hundred well-preserved tests present. PC 9-11 contain dominantly finely agglutinated assemblages, and a lower number of tests. The preservation of calcareous tests in these three samples is very poor, indicating re-working or calcium carbonate dissolution.

Cores 4 and 5 consisted of live sponge and sponge spicule mat respectively. Core 4 had a sparse foraminiferal assemblage but species present were similar or identical to those in the other cores.

The sediment in the samples varies from very fine sand to granule and pebble size clasts. To date no sedimentologic analyses have been attempted on this material.

Bathymetric Data (BAS)

At all gravity and piston core sites in McMurdo Sound water depth was measured to an accuracy of 1 m with a weighted terylene line. Along the Strand profile, McMurdo Sound has an assymetric cross-section with the longer limb on the western side and a maximum water depth of 611 m occurring two thirds of the way across the sound. The Butter Point profile is similar. The Ferrar profile runs down the centre of the valley formerly occupied by the Ferrar Glacier. Water depths in the valley were found to vary between 200 and 235 m but 15 km from the present glacier snout there is an abrupt rise in the sea floor and water depth decreases to 125 m. Further out water depth increases again to 180 m, the typical depth obtained from many measurements throughout the New Harbour area. The shallows at the mouth of the Ferrar Valley are thus anomalous. They are thought to be due to moraines left by the retreating Ferrar Glacier. Similar moraines, now submerged, have been reported from elsewhere in McMurdo Sound (Northey & Sissons (1974); Wong & Christoffel, in press).

Reference Pole at the snout of Ferrar Glacier (BAS)

A 75 × 50 mm pole with 500 mm square crossed red plywood vanes was set up on the floating snout of Ferrar Glacier. The pole is buried about 1.5 m in the ice and the top of the pole was measured to be 0.860 m above the ice surface on 9.11.79.

Sea Ice Gravity Survey (BAS)

Gravity was observed with a Worden gravity meter at 2 1/2 km intervals along all profiles except that at Cape Armitage (Fig. 10). All observations on the sea ice were repeated and the standard error of observation found to be 0.4 mgals. The positions of stations were determined by starting profiles at known locations and setting a course directly toward another known feature. Distances were measured by metre wheel. Precise surveys were subsequently made of the entire Strand Profile and of three beacons in the Ferrar Profile including one at the snout of the Ferrar Glacier. In all cases station positions are known to better than ± 200 m. Elevations on the sea ice are known to within 0.1 m. The total error in computing Bouguer Anomalies is less than 0.7 mgals.

Bouguer Anomalies computed for a crustal density of 2.67 gm/cc and water density of 1.00gm/cc were used to construct Figure 10, a Bouguer Anomaly map of McMurdo Sound Dry Valley region. The map includes additional data from land based surveys discussed later (Event 13). Bouguer Anomaly profiles along the lines of observation are given in Figure 11. The main feature is steep positive eastward gradient across the Victoria Land Coast previously described by Bull (1962) and Smithson (1972). Smithson attributed his results to crustal thinning under McMurdo Sound together with a +0.2 gm/cc intrusion at a depth of 4 km in basement under the Sound.

Our survey also shows a 15 mgal anomaly in the reverse sense to the main gradient and not found in the previous surveys. The anomaly has a wavelength of 5 km and is centred 10 km east of the Victoria Land Coast. Preliminary models show that it is consistent with a vertical 0.4 gm/cc density discontinuity having a throw of about 1 km and mid depth of about 1 km, the feature could be a basement fault down thrown to the east.

Figure 10 Preliminary Bouguer Anomaly Map of western McMurdo Sound with gravity and bathymetry profile lines shown.

Figure 11 Bouguer anomalies for profile lines shown in Figure 10.