Late Miocene-Middle Pleistocene Sequences in the Po Plain

Marine and Petroleum Geology xxx (2012) 1e32

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Marine and Petroleum Geology

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Late MioceneeMiddle Pleistocene sequences in the Po Plain e Northern Adriatic Sea (Italy): The stratigraphic record of modiﬁcation phases affecting a complex foreland basin

Manlio Ghielmi a,*, Matteo Minervini b,1, Claudio Nini c,1, Sergio Rogledi d, Massimo Rossi e a Eni S.p.A., Exploration & Production Division, STEX, Via Emilia 1, 20097 San Donato Milanese (MI), Italy b IEOC Egypt Branch, Exploration Department, 1, Rd 204, Degla Sq. Maadi, P.O. Box 52, New Maadi 11742, Cairo, Egypt c Eni Ghana E&P, 1st ﬂoor Una Home Bldg, 12 Airport Bypass Road, Airport City, PMB KA 185, Accra, Ghana d Via Pasino Sforza 8, 20078 San Colombano al Lambro (MI), Italy e Eni S.p.A., Exploration & Production Division, SPES, Via Emilia 1, 20097 San Donato Milanese (MI), Italy article info abstract

Article history: During the Late MioceneePleistocene interval, a complex system of elongate foredeeps, the Po Plain- Received 15 May 2012 Adriatic Foredeep (PPAF), developed in the eastern sector of the Po Plain and in the northern Adriatic Received in revised form e 1 October 2012 Sea, This system is the largest Late Miocene Pleistocene complex of foredeep depocenters of the Peri- e Accepted 30 November 2012 adriatic Basin with an overall length of 500 km and a width of 80 120 km. Available online xxx In the last 15 years, several Eni-Agip multidisciplinary studies analyzed the buried Late MioceneePleistocene succession of the central-eastern Po Plain and northern Adriatic Sea. Detailed revisions of biostratigraphy, Keywords: chronostratigraphy, sedimentology, seismic interpretation and sequence stratigraphy were performed using Late miocene the very large Eni subsurface dataset including over 500 deep exploration and development wells and regional Pliocene 2D and 3D seismic surveys. The large availability of subsurface data, the preservation and the relatively Pleistocene moderate structural deformation of the studied succession were essential factors for the generation of Messinian base-level changes a detailed three-dimensional geological model for the foredeep basins and also for the related ramp/foreland Northern Adriatic Sea and thrust-top basins areas. The model, which is presented in this paper, may be considered, for the large Po Plain volume of qualitative and quantitative information, as a reference model for tectonically active foredeep basins Turbidite sedimentation dominated by basin-scale sand-rich turbidite systems. Foredeep Allogroup During the Late Miocene, Pliocene and Early Pleistocene a severe tectonic activity affected the northern Tectonic sequence Apennine and the PPAF area. Due to the northern Apennine compressive tectonics, the PPAF underwent four regional phases of compressional deformation and depocenter migration towards the foreland (to the northeast). During these tectonic phases, four basin-scale tectonic unconformities were generated: the Latest Tortonian, the Intra-Messinian, the Intra-Zanclean and the Gelasian Unconformities. The sequence-stratigraphic analysis of the basin was based on the recognition of allogroups, i.e. major stratigraphic units bounded at base and top by the four regional tectonically-induced unconformities, and of their component sequences, mainly of tectonic origin, ranked on the basis of their physical scale. During the latest Miocene-to-Pleistocene time interval the foredeep shape was affected by a large variability in space through time, ranging from regular elongated shape to irregular shape, from simple foredeep to fragmented foredeep. A new evolutive model for the Apennine foredeep with two evolutive stages is proposed in this paper. The PPAF was a deep-marine basin with water depths usually exceeding 1000 m. Its latest Miocenee Pleistocene succession mainly consists of thick sequences of turbidite deposits. Basin-scale, sand-rich, highly-efﬁcient turbidite systems were largely predominant in the foredeep. Thick-bedded sand/sandstone lobes and thin-bedded ﬁne-grained turbidite basin plain deposits represent the most common turbidite facies associations. Paleocurrents are predominantly directed to the southeast, parallel to the foredeep main axis.

* Corresponding author. Tel.: þ39 0252063124; fax: þ39 0252062401. E-mail addresses: [email protected] (M. Ghielmi), [email protected] (M. Minervini), [email protected] (C. Nini), [email protected] (S. Rogledi), [email protected] (M. Rossi). 1 Present address.

The thick PPAF succession consists of the turbidites of the Bagnolo (latest Tortonianesyn-evaporitic Mes- sinian), Fusignano (post-evaporitic Messinian), Canopo (latest MessinianeZanclean), Porto Corsini (Zanclean ePiacentian), Porto Garibaldi (PiacentianeCalabrian) and Carola (CalabrianeLate Pleistocene) Formations. The ﬂuvio-deltaic systems of the Paleo-Adda, Paleo-Mincio and Paleo-Adige Rivers, located along the Alps margins of Lombardia and Veneto foreland, provided the bulk of the PPAF siliciclastic sedimentary input. With the partial exception of the post-evaporitic Messinian, the clastic supply from the northern Apennine belt was subordinate. The Messinian depositional systems underwent dramatic changes due to the combination of the salinity crisis and Intra-Messinian morphostructural reshaping. This behavior was not associated with a unique, dramatic lowering related to the Messinian Salinity Crisis (MSC) but was modulated by a combination of factors like climatic changes, deformation phases, isostatic rebound and sediment ﬂux. Stratigraphic relationships among decompacted coastal wedges suggest that the total lowering of the relative base-level did not exceed 900 m in the study area. This total drop was reached through three distinct base-level changes probably related to evaporation (ME2 Sequence), differential subsidence (ME3 Sequence) and regional uplift possibly associated to isostatic rebound (ME4 Sequence). The Zan- clean (Early Pliocene) relative sea-level rise is estimated on the order of 800e900 m, with a shoreline landward shift in the foreland of at least 70 km. In the study area, the MSC was therefore recorded by an overall strongly asymmetric cycle where discrete events of various origin punctuated a relatively gradual base-level fall. During the entire MSC, the PPAF depocenters remained in relatively deep-water or deep- water conditions, associated with turbiditic sedimentation and locally to bottom currents. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction 1.2. Methodology

Over the last two decades, Eni geologists and geophysicists have The Eni studies of PPAF are based on detailed well-log analysis performed several multidisciplinary studies of the buried and correlation strictly integrated with and calibrated by a dedicate Oligocene-to-Pleistocene depocenters of the Periadriatic Basin seismic interpretation of the 2D and 3D seismic surveys. Twenty- based on the modern principles of sequence stratigraphy. two stratigraphic surfaces corresponding to unconformities and Part of this activity was focused on the Late Miocene-to- correlative conformities of different order and importance were Pleistocene succession of the central-eastern Po Plain and correlated, at basin scale, on the well and seismic data applying northern Adriatic Sea through several multidisciplinary studies seismic stratigraphy principles (Vail et al., 1977). The main result of based on the vast Eni subsurface database (Dalla, 1994; Bettazzoli this integrated analysis was a detailed physical stratigraphic model and Visentin, 1997; Ghielmi et al., 1998; Amore et al., 2004; Nini of the Late Miocene-to-Pleistocene sequence strictly based on the and Visentin, 2004). Complete and detailed revisions of the considerable well and seismic dataset. This model not only extends biostratigraphy, chronostratigraphy, sedimentology, seismic inter- to the foredeep, but also to the ramp and part of the foreland, and to pretation and sequence stratigraphy were performed for this major the piggy-back basins area, along the outer and inner foredeep depocenter of the Periadriatic Basin, hereafter referred to as the Po margins respectively. The sequence stratigraphy of the succession Plain-Adriatic Foredeep (PPAF). The main aim of these studies (i.e. origin and areal extension of the unconformities, type of were: (1) to provide modern detailed geological models for the Eni stratigraphic units, hierarchy, etc.) was defined on the basis of the (Agip at that time) Exploration and Development Departments to evidences of the physical stratigraphic model. be used for their last phases of activity in this already mature area The sedimentological interpretation of the succession was based and (2) an improved characterization of the petroleum systems on the description and interpretation of bottom cores and well logs active in the area and of the related hydrocarbon trap types. Due to of the studied wells. Fundamental information for the paleoenvir- their corporate value, the results of these studies were released onmental interpretation was also provided by the seismic facies of only partially (Ghielmi et al., 2008a, 2008b, 2010a; Bertello et al., the deposits (amplitude, continuity, geometry, etc.). The facies 2009; Minervini et al., 2009). The different studies were eventu- models and turbidite systems classification of Mutti (1985), Mutti ally integrated in a single basin-scale model extended to both the and Normark (1987), Mutti et al. (1992, 1999, 2003) were adopted onshore and offshore areas of the basin. A synthesis of the latest as reference models in the sedimentological analysis and classifi- Miocene-to-Pleistocene tectono-sedimentary evolution of the cation of the PPAF turbidites. central-eastern Po Plain and northern Adriatic Sea, based on the Eni Paleontological analysis of foraminifera and nannoplancton model, is presented in this paper. assemblages were carried out on samples from bottom and sidewall cores, and on samples from ditch cuttings of selected strati- 1.1. Dataset graphic intervals. On Messinian succession palynological analysis were also performed. The chronostratigraphic and paleoecological The vast Eni subsurface database of the central-eastern Po Plain evidences were useful for the accurate datation of the unconfor- and northern Adriatic Sea includes both well and seismic data mities corresponding to allogroup and sequence boundaries, and acquired over 40 years of activity in this area. It is comprised of: (1) for supporting and verifying the well and seismic correlations as a regional 2D seismic survey of the eastern Po Plain area; (2) well as the environmental interpretations. a regional 3D seismic survey (“Adria 3D”) of about 12,000 km2 in Notwithstanding the changes introduced in June 2009 by the the northern Adriatic Sea; and (3) several Eni 3D seismic surveys Subcommission on Neogene Stratigraphy (SNS) of the International acquired for the development of onshore and offshore gas fields. I Commission on Stratigraphy (ICS) (Gibbard et al., 2010), which 500 explorative and development wells (with conventional and lowered the base of the Pleistocene to the base of the Gelasian stage image logs, ditch cuttings, bottom and sidewall cores, etc.) were (previously included into the Pliocene), the present work adopts selected among all the available data. the “old” chronostratigraphy (Gelasian as the last stage of Pliocene

2. Introduction to the northern Apennine and modern Apennine foredeep

The Po Plain and Adriatic foreland is shared by the converging South Alpine, Apennine and Dinaric chains. As well as in the bordering chain sectors, the compressional architecture of the area is overprinted on the polyphasic framework produced by the Mesozoic extensional phases. The Cenozoic compression reached the area at different times and with changing directions of tectonic Figure 1. Distribution of the Modern Apennine Foredeep (MAF) depocenters and movement: the Dinaric system (EeW compression) from Paleocene location of the Po Plain-Adriatic Foredeep (PPAF) at GelasianeLate Pleistocene time e after the last foredeep depocenter migration of the Gelasian Tectonic Phase (Ghielmi to Pleistocene, the Southern Alps system (N S compression) during et al., 2008a). Middle Eocene to Miocene Meso and Neoalpine Phases, and the

Figure 2. Model of the Po Plain-Adriatic Foredeep (PPAF) depocenters migration. For comparison also the inner and outer Marnoso-Arenacea Fm. depocenters (according to Ricci Lucchi, 1986) are indicated (Ghielmi et al., 2010a).

Apennines system (NNEeSSW compression) from Oligocene to consists of a thick succession of Upper Miocene (uppermost PlioceneePleistocene. TortonianeMessinian), Pliocene and Pleistocene turbidite deposits. The northern Apennines are part of an eastward migrating fold The present day eastern Po Plain and northern Adriatic Sea belong and thrust belt, developed since the Late Oligocene to present in to the largest of these depocenters, the Po Plain-Adriatic Foredeep relation to the passive westward subduction of the Adria Plate (PPAF) (Ghielmi et al., 2008b, 2010a). (Malinverno and Ryan, 1986; Ricci Lucchi, 1986; Patacca et al., 1990; Boccaletti et al., 1990; Doglioni, 1991; Argnani and Ricci-Lucchi, 3. The Po Plain-Adriatic Foredeep (PPAF) 2001) in the context of an A-type ensialic subduction (Bally and Snelson, 1980) as a consequence of the collision of the European The Po Plain-Adriatic Foredeep (PPAF) extends over the whole plate with the Adria microplate (Boccaletti et al., 1971; Elter, 1975; present day northern Adriatic Sea and also includes the eastern Marroni et al., 2002). The Adria paleomargin was characterised by portion of the Po Plain. It is the largest foredeep depocenter of the the development of the Apennine or Periadriatic Foreland Basin. MAF with an overall length of 500 km and a width of 80e120 km The foredeep corresponds to the most external and deepest (Ghielmi et al., 2010a)(Fig. 1). The foredeep is bounded at the depocenter of the foreland basin as in the general scheme of fore- outer margin by the Adriatic foreland ramp and at the inner margin land basin systems in Giles and DeCelles (1996). It is bounded at the by the outermost thrust-propagation folds of the northern Apen- outer margin by the foreland ramp, and at the inner margin by the nine thrust and fold belt. outermost submerged thrust-propagation folds of the Apennine The PPAF was affected by an active synsedimentary Apenninic fold belt (Pieri and Groppi, 1981; Pieri, 1983; Castellarin et al., 1985; compressional tectonics. Four major deformative phases were recog- Castellarin and Vai, 1986; Fantoni and Franciosi, 2010). The nized in the latest MioceneePleistocene interval (Gelati et al., 1987; northern Apennine Foreland Basin evolved through a number of Rossi and Rogledi, 1988; Ghielmi et al., 1998, 2010a; Rossi et al., 2002). successive regional tectonic phases, leading to a step wise outward They generated four major tectonically controlled unconformities: the migration of the Apennine thrust and fold belt. This evolution gave Latest Tortonian, the Intra-Messinian, the Intra-Zanclean and the way to a migrating set of asymmetric foredeeps (Ricci Lucchi, 1986) Gelasian Unconformities (Ghielmi et al., 2010a)(Fig. 2). These and associated piggy-back basins (according to Ori and Friend, repeated deformative events were responsible for the outward 1984) developed on the inner margin of the thrust sheets. A migration and change in shape of the foredeep depocenters from the similar tectono-sedimentary evolution of the foreland basin was former Outer Marnoso-Arenacea basin located in the Emilia-Romagna also recognized in the central and Southern Apennines (Ori et al., Apennines (which was deformed and included in the northern 1991; Doglioni et al., 1999; Casnedi et al., 2006; Milli et al., 2007, Apennine thrust and fold belt), to the last more external location in 2009 and references therein; Ghielmi et al., 2010b, 2011). In the Veneto and the northern Adriatic Sea. In the Emilia-Romagna sector, Tertiary Periadriatic Foredeep basins the sedimentation is relatively the foredeep depocenter migration occurred through alternating continuous and typically occurred in deep water environments stages of simple (LM and LP Allogroups) and fragmented (EM and EP with high sedimentation rates. The Late Miocene to present Allogroups) foredeep basins (Ghielmi et al., 2010a). Apennine Foredeep, hereinafter referred to as the Modern Apen- The Late MioceneePleistocene sedimentary inﬁll of the PPAF is nine Foredeep (MAF), is a large and elongate, undeformed or mostly represented by thick sequences of turbidites deposited in slightly deformed basin, stretching parallel to the local structural deep-marine environments (water depths usually exceeding axes of the northern, central and southern Apennines (Fig. 1). The 1000 m) (Dondi et al., 1982; Gelati et al., 1987; Ricci-Lucchi, 1986; MAF is subdivided by large structural lineaments into distinct and Boccaletti et al., 1990; Mattavelli et al., 1992; Ghielmi et al., 1998, relatively independent depocenters (Fig. 1). Its sedimentary inﬁll 2008b, 2010a). Sand-rich turbidite systems were volumetrically

