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Constraints on the dispersal of Mt. Vulture pyroclastic products: implications to mid-Pleistocene climate conditions in the foredeep domain of southern Italy

Contraintes sur la dispersion des roches pyroclastiques du Mont Vulture, sud de l’Italie : implications des conditions climatiques de l’avant-fosse au Pléistocène moyen
Giuseppe Corrado, Paola Di Leo, Paolo Giannandrea et Marcello Schiattarella

Résumés

Le volcan du Mont Vulture est situé sur la portion frontale des Apennins du sud de l’Italie. Ses produits datant de la période du Pléistocène moyen se sont répandus en direction du Sud-Est, jusqu’à l’intérieur du bassin adjacent de l’avant-fosse du Bradano. Leur position au sein des séries sédimentaires du Quaternaire fournit une clé de lecture pour comprendre l’évolution de l’avant-fosse. Les éléments géomorphologiques et stratigraphiques ont révélé que le bassin fluvio-lacustre de Venosa, situé à proximité de l’édifice volcanique, est le résultat du remplissage d’une paléovallée incisée dans des séquences Pléistocènes de l’avant-fosse. L’étude des niveaux de téphra associés aux paléosols du secteur plus au sud de l’avant-fosse (Irsina et Pomarico), revêt une importance capitale en vue d’une reconstruction du scénario morphoclimatique. Sur d’autres sites, les niveaux de ponces déposés durant les éruptions ainsi que les paléosols qui y sont associés, sont présents au sommet de la série sédimentaire du Pléistocène inférieur-moyen. Les analyses géochimiques et minéralogiques des paléosols et des roches pyroclastiques montrent que les ponces de Pomarico se sont accumulées dans un environnement alluvionnaire et pourraient être corrélées aux ignimbrites de Fara d’Olivo (âge < 687 ± 8 ka) présentes dans l’unité basale de la séquence volcanique du Mont Vulture. La présence d’halloysite‑7Å dans les paléosols qui ont été échantillonnés sur deux sites différents, nous suggère que les conditions d’altération au cours du Pléistocène moyen ont probablement eu lieu dans le cadre d’un climat humide, caractérisé par une alternance de saisons pluvieuses et durant lesquelles a eu lieu un processus de lixiviation, suivies par des périodes de sécheresse marquées par de très faibles précipitations.

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Notes de la rédaction

Soumis le 20 décembre 2016, reçu sous sa forme révisée le 25 mars 2017. Définitivement accepté le 16 mai 2017.

Texte intégral

We sincerely thanks David Karatson and Karoly Nemeth for the accurate revision of the early version of the manuscript. We wish to thank also Dr. Jean‑Pierre Scarpelli, Lecturer of French at the Basilicata University, for the translation of Résumé and Version française abrégée. This work has been funded by PO-FESR Basilicata 2007-2013 (MeTIBas Project), granted to Dr. Paola Di Leo, and by Basilicata University RIL 2015, granted to Professor M. Schiattarella.

1. Introduction and methods

1In this work, the mid-Pleistocene morpho-climatic scenario and the depositional setting of the foredeep domain of southern Italy (fig. 1), as defined by Selli (1962) and D’Argenio et al. (1973), has been reconstructed on the basis of a multiproxy approach. This includes geomorphological and stratigraphic investigations of the entire southern sector of the Bradano foredeep as well as geochemical and mineralogical analyses of paleosols and pyroclastic deposits and isotopic analysis of carbonate crusts parallel to soil horizons in paleosol sequences.

2The Bradano foredeep has been well studied for a long time from a geological viewpoint, but scarce data on the morpho-evolutionary steps of this relevant physiographic unit and no information about its paleoclimate scenarios have been obtained in the past. On the other hand, in spite of the paleomorphological meaning of their dispersal, the pyroclastic rocks from Mt. Vulture eruptions have been analysed until now only in the closeness of the volcano.

3The significant outcrops recognized in the Venosa lacustrine-alluvial basin, not far from the volcanic edifice of Mt. Vulture, and the tephra levels and associated paleosols from the southern zone of the study area (Irsina and Pomarico sites), are crucial for a complete reconstruction of the Pleistocene paleoclimatic and geomorphological scenario. The stratigraphic records within the Quaternary successions may represent a key to understand the paleo-environmental conditions of the south-Apennines foredeep.

4The Tyrrhenian Sea, the southern Apennines, and the Bradano Basin are the expressions of a single geodynamic system, representing respectively the back-arc basin, the orogenic chain, and its foredeep. The southern Apennines are an Adriatic-verging fold-and-thrust belt derived from the deformation of large Mesozoic–Cenozoic circum-Tethyan domains and associated Neogene–Pleistocene foredeep and satellite-basin deposits (Pescatore et al. 1999). Starting from the Tortonian, the orogen underwent low-angle extension which led to the exhumation of its non-metamorphic “core complex” (Schiattarella et al., 2006). This regional frame has been strongly complicated by Quaternary tectonics, responsible for the segmentation of the chain and its uplift. Also the Bradano foredeep underwent neotectonic rising, responsible for vertical incision of streams and formation of terraces.

5In this paper, we try to furnish some suggestions useful to the reconstruction of the paleodrainage of the southern sector of the foredeep basin (fig. 1) on the basis of both geomorphological (i.e. comparing elevations of reference levels, such as paleosurfaces and fluvial terraces) and statigraphical markers (i.e. correlating tephra layers). Our results represent necessary constraints to reconstruct ancient landscapes, calculate erosion rates, and correlate stratigraphic sequences all along the Bradano foredeep. To this scope, we analysed 13 rock and paleosol samples from three different sites (fig. 1) by means of a multiproxy approach.

