The volcaniclastic bodies in the Lower Miocene Bisciaro Formation (Umbria-Marche Apennines, central Italy)

Several sections of the Bisciaro Formation were investigated in order to determine the stratigraphic and areal distribution of peculiar volcaniclastic products within the lower Miocene strata of the Umbria-Marche Apennines. A distinctive concentration of thick-bedded volcaniclastic layers occurs in all the studied sections within the middle portion of the Bisciaro Formation. These volcaniclastic bodies (VB), 5 to 10 m thick, encompass the "Mega P' interval of Coccioni et al. (1988). They are composed of vertically stacked, amalgamated, ash layers reflecting distinct sedimentation episodes. Stratigraphic correlation of the VB across the study area is supported by integrated biostratigraphic data which indicate an early Burdigalian age for all the studied sections. A possible interpretation of the dispersal/emplacement mechanism suggests a remobilization process of volcanogenic material by more or less dilute density currents, possibly controlled by topography and synsedimentary tectonics