OFR 151.pdf - CRC LEME

5.5 Time Slice K-2. Aptian-Albian [115- 97.5 Ma]
5.5.1 Palaeogeography
Figure 5: Aptian (118 Ma) palaeogeography (from Veevers et al. 1991)
By basal Aptian time, Australia had begun to move away from Antarctica (Figure 5). This
involved stretching of the continental lithosphere beneath the Australo-Antarctic Rift System,
which extended to and around Tasmania by the end of the Albian [~97 Ma].
Valleys at the Western Australian end of the rift system were flooded by the ocean to form a
narrow seaway (Australo-Antarctic Seaway) whilst extensive brackish and freshwater
swamps occupied tectonic depressions to the south-east. Fluvio-lacustrine sediments
accumulating in one freshwater swamp in the Gippsland Basin (Koonwarra Fossil Beds)
preserve the remains of a bird, fish and insects and angiosperm flowers (Taylor and Hickey
1990). Incipient rift valleys began forming in the Coral Sea region (palaeo-northeastern
margin) and a volcanic arc (Eastern Cordillera) extended southwards down the Queensland
and New South Wales coasts (palaeo-eastern margin). Igneous activity affected much of
eastern Tasmania, then part of the palaeo-southern margin. Relative sea levels continued to
rise, resulting in the continent being subdivided into four large islands (Yilgarn-Pilbara Block,
Arunta-Kimberley Blocks, north-east Queensland, south-east Australia-Tasmania) separated
by shallow epicontinental seaways during the Aptian. These seaways extended from the Gulf
of Carpentaria (palaeo-northwestern margin) to near the Bight (palaeo-western margin) and
eastwards onto the palaeo-southern margin (present-day New South Wales and southeastern
Queensland).
Anoxic conditions developed in the central parts of the Carpentaria and Eromanga Basins
during late Middle Aptian to Early Albian time. The Toolebuc Formation is an example of
the distinctive condensed black shale sequences accumulating in many oceans at this time (cf.
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