Neogene-Quaternary sedimentary evolution in the western side of the northern Apennines (Italy)

The Burdigalian-Early Pleistocene deposits of the western side of the Northern Apennines have been subdivided into regional depositional units. Unit boundaries are represented by regional unconformities or by local unconformities and their correlative conforimities. They have been identified on the base of significant sedimentological changes (as lithological contacts or significant depositional environment changes). Neogene-Quaternary deposits are subdivided into: 1) The group of the Burdigalian-early Tortonian formations. These formations unconforrmably rest on the Ligurian Unit, their outcrops are rare and discontinuous and their stratigraphic relationships are unknown. Therefore, they are neither combined in a single depositional unit nor subdivided in distinct unit; 2) eight regional depositional units deposited during late Tortonian-Early Pleistocene: Lignitiferous Unit (late Tortonian), Acquabona-Spicchiaiola Unit (earliest Messinian), Castelnuovo Unit (early Messinian), Logo -Mare Unit (late Messinian), Pliocene I Unit (early Pliocene), Pliocene II Unit (early-middle Pliocene), Pliocene III Unit (middle Pliocene) and Chiani-Tevere-Montescudaio Unit (Early Pleistocene). At least four of the recognized unit boundaries have been related to extensive regional unconformities. They mark stratigraphic hiatuses and well pronounced erosive events due to tectonic uplifts. The first uplift event occurred during the Aquitanian-early Burdigalian, the second one during the middle Tortonian, the third one during the late Pliocene and the last uplift event occurred at the Early-Middle Pleistocene boundary. These regional unconformities allow to recognize three different sedimentary intervals. The first one is related to the deposition of the epiligurian formations and its age is Burdigalian-early Tortonian; the second one is late Tortonian-middle Pliocene, and it is represented by the Lignitiferous Unit, Acquabona-Spicchiaiola Urnt, Castelnuovo Unit, Lago-Mare Unit, Pliocene I Unit, Pliocene II and Pliocene III Unit; the third one is Early Pleistocene and it is represented by the Chiani-Tevere-Montescudaio Unit. The age of the sedimentary intervals and the areas where they developed indicate a Mio-Pleistocene sedimentary evolution from WSW to ENE.