6The mineralogical associations in the paleosols were identified by X-ray diffraction (XRD) using a Rigaku miniflex apparatus, operating under the following conditions: CuKα radiation, 0.02 steps, 0.5/min time, sample spinner (XRD analyses were performed at CNR-IMAA laboratories). The MacDiff software (4.2 version, see website at http://www.geol-pal.uni-frankfurt.de), with JCPDS mineralogical cards database, was used to identify the mineralogical phases.

7The total element composition of paleosols was determined by X-ray fluorescence spectrometry (XRF) of fused discs using a Philips PW 1480 instrument. Samples were homogenized in an agate mortar. Total loss on ignition (LOI) was gravimetrically estimated after overnight heating at 950°C. Isotopic compositions of carbonate crusts were detected using a VG SIRA‑10, stable isotope ratio mass spectrometer, determining the 13C/C and 18O/16O ratio.

Fig. 1 – Regional framework in which the study area is included and dispersal of Mt. Vulture volcano pyroclastic products.
Fig. 1 – Cadre géologique de la zone d'étude incluant la dispersion des produits pyroclastiques du volcan du Mont Vulture.

Fig. 1 – Regional framework in which the study area is included and dispersal of Mt. Vulture volcano pyroclastic products.  Fig. 1 – Cadre géologique de la zone d'étude incluant la dispersion des produits pyroclastiques du volcan du Mont Vulture.

2. Geological and geomorphological setting

8The NW-SE-trending Bradano foredeep basin is located between the Apennine chain and the Murgia carbonate platform of the Apulian foreland, in southern Italy (fig. 1). The eastern thrust front of the Lucanian segment of the chain is made of Mesozoic-Cenozoic clayey, sandy, and marly-calcareous successions, organized in more stacked tectonic units verging toward the Adriatic Sea (Patacca and Scandone, 2007, and references therein). Such units tectonically overlap the Apulian foreland system. The vertical and lateral stratigraphic variations of the foredeep units are due to the dynamic behaviour of the chain-foreland couple during Pliocene and Quaternary times. The lower plate (i.e. the Apulian platform) of such a system is segmented by transversal lithospheric faults which allowed a differential retreat responsible for the magma ascent of the Vulture volcano (Schiattarella et al., 2005).

9Located on the thrust front of the chain, at the north-western apex of the study area (fig. 1), Mt. Vulture volcano is made of mid-Pleistocene products spread along the adjacent Bradano foredeep comprising pyroclastic flow, pyroclastic fall, and epiclastic deposits. The above mentioned foredeep basin represents the southern segment of the Apennine foredeep (Selli, 1962; D’Argenio et al., 1973), filled by several kms-thick Pliocene-Pleistocene deposits (Tropeano et al., 2002). The intercalation of an Apennine allochthonous wedge marks a sharp stratigraphic discontinuity in coincidence of the passage from middle to upper Pliocene (Balduzzi et al., 1982). Such a limit separates the whole sequence in two parts which are, on a regional scale, correlative to the coeval deposits of the Ofanto Basin succession (Giannandrea et al., 2014). The lower part of the sequence is made of turbiditic sediments, middle Pliocene in age, whereas the upper part is formed of several hundred metres thick clayey deposits (Argille subappennine Fm) passing toward the top to regressive sands (Sabbie di Monte Marano) and conglomerate (Conglomerati di Irsina), upper Pliocene – lower Pleistocene in age, showing deltaic, beach, and alluvial facies (Ricchetti, 1967). Regional-scale unconformities are also present in the regressive sequence shown above, separating different marine to continental units such as the Venosa, Monte Vulture, and Monticchio supersynthems (Giannandrea, 2009). Most part of the volcanic products and epiclastic sediments of the area are included in the Monte Vulture supersynthem, forming a 35 to 80 m thick succession in the mid-Pleistocene Venosa Basin (Giannandrea, 2009).

10Monte Vulture is a strato-volcano composed of highly undersaturated alkaline-potassic to ultrapotassic rocks belonging to the Roman Magmatic Province and ranging in age from 687 ± 8 ka to 141 ± 11 ka (Villa and Buettner, 2009), represented by pumiceous to lapilli-dominated fall deposits, pyroclastic flows, lava flows, and various mud flow to debris flow deposits produced by lahars. It is a relatively small volcanic complex characterized by a central vent and parasitic cones, domes, and eccentric lava-plugs (Giannandrea et al., 2004). The earlier volcanic morphology has been modified by summit and lateral volcano-tectonic collapses. The volcano was grown on a structural high of the pre-Pliocene and Pliocene sedimentary bedrock. Since its activity developed in middle Pleistocene times, it was coeval with the recent deformation of the frontal (i.e. eastern) part of the south-Apennines chain (Schiattarella et al., 2005).

11Relevant outcrops of the mid-Pleistocene products of Mt. Vulture have been recognized in the Venosa lacustrine-alluvial basin, adjacent to the volcanic edifice, and in the southernmost sector of the Bradano foredeep. The Venosa Basin (Giannandrea, 2009) is constituted by the sedimentary infill of a paleovalley cut in the Pleistocene Bradano succession between the Venosa and Irsina villages (fig. 1). At other sites, such as in the Pomarico village area, pumice fall horizons and related paleosols are present on the top of the lower-middle Pleistocene sedimentary succession. At the southern termination of the foredeep, in the well-known Montalbano stratigraphic section, several tephra layers interbedded in marine sediments have been studied, dated, and partly correlated with the activity of Mt. Vulture volcano (Petrosino et al., 2015).

