Evolution of a Pyrenean molassic basin in the Western Mediterranean area: The Eocene-Oligocene Cixerri Formation in Southern Sardinia (Italy)

The sedimentary features of the Cixerri Formation (Eocene-Oligocene) and its evolution in space and time are described through mapping, investigation of all the significant outcrops, and also by analysing the depositional architecture of well-exposed and laterally extensive outcrops. The Cixerri Formation marks the middle Eocene Pyrenean unconformity and blankets a former structural ridge-and-swale morphology. This started by covering the flanks of the ridges with small alluvial fans and ultimately filled the swales with fluvial deposits in braided to meandering stream environments. In this latter sedimentation phase, the depositional environment of the formation evolved both upwards, and from west to east, to gradually lower energy environments, suggesting a transgressive-type evolution. The Eocene-Oligocene Cixerri Formation was deposited by a fluvial network with a regular flow regime under a humid-warm climate: This network was formed by a main W-E-directed trunk and a NW-SE-directed minor branch that ultimately joined together. The Cixerri Formation, which was fed by the erosion of a sector of the Pyrenean Chain, passes upwards to coarser deposits that indicate the transition from a quiet tectonic environment to an active transtensional tectonic phase (Apennine phase). A comparison with similar coeval formations in Northern Sardinia, Corsica, and the Western Mediterranean area (Balearic Islands, France) is attempted, before the significance of the formation in the Pyrenean orogeny is then discussed.