Reconstructing the Alpine orogenic wedge and its foreland basin: The petrologic transition from Cretaceous passive-margin to Eocene foreland sedimentation in northern Corsica (France)
Published:December 28, 2018
- PDF LinkChapter PDF
William Cavazza*, Giorgio Gandolfi, Luigi Paganelli†, 2018. "Reconstructing the Alpine orogenic wedge and its foreland basin: The petrologic transition from Cretaceous passive-margin to Eocene foreland sedimentation in northern Corsica (France)", Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson, Raymond V. Ingersoll, Timothy F. Lawton, Stephan A. Graham
Download citation file:
A broad synform in the Balagne region of northern Corsica (France) comprises the most complete remnant of the southwestern Alpine foreland basin and associated orogenic wedge, which have been otherwise fragmented and mostly eroded by a late Cenozoic postcollisional episode of microplate dispersal along the southern European continental margin. The Upper Cretaceous–Eocene turbidites of the Balagne region record the opening and subsequent progressive closure of the Ligurian-Piedmont ocean, the main branch of the Alpine Tethys in the Western Mediterranean. Sandstone detrital modes (gross and heavy-mineral compositions) of the Balagne turbidites can be compared with those of age-equivalent lithostratigraphic units of the western Alps and the Northern Apennines, thus defining broad sediment paleodispersal patterns and providing compelling paleogeographic constraints on the transition from pre-orogenic passive-margin to synorogenic foreland sedimentation. Upper Cretaceous turbidites of the Novella and Alturaia Formations were deposited along the northeastern (European) margin of the narrow Ligurian-Piedmont ocean. In contrast, the mixed carbonate/siliciclastic turbidites of the Upper Cretaceous Narbinco Formation have a distinct composition relative to the age-equivalent Novella and Alturaia Formations and cannot have been derived from the same sediment source area of the Helminthoid Flysch of the Northern Apennines and the Ligurian Alps. The Middle Eocene Balagne foreland basin fill represents a phase of sediment underfilling during the progressive flexure of the Corsican foreland in front of the advancing Alpine orogenic wedge. The basin-fill succession consists of, from bottom to top: (1) continental-to-transitional conglomerate and sandstone filling paleodepressions within the foreland basement complex; (2) thin and discontinuous nummulitic limestone capping—and partly lateral equivalent to—the basal conglomerate; (3) hemipelagic pelite; and (4) a thick turbidite section.
The Cenozoic postcollisional evolution of the Western Mediterranean region is marked by an episode of late Oligocene to Miocene rifting and microplate dispersal affecting the southern European continental margin (e.g., Alvarez et al., 1974; Malinverno and Ryan, 1986; Dewey et al., 1989; Gueguen et al., 1998; Ziegler et al., 2001). As a result of this episode, fragments of both the southwestern portion of the Alpine orocline and its foreland are presently exposed in Corsica, Sardinia, and Calabria (e.g., Caron and Bonin, 1980; Durand-Delga, 1978; Amaudric du Chaffaut et al., 1984; Bonardi et al., 2001; Schmid et al., 2004). Rocks exposed in these areas are integral components of the Alpine orogenic system, and the results of their study must be integrated with those obtained from the classic localities of the western Alps, in order to have a complete understanding of the Alpine geological domain. From this viewpoint, the island of Corsica (Figs. 1 and 2) is particularly relevant because (1) it offers a complete, well-exposed section of the Alpine orogenic wedge and its foreland, and (2) it largely escaped later deformation, and thus primary lithostratigraphic and structural features are relatively well preserved (e.g., Durand-Delga, 1984; Caron, 1994; Lahondère, 1996; Meresse et al., 2012; Vitale Brovarone et al., 2013).
