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Int J Earth Sci (Geol Rundsch)DOI 10.1007/s00531-014-0998-5

ORIGInal PaPER

Sedimentation in the Northern Apennines–Corsica tectonic knot (Northern Tyrrhenian Sea, Central Mediterranean): offshore drilling data from the Elba–Pianosa Ridge

Gianluca Cornamusini · Vincenzo Pascucci

Received: 4 april 2013 / accepted: 29 December 2013 © Springer-Verlag Berlin Heidelberg 2014

Keywords Stratigraphy · Petrography · Turbidite system · Geodynamic setting · northern Tyrrhenian Sea

Introduction

The Corsica–northern Tyrrhenian Sea–Inner northern apennines system is an important subject in the Central-Western Mediterranean, due to its peculiar tectonic and depositional history, recording evidence of the transition from orogenic to post-orogenic conditions. The structural complexity of this system, which includes two orogenic segments, the alps and the apennines, is well known (Prin-cipi and Treves 1984; Dewey et al. 1989; Carmignani et al. 1995, 2004; Jolivet et al. 1998; Mauffret et al. 1999; Brunet et al. 2000; Storti 2005; Schettino and Turco 2006; Molli 2008; Marroni et al. 2010; Molli and Malavieille 2011; argnani 2009, 2012; Carminati and Doglioni 2012; Turco et al. 2012). In contrast, the depositional/stratigraphic framework of the northern Tyrrhenian Sea is not well con-strained because interpretations are based mainly on seis-mics only (Gabin 1972; Bartole et al. 1991; Zitellini et al. 1986; Bartole 1995; Mauffret et al. 1999; Pascucci et al. 1999; Cornamusini et al. 2002; Contrucci et al. 2005).

In this paper, we present detailed sedimentologic and stratigraphical data from the study of two successions span-ning the Eocene to Early Miocene, explored through deep wells (Martina 1 and Mimosa 1, by agip Spa) along the offshore Elba–Pianosa Ridge (EPR) (Fig. 1).

These new results allow us to: (1) hypothesize possible scenarios for the definition of the depositional environ-ments and their evolution as occurred from the Eocene to the early Miocene; (2) compare, discuss and eventu-ally suggest a correlation at regional scale of the proposed stratigraphic units; (3) hypothesize potential detrital source

Abstract The northern Tyrrhenian Sea is located on the collisional zone between the alpine Corsica and the north-ern apennines and is a key area for gaining a better under-standing of the complex relationships between these two systems. The knowledge of the wide offshore part of this zone, located between Corsica (France) and mainland Italy, is based primarily on the analysis of several seismic pro-files tied to the outcropping geology and unpublished pre-liminary reports of few offshore wells. The here presented study of two offshore wells provides a revision of the sedi-mentology, biostratigraphy and petrography of the thick, mainly siliciclastic, Tertiary successions (about 3,600 m) composing the Elba–Pianosa Ridge (EPR), a structural/morphological high separating the Tuscan Shelf to the east from the Corsica Basin to the west. a comparison with similar deposits cropping out in the surrounding onshore areas (northern apennines, Corsica, Tuscan archipelago, Piedmont Tertiary Basin) provides additional constraints for refinement of the complex geodynamic and regional setting in which the EPR evolved.

Electronic supplementary material The online version of this article (doi:10.1007/s00531-014-0998-5) contains supplementary material, which is available to authorized users.

G. Cornamusini (*) Dipartimento di Scienze Fisiche, della Terra e dell’ambiente, Università di Siena, Siena, Italye-mail: cornamusini@unisi.it

G. Cornamusini Centro di Geotecnologie, Università di Siena, S. Giovanni V.no, Italy

V. Pascucci Dipartimento di Scienze della natura e del Territorio, Università di Sassari, Sassari, Italy

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areas; and (4) discuss data and interpretations in a basin evolutionary framework for the Corsica–northern apen-nines system during the Eocene–Early Miocene interval.

Geological setting of the Northern Tyrrhenian Sea within the framework of the Alps–Northern Apennines junction

Most authors (e.g., Boccaletti et al. 1980; Principi and Treves 1984; alvarez 1991; Jolivet et al. 1998; Carmig-nani et al. 1995; Molli 2008; Carminati and Doglioni 2012)

conclude that the northern apennines chain developed since the late Eocene as a consequence of the closure of the Tethys Ocean and the following collision. Other authors suggest the development of such a chain as a consequence of a transpressional/transtensional phase (Turco et al. 2012; argnani 2012) involving the adria microplate (“promon-tory” of the africa plate) and the Corsica–Sardinia Massif (part of the Iberia Plate, see in Frisch 1979; Stampfli et al. 1998; Handy et al. 2010; Schettino and Turco 2011).

During the Miocene–Pleistocene time interval, after the counterclockwise rotation of the Corsica–Sardinia Massif and the opening of the liguro–Provençal Basin (Vigliotti

Fig. 1 Geological sketch map of the Western alps–Corsica–north-ern Tyrrhenian Sea–northern apennines. The geological domains are (framed by gray dashed lines): a Corsica Island; b northern Tyrrhenian Sea; c Inner northern apennines; d Piedmont Tertiary Basin/Epiligurian Basins. The geological units are for the Western alps: 1 Upper Penninic Helminthoid Flysch units; 2 Schistes lus-trés nappes; 3 Middle Penninic Brianconnaise nappes; 4 Middle Pen-ninic basement nappes; 5 alpine foreland units; 6 external massifs; Corsica: 7 alpine non-metamorphic nappes; 8 metamorphic Schistes lustrés nappes; 9 Hercynian basement; northern apennines: 10 inter-nal ligurian units (Il); 11 external ligurian units (El) including the antola nappe; 12 Sub-ligurian units; 13 Tuscan nappe; 14 Tuscan metamorphic units; 15 Cervarola foredeep unit; 16 Umbria–Marche foreland unit; 17 Piedmont Tertiary Basin and Epiligurian units; 18 neogene-Quaternary post-collisional deposits of inner Tuscany, east-

ern Corsica and Po Plain; 19 neogene-Quaternary magmatic rocks of Southern Tuscany, a- volcanites, b- granites; 20 major onshore thrusts; 21 high-angle normal faults; 22 transcurrent faults; 23 sedi-ment thickness in seconds TWTt for the northern Tyrrhenian Sea; 24 Pliocene isobaths (in km) in the Po Plain and adriatic Sea; 25 loca-tions of the cited lithostratigraphic units and wells: 1 Martina 1 well; 2 Mimosa 1 well; A Macigno costiero Fm.; B Manciano Sandstones; C Ranzano Fm.; D loiano Sandstones; E aveto Sandstones; F Mt. Senario Sandstones; G Solaro and Prunelli flysch; H Palasca and lozari flysch; L Colle Reciso Fm.; M Marina del Marchese Fm.; N aghione Fm.; V Mortara volcanic center. Main tectonic lineaments: SVL Sestri–Voltaggio line, VVL Villalvernia–Varzi line, VEL Val Secchia line, SOF Solenzara Fault, LSL livorno–Sillaro line, PFL Piombino–Faenza line, AML arbia-Val Marecchia line, ALL albe-gna line (modified from Molli 2008)

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and Kent 1990; Gueguen et al. 1998; Speranza et al. 2002; Gattacceca et al. 2007), the back-arc rifting of the Tyrrhe-nian Sea developed (Boccaletti et al. 1971; Scandone 1979; Kastens et al. 1988; Dewey et al. 1989; Faccenna et al. 1997; Doglioni et al. 1998; argnani 2009).

To better understand the geodynamic puzzle character-izing the so-called Western alps–northern apennines junc-tion area (Molli et al. 2010 with references therein), it is nec-essary to briefly introduce four main geological domains: the Corsica Island, the inner northern apennines, the Pied-mont Tertiary Basin/Emilian Epiligurian Basins (PTB/EEB) and finally the northern Tyrrhenian Sea (Fig. 1).

The Corsica Island is part of the Corsica–Sardinia Mas-sif (alpine foreland of the European/Iberian Plate) and is normally subdivided into alpine and Hercynian domains (Fig. 1) (Molli 2008 with references therein). The alpine Corsica is formed by west-verging orogenic stacked nap-pes composed of high pressure metamorphic oceanic rocks, non-metamorphic ophiolitic units, continental crystalline slices and Paleogene flysch (Durand-Delga 1978, 1986; Malavieille et al. 1998; Marroni and Pandolfi 2003 with references therein) overthrusting the Hercynian basement. nappes are unconformably overlain by Miocene up to Plio-cene shallow marine and continental deposits, cropping out mainly along the eastern side of Corsica (aleria Plain) (loÿe-Pilot and Magné 1989; Bossio et al. 2000).

