The succession of Hirnantian events based on data from Baltica: brachiopods, chitinozoans, conodonts, and carbon isotopes

Abstract

The Hirnantian (late Ordovician) environment was complex and dynamic. Understanding correct order of events and their precise correlation with a time scale are extremely important for development of different kinds of environmental interpretations. The lower boundary of Hirnantian Stage is officially defined by the lowest occurrence of Normalograptus extraordinarius, base of major positive carbon-13 isotope excursion, and beginning of a pronounced sea-level fall associated with onset of a major glaciation (ICS website). Our aim is to check if these events are synchronous, particularly how situation is with respect to Baltic. Thus several sections were analysed using mainly East Baltic data (drill cores), but also data from elsewhere, including brachiopod, chitinozoan, and conodont biostratigraphy combined with graptolite and carbon isotope data. Brachiopod faunas of Pirgu and Porkuni stages are rather similar to those of Ellis Bay Formation of Anticosti Island, Quebec, but Pirgu assemblage, as well as that of lower Ellis Bay Fm. lack key elements of Hirnantian faunas. The primary criterion quoted above is stable, but auxiliary data like isotope curve are less convincing. When latter is used, it should be specified, e.g. that carbon isotope excursion begins usually slightly earlier (in Diceratograptus mirus Biozone), but it might be conventionally placed into N. extraordinarius Biozone when a biostratigraphical proxy is available. The peak of excursion is in lower N. persculptus Biozone, but main increase in values takes place in N. extraordinarius Biozone. The Spinachitina taugourdeaui Biozone marks at many localities bottom of Hirnantian, and only recent chitinozoan-graptolite data from topmost Lousy Cove Member (on Anticosti Island) suggest a mid-Hirnantian age, which is at variance with common correlation schemes.