Miocene climatic oscillations recorded in the Lakes Entrance oil shaft, southeastern Australia: reappraisal of the planktonic forminiferal record

Abstract

The stratigraphic section at Lakes Entrance accumulated on a narrow platform in a neritic environment, close to the interaction of the East Australian Current and the West Wind Drift. The biostratigraphic succession of planktonic foraminiferal events first presented by D.G. Jenkins in 1960 has been slightly revised and correlated with the integrated Miocene geochronology. To extend biostratigraphy to ecostratigraphy, we have revised the systematics and nomenclature of the planktonic taxa and profiled the faunal succession in sixteen assemblages falling into three groups. 1, From the later middle Miocene to late Miocene occurred assemblages XI to XVI, typified by the collapse of woodi/bulloides ratio and resurgence of spinose species, as well as a comeback by the cancellate and globorotaliid forms. 2, Assemblages IX to X range from the latest early Miocene to early middle Miocene (upper N7 to N10 equivalents), with greatest amplitudes in fluctuation in species diversity and other metrics. 3, The early Miocene contained assemblages I to VIII, in which a rising woodi/bulloides ratio was accompanied by abundant microperforates but decline in spinose and cancellate species and in the planktonic/benthic ratio. At the second order or 107 years' scale, the Exxon sealevel curve rises sporadically through the early Miocene 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 106 years' scale, there may be promise of synchrony between the Mi glacial cycles and the marginal sequences. It is noteworthy that, at Lakes Entrance, there are about ten putatively global sequence boundaries and ten Mi glaciations spanning the time in which 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 highs, in planktonic diversity and planktonic/benthic ratios, suggesting intensified estuarine-type runoff as the control in this neritic setting. The perturbation in all measures increased in the 1.5 m.y. spanning assemblages IX and X straddling the early/middle Miocene boundary. At this time of peak warming and transgression, of stratification of pelagic water and planktonic communities, and bunching of third-order sequences, the biosphere was at its most sensitive and volatile and most responsive to perturbations (which led the big drops in climate and sealevel).