Early Silurian Oceanic Episodes and Events
Journal of the Geological Society, London, Vol. 150, 1993, pp. 501-513, 3 figs. Printed in Northern Ireland Early Silurian oceanic episodes and events R. J. ALDRIDGE l, L. JEPPSSON 2 & K. J. DORNING 3 1Department of Geology, The University, Leicester LE1 7RH, UK 2Department of Historical Geology and Palaeontology, SiSlvegatan 13, S-223 62 Lund, Sweden 3pallab Research, 58 Robertson Road, Sheffield $6 5DX, UK Abstract: Biotic cycles in the early Silurian correlate broadly with postulated sea-level changes, but are better explained by a model that involves episodic changes in oceanic state. Primo episodes were characterized by cool high-latitude climates, cold oceanic bottom waters, and high nutrient supply which supported abundant and diverse planktonic communities. Secundo episodes were characterized by warmer high-latitude climates, salinity-dense oceanic bottom waters, low diversity planktonic communities, and carbonate formation in shallow waters. Extinction events occurred between primo and secundo episodes, with stepwise extinctions of taxa reflecting fluctuating conditions during the transition period. The pattern of turnover shown by conodont faunas, together with sedimentological information and data from other fossil groups, permit the identification of two cycles in the Llandovery to earliest Weniock interval. The episodes and events within these cycles are named: the Spirodden Secundo episode, the Jong Primo episode, the Sandvika event, the Malm#ykalven Secundo episode, the Snipklint Primo episode, and the lreviken event. Oceanic and climatic cyclicity is being increasingly semblages (Johnson et al. 1991b, p. 145). Using this recognized in the geological record, and linked to major and approach, they were able to detect four cycles within the minor sedimentological and biotic fluctuations.
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