The inner northern apennines are the result of com-plex orogenic stacking processes, which created a stacked units succession made of (from the base): (1) Tuscan low-grade metamorphic Carboniferous–Triassic units lying on a Paleozoic basement; (2) Triassic-lowermost Miocene car-bonate–shale–sandstone sedimentary unit (Tuscan nappe), belonging to the adria Plate continental margin; (3) Eocene–Oligocene transitional oceanic–continental Sub-ligurian Unit; (4) Middle Jurassic–Middle Eocene ligurian units derived from the ligurian Oceanic Domain and made of ophiolitic complexes and their sedimentary cover (Car-mignani et al. 2001, 2013 with references therein). These stacked units are unconformably overlain by Middle Mio-cene to Pleistocene sequences belonging to the post-colli-sional sedimentary cycle (Bossio et al. 1998; Cornamusini et al. 2011 with references therein).

The PTB is located onshore north of the northern Tyr-rhenian Sea (Fig. 1) where it seals the crustal “indentation” and junction between the apennines and the alps (Mutti et al. 1995; Barbieri et al. 2003; Carrapa et al. 2004). The PTB and the close and lateral EEB thrust-top basins are filled by marly-sandstone–limestone sequences, ranging from Middle Eocene up to Pliocene and characterized by sharp lateral sedimentary facies changes, mostly due to the synsedimentary activity of transversal tectonic lineaments (Castellarin 1994; Martelli et al. 1998; Di Giulio et al. 2002; Cibin et al. 2001, 2003).

The northern Tyrrhenian Sea is located at the center of the alpine Corsica–northern apennines knot. It is lon-gitudinally subdivided by the EPR, an important morpho-logical and structural high separating the Corsica Basin to the west from the Tuscan Shelf to the east (Figs. 1, 2). The EPR surfaces at Pianosa, western Elba, Scoglio d’affrica, Capraia and Montecristo islands (Fig. 2). These last two islands are formed of only neogene magmatic rocks. The west-central part of Elba Island consists of Upper Mio-cene granitoids (8.2–6.2 Ma) and subordinately of ligurian Jurassic metaophiolites, Upper Cretaceous sandy flysch (Elba Flysch), Middle Eocene shaly-arenaceous–calcareous flysch with ophiolitic breccias interlayered (Colle Reciso Formation, of Bortolotti et al. 2001) and Tuscan units (Coli et al. 2001). The Pianosa Island consists of lower Miocene (Burdigalian) to Pleistocene shallow marine to continen-tal deposits (Marina del Marchese, Golfo della Botte and Pianosa formations; Colantoni and Borsetti 1973; Bossio et al. 2000). The southernmost part of the EPR is charac-terized by Jurassic (Hettangian) shallow marine limestones (Calcare Massiccio Formation) belonging to the Tuscan nappe (Scoglio d’affrica Islet, Fig. 2) and by Upper Cre-taceous Helminthoid Flysch belonging to the ligurian units dredged during oceanic cruises (Bigi et al. 1989).

The Corsica Basin is filled with about 8.5 km of Eocene to present sedimentary deposits (Mauffret and Contrucci 1999; Mauffret et al. 1999). The basin has developed as an extensional basin since the early Miocene, with the mas-ter fault located on its western side (Gabin 1972; Mauffret et al. 1999).

The Tuscan Shelf is subdivided by magmatic intrusions and the metamorphic ridge of Elba Island, into a northern and a southern part (Pascucci et al. 1999), and is charac-terized by several small neogene sedimentary basins, each separated by structural ridges composed of Mesozoic or Tertiary rocks (Fig. 2). Such small basins started to form during late Serravallian–Messinian time following the extensional tectonics that affected the hinterland of the northern apennines (Pascucci et al. 1999).

Materials and methods

The studied wells (Fig. 3) are located on the eastern mar-gin of the Corsica Basin, south of Pianosa Island (Martina 1) and west-southwest of Montecristo Island (Mimosa 1) (Fig. 2). They reach, respectively, 3,183 m below sea floor, 80 m of sea depth and 3,606 m below sea floor, 188 m of sea depth.

The data set available in the past consisted of synthetic stratigraphies of the Martina 1 and Mimosa 1 wells and seismic lines shot by aGIP in the 1970s, 1980s and for the CROP Project (deep crust investigation) in the 1990s (e.g.,

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Finetti et al. 2001). The stratigraphy of the two wells has been here revised through new lithological, compositional and biostratigraphical investigations.

The lithological–compositional and grain-size charac-terizations have been performed through petrographical (qualitative and semiquantitative) analyses of about 400 samples obtained from cuttings of the complete Martina 1 well succession. The composition, occurrence/distribu-tion and frequency of mineralogical–lithological markers were defined (Fig. 4), and using the lithofacies distribu-tion along the successions, grain-size vertical trends were defined (Fig. 4). Sedimentological and modal petrographi-cal analyses on the sandstone fraction have also been car-ried out on 5 cores (four from Martina 1 and one from Mimosa 1), each 7–9 m thick, from different stratigraphic

levels (Fig. 4). Seventeen medium-coarse-grained sand core samples have been point-counted using the Gazzi–Dickinson method (Ingersoll et al. 1984 with references therein), for the total framework and the main (QFl + CE) compositions (Dickinson 1970; Zuffa 1980, 1991) and for the fine-grained lithic composition (Zuffa 1991; Cibin et al. 2001, 2003). Point-counting data and the related pet-rologic parameters have been used to compare the studied sediments with other clastic units cropping out in the sur-rounding regions.

The biostratigraphic framework proposed here is based on the revised agip foraminiferal, nannofossil and pol-len industrial unpublished reports (aGIP 1975, 1980, 1983) integrated with new nannofossil analyses. The new nannofossil analyses have been performed with a

Fig. 2 Structural sketch map of the northern Tyrrhenian area show-ing the inferred distribution of the Eocene, Oligocene and Miocene deposits drilled by the Martina 1 and Mimosa 1 wells. Main thrusts, faults and transversal lines for the offshore area are hypothetical. Data from Wezel (1982), Bigi et al. (1989), Bartole (1995), Pascucci

et al. (1999), Mauffret et al. (1999) and Cornamusini et al. (2002) used to construct hypothesized offshore geology. numbers close to magmatic rocks indicate the age in Ma (lustrino et al. 2011 and ref-erences therein)

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semiquantitative modality on samples from both cut-tings and cores. Calcareous nannofossil biozone schemes are those proposed by Martini (1971), Okada and Bukry (1980), Fornaciari and Rio (1996) and Catanzariti et al. (1997), whereas the distribution tables of the most sig-nificant taxa of the northern apennines deposits are from Perch-nielsen (1985), Catanzariti et al. (2002) and Cerrina Feroni et al. (2010).

Stratigraphy and depositional environment

Previous stratigraphic data

Previous studies correlated the drilled Eocene and Oligo-cene successions with, respectively, the ligurian Units and the Epiligurian Units of the northern apennines (Zitellini et al. 1986; Bartole 1990, 1995; Bartole et al. 1991; Sartori and Capozzi 1998). The Gruppo Bacini Sedimentari (1980) interpreted the entire Eocene–Oligocene interval as a single sequence unconformably overlying a calcschist basement of alpine affinity.

In Online Resource 1, biostratigraphic and paleoenvi-ronmental data from agip analyses (aGIP 1975, 1980, 1983) are reported.

Three unconformities (X, A and D) were distinguished from the projection of log data on seismic lines (Corna-musini et al. 2002) and were dated using biostratigraphic data (Figs. 3, 5). accordingly, the successions have been grouped in four unconformity bounded units (from the bot-tom): Sub1, Sub2, Lit 0 and Lit7. Seismic profiles crossing the Corsica Basin clearly show how these deposits thicken in the depocenter of the basin and thin toward the eastern margin of the basin (EPR) (Mauffret and Contrucci 1999; Mauffret et al. 1999; Pascucci 2005).

Unit Sub1 (Early–Middle Eocene)

Unit Sub1 has been drilled by both Martina 1 and Mimosa 1 wells (respectively, 1,750 and 1,600 m thick siliciclastic deposits) (Figs. 3, 4).