12The mid- to late Pleistocene Vulture volcano deposits (fig. 2) are subdivided in a number of units bounded by unconformities (Giannandrea et al., 2006). The application of the UBSU criteria (Chang, 1975; Salvador, 1987) to a complex area such as the foreland basin or satellite basins of the southern Apennines (Giannandrea et al., 2014) implies a long path of survey, sampling, dating, fieldwork refinements, and mapping extrapolation. The passage from both field and literature data collection (stratigraphic, sedimentological, biostratigraphical, tectonic, geomorphological, and geophysical data) to the hierarchical subdivision of the units by means of the correct recognition of the discontinuities implicates a continuous comparing of different information sources. In such a way, the boundaries of the units can assume the same meaning of the limits in the sequence stratigraphy. This means that the unconformity-bounded units are not “empty boxes” at all, but highly informative archives.

Fig. 2 – Volcanic succession of Mt. Vulture.
Fig. 2 – Chronologie des dépôts volcaniques du Mont Vulture.

Fig. 2 – Volcanic succession of Mt. Vulture.  Fig. 2 – Chronologie des dépôts volcaniques du Mont Vulture.

The mid- to late Pleistocene Vulture volcano deposits (Ar/Ar ages: 687 ± 8 ka to 141 ± 11 ka; Villa and Buettner, 2009) are subdivided into more units bounded by unconformities (Giannandrea et al., 2006).
Les dépôts du Pléistocène moyen et supérieur (âges Ar/Ar : 687 ± 8 ka à 141 ± 11 ka ; Villa et Buettner, 2009) sont subdivisés en plusieurs unités délimitées par des discordances (Giannandrea et al., 2006).

13The Monte Vulture and Monticchio supersynthems have been distinguished on a regional scale also in the sedimentary basins. The supersynthem boundary coincides with the Melfi Synthem, locally represented by a paleosol M18 after La Volpe and Principe (1994) suturing the faults (Schiattarella et al., 2005; Giannandrea et al., 2006). The Monte Vulture Supersynthem includes the Foggianello, Barile, and Melfi synthems, whereas the Monticchio Supersynthem, more limited in extension, contains the Valle dei Grigi - Fosso del Corbo and the Laghi di Monticchio synthems (tab. 1). The Foggianello Synthem groups three subsynthems linked to subsequent eruptive stages. Alluvial deposits and dykes crossing the pre-eruptive bedrock located north-east to the volcano are present at the base of the succession. The overlying synthems are respectively constituted by pyroclastic-fall deposits and ignimbrites (Fara d’Olivo Volcano-Stratigraphic Unit, after La Volpe and Principe, 1994). The Barile Synthem groups four subsynthems formed by both pyroclastic fall and flow products and lava flows and domes, constituting the central edifice of Monte Vulture. The Melfi Synthem is formed of lava flows from two different vents and fluvial-lacustrine deposits from two different catchments. Finally, the Valle dei Grigi - Fosso del Corbo and Laghi di Monticchio synthems include volumetrically reduced subsynthems which are related to vents characterized by the presence of mantle xenoliths.

Tab. 1 – Chronostratigraphy of the Mount Vulture volcano and surroundings.
Tab. 1 – Chronostratigraphie du volcan du Mont Vulture et ses environs.

Tab. 1 – Chronostratigraphy of the Mount Vulture volcano and surroundings.Tab. 1 – Chronostratigraphie du volcan du Mont Vulture et ses environs.

39Ar/40Ar ages after the review of Villa and Buettner (2009), except (1) after Bonadonna et al. (1998), and (2) after Laurenzi et al. (1993).
Àge 39Ar/40Ar d'après la recension de Villa et Buettner (2009), à l'exception de (1) d'après Bonadonna et al. (1998), et (2) d'après Laurenzi et al. (1993).

14In the Venosa Basin, alluvial and lacustrine deposits were deposited, consisting of conglomerate, clay, and tuff, which can be grouped into three synthems bounded by stratigraphic unconformities. Sediments are mainly volcaniclastic deposits, with minor layers of primary volcanic products (pyroclastic-flow and -fall deposits), which can be correlated with the Foggianello, Barile, and Melfi synthems of Monte Vulture (Giannandrea, 2009).

15The study areas including the sampling sites of Irsina and Pomarico are characterized by mesa-like landscapes, moulded in a clayey-sandy succession overlain by conglomerates forming the caprock of the mesas. At Irsina, the studied log is located immediately below a relict flat land surface at about 400 m asl, in turn morphologically inserted in a higher and slightly older paleosurface at about 540 m asl moulded in the Conglomerati di Irsina Fm. Beneduce et al. (2004) assumed that the fluvial net cutting in the Matera horst and its surroundings, not far from the study area (fig. 1), started to be entrenched during mid-Pleistocene times, on the basis of the analysis of a 50 cm-thick bed of reworked pyroclastic rocks included in the upper part of a clastic deposit associated to a fluvial terrace outcropping about 10 km southwest of Matera town. The volcanic minerals in the clastic component of this layer include sanidine, clinopyroxene and phlogopite, suggesting that the most suitable source supplying the volcanic mineral assemblage was the lower part of the Mount Vulture volcanic succession (Giannandrea et al., 2004, 2006).

3. Stratigraphic sections and sampling

16The three sampling sites of this study are comprised in the Lucanian segment of the foredeep basin of the southern Apennines (fig. 1), featured by uplift since the middle Pleistocene (Doglioni et al., 1996; Schiattarella et al., 2006). In the area between Venosa and Irsina, due to the infill of a previous fluvial incision in the Bradano foredeep deposits, a disconformity marks the contact between lower Pleistocene clay (Argille subappennine Fm) and mid-Pleistocene alluvial-lacustrine sediments. Such continental sediments are covered by few metres-thick eluvial and colluvial deposits with interbedded carbonate crusts. No information about pyroclastic-fall deposits from Monte Vulture volcano, either primary or reworked, is reported for sites to the south of Irsina village.

