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Deep gullies entrenchment in valley fills during the Late Holocene in the Basento basin, Basilicata (southern Italy)

Creusement de profondes ravines à l’Holocène supérieur dans les remblaiements fluviatiles du bassin de Basento, Basilicata (Italie du sud)
Marco Piccarreta, Domenico Capolongo et Maria Nilla Miccoli
p. 239-248

Résumés

Des profondes ravines se sont développées sur les versants légèrement pentus situés en marge des plaines alluviales de différents bassins hydrographiques de Lucanie, dans le sud de l’Italie. Dans cette région, les remblaiements de vallée sont principalement holocènes; des dépôts datant du Pléistocène supérieur, mal conservés, affleurent au niveau de terrasses situées le long de tous les principaux cours d’eau. La phase de dépôt holocène débute à partir de 7000 cal. BP et se termine vers 120 cal. BP. L’étude focalise sur l’incision des ravines durant les derniers 4500 ans ; elle s’appuie sur l’analyse chronostratigraphique détaillée des dépôts alluviaux du fleuve Basento. Quatre principales phases d’incision sont mises en évidence : 4500-4350, 1500, 900-800 et 120 cal. BP. La mise en place des ravines durant l’Holocène supérieur est en lien avec les changements du climat, de la végétation et des conditions des eaux souterraines. L’histoire environnementale dérive principalement des séquences polliniques du Lago Grande de Monticchio, où la mise en place du ravinement semble associée à des cycles sécheresse/humidité répétés. Quand le climat devient plus sec, la diminution du couvert végétal et l’augmentation du ruissellement le long des versants entraînent un approfondissement et un élargissement des ravines.

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Notes de la rédaction

Article soumis le 11 février 2011, accepté le 5 septembre 2011.

Texte intégral

Introduction

1The development of gullies and entrenched channels on valley fills (badland gullies) in semi-arid areas of Mediterranean landscape has been widely described in literature and usually attributed to environmental changes induced by either climatic oscillations or human activity, which increased runoff rates and/or decreased the protection provided by the vegetation cover (Vita-Finzi, 1969; Leopold and Vita-Finzi, 1998; Grove and Rackam, 2003). In several areas of Spain (Tabernas, Almeria; Harvey, 1987) and central-southern Italy (Tuscany, Latium, Basilicata) gulling is always linked with badland to constitute complex badland systems. According to the classification proposed by I.A. Campell (1989), badland gullies develop in three topographic or lithostructural settings: (i) gullies in alluvial-fill deposits in preformed valleys, (ii) gullies cut in bedrock on slopes or valley sides and (iii) gullies developing as headward extensions of a drainage system on undissected upland surface. In the semi-arid environment of Fossa Bradanica, Basilicata, (Southern Italy) badland gullies occur on the gently sloping valley floors of many alluvial basins. They are the part of the main fluvial network that is represented by entrenched channels in Holocene alluvial deposits incised up to 20 m, with a maximum width of a few hundreds of metres and with a maximum length of 20-30 km. The first researches on the Holocene alluvial deposits in Basilicata were carried out by R. Neboit (1977, 1983), H. Brückner (1983, 1990) and J. Abbott and S. Velastro (1995). Most recently, F. Boenzi et al. (2008) and M. Piccarreta et al. (2011) have brought new data concerning the stratigraphy and chronology of these deposits. However, all these previous studies have highlighted the nature of the deposits and the causes originating them, and have neglected on the incision phases. Thus, some questions are still open: when and how did badland gullies form in the Holocene alluvial fills? Are badland gullies linked to climate change or to human impact? A.T. Grove and O. Rackam (2003) postulate that at some time or times in the past 2700 years, climate and land use have combined to produce a period of intense gully erosion, but their hypothesis is not supported by explanatory comments. To address this question, a careful reconstruction of alluvial stratigraphy, erosion history and palaeoclimate record is needed. In this paper, we review previous studies on Holocene geomorphological activity in Basilicata, focusing on channel entrenchment (badland gullies formation) in the last 4500 years in the Basento basin, in relation to the characteristics and changes in climate, human impact, vegetation and groundwater conditions.

Study area

2The study area (fig. 1) is located in the Basento catchment, Basilicata region in southern Italy. It lies in the Fossa Bradanica, which is a sedimentary basin with a NW-SE trend limited by the southern Apennines and Apulian foreland. The area is mainly characterised by the deposition of open-shelf mudstones, consisting of clays and silts and referred as the Formation of the “Argille pleistoceniche subappenniniche” (Pleistocene Subappennine Clays), unconformably followed upwards by sandy-gravelly regressive deposits that constitute the upper part of the filling. According to different authors (Boenzi et al., 1971; Brückner, 1980; Amato, 2000; Bentivenga et al., 2004), these were displaced by faults or dissected by a series of multi-ordered marine terraces. In the Middle Pleistocene, the Fossa Bradanica trough began to uplift and the rivers started to cut the deep valleys perpendicularly to the coast and the lateral erosion to attack the hillslopes. The result was the increased dissection and the exposure of the highly erodible clayey bedrock. A sequence of slightly tilted relict slope pediments occurs in the middle and high parts of the hillslope, while well-preserved remains of two Pleistocene fluvial terraces are present in the lower parts. Towards the valley bottom, three levels of Holocene fluvial terraces as well as the modern floodplain of the Basento River are present. The hillslopes are cut by deep gullies (locally called fossi), which often erode to the upper ridge, making them susceptible to landsliding. Where clays crop out, the gully heads and sides exhibit patterns of badland erosion (calanchi in Italian).

