SILURIAN AND DEVONIAN BIOSTRATIGRAPHY AND PALAEONTOLOGY OF SARDINIA (ITALY) AND PALAEOGEOGRAPHIC IMPLICATIONS

Silurian and Devonian strata are exposed, discontinuously, only in Southern Sardinia. A detailed biostratigraphy has been achieved on the basis of conodonts and graptolites. Two distinct and peculiar situations in Silurian and Lower Devonian rocks occur in the southwestern and in the southeastern part of the island: they mainly resemble the coeval sequences exposed in Bohemia and in Thuringia respectively. Their mutual relation is still unclear, but they were likely deposited in different areas of the North Gondwana margin, several dozens of km from each other, and joined together by the Variscan Orogeny. In SW Sardinia, Silurian and Lower Devonian are mainly represented by black shales and Orthoceras limestones. Strong faunal affinities with Morocco, the Carnic Alps and Bohemia have been demonstrated by several authors, mainly on the basis of nautiloid cephalopods and bivalves. In SE Sardinia, corresponding sediments are represented by the classical Thuringian Triad: “Lower Graptolitic Shales”, “Ockerkalk”, “Upper Graptolitic Shales”. Apart from graptolites in shaly units,and conodonts in the Ockerkalk limestone, the faunal content is very poor. The loboliths of the pelagic crinoid Scyphocrinites, well known along the North Gondwana margin, occur across the S/D boundary both in SE and SW Sardinia, even if at slightly different stratigraphic levels. Rocks of upper Lower to Upper Devonian age crop out in SE Sardinia only. Emsian and Middle Devonian strata consist mainly of shales and marly limestones with tentaculites; the calcareous content seems to increase throughout the Middle Devonian up to the massive Clymeniae Limestones of Late Devonian-earliest Carboniferous age. A detailed biostratigraphy of the latter unit has been developed on the basis of conodonts.