17In the neighborhood of Pomarico village (fig. 1), we surveyed and sampled cm-sized whitish pumices on the top of the Bradano foredeep succession, interbedded to red-brown fine to medium-grained silty-clayey sand with an isolated metric intercalation of pebbly sandstones and covered by reddish silty-clayey sand and conglomerate (fig. 3B). Such a succession is about 24 m-thick and overlies the Conglomerati di Irsina formation by an erosional contact. From the bottom, this section consists of a 2 m-thick alluvial conglomerate with concave cross-bedding, overlain by 4.80 m-thick sandstone organized in decimetric beds, mainly massive at the base and with parallel- and cross-lamination toward the top (fig. 4). The sandstone contains locally lens-shaped horizons of small pebbles and abundant white pumices. An 11 m-thick layer of thin-laminated sandy clay with vegetation remains and intercalation of laminated sandstone in the uppermost 4 m-thick level, is present in the upper part of the section. On this level, separated by an erosional surface, a 6 m-thick horizon of reddish sand and conglomerate developed. Samples Pom 1 (pumice), Pom 2 (whitish sandstone), and Pom 3 (reddish sands) are taken from this succession (fig. 3B).

18The other two sampling sites are located near the villages of Irsina and Venosa, at the opposite ends of the Venosa Basin (fig. 1). At Irsina, two samples (Irs 1 and Irs 2) come from two different paleosols (fig. 3A) in the uppermost part of the alluvial conglomerate correlated to the epiclastic succession outcropping at Venosa. Such paleosols are about 1.50 m-thick, with a sandy-silt grain size, massive, and red-brown in colour. Both are covered by carbonate crusts which form a layer of maximum 10 cm in thickness. Near Venosa town, at the Fornace locality, three paleosols (fig. 3C) are interbedded in the uppermost part of the epiclastic succession (Giannandrea, 2009). These relict soils, red-brown in colour, show thickness variable from 54 cm to 1.90 m and are made of massive silty-clayey sand, with a granular and porous texture, and a skeleton of small pebbles. Every paleosol is covered by 2 to 5 cm-thick calcic crusts. Five samples (For 1, For 2, For 4, For 6, For 7) have been picked up in the sandy paleosols and three in the overlying crusts (For 3, For 5, For 8).

Fig. 3 – Sampling sites.
Fig. 3 – Sites d'échantillonnage.

Fig. 3 – Sampling sites.Fig. 3 – Sites d'échantillonnage.

See text for explanations about samples. A. Irsina site; B. Pomarico site; C. Venosa site (Fornace section).
Voir le texte pour l'information sur les échantillons. A. Site d'Irsina ; B. Site de Pomarico ; C. Site de Venosa (section de Fornace).

Fig. 4 – Stratigraphic log of the Pomarico section.
Fig. 4 – Log stratigraphique de Pomarico.

Fig. 4 – Stratigraphic log of the Pomarico section.Fig. 4 – Log stratigraphique de Pomarico.

1. Pumice (1-3 cm size); 2. Laminated sandy clay; 3. Deformed layers of medium- to fine grained sand and silt; 4. Medium-grained, oblique cross stratified sandstone; 5. Coarse- to medium-grained, thin horizontally laminated sandstone, with pebbles (2 cm size); 6. Coarse- to fine-grained massive sandstone; 7. Cross stratified conglomerate.
1. Ponce ; 2. Argile sableuse stratifiée ; 3. Couches déformées de sable à grain moyen à fin ; 4. Grès à grain moyen avec stratification croisée oblique ; 5. Grès grossier à moyen-grainé avec des cailloux ; 6. Grès massif ; 7. Conglomérat.

4. Multiproxy analysis

4.1. Age constraints by geochemical affinity

19Multivariate analysis of geochemical characteristics, using Principal Component Analysis (PCA) to extract factors behind, suggests that the pumices sampled near Pomarico village - reworked by run off and deposited in an alluvial environment - could be likely correlated to the Fara d'Olivo ignimbrites (age < 687 ± 8 ka) present in the upper part of the Foggianello Synthem (i.e. the basal unit of the Mt. Vulture succession (tab. 1, fig. 2). In terms of weathering, paleosols sampled at the same site can also be linked to the same ignimbrites. In the PCA plot from Figure 5 both Pomarico pumices and paleosols fall indeed in the direction of maximum variation of SiO2, Al2O3, K2O and Na2O and clearly overlap with the pyroclastic fall and ignimbrites from the Foggianello Synthem, reported in the plot as references. Such an interpretation is also supported by the presence of diopside, a pyroxen largely represented in the Fara d’Olivo ignimbrites, in the pumices and paleosols sampled at Pomarico site.

20On the other hand, PCA also indicates that the paleosols sampled at the Venosa site, along the Fornace section, as well as in the paleosols sampled at the Irsina site, contain reworked and weathered volcanic deposits related to the Valle dei Grigi - Fosso del Corbo Synthem (fig. 5). This unit is comprised in a time-span ranging from 484 ± 8 ka to 132 ± 12 ka (i.e. the upper part of the middle Pleistocene), as reported by Schiattarella et al. (2005) and Giannandrea et al. (2006), or from 494 ± 5 ka to 141 ± 11 ka, according to the age revision by Villa and Buettner (2009).