Fig. 1 – Geomorphological map of the study area
Fig. 1 – Carte géomorphologique de la zone d’étude

Fig. 1 – Geomorphological map of the study areaFig. 1 – Carte géomorphologique de la zone d’étude

1: main towns; 2: main streams; 3: marine terraces edge; 4: slope pediment; 5: alluvial deposits; 6: clays (affected by badlands); 7: coastal deposits; 8: sands and conglomerates; 9: landslide debris; 10: Pleistocene fluvial terrace; 11: Middle-Late Holocene fluvial terrace; 12: Late Holocene fluvial terrace; 13: recent fluvial terrace.
1 : ville principale ; 2 : rivière principale ; 3 : rebord de terrasse marine ; 4 : glacis ; 5 : remblaiements alluviaux ; 6 : argiles (ravinées) ; 7 : remblaiement marin ; 8 : sables et conglomérats ; 9 : dépôt de glissement de terrain ; 10 : terrasse fluviale pléistocène ; 11 : terrasse fluviales de l’Holocène moyen/supérieur ; 12 : terrasses fluviale de l’Holocène inférieur ; 13 : terrasse fluviale récente.

Methods

3This paper reviews previous studies on Holocene geomorphological activity in Basilicata, focusing on channel entrenchment occurred in the Basento basin in relation to the characteristics and changes in climate, human impact, vegetation and groundwater conditions. Phases of incision were detected from the detailed chronostratigraphical study of two sections along the Basento tributaries named Fosso Del Ponte and Fosso La Capriola (Boenzi et al., 2008; Piccarreta et al., 2011) and the comparison with the radiocarbon datings obtained by previous studies on alluvial deposits of Basilicata rivers (Neboit, 1977; 1983; Brückner, 1983; Abbott and Valastro, 1995; Small et al., 1998). Radiocarbon ages of the Basilicata data set were calibrated using the CALIB 6.0.1 14C program (Stuiver and Reimer, 2003; Reimer et al., 2009) and are reported in the text as “a cal. BP” with a precision of 2 σ. With the aim to better investigate the Holocene evolution of the valley and its fluvial terraces, a detailed geomorphological map of the middle-lower Basento valley was realised from field survey and aerial-photos interpretation. Information was collected on the lithological, mineralogical and pedological characteristics of the fluvial terraces, and stratigraphic sections of river terrace deposits were described.

4The connection between the badland gullies formation and the changes occurred in climate, human impact and vegetation during the last 4500 years has been pursued by correlating the alluvial stratigraphic record of Basento River and its low-order tributaries (Fosso La Capriola and Fosso Del Ponte) with several proxies: (i) palaeoenvironmental records for this region provided by the study of pollen sequences at Lago Grande di Monticchio (Allen et al., 2002); (ii) archaeological information on the Metapontine agriculture from 5th c. BC to Middle Ages (De Siena, 2001; Rescio, 2001; Costantini and Biasini Costantini, 2003; Castoldi, 2008); (iii) information about cereal production, droughts and floods found in the historical archives of the 13th-14th c. for the Campania and Basilicata regions (Boenzi and Giura Longo, 1994; Diodato, 2007).

The middle-lower valley of the Basento River

5The middle-lower valley of the Basento River has been studied in detail (fig. 2). In the middle sector, the river shows a braided pattern; the talweg enlarges progressively and is bordered by a large alluvial plain. The floodplain adjacent to the thalweg, based on historical documents, was frequently flooded up to the end of the last century and it is still flooded during extraordinary floods (Boenzi and Giura Longo, 1994; Clarke and Rendell, 2006). Toward the coast, in the lower valley, the river pattern changes into a typical meander pattern and five orders of river terraces are present: two orders belong to the Pleistocene age whereas the other three formed during Holocene. Pleistocene deposits occur as terraces (I and II orders) above the flood plain or as eroded benches buried beneath the younger alluvium. These are attributed to the late Pleistocene and consist of planar and through cross-bedded gravels and sands, indicating deposition under relatively high-energy conditions by a braided channel (Abbott and Valastro, 1995; Boenzi et al., 2008). Among the three orders of Holocene fluvial terraces (fig. 2 and fig. 3) the lowest (V order) is quite recent and it is present only in the middle valley at about 2 m above the modern channel. It was not present in the aerial photos of 1954, whereas it is well formed in the aerial photos of 1986. During this span of time, two extraordinary flood events (23-25 November 1959, 18-19 January 1972) occurred in the Basento basin. The medium terrace (IV order) consists of a series of alluvial strath and strath/fill benches mantled with relatively thin (1-3 m thick) sandy and silty alluvium at elevations of 5-10 m above the modern channel. The Upper-Holocene surface (III order) has been intensively studied in the last 30 years (Neboit, 1977, 1983; Brückner, 1983, 1990; Abbott and Valastro, 1995; Boenzi et al., 2008; Piccarreta et al., 2011) and it consists of a sequence of stacked, predominantly silty deposits with a total thickness exceeding 18 m. On the basis of several radiocarbon datings obtained from various outcrop successions, it seems that fluvial deposition in the area began earlier than 7000 a cal. BP (Brückner 1983, 1990; Abbott and Valastro, 1995) and ended around 120 a cal. BP (180 years ago; Piccarreta et al., 2011).

