Journal of the Virtual Explorer

It is represented by the high-grade metamorphic rocks of the Belgodere complex, which comprises leptynite–amphibolite associations with eclogite boudins, orthogneiss and metasediments derived from an Early Palaeozoic protolith affected by early-Devonian high grade metamorphism (Palagi et al., 1984; Menot and Orsini, 1990; Rossi et al., 2001; Rossi et al., 2009). The Belgodere complex was intruded at c. 340 Ma (Rossi et al., 1988; Menot and Orsini, 1990) by Mg-K plutons, mainly consisting of monzonites and associated ultrapotassic mafic rocks. During the emplacement of Mg-K intrusion, the Belgodere gneiss underwent anatexis under amphibolite conditions (Rossi et al 1991). A second composite magmatic association is testified by granitic and mafic intrusions dated at 300-280 Ma (Rossi et al., 2001; Tribuzio et al., 2008), by a volcano-sedimentary sequence including calc-alkaline ignimbrites and volcanoclastites and dykes swarms (Rossi et al., 2001). A third Permian magmatic cycle is associated with the alkaline volcano-plutonic magmatism (Menot and Orsini, 1990; Rossi et al., 2001; Tribuzio et al., 2008; Rossi et al., 2009) e.g. the Monte Cinto caldera and the metaluminous intrusion of Popolasca. The magmatic history of the basement ends with a later set of mafic dykes (Rossi et al., 2001).

The Hercynian “autochthonous” basement is covered by Mesozoic-Tertiary sediments, all characterized by the incomplete and rather thin sequences, with several unconformities (Durand Delga, 1976; Nardi et al., 1978; Rossi et al., 2001; Durand Delga et al., 2001). In the area of interest, Early? -Mid Triassic reddish to green conglomerates, pelites and sandstones (Petra Moneta Conglomerates), lie alternatively on the Hercynian basement and on the Permian volcano-sedimentary sequences. The cover sequence is completed by massive conglomerates characterized by basement-derived clasts (high and low grade metamorphic rocks, granites, volcanites) having a maximun thickness of c. 250m. Within it, conglomerate-matrix nummulites of Paleocene-Early Eocene age have been locally found (Amaudric du Chaffaut, 1973; Ferrandini et al., 2010). The Conglomerates grade to discontinuous levels of grey limestones and calcarenites (“Nummulitic limestones”) roughly stratified and characterized by Mid-Late Lutetian nummulites (Nardi et al., 1978; Ferrandini et al., 2010), in turn followed by grey sandstone, siltite, microconglomerates and pelites (Lozari sandstone) well exposed along the R.N. 199. Sandstone levels locally include, at their base, Upper Lutetian nummulites. The sequence ends with black shales, siltstone and fine grained sandstones forming the so called “Shaly Flysch” (“Flysch noir”, “Flysch Argilloso“), with a maximum thickness of c.250 m. The “Shaly Flysch” can directly overlie the basal conglomerates, the “Nummulitic limestones” or the Lozari sandstones. The maximum thickness of the “autochtonous” sedimentary cover is around 500 m. Fragments and/or slices of Mesozoic limestone, classically considered as small to large (decameter in size) olistoliths (Nardi et al., 1978; Durand Delga, 1978; Rossi et al., 2001), are included in the upper part of the Middle Eocene sequence near Pietralba).

The autochthonous basement and cover sequence were affected by polyphase deformation (Egal & Caron, 1989; Egal, 1992), which occurred at shallow crustal levels, at T < 280/300°C. The sedimentary sequence is characterized by heterogeneous deformation localized along bedding-parallel fault zones and distributed within folded domains. West vergent north-south trending parallel folds and top-to-the west fault zones, usually overprinted by late folds and faults, are common deformation features (Egal, 1992).