In the Martina 1 well, the lower part (from 2,880 m to the bottom) of the Unit Sub1 is composed mainly of silt-stones, shales and fine to medium-grained siliciclastic

a b

Fig. 3 Stratigraphic diagrams: a synthetic stratigraphy of the two wells Mimosa 1 and Martina 1, showing the unconformity bounded units. Main seismic unconformities (after Cornamusini et al. 2002): X at the base of the Unit Sub2; a at the base of the Unit lit0; D at the base of the Unit lit7. For explanations about depths, see the “Materi-

als and methods” Chapter; b chrono-biostratigraphic table of the two wells using nannofossil biozones. These are after Martini (1971) (nP biozones) and Fornaciari Rio (1996), Catanzariti et al. (1997) (MnP biozones)

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sandstones (Fig. 4). In particular, cores 3 and 4 show gray–green shales, bioturbated siltstones and very fine and thin sandstone beds, with plane-parallel or small-scale cross-lamination. The upper part of Unit Sub1 of the Martina 1 well (from 1,590 to 2,880 m) is represented mainly by coarse to fine sandstone, occasionally microrudite, with

minor mudstone beds. Sandstones are organized in graded or laminated dm thick beds, with plane-parallel or rip-ple cross-lamination, mud clasts and fluid escape struc-tures. In the central-lower portion of this upper part (from 2,100 to 2,900 m in Fig. 4), a thick coarse-grained silici-clastic sandstone body occurs. Details observed in core 2 (Online Resource 2) show the alternation of coarse sand-stone, fine sandstone/mudstone and silty marl beds. The latter are for the most part, bioturbated. Mudstones are rich in coarse (>2 mm) volcanic (acid-rhyolite to intermediate/basic-andesite) and fossil grains. The large size of the vol-canic grains with respect to the fine hosting sediments, as well as the unaltered appearance and single euhedral crys-tals (quartz and plagioclase) allow us to classify them as neovolcanic grains (V3 or V2 type of Zuffa 1987, 1991; Fig. 6). This classification is strengthened by the low amount of detrital volcanic lithic grains contained in the sandstones (see next chapter). Between 1,990 and 2,200 m,

Fig. 5 a line drawing relative to the seismic profile T12, with the Martina 1 well. X = bottom of Oligocene deposits (Sub2); A = bot-tom of upper Burdigalian deposits (lit0); B = bottom of upper Tortonian-lower Messinian deposits (Tir2); C = bottom of Pliocene

deposits (Tir4); D = bottom of Pleistocene deposits (lit7) (after Cor-namusini et al. 2002). location is shown in Fig. 2; b uninterpreted migrated T12 seismic profile

Fig. 4 Stratigraphic columns of the Mimosa 1 and Martina 1 sedi-mentary successions. The panel of the cuttings shows the distribu-tions of the marker granules or fragments as recognized from the cutting analysis (ser, serpentinites; bo, ophiolitic breccias; gab, gab-bros; Va, acid volcanites; Vi, intermediate volcanites; Vb, basic vol-canites). Core ubications and bed attitudes obtained through dipme-ter measurements are also shown. Bed stacking patterns with CU and FU upward trends are shown. The well depths include the hole below the sea floor (total drilling sediment thickness), the water depth and the drilling rotary table above sea level. They are, respectively, 3,182,5 m, 80,5 m and 33 m for the Martina 1 and 3,606, 188 and 32 m for the Mimosa 1

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coarse gravel-size clasts (Fig. 6) of ophiolite rocks (serpen-tinites, ophiolithic breccias, and minor gabbros) are widely dispersed in the sediments. The abundance of the ophiolitic clasts increases upward.

The Unit Sub1 in the Mimosa 1 well is composed of silt-stones and shales passing upward to medium-fine-grained sandstones (Fig. 4). Similar to the Martina 1 well, ophiolitic gravel-size clasts occur at about 3,000–3,050 m (Fig. 4). at about 2,420 m, a repetition of the succession interpreted as a thrust effect (Cornamusini et al. 2002) is recorded. The portion of the Unit Sub1 placed at the hanging wall of the thrust (Fig. 4) mainly consists of chaotic shales overlain by siltstones.

Paleoenvironmental significance

The sedimentological features of the lower part of the Unit Sub1 observed in Martina 1 cores allow interpretation of the fine deposits as heterolithic mudstone/sandstone facies (sensu Stow and Bowen 1980). The deposits are due to dilute flow turbidites or hyperpycnal flows (Mulder et al. 2003; Soyinka and Slatt 2008).

The upper part of the Unit Sub1 Martina 1, as revealed by core 2, shows sedimentary features typical of turbidite massive flows with densities ranging from high (gravelly flow) to low (dilute flows) (Mutti 1992; Mutti et al. 1999). Other sedimentary structures can be interpreted as formed by hyperpycnal flows (sandy hyperpycnites of Soyinka and Slatt 2008). This is because the occurrence of cm thick alternation of microrudites and medium-coarse sandstones with inverse then normal graded beds without erosional bases is indicative of waxing then waning flows behavior (Mulder et al. 2003). Moreover, the thick sandstone hori-zon characterizing the middle–upper part of the succession of Martina 1, topped by shale–siltstone lithofacies, shows

sedimentological and stacking pattern features interpretable as progradation and than retrogradation-deactivation of a turbidite lobe fan system.

The foraminifera contents for the unit documented in agip reports (aGIP 1975, 1980, see the Online Resource 1) agree with the interpretation of a shallowing upward trend from lower bathyal to outer neritic environment.

The coarsening upward trend of the Unit Sub1 of Mimosa 1 well could be related to a progradation of a distal turbidite system (Mutti and normark 1987).

The occurrence of ophiolites found in the Unit Sub1 (both wells) can be interpreted as mass-slide blocks from basinal highs or slopes. Moreover, the occurrence of neo-volcanic grains in the Middle Eocene mudstone beds is indicative of volcanic activity in the peribasinal areas (Zuffa 1991; Critelli and Ingersoll 1995).

Biostratigraphy

nannofossil analysis conducted on Martina 1 core 4 (Fig. 4) reveals a very poor association (list of taxa in Online Resource 3) referable to the early Eocene. Sam-ples collected from the core 3 (Fig. 4) are barren or show a poor nannofossil association (Online Resource 3) that is associated with the nP12–13 Zones of the Martini (1971) standard zonation. These data indicate that the rocks were deposited during the middle part of the Ypresian and are in agreement with age data coming from foraminifera analy-ses performed by agip (see Online Resource 1).

The nannofossil analysis on core 2 (2,200 m) reveals an association (Online Resource 3) characteristic of the nP16–nP17 interval of the Martini (1971) standard zonation (with lower–middle Eocene reworked taxa) relative to the Bartonian. These data agree with aGIP (1975) foraminifera determinations (Online Resource 1) that indicate the por-tion comprises between 2,300 and 2,900 m from the mid-dle part of the Middle Eocene, whereas the upward portion between 1,600 and 2,300 m is from the upper part of the Middle Eocene. Palynological data by aGIP (1980) for this interval are consistent with a Middle-? late Eocene age.

