ORDOVICIAN ALLUVIAL-FAN TO MARINE SHELF TRANSITION IN SW SARDINIA, WESTERN MEDITERRANEAN-SEA - TECTONICALLY (SARDIC PHASE) INFLUENCED CLASTIC SEDIMENTATION

A localized Lower Ordovician, early Caledonian structural event ("Sardic phase") is recorded in SW Sardinia by an angular and erosional unconformity, faulting, and mild folding. This event was accompanied by sedimentation of a transgressive, clastic unit ("Puddinga Beds") which is particularly well developed along a major fault system (Gonnesa fault zone). There, the "Puddinga Beds" range from basal interfingering fans (some formed by coarse carbonate and schist-clasts others by schist-clasts only), to fluvio-lacustrine mud and gravelly sandstone, to coastal muddy and sandy deposits, and to fossiliferous restricted marine shelf sediments. The existence of the slightly compressive "Sardic phase" in SW Sardinia and a quasi-coeval calc-alkaline volcanism and an extensional structural phase ("Sarrabese phase") in central-east Sardinia, suggest transcurrent faulting. The calc-alkaline acid volcanism is also interpreted by some authors to indicate that the rifted basins developed in backarc position. The non-volcanic sequence of SW Sardinia analyzed in this paper was probably formed in basins developed on the northern margin of the Gondwanaland craton. This Sardinian system may be the eastward extension of transcurrent rifts which have also been recently reported from NW Spain. Structural activity has been the predominant control on sedimentation; however, worldwide, recurring, low sea-level stands may have influenced the sedimentation of the lower part of the "Puddinga Beds", and the deglaciation of the Saharan Ice Sheet has contributed to the uppermost Ordovician-early Silurian deepening of the euxinic shelf.