Between the “autochthonous” basement and the overlying units belonging to the “Nappe Supérieure”, slices of a “parautchtonous” basement and cover sequences can be locally observed. These can be considered as the northward equivalent of tectonic units exposed along the western border of the “Hercynian Corsica” between Vecchio in the south and Popolasca-Ponte Leccia in the north, which are labelled “External continental units” (Fig.6; Bezert, 1990, Bezert and Caby, 1989; Molli, 2008). These units, which have also been studied near Corte (“Corte slices” of Amaudric du Chaffaut et al., 1973), are characterized by basement rocks (mainly Permian granites and Paleozoic host rocks) and a metasedimentary cover consisting of Mesozoic carbonates (Corte and Restonica marbles) including Late Jurassic and Late Cretaceous metabreccias. The sedimentary sequence ends with siliciclastic turbidites (metasandstones) dated to early Mid-Eocene on the basis of the presence of Nummulites biarritzensis; Discocyclina sp., (Bezert & Caby 1988). In the Balagne area very limited exposures of these units are reported. The Volparone breccias (Late Cretaceous? breccia which includes clasts of Malm limestone and rift-related metabasalts) in the Bocca di Fuata area are an example (Malasoma and Marroni, 2007).

The "Parautochthonous" or “External units” are characterized (Bezert & Caby 1988) by a low grade high pressure/low temperature metamorphic overprint (HP greenschist facies of Bousquet et al., 2008) recently constrained at ~0.7 GPa for a temperature of 325°C (Malasoma et al. 2006; Malasoma and Marroni, 2007). The high pressure/low temperature parageneses are mainly observable in suitable rock types, e.g. within the chlorite-phengite matrix of the metabreccias and in metapelites as well as within the mafic dykes. In the granitoids, millimetre-scale blue-amphibole bearing shear zones have been observed (Molli et al. 2005 and unpublished). These shear zones show evidence of east–west transport associated with top-to-the-west kinematics, similarly to structures described elsewhere all along the boundary between Alpine and Hercynian Corsica by Bézért (1990).

The “Nappe Supérieure” System includes two major tectonic units: the Nappe du Bas Ostriconi and the Balagne Nappe.

The Balagne Nappe

It includes Jurassic ophiolites, a supra-ophiolitic Late Jurassic to Late Cretaceous sedimentary sequence and Paleocene-early Eocene siliciclastic deposits (Fig. 2.23). The ophiolitic sequence comprises Jurassic basalts, mainly pillow lavas lying on gabbros and serpentinized mantle peridotites. U-Pb (SHRIMP) data on zircon from a trondhjemite vein intruded in a gabbro indicates late magmatism at 169±3 Ma (Rossi et al., 2002). Geochemical features (GLOM, 1977; Durand Delga, 1997; Saccani, 2003) of basalts reveal E-MORB affinity, typical of crust developed during the initial stages of oceanic spreading (Marroni & Pandolfi, 2006). The ophiolitic sedimentary cover is formed by radiolarites (Late Dogger-Late Malm, Bill et al., 2001; Dalenian et al., 2008 and ref. therein), Calpionella Limestone (Tithonian-early Berriasian) and early Cretaceous S.Martino fm (Marroni et al., 2000). The S.Martino fm. consists of c.100 m thick calcareous turbidites and shales interbedded with minor quartz-rich siltites, which have been correlated since the late seventies with the Palombini shales of Northern Apennine (Nardi, 1968; Durand Delga, 1978; Marroni et al., 2000). The S.Martino fm. is followed by Lydienne Flysch (Routhier, 1956), consisting of silicified thin-bedded calcareous and mixed turbidites (Pandolfi, 2007), grading to the medium to coarse grained arenites and rudites of the Novella Sandstone, which is part of the Late Cretaceous (Albian-Late Cenomanian) siliciclastic turbidite system (Nardi et al., 1978; Durand Delga et al., 1978; Durand Delga, 1984; Marino et al., 1995; Marroni et al., 2007). The Novella sandstones are not found everywhere and they are locally replaced by two different coarse grained deposits, named Toccone and Alturaja formations. The Toccone breccias consist of coarse grained polygenic breccias with clasts supplied from continental-crust units (low grade metamorphites, granites, volcano-sedimentary, triassic dolomites, liassic limestones) as well as from the reworking of an ophiolitic sequence (basalts, radiolarites, limestones, lydiennes; Rossi et al, 2001). The Alturaja fm. (Alturaja Arkose) consists of massive to roughly bedded conglomerates interbedded with minor turbiditic sandstones (Marroni et al., 2004). The clasts of this formation derived mainly from granitoids, low-grade metamorphic rocks and Si-rich volcanics (rhyolites to dacites). These deposits, dated to Late Barremian to Middle Aptian with palynological assemblages, are interpreted by Marroni et al. (2004) as the proximal part of a marine turbidite system very close to the source area. Marroni et al. (2006) considered the Alturaja fm. and the Toccone breccias heteropic with the Lydienne Flysch and Novella Sandstone. An alternative interpretation is favoured by Nardi et al. (1978), Durand Delga (1984) and Rossi et al. (2001), who proposed that the Alturaja fm. overlies unconformably the entire ophiolitic cover and locally rests above basalts (e.g. at Bocca di Fuata).

