Ammonite evidence for long-term sea-level fluctuations between the 2nd and 3rd order in the lowest Cretaceous

Abstract

The Berriasian to Barremian pelagic succession along the Rio Argos (SE Spain) has been subdivided into sequences which are precisely dated by ammonite biozones. Comparative analyses of ranges of Mediterranean ammonite species and elaborate studies of ammonite diversity revealed seven important changes in ammonite composition between (or even within) certain biozones. These changes are much greater than between most other biozones and represent important faunal turnovers. Similar changes occur at the same levels in other parts of the Mediterranean Region; they are therefore of supraregional significance. Each faunal turnover coincides with a prominent minimum in ammonite diversity and is immediately preceded by a marked maximum in diversity. The turnovers virtually coincide with particular sequence boundaries and are ascribed to severe eustatic sea-level falls with a far higher amplitude than those at most other sequence boundaries. The high diversities preceding the turnovers are ascribed to extra high eustatic sea-level stands, which correspond not only to bundles of extensively transgressive sequences in the Mediterranean Region, but also to major incursions of Tethyan organisms into the boreal seas of northern Germany and Russia. A causal relationship is assumed. The repeated high amplitude sea-level falls were ascribed to a long-term sea-level variation cycle, not apparent from the eustatic curve published by Haq et al. (1988).