Figure 3. Simpliﬁed Stratigraphy of the Po Plain-Adriatic Foredeep (PPAF) succession. Vertical scale in geologic time. Note that in the chronostratigraphy utilized by Eni the stage Gelasian is still included in the upper part of Pliocene according to Rio et al. (1998). Also the Eni nomenclature of allogroups and sequences, developed during the late 1990s and early 2000s, is still based on the former chronostratigraphy.

dominant in the PPAF sedimentary evolution. Very large dimensions downcurrent into turbidite basin plain deposits (according to Mutti characterize these basin-scale systems: widths up to 50e70 km, et al., 1999), made up of mud/mudstone with thin-bedded ﬁne- lengths usually exceeding 200/250 km, and thicknesses of several grained laminated sand/sandstone. On the basis of the subsurface hundreds of meters (Ghielmi et al., 2010a). They are almost entirely data (wells and seismic data), these systems are interpreted as composed of sheet-like sand/sandstone lobes, predominantly highly-efﬁcient (Type I) turbidite systems (Ghielmi et al., 1998, consisting of thick-bedded massive sand/sandstone, passing 2010a; this study), according to model for turbidite systems of

Figure 4. The MessinianePleistocene stratigraphic framework of the study area. Vertical scale in geologic time. The Messinian stratigraphy is representative of an idealized oblique transect located in the central-eastern Po Plain from Emilia to SW (Salsomaggiore Front in the Piacenza/Parma area) to Veneto to NE (Venezia area). The Plio-Pleistocene stratigraphy is representative of an idealized transect located in the eastern Po Plain (i.e. the central sector of the study area) from the Emilia to SW (Santerno Valley and Bologna areas) to the eastern Veneto to NE (Venezia area). Note that in the Pliocene and Pleistocene chronostratigraphy utilized by Eni the stage Gelasian is still included in the late Pliocene according to Rio et al. (1998).

Mutti (1985) and Mutti et al. (1999). Large-scale coarse-grained poorly-efficient (Type II) turbidite systems fed by lateral entry- canyon-fill and channel systems, genetically related to the latest points are usually more frequent than larger longitudinal highly- MioceneePleistocene highly-efficient turbidite systems, were efficient systems (Ghielmi et al., 2010a). locally recognized on the foreland ramp and at the foreland rampe As documented by Ghielmi et al. (1998, 2008b, 2010a), the foredeep transition corresponding to the main sedimentary entry- provenance of the bulk of the siliciclastic sediment supply of the points of the foredeep. The paleocurrents, parallel to the foredeep PPAF was from the Adriatic foreland, with the major entry-points main axis, are from NW to SE. concentrated in the foredeep apex area. The subsurface data Turbidite systems fed by lateral entry-points are also present in suggest that large volumes of clastics were supplied by the major the PPAF. They mainly consist of thick-bedded coarse-grained fluvio-deltaic systems active along the Alpine margins of Lombar- channel-lobe transition deposits rapidly grading downcurrent into dia and Veneto: the Paleo-Adda, Paleo-Mincio and Paleo-Adige lobe deposits. These smaller turbidite systems, interpreted as poorly- Rivers (Ghielmi et al., 1998, 2010a). With the partial exception of efficient (Type II) turbidite systems, were probably predominant in the post-evaporitic Messinian, only a moderate clastic supply was the Early Pliocene foredeep (PL1 Seq.) (Ghielmi et al., 1998, 2010a). provided by the northern Apennine belt, mostly due to local Thick successions of turbidites were also deposited in the submarine erosion and resedimentation of recent unconsolidated northern Apennine piggy-back basins related to the PPAF. In these deposits from the active structural fronts (Ghielmi et al., 2010a). basins the turbidites are often represented by fine-grained and The foreland ramp and foreland successions mainly consist of thin-bedded turbidite basin plain deposits (mostly mud/ mud and marls (often condensed) and sometimes of siliciclastic or mudstone). Coarser turbiditic deposits are also present with thick- bioclastic coastal/shelfal wedges. Hiatuses due to condensation or bedded sand/sandstone and gravel/conglomerate of lobe or no deposition episodes were also recognized (Ghielmi et al., 1998, channel-lobe transition facies associations. In these depocenters, 2010a).

4. Description of the tectono-sedimentary evolution of the According to the original purposes of the subsurface analysis, a great late miocene-to-pleistocene PPAF emphasis was usually dedicated to the local presence of potential sandstone or conglomeratic reservoirs. In each map, only the thrust 4.1. Introduction fronts and faults which were active or present (if already inactive) at that time are reported. The structures are not retro-deformed, The Late Miocene-to-Pleistocene tectonic and sedimentary therefore all fronts are represented at their present locations. All evolution of the northeastern part of the Periadriatic Basin largely sedimentation rates reported in this paper are referred to present coincides with the evolution of the PPAF, of its ramp and foreland day (not decompacted) thicknesses of the stratigraphic units. areas and of the related northern Apennine thrust-top basins. A synthesis of sedimentary facies, sequence stratigraphy and 4.2. EM allogroup tectono-sedimentary evolution of the Late Miocene-to-Pleistocene PPAF succession, based on the Eni geological model of the basin, has The incipient Emilia and Romagna Arcs subdivided the foredeep recently been presented (Ghielmi et al., 2008b, 2010a). This paper, of the Allogroup EM (latest TortonianeLate Messinian) into two largely based on those former articles, introduces new data and distinct depocenters where over 1000 m of turbidite sandstones and interpretations about the same stratigraphic interval over a more mudstones of the Bagnolo Fm. were deposited (Fig. 5). These sedi- extensive area. ments mostly refer to highly-efficient turbidite systems, with an Each allogroup, corresponding to a well defined step of the average sedimentation rate around 1 106 mm/My. In the Romagna geological evolution, will be described separately mainly through area, during the regional Latest Tortonian deformation phase (EM dedicated basin-scale facies association distribution maps of the allogroup boundary), the inner part of the outer Marnoso-arenacea Large-Scale Sequences. The maps are based on detailed correlation foredeep (Ricci Lucchi, 1986) was involved in the Apennine thrust of over 500 selected wells integrated with and calibrated by and fold belt. The new Early Messinian foredeep was located several a seismic interpretation of the Eni 2D and 3D seismic surveys. kilometers to the NE with respect to the previous one, thus recording Twenty-two seismic horizons corresponding to allogroup, large- an outward shift of the northern Apennine deformation. Therefore, and medium-scale sequence boundaries were interpreted at basin the Bagnolo Fm. depocenters overlie the external part of the outer scale. The maps are representative, at basin-scale, of the facies Marnoso-arenacea foredeep and the SerravallianeTortonian fore- distribution of time-equivalent or relatively time-equivalent depo- land ramp. sitional systems that can reach several hundreds of meters of Two component LSSs (large-scale sequences) can be identified thickness in the depocenters. Therefore, an overall synthesis was in the EM allogroup (Figs. 3 and 4), the ME1 Sequence (latest necessary in order to represent the variable, both laterally and Tortonianepre-evaporitic Messinian) and the ME2 Sequence (syn- vertically, distribution of different depositional environments. evaporitic Messinian), related to the hyperaline stage following the

Figure 5. Facies associations distribution map of the pre- and syn-evaporitic Messinian ME1 and ME2 Sequences (Allogroup EM). See text for the description (redrawn from Ghielmi et al., 2010a).

Figure 6. Overall geometry and internal architecture of a Messinian remnant along the Southern Alps margin (see Figs. 5, 13, 14 for location). a) seismic profiles; b) time slice. Note: 1) the major change from aggradation to forced regression across the Intra-Messinian unconformity (green surface, LM allogroup boundary) generated in relation to the combination of uplift along the Southern Alps thrust fold belt and climatic wetting; 2) the genesis of the remnant resulting from the basin-modification phase recorded by the intra- Zanclean submarine erosional unconformity (red surface), expressed in this location by a major drowning unconformity generated by the coalescence of retrogressive slump scars and submarine canyons. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Figure 7. Northern Adriatic Sea area: Top Miocene TWT seismic map of the dip morphological seismic attribute (Eni “Adria” 3D Seismic Survey) (See Figs. 5, 23 for location) (modiﬁed from Ghielmi et al., 2010a). These map, roughly corresponding to the Intra-Messinian Unconformity (LM allogroup boundary), shows: 1) the roughly NeS pre-evaporitic Messinian coastal wedge (ME1 Seq.); 2) the post-evaporitic Messinian incised-valleys of the ME4 Seq. (partly reactivated and modiﬁed as deep-water turbiditic canyons during Pliocene); 3) a large slump scour system produced in a foreland ramp setting by submarine slides during Pliocene; 4) the roughly NWeSE oriented onlap of the PiacentianeGelasian turbidites of the Porto Garibaldi Formation. The pre-evaporitic Messinian basin was deepening roughly towards the WSW, while the Gelasian foreland ramp towards the SW. The difference of about 25 is a consequence of the Apenninic deformation of this sector of Adria foreland progressively occurred during the Intra-Messinian, Intra-Zanclean and Gelasian major compressional events.

4.3. LM allogroup These sediments, mostly referred to highly-efficient turbidite systems, are volumetrically dominated by sandstone lobe deposits The boundary of Allogroup LM (late MessinianeZanclean) (Figs. 13 and 14). The main source area is probably located along the corresponds to a marked tectonic unconformity produced by the outer foredeep margin in eastern Lombardia, where both the fluvial so-called Intra-Messinian Phase. This tectonic event was respon- valleys and the submarine canyons underwent reincision, while the sible for the reactivation and uplift of the Emilia and Romagna Arcs fan delta systems became largely dominated by progradation and (Ghielmi et al., 2010a) and for the migration of the foredeep forced regression. Fluvial reincision of the EM valleys occurred, depocenters toward the foreland. The main consequences are: (1) where across the Intra-Messinian Unconformity extremely thick, the inner depocenter of the former fragmented EM foredeep is semi-arid, argillic paleosols occur (Minervini, 1999; Fantoni et al., incorporated within the northern Apennine thrust and fold belt as 2001). Close to the ME4 sequence boundary, a change to semi- a system of large piggy-back basins (Fig. 13); (2) the creation of humid paleosols (gley to pseudogley) is recorded, suggesting the a new simple foredeep in a more external position with respect to presence of a periodically higher water table. Other important the Emilia and Romagna Arcs and partially overlying the outer entry-points have been recognized more eastward, where large depocenter of the former EM fragmented foredeep. During this incisions cut the foreland ramp (Figs. 13 and 14). The steeper time, the foredeep extended from the MM to the northern Adriatic gradients recorded after the Intra-Messinian phase led to the local Sea for over 200 km in length and 25e40 km in width (Figs. 13 and erosion and remobilization of primary evaporites and to a renewed 14). The LM allogroup consists of three component LSSs: ME3, ME4 clastic input; while the uplift and consequent readjustment to and PL1 (Figs. 3 and 4). a lower base level generated a network of incised valleys and The post-evaporitic Messinian foredeep deposits consist of canyons. The ME3 Seq. records a major basin-modification phase 1500e2000 m of turbidite sandstones and conglomerates of the and an increasing rate of differential subsidence, whereas the ME4 Fusignano Fm. belonging to the ME3 and ME4 Sequences, with an Seq. records a dramatic reduction of tectonic subsidence at the average sedimentation rate ranging from 5 to over 6 106 mm/My. regional scale.

Figure 8. Regional seismic proﬁle (location in Figs. 5, 13, 14, 18) showing the stratigraphic relationships between coastal wedges developed from latest Tortonian to Zanclean (Early Pliocene) around the peripheral bulge separating the Southern Alps and the Northern Apennines foredeeps. High-magnitude relative base-level changes can be estimated (see text for discussion). The coastal wedges are later onlapped by Plio-Pleistocene aggradational turbidite systems of the PPAF.

Figure 9. Uppermost Tortonian to Zanclean (Lower Pliocene) depositional sequences and component depositional systems at the outer margin of the Apenninic foredeep. a) regional correlation panel (location in Figs. 5, 13, 14 and 18) showing the stratigraphic architecture visible in the regional proﬁle of Fig. 16. The stratigraphic expansion in the southwest records the uppermost TortonianeZanclean Apenninic foredeep ﬁlled by deep-water deposits. The relationships, in each sequence, between coastal onlaps and the position of shoreline and shelf breaks may be used in order to roughly evaluate the amount of relative base-level changes.

