A pre-Himantian (Late Ordovician) interval of global cooling - The Boda event re-assessed

Facies analysis suggests that the early-mid Ashgill (Late Ordovician) 'Boda event' of Fortey and Cocks [Fortey, R.A., Cocks, L.R.M., 2005. Late Ordovician global warming-The Boda event. Geology 33 (5), 405-408] was an episode of global cooling, not of warming as they proposed. Limestones are widely developed at this level from mid-latitude to high-latitude, commonly interrupting thick Ordovician successions of fine clastic sediments. Bryozoan carbonate mud mounds in a laterally extensive coolwater carbonate belt along the high palaeolatitude margin of W Gondwana are associated with glacial diamictites, and grew during glacial lowstand with sea ice prior to the Hirmantian glaciation. From high to mid-palaeolatitudes, contemporaneous cool-water carbonates with mud mounds, including stromatactis both in Gondwana and in Baltica, developed in mid-outer ramp settings during regression. In the dominantly clastic Welsh Basin of Avalonia, phosphatic facies on an Iapetus-facing carbonate ramp indicate upwelling cold waters down-ramp, yet up-ramp coralliferous-algal limestones suggest warm surface waters. We propose that the Late Ordovician cool-water carbonates developed in response to global cooling and expansion of Gondwanan terrestrial and sea ice that lowered eustatic sea level, and on the W Gondwanan shelf reduced or shut down fluvial clastic input. Previously poorly circulated, salinity-stratified oceans that had restricted benthic dispersal in mid-low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas. An earlier, Caradoc carbonate interval in low-mid palaeolatitudes that shares many facies characteristics with the Ashgill cool-water limestones, and correlates with a widely recognised isotope excursion, is interpreted as indicating an earlier global cooling episode. (c) 2007 Published by Elsevier B.V.