Radiolarian correlation of Jurassic siliceous successions of the Rosso Ammonitico Formation in the Southern Alps and Western Sicily (Italy)

Abstract

This paper deals with the radiolarian biostratigraphy of Middle-Upper Jurassic pelagic siliceous successions of the Southern Alps and Western Sicily (Italy). The crucial complement to this research is the occurrence of ammonites in the studied successions (Rosso Ammonitico Medio: the intermediate siliceous member of the Rosso Ammonitico Formation), as well as in the under- and overlying sediments (Rosso Ammonitico Inferiore and Rosso Ammonitico Superiore, respectively). The abundance of radiolarians in all successions allow to analyse them for a twofold purpose: to date directly most of the successions, and to improve the calibration of radiolarian biozones thanks to the occurrence of ammonites. The biostratigraphic analysis has been carried out using the Unitary Associations method, and six new radiolarian biozones have been defined: the combined occurrence of radiolarians and ammonites provided a new Bathonian to late Kimmeridgian radiolarian zonation for the Southern Alps and Western Sicily (Italy). The new Unitary Association Zones show a good reproducibility throughout the investigated successions, and make possible a first direct dating and correlation by radiolarians of the Rosso Ammonitico Medio. Furthermore, the radiolarian biozones reveal a significant diachronism for both the lower and the upper limit of the Jurassic pelagic siliceous facies in the Alpine and Sicilian sections. In the light of new radiolarian biozones, the age assignments of the Ceniga (Southern Alps) and the Sant'Anna (Sicily) sections, and the ranges of some taxa are discussed.