During Alpine peak collisional tectonics and prior to early Miocene counterclockwise rotation, the Corsica continental fragment was juxtaposed against Provençe. Low-temperature thermochronologic studies have shown that during the Alpine collision, virtually the whole of Corsica was covered by either the Alpine orogenic wedge or a thick succession of foreland deposits (Cavazza et al., 2001; Zarki-Jakni et al., 2004; Fellin et al., 2006; Danišík et al., 2007). The vast majority of such overburden has been lost to erosion. A large remnant of the Alpine orogenic wedge is now exposed in the northeastern portion of the island, whereas the exhumed Hercynian crystalline basement complex crops out in the eastern and southern portions of the island. Remnants of the Eocene foreland basin crop out discontinuously along the contact between the orogenic wedge and its foreland.
The largest and most complete remnant of the Eocene foreland basin in Corsica occurs in the Balagne region (Figs. 2 and 3; Nardi et al., 1978; Rossi et al., 2001). Its stratigraphy is similar to that of the western Alpine foreland basin on the continent (Maritime Alps, Haute Savoie, Glarus Alps), with the notable difference that shortening and thickening of the Alpine orogenic wedge ended in the Oligocene in Corsica, whereas it continued into the Neogene in the western Alps. The Eocene foreland deposits of the western Alps were deformed during progressive incorporation into the Alpine orogenic wedge, whereas the Balagne region largely escaped that event, and the original relationships can be still observed and investigated.
In this paper, we present new petrographic data on the Cretaceous–Eocene siliciclastic units of the Balagne region of northern Corsica and integrate such data with preexisting information in order to unravel the overall tectonic evolution of the western Alps. The results of this study place significant constraints on the transition from Cretaceous pre-orogenic passive-margin sedimentation to Eocene synorogenic foreland sedimentation in the western Alps and augment understanding of the paleogeographic implications of such a transition.
The island of Corsica features two rather different geologic provinces (Fig. 2). (1) The western province is characterized by Carboniferous–Permian granitoid rocks and acidic volcanic rocks related to the Hercynian orogeny, with Precambrian–Middle Paleozoic metamorphic host rocks and scattered outcrops of Upper Paleozoic sedimentary rocks. (2) The eastern province is a nappe stack dominated by Jurassic oceanic crust and its sedimentary cover (Durand-Delga, 1978), which underwent high-pressure–low-temperature metamorphism during the Alpine orogeny (Gibbons et al., 1986). Major thrusting occurred in the late Eocene (Caron, 1994).
The Balagne region of northern Corsica (Figs. 3 and 4) is somewhat transitional between the relatively undeformed Hercynian basement complex to the west and the metamorphosed and intensely deformed Alpine orogenic wedge to the east, as discussed below. The eastern and central parts of the Balagne region are composed of the highest structural unit of the Alpine orogenic wedge of Corsica, the Nappe Supérieure of the French authors (e.g., Durand-Delga, 1978, 1984; Malavieille, 1983). This nonmetamorphic to slightly metamorphic (anchizone) unit is preserved within a north-south–trending, kilometer-scale synformal structure and is composed of a pile of tectonostratigraphic subunits comprising an ophiolitic suite and associated deep-marine sedimentary deposits (Marroni and Pandolfi, 2003). The Nappe Supérieure of the Balagne region is the westernmost exposure of the Alpine orogenic wedge, but thermochronologic data have shown that almost all of Corsica was once buried below the Alpine orogenic prism (Cavazza et al., 2001; Zarki-Jakni et al., 2004; Fellin et al., 2006; Danišík et al., 2007), challenging the long-held notion that the western part of Corsica largely escaped the effects of the Alpine orogeny. The existence of a widespread Alpine tectonic overburden, now largely eroded, is also indicated by widespread Alpine-age ductile shear zones within the Variscan basement complex of central Corsica, kilometers to the west of the present-day front of the Alpine nappes (Di Vincenzo et al., 2016).
The western part of the Balagne region includes relatively small remnants of an Eocene foreland basin located between the flexured Hercynian crystalline basement complex and the Alpine orogenic wedge (Durand-Delga, 1978, 1984; Nardi et al., 1978). The Balagne part of the foreland basin is the northernmost occurrence in Corsica of an originally much more extensive Alpine foreland basin; other remnants crop out along the boundary between the Alpine orogenic wedge and the Hercynian basement complex in the Corte and Solenzara regions to the south (Fig. 2).
