Myths and legends in the correlation between the Boreal and Tethyan Realms. Implications on the dating of the Early Toarcian mass extinctions and the Oceanic Anoxic Event

This study constitutes a revision of the biostratigraphy of Early Toarcian age ammonites coming from different successions in the NW Europe and Mediterranean areas, complemented by new, important data from the Migiana di M. Malbe section (Perugia, Italy). The research has demonstrated that the problems of correlation between some Zones of the Standard and Mediterranean biostratigraphic scales in this period are doubtful. On the negative side, procedural problems and defects have emerged that greatly detract from the validity of some boundaries which were, almost unanimously, held to be synchronous. Undervaluation of the problems brought to light in this study has had repercussions both on the construction of the eustatic curves and on the dating of the Oceanic Anoxic Event (OAE), and consequently on the evolutionary models for the various areas of the Tethys and of NW Europe. The various datings obtained by different writers have led to the belief that the anoxic conditions became established diachronically in the two areas. However, ammonite dating of the black shale-like facies, made for the first time in the Umbria-Marche Apennines, shows instead that, at least between this area and Germany, the diachronism was only partial. In both areas, the establishment of "anoxic" conditions took place in the Dactylioceras semicelatum Subzone. Problems linked to ammonite faunal provinciality and evolution, complicated by tectono-eustatic phenomena that produce hiatuses in NW Europe, are at the root of the lack of continuity in tracing the Pliensbachian-Toarcian boundary between the NW Europe area and the rest of the world. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.