Fig. 2 – A: Geomorphological map of the middle-lower Basento valley. B: Alluvial sequences at Fosso Del Ponte and Fosso La Capriola. C: Longitudinal profiles of the Basento River and its terraces
Fig. 2 – A : Carte géomorphologique de la moyenne-basse vallée du Basento. B : Les séquences alluviales à Fosso Del Ponte et à Fosso La Capriola. C : Profils longitudinaux du fleuve Basento et de ses terrasses

Fig. 2 – A: Geomorphological map of the middle-lower Basento valley. B: Alluvial sequences at Fosso Del Ponte and Fosso La Capriola. C: Longitudinal profiles of the Basento River and its terracesFig. 2 – A : Carte géomorphologique de la moyenne-basse vallée du Basento. B : Les séquences alluviales à Fosso Del Ponte et à Fosso La Capriola. C : Profils longitudinaux du fleuve Basento et de ses terrasses

A – 1: thalweg; 2: present-day alluvial plain; 3: barrier beach and deltaic deposits; 4: I order (Late Pleistocene fluvial terrace, ca. 39000 BP); 5: II order (Late Pleistocene fluvial terrace, age?); 6: III order (Middle-Late Holocene fluvial terrace); 7: IV order (Late Holocene fluvial terrace, Little Ice Age); 8: V order (recent, after 1954); FP: Fosso del Piano; MG: Masseria Glionne; MF: Masseria Fiorentino; FC: Fosso La Capriola; FLC: Fosso La Canala. B – Alluvial sequences at Fosso Del Ponte and Fosso La Capriola. 1: pebbly alluvium; 2: sandy alluvim; 3: fine alluvium; 4: clays; 5: palaeosol; 6: AP3 tephra (ca. 2800 BP); 7: Avellino tephra (ca. 4300 BP); 8: erosive surface; 9: fossils; 10: charcoal; 11: woods (cortex); 12: potsherds (ca. 4500 BP); 13: burial (ca. 2500 BP). C 1: I order; 2: II order; 3: III order; 4: IV order.
A – 1 : talweg ; 2 : plaine alluviale actuelle ; 3 : cordon littoral et dépôts deltaïques ; 4 : ordre I (terrasses fluviales de Pléistocène supérieur, environ 39000 BP) ; 5 : ordre II (terrasses fluviales de Pléistocène supérieur, âge indéterminé) ; 6 : ordre III (terrasses fluviales de l’Holocène moyen/supérieur) ; 7 : ordre IV (terrasses fluviales de l’Holocène inférieur, Petit Âge Glaciaire) ; 8 : ordre V (récents, après 1954) ; FP : Fosso del Piano ; MG : Masseria Glionne ; MF : Masseria Fiorentino ; FC : Fosso La Capriola ; FLC : Fosso La Canala. B – 1 : alluvions caillouteuses ; 2 : alluvions sableuses ; 3 : alluvions fines ; 4 : argiles ; 5 : paléosol ; 6 : téphra AP3 (environ 2800 BP) ; 7 : téphra Avellino (environ 4300 BP) ; 8 : surface d’érosion ; 9 : fossiles ; 10 : charbons ; 11 : bois (cortex) ; 12 : tessons (environ 4500 BP) ; 13 : sépulture (environ 2500 BP). C 1 : ordre I ; 2 : ordre II ; 3 : ordre III ; 4 : ordre IV.

From Boenzi et al., 2008; Piccarreta et al., 2011 (fig. 2B).
D'après Boenzi et al., 2008; Piccarreta et al., 2011 (fig. 2B).

Fig. 3 – Panoramic view of the middle valley of the Basento River
Fig. 3 – Vue panoramique de la moyenne vallée du fleuve Basento

Fig. 3 – Panoramic view of the middle valley of the Basento River Fig. 3 – Vue panoramique de la moyenne vallée du fleuve Basento

1: Middle-Late Holocene fluvial terrace; 2: Late Holocene fluvial terrace; 3: recent fluvial terrace.
1 : terrasses fluviales de l’Holocène moyen/supérieur ; 2 : terrasses fluviales de l’Holocène inférieur ; 3 : terrasses fluviales récents.

Aerial photo courtesy of prof. Massimo Caldara.
Photos aériennes fournies par le prof. Massimo Caldara.