Figure 2C shows a W-E–oriented geological section from the Balagne region to the town of Bastia. This section has been the object of much research, and it is arguably the best-studied part of Corsica. We provide a short description of the main geological units exposed along this section, from west to east, here. The basement complex of the European plate (Hercynian Corsica) has been traditionally regarded as basically autochthonous and relatively undeformed, but this complex was actually affected by significant Alpine-age shearing (Di Vincenzo et al., 2016). Remnants of the Balagne foreland basin with mid-Eocene sedimentary deposits are overthrust by W-verging Alpine nappes (collectively called Balagne composite nappe; see Fig. 4) made of Jurassic ophiolites and their Cretaceous–Eocene, unmetamorphosed or slightly metamorphosed sedimentary cover. The highly deformed parts of the European continental margin, including the dome-shaped Tenda massif and slices of crystalline basement of the Oletta–Serra di Pigno–Farinole crystalline unit, are interspersed in the Schistes Lustrés nappe (see below). The Tenda massif is mostly made of Hercynian granitoid rocks and Paleozoic volcanic rocks that underwent medium-pressure metamorphic conditions during westward Alpine thrusting and were subsequently partly retrogressed under greenschist facies by ductile extension during Oligocene time (Waters, 1990; Fournier et al., 1991; Brunet et al., 2000; Rossetti et al., 2015). The Nebbio nappe is composed of rock units similar in terms of age and composition to those of the Balagne nappe and similarly unmetamorphosed or affected by very low-grade metamorphism. The Nebbio nappe overlies the previously deformed and metamorphosed Schistes Lustrés nappe and is unconformably overlain by the Miocene Saint Florent calcarenites (Dallan and Puccinelli, 1995; Cavazza et al., 2007). The Nebbio and Balagne nappes are among the uppermost tectonic units of the Alpine orogenic wedge of Corsica. The Schistes Lustrés nappe is composed of meta-ophiolitic rocks covered by metasedimentary rocks. It consists of eclogitic associations (Caron et al., 1981; Lahondère, 1988; Fournier et al., 1991) of Late Cretaceous age (Lahondère and Guerrot, 1997), which were retrogressed under blueschist-facies conditions (Gibbons et al., 1986; Lahondère, 1988; Waters, 1990) during Eocene time (Brunet et al., 2000). Westward emplacement of the Schistes Lustrés nappe onto the Hercynian basement and its Eocene sedimentary cover ended in late Eocene time (Durand-Delga, 1978). The basal thrust fault of the Schistes Lustrés nappe, emplaced during the Eocene (e.g., Durand-Delga, 1984), was tectonically reactivated as a low-angle detachment fault during the Oligocene (Daniel et al., 1996; Jolivet et al., 1990, 1991).
STRUCTURE AND STRATIGRAPHY OF THE BALAGNE REGION
Next, we present a brief description of the geological structure and stratigraphy of the Balagne region, summarized mostly from Marroni et al. (2000), Marroni and Pandolfi (2003), and Malavieille et al. (2011), with additions from Nardi et al. (1978) and Rossi et al. (2001). Descriptions of the structural and stratigraphic architecture of the region are given in Figures 3 and 4, respectively.