The Sequence ME3 (Fig. 13) is mainly characterized, in the clastics into deeper waters, until it became filled in its more basinal northern Apennine thrust-top basins (Gelati et al., 1987; Rossi and portion during latest Messinian time (ME4 Seq.). Rogledi, 1988; Ghielmi et al., 1998; Roveri et al., 1998; Rossi et al., In the foreland, the Sequence ME4 is represented only by thin 2002; Artoni et al., 2010), by large and locally very thick chaotic sections of fluvial conglomerates deposited in the valley axes complexes which form the lower stratigraphic portion of the Tetto (Figs. 9, 10D and 16). Both in the northern Apennine thrust-top Fm. (Fig. 11 in Ghielmi et al., 2010a) These mass-transport deposits basins and in the Southern Alps margin, the ME4 Seq. records the may exceed several hundreds of meters in thickness, wedging-out deposition of fluvio-deltaic systems (Fig. 14) dominated by cata- at the base of tectonic slopes. These sedimentary bodies typically strophic fluvial floods (Fig. 6) and developed in relation to the contained slide blocks, slumps, debris flows and large olistoliths combination of low-accommodation at basin margins and low- made of Messinian evaporites and Epi-ligurian rocks. salinity of basin waters (brackish- to fresh-waters typical of the The reworking index becomes very high in this setting, involving “Lagomare” event: Cita et al., 1975; Casati et al., 1976; Roveri et al., also Cretaceous Ligurian units. Typically, the mass-transport evap- 2001). These deposits, referred in the subsurface as the Sergnano orites evolve downcurrent into high-density turbidites made of Fm. in the Southern Alps margin (Fantoni et al., 2001; Minervini gypsorudites and then gypsarenites (Fig. 15)(Rossi et al., 2002; et al., 2008) and as the Cortemaggiore Fm. in the northern Apen- Roveri et al., 2003; Manzi et al., 2005; Minervini et al., 2008; Artoni nine margin (Rossi et al., 2002), formed active prograding wedges et al., 2010; Ghielmi et al., 2010a). Anoxic mudstones occur both in characterized by an increasing rate of by-passing and forced the thrust-top and in the foredeep basins (Fig. 11b), and locally can regression. The erosion and/or re-shaping of large-scale incised be characterized by bottom current reworking. In the foreland of the valleys took place over the whole outer margin of the foredeep Veneto Plain and northern Adriatic Sea, the allogroup boundary is during the period of subaerial exposure, passing basinward into represented by a deep erosional unconformity controlled by flexural a network of submarine canyons (Figs. 7, 8, 16 and 17). tilting and local uplift over the peripheral bulge (Fig. 8). This event The MioceneePliocene boundary represents a well known generated a relative base-level fall related to the Intra-Messinian regional marine ingression, although its nature is not completely phase (Fig. 9). The erosion of large-scale incised valleys developed clear yet (see Gennari et al., 2008 and references therein). Along the over the whole area during the period of subaerial exposure passing northern Apennine foreland basin inner and outer margins, the PL1 basinward into a network of submarine canyons (Fig. 13). The sequence boundary corresponds to this impressive flooding surface. truncated stratigraphic succession includes pre- and syn-evaporitic In these areas the Lower Pliocene unconformably overlies the Messinian clastics and evaporites and older Miocene deposits, and regressive wedge of the ME4 Seq. (Fig. 8; Fig. 12 in Ghielmi et al., locally reaches a maximum thickness of 400e500 m (Figs. 16 and 2010a) or older Messinian or pre-Messinian deposits (Figs. 10E, 16 17). This impressive system of valleys and canyons acted as by- and 19)(Ghielmi et al., 2010a). On the contrary, conformable passing areas for conveying large volumes of coarse-grained stratigraphic relationships occur at the MioceneePliocene boundary

Figure 10. Simpliﬁed model of the Messinian-to-Gelasian tectono-sedimentary evolution of the northern Adriatic Sea. This model is particularly referred to the central part of the northern Adriatic Sea. See text for the description. basinward, where the basal portion of this mainly Lower Pliocene were deposited in a more than 220 km WNWeESE elongated sequence probably contains also uppermost Messinian deposits. foredeep, extending from the MM (eastern Lombardia) to the In the foredeep the Sequence PL1 is made up by 500e700 m of northern Adriatic Sea. The foredeep was 20/25 km wide in the alternating turbidite thick-bedded sand, sandstone and polygenic Emilia-Romagna area, and up to 35/40 km in MM. The coarse- conglomerate of the Canopo Fm. (Figs. 3 and 4). These sediments grained channel-to-lobe transition deposits and the sand/

Figure 11. Shallow- and deep-water facies associations in the ME2 Sequence (see Fig. 5 for location). a) Stacking pattern and lateral facies changes in the ME2 Seq. of an Apenninic thrust-top basin. Note the upward decreasing in thickness of the individual evaporitic cycles (numbers refer to the cycles as defined in Vai and Ricci Lucchi, 1976), suggesting an overall decreasing in accommodation. This is recorded by the SW-ward (i.e., marginward) increase in thickness of the coastal plain mudstones which eventually capped the primary evaporites in the most marginward locations, probably due to the coastal plain progradation (modified from Rossi et al., 2002). b) Deep-water expression (recorded at both the inner and outer margin of the main foredeep) of the primary evaporites of the ME2 Seq., consisting of high-resistivity dolomicritic facies interbedded by anoxic mudstones. Note the stacking pattern similarity in spite of the long distance between the two wells. c) Facies recording toe-of-slope deposition immediately before and during the Messinian Salinity Crisis. Note the abundance of mud drapes and bioturbation generated in relation to bottom currents as a result of stratification in the water column. sandstone lobes are the predominant facies associations (Fig. 18). The available data demonstrate that during the Zanclean (Lower The average sedimentation rate is evaluated in 0.45e0.6 106 mm/ Pliocene), the central Lombardia and the western sector of MM My. Mainly on the basis of: the presence of several active entry- represented a relative paleo-high between the PPAF to the East and points located along both foredeep margins (indicated by the the Western Po Plain Foredeep (WPPF) located more to the west in local presence of coarse-grained facies), the predominance of Piemonte and western Lombardia (Minervini et al., 2008; Ghielmi sandstone lobes deposits in all the foredeep area, and the absence et al., 2010a). In these sectors the sedimentation of a relatively of a clear basin-scale downcurrent evolution of the turbidite facies, thin section of clay interbedded with coarse-grained turbidites of these sediments are interpreted as a complex of laterally coalescent the Caviaga Fm. (Dondi et al., 1982), attributed to poorly-efficient sand-rich poorly-efficient turbidite systems (Ghielmi et al., 2008b, turbidite systems, took place (Ghielmi et al., 1998, 2008b, 2010a). 2010a). These systems were fed by several sedimentary entry- If compared with the sand-rich turbidite systems of the other points located both on the inner and the outer margins of the Plio-Pleistocene sequences, the foredeep succession of PL1 Seq. is foredeep with local paleocurrents transversal to the main foredeep characterized by a relative abundance of conglomerate and pebbly axis. The area of the MM foreland and the inner foredeep margin of sandstone/sand. This distinctive sedimentary feature was probably the eastern Romagna sector have been recognized as the most due to an extensive sub-marine erosion and resedimentation of the probable candidate as source-areas (Fig. 18). coarse-grained deposits of the ME3 and ME4 Sequences originally

Figure 12. Seismic expression of the primary evaporites and of their lateral facies changes within the ME2 Seq. (vertically exaggerated; vertical scale in milliseconds) (Eni “Adria” 3D Seismic Survey). a) Stratigraphic relationships among the Messinian units in the northern Adriatic Sea area. Note in the ME1 Seq. the occurrence of both marine and coastal onlaps and the development of a prograding coastal wedge in its upper portion (Progradation towards WSW). Primary evaporites thin-out towards ENE and in turn show coastal onlap against the toeset of the ME1 coastal wedge. b) In this case, high amplitude evaporitic geobodies change NE-ward (i.e., toward the Adriatic foreland) into lower amplitude seismic facies interpreted to record more heterolithic, possibly coastal plain deposits at the outer margin of the foreland basin (see Fig. 7 for location). Note also the displacement of the primary evaporites (ME2 Seq.) and of the post-evaporitic deposits (ME3 Seq.) due to high-angle normal faults. These faults are the superficial expression of deeper and older high-angle normal faults reactivated during the Pliocene deformation phases of the Adriatic foreland. The present-day dip of the Messinian and older Miocene deposits towards southwest is due to repeated phases of subsidence and tilting occurred in the area during the Intra-Messinian Phase of deformation and mainly during Intra-Zanclean and Gelasian Phases. deposited directly on both the outer and inner foredeep margins Adriatic Sea, in the east, during the middle-upper part of the PL1 (the foreland ramp, and the thrust-top basins closest to the fore- Seq. (Figs. 8, 16, 17 and 18). It consists of coastal, deltaic, shelfal, deep respectively) during the major Late Messinian base-level falls. slope and genetically related deep-water gravitative sands included In the foreland paleogeographic domain, the sequence is rep- into the Eraclea Fm. (Eni subsurface lithostratigraphy) (Fig. 4). A resented by a transgressiveeregressive cycle. On top of the basal continental sedimentation probably occurred in the northern part Pliocene regional transgressive surface, marine clays of the San- of the foreland outside the study area. In a large area of northern terno Formation (Dondi et al., 1982) were deposited over a large Adriatic Sea foreland the gravity-flow deposits of the Gisella Fm. area of the foreland (Fig. 18). The regressive component of the cycle (Eni subsurface lithostratigraphy) have been recognized at the base consists of a predominantly southward-migrating prograding of the PL1 Seq. succession. This unit, 100/200 m thick, consists of complex developed from Lombardia, in the west, to the northern mostly fine-grained turbidite sand and sandstone lobes rapidly

Figure 13. Facies associations distribution map of the post-evaporitic Messinian ME3 Sequence (Allogroup LM). See text for the description (redrawn from Ghielmi et al., 2010a).

passing southward and downcurrent to alternating sand and clay of recalculated on the basis of well data after the decompaction of lobe fringe/basin plain depositional environments. These lower coastal wedges in the different Messinian sequences, permitted Zanclean sediments, rich in dolomitic and calcitic clasts, were a broad estimation of the magnitude of the downward shift of deposited into the drowned Messinian paleo-valleys probably by coastal onlap and to assess the subaerial or subaqueous nature of poorly-efficient turbidite systems fed from source areas located to the major Messinian incisions. the north in the Veneto Alps (Figs. 10E, 16, 17 and 19). The analysis During the syn-evaporitic Messinian (ME2 Seq.) the evaporative of the seismic sections and of well data of the Lower Pliocene drawdown, possibly coupled with sea-level lowering, generated Progradation, integrated with paleontological analysis, allowed the a basinward shift of the coastal onlap characterized by primary original water depth of this external foreland area to be evaluated evaporites onlapping over the toesets of the pre-evaporitic Messi- at about 700/800 m. nian coastal wedges of ME1 Seq. (Figs. 8, 9, 10B and 12). In the piggy-back basins depocenters of the northern Apennine Nevertheless this lowering was very important, however in the belt, the PL1 Seq. is predominantly represented by basinal study area it was not as high as claimed in the previous literature. turbiditic-hemipelagic clays (Fig. 18), while thin sections of Decompacted shoreline break seismic geometries suggest an condensed clay were deposited on the submarine structural paleo- amount of total lowering estimated between 300 and 500 m, highs. These fine-grained deposits, included into the Santerno derived by the computation performed in two distinct locations, Formation according to the Eni subsurface lithostratigraphy (Dondi both of them pertaining to the foreland ramp but showing different et al., 1982), are the subsurface equivalents of the well-known local tectonic settings. In the Veneto foreland, the basinward shift of Argille Azzurre Formation outcropping along the northern Apen- the shoreline break with respect to the end of ME1 Seq. has been nines from the Piemonte to the Marche (Gennari et al., 2008). A estimated in the order of at least 30 km (Figs. 5, 8 and 9). progradation from the southern basin margins towards NE devel- The primary gypsum and anhydrites of the ME2 Seq. (deposited oped in the upper part of the sequence along the inner basin in restricted-to-closed settings both in the Apennine thrust-top margin south of Piacenza. basins and over the foreland) grade basinward into dolomicrites and anoxic mudstones, sometimes found in close stratigraphic 4.3.1. The events of the Messinian salinity crisis in the northern relationships with canyon-fill or intra-slope turbidite systems, Apenninic foreland basin: constrains from physical stratigraphy and recording slope to toe-of-slope sedimentation. These slope facies well data occur in three main settings: (1) along the inner foredeep margin, During the Messinian, the style of depositional systems under- where they accommodate the differential subsidence generated by went dramatic changes particularly in relation to the combined the fault-bounded transition from the piggy-back basins to the effects of the salinity crisis and the morpho-structural reshaping foredeep; (2) at the outer ramp; (3) in the larger piggy-back basins. due to the Intra-Messinian Phase. Stratigraphic relationships, Along these depositional or morphostructural slopes, sparse core

Figure 14. Facies associations distribution map of the post-evaporitic Messinian ME4 Sequence (Allogroup LM). See text for the description (redrawn from Ghielmi et al., 2010a). data suggest the presence of bottom currents probably associated along the margins of the thrust-top basins, by chaotic complexes of to a relatively deep stratiﬁcation in the water column during late the ME3 Seq. containing large amounts of Cretaceous-to-Miocene ME1, ME2 and early ME3 Sequences (Fig. 11). The ME2 foredeep olistoliths and Messinian gypsum breccias, mainly in relation to depocenters remained in relatively deep-water conditions and the uplift generated by the Intra-Messinian Phase and leading to were locally associated to turbiditic sedimentation. A phase of the erosion and destabilization of the primary evaporites on large foredeep depocenter migration occurred during the severe Intra- areas of the northern Apennine thrust and fold belt. Messinian Phase, when a new depocenter was generated as Chaotic deposits are also present on same ﬂanks of the a simple foredeep of the LM Allogroup, hosting the post-evaporitic peripheral bulge of the Western Po Plain Foredeep (WPPF) (Figs. 13 Messinian turbidites of the Fusignano Formation (ME3 and ME4 and 15). Nevertheless the evaporative lowering of base level started Sequences) (Fig. 13). This major unconformity is overlain, mainly since the onset of the salinity crisis, however a network of incised

Figure 15. Stratigraphic relationships between the primary evaporites of the ME2 Sequence and the resedimented evaporites of the ME3 Sequence, separated by the Intra- Messinian unconformity affecting the peripheral bulge between the Northern Apennines and Southern Alps thrust fold belts (western L e Western Po Plain Foredeep; see Figs. 5 and 13 for location). The truncation and onlap terminations respectively below and above the LM major sequence boundary, as well as the different stacking pattern of the primary vs. resedimented evaporites are shown. Note the high resolution seismic imaging of the downcurrent ﬂow transformations in the resedimented evaporites.