Holocene alluvial stratigraphy and chronology

6The upper-Holocene terrace has been studied in detail at Fosso La Capriola site by F. Boenzi et al. (2008; fig. 2). It consists of a sequence of alternating silty-sand deposits with thin intercalated lenses occurring at elevations of 10-20 m above the present-day channel. These deposits represent a sedimentary sequence that accumulated by vertical accretion during valley-wide flooding. The lower part of the outcrop is made up of an irregular alternation of well-sorted coarse sand, massive loamy sand and finely laminated silt, followed by a massive silty sand layer bearing many pottery fragments and charcoals. The potsherds have been sampled and analysed by archaeologists from Soprintendenza Archeologica della Basilicata, and are to be referred to the Neolithic cultural phase known as “Diana” (Bianco, 1981). An AMS radiocarbon dating, obtained from a charcoal fragment, has given a 14C age of 4340±50 BP (i.e., 5041-4835 a cal. BP). An erosive surface separates these levels from the rest of the succession and defines a small channel whose filling started shortly before the deposition of a tephra layer. The tephra is made of fine grey ash of tephritic-phonolitic composition and its components are mainly represented by highly to moderately vesicular pumices with a subordinate amount of pyroxene and feldspar crystals. Lithological features, mineralogical and chemical composition and morphological characters of ash fragments suggest that this layer should be linked to the fallout phase of the Plinian eruption of Vesuvius occurred 4300 a cal. BP and named “Avellino” event (Wulf et al., 2004; Santacroce et al., 2008; Sulpizio et al., 2008; Wulf et al., 2008). Above the Avellino tephra, there is 0.85 m of pale-brown laminated silt. The bedding planes are often marked by millimetric laminas made of dark organic matter or reddish oxidised material. In this interval, a discontinuous palaeosol level is also present and two radiocarbon-dated charcoals have respectively given the 14C age of 3131±100 BP (i.e., 3582-3066 a cal. BP) and of 3059±95 BP (i.e., 3454-2976 a cal. BP). Another thin tephra horizon lies at about 4.7 m from the base of the sequence and it is composed of grey ash of tephritic-phonolitic composition, which main components are highly vescicular pumice fragments and subordinate feldspar and pyroxene crystals. The geochemical features suggest that this bed should be attributed to the AP3 eruption of Vesuvius (Andronico and Cioni, 2002; Santacroce et al., 2008) , which occurred between the “Avellino” and “Pompeii” events, at about 2800 a cal. BP. A 1.8 m succession of two palaeosols, separated by a 0.25 m sand layer, lies on the top of the “AP3” horizon and they are brown in colour, massive, rich in calcareous lumps and root marks and several landsnails have also been found. The last palaeosol is topped by a 0.70 m of brown-reddish, partially pedogeneised sand with scattered pebbles and within this layer a pot about 20 cm wide and 30 cm high has been found. Archaeologists from Soprintendenza Archeologica della Basilicata interpret the pot as the burial receptacle of a child dating back to around 2500 a cal. BP. The grave was excavated in the reddish sand deposit, which at that time represented the ground surface. This layer was buried under a 2.4 m thick succession of pale-brown fine sands which are either laminated (planar and cross lamination) or massive. The top of the sequence is characterised by a 4.0 m thick succession of three palaeosols (brown in colour) separated by laminated fine sand layers showing both planar and ripple structures. A 14C dating, obtained from an ox bone collected from the lower palaeosol, has given the age of 1710±40 a BP (i.e., 1708-1536 a cal. BP). An erosional surface cuts part of the sequence, defining a channel with a thickness of several metres. The deposit that fills the channel contains a set of lenses mainly made of planar laminated coarse sand, but thick lenses of gravel, laminated loam and thin horizons bearing dark organic matter (mainly tiny charcoals) have also been observed. One of the layers is rich in charcoals, that have a 14C age of 1553±60 a BP (i.e., 1555-1318 a cal. BP). The channel deposit is buried by a pale-brown massive sand layer, whose top is at 8.5 m above the ground. Above the massive sand, a 1.4 m thick palaeosol developed, bearing scattered tiny charcoals, whose fragments yielded respectively two 14C ages of 1130±40 a BP (i.e., 1147-958 a cal. BP) and 1090±40 a BP (i.e., 1070-928 a cal. BP). The succession is topped by 1.3 m of fine pale-brown sand with plane and cross laminations.

7The lower Holocene terrace has been studied in detail by M. Piccarreta et al. (2011) at Fosso Del Ponte site (fig. 2). It consists of a more than 2 m thick basal coarse gravel layer mantled with relatively thin (1-3 m thick) sandy and silty alluvium at elevation of 3-5 m above the modern channel. The sequence lies on clayey-sandy substrate, which may represent the top of the first Holocene terrace. A sample of charcoal and a sample of wood were respectively retrieved from this substrate and they provided the same 14C date of 970±30 a BP (i.e., 889-795 a cal. BP). The first 2 m of the successions mainly consist of coarse heterometric gravels with interbedded horizontal to cross-laminated sandy layers. A piece of wood sampled in the upper part of this layer yields a 14C age of 180±25 a BP (i.e., 223-138 a cal. BP). The succession ends with a 1 m thick layer of massive loamy alluvium from which a sample of charcoal (remains of a fire) dated by 14C 200±25 a BP (i.e., 215-145 a cal. BP) has been yielded. While it is quite clear that deposition ended around 150 cal. BP, the age of starting deposition cannot be determined due to the lack of available material for radiocarbon dating.

Palaeoenvironmental reconstruction for the last 4500 years

8For Basilicata region, detailed palaeoclimatic information has been provided by the pollen record of Lago Grande di Monticchio (Allen et al., 2002) for the last 4500 years (fig. 4); these data can be integrated by such proxy data as the archaeological information on the Metapontine agriculture from 5th c. BC to Middle Ages (De Siena, 2001; Rescio, 2001; Costantini and Biasini Costantini, 2003; Castoldi, 2008) and information about cereal production, droughts and floods found in the historical archives of the 13th-14th c. for the Campania and Basilicata region province (Boenzi and Giura Longo, 1994; Diodato, 2007).

