Paleoflora and environment of the Surat Basin at the time of the JK transition

Cooling, Jennifer1; Esterle, Joan1; McKellar, John2

1University Of Queensland, St Lucia, Australia, 2Department of Natural Resources, Mines and Energy, Brisbane, Australia

The Westbourne Formation to Mooga Sandstone interval of the Surat Basin contains the most complete record of deposition from Queensland from the time of the Jurassic–Cretaceous transition. As relatively few botanical macrofossils have been reported from this interval, and few of those have been well described, palynological studies, such as the one presented here, provide the best window into the region’s flora just before the arrival of the angiosperms. Palynological samples taken from three stratigraphic boreholes that intersected this interval produced a diverse, but relatively stable, microflora of 218 taxa. Fern spores are the most abundant component of assemblages, averaging 51 percent of all samples, in particular those of the Osmundaceae and Matoniaceae. Spores produced by the Dicksoniaceae, Gleicheniaceae, Horsetails, Marattiaceae, Schizaeaceae, Pteridaceae and Polypodiaceae are also represented. Averaging 20 percent of all assemblages the spores of the lycopods were the second most abundant group and were the most taxonomically diverse. Conifer pollen averaged 17 percent of all assemblages with that produced by the Araucariaceae being the most common of these. Pollen from the Cheirolepidaceae, Podocarpaceae, Pinaceae and Taxodiaceae is also recorded in these samples. Pollen produced by the seed ferns averaged five percent of all samples, while spores produced by bryophytes averaged less than two percent and the monocolpate pollen that may have been produced by some combination of cycads, ginkgoes and gnetales less than one percent of all assemblages. While the link between palynofloral and macro- floral abundances is not a direct one, being influenced by sedimentological and taphonomic effects, some broad conclusions about the parent flora can be made. The Surat Basin flora of the mid- Tithonian to early Hauterivian was a diverse one, with a floodplain (from where these samples were taken) flora dominated by members of the Matoniaceae and Osmundaceae along with numerous other species of ferns, lycopods and bryophytes. The somewhat drier upland flora is represented by the Araucariacean conifers with the seed ferns and the Podocarpacean and Cheirolepidacean conifers producing much of the remaining upland palynofloral assemblage.

With their high proportion of fern spores to conifer pollen, the majority of the samples from this project can be recognized as coming from a backswamp or floodplain facies. This paleoenvironmental interpretation supports the view that the Westbourne Formation to Mooga Sandstone sequence was, like much of the Australian Late Jurassic sequence, deposited in a highly fluvial environment of meandering and braided streams, shallow lakes and swampy floodplains. This interpretation is further supported when the contributions of different palynomorph ecogroups (PEGs) to the assemblages are considered and the results of polytopic vector analysis (PVA) of the assemblages.


Biography

Jennifer Cooling is a PhD candidate from the University of Queensland due to complete her project on the Palynology of the Jurassic-Cretaceous of the Surat Basin this year.

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