21It is worth noting that the volcanic component recognized in the paleosol sampled at the base of the Fornace section can be associated to the Melfi Synthem (tab. 1). For 1 sample (fig. 3) in fact separates in the PCA plot (fig. 5) from the other paleosols sampled along the Fornace section and falls in the area where the volcanic products from the Melfi Synthem are displayed.

Fig. 5 – Multivariate analysis of paleosol geochemical features using PCA method to extract factors.
Fig. 5 – Analyse en composantes principales (ACP) des caractéristiques géochimiques des palésols.

Fig. 5 – Multivariate analysis of paleosol geochemical features using PCA method to extract factors.Fig. 5 – Analyse en composantes principales (ACP) des caractéristiques géochimiques des palésols.

Variables are represented by the major element contents in paleosols (expressed in weight %). Synthems compositions are provided by Schiattarella et al. (2005).
Les variables sont représentées par le contenu des éléments majeurs dans les paléosols (exprimé en % en poids). Les synthems proviennent de Schiattarella et al. (2005).

4.2. Paleoclimate implications: geochemical and mineralogical proxies

22The scarcity of pedogenetic clay minerals in the paleosols sampled near Pomarico village (most of the clay minerals are inherited from pre-existing sediments) suggests that the paleosols likely accumulated in cold/arid climate condition (fig. 6A), since soil formation is inhibited in such environments (Birkeland, 1999). On the other hand, abundance of diagenetic illite (PCI), illite/smectite mixed layers and poorly crystallized kaolinite coupled to the presence of 7Å halloysite observed in the paleosols from Fornace section sampled at Venosa site (fig. 6B) clearly indicates a climate context where development of mature soil profiles is expected. Besides, the presence of a 7Å halloysite phase in both Pomarico and Irsina paleosols (fig. 6A, C) also implies that the weathering likely took place in a dry-humid climate with an alternation of rainy seasons, where an intense leaching occurred under the dry periods with scarce rains (Di Leo et al., 2009).

23Clay mineral content, normalized to quartz, is an indicator of pedogenesis in soil/paleosol sequences: increasing amount of pedogenic clay minerals are index of incipient pedogenesis (Velde, 1995). The selected mineralogical proxies suggest that Venosa paleosols from the Fornace section (For 1-7) (fig. 7) are the result of a pedogenesis occurred in a humid and warm climate. A more prolonged and intense pedogenesis at a stable land surface seems to be responsible for the Irsina paleosol development (Irs 1) (fig. 7).

Fig. 6 – XRD patterns of oriented mounts of clay fraction (< 2 μm) from paleosols.
Fig. 6 – DRX de lame orientée de la fraction argileuse (< 2 μm) des paléosols.

Fig. 6 – XRD patterns of oriented mounts of clay fraction (&lt; 2 μm) from paleosols.Fig. 6 – DRX de lame orientée de la fraction argileuse (&lt; 2 μm) des paléosols.

A. Paleosols sampled near Pomarico site; B. Paleosols sampled at Venosa site; C. Paleosols sampled at Irsina site.
A. Paléosols échantillonnés près du site Pomarico ; B. Paléosols échantillonnés près du site de Venosa ; C. Paléosols échantillonnés près du site d'Irsina.

Fig. 7 – Variation of clay minerals/quartz ratio in the paleosol sequences sampled at Venosa, Irsina and Pomarico sites.
Fig. 7 – Variation du rapport minéraux argileux/quartz dans les séquences de paléosols échantillonnées sur les sites de Venosa, Irsina et Pomarico.

Fig. 7 – Variation of clay minerals/quartz ratio in the paleosol sequences sampled at Venosa, Irsina and Pomarico sites.Fig. 7 – Variation du rapport minéraux argileux/quartz dans les séquences de paléosols échantillonnées sur les sites de Venosa, Irsina et Pomarico.

24Same conclusions can be drawn from the analysis of weathering index distribution (tab. 2, fig. 8) in the paleosols from Venosa and Irsina sites. Weathering indices are commonly used in comparing the extent of chemical alteration in different materials (Birkeland, 1999; Darmody et al., 2005). These indices are based on the principle that the ratio between concentrations of mobile (such as SiO2, CaO, MgO, and Na2O) and immobile elements (Al2O3, Fe2O3, TiO2) should decrease over time as leaching proceeds. Indeed, the concentrations of the most immobile elements should increase during weathering if surface erosion is negligible. Given the formulas of most weathering indices, decreasing values refer to greater weathering. Multivariate analysis carried out on paleosols from the Venosa, Irsina and Pomarico sites - performed using PCA method to extract factors and the weathering indices in the input matrix - indicates that a dry/cold climate characterized the Pomarico paleosols development whereas paleosols from Venosa and Irsina sites (fig. 8) are the result of a pedogenesis onset in humid and warm climate conditions.

Tab. 2 – Weathering indices applied to paleosols from Pomarico, Venosa, and Irsina sites.
Tab. 2 – Indices d’altération appliqués aux paléosols des sites de Pomarico, Venosa et Irsina.

Tab. 2 – Weathering indices applied to paleosols from Pomarico, Venosa, and Irsina sites.Tab. 2 – Indices d’altération appliqués aux paléosols des sites de Pomarico, Venosa et Irsina.

Fig. 8 – Multivariate analysis of weathering indices (Pulice et al., 2013 and references therein) of paleosols using PCA method to extract factors.
Fig. 8 – Analyse en composantes principales (ACP) des indices d'altération (Pulice et al., 2013) des palésols.

Fig. 8 – Multivariate analysis of weathering indices (Pulice et al., 2013 and references therein) of paleosols using PCA method to extract factors.Fig. 8 – Analyse en composantes principales (ACP) des indices d'altération (Pulice et al., 2013) des palésols.

