The “Bonarelli Horizon” in the central Apennines (Italy): radiolarian biostratigraphy

Abstract The “Bonarelli Horizon”, of Cenomanian to Turonian age, is a thin (approximately one metre) layer within the Scaglia Bianca Formation of the Umbria sequence of central Italy. It consists of black mudstones rich in organic matter, silty shales and beds of radiolarian siltones and fine sandstones. The abundance of organic matter reflects the anoxic conditions prevailing during its deposition. Several sections (Valle del Bottaccione, Valle della Contessa, Monte Petrano, Valle del Burano and Gola del Furlo) show that the Bonarelli Horizon includes a “lower” radiolarian assemblage characterized by the presence of Novixitus mclaughlini, Thanarla pulchra, Holocryptocanium astiensis, Archaeodictyomitra sliteri, Pseudodictyomitra carpatica , and an “upper” assemblage where Alievium superbum is present in association with Crucella cachensis and Pseudoaulophacus putahensis . The transition between the “lower” and “upper” assemblages represents a faunal event that, on the basis of current knowledge of radiolarian biostratigraphy, suggests a correlation with the Cenomanian-Turonian boundary.