OFR 151.pdf - CRC LEME

in the Bass Basin, the basal Seaspray Group in the Gippsland Basin, and the upper Buccleugh
beds, Ettrick Formation and Renmark Group in the Murray Basin. No carbonaceous
terrestrial sediments of confirmed Early Oligocene age have been recorded on the eastern
margin. Conversely, marls of this age are preserved in distal areas of the North West Shelf
(Quilty 1977, Apthorpe 1988) where rising sea levels allowed carbonate accumulation to
recommence during the Late Oligocene.
Gorter and Bayford (2000) have proposed that a small multi-ringed structure (Puffin
Structure) on the Ashmore Platform, North West Shelf was formed by the impact of a
meteorite or small asteroid into unconsolidated shallow marine carbonates sometime during
the Middle Miocene. By this time, the northern margin was at about palaeolatitude 20 0 S and,
during the mid-Miocene, began to collide with South-East Asia. One consequence was an
episode of major mountain-building in New Guinea from ~30 Ma onwards (New Guinea
Orogeny). Formation of the 3 km high island of Timor and the adjacent 3 km deep Timor
Trough foredeep reflect the same event.
At about the same time, coral reefs began to develop in the area of the present northern Great
Barrier Reef. To the south, the same major rise in relative sea-level caused Bass Strait to
become a shallow seaway whilst a somewhat smaller seaway developed across the top of the
Fleurieu Peninsula. Shallow epicontinental seas flooded the Eucla and western Murray
Basins (Brown and Stephenson 1991). In the offshore Gippsland Basin, thick carbonate
wedges developed over the paralic and other terrigenous sediments deposited during the
Palaeogene. Onshore terrestrial sequences include deposition of a great thickness of peat
(now brown coal measures) in the Latrobe Valley (Holdgate et al. 2000).
Gentle uplift split the Eyre Basin into two sub-basins, the Tirari Sub-basin to the west, and
Callabonna Sub-basin to the east, of Coopers Creek. River systems here and in inland southwest
Australia became very sluggish and began to break up into chains of lakes, whilst
Paleocene-Eocene deposits such as the Eyre Formation became deeply weathered and capped
by silcrete (Cordillo Surface). Sedimentation in the Lake Eyre Basin recommenced during
the latest Oligocene and Miocene (Alley 1998), leading to the deposition of the Oligocene-
Middle Miocene Etadunna Formation in the Tirari Sub-basin and the correlative Namba
Formation in the Callabonna Sub-basin. Intraplate volcanism continued to occur in
northwestern and eastern Tasmania and along the Eastern Highlands where some vents
developed into shield volcanoes. An example is the Ebor Volcano at Dorrigo on the New
England Plateau. This volcano was at least 45 km across and stood 800 m above basement
during the Early Miocene (Ashley et al. 1995). Fluvial sediments blanketing coastal lowlands
of southern New South Wales may date from the same period (Nott et al. 1991).
In northern Queensland, sequences of probable Oligocene-Middle Miocene age
unconformably overlie Mesozoic sedimentary sequences (Smart et al. 1980, Bain and Draper
1997). One possible Late Oligocene-Early Miocene formation in the Karumba Basin
(Bulimba Formation) is reported to include thin coals. Grimes (1980) suggests this formation
(and correlatives) were deposited, then exposed as the Aurukum Surface in the earliest
(Bulimba Cycle) of three cycles of erosion, deposition and weathering that shaped this basin
during the Tertiary. More generally, Idnurm and Senior (1978) have proposed that western
Queensland underwent extensive deep weathering during the Late Oligocene. Revisions to
the Australian apparent polar wander path suggest a Miocene age for this weathering cycle
(Idnurm 1985, 1986).
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