The Hercynian basement complex of the Balagne region is composed of gneiss, mica schist, and associated amphibolite of Paleozoic age intruded by late Carboniferous granitoids and cut by Permian dolerite dikes. The Hercynian basement complex of the Balagne region remained in subaerial conditions until the Eocene, as shown by nonconformable and discontinuous continental conglomerate (conglomerati di trasgressione of Nardi et al., 1978; Poudingues polygéniques of Rossi et al., 2001) filling the depressions of an irregular paleorelief and reaching a maximum thickness of ~300 m. The conglomerate marks the inception of a generalized subsidence in northern Corsica and is overlain by nummulite-bearing limestone of middle-late Lutetian age (Nardi et al., 1978) and by laterally equivalent shallow-marine sandstone (Arenarie di Lozari of Nardi et al., 1978). The superjacent unit is the ~250-m-thick Flysch Argilloso (clayey flysch; Nardi et al., 1978), also known as Flysch Noir (black flysch) in the French literature. This unit consists of relatively deep-marine, gray and blackish siltstone and claystone, with local sandstone. This basal succession of conglomerate, nummulitic limestone, sandstone, and shale was affected by polyphase deformation and can be considered parautochthonous. It records the progressive flexure of the European lithosphere in front of the advancing Alpine orogenic wedge, as discussed later. The basal succession is overlain by the semi-allochthonous, upper Lutetian–?Bartonian, 300-m-thick sandy siliciclastic turbidites of the Annunciata Formation (Arenarie di Palasca of Nardi et al., 1978). The Annunciata Formation is thrust over the basal succession (Figs. 3 and 4), as also shown by the presence at its base of metaconglomerates and slices of Hercynian basement complex (Marroni et al., 2001; Marroni and Pandolfi, 2003).
The parautochthonous to semi-allochthonous succession is overlain by the Nappe Supérieure, the highest structural element of the Alpine orogenic wedge in Corsica, consisting of two major tectonic units: (1) the Balagne nappe and (2) the Bas-Ostriconi nappe. The Balagne nappe consists of two juxtaposed tectonostratigraphic units, the Toccone and Navaccia Units (Figs. 3 and 4). Overall, the reconstructed stratigraphy of the Balagne nappe features a Jurassic ophiolitic sequence and its associated deep-sea sedimentary cover, including pelagic deposits as well as hybrid (i.e., mixed siliciclastic-carbonate) turbidites. The latter deposits are key layers in terms of paleogeographic reconstructions, as their provenance constrains the Cretaceous paleogeographic configuration of the Alpine Tethys. Hybrid arenites of the Balagne nappe studied during this research include the Alturaia Flysch (late Barremian–middle Aptian [Marroni et al., 2004]; Arcosi e conglomerati di Cima de l’Alturaia of Nardi et al., 1978; Formation de l’Alturaia of Rossi et al., 2001) of the Toccone Unit and the Novella Sandstone (Cenomanian [Nardi et al., 1978]; Arenarie de la Gare de Novella of Nardi et al., 1978; Grès de la gare de Novella of Rossi et al., 2001) of the Navaccia Unit.
The Bas-Ostriconi nappe is a structural unit cropping out in northern Balagne (Fig. 3), where it is thrust over the Eocene cover of the basement or the Balagne nappe (Nardi et al., 1978; Rossi et al., 2001; Pandolfi et al., 2016). The Bas-Ostriconi nappe is mostly composed of Upper Cretaceous carbonate (with subordinate siliciclastic) turbidites of the Narbinco Flysch (Flysch calcareo of Nardi et al., 1978; Marino et al., 1995; Rossi et al., 2001). The basement of this nappe is unknown. A polyphasic deformation history of Alpine age, characterized by four superimposed deformation phases, has been recognized in the whole Bas-Ostriconi nappe. This deformation history is correlative with that described for the Balagne Nappe (Marroni and Pandolfi, 2003; Pandolfi et al., 2016). According to Malavieille et al. (2011), the Bas-Ostriconi nappe shows NW-SE–striking and SW-vergent folds associated with an east-dipping cleavage and with top-to-the-west thrust faults. These structures were formed at relatively shallow crustal levels, at temperatures <280 °C (Vitale Brovarone, 2013). The Bas-Ostriconi nappe is structurally correlatable with the Macinaggio nappe of northeastern Cap Corse (Fig. 2). The Narbinco Flysch is considered to be a stratigraphic equivalent of the Macinaggio Flysch, and both lithostratigraphic units have been long correlated with the Helminthoid Flysch of the Northern Apennines and Ligurian Alps (e.g., Nardi, 1968; Durand-Delga, 1984; Dallan and Nardi, 1984).
