Introduction

Exposures of Beacon Supergroup sediments between Mt Metschel and Robinson Peak were examined during a 60 day. 950 km traverse from Scott Base. Although a significant number of days were lost to bad weather while travelling on the Ross Ice Shelf these delays were almost certainty less than the weather, mechanical, weekend and public holiday delays we would have experienced with helicopter support.

Travel was generally easy, the only exception being on the Lower Staircase, where freak, deep soft snow conditions made travel extremely slow.

Facies descriptions with thicknesses along with paleocurrent data were obtained from a number of localities on Mt Metschel, Portal Mountain, Mt Cream, Pivot Peak, Mt Fleming, Mt Bastion and Robinson Peak. These observations provide valuable information on ancient paleoslope direction and basin subsidence.

Detailed descriptions of Pivot Coal Measures were made at Pivot Peak and these will be used to compare these Devonian sediments with the better understood and more widespread Permian Weiler Coal Measures. At the same time vertical and lateral sample suits were collected and these will be used to study the effects of Ferrar Dolerite on carbon sulphur ratios and to examine the suitability of carbon sulphur geochemistry as a paleosalinity indicator for Beacon Rocks.

Extensive paleocurrent and facies analysis was conducted on exposures of Lashly A, where fining upwards cycles are preserved with low sinuosity channels.

Results

Loaded channels of Maya Arkose in Metschel Tillite (diamictite phase) at Mt Crean show that the time represented by the Pyramid Erosion Surface is geologically insignificant. This supports previous observations which suggested a gradation between Metschel Tillite and Weiler Coal Measures at Mt Fleming (Pyne 1986).

A pebbly carbonaceous shale (diamictite) occurs on a ridge south of Mt Fleming. This unit is of limited extent but is clearly exposed below the base of the Maya Arkose. Initial interpretations are that the carbonaceous material is primary, suggesting a temperate climate at the time of deposition. It is hoped that samples may contain spores suitable for dating.

Preliminary results suggest that the lower part of the Lashly Formation (Lashly A) was deposited in a fluvio-lacustrine setting, possibly initially fed by a large braided river which deposited Feather Conglomerate higher up on the flood plain. At several locations the Lashly-Feather boundary Is gradational and at Horseshoe Mountain facies characteristic of the Feather Conglomerate are interbedded with typical Lashly sediments over a 20 m interval. The Lashly A-B boundary becomes difficult to place north of Mt Bastion.

Structures resembling HCS and SCS were observed along with logs and peat rafts in Lashly B at Horseshoe Mountain. These are similar to HCS and SCS observed last year at Allan Hills in the upper Weller Coal Measures and suggests that similar depositional processes were acting. We do not know what flow conditions produce these structures but it is hoped that a model for their formation will be developed after the proposed Allan Hills project in 1990-91.

Initial interpretations based on isopach data, support our view that the Beacon was deposited in a tectonically quiescent intracratonic basin of substantial size and duration.

Skolithos was observed in non-marine sediments at Pivot Peak and Horseshoe Mountain No evidence of marine incursion was found and we now believe that the entire Beacon Supergroup in South Victoria Land is non-marine.

Geochemical data, including the planned carbon/sulphur profile of the Beacon Supergroup is not yet available.

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Note: Since this report was prepared, geochemical data has revealed that the Pivot Coal Measures Member is non-carbonaceous. The black finely divided material is titano-magnetite and the name Pivot Member is proposed as an alternative (Amot and Woolfe in press).