LATE DEVONIAN-EARLY CARBONIFEROUS CONODONT BIOSTRATIGRAPHY OF THE “CLYMENIAE LIMESTONES” OF SARDINIA, ITALY

The calcareous sediments of Late Devonian – Early Carboniferous age of south-eastern Sardinia are mainly represented by grey massive limestones known as “Calcari a Clymeniae” (=“Clymeniae Limestone”) because of the occurrence of ammonoids in some levels. The unit is not yet formally defined. Its apparent thickness reaches hundreds of meters, although it is highly probable that repetitions occur, owing to strong tectonics. Ammonoids, apart from crinoid stems, are the only abundant macrofossils; they seem to be concentrated at specific levels.Very rare brachiopods and fish teeth have been reported from the heavy fraction obtained in acid-leaching. The microfacies is always represented by “poorly fossiliferous micrite”, with the sparse fossil remains occurring only in ammonoids-bearing beds, where ostracods, small shells (bivalves?, brachiopods?), fragments of echinoderms and gastropods, as well as rare trilobite remains are present. A pelagic deposition environment is suggested for these limestones, on the basis of microfacies and absence of benthonic fauna; conodont biofacies also indicate an off-shore environment. One hundred-twenty conodont taxa (species, subspecies and morphotypes) belonging to thirteen genera (Alternognathus, Ancyrognathus, Bispathodus, Branmehla, Icriodus, Mehlina, Palmatolepis, Pelekysgnathus, Protognathodus, Polygnathus, Pseudopolygnathus, Scaphignathus and Siphonodella) have been descriminated. Sixteen famennian conodont zones in continuous sequence from Lower crepida to Lower praesulcata have been recognised, as well as two Tournaisian (Lower and Upper duplicata). Up to date, no evidence of the Upper praesulcata and of the sulcata zones have been found. Furthermore, in the postera interval it is impossible to discriminate between the Lower and the Upper postera Zone, because the marker of the latter is missing, as well as any other stratigraphically important taxon.