Figure 2.23. Balagne Nappe

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General stratigraphic setting of the Balagne Nappe (after Durand Delga, 1984 modified in Spella et al., 2009).

The sedimentary sequence of the Balagne Nappe is completed by the 200-300 m thick Eocene deposits of the Annunciata Fm. (Palasca Sandstone in Nardi et al., 1978). This formation, which is interpreted as a siliciclastic turbidite fed by a continental-crust source area, consists of meter thick medium to coarse grained sandstones alternated with siltites and shales,. A Middle Eocene depositional age (Lutetian 46-40 Ma) has been proposed based on the presence of Nummulites brogniarti (forme A), Discocyclina, Asterodiscus, Amphistegina (Nardi, 1968; Bonnal, 1972; Lacazeidieu, 1974; Durand Delga, 1976; Nardi et al. 1978). These findings have been confirmed recently with nannoplancton data in Marino et al., 1995 (Rossi et al., 2001).

The close sedimentological analogies and the similarities in age (Middle Eocene) of the Annunciata fm. and the Eocene cover of the “Autochthonous” basement represent a key point in the long standing debate about the Balagne region, in particular for the nature and significance of the ophiolitic Balagne nappe. This point is discussed below.

The Balagne Nappe has been deformed at shallow structural levels with max P not exceeding 0.3/0.4 GPa at a T ~150/200°C (Marroni and Pandolfi, 2003). Different generations of superimposed structures can be recognized in the supraophiolitic cover including an early slaty cleavage associated with meter to map scale tight folds and later crenulation cleavages associated with open to tight folds. Fold-facing and kinematics along localized zones of deformation point to a dominant top-to-the west sense of transport. In the Eocene Annunciata Flysch simpler deformation patterns, which developed at shallower depths, have been described (Egal, 1992). The Balagne Nappe system can be subdivided into several minor tectonic elements (Nardi et al., 1978; Durand Delga, 1978; Egal and Caron, 1988; Egal, 1992; Rossi et al., 2001; Marroni & Pandolfi, 2003), all of them characterized by constant and dominant top-to-the west kinematics.

The Balagne region has been the classical ground for geological debate since the beginning of the 20th century. A complete historical report of such history can be found in Nardi, 1968; Durand Delga, 1978; furthermore in Rossi et al. (2001) it is possible to trace back the issue to the early “autochthonist” vs. “allochthonist” debate, including the discussion on the sense of movement of the Nappe Supérieure from west to east (Steinman, 1907) or from east to west (Maury 1907). These topics were discussed also in a field trip organized in 1928 (first edition of CorseAlp!!) by French geologists (P. Termier, E.Maury, E.Raguin) with Alpine “tectoniciens” (Steinmann G., Staub R., Kober L., Tillman N.) as well as in the more recent field trip of the Réunion extraordinaire de la Societé Géologique de France in 1976 (Amaudric du Chaffaut and Campreon, 1976). The debate on the significance, origin and tectonic evolution of the Balagne Nappe and of the Nappe Supérieure is still ongoing.