Figure 16. a) TWT Seismic Map of the Intra-Messinian Unconformity in the northern Adriatic Sea (Eni “Adria” 3D Seismic Survey, see Fig. 14 for location). b) Well-log expression of the incised valley fill Upper Messinian fluvial conglomerates of the ME4 Seq. (See the TWT map for well location). c) 2D seismic section (vertically exaggerated) of the uppermost MessinianePlio-Pleistocene succession in the Venice Lagoon (See the TWT map for location). Note the high amplitude Upper Messinian conglomeratic incised valley fill (ME4 Seq.). The local direction of progradation is towards S for the Lower Pliocene Progradation and towards the NE for the Middle Pleistocene Po Plain Prograding Complex (modified from Ghielmi et al., 2010a).

valleys, submarine canyons and slump scars developed in relation Along the Southern Alps margin, ﬂuvio-deltaic systems turned to the Intra-Messinian Unconformity, which can be seismically from normal regression to forced regression (Fig. 6). In the ME1 Seq. traced above the primary evaporites across the foreland ramp and the rate of progradation of the ﬂuvio-deltaic systems was up to over the foreland (Figs. 8, 10, 13, 15, 18 and 19). 12.5 km/My, while above the LM unconformity it dramatically

Figure 17. Seismic section (vertically exaggerated) of the uppermost Messinianeto-Middle Pleistocene succession in northern Adriatic Sea (Eni “Adria” 3D Seismic Survey) (See Figs. 16a, 18, 20 for location). The picture is strongly vertically exaggerated. Note: 1) the irregularity of the LM unconformity produced by the Late Messinian subaerial exposure; 2) the Lower Pliocene Progradation (PL1 Seq.) whose local direction is towards the SSE; 3) in this offshore foreland area the Early Pliocene turbidite sedimentation (PL1 Seq.) was conﬁned into the Messinian paleo-valleys. These poorly-efﬁcient turbidites, directly fed by the progradation, were deposited in a foreland setting with a water-depth range of about 300e500 m (evaluation based on the geometry of the time-equivalent prograding wedge); 4) in this area the LP a.b. is represented by a surface of no deposition-condensation. At this location the hiatus/condensed section comprises the sediments of the PL4 and PS1 Sequences and of the basal part of the PS2 Seq.

Figure 18. Facies associations distribution map of the Zanclean (Early Pliocene) PL1 Sequence (Allogroup LM). The map, representative of the top of the sequence, shows the facies distribution at about 4.0 My. See text for the description (redrawn from Ghielmi et al., 2010a).

Figure 19. Seismic section strikes to the Naomi Canyon (vertically exaggerated e vertical scale in milliseconds) (Northern Adriatic Sea e Eni “Adria” 3D Seismic Survey; see Figs. 21, 23, for location). The canyon was deeply eroded into the Messinian primary evaporites (ME2 Seq.) and the pre-evaporitic Messinian shelfal deposits of the Clara Formation (ME1 Seq.). A first erosional phase occurred during the Late Messinian sea-level fall due to fluvial erosion in an incised-valley setting. After the Early Pliocene transgression the Messinian incised-valley was filled in by Lower Pliocene relatively deep-marine clays (PL1 Seq.). The Intra-Zanclean and Gelasian Phases were responsible for the tilting and deepening of this sector of the northern Adriatic Sea. Consequently to these phases the Naomi Canyon was re-eroded during Pliocene in a foreland ramp deep-marine environment by relatively small-volume turbidity currents from the outer margin. The erosion was finally infilled by GelasianeCalabrian highly-efficient turbidites of the Outer Porto Garibaldi Formation (paleocurrent from NW).

increased up to 100 km/My (Fig. 6). In the foredeep (Figs. 9 and 13), to be associated with a unique, dramatic lowering taking place from this turning point is highlighted by a dramatic increase of sedi- the onset of the Messinian Salinity Crisis. On the contrary the total mentation rates in ME3 and ME4 Sequences, which exceeded lowering of relative base-level, that in any case could not exceed 6 106 mm/My, almost one order of magnitude higher than in the 900 m in the study area (value calculated on decompacted Zanclean ME1 Seq. (about 1 106 mm/My). deposits of the Veneto onshore), could have been reached through While along the Southern Alps margin the forced regression three distinct phases of relative base-level changes taking place in started with the ME3 Seq., in the Apenninic thrust-top basins this approximately 500 ky. The main driver of the first event was main turning point only occurred in the upper part of the ME4 Seq., probably evaporation (during ME2 Seq.). In the second (ME3 Seq.) suggesting that the different rates of tectonic subsidence affecting the main driver was differential subsidence, so that marginward of different sub-basins strongly controlled the stratigraphic architec- tectonic hinge lines space subtraction and incision are recorded, ture. At the same time, when accommodation was minimal or while basinward space creation mostly occurred. In the third phase negative, most of the foreland area underwent subaerial exposure (ME4 Seq.), a regional uplift possibly associated to isostatic rebound leading to the reshaping of the large-scale erosional surfaces was the main mechanism generating latest Messinian maximum created in relation to the Intra-Messinian Phase (Figs. 6, 9, 10, 14e regression phase and the final reshaping of incised valleys. In this 16). On the basis of stratigraphic relationships between the Mes- time interval, moreover, the rate of progradation was extremely sinian decompacted coastal wedges, the downward shift of coastal high (approximately 200 km/My) due to the high sediment flux and onlap is estimated between 250 and 400 m (Figs. 8 and 9). Basin- the low-salinity Lagomare basin, both factors favoring the devel- ward the erosional depressions are filled by uppermost Messinian opment of large fluvio-deltaic systems dominated by hyperpycnal systems when accommodation increased after the forced regres- flows (Rossi et al., 2002). sion phase, and eventually by Zanclean (Lower Pliocene) deposits The base of the Pliocene corresponds to the well-known regional unconformably overlying Lower Messinian or older rocks in a more marine ingression. The Zanclean (Lower Pliocene) foredeep marginward position (Figs. 9, 10, 16 and 17). succession is made up of the sand-rich turbidites of the Canopo The Messinian landscape-seascape morphology had a stepped Formation (PL1 Seq.). In the foreland areas, the Zanclean of the PL1 nature mainly controlled by the evolution of peripheral bulges Seq. is represented by a transgressiveeregressive cycle with located in Veneto (PPAF) and in western Lombardia (WPPF) a reduced section of transgressive clay at the base overlain by a rapid between the Alpine and Apenninic foredeeps. It appears that the southward progradation of slope, shelfal, deltaic and coastal shoreline was able to move across the peripheral bulge located deposits of the Eraclea Formation (Figs. 8, 9, 16, 17 and 18). The between the PPAF and the Southern Alps/Dinaride foredeep only if maximum amount of relative sea-level rise related to the basal the total amount of relative base level changes (evaluated on the Pliocene transgression is estimated on the order of 800e900 m, with basis of the basinward shift of the Messinian coastal onlaps) was a shoreline landward shift in the Veneto foreland of at least 70 km. exceeding several hundreds meters, in relation to repeated phases In summary, in relation to the tectono-stratigraphic evolution of relative base-level drops. outlined so far, from our data it appears that: (1) the Messinian This evolution passed through various phases controlled by Salinity Crisis was recorded in the study area by an overall strongly a combination of factors like climatic changes, deformation phases, asymmetric cycle characterized by a relatively gradual lowering isostatic rebound and sediment flux. Probably this behavior has not punctuated by internal discontinuities and a much more rapid re-

flooding; and (2) during the whole Messinian Salinity Crisis the PPAF fed by source-areas located in the northwestern foredeep apex: in depocenters remained in relatively deep-water or deep-water the eastern Lombardia (MM) and western Veneto foreland. The conditions, associated with turbiditic sedimentation and some- distribution of the proximal coarse-grained turbidites suggests that times affected by bottom currents developed along depositional and the huge sediment supply was mainly provided by the Paleo- morphostructural slopes. Mincio and Paleo-Adige Rivers (Ghielmi et al., 2010a). The regional tectonic unconformity at the base of the allogroup 4.4. EP allogroup was generated by the Intra-Zanclean Phase: probably the most important Plio-Pleistocene compressional episode in the northern The EP Allogroup consists of sediments of the upper Zanclean and central Apennines. This major tectonic event caused important (Early Pliocene), of the Piacentian, and of the basal Gelasian (Figs. 3 paleogeographic changes in the PPAF area. In particular, it was and 4). The unconformable allogroup boundary corresponds to responsible for (Ghielmi et al., 2008b, 2010a): (1) a phase of rapid a severe Apennine tectonic event: the Intra-Zanclean Phase subsidence (marked by an abrupt relative sea level rise) of all the (Ghielmi et al., 2008b, 2010a). The subsurface data indicate the EP foreland from the Lombardia to the northern Adriatic Sea; (2) the Allogroup foredeep extended from the MM to the northern Adriatic deformation of the inner structural element of the Ferrara thrust Sea with a total length of over 300 km and an average width of system, with a partial migration towards the foreland of the fore- about 50 km (Figs. 20 and 21). The western part of MM and the deep, which was split into two separate sub-basins (“fragmented central Lombardia areas still represented a relative paleo-high, foredeep”) in the Romagna area; (3) the activation of a new where a mud-dominated turbiditic sedimentation interpreted as important thrust front system in the Romagna offshore area; (4) distal basin plain deposits of the westernmost WPPF took place. a phase of severe growth of the northern Apennine thrust belt and Coarse-grained turbidite facies (interpreted as poorly-efficient tilting of the related thrust-top basins; and (5) an important turbidite systems) fed by local entry-points located in the fore- northeastward movement of the Ligurian Nappe in the Emilia area land are locally intercalated into the succession (Figs. 20 and 21). from Reggio Emilia to Bologna. In the present day onshore portion of the foredeep, the turbidite The Sequence PL2 foredeep was 30/40 km wide (45 km in the succession is represented by about 2000 m of stacked sand lobes. In MM) and over 260 km long. Its sedimentary succession includes the the northern Adriatic Sea, these sediments grade downcurrent into upper Zancleanelower Piacentian foredeep turbidites of the Porto fine-grained basin plain deposits. The paleocurrents are parallel to Corsini Fm. (Figs. 3 and 4). In the MM, thick-bedded turbidite sand the main foredeep axis from NW to SE. These sediments were and gravels, interpreted as proximal lobe and channelelobe tran- deposited by the large volume highly-efficient bipartite turbidity sition deposits, have been recognized in front of the main entry- currents of the Porto Corsini and “inner” Porto Garibaldi Forma- points along the northern foredeep margin. In the eastern Po tions turbidite systems (Figs. 3 and 4)(Ghielmi et al., 2008b, 2010a). Plain the foredeep deposits are mainly represented by alternating These impressive highly-efficient turbidite systems were mainly thick-bedded coarse- to fine-grained turbidite sands and mud

Figure 20. Facies associations distribution map of the ZancleanePiacentian PL2 Sequence (Allogroup EP). The map, representative of the top of the sequence, shows the facies distribution at 3.4/3.3 My. See text for the description (redrawn from Ghielmi et al., 2010a).

Figure 21. Facies associations distribution map of the PiacentianeGelasian PL3 Sequence (Allogroup EP). The map, representative of the top of the sequence, shows the facies distribution at about 2.4 My. See text for the description (redrawn from Ghielmi et al., 2010a).

(Fig. 20). These turbidites, that can reach in the foredeep depo- already formed structural fronts (in particular the Emilia-Romagna centers a maximum thickness of 1100 m, are interpreted as prox- Arc) and a phase of differential subsidence. As a consequence, the imal and distal turbidite sand lobe and turbidite basin plain sediments of PL2 Seq. were deposited only in narrow and often iso- deposits (Ghielmi et al., 2010a). The deposition of mud with lated depocenters of the thrust-top basins while the sedimentation of intercalations of thin- to thick-bedded fine-grained sand occurred condensed clay of the Argille Azzurre/Santerno Fm. occurred on the in the foredeep sector located in the northern Adriatic Sea. These new large submarine paleo-highs (Fig. 20). In these depocenters the finer sediments are interpreted as basin plain and distal sand lobe succession is usually represented by 200/300 m of basinal fine- deposits. The Porto Corsini turbidites were deposited by highly- grained deposits (Argille Azzurre/Santerno Fm.). The average sedi- efficient turbidite systems. The turbidity currents flowed south- mentation rate is evaluated at about 0.5 106 mm/My. Small-scale eastward along the main axis of the foredeep, as clearly evidenced sand-rich turbidite systems, interpreted as poorly-efficient systems, by the general downcurrent grading into finer and more distal can locally occur. Shallow-water carbonate systems were active also turbidite deposits towards the SE. In the eastern Po Plain the in this domain as documented by the presence of skeletal packstones sedimentation rate is about 1.7 106 mm/My: the highest value for deposited in the photic zone on top of two of the highest structural the Plio-Pleistocene. In the MM and western Veneto foreland ramp, fronts: the Cortemaggiore Front in Emilia and the Santerno-Riolo large-scale submarine canyons were incised by large-volume Bagni-Marzeno Front in Romagna (Ghielmi et al., 2010a). These turbidity currents into the PL1 Lower Pliocene Progradation wedge. carbonates are included in the Calcari ad Amphistegina Fm. During the Intra-Zanclean Phase, the whole foreland area was (Cremonini et al., 1982) outcropping in the Romagna Apennines, affected by a major marine transgression related to the episode of whose they represent the in-situ carbonate systems. Along the inner active tilting and subsidence. In the Lombardia, Veneto and northern (southwestern) margin of the Piacenza sector, coastal and shelfal Adriatic Sea foreland, the rapid relative sea level rise was responsible clastics of the Castell’Arquato Formation unconformably overlie fine- for a general drowning of the PL1 prograding complexes and for grained basinal deposits of the PL1 Seq. The sharp basinward shift of a sharp landward shift of the shelfal and coastal/deltaic systems the Castell’Arquato clastic systems is related to the tectonic uplift of (Figs. 8, 16 and 17). The large foreland and foreland amp areas were the northern Apennine margin in this area. characterized by the deposition of condensed clay (Santerno Fm.) or The Sequence PL3 consists in the foredeep of the thick stratigraphic by a hiatus due to non deposition (Figs. 10F and 20). In the Venice succession of the Piacentian-basal Gelasian “inner” Porto Garibaldi sector of the foreland, up to 50 m-thick succession of skeletal pack- Formation (Figs. 3 and 4). As documented by seismic data, the PL3 stone was deposited on a relative high area far from the main clastic sequence boundary corresponds to a tectonic-controlled unconfor- inputs by shallow-water (photic zone) carbonate systems (Fig. 20). mity. In the foredeep the deformative event was responsible for the As indicated by clear angular unconformities in the seismic reactivation of the former structures, and also for the formation of new sections, during the Intra-Zanclean Phase, the northern Apennine belt structural fronts in the onshore Ravenna area and in the northern underwent a severe phase of deformation with the reactivation of the Adriatic Sea (Fig. 21). Furthermore, this deformative event marked