9From 4500 to ca. 4000 a cal. BP, an abrupt forest cover regression occurred, with its maximum peak at around 4200-4100 cal. BP. This abrupt collapse in the concentration of tree pollen has been recorded in many Mediterranean areas at that time (Jalut et al., 2000), suggesting that a dry phase should have taken place around 4200 a cal. BP. This aridification phase seems to be due to a general progression of a North African high-pressure cell towards the northeast affecting the Italian territory. This period is followed by a new cooling phase until 3500 cal. BP, as also testified by the glacier advance in the Alps and in the Apennine massifs from central Italy (Giraudi, 2005a), and by the highstands of lake level recognised in central Italy (Giraudi, 1998, 2004; Sadori et al., 2004; Magny et al., 2007). A new dry phase took place in the area from ca. 3000 to 2500 a cal. BP. The pollen records from Monticchio reveal that although the warm mixed forest is the dominant biome, its composition differs from the past for the absence of Abies and Taxus. In the considered span of time (4500-2500 a cal. BP), the inner areas of the middle Basento valley, corresponding to the territorial areas of Pisticci, Pomarico and Ferrandina, were mainly controlled by indigenous peoples (Enotri and Chones). These areas were wooded and uncultivated land, only devoted to grazing and to wood supply (Castoldi, 2008). The indigenous population preserved their cultural autonomy over the centuries, as shown by the crouched burial, which is evidenced by the child burial discovery within the Late-Holocene Fosso La Capriola valley fills (Boenzi et al., 2008). Very intensive was instead the human pressure on the coastal area, which was occupied by Greek people since 8th c. BC (Costantini and Biasini Coastantini, 2003). Here, several forested areas were cleared for olive and cereal cultivation. According to some authors (Neboit, 1977; Bruckner, 1983), the great expansion of agriculture as well as the forest removal favoured soil erosion on the clayey slopes and sediment storing in the valley floor. However, it should be considered that the land exploitation was mainly concentrated in the Ionian belt, especially in the territory of the ancient Metaponto colony (Chora; Adamasteanu, 1974; De Siena, 2001) and not in the hilly inner areas, where channels entrenchment had already begun. After 2500 a cal. BP, a period of hydrogeological unsteadiness started, due to a cool-wet climate coinciding with the Alpine glacier advance (Holzhauser et al., 2005). Historical data reveal that at Metaponto, the recurrent flooding events and the rising up of the groundwater table at around 2500-2300 a cal. BP forced the inhabitants to move towards higher areas and to build an artificial drainage network (Boenzi and Giura-Longo, 1994; De Siena, 2001). In the Monticchio sequence the last 2000 years are characterised by evidence of forest clearance and agricultural activity; Olea, cereal grasses and Juglans are characteristic, but also Castanea and Pistacia are most frequent. In the younger samples (younger than 1500 years), semi-desert taxa, and especially Artemisia, increase again, suggesting that the recent change in vegetation cover should probably been connected to climatic change and instability, or an interaction of human exploitation and climate change. At Metaponto, instead, the Chora, which had reached its full spreading at the beginning of 2300 a cal. BP, began to break up until the complete decline due to the war against Hannibal. After the Roman Empire decline, Basilicata began to depopulate and much of its territory was interested by large reforestation. At around 1200 a cal. BP, in the Monticchio Lake a new increase in arboreal pollen is documented and suggests moister climate conditions. This cooling period is also identified in central Italy by both the advance of Calderone glacier (Giraudi, 2005b) and the phases of lacustrine high level (Giraudi, 1998, 2004; Sadori et al., 2004; Magny et al., 2007). At around 1000 cal. BP and for the whole Middle-Ages period, the region, especially in its western part, was considered a perfect setting by the Benedictine monks, who occupied the hilltop forested areas which were gradually cleared and reclaimed (Rescio, 2001). At around 800 a cal. BP, a strong decrease in arboreal pollen suggests drier condition and this climatic feature endured for about 400 years. The indirect climatic data available show a warm climate with strong interannual contrasts and this is clearly reflected in the information about cereal production, droughts and floods found in the historical archives of the 13th-14th c. for the Campania and Basilicata regions (Boenzi and Giura Longo, 1994; Diodato, 2007). The warm-dry climate brought to a gradually decrease in vegetation cover, and the rills and gullies developing on the slopes eroded previous accumulations. Toward 390 a cal. BP, as is clearly identified also by the dendrochronological data of Pinus Leucodermis from Monte Pollino (Serre-Bachet, 1985), a cold period corresponding to the Little Ice Age (LIA) began. As stated by J. Guiot et al. (2005) in a most recent study on the last millennium summer temperature variations in western Europe, the LIA extends from 390 BP to 90 a cal. BP (i.e., 450-150 years ago). These data find a good relation both with the dendrochronological record and with the development of the temperate deciduous forest at Monticchio, which is once again the more frequent biome. The period of extensive deforestation in Basilicata occurred between the end of the 19th c. and the beginning of the 20th c. (Ticky, 1962). This phenomenon was due to the emergence of new social classes, the expansion of large estates and the increase in population.

Fig. 4 – Synoptical palaeoecological (pollen data from Allen et al., 2002) and morphological changes in the Basento valley (from Boenzi et al., 2008; Piccarreta et al., 2011) during the Late Holocene.
Fig. 4 – Changements paléoécologiques (données polliniques : Allen et al., 2002) et morphologiques (Boenzi et al., 2008 ; Piccarreta et al., 2011) synoptiques dans la vallée du Basento durant l’Holocène supérieur

Fig. 4 – Synoptical palaeoecological (pollen data from Allen et al., 2002) and morphological changes in the Basento valley (from Boenzi et al., 2008; Piccarreta et al., 2011) during the Late Holocene. Fig. 4 – Changements paléoécologiques (données polliniques : Allen et al., 2002) et morphologiques (Boenzi et al., 2008 ; Piccarreta et al., 2011) synoptiques dans la vallée du Basento durant l’Holocène supérieur

Chronology in calendar age. 1: pebbly alluvium; 2: sandy alluvium; 3: fine alluvium; 4: clays; 5: palaeosol; 6: AP3 tephra (ca. 2800 BP); 7: Avellino tephra (ca. 4300 BP); 8: erosive surface; 9: fossils; 10: charcoal; 11: woods (cortex); 12: potsherds (ca. 4500 BP); 13: burial (ca. 2500 BP).
Chronologie en âge calendaire. 1 : alluvions caillouteuses ; 2 : alluvions sableuses ; 3 : alluvions fines ; 4 : argiles ; 5 : paléosol ; 6 : téphra AP3 (environ 2800 BP) ; 7 : téphra Avellino (environ 4300 BP) ; 8 : surface érosive ; 9 : fossiles ; 10 : charbons ; 11 : bois (cortex) ; 12 : tessons (environ 4500 BP) ; 13 : sépulture (environ 2500 BP).