The present-day topography lies very low in the structural stack of the Balagne, Nebbio, and Macinaggio nappes of northern Corsica. Only a few, relatively thin outliers of these once much thicker and widespread piles of nappes remain, thus making it somewhat difficult to reconstruct their geometry and internal arrangement. Despite such intrinsic limitations, the study of these allochthonous Alpine units and the underlying Middle Eocene autochthonous to parautochthonous succession is crucial for the understanding of the transition from pre-orogenic Mesozoic passive margin to foreland lithospheric flexure and synorogenic sedimentation in the southwestern part of the Alpine orogen.
METHODS AND SAMPLES
Sandstone samples were taken from all suitable Eocene lithostratigraphic units of the Balagne foreland basin, i.e., from bottom to top (Fig. 4): (1) basal continental conglomerates (five samples), (2) the Lozari sandstone (five samples), (3) the Flysch Argilloso (five samples), and (4) the Annunciata Formation (nine samples). Eighteen samples of the Cretaceous sandstone of the Balagne composite nappe were analyzed from the following lithostratigraphic units: (1) the Alturaia Flysch of the Toccone tectonostratigraphic unit, (2) the Novella Sandstone of the Navaccia tectonostratigraphic unit, and (3) the Narbinco Flysch of the Bas-Ostriconi nappe (Fig. 4).
Framework detrital modes of sandstone samples were determined through point counting using the Gazzi-Dickinson method. This procedure, proposed by Gazzi (1966) and Dickinson (1970), and discussed by Gazzi et al. (1973), Ingersoll et al. (1984), and Zuffa (1985), minimizes the variation of composition with grain size. Thin sections were stained for K-feldspar. Five-hundred framework grains were counted for each sandstone sample and were assigned to 23 categories (Table 1). Criteria used for distinguishing lithic types, matrix types, and other components of samples are those of Dickinson (1970) and Graham et al. (1976).
Heavy-mineral concentrates for the analyzed sandstone samples were prepared following the procedures of Gazzi et al. (1973). Two-hundred translucent heavy-mineral grains in the 2–4 phi grain-size range were counted using a petrographic microscope and heavy liquids of known refractive indices. Heavy-mineral raw counts were then recalculated to produce the 14 categories included in Table 2. Further details on sample preparations and analytical techniques used to determine the framework compositions and heavy-mineral assemblages of the sandstone samples can be found in previous papers (Gazzi et al., 1973; Zuffa, 1985; Gandolfi et al., 1983, 2007).
PETROGRAPHIC RESULTS AND INTERPRETATION
Despite some limiting factors, such as compositional modifications occurring during weathering, sediment transport, deposition, and diagenesis (for a review, see Zuffa, 1985; Johnsson and Basu, 1993; Arribas et al., 2007; Garzanti, 2016), several studies have proposed a correlation between sandstone composition and specific plate-tectonic settings (e.g., Dickinson, 1970; Crook, 1974; Dickinson and Suczek, 1979; Ingersoll and Suczek, 1979; Valloni and Maynard, 1981; Dickinson et al., 1983; Ingersoll, 1990). Such broad correlation schemes have been tested extensively, and sandstone detrital modes are now routinely determined to constrain, in conjunction with other basin-analysis techniques, the plate-tectonic setting of ancient terrigenous successions.
Sandstone point-count data (gross composition and heavy minerals) from the study area were recalculated to produce the grain parameters indicated in Tables 1 and 2. Such parameters were then used to generate the compositional diagrams shown in Figures 5 and 6, which are discussed below.
