OFR 151.pdf - CRC LEME

mudstones (Kings Park Shale). Sporadic basaltic eruptions occurred in eastern Tasmania and
along the length of the Eastern Highlands into Queensland, where a number of terranes lay off
the northeastern margin. A series of fresh to brackish water lakes formed in fault troughs
aligned along the present Queensland coastline. Carbonaceous sediments accumulated in
these basins, and also in central Australia where the absence of piedmont deposits implies the
topography was relatively subdued (Alley 1998). Restricted to open marine conditions existed
along the southern margin where clastic sedimentation became episodic due to regional
tectonic and eustatic forcing. Bass Strait remained a swampy alluvial plain surrounding large
freshwater to saline lakes whilst the South Tasman Rise land-bridge had become partially
submerged.
7.5.2 Palaeobotany
The progressive replacement of cryptogams and archaic gymnosperms by rainforest
angiosperms appears to have been almost complete in the coastal vegetation in the Gippsland
and Bass Basins by the earliest Early Eocene. Prominent families included Casuarinaceae
(representing Gymnostoma rather than Allocasuarina/Casuarina), Euphorbiaceae and
Proteaceae. The relative abundance of Malvacipollis and Proteacidites pollen in coastal plain
and marginal marine sediments suggests Euphorbiaceae and Proteaceae were prominent in
riparian communities and may have included salt-tolerant (mangrove) species. Fossil pollen
data indicate that a number of angiosperm clades that reached their maximum diversity in
Australia during the Early Eocene (Martin 1994, Jordan et al. 1998). However a significant
number of rare taxa occurring in northern and central Australia during the Early Eocene are
not found in southeastern Australia until the Middle-Late Eocene.
7.5.3 Palaeoclimates
The Paleocene-Eocene transition in southeastern Australia is marked by the first occurrence
of fossil Nypa pollen (Spinizonocolpites prominatus). The presence of this pollen type and
Nypa macrofossils (Pole and Macphail 1996) confirms that very warm to hot conditions
persisted into late Early Eocene time within the Gippsland Basin (palaeolatitude 62 0 S) and
Macquarie Harbour in western Tasmania (palaeolatitude 66 0 S). Conditions in the northern
half of the continent during the early Early Eocene are less easily reconstructed due to the
paucity of reliably dated fossil assemblages although, surprisingly, conditions may have been
cooler in northeastern Australia than within the Australo-Antarctic Seaway to the south. For
example, there is no definite pollen evidence for palms in coastal southern Queensland despite
its relatively low (~50 0 S) palaeolatitude.
Palaeo-northern Australia
Occurrences of palm (and paucity of gymnosperm) pollen in Casuarinaceae-dominated
microfloras in the Bonaparte Basin are consistent with hot (megatherm range) and seasonally
very wet (perhumid) conditions during the late Early Eocene. The presence of freshwater
Sparganiaceae swamps along the coast complements sedimentary evidence for increased
continental run-off into the northern Carnarvon Basin in the Early Eocene, although at some
time during this period warm water faunas began to be replaced by cool water faunas
(Apthorpe 1988). Banksia macrofossils indicate that conditions in the Kimberley region were
either drier or soils has become strongly leached.
Conditions at similar palaeolatitudes in northeastern Australia appear to have been cooler
(lower mesotherm) and probably wetter during summer months (humid-perhumid), based on
macrofossils of Agathis, Gymnostoma and Lauraceae and microfloras, which include
rainforest taxa with mesotherm (Anacolosa, Tiliaceae) and microtherm (Dacrydium,
Lagarostrobos, Microcachrys) NLRs. The absence of palms is best explained by cold
currents flowing northwards along the eastern margin.
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