OFR 151.pdf - CRC LEME

SECTION 3 (CHRONOSTRATIGRAPHY)
Few rock sequences are suitable for isotopic dating. For example, the potassium/argon
(K/Ar) method is of limited use in Australia due to the restriction of suitable volcanic rocks to
the eastern margin, Tasmania and the Kimberley region. Other sequences have to be dated
via their fossil content or their stratigraphic relationships to other strata that preserve fossils.
This inevitably leads to circular reasoning when the same fossils are used as evidence of both
age and palaeoenvironment. In most instances however, age determinations can be validated
via correlation with other sections that can be dated by chronometric techniques.
3.1 Palynostratigraphic dating
Most of the zonation schema that use fossil pollen and spores to date and correlate Cretaceous
and Cenozoic sediments in Australia were developed by or for the petroleum exploration
industry. An exception is the Murray-Darling Basin in southeastern Australia where the
driver has been groundwater and salinity management.
In almost all instances, the zone boundaries are defined by the observed First (FO) and Last
(LO) occurrences of selected species (Concurrent Range zones). Computer models that can
simulate the fossil record have been used to analyse potential discrepancies (offsets) in the FO
and LO of a species in the stratigraphic record versus the probable earlier times of migration
and later extinction of the parent taxa (cf. Holland and Patzkowsky 1999, Nowak et al. 2000).
Offsets explain occasional anomalously early or late occurrences of particular fossil species.
The microfossils that are most useful as zone indicator species (zone index species) tend to be
relatively large and distinctively shaped/ornamented types that are easily seen using low
magnification or bright field microscopy. Many of the parent plants were small, rare or
under-represented plants that grew in the subcanopy or ground stratum, or in water.
Examples are two liverwort spore types, Foraminisporis wonthaggiensis and
Cingulatisporites bifurcatus whose FOs define the base of the Valanginian and Late Miocene
in southeastern Australia, respectively.
For these reasons, zone index species may be restricted to one sedimentary basin or
geographic region. An example is a fern now restricted to South America (Lophosoria)
whose fossil spores (Cyatheacidites annulatus) are widespread in Oligo-Miocene sediments
across southeastern Australia but are not known to occur in correlative sediments in northern
Queensland, central Australia or Western Australia; Foraminisporis wonthaggiensis is
common in Early Cretaceous sequences in eastern Australian but has not been recorded in
Cretaceous sequences in south-west or north-west Western Australia (M.K. Macphail and
A.D. Partridge pers. observations). Similarly, the age range (time distribution) and maximum
relative abundance (acme) of commonly occurring species will tend to vary from basin to
basin because of differences in local to regional environments. An example is an extinct
species of the rainforest genus Anacolosa whose fossil pollen (Anacolosidites acutullus) first
appears in northwestern Australia in the Late Cretaceous, but is not recorded southeastern
Australia until the Late Paleocene.
3.1.1 Cretaceous
Formal spore-pollen and dinoflagellate-based palynostratigraphies have been available since
the 1960s, e.g. Burger (1980) and Morgan (1980) for the Eromanga and Surat Basins, which
cover much of northern South Australia, central Queensland and northwestern New South
Wales respectively; Dettmann (1963), Stover and Evans (1973) and Stover and Partridge
(1973) for the Otway and Gippsland Basins in southeastern Australia; and Balme (1964) and
Backhouse (1988) for the Perth, Carnarvon and Canning Basins in Western Australia.
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