The lower part of the Unit Sub1 in the Mimosa 1 well (about 3,500 m depth) is characterized by a poor nan-nofossils association (Online Resource 3), with markers Chiasmolithus grandis, Sphenolithus radians, Tribrachia-tus orthostylus, and abundant Cretaceous and Paleocene reworked forms. It is associated with the interval nP11–nP12 of Martini (1971) standard zonation and is lower–middle Ypresian in age.

no significant nannofossil associations occur for the upper part of the unit (2,600–2,750 m, placed at the footwall of the thrust fault of Fig. 4). However, on the basis of foraminifera agip data (Online Resource 1), in particular the occurrence of Globorotalia bullbrooki, this interval is ascribed to the

Fig. 6 Photomicrographs of the cuttings of the Martina 1 well and main petrographic features of the sandstones: a at the center of the photograph is an euhedral zoned plagioclase (coeval) enclosed in mudstones, fragments of silty mudstones, siltstones, limestones (Unit lit0, 430 m bsf) (crossed nicols); b fossil in silty mudstone (bot-tom of the Unit lit0, 700 m bsf) (crossed nicols); c volcanic quartz grain (coeval) with typical embayment shape, enclosed in mudstones (Unit Sub2, 1,000 m bsf) (crossed nicols); d volcanic grain (probably coeval) with zoned plagioclase and other phenocrysts (Unit Sub2, 1,000 m bsf) (crossed nicols); e fragment of serpentinite with typical cellar structure (Unit lit0, 460 m bsf) (crossed nicols); f fragments of silty mudstone, with a few fragments of fossil, quartz, feldspar, sandstone and glauconite (Unit lit0, 380 m bsf) (crossed nicols); g medium-fine grain-size arkose of the core 1, Martina 1 (Unit Sub2) crossed nicols; h medium grain-size lithic arkose of the core 2, Mar-tina 1 (Unit Sub1) crossed nicols; i very coarse grain-size arenite of the core 1, Mimosa 1 (Unit Sub2) (uncrossed nicols), Q, quartz, Sh, deformed shale lithic, CE, carbonate extrabasinal clast; j coarse grain-size arenite of the core 1, Mimosa 1 (Unit Sub2) crossed nicols, Q, quartz, Sh, deformed shale lithic, F, k-feldspar, CE, carbonate extrabasinal clast

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Middle Eocene. The part of the Unit Sub1 above the detach-ment surface (from 2,050 to 2,420 m in Fig. 4) reveals a poor nannofossil association (Sphenolithus furcatolithoides, S. radians, S. spiniger), significant of the Middle Eocene and in particular of the nP15–nP16 interval of Martini (1971) standard zonation (upper part of the lutetian).

Unit Sub2 (Oligocene)

The Unit Sub2 unconformably lies on Unit Sub1 (uncon-formity X of Fig. 3) is 800 m thick in the Martina 1 well and is 1,600 m thick in the Mimosa 1 well.

Unit Sub2 in Martina 1 is mainly characterized by shales, siltstones, marly siltstones, silty marls and sand-stones. Sandstones occur in the central part of Unit Sub2 as stacked coarsening upward (CU) sequences transitioning upward to a fining upward (FU) sequence where marlstones occur (Fig. 4). In the upper part of the unit, microfossils, glauconite and volcanic grains (acid/rhyolite to intermedi-ate-basic/dacite–andesite) are dispersed within mudstone sediments (Fig. 6). The thickest succession of the Mimosa 1 well is organized in CU sequences (from shale/siltite to fine sandstone), transitioning upward to a 400-m thick sandstone horizon (sand/mud ratio S/M >1).

The sedimentary features recognized in the Martina 1 core 1 (Online Resource 2) consist mainly of interbeddings of heterolithic mudstone/siltstone and sandstone beds with S/M <1, organized in thin couplets with plane-parallel to gently dipping cross-lamination. Mudstones are charac-terized by intense bioturbation (with rare trace fossils like Planolites, Asterosoma and Skolithos). Thicker sandstone beds, fine to medium in grain-size, are mainly character-ized by plane-parallel, rippled and convolute lamination, normal grading and a few traction carpets.

The Mimosa 1 core 1 (2,450 m), collected below the detachment surface, and most likely in the lower strati-graphic part of Unit Sub2, shows alternation of sandstone beds (fine to very coarse grained, occasionally microrudite) and dm thick highly laminated (plane-parallel to cross-lam-ination) siltstone/mudstone couplets. Sandstone beds are massive or normally graded (Online Resource 2) and dis-play plane-parallel to cross-lamination. Mudstones occur at the top of the beds. amalgamation and mud clast horizons, as well as fluid escape structures occur. Heterolithic cou-plets show intense bioturbation, and the trace fossils Plano-lites (1–4 mm in diameter), Helminthopsis and Chondrites are found.

Paleoenvironmental significance

The Unit Sub2 in the Mimosa 1 well is characterized by the occurrence of two main sandstone bodies, early and late Oligocene in age, stacked in CU vertical trend sequences

and separated by a siltstone–shale horizon (Fig. 4). Sedi-mentological features suggest that the sandstones could have been deposited as turbidite sequences associated with the prograding lobes of a submarine fan system (Mutti and normark 1987).

The Unit Sub2 Martina 1 shows a sandstone body organ-ized in a CU sequence overlain by a FU sequence made of marly mudstones. These latter rocks are interpreted as the closure draping mud (marly mudstones) associated with the deactivation of the turbidite system (CU sandstones) or alternatively as the lateral migration of submarine fan lobes. The sedimentological features of the sandstone litho-facies (core 1 Mimosa 1 well) such as “Bouma sequences,” amalgamation and mud clasts are indicative of high density turbidite flows. The sedimentological features of the finer lithofacies (core 1 Martina 1 well), such as the mudstone/fine sandstone couplets, are indicative of dilute flow pro-cesses such as low density turbidity currents or hyperpicnal flows (Mulder et al. 2003; Soyinka and Slatt 2008).

The foraminifera associations (Online Resource 1) and the trace fossils (Zoophycos to Cruziana ichnofacies) also suggest a depositional paleoenvironment changing upward from bathyal to outer neritic.

Biostratigraphy

The nannofossil analysis performed on the Martina 1 core 1 (lower part of the unit) has revealed a very rich associa-tion (list of taxa in Online Resource 3) characteristic of the transition MnP22–MnP23 of the Catanzariti et al. (1997) zonation scheme, from the upper part of the Early Oligo-cene (Rupelian); Cretaceous and Paleogene reworking is also abundant. no significant nannofossil associations have been detected for the middle–upper part of the unit; how-ever, based on the foraminifera content (Online Resource 1) as analyzed and reported by aGIP (1975), it has been attributed to the Chattian.

The rich nannofossil association of Unit Sub2 of Mimosa 1 (Online Resource 3) recorded between 2,420 and 2,600 m (Fig. 4) is indicative of the transition MnP22–MnP23 of the zonation scheme of Catanzariti et al. (1997), early Oligocene (Rupelian) in age. The nannofossil association recorded between 2,050 and 1,300 m is indicative of the MnP21b-MnP22 interval of the Catanzariti et al. (1997) zonation (association with: Ericsonia formosa, Istmolithus recurvus, Reticulofenestra hillae, R. umbilica), of the early Rupelian. The middle part of the unit (1,000–1,300 m) has revealed a nannofossil association indicative of the MnP23 Zone of Catanzariti et al. (1997), relative to the Rupelian p.p. The upper part, from 1,000 m upward, is characterized by a nannofossil association (Cyclicargolithus abisectus, Dictyococcites bisectus, Helicosphaera recta) indicative of the MnP24–MnP25 interval of the Fornaciari and Rio

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(1996) zonation (late Oligocene) (Fig. 3). These data are generally in agreement with foraminifera data by aGIP (1983, see in Online Resource 1).

Unit lit0 (early Miocene)

The Unit Lit0 has been recognized only in the Martina 1 well and unconformably lies (unconformity a) above the Unit Sub2 (Fig. 3). It is 630 m thick (from 170 to 800 m) and is characterized by two lithologically distinct parts (Fig. 4). The lower part, almost 400 m thick, is composed mainly of mudstones (marls and marly siltstones). The upper part is characterized by the interlayering of sand-stone, siltstone, hybrid arenite and limestone (calcarenite, calcilutite, marly limestone) beds. The grain-size vertical trends show CU sequences. The sandstone beds are from fine to coarse grained with variable textural maturity. Usually, they are siliciclastic, but with significant carbon-ate (lithoclasts and bioclasts) and glauconite grains; the latter may also occur within limestones and mudstones throughout the whole unit (Fig. 4). limestone and marly limestone beds occur mostly between 400 and 210 m. Bioclasts are composed of planktonic and benthic fora-minifers, algae, bryozoans and corals. Sometimes lime-stones contain significant amounts of siliciclast grains dispersed within the micrite. Mainly acid-intermediate (rhyolite–rhyodacite to andesite) neovolcanic grains are diffuse in the mudstones of the lower part of the unit, whereas ophiolitic gravel-size clasts are recurrent in the upper mudstone horizon (Fig. 6).

Palaeoenvironmental significance

The general coarsening upward trend records a regres-sive trend of the overall depositional system. The benthic foraminifera association (aGIP 1975, 1980, see Online Resource 1) allowed ascribing the lower part of the unit to the outer neritic/bathyal zone. The lower part of the unit can be interpreted as a gently dipping ramp mainly characterized by hemipelagic sedimentation, where occasionally turbidite or hyperpycnal low density flows deposited. Conversely, the upper part of the unit is char-acterized by coarse siliciclastic and carbonate sandstone deposits, relative to clastic systems interacting with car-bonates, as shown by the occurrence of limestones and marly limestones. These systems probably developed in a neritic environment, as suggested by the foraminif-era association (Online Resource 1) and in agreement with the occurrence of glauconite grains. nevertheless, Mancin et al. (2009) interpreted a similar lower Miocene depositional change, from clastic to carbonate deposits, observed in the Mediterranean region, as due to regional scale climate effect.

