Journal of Mediterranean Earth Sciences Special Issue (2013), 41-44 The Cixerri Fm (Middle Eocene-Early Oligocene): analysis of a “Pyrenean” continental molassic system in southern Sardinia Luca G. Costamagna 1 , Andreas Schäfer 2 1 Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Cagliari, Italy 2 Steinmann Institute - Geology, Nussallee 8, Bonn, Germany INTRODUCTION The Cixerri Fm (Pecorini and Pomesano Cherchi, 1969) is a Middle Eocene to Early Oligocene terrigenous formation of siltites, subordinated sandstones and minor conglomerates with very rare intercalations of limestones and lignitiferous clays. Its thickness may reach 350-400 m, and was deposed in continental to possibly transitional environments (Barca & Costamagna, 2010). It represents the molassic post-Pyrenean phase sedimentation in Sardinia (Cherchi, 1979; Barca and Costamagna, 2000; 2010). Cixerri Fm outcrops are scattered in Southern Sardinia, missing only in its eastern part. Here the unit may rest conformably to unconformably over the Lower Eocene Lignitifero Fm or it may be posed unconformably over the Variscan metamorphic basement. Upwards it may pass sharply through an unconformity to the calc- alkaline volcanics of the Oligocene-Miocene cycle or to the terrigenous of the Ussana Fm (Pecorini and Pomesano Cherchi, 1969). In order to acquire a complete model of the sedimentary meaning of the Cixerri Fm in the frame of the Pyrenean orogeny and of the Western Mediterranean area, stratigraphical, sedimentological, architectural, paleohydrological and petrographical investigations have been carried out. Here the main results about paleohydrology, paleogeography, and depositional environment are briefly summarized. PALEOCIXERRI RIVER NETWORK PALEOHYDROLOGY Based on composition, textural features, channel fill architectures and channel parameter values (width, depth, orientation, storeys number, directional structures, etc.) a paleohydrological analysis of the Paleocixerri river network has been attempted. The Paleocixerri river had probably two main branches, the main one oriented E-W and flowing towards the East, with its catchment basin located far off the coast of the Iglesiente area, and a possibly minor one oriented roughly NW-SE, whose catchment basin was located far in central Sardinia and perhaps beyond, flowing towards the Southeast (Fig. 1). The main E-W reach flowed most likely across almost all the present Sulcis area that represented an alluvial plain at least 40 km wide from N to S. The E-W and NW-SE branches were likely separated by a pre-existing Iglesiente high: in fact, differently from the Sulcis area, the Iglesiente Variscan highland area is devoid of Cixerri Fm outcrops, suggesting that never the highlands were covered by sediments, but they only bordered it with limited thicknesses. Those branches joined perhaps in the Assemini area. The mean grain-size of the Cixerri Fm deposits decrease towards the East and the South, together with a vertical and lateral change of the fluvial style and the channel pattern, suggesting a trend towards lower energy settings. In the unit the best fluvial paleoenvironmental differentiation may be evidenced by comparing the lateral evolution of the lower part of the Cixerri succession from W to E. Here the grain-size range of the events filling the channels gave a qualitative valuation of the discharge fluctuation in the single flood event: these valuations show their maximum importance in the western outcrops (Iglesias-Domusnovas-Carbonia area), then decreasing gradually eastward. The western channels were large and shallow with limited lateral shift, while the eastern ones were more narrow and meanly deeper with a high rate of lateral migration. The competence of the river flow was higher in the West as well. The architectural analysis of the channels shows they were mainly multistorey-type, so suggesting a permanent fluvial network. This kind of fluvial network, together with the rare fossils (plants and vertebrates), suggests a warm-temperate to (sub?)tropical climate with well-distributed rainy events in the wet season. The local presence of calcrete horizon concentrations at several level of the succession may indicate times where the climate was more arid. It may be assumed the main Paleocixerri was a river with a channel width of 10 to 15 metres at the most, with a maximum depth of 2-3 m in the main reach. Crevasse splays are diffuse, indicating frequent flood events. Their recurring presence below the main channels suggests a progressive migration of the main channel belt. DEPOSITIONAL ENVIRONMENTS The Cixerri Fm is a continental unit deposed in the middle to lower, distal part of a foreland alluvial plain fed JME S Journal of Mediterranean Earth Sciences