CONODONT BIOSTRATIGRAPHY OF THE OCKERKALK OF SOUTH-EAST SARDINIA, ITALY (LATE SILURIAN)

The "Ockerkalk" (25 m thick) is an argillaceous flaser limestone, exposed in SE Sardinia, having a blue-grey colour weathered to ochre, from which its name is derived. The unit represents a calcareous “intermezzo” between two shaley units, the “Lower Graptolitic Shales” and the “Upper Graptolitic Shales” respectively. Loboliths and crinoid stems are the only macrofossils clearly visible in outcrop, as well as a few cephalopods. Trace fossils and very small solitary corals were also reported. Microbiofacies analysis reveals fine micritic limestones with a fossil content (ostracodes, brachiopods, thin-shelled bivalves, trilobite and echinoderm fragments, gastropods, sponge spiculae) scattered in the matrix and only locally concentrated in millimetric shell-lags of disarticulated debris. A rich conodont fauna has been reported from several sections and outcrops. Microbrachiopods and phyllocarid gnathal lobes were occasionally recovered in the conodont heavy fractions. The conodont fauna includes twenty-six multielement species, belonging to twelve genera, documenting eight conodont zones (Oz. exc. hamata, A. ploeckensis, Pol. siluricus, Pe. latialata, Oz. snajdri, Oz. crispa, Oz. remscheidensis and Oul. el. detortus) of early Ludlow-top Pridoli age. Sections in this Unit were the basis of the Ludlow-Pridoli part of the Silurian Conodont Zonation proposed by Corradini and Serpagli (1999, 2000).