Scientific Summary

Scientifically, much was accomplished during the field season, however, the initial objectives outlined above were extremely ambitious in view of the fact they were set before the capabilities of the drilling system were completely understood and tested under Antarctic conditions. Site selection for coring is critical and must be reviewed by both scientists and drillers prior to conception of logistical plans. The depth to ice-cemented sediment must be determined so that if necessary casing can be provided and used during coring. Core recovery in ice-free sediments is probably not possible and if this zone cannot be cleared by shovel it must cased off, if drilling below this zone is required. In logistics planning, allowance for bad weather is critical because of the time needed for 6 helo moves. November 2000 was one of the snowiest on record at Scott Base and given the variable conditions in the mountains, the weather must be suitable both at base and in the field for helo movement to occur.

At Allan Hills 2 core holes were drilled in Trudge Valley (Fig. 1). The site initially selected was on the top of the 'ramp', but high winds made drilling at this location impractical for field time allotted to the Allan Hills area. Therefore, to assess the thickness of the Sirius in Trudge Valley, we decided first to core the Sirius outcrop on the valley bottom NE of the 'ramp'. To our surprise, we found the top 1.5 m of Sirius was ice-free and core from this page break section was lost. At this hole, the Weller was encountered at a depth of 3.40 m. It was decided to drill the second hole in Trudge Valley in the basal part of the 'ramp' sediments which overlie the Sirius. The idea was that the top of the ice-cement would lie in the 'ramp' sediments and allow recovery of a complete section of the underlying Sirius. Unfortunately, the unconsolidated 'ramp' sediments made drilling extremely difficult and slow. Consequently, the first core recovered at a depth of 2.48 m was the Sirius, and it was not possible to core the contact between the 'ramp' sediments and the Sirius.

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A move to other sites in the Allan Hills area became impractical because of time constraints. Persistent 30 - 40 knot winds on the Upper Triangle essentially ruled out access to this area. Good outcrops surrounding the Lower Triangle and the probability of a thin Sirius section due to the 1-2 m thickness of the ice-free horizon, ruled out coring in this area as well.

At Table Mt, 6 core holes were drilled in an area where a ridge of Sirius is truncated by a debris flow of dolerite blocks which is marked by large (15 m across) polygonal ground (Fig. 2). Adjacent to the ridge is the Sirius regolith which is marked by small (5 m across) polygonal ground. These three outcrops are within a 100 m radius so a helo move of the compressor was not necessary. The primary objective was to take 3 m deep cores in each of these outcrops for Be/N inventory dating. The secondary objective was to investigate the nature of the polygonal ground and determine the depth of the 'active' layer. In this case, the term active layer does not refer to annual freeze-thaw, but the depth to which the polygonal ground extends. In the large polygons, 3 holes were drilled; one in the centre, one at the perimeter and one half way in between these holes. Due to the blocky nature of the debris flow, depth to the ice-cemented horizon varied from about 10 to 45 cm. TM-00-1 drilled in the centre of a polygon terminated at a depth of 6.3 m due to a stuck and un-recovered bit and core barrel. TM-00-2 was drilled half way between the centre and perimeter of the polygon to a depth of 2.37 m so that a 2 m thermistor probe could be installed in this hole. The hole at the perimeter, TM-00-3, was drilled to 2.70 m and terminated when drilling became difficult.

TM-00-4 was drilled at the top and southern-most end of a kilometre long ridge of Sirius (Fig. 2). The ridge is mostly capped by the dimict facies of the Sirius which overlies the well sorted sandy facies of the Sirius. The surface of the ice-cemented horizon at 48 cm was very even and flat which probably reflects the homogeneous nature of the sandy facies. Coring was fast (1 m /0.5hr) and drilling was terminated after 3 runs at 3.21 m.

A small polygon in the Sirius regolith was cored in its centre and perimeter. Depth to the ice-cemented horizon was less variable than in the large polygon and averaged about 15 cm. In hole TM-00-5 at the polygon centre, ice-cemented dolerite clasts (10-20 cm diameter) were present to 1.03 m deep where the well sorted Sirius sand was encountered. This may represent the depth of the active layer. Unfortunately, coring was terminated in a large dolerite clast (only known to be a clast after drilling the perimeter hole) at 1.83 m after drilling only 0.3 m in 1 hr. The perimeter hole, TM-00-6, encountered the sandy Sirius facies at about 0.88 m. However, at about 1.47 m a gravelly facies probably of fluvial origin was encountered. This facies continued to 4.40 m when coring was stopped for the move back to Scott Base.