Some authors e.g. Durand Delga (1976); Durand Delga (1984); Principi and Treves (1984); Durand Delga (1998); Marroni and Pandolfi (2003, 2007) support an original location of the Balagne Nappe and the Nappe Supérieure close to the Corsican continental margin (Balano-Ligure domain of Durand Delga, 1984). This interpretation is mainly based on the stratigraphic and geochemical characteristics of the ophiolites and the related sedimentary cover. Following Durand Delga et al. (2002) these features can be summarized as follows:

1) Presence of debris derived from continental crust at different stratigraphic levels within the ophiolitic sequence and cover series:

(a) Late Malm zircon-bearing sandstone associated with basalts, with zircon identical to those of the neighbouring granitic Variscan batholith of Corsica;

(b) continental debris within latest Malm limestone (S.Colombano);

(c) continental source area for Late Cretaceous and Eocene siliciclastic deposits correlated with Hercynian Corsica rock-types;

2) E-MORB composition of the oceanic basalts formed during early stage of oceanization in the ocean-continent (Corsica) transition area.

Other authors (Nardi, 1968; Mattauer and Proust, 1975; Nardi et al., 1978; Dallan and Nardi, 1984; Malavieille et al., 1998; Molli, 2008) considered the Balagne Nappe and the Nappe Superiore as derived from a more “internal" position, in an oceanic domain located far east from the Corsican margin (“ultra-Schistes Lustrés”) toward the “Apenninic” oceanic domain.

Our view (Molli and Malavieille, 2010) envisages the Balagne-Nebbio-Macinaggio units (Balagne and Nappe Supérieure system) as the relict of an intraoceanic accretionary wedge formed by offscrapping and shallow underplating of an oceanic part of Ligurian Tethys far from the Corsican continental margin. The Ocean-Continent transition originally located directly to the east of the Corsican continent is in our view sampled in the eclogitic and blueschist units of Serra di Pigno, Farinole and Mont San Petrone area (see Field trip 1 and Field trip 2.1, Vitale Brovarone et al., 2010; 2011; Vitale Brovarone 2011). In this context, the Balagne ophiolite could be derived from the opposite OCT, close to the microcontinent located east of the Mesozoic Corsican margin. The continental basement-derived Late Cretaceous sediments in the supra-ophiolitic cover could have been deposited in a forearc basin at the rear of a growing accretionary wedge. Therefore, the Nebbio-Calabria microcontinent (continental edge block) could have been the source area of the Late Cretaceous siliciclastic deposits, as also suggested by Malavieille et al. (1998); Michard et al. (2002); Molli and Malavieille (2010 and references therein); Vitale Brovarone (2011); Vitale Brovarone et al. (in prep).

Stop 3.1:

Locality: Colle di U’Vezzu (UTM 32T 521706 E 4720892)

Themes: General introduction to the geology of the Balagne region and to western limb of Tenda antiform, which are visible in the landscape

Stop 3.2

Locality: Ostriconi (UTM 32T 504843 E 4722876)

Themes: Nappe Supérieure (Flysch di Ostriconi), sedimentary and structural features.

This stop aims to illustrate some of the sedimentary and structural features of the Flysch d’Ostriconi a Late Cretaceous (Senonian) deposit belonging to the Nappe Supériore System (Nardi et al., 1978; Durand Delga & Magné, 1978). Although the best exposures are located along the new road cut, the old RN 199 allows observations of the calcareous-marly sequence (Fig. 2.24a) associated with siliciclastic deposits, including microconglomerates and sandstones sourced from continental basement (Fig. 2.24b). The sequence is affected by bedding-parallel extension of calcareous beds, testified by incipient to well developed boudinage structures and by asymmetric west-vergentfolds, ranging in size from one meter to several meters, associated with a east-dipping cleavage (Fig. 2.24c). Towards the basal contact of the unit it is possible to notice a marked increase of calcite/quartz veins sets (Fig. 2.24d), which are especially well exposed in the outcropsalong the coast.

Figure 2.24. Flysch d’Ostriconi

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Some stratigraphic and structural features of the Flysch d’Ostriconi. See text for details.

Stop 3.3

Locality: Cappella di San Sebastiano (UTM 32T 5032279 E 4715484)

Themes: Hercynian authochtonous basement and alpine sedimentary cover sequence.