Deep gullies entrenchment and environmental change

10The comparison between geomorphological processes and palaeoenvironmental data shows that there is a strong relation between phases of channel entrenching (i.e., badland gullies formation) and changes in climate and vegetation (fig. 4). From the detailed study of the chronostratigraphy of alluvial sequences in the Basento basin, and from the radiocarbon datings from alluvial deposits of Basilicata rivers, it emerges that 4 phases of incision occurred during the Late Holocene at around 4500-4350, 1500, 900-800 and 150 a cal. BP; the human pressure was neglectable as regards the channel incisions:

11- After around 4500 a cal. BP, the Mediterranean climate was marked by increased aridity, which was really intense in Basilicata, especially from 4400 to 4200 a cal. BP (Allen et al., 2002; Di Rita and Magri, 2009). Shortly before 4300 a cal. BP, 10 to 50 cm-thick “Avellino” tephra layers were deposited and in the Fosso La Capriola, the “Avellino” tephra marks an erosive surface which separates the first fills from the rest of the succession, defining a small channel whose filling started shortly before the ash deposition (Boenzi et al., 2008). By combining the evidence, it is possible to suggest that a downcutting phase should have taken place between 4550 and 4350 a cal. BP, during a dry climate period.

12- A new erosional phase started after 1620 a cal. BP and ended before 1435 a cal. BP in correspondence with new climatic shifts towards warmer and drier condition. Since the settlement density was quite low during the above-mentioned period in the investigated area, anthropogenic pressure cannot be considered as the cause of the deposition/incision phases occurring in this span of time.

13- The third phase of incision (after 900-800 a cal. BP) lasted probably for almost 300 years and was characterised by a strong rapid downcutting of the main drainages below the upper Holocene terrace to the present river level, followed by base level stability. All the five main rivers (Bradano, Basento, Cavone, Agri and Sinni) dug into their alluvial deposits for more than 15 m and these events occurred mainly during the Mediaeval Warm period, continuing throughout alternating phases of stability until the end of 300 a cal. BP. The overall dry conditions led to a drop in water tables and a reduction in the vegetation cover that protected the slopes from erosion. Low frequency, high magnitude flooding resulting from a phase of increased precipitation following a dry period would trigger badland gullies cutting. As argued by L. Leopold (1976) and L. Leopold and C. Vita-Finzi (1998), periods of erosion in semi-arid areas are relatively arid and may not have lower annual rainfall, but a higher frequency of intense storms. Therefore, erosion was associated with change in rainfall intensity, not the mean annual amount. This last feature has been widely recognised in the recent behaviour of Basilicata rivers and gullies (Piccarreta et al., 2006; Piccarreta and Capolongo, 2009). For example, over a 2-day period (November 24 and 25, 1959) there were 375 mm of rainfall, with 314.7 mm on 25th November, corresponding to more than half of the annual average. This event generated significant soil erosion and produced gullies greater than 2 m in depth, contributing to the evolution of badlands (fig. 5).

14- From 300 a cal. BP (360 years ago), during the LIA, a cooling phase associated with an increase in fluvial geomorphic activity occurred in Europe (Vita-Finzi, 1969; Grove, 2001; Grove and Rackam, 2003; Macklin et al., 2006), with rapid deposition of coarse bedload-dominant material and sandy-loamy alluvium to form a younger Holocene terrace. This aggradational phase continued until around 120 a cal. BP (i.e., 180 years ago; Piccarreta et al., 2011). After 120 a cal. BP, the streams began to downcut, as testified by the presence of well-formed fluvial terraces in the old topographic map of the Basilicata region realised in AD 1875.

Fig. 5 – More than 2 m deep gully after the storm events of 24-25 November 1959
Fig. 5 – Ravine de plus de 2 m de profondeur mise en place après la tempête du 24-25 novembre 1959

Fig. 5 – More than 2 m deep gully after the storm events of 24-25 November 1959 Fig. 5 – Ravine de plus de 2 m de profondeur mise en place après la tempête du 24-25 novembre 1959

Photo: prof. Federico Boenzi.
Photo : prof. Federico Boenzi.

Conclusions

15Evidence provided by the alluvial sequences of the Basento River shows that badland gullies in valley fills in Fossa Bradanica, Basilicata, formed during the last 4500 years as a result of changing climate and biotic conditions. Four main phases of deep gullies/channel entrenching have been documented respectively at around 4500-4350, 1500, 900-800 and 120 a cal. BP. Badland gullies formation appears to be linked to repeated dry-wet cycles. When the climate became drier, decreasing plant cover and increasing runoff and discharge from rainfall led to the deepening and widening of river channels.

We thank the Editors and three anonymous reviewers for their constructive revision of the original manuscript.

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Annexe

Version abrégée en français

Dans la Fossa Bradanica, zone semi-aride située en Lucanie, Italie méridionale, de profondes ravines se sont développées sur les versants légèrement pentus situés en marge des plaines alluviales de différents bassins hydrographiques (Neboit, 1977, 1983). Les ravines ont été creusées de façon cyclique dans les dépôts alluviaux holocènes jusqu’à une profondeur maximale d’environ 20 m, une largeur maximale de plusieurs centaines de mètres et une longueur maximale de 20-30 km. L’étude de la dynamique récente des réseaux de ravines et son impact sur l’environnement a ouvert le champ à de nombreuses questions scientifiques jusque-là peu abordées : 1) Quand et comment les ravines se formèrent dans les remblaiements alluviaux holocènes ? Les ravines sont-elles liées aux changements climatiques ou à l’impact anthropique ? Pour répondre à de telles questions, une reconstitution précise de la stratigraphie alluviale, de l’histoire des processus érosifs et du climat a été nécessaire.