25Stable isotopes in carbonate crusts (δ18O and δC) formed in paleosol sequence can be used as a mechanism for paleoclimate reconstruction (Gallant et al., 2014). The Fornace section, at the Venosa sampling site, is characterized by the formation of three mature soil profiles (For 2, For 4, and For 7), all marked by well-developed calcic crusts (For 3, For 5 and For 8). Observed oxygens isotopes (δ18O) values in the pedogenic carbonate (fig. 9) reflect climate amelioration conditions associated with transition from glacial into interglacial. Such an interpretation is feasible given the similarity between the δ18O values of calcic crust developed at top of paleosol For 4 and those from interglacial soil carbonates elsewhere in Europe, found throughout the MIS11 (Gallant et al., 2014). The value of carbon isotope (δ13C) (fig. 9) reveals that plants using the C3 photosynthetic pathways were the dominant vegetation type during the accumulation and alteration of the paleosols sequence at the Fornace section (Venosa site). No significant contribute is due to the impact of major Mount Vulture eruptions, since the pattern is not complicated by volcanic sources.

Fig. 9 – Stable isotopes (δ18O and δ13C) in carbonate crusts from the Fornace stratigraphic section (Venosa site).
Fig. 9 – Isotopes stables (δ18O and δ13C) dans les croûtes carbonatés de la section stratigraphique de Fornace (site de Venosa).

Fig. 9 – Stable isotopes (δ18O and δ13C) in carbonate crusts from the Fornace stratigraphic section (Venosa site).Fig. 9 – Isotopes stables (δ18O and δ13C) dans les croûtes carbonatés de la section stratigraphique de Fornace (site de Venosa).

26A synoptic framework of the relationships among paleosols, tephra layers, ages, and climate conditions is shown in Figure 10. It represents our attempt to contextualize field observations and analytic data in the frame of the Pleistocene climate changes.

Fig. 10 – A synoptic framework of the relationships among paleosols, tephra layers, ages and climate, based on the scheme of IOS stages elaborated by Railsback et al. (2015).
Fig. 10 – Cadre synoptique des relations entre les paléosols, les niveaux de téphra, les âges et le climat, basé sur le schéma des stades isotopiques de l'oxygène (OIS) élaboré par Railsback et al. (2015).

Fig. 10 – A synoptic framework of the relationships among paleosols, tephra layers, ages and climate, based on the scheme of IOS stages elaborated by Railsback et al. (2015).Fig. 10 – Cadre synoptique des relations entre les paléosols, les niveaux de téphra, les âges et le climat, basé sur le schéma des stades isotopiques de l'oxygène (OIS) élaboré par Railsback et al. (2015).

5. Conclusion

27The paleoclimatic implications of the study of the paleosols scattered in the mid-Pleistocene sequences of the Bradano foredeep and the dispersal of pyroclastic products from Mt. Vulture volcano may help to better outline the morphological and sedimentary evolution of one of the key areas of the Mediterranean region. Further, data from the studied successions may give new insights into the reconstruction of the Quaternary history of the study area if related to the arrangement of relict land surfaces, which constitute most of the hilltops of the foredeep relief and are featured by the continental sequences of tephra, paleosols, and alluvial sediments illustrated in this paper. At present, understanding the configuration of the mid-Pleistocene landscape of the foredeep can only be based on two different hypotheses: the terraced surfaces (about 540 m asl at Irsina and 450 m asl at Pomarico) at the top of the sampling sites (i) were connected by low-angle morphological scarps (see for example Lazzari and Pieri, 2002) or (ii) formed an unique paleosurface with a regional dip not exceeding 3° (calculated on the basis of the gradient between Pomarico and Irsina surfaces).

28Concerning the evolution of the drainage network, most part of the fluvial incision responsible for the genesis of the mesas of the Bradano foredeep seems to be achieved in a relatively short and recent time-span (i.e. starting from the upper part of the middle Pleistocene) – as suggested by the age of the volcaniclastic markers (i.e. Valle dei Grigi - Fosso del Corbo Synthem) cut by stream erosion and interbedded in the uppermost part of the alluvial successions surveyed at Venosa and Irsina – and not spanned in the entire chronological interval of the progressive emersion of the foredeep basin from the sea. However, slightly pronounced paleovalleys in which those alluvial successions were deposited, moulded in a more continuous plateau, set up during the early middle Pleistocene.

29It is worthy to note that, on a regional scale, the onset of the tectonic uplift in similar coastal areas located to the east of the orogenic chain is also hypothesized in the late middle Pleistocene for the staircase of alluvial terraces of the Ionian coast of northern Calabria (Robustelli et al., 2009).

30The landscape response to the dynamic behaviour of the Bradano foredeep may be due to a change in the rates of the tectonic and geomorphic processes during the middle Pleistocene, when the general uplift of the chain-foredeep system promoted the regressive incision of the major streams -thus favouring fluvial piracy- under the temperate climate conditions of southern Italy that followed the Mindel-Riss Interglacial Stage.

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Annexe

Version française abrégée

Le volcan du Mont Vulture est situé sur la portion frontale des Apennins du sud de l’Italie. Ses produits, issus de la période du Pléistocène moyen, se sont répandus en direction du Sud-Est, jusqu’à l’intérieur du bassin adjacent à l’avant-fosse du Bradano. Leur position au sein des stratifications sédimentaires du Quaternaire représente une clé de lecture pour comprendre l’environnement paléomorphologique de l’avant-fosse. L’ensemble du bassin de l’avant-fosse s’étend selon une direction NO-SE. Il est situé entre la chaîne de l’Apennin méridional et la plate-forme carbonatée des Murge, représentant l’avant- pays Apulien (fig. 1).

