OFR 151.pdf - CRC LEME

5.2.6 South-East Australia
Late Neogene microfloras come from a diverse spectrum of localities and depositional
environments in southeastern Australia. End-member examples are Lake George at 673 m
elevation on the Southeastern Highlands and a bioclastic carbonate sequence (Hapuku-1)
located about 50 km off the present-day coast of southeastern Victoria (offshore Gippsland
Basin).
1. Southeastern Highlands of New South Wales
Sediments infilling Lake George provide snap-shot records of Late Neogene climates at
moderate (~650 m) elevations on the Southeastern Highlands near Canberra (Kershaw et al.
1991). Episodes of deep weathering have destroyed most of the organic record, including
plant microfossils. Exceptions are thin intervals dated by palaeomagnetism as late Early
Pliocene and late Late Pliocene.
Microfloras recovered from the earlier interval are dominated by Podocarpus-Prumnopitys,
Casuarinaceae, Myrtaceae (including Eucalyptus) and Nothofagus. The last is dominated by
Brassospora spp. but includes significant numbers of Fuscospora and Lophozonia types.
Nothofagus is absent in the younger interval where the microfloras are dominated by
Asteraceae, Casuarinaceae, Chenopodiaceae and Poaceae. Araucariaceae pollen is not
recorded in either section and podocarps only range as high as the basal microflora in the
younger interval.
McMinn (1981c) has recorded a gymnosperm-dominated microflora of possible Pliocene age
from Wingecarribee Swamp near Bowral.
Inferred climate
The presence of Nothofagus (Brassospora) and N. (Lophozonia) spp. but not Araucariaceae is
reliable evidence that local conditions were uniformly very wet (perhumid), and mean
temperatures cool-cold (microtherm range) during the early Late Pliocene. If the steep
dissected fault scarp forming the western margin of Lake George provided a refugia for
rainforest taxa, then regional climates could have been much drier (subhumid) or more
strongly seasonal than is indicated by the fossil pollen data. The first appearance of
Asteraceae-Poaceae dominated communities may reflect the onset of seasonally very cold
(lower microtherm) and winter-wet climates during the Plio-Pleistocene transition (cf.
Truswell 1993).
2. South-west slopes of New South Wales
Microfloras preserved in the Upper Lachlan Valley appear to represent plant communities
growing at relatively low (90-120 m) elevations on the south-west slopes of the Southeastern
Highlands.
Martin (1973, 1991b) has proposed that microfloras preserved in the major alluvial
formations infilling these valleys (Lachlan and Cowra Formations) were deposited during the
Pliocene and Pleistocene, respectively. Microfloras ‘assigned’ to the Late Miocene and Late
Pliocene ages are wholly dominated by Casuarinaceae and Myrtaceae but recounts of
microfloras recovered from the Lachlan Formation (Macphail 1997b) support the Late
Miocene-Early Pliocene age and confirm the survival of Nothofagus (Brassospora) spp. in
rainforest communities co-dominated by N. (Fuscospora) and N. (Lophozonia) spp.,
podocarps and Myrtaceae. By this time Araucariaceae are rare or absent. Martin (1991b) has
proposed that trends in pollen dominance reflect the transient expansion (Early Pliocene) of
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