Please cite this article in press as: Ghielmi, M., et al., Late MioceneeMiddle Pleistocene sequences in the Po Plain e Northern Adriatic Sea (Italy): The stratigraphic record of modification phases affecting a complex foreland basin, Marine and Petroleum Geology (2012), http://dx.doi.org/ 10.1016/j.marpetgeo.2012.11.007 M. Ghielmi et al. / Marine and Petroleum Geology xxx (2012) 1e32 21 a sharp forestepping of the highly-efficient turbidite systems of the Sea areas. Its regional tectonic unconformity marks the base of the “inner” Porto Garibaldi Formation. The deposition of thick-bedded Allogroup LP which includes Gelasian-to-Upper Pleistocene coarse- to fine-grained sand, interpreted as proximal and distal sand deposits. As well documented by the seismic and well data, this lobes facies association, occurred over most of the foredeep, while major compressional event is correlated with the final structura- fine-grained and thin-bedded deposits of turbidite basin plain were tion of the Ferrara thrust system, the growth of its NW and SE deposited downcurrent in the more distal sector of northern Adriatic lateral ramps, and the uplifting and tilting toward SW of the related Sea (Fig. 21). In the westernmost MM, turbidite coarse-grained sand, thrust-top basins and of the whole present day Bologna area pebbly sand and gravel of channel-lobe transition and proximal lobe (Ghielmi et al., 2008b, 2010a). In this sector, also the deeper were deposited. On the northern margin of this area a large-scale Mesozoic carbonate units were involved in the compressional channel complex were identified through seismic interpretation deformation. A severe phase of deformation involved also the and well data. The foredeep succession of the PL3 Seq. has a maximum northern Adriatic thrust-and-fold belt (Figs. 22 and 25). The avail- thickness of 1000e1100 m and shows an increase of the sandemud able data indicate that the Gelasian Phase was responsible for ratio with time. The average sedimentation rate is about a dramatic reshaping of the Po Plain and northern Adriatic Sea 1.0 106 mm/My. The accumulation of this thick sequence of turbi- paleogeography. The onshore Emilia-Romagna sector and the inner dites led to the gradual infilling of the foredeep depocenters. As part of the northern Adriatic Sea, belonging to the foredeep area of a consequence the PL3 foredeep progressively expanded its area the LM and EP Allogroups, were incorporated within the Apennine reaching 45e50 km of width and over 300 km of length. thrust and fold belt as large piggy-back basins (Figs. 22 and 23). To In the Bologna area, due to the aggradation of the thick turbiditic the west, the growth of the northwestern lateral ramp of the Fer- succession, the Emilia Arc thrust folds of the inner foredeep margin rara thrust system, caused the separation of the MM sector which were gradually onlapped and, in the upper part of the sequence, was included in the westernmost Western Po Plain Foredeep finally overpassed with the foredeep turbidity currents overflowing (Minervini et al., 2008; Ghielmi et al., 2008b, 2010a). In the entire also into the piggy-back basin. foreland and ramp domain, the Gelasian Phase was responsible for In large sectors of the MM and western Veneto foreland, the a new episode of subsidence and tilting towards the foredeep as succession of the PL3 Seq. (maximum thickness 100e150 m) is evidenced in the seismic profiles by clear angular unconformities. represented by an overall regressive cycle with, in the lower part, The new foredeep (up to 350-km long, 80-km wide) was located ramp to shelfal silty clay (Santerno Formation) and, in the upper more to the east, in present day eastern Veneto Plain, northwestern part, predominantly southward prograding shelfal and coastal Emilia-Romagna and northern Adriatic Sea areas (Ghielmi et al., deposits of the Bovolone Formation (Eni subsurface lithostratig- 2008b, 2010a). raphy) in the upper part. A muddy deposition took place in the The foredeep sequence consists of a up to 2800 m thick foreland ramp area. In these areas the siliciclastic sedimentation of succession of sand-rich turbidites of the “Outer” Porto Garibaldi the PL3 Seq. developed after the long period of non deposition or Formation (PL4 Seq. e Figs. 23 and 24) and Carola Formation (PS1 condensation of the PL2 Seq. On the contrary, in the ramp and and PS2 Sequences) (Fig. 26; Figs. 20e22 in Ghielmi et al., 2010a). foreland areas of eastern Veneto Plain and northern Adriatic Sea, The main source areas of the new foredeep were the central- the sequence consists of a thin section of marine clay (Santerno eastern Veneto foreland during the Gelasian and Calabrian (Early Fm.) or is represented by a non depositional hiatus (Fig. 17). The Pleistocene) and the Po River delta in the Middle-Late Pleistocene. large distance from the main source areas was probably responsible The Sequence PL4 succession includes Gelasian and basal Cala- for the very low sedimentation rates in these areas. brian sediments (Figs. 3 and 4). The foredeep succession consists of In the piggy-back basins of the Emilia-Romagna area, the PL3 a thick accumulation of turbidites of the “Outer” Porto Garibaldi succession mostly consists of fine-grained basin plain deposits of Formation which attains a maximum thickness of about 1200 m the Argille Azzurre/Santerno Formation. In some depocenters the (sedimentation rate about 1.5 106 mm/My). The new foredeep, thickness can attain 400/600 m and up to 800e1000 m in the mainly located in the northern Adriatic Sea, overlies the external highly subsiding Bologna Syncline. Resedimented bioclastic cal- part of the foredeep and the foreland ramp of the former EP carenites locally occur at the base and within the succession. These Allogroup. It shows a NWeSE regular elongated shape with a length layers originated from the partial resedimentation into the deep of over 200 km and a width of 35e45 km (decreasing to 20e25 km basins (mainly triggered by tectonics) of the Calcari ad Amphiste- in the SE sector) (Fig. 23). The sedimentation of monotonous thick- gina shallow-water carbonates of the PL2 Seq. located on the top of bedded amalgamated coarse- to fine-grained mainly massive the highest thrust fronts. Turbidite sand lobe deposits and basin sands, interpreted as basin-scale proximal and distal turbidite sand plain interbedded clay and sand commonly occur only in the upper lobe deposits, took place in the northwestern sector of the fore- part of the PL3 succession of the Bologna Syncline (Fig. 21). As deep. Towards the southeast, these deposits grade into alternation noted above, the deposition of turbidite sands in this piggy-back of mud and fine-grained mainly laminated sand interpreted as basin was due to the local flowing of the foredeep sand-rich distal turbidite basin plain deposits. Along the outer foredeep turbidity currents into the piggy-back basin. No-deposition margin, the sand-rich turbidite lobes onlapped a condensed section hiatuses or, more frequently, the sedimentation of condensed clay of Pliocene foreland ramp clays with no or very reduced lateral of the Argille Azzurre/Santerno Fm. occurred on the submarine facies change (Figs. 7, 10G and 24). The main entry-points were structural paleo-highs. Shallow-water skeletal carbonate systems located in the Veneto foreland, as indicated by the local presence of of the Calcare ad Amphistegina Formation were possibly still active deep-marine turbidite pebbly sands and gravels (Fig. 23). Impor- on some of the highest structural fronts in the lower part of the PL3 tant volumes of Piacentian turbidites were also destabilized by the Seq. Shelfal deposits of the Castell’Arquato Formation, showing an Gelasian tectonic activity along the southeastern lateral ramp of the overall regressive trend, are preserved in the subsurface of the Ferrara thrust system. The available seismic and well data suggest southwestern margin of the Piacenza area. that also the Gelasian and lower Calabrian sand-rich “Outer” Porto Garibaldi turbidites were deposited by very large-volume turbidity 4.5. LP allogroup currents that flowed towards SE along the axis of the foredeep. These deposits are attributed to basin-scale highly-efficient turbi- The Gelasian Phase was responsible for the last foredeep dite systems (Ghielmi et al., 2008b, 2010a). Along the inner fore- depocenter migration in the Eastern Po Plain and northern Adriatic deep margin, submarine collapses of PL3 “Inner Porto Garibaldi”

Figure 22. Seismic section of the Plio-Pleistocene succession in the northern Adriatic Sea fold belt (vertically exaggerated e vertical scale in milliseconds; see Figs. 18, 20, 21, 23 and 26 for location) (Eni “Adria” 3D Seismic Survey). It is possible to observe: 1) the frontal part of the northern Apennine thrust and fold belt; 2) the substantially undeformed foredeep of the LP Allogroup; 3) the progradational seismic geometries of the Pleistocene Progradation Complexes. Note also the partial submarine truncation of the PL3 Seq. along the Thrust front n. 2 occurred mainly during the Gelasian Phase. deposits from the active fronts of the northern Adriatic thrust and domain produced by the Gelasian Phase. In these areas the fold belt (Figs. 22 and 25) provided minor volumes of sediments to sequence boundary is marked by a transgressive episode. In the poorly-efﬁcient sand-rich turbidite systems intercalated into the northern Adriatic Sea ramp and foreland, the sequence is repre- longitudinal turbidite systems deposits (Fig. 23). sented by condensed clays or by a non depositional hiatus. In the In the MM depocenter, now belonging to the Western Po Plain northern MM and western Veneto (Fig. 23), now belonging to the Foredeep, the sedimentation of an over 1000 m thick succession of WPPF foreland, the transgression corresponds to a general land- sand-rich turbidites occurred (Fig. 23). These turbidites were ward shift of the shelf and coastal systems. In this area the basal deposited in a ponded foredeep basin by highly-efﬁcient turbidite marine ingression was followed by the deposition of a 50e100 m systems. thick succession of ramp-to-outer shelf silty clay (Santerno As noted above, the PL4 sequence boundary corresponds to Formation) passing upward to regressive coarser-grained shelfal, a major phase of subsidence and tilting of the foreland and ramp coastal and deltaic deposits of the Bovolone Formation.

Figure 23. Facies associations distribution map of the GelasianeCalabrian PL4 Sequence (Allogroup LP). The map is representative of the facies distribution at the Gelasiane Calabrian boundary (upper part of the sequence). See text for the description (redrawn from Ghielmi et al., 2010a).

Figure 24. Well correlation of the foredeep turbiditic succession of the Inner (upper part) and Outer Porto Garibaldi Formations (Sequences PL3 p.p., PL4 respectively). The correlation e vertically exaggerated e extends across the main foredeep axis of the northern Adriatic Sea perpendicular to the main paleocurrents direction (see Figs. 21, 23 for location). Note: 1) the remarkable high lateral continuity and tabular geometry of the Porto Garibaldi highly-efﬁcient turbidite systems; 2) the angular unconformity due to onlap at the boundary of the tectonic-controlled Large- (PL4) and Medium-Scale (PL4b, PL4c) Sequences; 3) the higher sand-mud ratio of the Outer Porto Garibaldi Fm. (PL4 Seq.) at this location. This major facies change is related to the LP allogroup boundary; 4) the stratigraphic closure on the foreland ramp of the Piacentianelower Gelasian turbidites of the Inner Porto Garibaldi Fm. (PL3 Seq.) occurres with an

evident lateral facies variation. On the contrary, the lateral facies variation is very reduced (absent in some stratigraphic interval) in the Gelasian Porto Garibaldi deposits (PL4 Seq.). The different sedimentary behavior of the turbidite 23 deposits is related to the different location of the deposits with respect to the main sedimentary entry-points: more distal in the PL3 Seq., more proximal in the PL4 Seq. 24 M. Ghielmi et al. / Marine and Petroleum Geology xxx (2012) 1e32

Figure 25. Seismic section of the Plio-Pleistocene succession in the northern Adriatic Sea (Eni “Adria” 3D Seismic Survey; vertically exaggerated; vertical scale in milliseconds; see Figs. 18, 20, 21, 23 and 26 for location). The central part of this seismic section runs parallel to the well correlation of Fig. 24. Note: 1) the most external and recent thrust- propagation folds of the northern Apennine thrust and fold belt; 2) the undeformed to slightly deformed foredeep of the LP Allogroup limited along the inner margin by the thrust front system n. 3; 3) the onlap of the turbidites of the Porto Corsini (PL2 Seq.), “inner” and “outer” Porto Garibaldi (PL3 Seq. and PL4 Seq. respectively), and Carola (PS1 Seq.) Formations on the SW-dipping foreland ramp; 4) the remarkable high lateral continuity and tabular geometry of the highly-efﬁcient turbidite systems of the same formations and sequences listed above; 5) the reduced thicknesses of the ME1, ME2 and ME3 Sequences, consisting in this area of shelf, coastal and evaporitic sediments deposits in a foreland domain; 6) the seismic progradational geometries of the Pleistocene progradations (upper part of seismic section). It is possible to observe also the merging between Pleistocene Apenninic Prograding Complex (Calabrian-Middle Pleistocene in age at this location) from the southwest margin of the basin, and the axial Po Plain Progradation Complex (Middle Pleistocene), the local directions of progradation are towards the NE and towards the SE respectively. Both the progradations consists of slope, shelfal and coastal deposits of the Ravenna Formation. The thrust front n. 1, uplifted during the Intra-Zanclean Phase, represented during the Zancleanelower Gelasian the inner limit of the EP Allogroup Foredeep (PL2 and PL3 Sequences). The more external thrust front system n. 3, involved in a ﬁrst phase of activity at PL3 sequence boundary but reactivated and completely uplifted during the Gelasian Phase, represented the inner limit of the Gelasian-Middle Pleistocene LP Allogroup Foredeep (PL4, PS1 and PS2 Sequences). The gradual decrease of the thrust propagation in the PPAF area during Pleistocene, after the latest TortonianeGelasian parossistic activity, is suggested by the gradual onlap of the PS1 turbidites on the front n. 3, and by the substantially undeformed sediments of the PS2 Seq. deposited above the same front.