The decrease in volcanic grains at the top of the succes-sion indicates that during Burdigalian time, the volcanic activity was reduced. The occurrence of ophiolithic grains as detritus within mudstone instead documents that ophi-olitic highs were still present inside the sedimentary basin, but not in the sandstone clast sources.

Biostratigraphy

On the basis of foraminifera association (Online Resource 1), in particular of the taxa Globigerinita dissimilis, Glob-orotalia acrostoma and Globigerinoides bisphericus (aGIP 1975, 1980), the deposition of Unit Lit0 occurred during the early Miocene (Globigerinoides trilobus Zone of Iacca-rino 1985, late Burdigalian). The unconformity separating Unit Lit0 from Unit Sub2 encompasses the latest Chattian, the whole aquitanian and the base of the Burdigalian.

Unit lit7 (Pleistocene)

The Unit Lit7 unconformably lies above the Unit Sub2 in the Mimosa 1 well and above the Unit Lit0 in the Mar-tina 1 well (Fig. 3). Unit Lit7 is Pleistocene in age and mainly consists of hybrid siliciclastic deposits. The main foraminifera taxa detected are (Online Resource 1): Elphidium crispum, E. complanatum, Ammonia sp., Cibi-cides lobatulus, Nonion sp., Uvigerina peregrina, Glo-bigerinoides trilobus, Globigerina spp., Miliolidae, and Ostracods, Bryozoans, Echinoids and Red algae, suggest-ing a littoral environment (aGIP 1975).

Detrital composition of sandstones

Sandstones of the Eocene Unit Sub1 (Martina 1, core 2) are composed mainly of quartz, feldspar, lithics and micas (Fig. 6). Monocrystalline quartz grains with undu-late extinction occur more than composite quartz, whereas k-feldspar (orthoclase and microcline) clasts occur more than plagioclase. Intrabasinal grains like mud clasts and bioclasts (i.e., macroforaminifera—mainly nummulites—and algae) are rare. Matrix is scarce or absent (arenite), rarely more than 10 % (greywacke), whereas secondary calcite is recurrent. among sedimentary lithic clasts, mic-ritic limestones are more abundant than fine siltstones and shales, whereas coarse siltstones and carbonate sandstones are very rare. Metamorphic lithic clasts are represented by gneiss, quartzite and micaschist, with minor low-grade schist. Volcanic lithics are very subordinate and include dacites, andesites, rhyolites and rhyodacites.

The mean QFl + CE composition of the sand-stones of the Unit Sub1 Martina 1 well (core 2) is Q58.8F27.3l + CE13.9 (arkose–lithic arkose) (Online

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Resource 4, Fig. 7a), whereas the mean composition of the fine-texture lithic fraction is lm43.0lv5.7ls + CE51.3 (Online Resource 4, Fig. 7b).

Sandstones of the Oligocene Unit Sub2 (Martina 1, core 1) are quite similar to those of the Unit Sub1 and are mainly composed of quartz, feldspar and fine-texture lithic fragments (Fig. 6). Main differences between the units are the relationships between the fine-texture lithic fragments (Fig. 7b), with a small decrease in the metamorphic lithics in Unit Sub2 associated with a small increase in the sedi-mentary lithics. a decrease in gneiss and micaschists, and a significant increase in low-grade schists is observed in the Unit Sub2 metamorphic lithics. acidic volcanites outnum-ber intermediate-basic volcanites in Unit Sub1. The main sandstone mean composition (Online Resource 4, Fig. 7a) is Q59.8F25.0l + CE15.2 (lithic arkose); the mean fine-tex-ture lithic composition (Online Resource 4, Fig. 7b) is lm34.4lv7.5ls + CE58.1.

The sandstones of the Unit Sub2 Mimosa 1 (core 1) dif-fer from those of Martina 1 because of lesser amounts of quartz and metamorphic lithics and a greater amount of sedimentary lithics such as shales and subordinately extra-basinal carbonates, particularly biomicrites and marlstones.

Fine siltstones are rare, whereas coarse siltstone and sand-stone fragments are absent.

The mean composition of the main Sub2 sandstone is Q49.6F20.7l + CE29.7 (arkosic litharenite) (Online Resource 4, Fig. 7a), whereas the mean composition of the fine-textured lithic fraction is lm24.6lv8.4ls + CE67.0 (Online Resource 4, Fig. 7b).

Discussion

Depositional evolution

The sedimentary successions encountered by the two wells Martina 1 and Mimosa 1 indicate that from the Early Eocene up to the Early Miocene, the EPR–proto–Corsica Basin system was characterized by the development of turbidite fans, ranging from bathyal up to neritic environ-ments. In particular, the Unit Sub1 developed during the Early Eocene as a fine-distal siliciclastic turbidite system. a hiatus encompassing, at least, the base of the lutetian separates this system from an upper turbidite fan system settled during the Middle Eocene (?lutetian–Bartonian).

a

b

c

Fig. 7 Sandstone modal composition ternary plots of the EPR succession. The statistical confidence intervals have been repre-sented through a classical “Dickinson model.” a Main composition (QFl + CE); b fine-grained lithic composition (lmlvls + CE). c Sandstone modal composition ternary plots comparing the QFl + CE and the fine-grained lithic compositions of the EPR units,

the PTB and Epiligurian basins, and the inner Oligocene northern apennines foredeep basin. Compositional groups and subgroup are petrofacies of the Table 1. (Petrographic data referred to lithostrati-graphic units from: Cibin 1989, 1993; Di Giulio 1991; Martelli et al. 1998; Cornamusini 2001)

Int J Earth Sci (Geol Rundsch)

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This fan was more proximal and coarser than the previ-ous and was characterized by coarsening to fining upward trends.

During the Oligocene, a turbidite fan (Unit Sub2) devel-oped as well. It shows a diachronous base marked by lower Rupelian strata overlying lower and Middle Eocene strata (Mimosa 1 well) and middle–upper Rupelian strata overly-ing Middle Eocene strata (Martina 1 well).

The Unit Sub2 turbidite fan system ranges from upper bathyal to outer neritic zones. The complete unit (also the thickest at 1,600 m) was recovered from the Mimosa 1 well. It ranges from early Rupelian to Chattian in age and consists of a general thickening-coarsening upward trend testifying to the general progradation of the system. The minor coarsening upward packages suggest the develop-ment of stacked turbidite fan lobes. The sandy lobe depos-its, mainly in the middle–upper part, are interlayered with very fine sediments (shales and siltstones), probably indi-cating prolonged inactive turbidite phases or alternatively a lateral migration of the sandstone lobes. Fan deactivation occurred in the Chattian (upper part of the Martina 1 well), as indicated by the presence of shales, siltstones and marls at the top of Unit Sub2.

a discontinuity marks the base of the Burdigalian Unit Lit0. This is, however, not enhanced by a significant litho-logical change (Fig. 4). nevertheless, a lacuna (seismic unconformity a) encompassing the uppermost Chattian, aquitanian and lower Burdigalian has been recognized. The depositional system of the Unit Lit0 developed as a coarsening and shallowing upward clastic system passing from offshore marls to lower shoreface coarse siliciclastic-carbonate deposits.

The occurrence of volcanic clasts dispersed within fine sediments (neovolcanic constituents, see Zuffa 1991), all along the succession, records synsedimentary volcanic activity. Volcanic processes were active in the Mortara center since the Eocene (Dalla et al. 1992; Cibin et al. 2001; Di Giulio et al. 2001; Mattioli et al. 2002) and in the Corsica–Sardinia and Periadriatic magmatic belts during Oligocene–Miocene (Dal Piaz and Venturelli 1983; Zattin and Zuffa 2004; Garzanti and Malusà 2008; lustrino et al. 2011).

