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The Holocene evolution of the Kalamas delta (northwestern Greece) derived from geophysical and sedimentological survey

L’évolution holocène du delta du Kalamas (Grèce du NW) dérivée des études géophysiques et sédimentologiques
Antoine Chabrol, Georges Apostolopoulos, Kosmas Pavlopoulos, Eric Fouache et Charles Le Cœur
p. 45-58

Résumés

Cet article a pour but de présenter les dynamiques holocènes responsables de la création et de l’évolution du delta du Kalamas (65 km², plus petit delta d’Épire, Grèce du NW). L’étude de ce delta, qui n’a jamais fait l’objet de recherches paléo-environnementales, ne manque pas d’intérêt géomorphologique et archéologique. Les vestiges archéologiques y attestent une occupation continue depuis le Paléolithique supérieur (site de Pirgos Raghiou). En lien avec les deux sites archéologiques de Mastilitsa (Âge du Bronze) et de Pirgos Raghiou (occupation continue depuis le Paléolithique supérieur), l’article présente les premiers résultats concernant l’évolution holocène de ce delta. La méthodologie adoptée croise les données géophysiques (7 profils de résistivité électrique le long de 3 transects) et sédimentologiques dérivées de 14 carottages. Les résultats sont présentés selon deux axes complémentaires : le premier souligne l’existence de plusieurs paléochenaux holocènes enfouis à proximité des sites archéologiques. Le second présente une première synthèse des environnements successifs depuis 7440 BP. Ces résultats, associés à une série de 3 datations 14C, permettent d’ouvrir des perspectives d’étude intéressantes sur les changements d’environnements survenus au début de l’Holocène, en lien avec les fluctuations climatiques (événement 8200 BP notamment).

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Notes de la rédaction

Article soumis le 28 février 2011, accepté le 15 juillet 2011.

Texte intégral

Introduction

1Across the Mediterranean basin, the glacial-interglacial transition (15000-6000 cal. BP; Alley et al., 2005; Wilson et al., 2008) is associated with the transition from hunter-gatherer societies of the Upper Paleolithic and Mesolithic farmer societies of the Neolithics (Kozlowski, 2005; Bar Yosef, 2006). It is also the time of a major bio-climatic transition between cold steppe environments of the last glacial maximum and sub-Mediterranean environments corresponding to a climate similar to the current interglacial (Bar-Matthews et al., 1997; Allen et al., 2002; Lawson et al., 2004; Bordon et al., 2008). The possible links between these major cultural changes and bio-climate changes have already been extensively debated. However, they resume their relevance today as the scientific advances in the field reveal palaeoclimatic and archaeological issues far more complex (Magny, 1993; van Geel et al., 1996; Kukla et al., 1997) across the Mediterranean world (Cullen et al., 2000; De Menocal, 2001; Lespez, 2003; Staubwasser and Weiss, 2006). These transitions and climate fluctuations have caused major palaeogeographic changes, which have not been adequately defined in terms of their magnitude and spatial and temporal limits. Global warming between 15000 BP and 6000 BP, despite the fluctuation of the Younger Dryas cooling, resulted in a rapid sea-level rise (-120 m to -10 m from the present mean sea level) and consequently, all coastal areas frequented between the Upper Paleolithics and the Neolithics had been submerged, sometimes over considerable distances. Archaeological remains may also have been fossilised by the sediments of the prograding alluvial deltas (Fouache and Pavlopoulos, 2005).

2For several decades, research has focused on the deltas and coastal plains of Greece (Kraft et al., 1977; Besonen, 1997; Fouache, 1999; Besonen et al., 2003; Fouache, 2003; Brückner, 2005; Vött et al., 2006; Ghilardi, 2007; Vött, 2007; Ghilardi et al., 2008; Engel et al., 2009; Ghilardi et al., 2011). At the interface between sea and land, these areas are suitable for palaeenvironmental an geoarchaeological studies. However, the delta of the Kalamas River (ancient name Thyamis), located in Epirus (northwestern Greece; fig. 1 and fig. 2) has never been studied from geoarchaeological point of view. Located to the east of the Corfu island, the Corfu strait was covered by a lake which was submerged during the post-glacial sea-level rise (Perissoratis and Conispoliatis, 2003) and may be considered as a key region for the neolithisation (Guilaine, 2000; Rasse, 2008; Velichko et al., 2009). The number of archaeological settlements in the delta (from the Upper Palaeolithic to the Byzantine times) highlights the interest of palaeoenvironmental studies. The mechanisms of the Holocene progradation of the Kalamas delta are not known. The sparse research carried out till now concerns only the recent prodelta evolution and configuration (Kapsimalis et al., 2010), and the geoarchaeological and geological research conducted in the Voïdomatis basin (Bailey et al., 1990; Woodward et al., 1992).

3In order to understand the geomorphological processes responsible for the construction and evolution of this delta, a dual geophysical/sedimentological approach has been chosen using short sediment corings, geoelectrical, seismic refraction and Ground Penetrating radar (GPR) techniques. Moreover, three AMS radiocarbon datings, grain-size analyses, magnetic susceptibility and microfaunal analyses have provided supplementary data in the present investigation. The research, based on a geoarchaeological approach (Fouache et al., 2010), aims to detail the transition from a marine to a terrestrial environment, to better understand the Holocene buried palaeochannels and thereby to provide an original understanding of the evolution of the Kalamas delta.

Fig. 1 – Location map of the area of study based on ASTER 30 m resolution DEM.
Fig. 1 – Carte de localisation du terrain d’étude réalisée à partir des MNT ASTER (résolution: 30 m).

Fig. 1 – Location map of the area of study based on ASTER 30 m resolution DEM. Fig. 1 – Carte de localisation du terrain d’étude réalisée à partir des MNT ASTER (résolution: 30 m).

