Oligocene ash-layers in an Epiligurian unit of Northern Apennines near Bologna (Italy): how far was the volcano?

A volcanoclastic ash-layer, 1 m thick, has been found interbedded in the Antognola Formation (Epiligurian successions, Oligo-Miocene) in the Sillaro riverbed (Bologna foothils, Northern Apennines). Five micropaleontological samples have provided a Chattian- Aquitanian age for the section comprehending the body. Even if volcanoclastic layers are rather frequent in the Oligocene and early Miocene of the Northern Apennines (particularly in the Antognola Formation), this finding is unusual because of the coarse grain size and the presence of medium scale cross-bedding. The deposit was investigated with an interdisciplinary approach using compositional, sedimentological and stratigraphic studies. The ash-layer is a crystal-tuff composed up to 50% of plagioclase crystals associated with secondary smectite, harmotome and heulandite. This mineralogy supports an origin from intermediate products of calc-alkaline volcanic suite. The secondary minerals suggest different stages of alteration, the first one producing smectite and harmotome from probably amorphous original materials and praecursor smectite. No other beds of the same composition occur throughout the 300 m thick section. The ash-layer consists of at least 3 depositional events related to high density turbidity current. The sequence below and above the ash-layer is constituted mostly of gravity emplaced, fine grained deposits, alternating with thin hemipelagic layers. Gravity flow deposits are represented by thin bedded turbidites (TBT), slump deposits, and debris flow deposits indicating a slope association. As to the possible volcanic sources, the known active volcanic provinces at the time of the deposition were: 1) Periadriatic magmatism (along the Insubric Line in the Alps); 2) Mortara volcanic body (buried under the Po Plain); 3) Pieve S. Stefano volcanic body (buried under the central part of the Northern Apennines); 4) Sardinia volcanic arc; 5) Provence volcanic district (south of France). The dynamics of emplacement of the ash layer and its grain size are incompatible with all of these known sources. We propose a closer location of the volcanic source, even in an intrabasin position (Epiligurian basins were located to the W-SW of the present location) This volcanic center can be related to a transversal transcurrent lineament (Sillaro line).