Heavy-mineral associations as tracers of limited compositional mixing during turbiditic sedimentation of the Marnoso-arenacea Formation (Miocene, Northern Apennines, Italy)

Petrographic and mineralogical analyses of 567 sandstone samples indicate that the turbidite beds of the Marnoso-arenacea Formation (Miocene, Northern Apennines foredeep basin) are the product of distinct yet contemporaneous detrital inputs derived from different source areas. These inputs were deflected along the main axis of the basin and flowed side by side with only minor mixing, as shown by the detrital modes of discrete stratigraphic intervals and of multi-sourced marker beds traceable in the basin over long distances. The bulk of the basin fill is made of sandstone turbidite beds with southeastward paleocurrents, and can be subdivided into three petrofacies (A, B and C), based on heavy-mineral and framework compositions. A is characterised by zircon, tourmaline and rutile (ZTR), garnet, staurolite, and abundant plutonic rock fragments; B features ZTR, garnet, epidote-group minerals, sphene, staurolite, kyanite, glaucophane and abundant metamorphic rock fragments; C contains ZTR, garnet, very abundant epidote-group minerals, hornblende, sphene, staurolite, kyanite, glaucophane and abundant metamorphic rock fragments. Petrofacies A and B are time-equivalent (Langhian-Serravallian), while petrofacies C ranges from the late Serravallian to the late Tortonian. Petrofacies A occurs mainly in the thrust sheets closest to the Apenninic (SW) side of the basin, petrofacies B in the thrust sheets toward the Adriatic (NE) side, and petrofacies C comprises the late-stage deposits in the evolution of the basin, when the depocenter shifted further to the northeast. Petrofacies A is made of detritus derived from the Western Alps and/or from the Corsica-Sardinia massif, while petrofacies B and C was derived from the terrains of the Central Alps. The southern portion of the basin is still volumetrically dominated by turbidite beds derived from the northwest but is also characterised by detrital inputs derived from south and south-west (petrofacies D, E and F). These produced turbidite beds which are interbedded with the predominant NW-derived beds and have a distinctive composition characterised by abundant bioclasts (D; including the Contessa megaturbidite marker bed) plus mudrock lithics and staurolite (E), or plus mudrock lithics, very abundant staurolite and chrome spinel (F).