Bann, Dr Glen1, Graham, Ian2, Jones, Brian1
1University of Wollongong, Wollongong, Australia, 2University of NSW, Kensington, Australia
A suite of newly identified mafic and felsic tuffs are described from the Shoalhaven and Talaterang Groups of the southern Sydney Basin. This includes the Clyde Coal Measures, Wasp Head, Pebbly Beach and Snapper Point Formations, Wandrawandian Siltstone, Nowra Sandstone, Berry Siltstone and the lower Broughton Formation. The tuffs are readily observed from outcrops in the field however, so far, have proved very difficult to discern in drill cores.
The mafic tuffs commonly comprise abundant biotite and muscovite grains, which are often deformed, K feldspar, plagioclase, volcanic quartz, with embayments, quartz shards and rare euhedral zircons. The felsic tuffs contain abundant volcanic and metamorphic quartz, plagioclase, more common pumaceous material, less micas and very rare or absent, shards. All tuffs contain carbonaceous material of various amounts and both are commonly reworked, although a lack of abrasion on the phenocrysts in the mafic tuffs suggests that the material has not travelled far from its source. Numerous dropstones of the same tuff material are common throughout the sequence, with volcanic types dominating in the east and metamorphic cratonic types in the west.
The Koo Lee Tuff Member, the largest of the mafic eruptions with a maximum thickness of almost 3m, is stratigraphically located within the lower Broughton Formation and due to its explosiveness and volume, has been deposited across a large area of the basin, hence outcrops in a number of locations. This provides the opportunity to identify eruption and emplacement mechanisms plus lateral changes in the deposit as well as providing a chronological time line through the southern Sydney Basin.
Volcanic detritus from island volcanoes to the south-east inundated sediment derived from the craton to the west during this period. Evidence from the presence of predominantly Cruziana ichnogenera and glendonites throughout the succussion, in addition to wavy contact surfaces beneath coarser sands and sporadic volcanic derived clasts suggest deposition was dominated by episodic storm activity in cold climate conditions with periodic coastal ice sheets depositing the clasts, or dropstones. Very fine-grained carbonaceous horizons indicate that deposition was also periodically dominated by extended low energy conditions. These deposits represent small or distant components of much larger volcaniclastic aprons surrounding a series of vents to the south-east. Evidence suggests a proximal source from island volcanoes ranging from mild Strombolian to the violently explosive Vulcanian or Plinean phreatomagmatic type eruptions. The association with the Late Permian Gerringong Volcanics and these earlier eruptions is presently unclear. The felsic eruptions are more distal, possibly associated with a large felsic provenance in the Zealandia craton to the south east.
The tuffs are often associated with trace fossil escape burrows, both successful and unsuccessful, and marine body death assemblages, commonly within the tuffs themselves but also found both below and above the tuffs. The effects of the eruptions and the tuffs on the local biota at the time will include changes in pH and Eh, elevated water and substrate temperatures, chemical toxicities such as Hg and As, during and post eruption, and an increase in turbidity. Effects will impact different species, with the more significant eruptions impacting everything. The more proximal eruptions, such as the Koo Lee Tuff, will also destroy habitat, displacing the animals.
It is therefore apparent that volcanism was controlling and dominating the deposition and conditions during early stages of the formation of the southern Sydney Basin.
This work has been ongoing since completing an Honours thesis on the early volcanism of the southern Sydney Basin in 1999, was drastically highjacked for many years by a PhD which had nothing to do with this, glad to be back and making interesting and challenging discoveries