Foredeep turbidites of the Miocene Marnoso-arenacea Formation (Northern Apennines)

The Marnoso-arenacea Formation (MAF, Langhian-Tortonian) was deposited in an elongate, NW-stretched foredeep basin formed in front of the growing Northern Apennines orogenic wedge (Figs. 2, 3A). These types of deposits have always had a fundamental role in the hystory of turbidites, because a great part of the models and facies schemes proposed in the literature have often been developed on these types of deposits. Among foredeep turbidites, the MAF is probably the most famous, the best exposed and less structurally deformed, due to its relatively external position within the Apenninic orogen. These characteristics have often favoured detailed physical stratigraphy studies, such as the pioneering ones by Ricci Lucchi and his co-workers (see for example Ricci Lucchi & Valmori, 1980). As indicated in figure 3, an idealized transect oriented perpendicularly to the main structural axes shows that sedimentation of a foreland region takes place in three distinct and coeval basins including: a) wedge-top basins, characterized by alluvial, deltaic and mixed depositional systems; b) a foredeep basin, characteristically in-filled with deep-water basinal turbidites; c) an outer and shallower ramp developed on the passive foreland plate. The progressive thrust propagation toward the outer margin of the basin produces a vertical superimposition of three depositional systems that, from base to top, are: (1) highly efficient basinal turbidite systems and associated hemipelagic deposits; (2) mixed depositional systems, in which turbidite-like bodies are deposited by poorly efficient gravity flows in a structurally confined basin. They can be associated to prodeltaic sediments, both vertically and laterally; (3) flood-dominated deltaic systems (see Mutti et al., 2003). The vertical stacking pattern of the MAF, illustrated in figures 4 and 33, is characterized by same vertical stratigraphic evolution in which at least three main depositional systems can be recognized and are represented by Langhian to Serravallian high-efficiency basinal turbidites, Tortonian low-efficiency mixed turbidites and shallow water Messinian euxinic shales and evaporites (Ricci Lucchi, 1978, 1981, 1986; Mutti et al., 2002a; Roveri et al., 2003; Tinterri & Muzzi Magalhaes, 2011). The MAF, therefore, consists of a shoaling-up stratigraphic succession, which results from the progressive closure of the foredeep due to the north-eastward propagation of the main thrust front of the MAF. Consequently, this eastward thrust propagation has produced a progressive uplift of the inner portions of the foredeep and a subsequent shifting in the same direction of themain depocentres. For this reason, Ricci Lucchi (1986) introduced the concepts of inner stage or basin (Langhian-Serravallian in age) and outer stage or basin (Tortonian in age). The first one is characterized by deep water high efficiency basinal turbidites, while the second one consists of low-efficient mixed turbidites in a shallower and more confined basin. The passage between inner and outer stages is recorded by an important tectonic phase (upper Serravallian in age) characterising the basal part of Unit V by Muzzi Magalhaes & Tinterri (2010), which is time equivalent to the Firenzuola and Paretaio systems (Figs. 4 and 33). The MAF stratigraphic succession, therefore, can be described in three stages: 1) a Langhian-Serravallian inner basin; 2) an Upper Serravallian phase that records the transition between inner and outer basins and 3) a Tortonian outer basin (see Fig. 33). These three stages or basins are characterized by three different facies associations related to the progressive increase, over time, of the structural control and the associated morphologic confinement. This fact, influencing especially the erosive degree and the deceleration rate of the turbidity currents, induces the formation of different bed types. The MAF foredeep can be considered as a complex foredeep (as meant by Ricci Lucchi, 1986) characterized by sin-sedimentary structural highs and depocenters related to the main thrust fronts within the MAF foredeep, which significantly control the lateral and vertical distribution of turbidite facies (see Muzzi Magalhaes & Tinterri, 2010; Tinterri & Muzzi Magalhaes, 2011). Therefore, after a short and general introduction to the geology and stratigraphy of the northern Apennines, the main targets of this field trip will be the stratigraphy, facies and processes of foredeep turbidites of the MAF outcropping in the north-eastern Apennines, focusing especially on two specific aspects of the MAF sedimentation: 1) the synsedimentary structural control affecting the MAF turbidites deposited in an elongate, NW-stretched complex foredeep basin formed in front of the growing Northern Apennines orogenic wedge and 2) the vertical facies changes of the MAF stratigraphic succession (more than 4000m thick) in relation to the progressive closure, uplift and consequent fragmentation of the foredeep due to the north-eastward propagation of the Apennine orogenic wedge (Fig. 33).