Università degli Studi di Cagliari Facoltà di Scienze MM. FF. NN. Scuola di Dottorato in Ingegneria e Scienze per l’Ambiente e il Territorio Dottorato in Scienze della Terra Ciclo XXVII Settori scientifici disciplinari di afferenza GEO/01, GEO/02 Selected Famennian (Late Devonian) events (Condroz, Annulata, Hangenberg) in Sardinia and in the Carnic Alps: conodont biostratigraphy, magnetic susceptibility and geochemistry. Presentata da: Dott. Angelo Mossoni Coordinatore Dottorato Prof. Marcello Franceschelli Tutor/Relatore Prof. Carlo Corradini Esame finale anno accademico 2013 – 2014 2 SUMMARY Summary ...................................................................................................................................... 3 Abstract ....................................................................................................................................... 5 Introduction ................................................................................................................................. 6 Condroz Event ................................................................................................................................. 9 Annulata Event ............................................................................................................................. 10 Hangenberg Event ........................................................................................................................ 11 Methods .................................................................................................................................... 13 Studied sections ......................................................................................................................... 16 Carnic Alps ................................................................................................................................. 17 Geological setting .............................................................................................................................. 18 Pizzul West section ............................................................................................................................ 24 Pramosio Bassa section ..................................................................................................................... 32 Sardinia ...................................................................................................................................... 37 Geological setting .............................................................................................................................. 38 The Gerrei Tectonic Unit ............................................................................................................... 38 Bruncu Bullai section ......................................................................................................................... 44 Monte Taccu section ......................................................................................................................... 51 Corona Mizziu I and Corona Mizziu II sections .................................................................................. 57 Corona Mizziu I .............................................................................................................................. 59 Corona Mizziu II ............................................................................................................................. 62 Discussion and Conclusions ......................................................................................................... 64 Discussion .......................................................................................................................................... 64 Condroz Event – Comparation between PZW and CM II sections. ................................................... 68 Annulata Event – Comparation between PB and CM I sections. ...................................................... 71 Hangenberg Event – Comparation between MT and BTE sections .................................................. 73 Comments on the magnetic susceptibility ........................................................................................ 75 Conclusions ....................................................................................................................................... 76 Sistematic .................................................................................................................................. 78 Acknowledgments .....................................................................................................................145 References ................................................................................................................................146 Plates ........................................................................................................................................158 3 4 ABSTRACT 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. A 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. Six 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. A 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. The 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. As 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. The 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. 5 INTRODUCTION During the geological Eras, the Earth has gone through climatic changes that have affected the planet. These changes, caused by several factors, have interested big variations in the eustatic sea-level, and can be described as “Eustatic Events”. Few of these events have affected the living beings, sometimes severely, and can thus be considered as “Extinction Events”. A multidisciplinary approach is necessary to study the events that have been registered in the rocks. The study of the magnetic susceptibility is considered as an index of detrital input in sedimentary environments, also its potential as a tool for correlating sections from different basins was witnessed in several areas. This work has to be set within this context. MS analysis as a tool to study sedimentary rocks The low field magnetic susceptibility (MS) measure the intensity of magnetization of an object, induced 3 by the presence of an external magnetic field. The MS of a rock sample (χLF=m /kg) is measured at very low fields, not exceeding 500 mT, and measures the induced magnetization of diamagnetic, paramagnetic, antiferromagnetic and ferromagnetic minerals. The use of magnetic susceptibility as a tool in different research fields has been extensively shown in the last 30 years, from enhanced magnetism of the soil for archaeological studies (Becker & Fassbinder 1999), environmental pollution (Flanders 1994), to different geological and stratigraphic fields. The use of magnetic susceptibility as a palaeoclimatic proxy for Holocene, Pleistocene, and Tertiary sediments has been proved valuable since the