Sandstone provenance

When the compositions of the EPR sandstones are com-pared within the regional framework (eastern Corsica, northern apennines and Piedmont Tertiary Basin, Fig. 7c; Table 1), in addition to differences, some similarities can be shown: (a) the Eocene Unit Sub1 is comparable to (Fig. 7c; Table 1) the loiano Sandstones (Epiligurian Basin, in Cibin 1989; Cibin et al. 2003) and the Corsica Eocene sandstones (i.e., Solaro and Palasca sandstones,

in Sagri et al. 1982) and Elba Flysch (Colle Reciso Fm, in Bortolotti et al. 2001); (b) the Oligocene Unit Sub2 shows some bland similarities with the coeval northern apen-nines foredeep deposits (Petrofacies a in Table 1; Fig. 7c; Cornamusini 2001; Elter et al. 1999) and greater similari-ties with the PTB Oligocene sandstones (albergana Mb.—Ranzano Fm.; Cibin 1993; Martelli et al. 1998).

Moreover, the slight compositional differences between Sub1 and Sub2 sandstones of the Martina 1 emphasize that they belong to a single sedimentary system, involv-ing a similar provenance for the whole Eocene–Oligocene time interval. The differences, however, include an upward decrease in the involvement of the metamorphic rocks (passing from medium to low grade) and an increase in mudstone rock clasts, whereas carbonate clasts decrease.

The location of the detrital sources for the Eocene–Oligocene regional system is, however, a strongly debated topic and needs a discussion based on the literature and the regional framework with the support of geodynamic mod-els, such as those of Dewey et al. (1989), Stampfli et al. (1998), Handy et al. (2010) and Turco et al. (2012).

Cibin et al. (2001) considered the Western alps as the main source areas for the PTB Eocene deposits, which was confirmed by Zattin and Zuffa (2004) through fission track analysis. Conversely, reinterpreting compositional data from the literature (Cibin 1993; Di Giulio 1991) and paleocurrent data, Mutti et al. (1995) suggested that the sources of Upper Eocene deposits (i.e., the lower part of the Ranzano Formation) of the PTB/EEB (Fig. 1) were located in the Corsica–Sardinia Massif, similar to what was proposed by Sagri et al. (1982) for the Eocene flysch crop-ping out in eastern Corsica (Palasca Sandstones) (Figs. 1, 8, 9). Still, Cibin et al. (2001) outlined that the ophiolitic detritus appeared in the sedimentary record of the PTB deposits since the lower Rupelian and referred to this time as the beginning of the subaerial exposure of the ligurian–Penninic orogenic wedge. It is notable, however, that Mar-telli et al. (1998), Cibin et al. (2001, 2003) and Zattin and Zuffa (2004) indicated sharp compositional differences between the Epiligurian sandstones occurring north and south of the Val Secchia lineament (Fig. 1). This lineament was an important tectonic structure crossing transversally the northern apennines and defining northern (Emilian apennines with northern Epiligurian basins) and south-ern (southern Epiligurian basins) thrust-top basin systems (Figs. 8, 9). In this regard, it is important to emphasize that for the Eocene southern Epiligurian deposits (loiano Sandstones), two alternative sources have been hypoth-esized: (1) a continental basement source as suggested by the presence of abundant quartz, feldspar, granite and gneiss rock fragments in the composition (Table 1) (Cibin 1989, 1993); or (2) a source from the recycling of older sandstone units stacked in the orogenic wedge (Cibin et al.

Int J Earth Sci (Geol Rundsch)

1 3

Tabl

e 1

Com

para

tive

pane

l of

the

petr

ogra

phic

al c

ompo

sitio

n fo

r th

e sa

ndst

ones

of

the

mai

n si

licic

last

ic s

ucce

ssio

ns o

f th

e n

orth

ern

ape

nnin

es–n

orth

ern

Tyrr

heni

an S

ea s

yste

m

Mai

n an

d fin

e-te

xtur

e lit

hic

com

posi

tions

and

cha

ract

eris

tic r

ock

frag

men

t as

sem

blag

es h

ave

been

sho

wn.

Pet

rofa

cies

sub

divi

sion

der

ives

fro

m t

he e

labo

ratio

n of

the

thr

ee a

bove

par

amet

ers.

N

D n

ot d

eter

min

ed. n

umbe

rs b

etw

een

brac

kets

indi

cate

bib

liogr

aphi

c so

urce

: 1 th

is p

aper

, 2 C

orna

mus

ini (

2001

), 3

Cib

in (

1993

), M

arte

lli e

t al.

(199

8), 4

Di G

iulio

(19

91),

5 F

onta

na (

1980

), 6

a

iello

(19

75),

7 E

lter

et a

l. (1

999)

, 8 S

agri

et a

l. (1

982)

Sand

ston

e un

it (a

ge)

Mai

n co

mpo

sitio

nFi

ne te

xt, l

ithic

com

posi

tion

Roc

k fr

agm

ents

Petr

ofac

ies

EPR

Uni

t Sub

2 M

artin

a 1

(lat

e O

ligoc

ene)

(1)

Q59

.8F 2

5.0l

C15

.2l

m34

.4lV

7.5l

sC58

.1G

rani

toid

and

met

amor

phic

with

few

lim

esto

nes

and

othe

r se

di-

men

tary

aa

EPR

Uni

t Sub

1 M

artin

a 1

(Mid

dle

Eoc

ene)

(1)

Q58

.8F 2

7.3l

C13

.9l

m43

.0lV

5.7l

sC51

.3G

rani

toid

and

met

amor

phic

with

few

lim

esto

nes

and

othe

r se

di-

men

tary

aa

EPR

Uni

t Sub

2 M

imos

a 1

(Ear

ly O

ligoc

ene)

(1)

Q49

.6F 2

0.7l

C29

.7l

m24

.6l

v 8.4

lsC

67.0

Gra

nito

id a

nd m

etam

orph

ic w

ith li

mes

tone

s an

d sh

ales

aa

“Mac

igno

cos

tiero

” Fm

.—T

usca

n fo

rede

ep (

lat

e O

ligoc

ene)

(2)

Q56

.5F 1

9.2l

C24

.3l

m64

.2l

v 20.

2lsC

l5.6

Gra

nito

id a

nd m

etam

orph

ic w

ith r

iolit

e an

d fo

ssils

ab

Ran

zano

Fm

.-Pi

zzo

d’O

ca M

b.—

nor

ther

n a

penn

ines

E

pilig

uria

n/ep

isut

ural

bas

ins—

(lat

e E

ocen

e) (

3)Q

39.0

F 27.

0lC

34.0

lm

92. 0

lv 5

.0l

sC3.

0G

rani

toid

and

low

- to

med

ium

-gra

de m

etam

orph

ica

c

Ran

zano

Fm

.-V

al P

esso

la M

b.—

nor

ther

n a

penn

ines

E

pilig

uria

n/ep

isut

ural

bas

ins—

(Ear

ly O

ligoc

ene)

(3)

Q26

F 22l

C52

lm

43l

v 41l

sC16

Serp

entin

ite a

nd l

igur

ian

sedi

men

tary

, med

ium

- an

d lo

w-g

rade

m

etam

orph

icB

a

Ran

zano

Fm

.-V

al P

esso

la 2

Mb.

—n

orth

ern

ape

nnin

es

Epi

ligur

ian/

epis

utur

al b

asin

s—(E

arly

Olig

ocen

e) (

3)Q

43F 3

9lC

18l

m12

lv 3

1lsC

57G

rani

toid

ad

S.Se

bast

iano

Cur

one

Fm.—

Eas

tern

Ter

tiary

Pie

dmon

t Bas

in -

(Ear

ly

Olig

ocen

e)—

(4)

Q30

F 10l

C60

lm

69l

v 14l

sC17

Serp

entin

e sc

hist

and

HP/

lT m

etam

orph

icC

a

Ran

zano

Fm

. -V

aran

o de

’ Mel

egar

i Mb.

—n

orth

ern

ape

nnin

es

Epi

ligur

ian/

epis

utur

al b

asin

s—(E

arly

Olig

ocen

e) (

3)Q

16F 1

2lC

72l

m30

lv 2

2lsC

48H

elm

inth

oid

limes

tone

, silt

ston

e an

d fe

w s

erpe

ntin

ite, l

ow-g

rade

m

etam

orph

icD

a

Ran

zano

Fm

. – a

lber

gana

Mb.