In a standard NCE-CE-CI ternary diagram (Fig. 5; Zuffa, 1985), where NCE is noncarbonate extrabasinal, CE is carbonate extrabasinal, and CI is carbonate intrabasinal, all four Eocene sandstone units (i.e., basal conglomerates, Flysch Argilloso, Lozari Formation, and Annunciata Formation) and the Barremian–Aptian Alturaia Formation plot on the NCE (noncarbonate extrabasinal) apex. Within the Eocene units, Paleocene to early Eocene carbonate pebbles and cobbles are rare in the basal conglomerates (Nardi et al., 1978; Rossi et al., 2001), whereas Mesozoic carbonates are locally present as olistoliths in the Flysch Argilloso and as pebbles/cobbles in the basal section of the overlying Annunciata Formation (Rossi et al., 2001). Such inverted stratigraphy of the carbonate conglomerate clasts points to the progressive unroofing of a rather thin and discontinuous Upper Triassic to Lower Eocene carbonate succession covering the Hercynian basement complex during the progressive flexure of the foreland in front of the advancing Alpine thrust sheets. As is commonly the case (see Ingersoll et al., 1987), carbonate clasts survived only in the coarser grain sizes and are absent in most sandstone samples. Among the Eocene lithostratigraphic units, only Lozari Formation sandstone contains a small proportion (≤1%) of carbonate extrabasinal clasts. The Alturaia Formation sandstone has an arkosic (i.e., quartzofeldspathic) composition and is devoid of carbonate components; rare carbonate clasts are locally present where the Alturaia Formation directly overlies the basalts of the Balagne ophiolites.
As to the carbonate composition of the other Cretaceous sandstone units, modal analyses of the Cenomanian Novella Sandstone include, on average, 18.5% carbonate extraclasts (Table 1), mostly micritic limestone of difficult age attribution. The Narbinco Flysch (Cenomanian–Campanian) is the only studied sandstone unit with a significant fraction of carbonate intraclasts (Table 2; Fig. 5). Some of the clasts classified as carbonate extraclasts may also be intrabasinal, as it was difficult in some cases to apply the criteria discussed by Zuffa (1985) to discriminate between intrabasinal and extrabasinal carbonate clasts (see also Critelli et al., 2007).
In a QmFLt ternary diagram (Fig. 5; Dickinson 1970), where Qm is monocrystalline quartz, F is total feldspar, and Lt represents lithic fragments, the Eocene units show a fairly coherent correlation between stratigraphic position and lithic content. The basal conglomerate contains a modal average of 30.5% aphanitic lithic fragments derived from the Hercynian basement complex (felsitic volcanics and low-grade metamorphics); the average percentage decreases progressively up section to 21.8% in the pelitic-arenaceous flysch and 11.9% in the Annunciata Formation. The Lozari sandstone contains only an average of 5.5% aphanitic lithic fragments, but this thin and discontinuous unit has a local provenance, and this may well account for this discrepancy from the general trend. As to the Cretaceous turbidite units, the Alturaia Formation has a very simple arkosic composition and plots along the QmF side of the ternary plot within the “basement uplift” field of Dickinson (1970). Compared to the Alturaia Formation, both the Narbinco Flysch and the Novella Sandstone have a higher content of aphanitic lithic fragments, with volcanic lithic fragments averaging 8.4% ± 4.0% and slate/phyllite fragments averaging 6.0% ± 2.3% (Table 1).
The heavy-mineral composition of all studied lithostratigraphic units is simple, with zircon, tourmaline, and rutile (ZTR) averaging 88.8% ± 9.4% in the Cretaceous formations and 89.0% ± 15.0% in the Eocene formations. Other heavy-mineral species include, in order of decreasing abundance, monazite, xenotime, garnet, allanite (and other epidotes), Cr-spinel (picotite), sphene, and other rare minerals (Table 2; Fig. 6). All sandstone samples from the Eocene lithostratigraphic units have a coherent heavy-mineral composition characterized by ZTR, monazite, xenotime ± small quantities of garnet, epidotes, and sphene. Such a simple heavy-mineral assemblage points to a provenance from the Hercynian crystalline basement complex, dominated by granitoid plutons intruding medium-grade metamorphic rocks. Among the Cretaceous sandstone units, the Narbinco Flysch has an extremely simple heavy-mineral assemblage featuring only ZTR grains in all samples except one (Table 2; Fig. 6). Samples from the Alturaia Formation have a heavy-mineral composition very similar to that of the Eocene units, with ZTR, monazite, xenotime, and subordinate garnet. Samples from the Novella Sandstone have the most varied heavy-mineral assemblage, with characteristic Cr-spinel, diallage ± augite and hornblende, marking a provenance from an oceanic lithospheric section.
