Journal of the Virtual Explorer

The Ligurian Alps show, unconformable on both the Brianconnais and Dauphinoise Mesozoic successions, a Mid-Late Eocene succession, known as the “Priabonian Trilogy” Auct., interpreted as deposited during the early stages of subsidence of an underfilled Alpine foreland basin (Sinclair, 1997; Allen et al. 1991; Ford et al. 1999) and made up of three formations, namely the carbonate-ramp deposits known as Nummulitic Limestone, the Globigerina Marls and Ventimiglia Flysch (Lanteaume 1958; 1968; 1990; Campredon, 1977, Varrone, 2004).

This last formation consists of a several hundred-meter thick siliciclastic turbidites, laterally equivalent of the Annot sandstones, (Stanley 1961) referred to the Priabonian – Lower Oligocene? (Vanossi 1990). A basal unconformity separates the Brianconnais-Dauphinoise Mesozoic sediments (Santonian to Campanian in age) from the Eocene ones. This discontinuity surface is characterised by evidences of subaerial exposure and can be related to a period of significant uplift, emersion and erosion of the substratum in which some hundreds meters of Upper Cretaceous strata were removed. In the Dauphinoise domain, Lutetian continental deposits of Microcodium Formation, (Faure-Muret and Fallot 1954; Bodelle and Campredon 1968, Varrone, 2004) are also present, directly above the Cretaceous unconformity.

During the Late Eocene a marine transgression (“EpimesoAlpine” basin sensu Mutti et al., 1995), developed soon after the inception of the continental subduction of the European/Briançonnais margin of the LA, with a deep basin developed on a substratum consisting of the former exhumed alpine nappe stack. The sediments of that basin are presently preserved at the base of the south TPB succession (i.e. Borbera-Grue area), in Monferrato. In detail, Late Eocene basinal mudstones and quartz-feldspathic arenites were deposited over non-metamorphic Ligurian substrata (D0 stratigraphic regional discontinuity of Fig. 4), and are unconformably overlain by marginal marine ophiolitic-rich sandstones (Cavanna et al. 1989; Di Giulio 1989; Mutti et al. 1995; Dela Pierre et al., 2003). The present central and southern part of TPB, resting on metamorphic units, were in a more marginal setting at that time, as recorded by deposition of fluvio-lacustrine facies overlain by alluvial fan deposits (Rossi et al., 2009).

During the Oligocene, at a regional scale, sediments were mainly accommodated in two complex structural depressions consisting of fault-bounded, partly coalescing depocentres, resting to the south-west and north-east respectively of a major structural divide, representing the Alto Monferrato high.

In the southern areas (i.e. the region extending from the Alto Monferrato to the Monregalese-Saluzzese), a number of minor sub-basins developed over the Alpine axial sector. More to the east a more continuous depression developed and extends to the north in the subsurface towards the Monferrato area (Mosca et al., 2009; Rossi et al., 2009). The deposition during Early Rupelian was typically dominated by alluvial fan-to-fan delta conglomerates overlain by a complex alternation of paralic, marginal marine and shelfal facies, resting on different types of rock substrate (D1 regional discontinuity). Since Late Rupelian, a transgression associated with the occurrence of drowning-platform unconformities led to a dramatic marginward shift, toward the south and the west, of the fluvio-deltaic systems, that were replaced basinward by shelfal gravity flow-dominated coarse-grained bodies encased in marine mudstones.

Along southern basin margins, the deposition records a SW-ward diachronism of fluvio-deltaic deposits in relation to a regional transgression from the NE (Rossi et al., 2009).

The Chattian-Aquitanian succession was dominated by turbidite systems which, along the western Alpine margin (Saluzzese and Monregalese areas; Mosca, 2006, Rossi et al., 2009) and the adjacent Southalpine thrust-fold-belt are overlain by a gravel-rich alluvial fan to fan-delta unit that prograded eastwards and northwards. In large part of the north TPB and south TPB, progressive drowning of the platform depositional areas occurred, and slope marly sediments were widespread. Siliceous sediments formed in Aquitanian- Early Burdigalian over large part of the TPB, as recorded in this time span in several sectors of the Central Mediterranean area.

Successively, in the Early Burdigalian, an important inversion of the basin occurred, giving origin to chronostratigraphic gaps both in the Alto Monferrato and in the north TPB sector, where carbonate shelf sediments were deposited unconformably (D2 regional discontinuity) on the Late-Oligocene –Aquitanian sediments (d’Atri 1990; Piana et al. 1997; Dela Pierre et al, 1995).