Figure 26. Facies associations distribution map of the Middle Pleistocene PS2 Sequence (Allogroup LP). The map is representative of the facies distribution of the middle-lower part of the sequence (top of the PS2a Medium-Scale Seq) and is referred at about 0.6 My. See text for the description (redrawn from Ghielmi et al., 2010a).

As well documented by the seismic and well data, a wide area of indicated by the minor extension in the foredeep of the sand-rich the Emilia-Romagna was included in a large piggy-back basin proximal and distal sand lobes facies associations (Figs. 20, 22 in bordered by the Ferrara thrust system to the northeast and by the Ghielmi et al., 2010a). The lower efficiency is attributed to Ligurian Units to the southwest (Fig. 23). This depocenter was about a reduction in volume of the turbidity currents, and is interpreted 140 km long and 20e30 km wide: an exceptional extension in as a consequence of the tectonic activity decrease during this time comparison to the other Late Miocene-to-Pleistocene thrust-top interval (Ghielmi et al., 2010a). basins of the northern Apennine belt. Its unusual extension was due The deposition of ramp and foreland mud of the Santerno to a gradual reduction of the tectonic activity, occurred in the Formation took place in the Veneto and northern Adriatic foreland Bologna area during Piacentian, of the Emilia Arc thrust fronts, areas of the PPAF (Figs. 20, 22 in Ghielmi et al., 2010a). In contrast, which were completely onlapped by a thick succession of in Lombardia and western Veneto foreland (now belonging to the PiacentianeCalabrian turbidites. A second large piggy-back basin WPPF domain) a regressive progradational cycle of slope, shelfal, was represented by the elongated Ravenna Syncline. In the north- coastal and deltaic deposits developed. western sectors of both depocenters, the succession (about 600/ The available seismic and well data evidence that most of the 700 m thick in the Bologna area) consists of thick-bedded sand structural highs of the piggy-back domain were progressively lobes. These deposits grade southeastwards into alternations of onlapped by the turbidite sediments of the PS1 Seq., probably due fine-grained sand and clay interpreted as distal sand lobe and to the combination of the thrust fronts reduced uplift and of the turbidite basin plain deposits, and, finally, into distal basinal mud high turbiditic sedimentation rate (Fig. 22). In the upper part of the (Fig. 23). These turbidites, attributed to highly-efficient turbidite Calabrian, the combination of these factors led to the formation of systems, were fed mainly by the deep erosion of the uplifted Ferrara a unique wide depocenter extending for about 160 km from the thrust system lateral ramps. In the more external Ferrara Syncline, Emilia area to the northern Adriatic Sea (and 25e40-km wide) isolated from the main sedimentary input, the sedimentation of limited by the emerged northern Apennine Chain to the southwest condensed mud took place (Fig. 23). In the southwestern part of the and by the Ferrara and northern Adriatic thrust systems to the study area, the Gelasian Phase was responsible for the reactivation northeast (Ghielmi et al., 2010a). The sedimentation of sand lobe of the Emilia Arc. In the thrust-top basins of this area a northeast- and turbidite basin plain facies, referred to highly-efficient turbidite ward progradation of slope, shelfal and coastal systems of the systems, occurred in the northeastern and central sectors of this Castell’Arquato Fm. Developed, probably as a consequence of an large piggy-back basin. The paleocurrents flowed along the basin important relative sea level fall related to the tectonic uplift of the axis towards southeast. While smaller, probably poorly-efficient, northern Apennine margin (Argnani et al., 2003, and references turbidite systems fed by lateral entry-points probably predomi- therein; Fig. 11 in Ghielmi et al., 2010a), while basin plain mud of nated in the onshore and offshore of its southeastern sector. During the Argille Azzurre/Santerno Formation was still deposited in the the Calabrian, a partial connection of the northwestern apex of this deeper basin depocenters. large basin with the MM occurred. The 3D seismic data interpre- The Sequence PS1 succession consists of Calabrian (Lower tation evidenced the presence of bottom-current deposits close to Pleistocene) deposits (Figs. 3 and 4). The basal sequence boundary the slope base in the piggy-back area of the northern Adriatic Sea corresponds to a tectonic-controlled unconformity as evidenced by (offshore Ravenna Syncline). An active progradation of slope, the subsurface data. After the severe Gelasian tectonics, a gradual shelfal, coastal and deltaic systems from all Apennine margin took decrease of the northern Apennine compressional deformation place. The progradation caused the rapid infilling of some marginal occurred in the study area during the Calabrian. The analysis of the depocenters behaving as piggy-back basins since the Late Messi- seismic data points out, in fact, a reduced activity of the existing nian (Ghielmi et al., 2010a). The sedimentation of condensed mud thrust-propagation folds (Figs. 22 and 25) and the formation of only occurred in the more external synclines of the Ferrara thrust a few new structural fronts in the Veneto onshore area (Fig. 20 in system, still isolated from the main sedimentary input (Fig. 20 in Ghielmi et al., 2010a). Consequently, during the PS1 Seq. (and of the Ghielmi et al., 2010a). later Middle-Late Pleistocene PS2 Seq.) the PPAF paleogeography During the PS1 Seq., an impressive progradation towards ESE, was very similar to the previous PL4 Seq. with a single simple several hundred meters thick, developed along the main foredeep foredeep depocenter (50e70 km wide, over 260 km long) located in axis in the present day central Po Plain area (Lombardia). This eastern Veneto Plain, northeastern Emilia-Romagna and northern progradation complex, consisting of slope, shelfal, coastal and Adriatic Sea (Figs. 22 and 25). The Ligurian Units of the Bologna- deltaic systems, was genetically related to the activity of the Po Reggio Emilia sector were involved in an important northeast- River and of its tributaries. During its fast advance, the Po Plain ward displacement probably driven by thin-skinned gravity Prograding Complex progressively merged with the Apenninic and tectonics as suggested by the geometry of their shallow thrust Alpine lateral progradations from the inner and outer foredeep planes (Argnani et al., 2003). margins, respectively (Fig. 20 in Ghielmi et al., 2010a). At the end of The foredeep sequence, up to 800/900 m thick, consists of the Calabrian (top of the PS1 Seq.) the sedimentary infilling of the turbidite deposits of the upper part of the “outer” Porto Garibaldi WPPF was complete (see in Fig. 26 the shelf-edge position at about and of the lower part of the Carola Formations (Figs. 3 and 4). In the 0.9 My). Veneto/Emilia-Romagna onshore and in the northern Adriatic Sea, The Sequence PS2 is made up of Middle-Upper Pleistocene the foredeep succession is predominantly composed of thick- deposits (Figs. 3 and 4). Due to the presence of basin-scale pro- bedded massive sands interpreted as turbidite sand lobe deposits. grading complexes, its succession is interpreted as an overall These proximal turbidites grade southeastward into more distal regressive sedimentary cycle. In the foredeep, the PS2 sequence alternation of mud and thin-bedded fine-grained laminated sand boundary is marked by the sharp superposition of sand lobe interpreted as turbidite basin plain deposits. These sediments were deposits on alternation of turbidite mud and sand of the older PS1 deposited by southeasterly directed highly-efficient turbidity Seq. interpreted as turbidite basin plain and distal sand lobe currents that flowed along the axis of the foredeep. The sedimen- deposits (Fig. 26; Fig. 22 in Ghielmi et al., 2010a). The rapid for- tation rate of the Calabrian foredeep succession is about 1.2/ estepping of the PS2 turbidite sand lobes is attributed to the 1.3 106 mm/My. The main source-area was still represented by concurrence of an abrupt increase of the grain-size and volume of the Veneto foreland. The PS1 Seq. corresponds to an evident phase the sediment supply due to the onset of Middle Pleistocene glaci- of backstepping of the highly-efficient turbidite systems, as ations in the Alps (Muttoni et al., 2003), and of a minor Apennine

Please cite this article in press as: Ghielmi, M., et al., Late MioceneeMiddle Pleistocene sequences in the Po Plain e Northern Adriatic Sea (Italy): The stratigraphic record of modification phases affecting a complex foreland basin, Marine and Petroleum Geology (2012), http://dx.doi.org/ 10.1016/j.marpetgeo.2012.11.007 26 M. Ghielmi et al. / Marine and Petroleum Geology xxx (2012) 1e32 compressional phase roughly coincident with the major climatic angle dipping foreland respectively (Fig. 26). The combination of change. During the PS2 Seq., the deep-water sedimentation was the gentle dip of the foreland (about 2) and of the high sedi- limited to the foredeep and to part of the ramp and foreland areas, mentation rate was responsible for a rapid extension towards the while the deposition of continental (i.e. fluvial and floodplain northeast of the foredeep which rapidly reached a maximum deposits), deltaic coastal, shelfal and slope deposits took place over extension with a width up to 75 km and a length of over 275 km. large sectors of the study area (Fig. 26). The Middle-Late Pleistocene foredeep turbidite systems were As the upper Calabrian, also the Middle-Late Pleistocene was fed directly by the Po Plain Prograding Complex. The seismic characterized by a reduced Apennine compressional deformation interpretation of the “Adria” 3D regional seismic survey pointed out and by a clear decrease of the thrust propagation, as suggested by the presence of a major delta systems at the northwestern apex of seismic interpretation and detailed well correlation. Moreover no the foredeep interpreted as the Po Paleo-delta (Fig. 26). Secondary new thrust front was activated during this time interval in the entry-points, directly fed by the Veneto Alpine foreland, were study area (Figs. 22 and 25). Along the southeastern margin of the located in the northern Adriatic area. The paleocurrents of the study area, the Ligurian Units of the Bologna-Reggio Emilia sector turbidity currents were still directed mainly to the southeast, were thrusted over Calabrian (Lower Pleistocene) deposits during parallel to the foredeep axis. The deposition of the Carola sand-rich their northeastward displacement. As noted before, this Pleistocene turbidites occurred in the more proximal northwestern sector of Ligurian deformation is interpreted as a thin-skinned gravity the foredeep. Southeastward these deposits grade into alternation tectonics genetically related to the Pleistocene uplift of the of turbidite mud and sand interpreted as distal sand lobe and northern Apennines, according to the interpretation of Argnani turbidite basin plain deposits. et al. (2003). The turbidite succession is still made up of vertically stacked The PS2 Seq. was dominated by the impressive Po Plain and sheet-like sand bodies roughly tabular and continuous at basin- Apenninic Prograding Complexes (Figs. 22 and 25; Figs. 7 and 22 in scale. But the 3D seismic data analysis clearly shows that these Ghielmi et al., 2010a). The two progradations, several hundred sand bodies, usually 20e50 m thick, are different from the sand lobe meters thick, consist of slope, shelfal, coastal and deltaic deposits of deposits of the MessinianeCalabrian highly-efficient turbidite the Ravenna Formation (Figs. 3 and 4). The development of the two systems. They consist of complex assemblages of laterally and rapid progradations is attributed to the large availability of silici- vertically stacked turbidite channels, channel-levee and sand lobe clastic sediments produced in the Alps and northern Apennine deposits (Fig. 27). The seismic data show that channels can range during the Middle Pleistocene glaciations, and to the related pro- from straight to meandering. As described above, these deposits longed lowstand phases. Probably also the reduced subsidence grade downcurrent into thick-bedded sand of distal sand lobe and related to the weak Apenninic compressional deformation favored into thin-bedded fine-grained deposits of turbidite basin plain. the progradation. The Apenninic progradation was responsible for The passage from the MessinianeCalabrian highly-efficient the final sedimentary filling of the piggy-back basins located in the turbidite systems to these less efficient systems must be related northern Adriatic Sea (Figs. 22 and 25). The more important Po Plain to the activity of the Po River delta as major feeder system of the Prograding Complex rapidly advanced southeastward along the foredeep. The well-log data show that the turbiditic succession of foredeep axis, from the central Veneto Plain to its present day posi- these systems is characterized by a well developed cyclicity. This tion in the central Adriatic Sea. The average progradation rate was cyclicity, interpreted as a climatic/eustatic-driven cyclicity, is evaluated, on the basis of the available data, in about 350 km/My, the expressed by the alternation of the thick sand-bodies, probably highest value of the Late Miocene-to-Pleistocene interval (Ghielmi deposited during the lowstand periods related to the Middle-Late et al., 2010a). During its advancement towards the southeast, the Pleistocene glaciations, with typically 5e10 m thick mud interval, axial Po Plain progradation merged progressively with the Apenninic probably deposited during the interglacial highstand periods. progradation from southwest. The southeastward progradation of the Po Plain Complex was also responsible for the rapid reduction of 5. Discussion on the main results of the study and on their the foredeep area during the Middle-Upper Pleistocene. regional implications The foredeep of the PS2 Seq. is located in the same area of the former PS1 foredeep and shows also the same elongate shape. In 5.1. The Po Plain-Adriatic Foredeep the northern Adriatic Sea and in the onshore area a 500/600 m thick succession of Middle Pleistocene turbidites was deposited The present day eastern Po Plain and northern Adriatic Sea with a sedimentation rate of about 1,5 106 mm/My. The very high belong to the largest foredeep system of the Periadriatic Basin: the sedimentation rate of the Middle-Upper Pleistocene turbidites are Po Plain-Adriatic Foredeep (PPAF). The presence in the Po Plain- attributed to the large availability of clastics combined with the northern Adriatic Sea area, of two independent foredeeps, the relative limited extension of the deep-water areas due to the rapid PPAF and the Western Po Plain Foredeep (located westward in Po Plain and Apenninic progradations. The Carola Formation Piemonte and western Lombardia) from the Late Miocene up to the turbidites are conformably overlain by about 1000 m of regressive Early Pleistocene, was demonstrated by the Eni studies on the basis siliciclastic deposits of the Middle-Upper Pleistocene. This thick of both seismic interpretation and facies distribution patterns overall regressive sequence consists, in the lower part, of slope, (Ghielmi et al., 2008b; Minervini et al., 2008). The large structural shelfal, deltaic and coastal deposits of the prograding complexes lineaments subdividing the two foredeep depocenters changed (Ravenna Formation), and, in the upper part, of alternating conti- through time due to the repeated tectonic reshaping of the Po Plain nental, coastal/deltaic and shallow-marine sediments deposited area: the Lachiarella High during Messinian, the central Lombardy during the Middle-Late Pleistocene climatic-driven sea-level fluc- threshold during ZancleanePiacentian, the Ferrara thrust system tuations. In this area the PS2 Seq. reaches a total thickness of 1400/ and the related western lateral ramp during GelasianeCalabrian. 1,500, corresponding to a calculated average sedimentation rate of 1.6 106 mm/My. 5.2. The foredeep depocenter migration and deformation During this period, the slope systems of the Po Plain Prograding Complex bounded the foredeep apex towards the northeast. The The PPAF was affected by severe synsedimentary Apenninic foredeep inner and the outer margins were represented by the toe compressional tectonics. The Eni studies of the area point out that of the slope of the lateral Apenninic progradation and by a low- its geological evolution in the latest MioceneePleistocene time

