MIOCENE UPWELLING EVENTS - NERITIC FORAMINIFERAL EVIDENCE FROM SOUTHERN AUSTRALIA

The neritic stratigraphic section in the Lakes Entrance oil shaft in G ippsland, Victoria, southeastern Australia, records four major upwelli ng events at the third-order or 10(6) years scale. The first and secon d occurred during the Janjukian (latest Oligocene to Early Miocene; at about 24.5 and 22 Ma); the third in the late Longfordian (late Early Miocene; 17.5-17 Ma); and the fourth at about 9-8 Ma in the Mitchellia n (Late Miocene). The evidence for upwelling consists of concurrent si gnals from the planktonic and the benthic foraminifera. The four upwel ling events are characterized by a low specific diversity in the plank ton, a high abundance of the planktonic species Globigerina bulloides, and a high sedimentation rate (> 2 cm/ka). Among the benthos, such in faunal species as Uvigerina proboscidea increased in abundance and the mixing of normally separate deeper and shallow-water taxa was common. The ratio of epifaunal to infaunal benthic foraminifera increases thr ough the Miocene as a second-order secular trend (10(7) a) but it is m arked by four strong reversals which are the four events. It is sugges ted that the northerly fluctuation of the Subtropical Convergence was largely responsible for the fourth of these events. The others, howeve r, are on the rising second-order trends in global warming and sea lev el and they alternate with warming events recognized on the occurrence of larger benthic foraminifera. The 'upwelling events' and the 'warmi ng events' occur at the same third-order scale as the putative eustati c cycles of sequence stratigraphy and the Mi glaciations based on ocea nic delta(18)O fluctuations. The upwelling events are neritic signals of global oceanic changes, of which the most marked is at the third, s ituated at the onset of the Monterey carbon isotope excursion. The Mio cene climatic optimum at the zeniths of high sea level and warm climat e lacks an 'upwelling event'. At this time there was a sharp reversal to oligotrophic conditions, a reversal seen also in the shallower and more restricted sea in the Murray Basin.