Selected Famennian (Late Devonian) events (Condroz, Annulata, Hangenberg) in Sardinia and in the Carnic Alps: conodont biostratigraphy, magnetic susceptibility and geochemistry.

摘要

The Condroz, the Annulata and the Hangenberg are three events occurred in the Famennian. Causes and consequences are different for each one. The Condroz event is related to a sharp regression associated with an extinction of few ammonoid groups. The Annulata event to a transgression associated with a global spread of the ammonoid Platyclymeniae annulata. The Hangenberg event is a major event, that took place in a complex phase of eustatic variations that affected severely the fauna, and it is considered as one of the biggest extinction in the history of the earth.udA multidisciplinary approach is here provided to investigate the selected events. A biostratigraphic study based on conodonts was carried out to individuate the precise biostratigraphic position when the events occurred. The evolution of the low field magnetic susceptibility and the major elements geochemistry (Al, Si, K and Ti) have been investigated to study the evolution of the detrital input across the events.udSix different section were studied in two different areas of North Gondwana where the Famennian rocks are well exposed. Two sections bearing the Condroz and the Annulata events were studied in the Carnic Alps, four sections containing the three events were studied in the South East of Sardinia.udA detailed biostratigraphic study is here provided from both areas. More than 28 000 conodonts have been collected and discriminated into 101 taxa. Each taxon is described and illustrated. Ten conodont Zones, from the Upper rhenana Zone (Frasnian) to the Lower expansa Zone (Famennian) have been recognized in the Carnic Alps. Nineteen conodont Zones, from the Lower crepida Zone (Famennian) to the Upper duplicata Zone (Tournaisian) have been discriminated in the Sardinian sections.udThe three events have been recognized in both areas. As for the Condroz and for the Annulata events, even if there is no change in lithology, the curves obtained by the MS and the major elements tends to show distinctive peaks time-equivalent to the events. In addition, a stratum rich in ammonoids marks the Annulata event in the field.udAs for the Hangenberg event, marked in the field by a thin level of black shales, while the curves of the magnetic susceptibility and the major elements show a constant trend immediately before the main event (Hangenberg equivalent shales), in the earliest Carboniferous the magnetic curve shows distinctive fluctuations. These fluctuations are here explained by a phase of oceanic instability after the event. Despite there are no changes in the MS and in the major element data, the fauna is impoverished and seems to be affected by an earlier phase of the main event already in the Lower praesulcata Zone.udThe major detrital input in the Carnic Alps sections may indicate a closer distance to the source for the latter respect to the Sardinia-Corsica microplate, therefore a new palaeogeographic interpretation for the two areas investigated in the Upper Devonian is here hypothesize.