Représentative de l’ensemble de la Fossa Bradanica, la zone d’étude (fig. 1 à fig. 3) se situe dans le bassin hydrographique du fleuve Basento. La lithologie du bassin est marquée par la présence d’argiles limoneuses pléistocènes (« Argille Subappennine ») déposées sur des conglomérats sableux disposés en terrasses marines (Boenzi et al., 1971; Brückner, 1980). La chronostratigraphie des dépôts alluviaux des différents fleuves lucaniens de l’Italie méridionale a fait l’objet de nombreuses études au cours des trente dernières années (Neboit, 1977, 1983; Brückner, 1983, 1990; Abbott et Valastro, 1995; Boenzi et al., 2008; Piccarreta et al., 2011). Les accumulations sont principalement holocènes; des dépôts datant du Pléistocène supérieur, mal conservés, sont présents dans deux terrasses situées le long de tous les principaux cours d’eau. Les dépôts holocènes s’observent au niveau de deux terrasses fluviales dont l’une se rapporte à l’Holocène moyen/supérieur et l’autre au Petit Âge Glaciaire. La phase de remblaiement alluvial holocène a débuté à partir de 7000 cal. BP (Brückner, 1983, 1990; Abbott et Valastro, 1995) pour se terminer vers 120 cal. BP (Piccarreta et al., 2011). Cette étude détaille les données chronostratigraphiques obtenues grâce à des reconstitutions détaillées et précises des dépôts alluviaux (terrasses de l’Holocène supérieur à Fosso La Capriola: Boenzi et al., 2008 ; terrasses de l’Holocène inférieur à Fosso del Piano : Piccarreta et al., 2011 ; fig. 2). Sur la base des travaux conduits par F. Boenzi et al. (2008), nous observons que le remblaiement holocène des vallée du fleuve Basento et de ses affluents est marqué par une alternance de phases d’accumulation (Néolithique supérieur ; 2800-1620 cal. BP, avec une interruption autour de 2500 cal. BP ; 1440-1000 cal. BP) et d’incision (4500-4350 cal. BP ; 1620-1500 cal. BP ; après 900 cal. BP). M. Piccarreta et al. (2011) soulignent que le remblaiement relatif au Petit Âge Glaciaire devrait être postérieur à 250 cal. BP, pour terminer autour de 120 cal. BP. Une nouvelle phase d’incision a donc entraîné l’approfondissement (maximum : 5 m) du fleuve Basento dans ses propres dépôts alluviaux.

Dans le but de corréler les épisodes d’incision avec les changements du climat et du couvert végétal au cours de l’Holocène moyen et supérieur, les données chronostratigraphiques ont été mises en relation avec les données paléoclimatiques résultant de l’étude des séquences polliniques du Lago Grande de Monticchio (Allen et al., 2002) et des archives historiques des XIIIe-XIVe s. des régions de Campanie et de Lucanie, concernant la production de céréales, les sécheresses et les inondations (Boenzi et Giura Longo, 1994; Diodato, 2007). La comparaison entre les phénomènes géomorphologiques observés in situ et les données paléoenvironnementales montre l’existence d’une forte corrélation entre les phases d’incision (formation des ravines et mise en place des paysages de badlands) et les changements du climat et du couvert végétal (fig. 4). Durant les phases d’incision, le climat de la zone étudiée est toujours sec, comme le prouve la chute considérable du taux de pollens arboréens au profit de l’augmentation de certains taxons herbacés. La mise en place des badlands et des formes de ravinement associées semble donc en lien avec des cycles sécheresse/humidité répétés. Quand le climat devient plus sec, la diminution du couvert végétal et l’augmentation du ruissellement le long des versants entraînent un approfondissement et un élargissement des ravines.

