Strontium isotope dating of bivalve faunas from the Upper Miocene Cacela Formation, eastern Algarve, Portugal: evidence from Messinian bivalve fauna

Abstract

The study is focused on the determination of the geological age of bivalve fauna recovered from four fossiliferous sequences in the Cacela Formation of Algarve, southern Portugal, by means of the strontium isotope stratigraphy. Forty two analyses of bivalve samples from Barroquinha, Cabanas, the Cacela River and Lacem were performed. The estimated age based on the 87Sr/86Sr ratios in bivalve shell samples from the Cacela River (a reference section for the Cacela Formation) is 7.6 ± 0.4 Ma and is consistent with a late Tortonian age suggested by micropalaeontological data (calcareous nannofossils and planktonic foraminifers). Moreover, strontium isotopic data record that the oldest age of the bivalves of the Cacela Formation (Lacem site) is at least 8.9 ± 0.4 Ma whilst the age of about 7.0 ± 0.5 Ma can be inferred for bivalves from Barroquinha. An intriguing new datum comes, however, from the bivalve strontium isotopic data of the Cabanas section which records the estimated age of about 6.7 ± 0.4 Ma and, therefore, an early Messinian age. This is the first report of the early Messinian marine bivalve fauna in Portugal. Sixty one bivalve species have been recovered in the Messinian of Cabanas. Four of them: Pseudopythina macandrewi (P. Fischer), Coripia corbis (Philippi), Glossus (Glossus) humanus (Linnaeus) and Cyathodonta dollfusi (Cossmann et Peyrot) have previously not been reported from the Miocene of southern Portugal.