Wenlock (Silurian) oceanic episodes and events

Abstract

Biotic and sedimentological cycles in the Wenlock are related to episodic changes in oceanic state. These may be explained by alternations between primo and secundo conditions, the former being characterized by cold oceanic bottom waters and high nutrient supply, and the latter by salinity-dense bottom waters and diminished nutrient supply. The lithological pattern, together with the turnover of conodont faunas and of other fossil groups, permits the identification of a sequence of episodes and events in the Wenlock, following the extinctions associated with the earliest Wenlock Ireviken Event: the Vattenfallet Secundo Episode, the Sanda Primo Episode, the Boge Event, the Lansa Secundo Episode, the Allekvia Primo Episode, the Valleviken Event, the Hellvi Secundo Episode, the Mulde Event and the Klinte Secundo Episode. The Mulde Event differs from others in separating two secundo episodes, and may represent an interval when deep waters remained saline and vertical circulation virtually ceased. The effects of the events and episodes were stronger in the early and late Wenlock than in the middle part of the epoch.