MIOCENE CLIMATIC OSCILLATION RECORDED IN THE LAKES ENTRANCE OIL SHAFT, SOUTHERN AUSTRALIA - REAPPRAISAL OF THE PLANKTONIC FORAMINIFERAL RECORD

The stratigraphic section at Lakes Entrance accumulated on a narrow pl atform in a neritic environment, close to the interaction of the East Australian Current and the West Wind Drift. The biostratigraphic succe ssion of planktonic foraminiferal events first presented by D.G. Jenki ns in 1960 has been slightly revised and correlated with the integrate d Miocene geochronology. To extend biostratigraphy to ecostratigraphy, we have revised the systematics and nomenclature of the planktonic ta xa and profiled the faunal succession in sixteen assemblages falling i nto three groups. 1, From the later middle Miocene to late Miocene occ urred assemblages XI to XVI, typified by the collapse of woodi/bulloid es ratio and resurgence of spinose species, as well as a comeback by t he cancellate and globorotaliid forms. 2, Assemblages IX to X range fr om the latest early Miocene to early middle Miocene (upper N7 to N10 e quivalents), with greatest amplitudes in fluctuation in species divers ity and other metrics. 3, The early Miocene contained assemblages I to VIlI, in which a rising woodi/bulloides ratio was accompanied by abun dant microperforates but decline in spinose and cancellate species and in the planktonic/benthic ratio. At the second order or 10(7) years' scale, the Exxon sealevel curve rises sporadically through the early M iocene and falls sporadically to its lowest level in the late Miocene, broadly congruent with the pelagic oxygen isotopes, which indicate an early Miocene rise and a major decline into the late middle Miocene. At the third order and 10(6) years' scale, there may be promise of syn chrony between the Mi glacial cycles and the marginal sequences. It is noteworthy that, at Lakes Entrance, there are about ten putatively gl obal sequence boundaries and ten Mi glaciations spanning the time in w hich fifteen neritic faunal assemblages were recognized. Spikes in the woodi/bulloides ratio fit between the Mi glaciations and fall in the vicinity of maximum flooding surfaces. However, they fit lows, not hig hs, in planktonic diversity and planktonic/benthic ratios, suggesting intensified estuarine-type runoff as the control in this neritic setti ng. The perturbation in all measures increased in the 1.5 m.y. spannin g assemblages IX and X straddling the early/middle Miocene boundary. A t this time of peak warming and transgression, of stratification of pe lagic water and planktonic communities, and bunching of third-order se quences, the biosphere was at its most sensitive and volatile and most responsive to perturbations (which led the big drops in climate and s ealevel).