The Early Aptian of Aralar (northern Spain): stratigraphy, sedimentology, ammonite biozonation, and OAE1

Abstract

The stratigraphy, sedimentology, ammonite faunas, and geochemistry of the early Aptian of the Aralar Mountains in northern Spain are described. This area contains a 983-m-thick succession of sediments of early Aptian age. The lithological succession evolves from lutites, marls, and calcarenites of the Errenaga Formation to rudist micritic limestones of the Sarastarri Formation, and finally marls, lutites, and sandstones of the Lareo Formation. Late Aptian calcarenites and marly limestones rest on top of the Early Aptian succession. The sedimentology indicates general deposition in a shallow marine environment, corresponding to mixed siliciclastic-carbonate ramps and a shallow-water carbonate platform. Ammonite occurrences allow recognition of the Deshayesites oglanlensis, D. weissi, D. deshayesi, and Dufrenoyia furcata Zones of the Mediterranean Tethyan belt scheme. A transition between the deshayesi and furcata Zones with the co-occurrence of the ammonite genera Deshayesites and Dufrenoyia is described for the first time in the Aralar succession and is currently unique. Black shales have been recognized at the top of the Barremian succession, in the D. deshayesi-D. furcata transition Zones, and in the D. furcata Zone. They correspond to three periods within the OAE1 interval. Geochemical TOC analyses indicate variations referable to cyclical climatic changes caused by orbital forcing. d13C and d18O geochemical signatures in Aralar serve for identifying and dating with greater precision to some of the previously recognised worldwide early Aptian major events: two negative and one positive carbon isotope curve shifts and two warming phases.