Early Permian silicified floras from the Perdasdefogu Basin (SE Sardinia): comparison and bio-chronostratigraphic correlation with the floras of the Autun Basin (Massif central, France)

摘要

In the Perdasdefogu Basin (Ogliastra, Sardinia) plants occurring in angular cherts are preserved as siliceous permineralizations. The plant assemblage consists of the genera Sphenophyllum, Arthropitys, Astromyelon, Palaeostachya, Pecopteris, Scolecopteris, Stewartiopteris, Psaronius, Ankyropteris, Anachoropteris; furthermore, probable medullosan wood and ovules, cordaitean stems, leaves and ovules, and Dadoxylon wood have been recorded. Arborescent marratialean ferns are the dominant elements with Pecopteris and Scolecopteris leaves being the most common; the calamiteans are the second in abundance. Generally the chert blocks contain a large number of plant fragments - roots and leaves are being almost equally represented - suggestive of a coal ball-like plant accumulation. More rarely a chert block may consist of a single plant part, for example a piece of wood. The preservation and accumulation of the plants are closely comparable to the silicified plant assemblages described from the Lower Permian of Autun and the upper Pennsylvanian of Grand-Croix (Massif central, France) and from the Early Permian D??hlen Basin (Germany). It is certainly significant that the assemblage from the Perdasdefogu Basin is dominated by tree ferns and calamites like the coeval silicified vegetation from Autun. However, like in Autun, the permineralized flora contrasts with the underlying compression floras dominated by conifers and peltasperms. The Perdasdefogu macroflora record suggest a middle-upper Autunian age which corresponds to the Surmoulin and Millery formations of the Autun Basin. This is in agreement with the Asselian-Sakmarian transition as pointed out by the amphibian species found in the same formation, and perfectly correlatable with the same association found in Gottlob-lake (Thuringian Forest Basin). Because of its rich fossil content, the Perdasdefogu Basin represents a reference succession for the Autunian of the entire westernmost palaeo-Tethyan domain and its macrofloral record.