Late Cretaceous Paleogeography and Sediment Dispersal System
By late Early Cretaceous time (ca. 130–125 Ma), the paleogeography of the Alpine-Mediterranean domain was characterized by two parallel oceanic basins: the Pyrenean-Valais ocean north of Iberia and the Alpine Tethys south of it and extending eastward (Fig. 7A; for a review, see Cavazza et al., 2004). These two oceanic basins were separated by the eastward-tapering Iberian-Briançonnais continental block, including Hercynian Corsica. Based on their age, facies characteristics, and structural position, the Cretaceous turbidite units analyzed in this study were deposited in the Ligurian-Piedmont segment of the Alpine Tethys, as discussed below.
Integration of a number of studies on the heavy-mineral distribution and sediment dispersal pattern in the Upper Cretaceous flysch deposits of the western and central Alps and the Northern Apennines (Stanley, 1965; Wildi, 1985; Bernoulli and Winkler, 1990; Argnani et al., 2006; Thum et al., 2015, and references therein) defines three broad heavy-mineral associations that coincide with distinct paleogeographic domains. (1) A ZTR association is typical of the northern Tethyan domain (Valais and northern Ligurian-Piedmont basin) and is interpreted as representative of detrital derivation from the granitoid rocks of the southern European continental margin. (2) A garnet-dominated association is typical of the southern Ligurian-Piedmont (including the Northern Apennines) and Rhenodanubian domains, and it represents a provenance from the Austro-Alpine and South Alpine domains, i.e., the northern Adriatic continental margin. (3) An association characterized by its minor content of Cr-spinel and high-grade metamorphic minerals such as staurolite, kyanite, and sillimanite is typical of the Lombardian flysch of the southern Alps, derived from the core of the Alpine orogen and the associated ophiolitic successions.
Such broad definitions of the overall Cretaceous sediment provenance and paleodispersal pattern in the Alpine region are clearly a simplification of a complex paleogeography. Nevertheless, even a broad paleogeographic attribution represents a useful first-order interpretative tool, particularly in areas such as Corsica, where the superposition of various tectonic events and later widespread erosion have largely obscured the relationships between the various tectonostratigraphic units. From this standpoint, the petrologic characteristics of the Cretaceous sandstone units studied in this paper (i.e., Alturaia Formation, Novella Sandstone, and Narbinco Flysch) point to deposition along the European margin of the Liguria-Piedmont ocean and its adjacent offshore domain (Fig. 8). This conclusion is in line with previous interpretations (Nilsen and Abbate, 1983; Bernoulli and Winkler, 1990; Argnani et al., 2004, 2006; Bracciali et al., 2007; Pandolfi et al., 2016). Although largely diachronous, the three Cretaceous turbidites units are depicted in the same paleogeographic sketch of Figure 8 because the overall geodynamic setting was identical.
The siliciclastic components of the Narbinco turbidite sandstone are compositionally different from those of the age-equivalent Helminthoid Flysch of the Northern Apennines (Valloni and Zuffa, 1984) and the Ligurian Alps, to which they have been routinely correlated in terms of both heavy-mineral and framework compositions. The Narbinco sandstone has a simple ZTR heavy-mineral assemblage, with small quantities of monazite and xenotime, and garnet being almost absent (Table 2). Conversely, siliciclastic sandstone of the Helminthoid Flysch characteristically has a more varied assemblage, with a high content of garnet and significant quantities of staurolite (Argnani et al., 2006; see also Fig. 4). These significant compositional differences warrant the interpretation proposed above.
Eocene Paleogeography and Sediment Dispersal System
In spite of their relatively small extent, outcrops of Eocene foreland sedimentary deposits of the Balagne region are important for understanding the relationships between the Alpine orogenic wedge and its foreland basin. Eocene sedimentary rocks of northern and central Corsica constitute the southernmost exposure of foreland deposits of the western Alps (Fig. 1). As such, they provide compelling constraints on the areal extent of the Alpine collisional belt.