L’étude multidisciplinaire des dépôts de nature pyroclastique du bassin fluvio-lacustre de Venosa, situé à proximité de l’édifice volcanique, ainsi que des niveaux de téphra associés aux paléosols du secteur plus au sud de l’avant-fosse dans les environs immédiats de la Commune de Pomarico, revêt une importance stratégique en vue d’une reconstruction du scénario morphoclimatique mais aussi sur celui lié aux stratifications. Les recherches de nature géomorphologique et stratigraphique ont révélé que le bassin de Venosa s’est constitué à travers le remplissage d’une paléovallée incisée à l’intérieur de la succession datant du Pléistocène de l’avant-fosse du Bradano.

Sur d’autres sites, comme celui de Pomarico, les niveaux de ponces tombés durant les éruptions ainsi que les paléosols associés, sont présents au sommet de la succession sédimentaire du Pléistocène inférieur-moyen (fig. 3). Les analyses géochimiques et minéralogiques des paléosols et des roches issues des coulées pyroclastiques montrent que les ponces de Pomarico se sont amalgamées et ensuite stratifiées dans un environnement alluvial et pourraient donc être corrélées aux ignimbrites de Fara d’Olivo (âge < 687 ± 8 ka) présentes dans l’unité basale de la succession volcanique du Mont Vulture (fig. 4). En outre, la présence d’halloysites 7Å dans les paléosols qui ont été échantillonnés dans deux sites différents (fig. 5), suggère que l’altération au cours du Pléistocène moyen s'est probablement réalisée dans un climat humide, caractérisé par une alternance de saisons pluvieuses durant lesquelles a eu lieu un processus de lixiviation, suivies par des périodes de sécheresse ayant été marquées par de très faibles précipitations (fig. 6-7).

Les données analysées suggèrent de concentrer les futures recherches vers la compréhension du paysage physique tel qu’il pouvait l’être durant le Pléistocène moyen dans l’aire de l’avant-fosse et ce, en faisant appel à deux hypothèses : les superficies des plateaux (aux alentours de 540 m d'altitude à Irsina, et de 400 m à Pomarico), placées légèrement au- dessus des horizons échantillonnés, (i) étaient reliées entre-elles par des talus morphologiques caractérisés par un angle bas ou encore (ii) formaient une seule et unique paléosuperficie ayant une inclinaison régionale ne dépassant pas les 3° (calculée sur la base du gradient de la pente, entre les superficies de Pomarico et celle d’Irsina).

Quant à l’évolution du réseau hydrographique, une grande partie de l’incision fluviale, responsable de la genèse des reliefs de type mesa de l’avant-fosse, semble être apparue au cours d’un intervalle temporel relativement court et récent -qui remonte à la partie haute du Pléistocène moyen, comme nous le laisse entendre l’âge des téphra creusés par l’action des cours d’eau et qui sont intercalés dans la partie supérieure des successions alluviales déjà étudiées- et non à l’intérieur d’un laps de temps compris entre l’émergence du bassin de l’avant-fosse et les remplissages successifs datant de la période du Pléistocène supérieur-Holocène des vallées. Ce résultat peut être dû à un changement des taux d’incision dérivant de processus de nature tectonique et géomorphologique de la période du Pléistocène moyen, dans un contexte marqué par le soulèvement général du système chaîne-avant-fosse qui a provoqué l’érosion régressive des cours d’eau principaux -ce qui a peut-être également favorisé des phénomènes de capture fluviale- et ce, dans le cadre de conditions climatiques caractérisées par des périodes tempérées faisant suite à la période interglaciaire Mindel-Riss (fig. 8-9).

