Resumen de The Middle Triassic of Sardinia (Muschelkalk) in the frame of the Western Tethys evolution
The sedimentary record of the Middle Triassic successions of Sardinia Island (Italy) has been long recognized as a key element in understanding the palaeogeographical evolution of the Tethys W sector. It includes the possible regional and interregional correlations of this sedimentary record with the other European sectors (Germanic basin, Spain, France, and the Southern Alps), and in understanding the evolution of the W Tethys Sea throughout this period. The aim of this PhD project is to integrate the present knowledge of the Sardinian continental-transitional Lower-Middle (Buntsandstein) and marine Middle (Muschelkalk) Triassic successions and to better define their stratigraphic, chronostratigraphic, paleoenvironmental and paleogeographical context. This has been achieved by providing new stratigraphic, sedimentological, paleontological, sequence stratigraphy and geochemical data. Following a stratigraphic survey fieldwork, 12 sections have been studied and sampled in detail as representative of the different fms. The facies associations and analysis made possible to distinguish 4 main broad sedimentary environments: 1) sabkha - muddy coastal plain, 2) carbonate tidal flat, 3) shallow low-energy subtidal, 4) calcarenitic shoals and 5) middle ramp. The palynological associations data have been obtained from re-evaluating previous works and the new associations obtained from 3 stratigraphical sections: Su Passu Malu section (Campumari, SW Sardinia), Arcu is Fronestas section and Escalaplano section (Escalaplano, Central Sardinia). The integration of these data with the stratigraphic and sedimentological data made possible to establish a) the lack of paleontological evidence for the early Anisian in Muschelkalk facies in Sardinia (Italy) b) a Pelsonian age for the Escalaplano Formation, an Illyrian age for the Su Passu Malu Member (Campumari Formation). The study of the ammonites, bivalves and foraminifera with the sedimentological data allowed to precise the progressive W settling of carbonate platforms, and the development of new migration and irradiation routes for Sephardic faunas from the SE (Palaeotethys) and Alpine faunas (Neotethys) from the NE. Based on new findings and the revision of palaeontological associations previously reported, comparisons were made with associations of adjacent paleogeographic domains (Balearic Islands, Levantine area of Spain), refining their palaeoecological distribution. The sequence sedimentology and stratigraphy studies reflect the broad carbonate ramp developed during a period of approximately 5-6 million years (late Illyrian to latest Longobardian) and records a major transgressive - regressive cycle, which can be assimilable to a 2nd order cycle in the sense of Vail et al (1991). The sequence is constituted by the upper part of the Buntsanstein (Röt facies), the different formations that represent the Muschelkalk and the lower part of the Keuper. Interregionally, this 2nd order depositional sequence of the Sardinian Muschelkalk finds its equivalence in different basins of Spain (Mega-Depositional Sequence-II of Escudero-Mozo, 2015 and equivalents). The paleogeographic analysis, allow to define a) the different stages of the transgression during the late Anisian-early Ladinian; b) that Sardinia, together with Minorca and the E area of Castellón (E Iberia), constituted a topographical high until the middle-late Illyrian (late Anisian), when an important change in the configuration of the W Tethys realm occured. Up until that age, these areas played a key role by acting as a barrier between the Paleotethys and Neotethys seas; c) from the late Illyrian to the early Fassanian, the new transgression of the Tethys Sea progressively covered Sardinia and later the neighbouring E Iberia and Minorca areas. During this stage of the Tethys transgression, almost all the elevated areas of the Western Tethys domain were finally covered.
