Abstract

The chronostratigraphy of the lower Miocene deposits of northwestern Sardinia is refined in the Castelsardo Basin. The combination of new ⁴⁰Ar/³⁹Ar isotopic datings, 19 micropalaeontological datings (calcareous nannofossils, planktonic and large benthic foraminifers) and 16 palaeomagnetic polarity measurements led to a new chronostratigraphic framework. The continental deposits of the first megasequence (Valledoria and Casteldoria members) are possibly Aquitanian. The overlying marine sediments are early Burdigalian (Vaginella depressa Molasse). The marine transgression can be dated to around 19.7 Ma. Marine conditions ceased 18.83±0.13 Ma, the age of the index aerial τ2 ignimbrite. The continental part of the second megasequence (“Lacustre” unit) is early-middle Burdigalian, between 18.8 and 18 Ma. The overlying marine part (Campulandu and Sedini members) is middle-late Burdigalian. The second marine transgression began in the earliest-late Burdigalian, during the Chron C5Dr reversal, at the time of the eustatic Bur 4 event.
The Castelsardo basin went through two main extensional tectonic episodes. The first one occurred as the Valledoria Member was deposited and possibly dates back to the Aquitanian. The second one occurred as the uppermost part of the Vaginella depressa Molasse was deposited during the early Burdigalian. The second extensional episode is related to a rapid rotation of Sardinia. Palaeomagnetic results confirm that Corsica and Sardinia acted as a single block which suffered a 28±17^° ccw rotation after 19.2 Ma. The Castelsardo basin is held to be an Aquitanian–early Burdigalian intra-arc rift basin that later evolved in a thermally controlled subsident margin of the Ligurian–Provençal Basin during the middle-late Burdigalian.

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