During the Late Burdigalian - Early Langhian, a basin-wide turbidite system was deposited, showing lap-out terminations (D2a discontinuity) and lateral fringing toward structurally-high areas located to the east (Alto Monferrato area) and to the north (Monferrato area). Since the deposition of this succession, the TPB is characterized by a more regular physiography, being a larger and more uniform basin, bounded to the north and to the south by uplifting areas. In this time, the major depocentre was located in the Langhe area.

Since the Langhian, (D3 discontinuity) major depocenters for marine and turbidite systems were progressively shifted northward below the present Savigliano and Alessandria basins (Falletti et al., 1995; Mosca, 2006; Mosca et al., 2009), due to the combination of marginal uplift, basinward tilting and outward progradation.

The ongoing shortening led to the progressive reduction of the sediment accomodation space until the early Tortonian (D4 discontinuity), when homogenous marly sediments were unconformably deposited in most part of the TPB.

The Messinian Salinity Crisis was superimposed to this framework and most of the evaporites were widespread resedimented as mass flow deposits (D5 discontinuity) (Dela Pierre et al. 2002a; Irace et al., 2005; Mosca 2006; Mosca et al., 2009; Rossi et al., 2009).

The westerly propagation of the north-vergent structures involved the north-western TPB since Late Miocene time. The occurrence in this area of Southalpine crust at shallow levels could have represented a major obstacle to the progressive westward propagation of the north-verging thrust systems. As a consequence, the regional N-S shortening was accommodated more to the south-west, as recorded since Late Miocene by pronounced activity of the fault systems at present buried in the Savigliano basin area (Mosca et al., 2009)

In this framework the Savigliano and Alessandria depocentres evolved as highly subsiding sub-basins close to the lateral ramps of the north-verging Torino Hill-Monferrato tectonic arc.

Pliocene sediments record a re-establishment of normal-marine conditions after the Messinian Salinity Crisis: clay-rich open marine facies were deposited along previous basin margins (D6 discontinuity). Sedimentation continues upward with sand-rich marginal marine deposits ranging from Early to Middle Pliocene, followed in the Pleistocene by mainly continental deposits. Pliocene and younger deposits exceed 2 km in the Savigliano and Alessandria Basin depocenters, while they are only a few hundred meters thick in their interposed Asti region.

North of the VVL the TPB corresponds to the basin unconformably formed onto the Ligurian Units of the Northern Apennine. The sedimentary succession deposited in this basin is known as Epiligurian Succession (Ricci Lucchi, 1986) and it is now exposed in several scattered outcrops along the Emilian side of the Northern Apennine from the Oltrepò Pavese area to that of Bologna (Fig.2).

The deposits of the Epiligurian Succession consist mainly of terrigenous clastics even if they include facies ranging from pelagic and hemipelagic deposits to siliciclastic turbidites and to shelf sandstones and calcarenites; the whole succession shows a maximum thickness of more than 5000 m. The Epiligurian Succession is characterized by ages ranging from Middle Eocene to Late Miocene-Pliocene and overlies the at least in part already deformed Ligurian Units. Starting from the early Oligocene the evolution of the Epiligurian Succession was related to the NNE-verging migration of the Apenninic accretionary wedge, which progressively incorporated the foredeep units of the Adria continental margin (Fig.4,6).

The Epiligurian Succession was therefore affected by contractional tectonics (thrusting and folding) as well as extensional deformation (low angle normal faults) which can be related with internal dynamics of evolving NA wedge (Costa and Frati 1997; Botti et al., 2006; Molli et al., 2000; Di Giulio et al., 2002; Molli, 2005).

From a stratigraphic point of view, the Epiligurian Succession consists of unconformity-bounded units whose boundaries (major unconformities) are strongly controlled by sub-marine tectonics which often generated slumpings and olistostromes (sedimentary melanges). Five major unconformities that define the main lithostratigraphic units (Monte Piano, Ranzano; Antognola, Bismantova, Termina, Gessoso-solfifera and Argille Azzurre formations) occur.

The sedimentation of the Epiligurian Succession began in the Middle Eocene (Lutetian p.p.-Bartonian) with marls (Monte Piano Fm) deposited in a deep marine environment (Di Giulio et al., 2002), locally preceded by muddy chaotic deposits and/or sandstone turbidites (Loiano Sandstone). The overlying basal unconformity of the Ranzano Fm, which often affects the Ligurian Units, testifies the vertical movements and submarine erosion of the Ligurian wedge from the Upper Eocene to the Lower Oligocene. The Ranzano Fm. is unconformably overlyain by the Antognola fm. starting from the Rupelian (Lower Oligocene) a system of siliciclastic turbidites (sandstones and conglomerates) recording a continuous subsidence of the Ligurian wedge up to Chattian-early? Aquitanian.

