OFR 151.pdf - CRC LEME

Palaeo-central Australia
Nothofagus (Brassospora) spp. and rainforest gymnosperms remained common only in sites
where the local topography can be expected to have maintained cool, humid microclimates,
such as sheltered valleys in the Macdonnell Ranges, or around permanent springs. This
observation, allied to lithostratigraphic evidence for the fragmentation of river systems into
stagnant ponds, indicates that summer rainfall had decreased significantly or become
unreliable since the Late Eocene. Nonetheless, the remains of fish, reptiles, birds and
mammals in the Oligocene-Middle Miocene Tirari and Namba Formations show that climates
in northern South Australia were still substantially wetter than at present (cf. Martin 1998a).
Paradoxically, pollen evidence from the upper reaches of the Darling River imply climates
became effectively wetter and/or less seasonal during the Early-Middle Miocene, allowing
evergreen rainforest species to survive (or extend) along river corridors as far west as Broken
Hill. Floristic impoverishment of the flora remains the strongest evidence that mean
palaeotemperatures also decreased (lower mesotherm) or the annual temperature range
become more extreme in central Australia during the Oligo-Miocene.
Palaeo-southern Australia
During the Oligo-Miocene, temperate rainforest dominated by Nothofagus (Brassospora) and
Podocarpaceae became increasingly restricted to Tasmania and southeastern Victoria
although individual Brassospora species such as Nothofagidites falcatus and members of
other subgenera, e.g. Lophozonia (Nothofagidites asperus), maintained a much wider
distribution across southern and eastern Australia.
1. South-west and central southern Australia:
Oligo-Miocene microfloras from the Norseman area in south-west Western Australia closely
resemble those found in central Australia and it is reasonable to assume that microclimates
within deeper valleys incised into the southern Yilgarn Craton were relatively cool (lower
mesotherm) and possibly uniformly wet (perhumid). Conditions in the St. Vincent Basin
were similar during the Oligocene but became warmer (upper mesotherm) during the
Miocene. Seasonality appears to have increased from weak in the late Early Miocene to
strong in the Middle Miocene, resulting in the near complete elimination of Brassospora spp.
from the Adelaide coastal plain. Foraminifera and mollusc faunas indicate significant
warming of surface waters within the Bight during the Early Oligocene and Miocene, with
maximum warmth being reached in the early Middle Miocene (McGowran and Li 1997, Li
and McGowran 1997). On present indications, SSTs in the Eucla and St. Vincent Basins were
warmer than in the Otway, Murray, Gippsland and Bass Basins to the east.
2. Otway Basin
Oxygen isotope stratigraphies from the Browns Creek-Castle sections in the Torquay Subbasin,
southwestern Victoria, show that SSTs decreased by up to 7 0 C, reaching a minimum
value of ~13 0 C during the Eocene-Oligocene transition (Kamp et al. 1990). This was
followed by a rapid warming during the Early Oligocene as circulation within Bight was
effectively decoupled from the developing Circumantarctic Current.
3. Murray Basin
Microfloras from the Murray Basin provide highly detailed records of the flora and vegetation
growing in eastern South Australia, northwestern Victoria and southwestern New South
Wales, but the climatic inferences are constrained by sample quality, strong edaphic control
of community composition, and the potentially enormous size of the pollen source area.
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