Sau, Antonio and Lecca, Luciano and Secchi, Francesco and Fercia, Maria Luisa (2005) La seconda fase del Rift Sardo: vulcanismo ed evoluzione dei sub-bacini di Ardara-Chilivani e Bonorva (Sardegna settentrionale). Bollettino della Società geologica italiana, Vol. 124 (1), p. 3-20. ISSN 0037-8763. Article.
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The Oligo-Miocene Sardinian Rift (27-15 Ma; CHERCHI & MONTADERT, 1982; LECCA et alii, 1997), is a typical intra arc-massif basin (sensu DICKINSON, 1974) made up of several sub-basins filled by thick erogenic volcanic sequences and marine sediments. This arc is linked to the subduction of the Neotethys oceanic plate under the European plate and to the Africa-Europe convergence. The displacement of the arc or, in other words, the drift of the Sardinia-Corsica Block, is related to the orogenesis of the northern Apennine and of the Maghrebid-Sicilian chain. Despite the key-role played by the Sardinia Rift in the knowledge of the geodynamic evolution of the western Mediterranean basin, the stratigraphy of volcanic sequences is still incomplete because large portions of this rift still require detailed mapping. At the regional scale, the Sardinia Rift shows marked differences with regard to volcanic and tectonic style. In northern Sardinia, the rift is arranged into several half-graben type sub-basins, related to tilted block and horst block systems. The basins are schematically attributable to the relative mobility of the main blocks with minor relative strike-slip to transtensional movements along NE-SW trending faults among the sub-blocks of eastern Sardinia. The Chilivani-Ardara and Bonorva sub-basins contain a poorly known thick pyroclastic sequence buried by epicontinental marine sediments. On the basis of field, petrographic and volcanological criteria, the volcanic pile is constituted, from bottom to the top, by early volcanics of broadly andesitic composition (M. Cuguttada Unit: MC), densely welded and rheomorphic ignimbrites (WI) followed by ash and pumice pyroclastic flow deposits named throughout this paper Pianu Ladu (PL), Pianu de Puma (IC) and Chilivani Unit (CH). Epiclastic deposits (EVL) are observed locally at different stratigraphic levels (the so-called "lacustre", VARDABASSO & ATZENI, 1962; PECORINI, 1962). Marine deposits onlap the volcano-sedimentary sequence in the upper Burdigalian-Serravallian interval. Published K-Ar (LECCA et alii, 1997) and Ar-Ar (EDEL et alii, 2001) radiometric data obtained for the pyroclastic flow deposits indicate a quite narrow time span in the range of 20-18 Ma. With regard to the regional stratigraphic picture proposed by LECCA et alii (1997), MC andesites belong to the Al andesites; WI are the densely welded reomorphic ignimbrites, whereas the PL, IC and CH ash and pumice flow-Units constitute the AP2 group. In detail, pyroclastic units belonging to the latter group show the following: Pianu Ladu Unit (PL). The PL unit is a light grey-coloured ash and pumice pyroclastic flow deposit, outcropping with continuity at the northern edge of the Chilivani-Ardara sub-basin. It lies normally on WI ignimbrites or locally on reddish matrix-supported conglomerates made up of clasts of Palaezoic basement. Content of pumice and xenoliths (accidental and cognate), ranges from 20 to 10% moving from S. A. of Bisarcio towards M. Ladu, while size and degree of welding decrease. Pianu de Puma Unit (IC). The IC Unit constitutes discontinuous outcrops in the Bonorva basin as well as in the southern part of Chilivani-Ardara. It lies with nonconformity on the Palaeozoic basement and/or on the WI. Locally (Pianu de Puma), it is preceded by greenish to yellow coloured epiclastic deposits at least 5 m thick, containing rare clasts of the Palaeozoic basement dispersed in an ashy matrix.
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