OFR 151.pdf - CRC LEME

3.2 Microfloras
Many of the macrofossil deposits preserve diverse microfloras, leading to parallel, albeit often
different palaeoclimatic interpretations. Regions for which only microfossil data are available
are the Bight region (Eucla Basin) and Alice Springs district (Ayers Rock, Hale, Huckitta,
Santa Teresa, Ti-tree Basins) (Milne 1988, Macphail 1996c, 1997a, Clarke 2000).
Of particular importance are Middle (Lower N. asperus Zone) and Late (Middle N. asperus
Zone) Eocene sequences preserved in the offshore Gippsland and Bass Basins. Reasons
include the high exposure of the basins to events occurring in the Southern Ocean, the
unequalled density of drilling and sampling (up to 120 conventional and sidewall core
samples per well) and concomitant highly detailed lithostratigraphic and geophysical database
(cf. Glenie 1986, Macphail et al. 1994). Unlike basins in central, western and northern
Australia, sufficient assemblages have been analysed over the past three decades to be certain
that a number of fossil species found in central Australia and the Murray Basin during the
Middle-Late Eocene had not migrated as far south as the Gippsland and Bass Basins.
Elsewhere, much potentially fossiliferous material recovered in western New South Wales,
Victoria and South Australia in the 1970s-1980s, was discarded before being pollen-analysed.
Nevertheless the existing microfossil database for the Murray-Darling Basin is second only to
the Gippsland Basin in terms of sample numbers. The basin covers a greater geographic area
(~300,000 km 2 ) than any other Cenozoic basin in Australia (Macphail 1999). Foraminiferal
data provide independent age control in the south-west of the basin (Lablack 1991) but some
age determinations and species time distributions may have compromised by the nature of the
material (cuttings) made available for analysis. Detailed reviews of the microfloras have been
published for the western (Truswell et al. 1985, Martin 1991a), central (Macphail and
Truswell 1989, 1993, Macphail 1999) and eastern (Martin 1973, 1987, 1993) sectors of the
Murray Basin. Martin (1993) has summarized changes in the relative proportion of major
vegetation types within five sectors of the basin via pollen groups averaged for each
palynological zone.
A feature common to all microfloral assemblages from southern and central Australia, is the
high relative abundance and diversity of Nothofagus (Brassospora) spp., especially
Nothofagidites emarcidus-heterus. Nevertheless floristic differences point to significant
bioclimatic gradients between the north/west and south/east regions of the continent. For
example, megathermal taxa such as Nypa are confined to northern Australia and the eastern
Eucla Basin whilst Nothofagus (Lophozonia) spp. are frequent only in eastern Australia and
Tasmania but are rare or absent in southwestern and northwestern Australia (Stover and
Partridge 1982).
3.2.1 North-West Australia
Middle-Late Eocene microfloras are preserved in some bioclastic carbonate facies in the
Bonaparte Basin (Cobia-1, Jacaranda-1). These are dominated by dinoflagellates, and the
minor spore and pollen component almost certainly represents the more prolific pollen and
spore producers in the riparian and regional dryland vegetation. Because of poor
preservation, processing may have concentrated downhole contaminants. Age determinations
are mostly based on marine microfossils.
1. Bonaparte Basin
Yields from Cobia-1 (365-380 m) are inadequate to determine relative abundance but the
most frequently recorded taxa are Casuarinaceae (Gymnostoma) and unidentified tricolporate
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