The “Autuniano Sardo”: basic features for a correlation through the Western Mediterranean and Paleoeurope.

In recent years, a team of Italian and foreign researchers have carried out detailed multidisciplinary studies on the main continental «Autunian» (Gzhelian-early Sakmarian, sensu BROUTIN et alii, 1999) basins of Sardinia. These investigations allowed us to evaluate better the general stratigraphy, but also the geodynamic activity and paleogeography of the island during this crucial late Palaeozoic interval. During the first (Upper Carboniferous to «Autunian») sequence, the well-known transtensional tectonics which deeply affected the dismantling of the Variscan orogen led to the opening of a number of intramontane basins, filled with lacustrine-to-alluvial sediments. In central-eastern Sardinia, the most complete volcano-sedimentary successions are those of Lake Mulargia, Escalaplano, Perdasdefogu and Seui-Seulo. Thinner but significant first sequence deposits also crop out in NW (Nurra, Lu Caparoni) and SW Sardinia (Sulcis, Guardia Pisano). In more detail, important and common features characterise the successions informally called here the «Autuniano Sardo», and can be summarized as follows: 1) grey to blackish lacustrine-to-alluvial siliciclastic deposition, intercalated with anthracite-bearing levels and freshwater limestones, indicating a variety of paleoenvironments, from perennial to ephemeral lake basins. These facies are consistent with a tropicallike, mostly humid climate; 2) almost all the sedimentary successions are intensively and repeatedly interspaced with volcanic and volcaniclastic calcalkaline products, both derived from basinal and extrabasinal eruption centres. Lower Permian volcanic complexes crop out in various part of Sardinia and they are probably located at the intersections of deeplyrooted fault systems; 3) most of these transtensional intramontane basins shows a clearly asymmetric inner geometry (half-graben), controlled by major normal faults and tilted blocks, and by transverse structures. However, transpressions are also locally evident (such as in the Seui Basin); 4) the paleontological record in these troughs shows various ecosystems which are always rich in meso- to xerophile macro- and microfloras of typical «Autunian» aspect, ostracods, algae, stromatolites, mollusca and, locally, vertebrate remains (amphibians and fishes). In particular, the recent discovery of an abundant macroflora, ostracods and fish remains in the Montarbu Basin (Barbagia di Seùlo) improves our knowledge of the Late Palaeozoic paleontological content of central Sardinia. The richness in fossils of these «Autunian» basins is of great help for paleogeographical reconstructions and correlations with the Western Mediterranean (SE Spain, the Catalonian Pyrenees, Provence, NW Africa), and also with the inner Variscides (French Massif Central basins, Thuringian Forest, etc.).