During this time span the NA accretionary wedge enlarges by frontal accretion at the toe wedge e.g. within the sub-Ligurian/Canetolo system (Bratica-Salsominore and Coli-Marra units) and by underplating of the distal Adria continental margin as recorded by the Tuscan units stacking.

The unconformity at the base of Contignaco Fm. is possibly the response of a low-angle normal fault-related subsidence during early exhumation and sin-contractional stacking of deepest parts of the wedge e.g. Tuscan units (Molli et al., 2002; Molli, 2005; Fellin et al., 2007; Molli, 2008).

The following Burdigalian regional major basal unconformity of the Bismantova Fm. shelf deposits (upper Burdigalian-Serravalian), can be connected with a major step of the migration of the Ligurian Units onto the Northern Apennine foredeep and in particular that corresponding with the emplacement onto the Cervarola Fm. of the Bobbio tectonic window (Labaume and Rio, 1994).

The base of the Bismantova Fm. represents also the beginning of a new Middle-Upper Miocene sedimentary cycle of the Epiligurian succession; the middle Miocene-Lower Miocene depositional cycle shows an overall shallowing trend which ends with a silica-rich marly sedimentary horizon (Contignaco Fm.) that records a Mediterranean scale biogenic-volcanogenic episode correlating through all domains of the Northern Apennine (Amorosi et al., 1995). The regional unconformity below the Contignaco Fm. does not correspond to the base of the siliceous zone itself, but rather to the base of widespread resedimented deposits in debris flow or turbidite facies (Canossa Melange or Anconella Sandstone), marking an important Aquitanian tectonic event which affected the Northern Apennines (Ligurian wedge emplacement above the Coli-Marra Subligurian Unit and the Tuscan Nappe Macigno Fm). The Middle-Upper Miocene sedimentary cycle of the Epiligurian succession is characterized by a shallow to deep marine cycle evolving from the Bismantova Fm. to the Termina Fm. which is characterized by mudstone slope deposits with intercalations of thick bodies of resedimented sandstones and sedimentary melanges. These features indicate that the thrust top basins (where the Epiligurian succession were deposited) felt the effect of the Ligurian wedge evolution and also the base of the Termina Fm. marks a regional discontinuity possibly connected with the progressive thrusting of the underlying foredeep units and with the unroofing and exhumation processes the Tuscan metamorhic units of the Alpi Apuane region suffered from Middle Miocene to Middle Pliocene times (Molli, 2008).

The Termina Fm. evolved upward to the Gessoso-solfifera Fm. (Messinian) which is made up of both primary and clastic, resedimented evaporites with interbedded organic-rich shales, deposited during the evaporitic and postevaporitic stages of the Messinian salinity crisis.

The overlying major unconformity in the Epiligurian succession corresponds to the base of the upper Messinian Colombacci Fm. (mainly continental clastic deposits derived from Apenninic sources with subordinate clays and marly limestones) which postdates the intra-Messinian tectonic event of the Northern Apennine evolution, correlated with the beginning of the second stage of Northern Tyrrhenian Basin rifting.

The upper part of the Epiligurian succession, overlying the Colombacci Fm., is represented by a thick (more than 1500 m) mudstone-dominated succession deposited in a relatively deep marine environment (Argille Azzurre Fm.) spanning in age from lower Pliocene to Pleistocene. The Argille Azzurre Fm presents two regional unconformities: the lower one occurred in the lower Pliocene (G. puncticulata phase; Cerrina Feroni et al., 2002, 2004; see also Vai, 1992) and recorded the involvement in the accretionary wedge of the youngest foredeep deposits (Marnoso-Arenacea Fm.) and the in-sequence overlying mainly marly post-foredeep deposits of lower Pliocene age (e.g. Cella Marl and Riolo Terme Fm.). The base of the lower Pliocene sedimentary cycle is locally marked by conglomerate and sandstone bodies (Monterumici Fm.; Borello Sandstone). The top of the lower Pliocene cycle is limited by the Middle Pliocene regional unconformity, well marked in the Romagna sector also by shelf carbonate deposits (Spungone Fm.), which points to the still active tectonics (thrusting and folding deformation) of the Northern Apennines during the Middle Pliocene.

