CALCAREOUS MICROPLANKTON AND POLARITY REVERSAL STRATIGRAPHIES OF THE UPPER EOCENE BROWNS-CREEK CLAY IN THE OTWAY BASIN, SOUTHEAST AUSTRALIA- MATCHING THE EVIDENCE

We attempt to correlate the Browns Creek Clay type section--one of the most important Palaeogene marine reference sections in southern Austr alia--with the geomagnetic polarity time scale. The correlation depend s on the choice of bioevent ties between the Browns Creek Clay and oth er sections with well-defined polarity reversals at similar mid to hig h southern latitudes. Of the various possible ties, we discuss two tha t best fit the available evidence, nominating our preference. The firs t correlation is a one-to-one match of reversals between the Browns Cr eek Clay record and the Geomagnetic Polarity Time Scale interval Chron 18n.1 to Chron 16n.2. This is achieved via the Weddell Sea record (OD P Leg 133 Site 689), using as the baseline tie the disappearance event of the foraminiferid Acarinina collactea. Inherent in this correlatio n are misfits with other more reliable bioevents and the utilisation o f two short normal polarity events in the Browns Creek record, each ba sed on a single stratigraphic sampling level. The second and preferred correlation is via the South Atlantic record (DSDP Leg 73 sites) and uses two reliable calcareous nannofossil bioevents: the appearance of Isthmolithus recurvus and the disappearance of Cyclicargolithus teticu latus. It suggests a match between the Browns Creek Clay polarity reve rsal record and a Geomagnetic Polarity Time Scale interval that starts in Chron 16n.2 and ends in Chron 15n. Although in this correlation th e two brief normal polarity events have no counterparts in the Geomagn etic Polarity Time Scale as it is currently recognised, it is preferre d because of stronger biostratigraphic links. The correlation suggests that the lower part of the Browns Creek Clay (the Turritella clays an d Notostrea greensand) was deposited in similar to 1.3 million years, about half the time suggested by the first correlation, and dates at c a 35.0 Ma the Hantkenina alabamensis primitiva interval which is an im portant biostratigraphic marker widespread in southern Australia.