OFR 151.pdf - CRC LEME

3.2 Microfloras
Partridge (1999) has challenged earlier interpretations that Plicatella (al. Appendicisporites)
distocarinata Zone microfloras are preserved in the Gippsland Basin. If correct, then the only
reliable palynostratigraphic evidence for the nature of the Cenomanian vegetation in southern
Australia comes from basins in, or bordering, the Great Australia Bight (Otway, Duntroon,
Eucla and Perth Basins). Most of these microfloras are preserved in marine facies and are
likely to be biased towards well-represented species such as gymnosperms due to the Neves
Effect.
The only detailed correlative records for northern Australia come from two boreholes
(Bathurst-1, Bathurst-2) drilled on Bathurst Island, 65 km north of Darwin (Norvick and
Burger 1975, Burger 1975) and in the Carpentaria Basin (BMR Dobbyn-1) in northwestern
Queensland (Playford et al. 1975). During the Cenomanian, this region was part of a very
shallow, regressive sea located at a palaeolatitude of ~49 0 S on the palaeo-northwestern
margin. Non-marine Cenomanian sediments are preserved further to the south in the
Eromanga and Surat Basins.
3.2.1 North-West Australia
1. Bathurst Island
Microfloras recovered from the Bathurst-2 well (Norvick and Burger 1975) appear to
represent two distinct vegetation types: (1) fern swamps growing along the palaeoshoreline
and (2) mixed Communities of Cheirolepidiacean conifer woodland and Austral Conifer
Forest, which may have been restricted to adjacent uplands.
Dominant taxa within the forests and/or woodlands were Cheirolepidaceae (Corollinia),
pteridosperms (Alisporites, Vitreisporites), brachyphyll araucarians (Balmeiopsis,
Hoegisporis) and Microcachrys (Microcachrydites antarcticus). It is possible the last species
was a shrub. The relative abundance of individual angiosperm species is very low but the
total angiosperm count is significant. The only types referable to modern taxa (in
parentheses) are Clavatipollenites (Chloranthaceae), Liliacidites (Palmae, Iridaceae or
Liliaceae) and Dicolpopollis (Palmae).
Coastal communities were dominated by cryptogams. The more common taxa are:
Triporoletes (liverworts), Perotriletes and Retitriletes (Lycopodiales), Crybelosporites
(aquatic ferns), Laevigatosporites, Cyathidites, Clavifera and Gleicheniidites (ferns). Two
extinct species of Anemia (Plicatella distocarinatus, Ruffordiaspora australiensis) are
common in many samples. Sphagnum spores occur only sporadically.
Inferred climate
Norvick and Burger (1975) suggest that the lowland vegetation grew in a temperate to warm,
possibly dry climate. This is inconsistent with the observed high diversity of cryptogams
(including obligate aquatic types).
The preferred explanation is that humidity was seasonally high (perhumid and/or monsoonal)
due to warm sea surface temperatures off the palaeo-north coast. Whether rainfall was
uniformly distributed or strongly seasonal is unclear. The relative paucity of Austral Conifer
Forest and virtual absence of Sphagnum is consistent with very warm (upper mesotherm)
temperatures and/or a pronounced dry season.
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