Meeting: Thursday 7 June 2012 at 7.45 pm BRIAN DAILY MEMORIAL LECTURE Ice, impact and the rise of the Ediacarans Associate Professor Vistor Gostin Extreme cold and a giant asteroid impact in South Australia may have influenced the evolution of the Ediacarans, some of the earliest complex life on Earth, (635 – 542 million years ago). The presence of isolated pebbles, sandstone aggregates and ≤2 mm siltstone pellets that occur in outcrops of marine mudstone of the ~580 Ma Bunyeroo Formation in the Adelaide Geosyncline, as seen in the central Flinders Ranges, and siltstone pellets in drillcore of the correlative Dey Dey Mudstone in the Officer Basin, SA, are interpreted as dropstones, frozen aggregates and till pellets dispersed by floating ice. Such ice-rafted material occurs <10 m both below and above the ejecta horizon in the Bunyeroo Formation derived from the ~300-km distant Acraman asteriod impact structure, which is in the Gawler Craton, about 300 km west of the Flinders Ranges. The original impact crater probably had a transient cavity of approximately 40 km followed by a collapse crater perhaps 90 km across. This large impact produced ejecta that covered about a 540 km radius. Till pellets underlie distal ejecta in the Dey Dey Mudstone, implying the impact occurred probably during the ~580 Ma Gaskiers glaciation recognized on most continents. The Ediacaran succession in South Australia followed this glaciation. The impact is followed by a negative carbon isotopic excursion and rapid diversification of acanthomorph acritarchs, climaxing with the advent of metazoans. The Ediacaran biotic evolution may have been stimulated by release from the combined environmental stresses of glaciation and giant asteroid impact. Victor Gostin is a retired Associate Professor, and an Honorary Visiting Research Fellow in Geology and Geophysics at the University of Adelaide. A graduate of Melbourne University and holding a Ph.D from the ANU, Canberra, Victor lectured in earth sciences at Adelaide University from 1970 to 2001. His scientific interests include the origins and evolution of the solar system and of life, meteorite impacts, earth history, environmental geoscience, and the effects of natural phenomena on the course of human history. His other interests include sketching the Australian outback. Victor is keen to popularize earth and planetary sciences to the community through lectures and radio (ABC radio891). As a result of recognising and proving that a unique rock layer in the ancient rocks of the Flinders Ranges, South Australia, was derived from a giant meteorite impact 350km away, he has been honoured by having an asteroid named after him (3640GOSTIN). The meeting will be held in the Mawson Lecture Theatre, Mawson Laboratories, corner of Victoria Drive and Frome Road, University of Adelaide at 7.45 pm. Members and visitors are warmly invited to attend. We are obliged for security reasons to keep the front door of the building locked, so please use the special Field Geology Club doorbell to the side of the door for admittance. Frances Williams
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