OFR 151.pdf - CRC LEME

(Morgan 1977, McMinn 1981a, Martin 1988, 1991b, 1997b, Macphail 1996c, 1997a). The
age determinations are based on floristically impoverished Nothofagidites assemblages that
lack index species found in younger (Late Miocene-Pliocene) or older (Eocene) microfloras.
None of the zone index species used to subdivide Oligocene-Middle Miocene time in
southern Australia have been recorded to date in central Australia and the age determinations
remain provisional.
1. Callabonna Sub-basin
The one known assemblage from the Namba Formation is moderately diverse, with
dominance shared between Cyathea (10%), gymnosperms (total 21%) and Restionaceae
(39%). Frequent to common types are Podocarpus-Prumnopitys (9%), Dacrydium (5%),
Lagarostrobos (3%), Phyllocladus (3%), Nothofagus (Brassospora) spp. (4%), Sparganiaceae
(5%) and Cyperaceae (2%). Rare taxa include Araucariaceae (Agathis/Wollemia),
Anacardiaceae, Caesalpinaceae and Eucalyptus. Proteaceae are absent.
2. Alice Springs District
Pollen dominance is shared between Gleicheniaceae, Podocarpaceae (Podocarpus-
Prumnopitys), Casuarinaceae and Nothofagidites (Brassospora) spp. For example the
Borehole RN 16861 is wholly dominated by Nothofagus (Brassospora) spp. (59%) with lesser
amounts of Casuarinaceae (27%), Podocarpus-Prumnopitys (7%) and Dacrydium (4%). Low
numbers of Botryococcus cysts imply the depositional environment was an ephemeral,
brackishwater lake.
Correlative Nothofagus (Brassospora) microfloras from the Burt Plain Basin occasionally
include frequent to common numbers of Blechnaceae, Araucariaceae (Araucaria), Malvaceae
(Malvacearumpollis) and unidentified tricolporate types. Rare taxa include Pteris,
Araucariaceae (Agathis/Wollemia-type), early records of Sprengelia-type (Epacridaceae) and
Grevillea-Hakea (Proteaceae), Menyanthaceae (cf. Villarsia), Onagraceae (cf. Epilobium) and
an unidentified Sparganiaceae. Cyperaceae, Myrtaceae (including Eucalyptus) and
Restionaceae are very rare or absent.
Assuming that the Namba and Alice Springs sequences are broadly contemporary, relative
abundances appear to have been shaped by the depositional environment and regional
topography. For example, the prominence of Nothofagus (Brassospora) spp. in the Alice
Springs district compared to the Lake Eyre Basin almost certainly is due to the proximity of
uplands such as the McDonnell Ranges. Conversely sedges and other swamp taxa are better
represented in the low-lying Lake Eyre Basin. Apart from Casuarinaceae, which may include
the dry sclerophyll genera Allocasuarina/Casuarina as well as Gymnostoma, it is unclear
which taxa occupied dry interfluve sites. Halophyte taxa such as Chenopodiaceae-
Amaranthaceae (Chenopodipollis chenopodiaceoides) or Wilsonia (Tricolpites trioblatus)
appear to be absent although both occur in saline-influenced deposits in the Murray Basin by
this time.
Inferred climate
The floristically and ecologically heterogeneous composition of the microfloras in central
Australia point to relatively cool (lower mesotherm) and seasonally wet (humid) conditions in
which mean summer rainfall was inadequate to support rainforest trees away from stream
banks or sheltered gullies. The ephemeral nature of depositional environments also point to
dry summers during which rivers fragmented into brackishwater swamps.
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