Along the DN 163 toward Palasca, walking towards the old chapel of San Sebastiano, we will cross the unconformity separating the Hercynian basement from its sedimenatry cover belonging to the Alpine cycle. The basement rocks, characterized by a steep foliation, mainly consist of migmatitic gneiss (Fig. 2.25b) and minor bodies of orthogneiss dated at 338 Ma (Menot et al., 1996) belonging to the Belgodere gneiss complex. These rocks are intruded by by Carboniferous pegmatoidal dykes.

The basement rocks are uncomformably covered by polygenic conglomerates (Fig. 2.25a) called ‘Poudinges de Palasca’, with clasts of granitoids, gneisses (dominant) and rare Permian cover rocks ranging in size from a few centimetres to half a meter. The conglomerates, dated to the Paleocene-early Eocene (Rossi et al., 2001), show a variable thickness ranging from a few meters up to 300 meters and are locally associated with coarse sandstone.

Figure 2.25. Authochtonous Hercynian basement and its alpine sedimentary cover sequence.

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(a) Paleocene-early Eocene Conglomerates; (b) Belgodere gneiss.

From the San Sebastiano chapel looking to the SE (Fig. 2.26) its is possible to observe the geometry of the nappe stack of the Balagne region. From SE (right) to NW (left) and from bottom to the top, the following units can be recognized:

1) the Hercynian basement and its Paleocene-early Eocene sedimentary cover (including the Flysch noir in this landscape view);

2) the Annunciata unit (early Mid Eocene sandstone) with a base of tectonic slices including exotic Jurassic limestones, slices of external continental units and elements of ophiolitic Balagne Nappe (ophiolitic breccias and Lydienne) (south of the view);

3) the Toccone unit, mainly consisting of Cretaceous Lydienne Flysch and ophiolitic breccias (Brecce di Toccone);

Figure 2.26. San Sebastiano Chapel

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(a, b) Panoramic view from San Sebastiano Chapel of the nappe stack geometry of the Balagne region.

A detail of the main basal thrust of the Annunciata unit on the authochtonous basement and cover can be seen in Fig. 2.27

Figure 2.27. A schematic field drawing of the main basal thrust of the Annunciata unit on the authochtonous.

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Stop 3.6

Locality: Col de San Colombano (UTM 32T 505813 E 4714121)

Themes: Panoramic view of the north Balagne Region, the ophiolitic Navaccia unit, main lithotypes and their structural features.

From the Col de San Colombano looking to the north a panoramic view of Balagne Region can be observed (Fig. 2.28) including, from left to right, the Hercynian basement, the Annunciata Eocene Sandstone (with beautiful west vergent fold systems) and the Balagne units. Further to the north, the central ridge with vegetation is located within the Nappe Superiore (Ostriconi), whereas towards the east the Cima delle Forchie unit (mainly granitoids), the volcano-sedimentary and Tenda orthogneiss, can be easily recognized. Polygenic Cretaceous Conglomerates (Alturaja fm) can be observed to the right, close to the lookout point.

Figure 2.28. Col de San Colombano

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A Panoramic view of the Balagne Region From the Col de San Colombano looking toward north. See text for description.

The walk from the San Colombano Pass to the hill on the east allows to see different lithotypes and sub-units of the Balagne Nappe System (San Colombano, Toccone and Alturaja sub-units of Marroni & Pandolfi, 2003) and their internal structures (Fig. 2.29).

Figure 2.29. San Colombano track

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Some rock-types and structures observable along the San Colombano track, see text for descriptions.

Most lithologies of the Balagne ophiolitic sequence can be analyzed here, including: Jurassic basalts (mainly pillow lava), Late Jurassic red Cherts (Late Dogger-Late Malm in age, Bill et al., 2001; Dalenian et al., 2008 and ref. therein) (Fig. 2.29a,b), Calpionella limestone (2.29a), including the ‘Gran Rocher limestone,’ which is well known in Corsican geological literature

The Calpionella and Grand Rocher limestone (Tithonian-early Berriasian) here include levels of coarse to medium grained breccia in carbonate matrix. Clasts consist mainly of continental basement (low and medium/high grade rocks, igneous intrusives and volcanic-subvolcanics rocks) as well as Triassic-early Jurassic carbonates. The Calpionella limestone is followed by the early Cretaceous S.Martino fm (Fig. 2.29c) (Marroni et al., 2000 cfr. Palombini shales of Northern Apennine) and Lydienne Flysch (Fig. 2.29d) (Late Hauterivian-Early Barremian Marroni et al., 2000), thin bedded turbidites locally with fine to coarse grained sandstone layers (mapped as ‘Novella Sandstones’ when especially abundant) and/or ophiolitic breccias (Fig. 2.29e; Toccone Breccia). The sequence is capped by a thick sequence of coarse grained conglomerates (Alturaja fm) early-Mid Aptian in age (Marroni et al., 2004), which will be observed at the top of the hill (Fig. 2.29f).

