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Triassic strata at Mt. Bastion (K7A) - B. Walker.

The Triassic Beacon Supergroup of south Victoria Land is divided into 5 stratigraphic units, the Fleming Member of the Feather Conglomerate and Members A-D of the Lashly Formation, and all are exposed at Mount Bastion. Preliminary investigations show that at least 4 distinct paleohydraulic regimes operated during the deposition of the Triassic strata.

The Fleming member consists of medium to large trough-x-bedded, medium to coarse grained quartzose sandstones with intervals of planar-x-beds. Paleocurrent variability is low as was also found for the other members. The Fleming member is considered to have been deposited in a braided stream environment where dune migration along the floors of wide but shallow river channels and transverse bar formation were the dominant processes.

Member A of the Lashly Formation has a greater diversity of sedimentary structures and variation in grain size. A change in grain composition from quartzose to volcaniclastic is evident from the weathering behaviour of the sandstones. The 80 metres of member A can be broadly divided into two facies, a trough-x-bedded medium grained sandstone facies and a parallel and ripple laminated very fine sandstone to siltstone facies with mudstones containing white rootlets. River channel cross-sections can be clearly seen at some exposures and show channel widths in the order of 100 metres with depth of 3 to 5 metres. Both facies are considered to be "in channel" facies with the mudstones representing still water deposition and alter plant colonisation in abandoned channels. The trough-x-bedded sandstone facies may represent deposition of sand within channels during floor events. A 0.5 metre thick sandstone with very low angle cross-stratification interbedded with horizontal laminae is at present unique to the section at Mount Bastion and requires further investigation as does a cross-stratified sedimentary structure whose geometry has not been reported in the scientific literature.

Member B is 96 metres thick and consists almost entirely of medium grained well-sorted sandstone. The bedforms are of a larger scale than found elsewhere with individual trough and planar-x-beds several metres thick. Large scours are common and often associated with thick (up to 14 metres) massive sandstones containing large silicified logs. Channel cross-sections exposed within a bluff at the northern end of Mount Bastion indicate channel depths of over 10 metres. Member B is interpreted as representing the deposits of wide and at times deep rivers that transported large amounts of sand, large mudstone blocks (up to 1 metre across) and plant debris during the peak flow of big floods. Channel avulsion was common accounting for the occasionally recorded 90° paleocurrent different from the general flow direction. In at least one place channel avulsion has cut through 20 metres of ripple laminated carbonaceous and non-carbonaceous fine sandstone. As yet unexplained is a 180° paleocurrent reversal that occurs within an interval of 5 metres and can be traced for the length of the outcrop (400m).

A complete change in paleohydraulic regime occurs in Member C which consists of thinly bedded carbonaceous and non-carbonaceous ripple laminated very fine sandstone and siltstone The 78 metres of Member C includes 2 thick sandstone units which suggest inundation of coarser flood debris onto what may have been a shallow lake environment. Conditions were at times stable long enough to allow for the accumulation of peat which eventually formed thin coal seams. Plant material is abundant suggesting that the Triassic landscape was well covered in vegetation.

Member D is 205 metres thick and consists largely of trough and planar-x-bedded medium to coarse grained sandstones. It is similar to parts of Member B and the Fleming member and similar conditions for its deposition are envisaged. Towards the top of Member D several sandstone units show a return to a quartzose composition last seen in the Fleming Member.