—n

orth

ern

ape

nnin

es

Epi

ligur

ian/

epis

utur

al b

asin

s—(E

arly

Olig

ocen

e) (

3)Q

46F 3

8lC

16l

m21

lv 4

4lsC

35G

rani

toid

, gne

iss

with

aci

d vo

lcan

ic a

nd s

edim

enta

rya

d

Ran

zano

Fm

.—l

agri

mon

e M

b.—

nor

ther

n a

penn

ines

E

pilig

uria

n/ep

isut

ural

bas

ins—

(Ear

ly O

ligoc

ene)

(3)

Q10

F 4l

C86

lm

70l

v 6l

sC24

Serp

entin

e sc

hist

, HP/

lT a

nd lo

w-g

rade

met

amor

phic

, met

ased

i-m

enta

ryD

b

Font

anel

le-I

nci

sa F

m.—

Eas

tern

Ter

tiary

Pie

dmon

t Bas

in—

(lat

e E

ocen

e)—

(4)

Q47

F 26l

C27

lm

93l

v 4l

sC3

Gra

nito

id a

nd s

chis

tsa

c

loi

ano

Sand

ston

es—

nor

ther

n a

penn

ines

Epi

ligur

ian

basi

ns—

(Mid

dle

Eoc

ene)

(5)

Q56

F 41l

C3

nD

Gra

nito

id, g

neis

s, r

hyol

itea

a

Bis

man

tova

San

dsto

nes—

nor

ther

n a

penn

ines

Epi

ligur

ian

basi

ns—

(Mid

dle

Mio

cene

) (5

)Q

55F 2

4lC

21n

Da

Der

nice

-Bos

men

so F

m.—

Eas

tern

Ter

tiary

Pie

dmon

t Bas

in—

(Ear

ly

Olig

ocen

e) (

4)Q

34F 2

7lC

39l

m40

lv 4

1lsC

19Se

rpen

tinite

, bas

alt,

ligur

ian

sedi

men

tary

and

bio

clas

tsB

a

Man

cian

o Sa

ndst

ones

—T

usca

ny E

pilig

uria

n B

asin

—(M

iddl

e

Mio

cene

) (5

)Q

58F 2

8lC

14n

DG

rani

toid

, met

amor

phic

and

min

or v

olca

nic,

fos

sils

and

rar

e sa

nd-

ston

e, li

guri

an li

mes

tone

and

oph

iolit

ea

a

Petr

igna

cola

San

dsto

nes—

nor

ther

n a

penn

ines

for

edee

p—(E

arly

O

ligoc

ene)

(6)

Q7F

48l

C45

nD

Vol

cani

cE

a

ave

to S

ands

tone

s Pe

tr. a

—n

orth

ern

ape

nnin

es f

ored

eep—

(Ear

ly

Olig

ocen

e) (

7)Q

41F 2

8lC

29l

m60

lv 2

0lsC

20M

ediu

m-

to lo

w-g

rade

met

amor

phic

, few

lim

esto

nes

and

bioc

last

s,

rare

vol

cani

ca

b

ave

to S

ands

tone

s Pe

tr. B

—n

orth

ern

ape

nnin

es f

ored

eep—

(Ear

ly

Olig

ocen

e) (

7)Q

12F 3

2lC

56l

m15

lv 8

0lsC

5V

olca

nic

Ea

Mon

te S

enar

io S

ands

tone

s—n

orth

ern

ape

nnin

es (

Olig

ocen

e) (

6)Q

45F 3

7lC

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2001). Similarly, Cibin et al. (2003) suggested a rework-ing of older sandstone units for the Rupelian sandstones of the albergana Mb. (Ranzano Fm.) of Martelli et al. (1998) (southern Epiligurian deposits), but without excluding a priori a first-cycle erosion of a continental basement.

In this scenario, the provenance for the Sub1 and Sub2 sandstones could fit with source areas characterized by both: (a) alpine Corsica unmetamorphic fine-grained sedi-mentary covers (the detrital sandstone framework is char-acterized by the lacking of ophiolites and by the occur-rence of limestone and shale clasts); and (b) basement of the Hercynian Corsica (or Tenda/nebbio Massif) or also of the nearby Calabrian block (see in Turco et al. 2012), as shown by the occurrence of metamorphic and granitoid rock fragments. nonetheless, the involvement of the Cor-sica basement sources requires consideration of additional scenarios. Partial sourcing from the Hercynian Corsica is considered difficult due to the occurrence of the interposed growing alpine Corsica chain and of the developing west-ern Corsica foreland basin. But the reduced flexural defor-mation of the Corsica foreland basin (Waters 1990; argnani 2012) as strengthened by the occurrence of flysch deposits (Solaro flysch) lying unconformably on both basement and

alpine stacked units (amaudric du Chaffaut 1973) allows the speculation that drainage clast paths could partially bypass the growing chain and feed easternmore basins, such as those of the Sub1 and Sub2 deposits (Fig. 9). This may have occurred through transversal basin-chain line-aments, like those segmenting the Epiligurian basins (for example the Val Secchia line, Castellarin 2001; Cibin et al. 2001). However, it is difficult to explain the source of the quartz–feldspar–crystalline lithics in a different way, unless the Calabrian block represented a source, as inferred by the models of Turco et al. (2012). about this last point, it is worth noting that some Southern apennines/Calabrian sedimentary units, such as the Burdigalian portion of the Cilento Group (amore et al. 1988), show remarkable lith-ological affinities with EPR units. Furthermore, a source from the Western alps similar to that for the northern Epiligurian and PTB basins is unlikely due to the differ-ences in composition (the petrographical markers of the PTB/EEB successions tracking the orogen uplift are lack-ing, see Table 1), the transversal segmentation of the basins and the long distance from source to basin (particularly for coarse and texturally less mature deposits). additionally, the occurrence of significant reworking of older sandstone

Fig. 8 Synthetic stratigraphic panel comparing the successions of the EPR–proto–Corsica basin system (eastern Corsica, Pianosa Island, Western Elba Island, Martina 1 and Mimosa 1 wells) and of the

Piedmont Tertiary Basin (East PTB, northern Epiligurian, Southern Epiligurian). VSL Val Secchia line, VVL Villalvernia–Varzi line

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successions, as invoked by Cibin et al. (2001) for the south-ern Epiligurian deposits (loiano Fm.), is also improbable, due to the absence of sandstone and coarse siltstone lithics in the detrital framework of the EPR sandstones. Moreover, the infrequent occurrence of ophiolite breccias interlayered within Eocene Sub1 deposits, as well as in some coeval Corsica flysch, combined with the lack of ophiolite clasts within the sandstones, indicates that ophiolitic units were partly exposed in peribasinal sectors, but conversely they were totally absent in the sandstone source areas. never-theless, the EPR sandstones also show some compositional similarities with Oligocene northern apennines foredeep deposits (e.g., Macigno Fm., see Table 1), even if settled in a different basin framework, that Carrapa and Di Giulio (2001) and Dunkl et al. (2001) demonstrated as sourced from Western alps.

In addition, it is important to emphasize that during the Eocene–Early Oligocene, the alpine Corsica was not greatly uplifted. Fission track thermochronology, radio-metric and structural data, have established that the alpine Corsica had the major exhumation during Oligocene–Miocene times (Daniel et al. 1996; Jakni et al. 1998;

Brunet et al. 2000; Danišik et al. 2007; agard et al. 2009), although a younger exhumation age (Early–Middle Mio-cene) was estimated by Cavazza et al. (2007), Zarki-Jakni et al. (2004) and Fellin et al. (2005). In agreement with the previous statement, it is worth noting that the Miocene eastern Corsica deposits (Orszag-Sperber and Pilot 1976) (Fig. 1) record detritals derived from alpine units only since the early Tortonian, while the older clastic deposits were mainly sourced from Hercynian granitoids (Bossio et al. 2000; loÿe-Pilot et al. 2004).

