OFR 151.pdf - CRC LEME

several taxa, which first appear in the Middle N. asperus Zone in the Gippsland Basin, are
recorded in early Eocene (Malvacipollis diversus Zone Equivalent) and Middle Eocene
(Lower N. asperus Zone Equivalent) microfloras in basins in south-west and central Australia,
e.g. Anacolosidites luteoides and Tricolpites thomasii.
In general terms, angiosperms are more prevalent than gymnosperms (av. 60-75% versus 15-
30%) throughout the Middle-Late Eocene, with the most common types being Nothofagus
(Brassospora) spp. and Podocarpaceae (Dacrydium, Lagarostrobos, Podocarpus-
Prumnopitys), respectively. Values of Nothofagus (Brassospora) increase from ~25% in the
early Middle Eocene to >50% in the late Late Eocene whilst Casuarinaceae and Proteaceae
decline from ~20% to less than 1-10% over the same interval (Partridge 1999). Coal
microfloras are often dominated by Lagarostrobos whilst Araucariaceae (Araucaria,
Agathis/Wollemia) tend to be more abundant in marine facies (Neves Effect). Proteacidites
asperopolus is often common in Lower N. asperus Zone assemblages whilst high values of P.
pachypolus are confined to the Middle N. asperus Zone.
Unlike other Australian basins, the Gippsland Basin database is sufficiently comprehensive to
use both quantitative data and the first and last appearances of rare taxa as reliable proxy
evidence for bioclimatic change within the Middle-Late Eocene. For example geological and
sequence stratigraphic evidence support palynostratigraphic evidence that Nothofagus
(Brassospora) spp., including one newly evolved canopy species (Nothofagidites falcatus),
expanded very rapidly across the basin during the early Middle Eocene. Late Eocene
rainforest appears to have been more diverse than Middle Eocene rainforest, due to the
immigration of herbaceous and woody taxa, e.g. Sparganiaceae (Aglaoreidia qualumis),
Anacolosa (Anacolosidites sectus), Loranthaceae (Tricolpites thomasii) and a range of extinct
and extant Proteaceae. The latter includes Isopogon (Proteacidites truncatus); the former
include Proteacidites confragosus, P. reticulatus and Triorites magnificus. The same
diversification is reflected in the greater proportion of pollen taxa with complex
ornamentation.
Inferred Climate
Conditions in the Gippsland Basin cooled markedly (to lower mesotherm values) during the
Early-Middle Eocene transition although conditions remained wet to very wet (perhumid).
Temporary warming, indicated by the appearance of Triorites magnificus during the Late
Eocene, may correlate with transient warming of sea surface temperatures recorded elsewhere
along the southern margin (B. McGowran pers. comm.).
3.2.7 Tasmania
Middle-Late Eocene facies (upper Eastern View Group) are intersected in the majority of
wells drilled in the Bass Basin and onshore sub-basins in northern Tasmania but few data are
available in well completion reports. It is unclear whether the plants producing Middle N.
asperus Zone index species e.g. Anacolosidites sectus, Tricolpites thomasii and Triorites
magnificus ever extended southwards into northeastern, central or southeastern Tasmania.
1. Bass Basin
Morgan (1986b) indicates Lower and Middle Nothofagidites asperus Zone Equivalent
microfloras are dominated by Nothofagus (Brassospora) with lesser amounts of
Casuarinaceae. The same dominance is observed in Squid-1, where two extinct Proteaceae
(Proteacidites asperopolus, P. leightonii) are abundant in Lower Nothofagidites asperus Zone
Equivalent (Martin 1985).
2. South-east Tasmania
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