Launceston’s Tamar Graben: Weighing the biostratigraphic evidence.

Plant fossils preserved in sediments in the Tamar Graben are a natural archive of past environments in the Tamar
Valley at, and to the north of, Launceston over the past 70 million years (Ma) – a period of geologic time during
which Tasmania evolved from a promontory of mainland Australia separated from Antarctica by rift valleys
(Southern Margin Rift System), into an island over 2500 km north of the ‘ice’ continent. Like most Late Cretaceous–
Neogene sites in inland central, southern, and western Tasmania, the geohistory of this graben is based on
a combination of isotopically dated basalts and/or the stratigraphic distribution of fossil pollen and spores (miospores)
preserved in fluvio-lacustrine sediments. The biostratigraphic age control for such sites however, assumes
that the age range of these miospores in geologic time is the more or less the same as in the offshore Gippsland
and Bass basins in Bass Strait.
In this bulletin, we test this assumption for the Tamar Graben by comparing the age ranges of the fossil pollen
and spore species (morphospecies) preserved in nine coreholes with their age ranges in the offshore Gippsland
and Bass basins. We conclude that the pollen and spore-based biostratigraphies developed for these basins do
provide a reliable framework for dating sediments in the Tamar Graben at the geological epoch and sub-epoch
scale i.e., over time scales usually greater than c. 3-10 Ma. However, a significant number of the inferred palynostratigraphic
ages in the Tamar Graben involved selective ‘weighting’ of the biostratigraphic evidence. How
applicable this approach is to dating sediments elsewhere in Tasmania is less certain. Improving the age and correlation
of strata of particular interest within the graben will likely require detailed seismic surveys or enhanced
isotopic dating of interbedded basalts.
Primary reasons for ancillary dating include: (i) mixing of different age fossil pollen and spore assemblages
(microfloras) due to the narrow 5 km width of the graben and endemic reworking of older deposits; (ii) uncertain
impact of Paleogene volcanic eruptions on plant communities growing within the graben; (iii) absence of many
of the key fossil species (zone index species) whose age range is used to define biostratigraphic zones in the
Gippsland and Bass Basins; (iv) apparent differences in the age-range of other less biostratigraphically-reliable
morphospecies (zone accessory species) between the Gippsland and Bass basins and the Tamar Graben; (v) significant
numbers of morphospecies that potentially represent different plant species to those whose closely related
pollen were first formally described in the Gippsland Basin (geographic variants); and (vi) significant numbers of
pollen and spore morphotypes that are undescribed or only have informal fossil species names in Industry reports
for the Gippsland and Bass basins (manuscript or ms species).
A number of these caveats are unlikely to be resolved without improved independent age control and the development
of regional biostratigraphies for Tasmania. The latter will be best achieved by palynostratigraphic analysis
of cores from inland basins where deposition of sediments during the Paleogene-Neogene has been less subject to
fluvial reworking (e.g., the Longford Basin for northern Tasmania, and Macquarie Harbour Graben for western
Tasmania).