Basin geodynamic setting

Taking into account the regional/geodynamic framework established in some literature models with particular focus on the recent kinematic model of Turco et al. (2012), the Oligo-Miocene succession was deposited in wedge-top basins linked with the collapse, extension and thinning that affected the orogenic wedge (Ciarcia et al. 2009; Vitale et al. 2011). Differently, if we consider other models (e.g., Castellarin 1994, 2001; Cibin et al. 2001; Stampfli et al. 1998; Molli 2008; Handy et al. 2010; Marroni et al. 2010;

Fig. 9 Hypothesized paleogeographic/depositional model for the Corsica–northern apennines system, during late Oligocene: LVV Villalvernia–Varzi lineament, LVS Val Secchia lineament. The north-ern part of the reconstruction is based on the schemes of: Castella-rin (1994, 2001); Cibin et al. (2001, 2003); Carrapa and Di Giulio

(2001); Dunkl et al. (2001); Finetti et al. (2001); Carrapa et al. (2004); Garzanti and Malusà (2008); Molli et al. (2010); the southern part, particularly concerning the foredeep basins, is based on data by Cornamusini and Costantini (1997) and Cornamusini (2004a, b)

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argnani 2012), such units could be considered in episutural or thrust-top basins (Fig. 9). The here defined EPR-Corsica Basin system was located south of the wide thrust-top sys-tem of the PTB/EEB and north of the Southern apennines “ligurian” Basin, as shown in Turco et al. (2012). The EPR-Corsica Basin most probably developed before and lasted longer than the PTB, as demonstrated by the Early Eocene age of the lower part of the Unit Sub1, whereas the oldest deposits of the PTB are Middle Eocene in age (Bartonian) (i.e., Monte Piano Marls, Di Giulio et al. 2002) (Fig. 8). Such age differences emphasize the importance and role that transversal tectonic lineaments played in the control of sedimentation and basin development (Castella-rin 1994; Pascucci et al. 2007), such as the Val Secchia lin-eament that dissected the EEB in a northern and a south-ern portion, or the regional E–W strike-slip faults of Turco et al. (2012). During the aforementioned event in the Early to Middle Eocene, the southern EEB deposits (i.e., the loi-ano Sandstones cropping out in northern apennines close to the Bologna city; Cibin 1989) and the EPR-Corsica Unit Sub1 deposits settled. This depositional phase marks the first structuration of these basins (Cibin 1989; Cibin et al. 2001; Catanzariti et al. 2002).

Compositional data also indicate that some similarities between the Sub1 sandstones, the loiano Sandstones and the Eocene Corsica flysch occur (Table 1). Therefore, con-sidering that the Eocene Corsica flysch rests on the Hercyn-ian Basement and on the deformed alpine units (Durand Delga 1978; Egal 1992; Molli and Malavieille 2011 with references therein), they can be considered as the infilling of the Corsican foreland basin (Molli and Malavieille 2011 cum bib) and/or, at least in part, of the western side of the thrust-top proto–Corsica Basin (Mauffret et al. 1999; Bor-tolotti et al. 2001). This latter basin is partly structured on remnants of oceanic crust (Turco et al. 2012; “trapped oce-anic crust” of Bortolotti et al. 2001; Balestrieri et al. 2011).

On the basis of the above discussion, it is reasonable to consider that during the Eocene, the wide and complex EPR–Corsica Basin system developed as a thrust-top sys-tem on the accretionary wedge. This sector was character-ized by a high subsidence rate allowing the deposition of 3,000 m thick Eocene strata (Mauffret et al. 1999). Simi-larly, the EPR Sub2 Unit represents the Oligocene clastic sedimentary infill of the evolving basin system (Figs. 9, 10), as well as the Oligocene albergana Sandstones of the Ranzano Fm. represents the contemporaneous infill of the

a

b

c

Fig. 10 Simplified 2D schematic panels of a transect across eastern Corsica–inner northern apennines showing the hypothesized struc-tural location of the EPR deposits: a Middle Eocene; b Oligocene; c Middle Miocene. OPH Ophiolite nappes, LIG ligurian nappes,

SUBL Subligurian nappe, SL alpine metamorphic Schistes lustrés, OC oceanic nappes, EOC Eocene deposits, OLIG Oligocene depos-its, MIO Miocene deposits. Vertical scale is exaggerated

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southern Epiligurian basins. The kinematic model of Turco et al. (2012) explains that accommodation space was pro-vided by extension linked with a longitudinal stretching of the accretionary wedges (phase 5 of the authors), deter-mining the structuration of wedge-top basins (Vitale et al. 2011). Moreover, following this model, the Priabonian stratigraphic gap between Sub1 and Sub2 units can be eas-ily explained and related to the final phase of the accretion of the Corsica prism. Starting from the Oligocene, a new left-lateral strike-slip fault zone formed (Turco et al. 2012), which was possibly responsible also for the tectonic dou-bling effect seen in the Mimosa1 succession.

The age and stratigraphic position of Unit Lit0 sug-gest it formed lateral of the lower Miocene formations cropping out at the Pianosa Island (Marina del Marchese Fm) and in the eastern Corsica aleria Plain (aghione-St. antoine fms) (Fig. 1). The latter Corsica formations have been interpreted as the first extensional syn-rift deposits (Fig. 10) of the northern Tyrrhenian Sea (Bartole et al. 1991; Jolivet et al. 1998; Carmignani et al. 1995; Bossio et al. 2000; loÿe-Pilot et al. 2004; Zarki-Jakni et al. 2004) or as extensional “intra-wedge” deposits (Speranza et al. 2002). In particular, Ciarcia et al. (2009, 2012) and Turco et al. (2012) set them or similar deposits, in extensional wedge-top basins, standing on to the stretched and thinned orogenic wedge. Therefore, the basal unconformity a marks the transition from a compressional to an extensional tectonic regime (Jolivet et al. 1998; Pascucci et al. 1999; Brunet et al. 2000), at least for the western Corsica Basin. Therefore, the lower Miocene deposits of the EPR can be interpreted as the infilling of extensional basins, just prior to the northern Tyrrhenian rifting phase (Turco et al. 2012).

Differently from the northern PTB-Epiligurian basin system, where the “alpine” deformative phases (ligurian I and III phases in Mutti et al. 1995) determined strong sed-imentary variations in detrital sources (Cibin et al. 2003; Carrapa et al. 2004 and references therein), in the EPR-Corsica system those phases do not seem to have caused major paleodrainage changes. In fact, clastic sources remained more or less the same from the Eocene to the Early Miocene.

Conclusions

The data presented here bring new constraints for the char-acterization of the depositional systems occurring along the EPR, between the Corsica and the northern apennines, since the Eocene until the Early Miocene, a crucial time interval during which the tectonic system evolved from compressional to extensional.

In this view, the EPR successions represent the marginal and easternmost filling of the proto-Corsica Basin. The

occurrence of siliciclastic successions, quite composition-ally homogeneous from bottom to top, but with interposed unconformities, suggests the existence of almost steady sedimentary systems, interrupted by tectonic pulses from alpine–apennine orogenic tectonic events. Sedimentary systems are represented by prograding/retrograding tur-bidite fans developed from bathyal to neritic environments, particularly during the Eocene and Oligocene.

The sedimentary basin where the EPR sediments accu-mulated is interpreted as representing the southernmost prolongation of the thrust-top PTB–Emilian Epiligurian basin. The basin probably had connections with the south-ernmore wedge-top basins of the Southern apennines.

The mixed source systems are derived from crystalline basement and limestone-shaly sedimentary covers. The latter can be observed in the unmetamorphosed ligurian units crop-ping out in the alpine Corsica, whereas the former cannot be well constrained. Because it is very improbable for the base-ment clasts to be sourced from the Western alps and from the recycling of older sediments, it is here hypothesized that there was a partial siliciclastic detrital source from the closest base-ment of the Hercynian Corsica and/or Calabrian block.

In conclusion, the EPR Tertiary sedimentary successions record continuously the various phases of the Corsica–Tyr-rhenian Basin from the alpine collision to the post-colli-sional rift, responsible for the opening of the northern Tyr-rhenian Sea.

Acknowledgments The present research has been developed within a collaboration between the University of Siena and the agip-EnI Spa. We are grateful to the EnI Spa to give us the availability to study their own materials and in particular to Saverio Merlini for the fruitful discussions and contributions to the research. We are indebted to antonio lazzarotto who supported, discussed and stimulated us, to anna Maria Bambini for her support and collaboration on the nanno-fossil association analyses, to luca Maria Foresi and alessandro Ter-zuoli for discussions and support about the foraminiferal assemblages and for the ichnofacies analyses, respectively. We are also grateful to Salvatore Critelli, Giancarlo Molli, Enrico Pandeli and Michel Séranne for critical readings of early versions. Particular thanks to the reviewer andrea Di Giulio and an anonymous reviewer, whose suggestions (in particular about the kinematic model for the latter) largely improved the paper. Barbara Terrosi and antonella Mancini helped us in drawing some figures. Finally, we thank linda Pickett for the English revision of the text.

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