Calcareous microplankton biostratigraphy of the Eocene Browns Creek Clay in the Aire District, Otway Basin of southeastern Australia: an update

In updating the calcareous microplankton stratigraphic record of the lower part of the Browns Creek Clay (the Turritella clays and Notostrea greensand), Otway Basin of southeastern Australia, unexpected problems with significant consequences were encountered. Assemblages from the base of the formation include two key species, the nannofossil lsthmolithus recurvus (hitherto unknown from this level) and the foraminiferid Acarinina collactea (unknown from this formation until fairly recently)- in association with Neococcolithes dubius, Chiasmolithus oamaruensis, Cyclicargolithus recticulatus, Discoaster saipanensis, Globigerinatheka index, Tenuitella aculeala, T gemma and T insolita. The stratigraphic range of I. recurvus is disjunct (in this respect, I. recurvus resembles the two Eocene foraminiferids T aculeata and G. index in southern Australia): in its lower range I. recurvus is rare, but it is common in its upper range-with a substantial gap in between, within which N. dubius disappears. Elsewhere, including other southern high-latitude sections, A. collactea is known to disappear well below the first appearance datum (FAD) of I. recurvus, and the split stratigraphic range of I. recurvus has not been explicitly reported. Reworking could be the cause of the overlap in the stratigraphic ranges of I. recurvus and A. eollactea at Browns Creek, although a seemingly wide geographic distribution of the association of these two species and N. dubius in southern Australia has been indicated herein. Evidence (including the presence of rare T gemma) has been discussed , favouring I. recurvus (in preference to A. collactea) as a prime evidence for dating the base of the Browns Creek Clay. This base has been correlated with the sequence boundary between the third order cycles 4.1 and 4.2, and dated late Eocene, at about 36.2 Ma. Cyclicargolithus reticulatus disappears near the Turritella clays/Notostrea greensand boundary - within the foraminiferid Hantkenina alabamensis primitiva interval, which has long been accepted as isochronous across southern Australia. The event of disappearance of C. reticulatus is thought to be at 34.9 Ma, in agreement with a previously given K-Ar age limits for the Hantkenina interval. The combined evidence of I. recurvus and C. reticulatus assigns the lower part of the Browns Creek Clay a late Eocene age, equivalent to low either Zone NP19/20 or Subzone CP15b. A correlation with the foraminiferal zonal interval high P15 to within P16 of the tropics is possible.