A Cenozoic-Style Scenario for the End-Ordovician Glaciation

International audienceThe end-Ordovician (Hirnantian) was an enigmatic interval in the Phanerozoic, known for massive glaciationpotentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and adevastating extinction event. Such linkage of eustatic, biological and isotopic records to the climatically forceddevelopment of an ice sheet can only be contemplated within a framework of high-resolution sequence stratigraphythat integrates allo-, chemo- or biostratigraphic markers. We develop sequence stratigraphic correlations fortwo superbly exposed and exceptionally well-developed latest Ordovician successions, the Anti-Atlas of Moroccoand Anticosti Island in Canada. Both offer sections, on a 100-km scale, from the basin edge to the axis of activesedimentary depocentres. Relative to the end-Ordovician ice-sheet centre (present-day north-central Africa), theyprovide a near-field (Anti-Atlas, siliciclastic platform) and a far-field (Anticosti Island, mixed carbonate and siliciclastic)stratigraphic records. These two successions, up to 300 and 100m thick, respectively, were deposited inbasins with notable subsidence rates and significant (ca. 100 m) initial water depths, enabling the development ofcomprehensive archives of the latest Ordovician glaciation. This framework, driven by glacio-eustatic cycles tiedto the evolution of polar continental-scale ice sheets over west Gondwana, enables the correlation of eustatic cyclesat a level that is beyond the resolution capability of most absolute dating methods and of biozones, the latter typicallyof Myr duration. A proposed Cenozoic-style scenario including three main glacial cycles and higher-orderphenomena necessitates the revision of the end-Ordovician, glaciation-related sequence of events