The CA-IDTIMS Method and the Calibration of Endemic Australian Palynostratigraphy to the Geological Timescale

Abstract

The Permian portion of the International Geological Timescale has marine fossil zonations as its prime correlative tools. These comprise conodont, ammonoid, fusulinid and benthic foraminifera zonations, all defined in the northern hemisphere. During the Permian, eastern Australian was in high southern latitudes and largely nonmarine, making spore-pollen zonation the most effective local biostratigraphic scheme. Correlating this zonation to the global geological timescale has proven difficult at best. The reasons for this are twofold. Firstly, conodonts and fusulinids have never been found in the eastern Australian successions, and ammonoids are very rare. Secondly, the high latitude flora of the Permian (the Glossopteris flora) was largely endemic to the circumpolar Gondwanan continents, so precise correlation to the northern hemisphere is almost impossible. Despite this, a tentative calibration of the spore-pollen scheme against the timescale has been in use for some time, and is largely based on rare marine fossil occurrences. The recently developed Chemical Abrasion-Isotope Dilution Thermal Ionisation Mass Spectrometry (CA-IDTIMS) technique, coupled with the plethora of tuffs in the nonmarine eastern Australian Permian, has provided an opportunity to calibrate the spore-pollen scheme directly to the International Geological Timescale. This has shown that the previous, tentative calibration is in need of considerable revision. For instance, the base of the Dulhuntyispora parvithola Zone was calibrated at about 263.5 Ma, in the early half of the Capitanian Stage. Our data clearly show that it lies within the lower half of the Wuchiapingian, at about 257.8 Ma. The base of the underlying Dulhuntyispora dulhuntyi Zone was calibrated at the base of the Capitanian (265.2 Ma). Our data demonstrate that it is, instead, early Wuchiapingian, at about 258.2 Ma. Preliminary data in the Triassic and Cretaceous indicate that similar dramatic changes in calibration may be required, with the base of the Triassic APT 4 being as much as 16 million years younger than currently assigned (middle Norian rather than late Ladinian), while the Cretaceous Cassiculosphaeridia delicata dinocyst zone may be as much as 3 million years younger, being early Valanginian rather than middle Berriasian. These adjustments to the ages and durations of Australian biozones can have profound impacts on the evaluation of depositional rates and burial history models used in petroleum exploration.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015