Depositional history of the Epiligurian wedge-top basin in the Val Marecchia area (northern Apennines, Italy): a revision of the Burdigalian-Tortonian succession

The Burdigalian-Tortonian Epiligurian succession in the Val Marecchia area comprehends different lithostratigraphic units deposited in a wedge-top basin during the northeastern migration of the thrust belt. The succession includes shallow-water carbonates passing to mixed carbonatesiliciclastic and to fine-grained pelitic sediments, capped by fluvio-deltaic coarse-grained deposits. Detailed field work and stratigraphy has allowed to characterize depositional units and unconformities and to delineate the sedimentary and tectonic evolution of the basin. Tectonics exerted a primary control at different stages. During the Burdigalian, a general uplift of the area allowed the onset of shelfal carbonate sedimentation on underlying Ligurian and Epiligurian deepwater sediments. At the Serravallian the sedimentation was influenced by the thrust reactivations which caused a marked asymmetry in the basin geometry and fill. The subsidence increase in the rear part of the basin determined the deposition of a thick succession of relatively deep fine-grained sediments (up to 800 m water-depth) (Serravallian, MNN6a through MNN6b subzones based on nannofossil biostratigraphy) and fossiliferous clays (lower Tortonian, biozones MNN8b-MNN9). Conversely, uplift is activated in the frontal part of the basin, causing the partial erosion of the Burdigalian–Langhian shallow-water carbonates. A relevant amount of this carbonate detritus is delivered to the foredeep, supplying the Marnoso-arenacea Fm. A general uplift of the area in the late Tortonian leads to the deposition of fluvio-deltaic conglomerates supplied by emerged rear sectors of the basin.