Late Ordovician Eustasy And Glacial Cycles Across The North Gondwana

The Late Ordovician glaciation was initially (1960-1990) thought to represent a long-lasting icehouse period. In the nineties, the idea was imposed of a much shorter glacial event, possibly catastrophic at the geological timescale. The corresponding glaciation was limited to the Hirnantian, or even to a part of this 1-2 My stage, as suggested by short-lived isotopic excursions and correlative biological turnovers. The sequence stratigraphy of Late Ordovician successions in the palaeo-high latitude North Gondwana setting reveals that at least 3 glacial event similar to the Hirnantian event occurred in the Katian, as indicated by the stacking-pattern of shelf successions including forced regressive system tracts. The latter seem to have their counterpart in low-latitude settings, either in the form of fully developed third-order regressive-transgressive cycles (Laurentia) or in karst horizons (Baltica). The synchronous character of these inferred worldwide glacioeustatically controlled event is however difficult to ascertain as biostratigraphy is essentially based on endemic (Laurentia/Baltica/Gondwana) faunal assemblages. The view of a long-lasting glaciation (> 25 My) including discrete short glacial events (< 1 Ma, intra-Katian events, Hirnantian, base Wenlock) prevails today. The tempos and forcings of these glacial events are however largely debatable, what made their correlation at continental scales questionable. Until recently, most of the glacial features (tillites, striated surfaces, tunnel valleys) were ascribed to the Hirnantian glacial event(s). Only during the corresponding time interval ice fronts indeed reach sedimentary basins around the Gondwana supercontinent, and hence were related to the widespread and well-known glacial record.