While the alpine wedge (Penninic and Ligurian units) at the Alps-Apennines junction was buried under deep-water sediments, a foredeep formed on Adria continental crust (Menard, 1988; Doglioni, 1993). The Adriatic foredeep was initially starved (Gallare-Aveto stage of Garzanti and Malusà, 2008), as attested by the Eocene - lower Oligocene Gallare Marl and Chiasso Fm to the north (Di Giulio et al., 2001), and by the Canetolo Complex and Aveto Sandstone to the south (Catanzariti et al., 1996; Elter et al., 1999). Huge detrital supply started abruptly in late Oligocene times (Rögl et al., 1975; Catanzariti et al., 1996), soon after the climax of Periadriatic magmatism and the onset of denudation in the central Alps (Gansser, 1982). The Gonfolite clastics thus accumulated proximally in the Southalpine foredeep (Gelati et al., 1988), while the Macigno turbidites accumulated distally in the Apenninic foredeep (Di Giulio, 1999).

The coarse-grained Gonfolite clastic wedge is discontinuously exposed north of Milano, and extends for ca 40 km N-S and ca 200 km E-W in the subsurface of the Po Plain (Pieri and Groppi, 1981; Di Giulio et al., 2001; Mosca et al., 2009; Rossi et al., 2009). The basal Como Conglomerate consists of 2 km-thick conglomerates and pebbly sandstones of late Chattian - early Burdigalian age, lying over mid-Oligocene marls. The Como Conglomerate is interpreted as a fan delta fed from the north and passing southward to a deep-sea fan (Gelati et al., 1988). Provenance from the rapidly exhumed Bregaglia Pluton and wallrocks has long been documented (Heim, 1919; Wagner et al., 1979; Bernoulli et al., 1993; Malusà et al. 2010). Gonfolite turbidites accumulated until the middle Miocene (Sciunnach and Tremolada, 2004) and were next accreted as S-vergent thrust sheets at the front of the Southern Alps, and unconformably sealed by Messinian deposits (Pieri and Groppi, 1981).

The foredeep clastic wedges of the Northern Apennine are thick successions of turbidite sandstones feeding longitudinal basins ahead the thrust fronts (Ricci Lucchi, 1986) as well as in depozones on top of the wedge itself. They are classically subdivided into tectonostratigraphic units arranged as thrust sheets, bound on top by Ligurian units and detached at different levels in the underlying stratigraphic sequence. From SW to NE, major units are the chiefly Chattian-lower Aquitanian Macigno Fm, the upper Chattian-Aquitanian Modino Fm, the upper Chattian-Burdigalian Cervarola Fm, and the Burdigalian-lower Messinian Marnoso-Arenacea Fm (Cerrina Feroni et al., 2004; Catanzariti et al., 2009).

The Macigno turbidites have a fairly constant thickness (2.5 to 3 km) over most outcrop areas, extending ca 50 km NE-SW and 250-300 km NW-SE. Sedimentation was contiguous with that of Gonfolite clastics (Lorenz, 1984; Gelati et al., 1988). Detrital modes indicate a prominent crystalline source, with upward decreasing subordinate supply from intermediate volcanic rocks, and negligible sedimentary detritus (Di Giulio, 1999). Sedimentary detritus becomes more important in the youngest units (Valloni and Zuffa, 1984). Based on basin-fill patterns, paleocurrents, petrographic signatures, and geochronological data, a generic axially Alpine provenance has long been inferred (Lorenz, 1984; Valloni and Zuffa, 1984), with the exceptions of the episodic calcareous megaturbidites fed from the south (Gandolfi et al., 1983; Talling et al., 2007) and some debated lateral radial supply for the southern Tuscany Macigno (Cornamusini et al., 2002; Butler 2009). Nevertheless, identification of specific sources within the Alps is controversial (see Garzanti and Malusà, 2008). Focused erosion of the Lepontine Dome has been recently recognized as the dominant source of detritus feeding the Adriatic foredeep, based on independent lines of evidence including petrographical and geochronogical signatures of the clastic wedges (Garzanti and Malusà, 2008). The marked topographic gradient between the rapidly exhumed Central Alps and the rapidly subsiding Adriatic foredeep favoured long-distance sediment transfer, which continued through the Miocene.

Foredeep sandstones of the Northern Apennine are topped by slope marlstones and in places by shelf calcareous sandstones that document the uplift and deactivation of the foredeep (closure facies sensu Ricci Lucchi, 1986b). Synsedimentary emplacement of the Ligurian wedge onto the foredeep succession is attested by large chaotic beds and olistostromes of wedge Ligurian or Subligurian affinity (Elter and Trevisan, 1973; Castellarin et al., 1987; Labaume and Rio, 1994; Argnani and Ricci Lucchi, 2001; Lucente and Pini, 2003).