Figure 27. Middle Pleistocene channels and lobes of the poorly-efﬁcient turbidite systems of the upper part of the PS2 Seq. (Central Adriatic Sea; Allogroup LP, PS2b Medium-Scale Seq.; Carola Formation; see Fig. 26 for location). a) RMS Amplitude map; b) Coherency map (Eni “Adria” 3D Seismic Survey). Paleocurrents are directed to SE (from Mavilla and Ardenghi, 2004).

interval was punctuated by repeated phases of outward migration demonstrated by the detailed analysis of the seismic horizons and shape change of the foredeep depocenters from the former geometry and terminations along the basin margins. In summary, Marnoso-Arenacea basin located in the Emilia-Romagna Apennines the Late Miocene-to-Pleistocene northern Apennine compressive (which was deformed and included in the northern Apennine deformation occurred through a succession of discrete and rela- thrust and fold belt), to the last and more external Gelasian-Upper tively short deformation phases of different magnitudes separated Pleistocene foredeep located in Veneto and the northern Adriatic by relatively longer intervals of quiescence or very low tectonic Sea. These results confirm the validity of the former conceptual activity. The same conclusions are also provided by basin-scale models presented for this area (Ricci Lucchi, 1986; Argnani and studies of Late Miocene, Pliocene and Pleistocene foredeeps of Ricci-Lucchi, 2001), and also introduce a large volume of new the central and Southern Apennines (unpublished or partly pub- detailed data and interpretations about the evolution of this sector lished Eni studies). of the Periadriatic Basin. As demonstrated by the paleogeographic reconstructions based on Four major regional unconformities were generated by the major seismic and well analysis, the PPAF shape was characterized by a large Apenninic deformative events in the studied time interval: the variability ranging from regular elongated shapes (e.g. LP Allogroup Latest Tortonian, the Intra-Messinian, the Intra-Zanclean and the foredeep; Fig. 23) to irregular shapes (e.g. EM Allogroup foredeep; Gelasian Unconformities. The existence and tectonic origin of the Fig. 5). Firstly, during the time the foredeep depocenter migration took first two unconformities were already known in the scientific liter- place through alternating stages of simple (LM and LPAllogroups) and ature due to some papers largely based on subsurface data of the Po fragmented (EM and EP Allogroups) foredeeps. Secondly, the shape Plain area (Gelati et al., 1987; Rossi and Rogledi, 1988; see also Dalla (and the deformation) of such a large foredeep can be different in the et al., 1992, for the correct chronostratigraphic attribution of the different sectors of the basin. This is the case of the EP Allogroup Latest Tortonian Unc.). The presence of two unconformities in the represented by a larger simple foredeep depocenter in the north- Zanclean and Gelasian successions was documented by Patacca et al. eastern sector, by a fragmented foredeep in the central sector, by (1990) in the outcrops of the Apennines. However their character of a simple foredeep in the northern Adriatic area (Figs. 20 and 21). regional unconformities related to two major compressive events, The available data suggest that the migration and the shape of the Intra-Zanclean and Gelasian Phases, responsible for the foredeep the foredeep depocenters were controlled by the Mesozoice migration was demonstrated by the recent Eni studies of the area Tertiary substratum. The presence of a thick and rigid Jurassic (Ghielmi et al., 1998, 2010a; this study). carbonate plateau in the Ferrara area (unpublished Eni data) was The migration of the foredeep depocenter and of the northern responsible, in the Zanclean and Piacentian, for a local reduced Apennine thrust and fold belt towards the foreland was not subsidence and for the deposition in that area of a relatively thin continuous. The Eni studies, based on a large subsurface dataset, succession of turbidites of the Porto Corsini and Porto Garibaldi demonstrate that, during the studied interval, the foredeep depo- Fms.; later, in the Gelasian, it hampered the creation of the new center was involved in four discrete phases of rapid migration foredeep depocenter of the LP allogroup in that area (the new towards the foreland. The available data show an evident tectonic foredeep is located towards east in the Veneto and northern Adri- cyclicity during the Messinian-to-Gelasian time interval with the atic sea). While the local presence of a rigid substratum in the occurrence of a foredeep depocenter migration phase every 1,5e1,8 Mantova Monocline, the Trento Plateau, hampered the propagation My. During the relatively long time intervals between two subse- of the Apenninic thrust and fold belt towards the foreland during quent migration phases, the foredeep and northern Apennine belt the Pliocene and Pleistocene. were affected only by a relatively moderate deformation usually responsible for the reactivation of the already existing thrust- 5.3. The deformation along the foredeep marginal areas: foreland propagation folds. Also this less severe deformation was not and piggy-back basins area continuous but represented by discrete deformation phases responsible for the generation of the tectonic unconformities of the Large sectors of the Adria foreland were gradually affected by the Large, Medium and Small-Scale Sequences of tectonic origin as northern Apennine tectonics as a consequence of the progressive

Please cite this article in press as: Ghielmi, M., et al., Late MioceneeMiddle Pleistocene sequences in the Po Plain e Northern Adriatic Sea (Italy): The stratigraphic record of modification phases affecting a complex foreland basin, Marine and Petroleum Geology (2012), http://dx.doi.org/ 10.1016/j.marpetgeo.2012.11.007 28 M. Ghielmi et al. / Marine and Petroleum Geology xxx (2012) 1e32 migration of the deformation towards the NE. In particular during in the piggy-back basins (due to thrust fronts activation or reac- the major deformative events, those areas were subjected to severe tivation), and also along the foredeep outer margin and in the subsidence and tilted towards the Apennines (Figs. 10 and 12), ramp/foreland (due to the foreland tilting). In the foredeep the probably in relation to the Adria microplate subduction in combi- allogroup boundaries usually correspond to major turbidite facies nation with flexural subsidence related by active thrusting in the changes as evidenced by the well analysis and correlation (Fig. 24). northern Apennine thrust and fold belt. As indicated by clear seismic It was emphasized earlier that: (1) in the piggy-back basins domain evidence, the regional flexuration of the foreland was commonly an allogroup boundary is marked by a relative sea level fall and by responsible for the reactivation of pre-existing structural systems a forestepping of the coastal and marginal marine depositional composed by conjugate high-angle normal faults (Fig.12). The rapid systems; (2) in the ramp and foreland areas, on the contrary, an subsidence and tilting related to the major deformative phases, were allogroup boundary is represented by a major drowning uncon- also responsible for abrupt relative sea-level rises in the foreland formity marked by a rapid landward shift of marginal marine and and foreland ramp domain. Detailed seismic and well correlation coastal systems. An allogroup boundary is therefore characterized supported by biostratigraphic analysis indicated, in fact, that the by opposite relative sea level variations in the inner domain of the Intra-Zanclean and Gelasian Unconformities (corresponding to the piggy-back basins (relative sea-level fall), and in outer ramp and EP and LP allogroup boundaries) can be correlated, in the ramp and foreland area (rapid relative sea level rise), as a consequence of the foreland areas, to regional drowning-platform unconformities. The different local deformation (uplift versus subsidence and tilting). best example is represented by the fast regional transgression The allogroups can be further subdivided into lower rank registered in all the foreland area at the EP allogroup boundary and unconformity bounded units. These component sequences were responsible for the abrupt drowning of the PL1 Lower Pliocene ranked on the basis of their physical scale (Large-, Medium- and Progradation (Figs. 8, 9, 16C, 18, 20). Small-Scale Sequences). During the latest Tortonian-Gelasian On the contrary, in the northern Apennine thrust and fold belt, (interval characterized by an active Apenninic deformation), the the same major tectonic events were responsible for severe uplift Large-Scale Sequences (time span from 0.2/0.3 to 1.3 My) are and tilting of the thrust-top basins. In this domain the allogroup mainly of tectonic origin, as suggested by tectonic-driven angular boundaries are, in fact, marked by abrupt relative sea level falls and unconformities. The subsurface data also suggest that the climatic/ by sharp basinward shifts of the marginal marine depositional eustatic-driven sequences were more frequent during Calabrian, systems, as well interpreted from seismic and well data analysis and dominant, with well-developed 4th/5th order (100 ky) depo- performed in these areas. sitional sequences, during the Middle-Late Pleistocene. This important change is interpreted as due to the Pleistocene strong 5.4. The Pleistocene deformation climatic variations combined with the decrease of the compressive deformation. As shown by the seismic data analysis, the Calabrian-Late The well data also show a well developed small-scale and high- Pleistocene interval was characterized, in all the study area, by: frequency cyclicity affecting all the latest Miocene-to-Pleistocene (1) a reduced activity of the thrust-propagation folds (Figs. 22 and turbidite succession. This cyclicity is typically expressed by the 25); (2) the activation during the Calabrian of only a few new thrust rhythmic alternation of some tens of meter-thick turbidite sand fronts in the frontal part of the thrust-and-fold belt (no new fronts lobe packages and clay intervals (Fig. 24; see also Fig. 22 in Ghielmi formed during the Middle-Late Pleistocene); (3) no new phase of et al., 2010a). This small-scale cyclicity cannot be explained as migration of the deformation towards the foreland (Ghielmi et al., a tectonic-driven cyclicity, and is interpreted as the sedimentary 1998, 2010a; Argnani et al., 2003; this study). These clear events response in foredeep turbidites to the highest frequency Milanko- are interpreted as the consequences of a gradual decrease of the vitch climatic/eustatic cyclicity. In summary, the larger sequence northern Apennine compressional deformation during this stratigraphic units of the PPAF are driven by tectonics, i.e. interval. Due to the combination of the reduced uplift of the thrust Allogroups and Large-Scale Sequences. While at the scale of the fronts and of the moderate to absent subsidence of the depocenter smaller scale sequence stratigraphic units, the climatic/eustatic areas, with the high turbiditic sedimentation rate, the foredeep and cyclicity predominates. piggy-back basin depocenters were gradually filled and most of the structural highs were progressively onlapped by turbidite deposits. 5.6. The foredeep turbidite systems The decrease, during the Pleistocene, of the compressional deformation in the study area was already pointed out by Argnani et al. The latest MioceneePleistocene succession of the PPAF is mainly (1997, 2003). These authors interpreted this major change as represented by thick sequences of turbidites deposited in deep- related to a two-phase evolution of the northern Apennine defor- marine environments. The large availability of subsurface data for mation in the Plio-Pleistocene. During the Pliocene significant the Eni studies (including bottom cores, well logs and seismic data) horizontal motion occurred giving rise to major thrusting and allowed a detailed characterization of these deposits at the scale of folding on the Po Plain-Adriatic side of the chain. The magnitude of the whole basin. The highly-efficient turbidite systems were largely horizontal displacement decreased in time towards the end of dominant in the PPAF and in the largest piggy-back basins in good Gelasian and during Calabrian. The Middle-Upper Pleistocene agreement with the sedimentological model of other Tertiary evolution, on the other hand, indicates a predominance of vertical foredeep turbidite systems of the Northern Apennines, Alps and motion, with uplift and widening of the northern Apennine range. Pyrenees. As demonstrated by well log correlation and seismic interpretation, these large-scale systems are characterized by 5.5. The sequence stratigraphy a remarkable basin-scale tabular geometry, widths up to 50/70 km, lengths usually exceeding 200/250 km and thicknesses of several Four major regional tectonically-induced unconformities were hundreds of meters. They are mainly composed of proximal and recognized in the PPAF succession. The four major stratigraphic distal sand/sandstone lobes, which slowly grade downcurrent into units bounded at base and top by these major unconformities were thinner bedded and finer grained turbidite basin plain deposits. interpreted as allogroups according to their definition in the NACSN According to the model of Mutti et al. (1999), these turbidites are (1983). The allogroup boundaries correspond to evident (in seismic interpreted as the deposits of very large-volume bipartite turbidity data) tectonic-driven angular unconformities in the foredeep and currents that flowed along the axis of the foredeep. The

