A detailed stratigraphic and facies analysis has been carried out on the Guardia Pisano sedimentary succession (Barca et alii, 1992; Pittau et alii, 2002), the most complete Permian section cropping out in SW Sardinia. This succession has been subdivided into three lithostratigraphic units. The lower unit (approximately 14.46 m) is formed by laminated, grey to blackish pelites and rare fine sandstones containing both acidic volcanic intercalations (lavas and pyroclastics) and epiclastites deriving from the erosion of volcanic products. Here, siltitic-clayey horizons containing a rich pollen assemblage suggesting an Asselian age (Lower Permian) has been found (Pittau et alii, 2002). The depositional environment of this unit was a palustrine to alluvial plain (meandering stream) under probably humid climatic conditions, characterized by a certain degree of tectono-magmatic activity. The intermediate unit (approximately 59 m) is built up by reddish siltites and clayey siltites containing subordinate intercalations of micaceous sandstones and rare conglomerates: the high content of micas and K-feldspars of these sandstones is probably due to the erosion of former volcanites. This unit was laid down into a meandering stream environment under conditions of regular rainfall. The upper unit (approximately 45.8 m) is constituted by rapid alternations of reddish pelites and sandstones with sheet-like and lens-shaped conglomeratic-sandstone intercalations, deposited into a meandering environment under more pronounced subarid conditions with irregular, catastrophic rainfall episodes: several crevasse splays have also been recognized here. These last two units are referable to the "New Red Sandstone" informal unit, showing a red beds facies, and having a sub-continental aspect in the frame of the Permian of Europe. The sedimentological features evidenced suggest a gradual evolution from (warm?) humid to subarid climates during the time of deposition of the succession. The volcanism, aged 297+ or -5 Ma (Pittau et alii, 2002), pertinent to the lower unit, post-orogenic in type, has been linked to the first opening stages of this Permian basin. Thanks to its extremely good exposure, the succession described, especially the part pertaining to the "New Red Sandstone", has been analysed and subdivided into channelized and overbank elements, using the Miall's (1985, 1996) "Depositional Architecture" criteria, so obtaining a clearer understanding of the palaeoenvironments and their evolution.

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