Geochimica e petrografica di black shales del Livello Bonarelli, sezione di Novara di Sicilia (Monte Peloritani), Sicilicia nord-orientale: Implicazioni paleoambientali

Organic-rich black shales rhythmically alternating with mar stones and mudstones crop out - in association with the Unit of Varicolourec Clays (Cretaceous-Palcogene) - in an area near the town of Novara di Sicilia (Pelorilani Mountains, northeastern Sicily). These deposits, belonging to the Upper Cenomanian, are identified as "Livello Bonarelli" representing the sedirr entary expression of the anoxic event OAE2. The alternances (about 11 m thick) pass downwards to more irregular alternances (about 9 m thick) of marly limestones, light marlstones, dark marlstones, mudstones, and calcarenites with only one black shale horizon intercalated. Geochemical and petrographic features of the black shales (a well preserved mm-lamination, common disseminated pyrite in very small crystals, high TOC percentages, negative values of Mn* and very high values of V/(V+Ni), together with abundant radiolarians and high values of the Si/Al ratio) identify a depositional environment characterized by very poor oxygen levels associated to high productivity of near-surface waters. Geochemical tracers of paleoredox conditions suggest an evolution from dysoxic-anoxic conditions, during the deposition of marlstones and mudstones from the lower part of the section, towards anoxic-euxinic conditions in coincidence with the enhanced frequency of black shale layers. A comparison with some deposits of the Livello Bonarelli cropping out in other areas in Italy discriminates the black shales studied for their high smectite proportions, low values of K/Al ratio and limited Ba values.