5.9. The Middle-Late Pleistocene foredeep turbidite systems 5.7. The foredeep feeder systems A new type of foredeep turbidite system developed in the PPAF The Eni sedimentological models, based on well and seismic in the Middle Pleistocene. The interpretation of the “Adria” 3D analysis and correlation, indicate for the first time that huge seismic survey demonstrated that the new systems are extensively volumes of clastics, including gravel, sand and mud, were supplied channelized in the foredeep area with straight, sinuous and by the erosion of the Alps and transferred to the PPAF by major meandering turbidite channels (Fig. 27). The turbidite sand bodies fluvio-deltaic systems, the Paleo-Adda, Paleo-Mincio and Paleo- are complex assemblages of laterally and vertically stacked turbi- Adige Rivers, active along the Southern Alps margins of Lombar- dite channel, channel-levee and sand lobe deposits. These deposits dia and Veneto. Due to the large northeastward extent of the grade downcurrent into distal lobe sands and into fine-grained marine basin, shown by the subsurface data, the paleo-deltas of deposits of turbidite basin plain. The paleocurrents are mainly these large fluvial systems developed close to the Southern Alps parallel to the foredeep axis. These deposits show a well developed thrust and fold belt, probably along the margin of narrow high- cyclicity expressed by regular intercalation of deep-marine clays gradient braid-plains. During the Plio-Pleistocene, these rivers corresponding to episodes of deactivation of the turbidite sand were very probably dominated by catastrophic floods due to their deposition. This cyclicity, not correlatable to sequence boundary of high-gradient alpine drainage areas and braid-plains. The floods tectonic origin, is interpreted as a high-frequency climatic/eustatic- reached the delta area through steep and narrow Alpine fluvial driven cyclicity. This turbidite sedimentation is interpreted as valleys without dissipating their high energy and were able to related to small-volume turbidity currents, if compared with the generate hyperpycnal flows (according to the models of Mutti et al., large-volume turbidity currents of the highly-efficient turbidity 1996, 1999). These flows, after a phase of acceleration and erosion systems, which flowed along the foredeep confined into turbidity along the long and steep submarine foreland ramp, were able to channels. The seismic and well correlations suggest that these reach the foredeep generating the large volume bipartite turbidity turbidite systems are genetically related to the activity of the Po currents of the PPAF turbidite systems. River delta as the major feeder system of the foredeep. These The available data suggest a subordinate clastic supply to the systems of the PPAF are still poorly understood and further detailed foredeep from the northern Apennine belt. Moreover, the recogni- analysis is needed. tion in the seismic interpretation of clear erosional surfaces on some of the thrust fronts along the foredeep margin, suggest that the 5.10. The Po Plain progradation complex clastic supply from the inner foredeep margin was mainly provided by local submarine erosion of recent unconsolidated deposits from In the study area and also in the central Adriatic Sea, the Pleis- active structural fronts. Two different factors were probably tocene was characterized by the development of basin-scale pro- responsible for the moderate Apenninic clastic supply: a) the rela- grading complexes consisting of slope, shelfal, deltaic and coastal tively moderate subaerial expression of the northern Apennines up deposits. New detailed information about the direction of pro- to the ZancleanePiacentian were probably responsible for a reduced gradation, age and progradation rate of the Plio-Pleistocene pro- clastic supply; b) the presence of a system of piggy-back basins along grading complexes were provided by the seismic analysis carried the northern Apennine margin which trapped the clastic inputs out on the entire Po Plain and northern and central Adriatic Sea from the fluvio-deltaic systems of the northern Apennine. through the Eni studies. The Po Plain Prograding Complex rapidly advanced towards the SE along the foredeep axis from western 5.8. The Messinian salinity crisis Lombardia as far as its present day position in the central Adriatic Sea just SE of Ancona (Figs. 7 and 22 in Ghielmi et al., 2010). During The tectonic deformation also interplayed with the events its development, it progressively merged with the Apenninic and which determined the Messinian Salinity Crisis. Stratigraphic Alpine progradations from the inner and outer foredeep margins relationships were recalculated, on the basis of well data, after the respectively, as clearly indicated by the seismic interpretation decompaction of coastal wedges in the different Messinian (Fig. 25). These prograding complexes, up to several hundred sequences. This led to a broad estimation of the magnitude of the meters thick and mainly consisting of slope silty clay, were

Please cite this article in press as: Ghielmi, M., et al., Late MioceneeMiddle Pleistocene sequences in the Po Plain e Northern Adriatic Sea (Italy): The stratigraphic record of modification phases affecting a complex foreland basin, Marine and Petroleum Geology (2012), http://dx.doi.org/ 10.1016/j.marpetgeo.2012.11.007 30 M. Ghielmi et al. / Marine and Petroleum Geology xxx (2012) 1e32 responsible for the final complete infilling of the WPPF during the extensive Eni seismic and well database, whose analysis and GelasianeCalabrian (Minervini et al., 2008) and later of the PPAF interpretation provided an unusual large volume of qualitative and during the Calabrian-Late Pleistocene. This basin-scale major quantitative information. For these reasons, in the authors’ opinion, regressive cycle was obviously a consequence of the well-known the geological model of the Po PlaineAdriatic Foredeep (PPAF) may Early Pleistocene climatic changes and of the Middle-Late Pleisto- be used as a reference model in the analysis of tectonically active cene glaciations (long periods of sea level lowstand and large foredeep basins dominated by basin-scale sand-rich highly- availability of siliciclastic sediments). However, the subcrop data efficient turbidite systems. show evidence of an acceleration of the progradation from the The sequence-stratigraphic analysis was based on the recogni- basin margins during the upper Gelasianebasal Calabrian roughly tion of allogroups (NACSN, 1983), i.e. major stratigraphic units correspondig to the beginning of the compressive deformation bounded at base and top by major regional tectonically induced decrease in the PPAF area. Therefore also the decrease of new unconformities; and of their component sequences, mainly of accommodation space creation probably played a major role for the tectonic origin, ranked on the basis of their physical scale (Large/ development of the Pleistocene basin-scale progradations. Medium/Small-Scale Sequences). This approach, extensively applied by Eni geologists for the analysis of the Miocene-to- 5.11. The foredeep evolution Pleistocene buried depocenters of the Apennine Foreland Basin, is considered by the authors as a stratigraphic methodology useful for The major Apenninic deformative events were responsible for the analysis of tectonically active basins in compressional settings. the formation of new foredeep depocenters. As documented by the A system of large and elongate foredeeps developed in the subsurface data, the newly formed foredeeps were typically narrow northern Adriatic Sea and in the central-eastern part of the Po Plain and elongated in shape, and bounded by steep margins (the fore- during the Late MioceneePleistocene interval. This system, in the land ramp and by active thrust-fronts on the outer and inner Eni studies referred to as Po Plain-Adriatic Foredeep (PPAF), is the margins respectively). During their sedimentary infilling the fore- largest Late Miocene-to-Pleistocene foredeep complex of the Peri- deeps increased their lateral extent, while the water depth gradu- adriatic Basin with an overall length of 500 km and a width of 80e ally decreased. 120 km. It is bounded at the outer margin by the Adriatic foreland On the basis of the PPAF geological model, and of evidence from ramp and at the inner margin by the northern Apennine thrust and other foredeeps of the central and southern Apennines (Messinian fold belt. Laga, Plio-Pleistocene Bradanic and Pleistocene Hyblean Foredeeps, As a consequence of the Messinian-to-Gelasian severe northern mainly unpublished Eni models), a new model with two evolutive Apennine compressive tectonics, the PPAF underwent four regional stages is proposed for the evolution of a foredeep depocenter. After phases of compressional deformation and depocenter migration its formation, during the first stage, the new foredeep is usually towards the foreland to the northeast. During these tectonic phases, characterized by: (1) a relatively narrow and elongated shape four major basin-scale tectonically induced unconformities were (commonly 30e40 km wide in the PPAF); (2) higher water depth generated: the Latest Tortonian, the Intra-Messinian, the Intra- (probably 1500/2000 m of depth); (3) the maximum efficiency of Zanclean and the Gelasian Unconformities. The occurrence of foredeep turbidite systems (ponding of the turbidity currents may a foredeep depocenter migration phase every 1,5e1,8 My during also occur); (4) higher sedimentation rate (usually exceeding the studied interval, suggests the existence of a tectonic cyclicity 1.5 106 mm/My in the PPAF); (5) an outer foredeep margin rep- controlling the large-scale stratigraphic organization of the basin resented by a relative high-angle foreland ramp; often the foredeep fill. turbidites lap onto the ramp without lateral facies variation; (6) During the PPAF evolution the foredeep shape was affected by condensed sedimentation or no depositional hiatus in the foreland a large variation, ranging from regular and elongated to irregular, ramp and outer foreland areas. from simple foredeep to fragmented foredeep. Differences of shape The second stage (or “stage of maturity”) is usually character- (and structural deformation) have sometimes affected different ized by: (1) a wider foredeep area (50e60 up to 75 km wide in the sectors of the same foredeep. PPAF) due to the gradual sedimentary infilling of the foredeep; (2) The presence of a rigid substratum in the foreland locally lower water depths (probably 1000e1500 m of depth or lower in hampered the foredeep depocenter migration and/or the propa- the final stage of infilling); (3) lower sedimentation rates (usually gation of the northern Apennine thrust and fold belt towards the about 1 106 mm/My in the PPAF); (4) a possible decrease in foreland. efficiency of the turbidite systems (indicated by a basin-scale A new evolutive model for the Apennine foredeeps with two backstepping of the turbidite sand lobes); (5) an outer foredeep evolutive stages, “active” foredeep and “mature” foredeep, is margin represented by a low angle foreland area (about 2 in proposed in this paper. northern Adriatic Sea); along this foredeep margin, lateral facies The latest Tortonianeto-Calabrian sand-rich highly-efficient changes (“lateral fringing”) of the turbidites commonly occur; (6) turbidite systems of the PPAF are characterized by very large the development of major regressive cycles in the foreland area. dimensions: usually over 200e250 km long, 50e70 km wide, several hundreds of meters thick. These systems are almost entirely 6. Conclusions composed of basin-scale sheet-like sand/sandstone lobes, mainly consisting of thick-bedded massive sand/sandstone, passing The Late Miocene-to-Pleistocene tectono-sedimentary evolu- downcurrent into turbidite basin plain deposits made up of alter- tion of the eastern Po Plain and northern Adriatic Sea areas pre- nating mud/mudstone and thin-bedded fine-grained laminated sented in this paper, represents the synthesis of several Eni sand/sandstone and silt/siltstone. The palaeocurrents are to SE, multidisciplinary studies carried out in the study area over the last parallel to the foredeep main axis. Turbidite systems fed by lateral two decades. The large availability of subsurface data from the vast entry-points are also present in the PPAF. They mostly consist of Eni database, the preservation and the relatively moderate struc- thick-bedded coarse-grained channel-lobe transition deposits tural deformation of the succession were essential factors for the rapidly grading downcurrent into thick-bedded sand/sandstone generation of a new detailed geological model for the Late lobe deposits. Mioceneeto-Pleistocene foredeeps and the related ramp/foreland The PPAF siliciclastic sediment supply was provided by the and thrust-top basins areas. The model is strictly based on the major fluvio-deltaic systems of the Paleo-Adda, Paleo-Mincio and

Paleo-Adige Rivers located along the Alps margins of the Lombardia Bettazzoli, P., Visentin, C., 1997. L’Avanfossa della Porto Garibaldi: studio sed- ’ and Veneto foreland. With the partial exception of the post- imentologico e geominerario degli onlap sull avampaese. Eni-Agip Internal Report, San Donato Milanese (Italy). evaporitic Messinian, the contribution of clastics to the foredeep Boccaletti, M., Elter, P., Guazzone, G., 1971. Plate tectonics models for the develop- from the northern Apennine belt was subordinate. ment of the western Alps and northern Apennines. Nature 234, 108e111. The dramatic changes undergone by the Messinian depositional Boccaletti, M., Calamita, F., Deiana, G., Gelati, R., Massari, F., Moratti, G., Ricci Lucchi, F., 1990. Migrating foredeep-thrust belt system in the Northern Apen- systems were not associated with a unique, dramatic lowering nines and Southern Alps. Paleogeogr. Paleoclim. Paleoecol. 77, 3e14. related to the Messinian Salinity Crisis; rather, their evolution was Boni, A., 1967. Note Illustrative alla Carta Geologica d’Italia, F. 59 Pavia. Servizio modulated by a combination of factors like climatic changes, Geologico d’Italia. fl Casati, P., Bertozzi, P., Cita, M.B., Longinelli, A., Damiani, V., 1976. Stratigraphy and morphostructural reshaping, isostatic rebound and sediment ux. paleoenvironment of the Messinian “Colombacci” formation in the periadriatic Stratigraphic relationships among decompacted Messinian trough; a pilot study. In: Catalano, R., Ruggieri, G., Sprovieri, R. (Eds.), Messinian coastal wedges suggest that the total lowering of the Messinian Evaporites in the Mediterranean. Memorie della Società Geologica Italiana 16, pp. 173e194. Salinity Crisis relative base-level apparently did not exceed 900 m Casnedi, R., Ghielmi, M., Rossi, M., Cazzola, L., Serafini, G., 2006. Geometrical in the study area. This total drop was reached through three distinct analysis and seismic modelling of an outer foredeep margin in the lower relative base-level falls probably related to: evaporation (ME2 Seq.), Messinian Laga turbidite complex (Central Apennines, Abruzzo, Italy). Boll. Soc. e differential subsidence (ME3 Seq.) and regional uplift possibly Geol. It. 125, 203 220. Castellarin, A., Vai, G.B., 1986. Southalpine versus Po Plain Apenninic arcs. In: Origin associated to isostatic rebound (ME4 Seq.). The Early Pliocene of Arcs. Development in Geotectonics, 21, pp. 253e280. relative sea-level rise has been estimated on the order of 800e Castellarin, A., Eva, C., Giglia, G., Vai, G.B., 1985. Analisi strutturale del Fronte e e 900 m, with a shoreline landward shift in the foreland of at least Appenninico Padano. Giorn. Geol. Bologna (Italy), Serie 3 47 (1 2), 47 76. Cita, M.B., Wright, R.C., Ryan, W.B.F., Longinelli, A. (Eds.), 1975. Messinian Paleon- 70 km. It appears therefore that the Messinian Salinity Crisis was vironments. Initial Reports of the DSDP, vol. 42. U.S. Government, Washington, recorded in the study area by an overall strongly asymmetric cycle pp. 1003e1035. ’ where discrete events of various origin punctuated a relatively Cremonini, G., D Onofrio, S., Francavilla, F., Marabini, S., Ricci Lucchi, F., Ruggeri, G., 1982. Lo “spungone” del Pliocene romagnolo. In: Cremonini, G., Ricci Lucchi, F. gradual base-level fall. (Eds.), Guida alla Geologia del margine appenninico padano: Guide Geologiche In the foredeep domains, during the whole Messinian Salinity Regionali. Società Geologica Italiana, Bologna (Italy), pp. 171e180. Crisis, relative foredeep depocenters persisted, recorded by rela- Dalla, S., Rossi, M., Orlando, M., Visentin, C., Gelati, R., Gnaccolini, M., Papani, G., Belli, A., Biffi, U., Catrullo, D., 1992. 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