All the units are strongly deformed, with localized shear zones characterized by foliated cataclasites that accommodated top-to-west displacement (Fig. 2.29c,d). Folds ranging in size from a few meters to several tens of meters are also common (Fig. 2.29b).

Stop 3.7

Locality:km 59 railway track (UTM 32T 510165 E 4710559)

Themes: Jurassic ophiolites and their sedimentary cover

A section through the top of the Tethyan ocean floor can be observed along the railway. Pillow lavas and lava tubes are covered by pillow breccias, themselves capped by deep marine sediments. The sedimentary sequence consists from bottom to top of red radiolarites (late Dogger, late Malm), interbedded calcareous turbidites and shales and late Cretaceous Lydienne Flysch.

Walking further along the road, a large body of pillow lavas allows observations of well preserved features of submarine volcanism.

Stop 3.8

Locality: Carrière de Taverna (UTM 32T 515768E 4693748)

Themes: Francardo Miocene basin. Sedimentary features and tectonic setting.

During the Oligocene, drifting and subsequent rotation of the Corsica-Sardinia block resulted in the development of the gulf of Lion oceanic domain. In Miocene time, rifting resumed and the subsequent cooling of the oceanic lithosphere led to thermal subsidence and marine invasion in parts of Alpine Corsica, where Miocene sedimentary basins controlled by transtensional faults were formed.

The last stop of the day is in the Francardo basin, which was related to these processes

Outcrops in the Taverna quarry allow observations of the different structures and sedimentary facies that characterize the paleoenvironmental and tectonic setting of this basin.

Sediments were deposited unconformably on, the autochthonous hercynian basement made by Permian alcaline granitoïds, Eocene para autochthonous units, pre-piemontese Caporalino-Pédani allochthonous units to the west and the Ligurian ophiolitic units to the North and East. Coarse detrital sediments deposited on paleoreliefs marks the base of the basin. The main stratification presents a slight dip (10-15°) to the west. North-South oriented high-angle normal faults are related to a transtensional tectonic setting during basin formation. They developed before tilting of the series related to a regional scale synformal structure that is bounded by similar normal faults to the west and to the east.

The Taverna formation, which crops out in the quarry, consists from bottom to top of :

- sandstones and clays including pit layers,

- marls and fossiliferous sandstones interlayered with limestones layers and conglomerates,

- conglomerates deposited in a shallow marine environment (presence of ripple-marks and oyster debris) covered by a thick sandstone sequence.

This formation contains Mid-Burdigalian to Upper Burdigalian fossils (Fig. 2.30). These fossils are typical of a shallow marine littoral. Clear water ostracods and fish living in brackish water characteristic of lagoons are also found. Fossils of plants and coal found in grey marls suggest the vicinity of the coast. All fossils indicate a hot climate.

Figure 2.30. Burdigalian fossils

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1: Polygenic conglomerate associated with medium to fine sandstone and siltstones; 2: échantillon TAV 21, south face of quarry. Croûte carbonatée, stromatolite; 3: Balanus sp., 4: Tellina sp.; 5: Corbicula sp.; 6: Arca sp.; 7: Aphanius sp.; 6: Empreinte de feuille, bloc éboulé, front de taille nord de la carrière.

Analysis of conglomerates suggests that during basin develoment, the main source of sediments was from proximal reliefs located to the west. Indeed, most of the pebbles consist of rocks types typical of the crystalline basement and of its Mesozoic and Eocene cover. The absence of metamorphic rocks from the ophiolitic schistes lustrés nappe may indicate that during the Miocene these rocks remained below sea level.