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Table des illustrations

Titre Fig. 1 – Regional framework in which the study area is included and dispersal of Mt. Vulture volcano pyroclastic products. Fig. 1 – Cadre géologique de la zone d'étude incluant la dispersion des produits pyroclastiques du volcan du Mont Vulture.
URL http://geomorphologie.revues.org/docannexe/image/11731/img-1.png
Fichier image/png, 1,1M
Titre Fig. 2 – Volcanic succession of Mt. Vulture. Fig. 2 – Chronologie des dépôts volcaniques du Mont Vulture.
Légende The mid- to late Pleistocene Vulture volcano deposits (Ar/Ar ages: 687 ± 8 ka to 141 ± 11 ka; Villa and Buettner, 2009) are subdivided into more units bounded by unconformities (Giannandrea et al., 2006).Les dépôts du Pléistocène moyen et supérieur (âges Ar/Ar : 687 ± 8 ka à 141 ± 11 ka ; Villa et Buettner, 2009) sont subdivisés en plusieurs unités délimitées par des discordances (Giannandrea et al., 2006).
URL http://geomorphologie.revues.org/docannexe/image/11731/img-2.png
Fichier image/png, 810k
Titre Tab. 1 – Chronostratigraphy of the Mount Vulture volcano and surroundings.Tab. 1 – Chronostratigraphie du volcan du Mont Vulture et ses environs.
Légende 39Ar/40Ar ages after the review of Villa and Buettner (2009), except (1) after Bonadonna et al. (1998), and (2) after Laurenzi et al. (1993).Àge 39Ar/40Ar d'après la recension de Villa et Buettner (2009), à l'exception de (1) d'après Bonadonna et al. (1998), et (2) d'après Laurenzi et al. (1993).
URL http://geomorphologie.revues.org/docannexe/image/11731/img-3.jpg
Fichier image/jpeg, 424k
Titre Fig. 3 – Sampling sites.Fig. 3 – Sites d'échantillonnage.
Légende See text for explanations about samples. A. Irsina site; B. Pomarico site; C. Venosa site (Fornace section).Voir le texte pour l'information sur les échantillons. A. Site d'Irsina ; B. Site de Pomarico ; C. Site de Venosa (section de Fornace).
URL http://geomorphologie.revues.org/docannexe/image/11731/img-4.png
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Titre Fig. 4 – Stratigraphic log of the Pomarico section.Fig. 4 – Log stratigraphique de Pomarico.
Légende 1. Pumice (1-3 cm size); 2. Laminated sandy clay; 3. Deformed layers of medium- to fine grained sand and silt; 4. Medium-grained, oblique cross stratified sandstone; 5. Coarse- to medium-grained, thin horizontally laminated sandstone, with pebbles (2 cm size); 6. Coarse- to fine-grained massive sandstone; 7. Cross stratified conglomerate.1. Ponce ; 2. Argile sableuse stratifiée ; 3. Couches déformées de sable à grain moyen à fin ; 4. Grès à grain moyen avec stratification croisée oblique ; 5. Grès grossier à moyen-grainé avec des cailloux ; 6. Grès massif ; 7. Conglomérat.
URL http://geomorphologie.revues.org/docannexe/image/11731/img-5.png
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Titre Fig. 5 – Multivariate analysis of paleosol geochemical features using PCA method to extract factors.Fig. 5 – Analyse en composantes principales (ACP) des caractéristiques géochimiques des palésols.
Légende Variables are represented by the major element contents in paleosols (expressed in weight %). Synthems compositions are provided by Schiattarella et al. (2005).Les variables sont représentées par le contenu des éléments majeurs dans les paléosols (exprimé en % en poids). Les synthems proviennent de Schiattarella et al. (2005).
URL http://geomorphologie.revues.org/docannexe/image/11731/img-6.png
Fichier image/png, 957k
Titre Fig. 6 – XRD patterns of oriented mounts of clay fraction (< 2 μm) from paleosols.Fig. 6 – DRX de lame orientée de la fraction argileuse (< 2 μm) des paléosols.
Légende A. Paleosols sampled near Pomarico site; B. Paleosols sampled at Venosa site; C. Paleosols sampled at Irsina site.A. Paléosols échantillonnés près du site Pomarico ; B. Paléosols échantillonnés près du site de Venosa ; C. Paléosols échantillonnés près du site d'Irsina.
URL http://geomorphologie.revues.org/docannexe/image/11731/img-7.png
Fichier image/png, 1,5M
Titre Fig. 7 – Variation of clay minerals/quartz ratio in the paleosol sequences sampled at Venosa, Irsina and Pomarico sites.Fig. 7 – Variation du rapport minéraux argileux/quartz dans les séquences de paléosols échantillonnées sur les sites de Venosa, Irsina et Pomarico.
URL http://geomorphologie.revues.org/docannexe/image/11731/img-8.png
Fichier image/png, 736k
Titre Tab. 2 – Weathering indices applied to paleosols from Pomarico, Venosa, and Irsina sites.Tab. 2 – Indices d’altération appliqués aux paléosols des sites de Pomarico, Venosa et Irsina.
URL http://geomorphologie.revues.org/docannexe/image/11731/img-9.jpg
Fichier image/jpeg, 408k
Titre Fig. 8 – Multivariate analysis of weathering indices (Pulice et al., 2013 and references therein) of paleosols using PCA method to extract factors.Fig. 8 – Analyse en composantes principales (ACP) des indices d'altération (Pulice et al., 2013) des palésols.
URL http://geomorphologie.revues.org/docannexe/image/11731/img-10.png
Fichier image/png, 823k
Titre Fig. 9 – Stable isotopes (δ18O and δ13C) in carbonate crusts from the Fornace stratigraphic section (Venosa site).Fig. 9 – Isotopes stables (δ18O and δ13C) dans les croûtes carbonatés de la section stratigraphique de Fornace (site de Venosa).
URL http://geomorphologie.revues.org/docannexe/image/11731/img-11.png
Fichier image/png, 540k
Titre Fig. 10 – A synoptic framework of the relationships among paleosols, tephra layers, ages and climate, based on the scheme of IOS stages elaborated by Railsback et al. (2015).Fig. 10 – Cadre synoptique des relations entre les paléosols, les niveaux de téphra, les âges et le climat, basé sur le schéma des stades isotopiques de l'oxygène (OIS) élaboré par Railsback et al. (2015).
URL http://geomorphologie.revues.org/docannexe/image/11731/img-12.png
Fichier image/png, 2,1M
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Référence électronique

Giuseppe Corrado, Paola Di Leo, Paolo Giannandrea et Marcello Schiattarella, « Constraints on the dispersal of Mt. Vulture pyroclastic products: implications to mid-Pleistocene climate conditions in the foredeep domain of southern Italy », Géomorphologie : relief, processus, environnement [En ligne], Articles sous presse, mis en ligne le 19 juin 2017, consulté le 25 juin 2017. URL : http://geomorphologie.revues.org/11731

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Auteurs

Giuseppe Corrado

Dipartimento di Scienze e Tecnologie, Parthenope University of Naples – I‑80134 Naples, Italy (giuseppe.corrado@uniparthenope.it).

Paola Di Leo

CNR-IMAA – I‑85050 Tito Scalo (Potenza), Italy (paola.dileo@imaa.cnr.it).

Paolo Giannandrea

Dipartimento di Scienze, Basilicata University – I‑85100 Potenza, Italy (paolo.giannandrea@unibas.it).

Marcello Schiattarella

Dipartimento delle Culture Europee e del Mediterraneo (DiCEM), Basilicata University – I‑75100 Matera, Italy (marcello.schiattarella@unibas.it).

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