Radioisotope dating of tephra layers challenges Messinian stratigraphy at Cabo De Gata (SE Spain)

Abstract

This study presents the first radioisotope ages of tephra deposits interbedded with the carbonate succession of the Cabo de Gata Range in SE Spain. These carbonate rocks are a reference section of the Miocene sedimentary record for the western Mediterranean, which is marked by the widespread development of coral reefs dominated by the genus Porites. Although Porites reefs at Cabo de Gata, and across the western Mediterranean, have been assigned to the late Tortonian–Messinian, two new 40Ar/39Ar ages (11.71 ± 0.11 Ma just below the base and 11.64 ± 0.18 Ma in the Porites interval) constrain their formation in the type section of La Molatilla–La Molata to the terminal Serravallian–earliest Tortonian. The radioisotope dating of an additional tephra layer interbedded with coastal oyster- and bryozoan-bearing calcirudites, which overlie the volcanic basement in the Los Escullos area and were previously assigned to the late Tortonian, yields an earliest Tortonian age of 11.29 ± 0.03 Ma. The Los Escullos tephra lies c. 4 m above sea level and about 100 m below terminal Serravallian tephra at La Molatilla–La Molata, suggesting a major sea-level drop of tens of meters during the earliest Tortonian. These findings highlight a more complex Miocene chronostratigraphy in the Cabo de Gata than previously recognized, and indicates that lithostratigraphic units with similar facies and comparable stratigraphic positions, such as skeletal calcarenites and calcirudites overlying the volcanic basement or Porites reefs, may vary significantly in age here and across the western Mediterranean.