Stratigrafia, analisi di facies, paleogeografia ed inquadramento regionale del Giurassico dell'area dei Tacchi (Sardegna centro-orientale)

New investigations carried on the Jurassic successions of the Tacchi area (Eastern Sardinia) have given a more detailed knowledge of the local stratigraphy and sedimentology. Geological mapping and litho-sedimentological analysis heve provided a detailed framework for the vertical and areal development and evolution of the sedimentary facies, and also allowed an accurate paleogeographical construction. The Genna Selole Fm. (Bajocian-Bathonian) (DIENI et alii, 1983), constituting the base of the Tacchi Jurassic succession, is built of siliciclastic to mixed, siliciclastic-carbonate, deposits. It was laid down in depositional environments ranging from distal alluvial fan to transitional lagoonal-littoral. In places the Genna Selole is absent and so the marine Dorgali Dolostones Fm. (Bathonian-Kimmeridgian) (DIENI et alii, 1983), directly overlies the folded Paleozoic basement. The omission of the Genna Selole Fm. suggests the presence of uplifted areas of erosion and alteration whose location has been controlled by structural, syn-depositional factors, as a possible Variscan paleo-high. Based on new evidence, the Genna Selole Fm. has been subdivided into three lithofacies: A) a conglomeratic lithofacies (Laconi-Gadoni Lithofacies); B) a sandyclayey lithofacies (Nurri-Escalaplano Lithofacies); and a mixed siliciclastic- carbonate lithofacies (Ussassai-Perdasdefogu Lithofacies), this latter constituting the gradual passage to the following Dorgali Dolostones Fm. In addition, features observed in the Genna Selole Fm. (slumps, synsedimentary faults, neptunian dykes, seismic breccias) suggest active coeval extensional tectonics. The thickness of the Genna Selole Fm. is between 0 and 50 m. We propose two others additional parastratotypes for this formation in the Escalaplano and Perdasdefogu areas, characterized by good continuity and exposure, and so integrating the stratotype described by DIENI et alii (1983). The comprehensive analysis of the Genna Selole Fm. isopachs and of its lithofacies in the Tacchi area evidences the presence, and the subsequent gradual burial and flooding, of a morpho-structural high («alto morfostrutturale barbaricino») developed in Central Sardinia, due to the coeval extensional tectonics prevailing during the Lower and Middle Jurassic. The upper Dorgali Dolostones Fm., up to 250- 300 m thick, is characterized by different lithofacies all referable to inter- to subtidal environments of a carbonate shelf (stromatolitic mounds, lagoonal sediments, storm layers, flaser bedding, herringbone cross stratification, oolitic shoals and bars, possible internal breccias of tectonic origin), extending between the carbonate lagoon and the platform break. The most widespread deposits are irregular alternations of calcarenitic storm layers and bioturbated mudstones. In the easternmost areas (Tacchi of Seui, Ulassai and Tertenia- Jerzu), 300 m of the Dorgali Dolostones Fm. are overlain abruptly by little, previously unknown, calcareous outcrops, having a maximum thickness of 40 m, referable to the M. Tului and M. Bardia Fms (AMADESI et alii, 1960). These are formed by oolitic calcarenites, oolitic-bioclastic calcirudites-calcarenites, coralline framestones, subordinated calcilutites and possible accumulations of fore-reef breccias. The presence of Ellipsactinia suggests an Upper Jurassic age. The depositional environments of the Monte Bardia and Monte Tului Fms. could be referred to platform margin areas where oolitic bars and limited bioconstructions (patch-reef) developed. Based on the described data, and taking in account the low protecting bioconstruction belt, the whole sedimentation environment can be referred to a shelf with ramp configuration. The overall depositional framework of the Jurassic succession can be related to a progressively deepening transgressive sequence, maybe correlatable with the extensional tectonics causing the development of the Tethys sea. Finally, an attempt has been made to hypotesize a regional framework of the Tacchi area both within the Sardinian Mesozoic successions, and in the Tethyan context