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Table des illustrations

Titre Fig. 1 – Geomorphological map of the study areaFig. 1 – Carte géomorphologique de la zone d’étude
Légende 1: main towns; 2: main streams; 3: marine terraces edge; 4: slope pediment; 5: alluvial deposits; 6: clays (affected by badlands); 7: coastal deposits; 8: sands and conglomerates; 9: landslide debris; 10: Pleistocene fluvial terrace; 11: Middle-Late Holocene fluvial terrace; 12: Late Holocene fluvial terrace; 13: recent fluvial terrace.1 : ville principale ; 2 : rivière principale ; 3 : rebord de terrasse marine ; 4 : glacis ; 5 : remblaiements alluviaux ; 6 : argiles (ravinées) ; 7 : remblaiement marin ; 8 : sables et conglomérats ; 9 : dépôt de glissement de terrain ; 10 : terrasse fluviale pléistocène ; 11 : terrasse fluviales de l’Holocène moyen/supérieur ; 12 : terrasses fluviale de l’Holocène inférieur ; 13 : terrasse fluviale récente.
URL http://geomorphologie.revues.org/docannexe/image/9856/img-1.png
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Titre Fig. 2 – A: Geomorphological map of the middle-lower Basento valley. B: Alluvial sequences at Fosso Del Ponte and Fosso La Capriola. C: Longitudinal profiles of the Basento River and its terracesFig. 2 – A : Carte géomorphologique de la moyenne-basse vallée du Basento. B : Les séquences alluviales à Fosso Del Ponte et à Fosso La Capriola. C : Profils longitudinaux du fleuve Basento et de ses terrasses
Légende A – 1: thalweg; 2: present-day alluvial plain; 3: barrier beach and deltaic deposits; 4: I order (Late Pleistocene fluvial terrace, ca. 39000 BP); 5: II order (Late Pleistocene fluvial terrace, age?); 6: III order (Middle-Late Holocene fluvial terrace); 7: IV order (Late Holocene fluvial terrace, Little Ice Age); 8: V order (recent, after 1954); FP: Fosso del Piano; MG: Masseria Glionne; MF: Masseria Fiorentino; FC: Fosso La Capriola; FLC: Fosso La Canala. B – Alluvial sequences at Fosso Del Ponte and Fosso La Capriola. 1: pebbly alluvium; 2: sandy alluvim; 3: fine alluvium; 4: clays; 5: palaeosol; 6: AP3 tephra (ca. 2800 BP); 7: Avellino tephra (ca. 4300 BP); 8: erosive surface; 9: fossils; 10: charcoal; 11: woods (cortex); 12: potsherds (ca. 4500 BP); 13: burial (ca. 2500 BP). C – 1: I order; 2: II order; 3: III order; 4: IV order.A – 1 : talweg ; 2 : plaine alluviale actuelle ; 3 : cordon littoral et dépôts deltaïques ; 4 : ordre I (terrasses fluviales de Pléistocène supérieur, environ 39000 BP) ; 5 : ordre II (terrasses fluviales de Pléistocène supérieur, âge indéterminé) ; 6 : ordre III (terrasses fluviales de l’Holocène moyen/supérieur) ; 7 : ordre IV (terrasses fluviales de l’Holocène inférieur, Petit Âge Glaciaire) ; 8 : ordre V (récents, après 1954) ; FP : Fosso del Piano ; MG : Masseria Glionne ; MF : Masseria Fiorentino ; FC : Fosso La Capriola ; FLC : Fosso La Canala. B – 1 : alluvions caillouteuses ; 2 : alluvions sableuses ; 3 : alluvions fines ; 4 : argiles ; 5 : paléosol ; 6 : téphra AP3 (environ 2800 BP) ; 7 : téphra Avellino (environ 4300 BP) ; 8 : surface d’érosion ; 9 : fossiles ; 10 : charbons ; 11 : bois (cortex) ; 12 : tessons (environ 4500 BP) ; 13 : sépulture (environ 2500 BP). C 1 : ordre I ; 2 : ordre II ; 3 : ordre III ; 4 : ordre IV.
URL http://geomorphologie.revues.org/docannexe/image/9856/img-2.png
Fichier image/png, 565k
Titre Fig. 3 – Panoramic view of the middle valley of the Basento River Fig. 3 – Vue panoramique de la moyenne vallée du fleuve Basento
Légende 1: Middle-Late Holocene fluvial terrace; 2: Late Holocene fluvial terrace; 3: recent fluvial terrace.1 : terrasses fluviales de l’Holocène moyen/supérieur ; 2 : terrasses fluviales de l’Holocène inférieur ; 3 : terrasses fluviales récents.
Crédits Aerial photo courtesy of prof. Massimo Caldara. Photos aériennes fournies par le prof. Massimo Caldara.
URL http://geomorphologie.revues.org/docannexe/image/9856/img-3.png
Fichier image/png, 2,6M
Titre Fig. 4 – Synoptical palaeoecological (pollen data from Allen et al., 2002) and morphological changes in the Basento valley (from Boenzi et al., 2008; Piccarreta et al., 2011) during the Late Holocene. Fig. 4 – Changements paléoécologiques (données polliniques : Allen et al., 2002) et morphologiques (Boenzi et al., 2008 ; Piccarreta et al., 2011) synoptiques dans la vallée du Basento durant l’Holocène supérieur
Légende Chronology in calendar age. 1: pebbly alluvium; 2: sandy alluvium; 3: fine alluvium; 4: clays; 5: palaeosol; 6: AP3 tephra (ca. 2800 BP); 7: Avellino tephra (ca. 4300 BP); 8: erosive surface; 9: fossils; 10: charcoal; 11: woods (cortex); 12: potsherds (ca. 4500 BP); 13: burial (ca. 2500 BP).Chronologie en âge calendaire. 1 : alluvions caillouteuses ; 2 : alluvions sableuses ; 3 : alluvions fines ; 4 : argiles ; 5 : paléosol ; 6 : téphra AP3 (environ 2800 BP) ; 7 : téphra Avellino (environ 4300 BP) ; 8 : surface érosive ; 9 : fossiles ; 10 : charbons ; 11 : bois (cortex) ; 12 : tessons (environ 4500 BP) ; 13 : sépulture (environ 2500 BP).
URL http://geomorphologie.revues.org/docannexe/image/9856/img-4.png
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Titre Fig. 5 – More than 2 m deep gully after the storm events of 24-25 November 1959 Fig. 5 – Ravine de plus de 2 m de profondeur mise en place après la tempête du 24-25 novembre 1959
Crédits Photo: prof. Federico Boenzi. Photo : prof. Federico Boenzi.
URL http://geomorphologie.revues.org/docannexe/image/9856/img-5.png
Fichier image/png, 1,6M
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Marco Piccarreta, Domenico Capolongo et Maria Nilla Miccoli, « Deep gullies entrenchment in valley fills during the Late Holocene in the Basento basin, Basilicata (southern Italy) », Géomorphologie : relief, processus, environnement, vol. 18 - n° 2 | 2012, 239-248.

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Marco Piccarreta, Domenico Capolongo et Maria Nilla Miccoli, « Deep gullies entrenchment in valley fills during the Late Holocene in the Basento basin, Basilicata (southern Italy) », Géomorphologie : relief, processus, environnement [En ligne], vol. 18 - n° 2 | 2012, mis en ligne le 02 novembre 2014, consulté le 01 octobre 2016. URL : http://geomorphologie.revues.org/9856 ; DOI : 10.4000/geomorphologie.9856

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Auteurs

Marco Piccarreta

Università degli Studi di Bari - Dipartimento di Scienze della Terra e Geoambientali - Via Orabona, 4 - 70125 Bari - Italy (marcopiccarreta@geo.uniba.it)

Domenico Capolongo

Università degli Studi di Bari - Dipartimento di Scienze della Terra e Geoambientali - Via Orabona, 4 - 70125 Bari - Italy (capolongo@geo.uniba.it)

Maria Nilla Miccoli

Università degli Studi di Bari - Dipartimento di Scienze della Terra e Geoambientali - Via Orabona, 4 - 70125 Bari - Italy (nillamiccoli@geo.uniba.it)

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Droits d’auteur

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