Palynology of the Oligocene-Miocene sequence in the Oakvale-1 corehole, western Murray Basin, South Australia

Palynological analysis of the marine Oligocene-Miocene Geera Clay and Renmark Group in the Oakvale-l corehole in the western Murray Basin has shown diverse and well-preserved assemblages of spores, pollen, and dinoflagellates throughout the sequence. Pollen of Nothofagus is present throughout, with Nothofagidites emarcidus the most common form . Myrtaceous pollen is abundant: most types are referable to closed forest genera such as Syzygium, Acmena and Tristania, although a significant component of Eucalyptus type pollen is present. Podocarpaceae are common, and include types comparable to the extant Podocarpus, Dacrycarpus, Phyllocladus, Microcachrys and Dacrydium. Araucariaceae, probably as Araucaria, locally reaches high frequencies. Casuarinaceae is consistently present, and Cyperaceae and Poaceae at some levels reach frequencies in excess of 10 per cent. A group of pollen and spores that were first recorded from Tertiary strata in the modern tropics is present, although in low numbers; these include Polypodiisporites usmensis, Margocolporites vanwijhei, and a form similar to Perfotricolpites digitatus. The site provides good fossil records for a number of extant Australian taxa - Acacia pollen (as Acaciapollenites myriosporites) is present from the late Oligocene, and Gyrostemonaceae pollen was recorded from the same interval - and also the first fossil record in Australia for pollen of Utricularia (as Polycolpites sp.) and Gardenia (as Triporotetradites sp.). There is clear evidence too of diversity within the Cyperaceae by the late Oligocene. Recycled Permian and early Cretaceous spores and pollen are most common in the upper part of the Geera Clay. Dinoflagellate cysts occur throughout the section. The assemblages are dominated by the Spiniferites ramosus complex, with Hystrichokolpoma rigaudae, Lingulodinium machaerophorum, Operculodinium centrocarpum, and Systematophora placacantha the most common of the other components. There is a general similarity to coeval assemblages from Europe and elsewhere, but one major difference is the absence of peridinioid forms such as Deflandrea, Wetzeliella sensu lato, and Palaeocystodinium. These apparently did not persist into the late Oligocene in this region. The base of the Triporopollenites bellus Zone of the Gippsland Basin has been tentatively identified at 80 m.  A quantitative zonation for this corehole sequence, based on the frequencies of the major pollen taxa, has been statistically calculated, enabling a division into two major zones; the younger of these has been further divided into four sub-zones. The basal Zone II, incorporating the Olney Formation and lower Geera Clay, is characterised by high Myrtaceae and high Nothofagus brassi type values. N. brassi, however, is lower here than at coeval sites in southeastern Australia. These assemblages are considered to indicate the presence of evergreen rainforest with abundant myrtaceous trees, in association with the N. brassi producers. A climatic regime of high, year-round precipitation may haye supported this forest type.  In the late Oligocene, the vegetation changed; N. brassi became reduced in importance, and Araucaria became more prominent. This has been interpreted as reflecting a drier type of rainforest growing under a mildly seasonal moisture regime.

Tags: Earth Sciences, GA Publication, Journal, Published_External, SA
Organization: Australian Ocean Data Network