The Eocene stratigraphic record in the study area (Figs. 3 and 4; Nardi et al., 1978; Jourdan, 1988; Rossi et al., 2001; Marroni and Pandolfi, 2003; our observations) defines a clear transgressive pattern, with (1) a deeply incised paleotopography filled by coarse-grained fluvial/alluvial clastics, overlain by (2) transitional and shallow-marine sedimentary deposits, and (3) a thick turbidite succession. This vertical arrangement is typical of progressive lithospheric flexure during the early stage of a foreland basin, when the creation of accommodation exceeds sediment accumulation (Sinclair, 1997). The stratigraphy of the study area mirrors that in the western and central Alps (for a summary, see Gupta and Allen, 2000; Allen et al., 2001). Correlation of foreland deposits from the central and western Alps toward the south in Corsica shows that the orogenic wedge and its associated foreland basin were rather rectilinear and that most oroclinal bending of the western Alps is post-Eocene in age.
Mid-Eocene paleogeography and paleotectonics of the Western Mediterranean region are characterized by the Alpine metamorphic peak, indicating deep indentation and mechanical coupling between the European and Adria plates. The paleogeographic reconstruction by Stampfli and Hochard (2009) at 40 Ma (Bartonian) shows an incipient collision between Corsica and the eastern margin of Adria (Fig. 7B). This is at odds with our stratigraphic analysis, which shows a clear flexural trend from the Lutetian, thus indicating that the Alpine orogenic wedge was already overthrusting the Corsican continental lithosphere during the Lutetian.
The Balagne region of northern Corsica (France) includes (1) a middle Eocene parautochthonous succession deposited during the progressive flexure of the Corsican foreland in front of the advancing west-vergent Alpine orogenic wedge, and (2) the overlying, slightly metamorphosed Balagne nappe. The nappe is the structurally highest element of the Alpine orogenic wedge of Corsica, composed of a highly imbricated stack of thrust sheets comprising both ophiolitic units (Middle–Late Jurassic) and deep-marine sedimentary deposits (latest Jurassic–Late Cretaceous).
Petrologic characteristics of the Cretaceous and Eocene sandstone units of the Balagne region reflect two contrasting geodynamic settings. The mid-Eocene succession records the progressive flexure of the European continental lithosphere in front of the advancing Alpine orogenic wedge and the ensuing creation of accommodation space during the underfilling stage of the foreland basin. The Cretaceous Novella and Alturaia Formations were deposited along the northeastern (European) passive margin of the narrow Ligurian-Piedmont ocean. The mixed carbonate/siliciclastic turbidites of the Narbinco Formation have a distinct composition and cannot have been derived from the same sediment source area of the Helminthoid Flysch of the Northern Apennines and the Ligurian Alps, as previously thought.
This paper is dedicated to the late Luigi Paganelli, a long-time colleague and friend who provided expertise, data, and insights for this publication. Late Cretaceous paleogeographic and paleoenvironmental interpretations of the Ligurian-Piedmont realm were discussed with Gerard Stampfli. Laurent Thum provided the original digital files for Figures 7 and 8. Thanks are due to Salvatore Critelli, Tim Lawton, and Michelangelo Martini for their careful reviews of an earlier version of the manuscript. This research was funded by MIUR (Italian Ministry of Education, University and Research) and the University of Bologna.
Figures & Tables
Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson
CONTAINS OPEN ACCESS
- Alpine Orogeny
- carbonate rocks
- clastic rocks
- continental margin
- foreland basins
- Liguria Italy
- Ligurian Alps
- Northern Apennines
- orogenic belts
- passive margins
- Piemonte Italy
- plate tectonics
- sedimentary basins
- sedimentary rocks
- Southern Europe
- tectonic wedges
- Upper Cretaceous
- Western Alps
- Western Europe
- Helminthoid Flysch
- Ligurian-Piedmont region
- Balagne France
- Novella Formation
- Alturaia Formation