The delta of the Kalamas River is located just near the Albanian boundary. The deltaic sediments are constrained by the altitudinal range 0-14 m. United Transverse Mercator WGS 84 coordinates.
Le delta du Kalamas se trouve à proximité immédiate de la frontière avec l’Albanie. Les dépôts deltaïques sont situés à des altitudes comprises entre 0 et 14 m. Le système de coordonnées est exprimé en United Transverse Mercator WGS 84.

Fig. 2 – Geomorphological map of the delta including the archaeological settlements, the cores and the geophysical transects.
Fig. 2 – Carte géomorphologique du delta comprenant les types de vestiges archéologiques ainsi que l’emplacement des carottages et des transects géophysiques.

Fig. 2 – Geomorphological map of the delta including the archaeological settlements, the cores and the geophysical transects.Fig. 2 – Carte géomorphologique du delta comprenant les types de vestiges archéologiques ainsi que l’emplacement des carottages et des transects géophysiques.

1: 'new' Kalamas direction, since 1958 and the construction of the dam of Ragio; 2: 'old' Kalamas direction, before 1958; 3: channel abandoned after the construction of the dam and the incision of the river; 4: palaeochannels discovered with aerial photos and SPOT and LANDSAT ETM+ images; 5: swamps and marshes. The swamps are Natura 2000 zones; 6: Ionian Sea; 7: Holocene deltaic deposits; 8: Pleistocene alluvial deposits; 9: Late Pleistocene deposits; 10: alluvial fan; 11: calcareous clay; 12: flyschs; 13: limestone; 14: limestone; 15: breccia; 16: Late Palaeolithic site; 17: Neolithic site; 18: Bronze Age site; 19: Classical site; 20: Roman site; 21: Fortress; 22: probable buried harbour; 23: fault; 24: overlap; 25: core; 26: geophysical transect.
1 : nouveau cours du Kalamas depuis 1958 ; 2 : ancien cours du Kalamas avant 1958 ; 3 : méandre abandonné après la construction du barrage et l’incision du fleuve ; 4 : paléo-cours du Kalamas découverts grâce à l’étude des photographies aériennes et des images satellite SPOT et Landsat ETM+ ; 5 : marais et zones humides. Zones classées Natura 2000 ; 6 : mer Ionienne ; 7 : dépôts deltaïques d’âge holocène ; 8 : dépôts alluviaux pléistocènes ; 9 : dépôts de la fin du Pléistocène ; 10 : cône alluvial ; 11 : marnes ; 12 : flyschs ; 13 : calcaires ; 14 : calcaires ; 15 : brèches triasiques ; 16 : site du Paléolithique supérieur ; 17 : site néolithique ; 18 : site de l’Age du Bronze ; 19 : site d’époque Classique ; 20 : site d’époque romaine ; 21 : forteresse ; 22 : probable port enfoui ; 23 : faille ; 24 : chevauchement ; 25 : carottage ; 26 : transect géophysique.

Regional setting

4The delta of the Kalamas River is the northern delta of Epirus (northwestern Greece) and is located near the Albanian border and opposite to the island of Corfu (fig. 1). The Epirus region is mainly mountainous and the deltaic plains formed by the rivers Acheron and Kalamas are the only low lands of the Ionian coast. The Kalamas River, which originates from the Mount Douskos (Greece, 1300 m asl) has a length of 115 km for a catchment area of 1895 km². The riverine sediments load discharges in the Corfu straits (Ionian Sea) and forms a small delta of 64 km². Elevations in the delta plain range between mean sea level in Saghiada and 14 m at the entrance of the river in the plain (fig. 1 and fig. 2). The associated wetlands were designated as a Natura 2000 site by the European Union (Karimbalis et al., 2006).

Geological and geomorphological background

5The studied area belongs to the Ionian zone that extends from Albania to Epirus, the Ionian islands, Acarnania and Western Peloponnesus (Aubouin, 1959; Bousquet, 1976; Fouache, 1999). The area consists of a series of NNW-SSE trending folds and faults blocks which form a series of parallel limestone mountain ranges with intervening flysch basins (Besonen, 1997). The faults and thrust fault blocks have been formed in a sequence of compressional orogenic events since the Late Jurassic (IGRSS/IFP, 1966). The rough relief is resulted from the structure and the contrasting lithologic properties of the limestone and flyschs. The relief is even more pregnant along the coasts where the rock cliffs plunge into the sea and conversely, very flat coastal plains give way in abrupt topographic discontinuity to carbonate valley walls (IGRSS/IFP, 1966; Bousquet, 1976; Besonen, 1997). Topographic irregularities occurred in the plain (Mavron Oros mount) are outcrops of the great limestone syncline of Igoumenitsa (IGRSS/IFP, 1966). At the dawn of delta’s creation, this mountain was probably an island, like the one where lies the settlement of Pirgos Raghiou (fig. 2). From regional tectonic studies (Moretti et al., 2004), various indications suggest that the delta is subsiding, contrary to the rocky coasts of southern Epirus and northeastern Corfu (Bousquet, 1976; Pirazzoli, 1980; Pirazzoli et al., 1994; Waters, 1994; Pirazzoli, 2005; Vött, 2007). This subsidence is complex and has several causes. Since 1958, an extended program of irrigation changed the topography of the delta. The diversion to the north of the 'old' river course and the construction of a dam near the village of Raghio changed the topography and the physiography of the delta. The subsidence of the plain is likely to be partly from agricultural pumping activities, as it is determined in other deltas of Greece (Stiros, 2001; Psimoulis et al., 2007). This subsidence is also maintained by the thickness and the weight of the Holocene sediments and the existence of active marginal faults (Bousquet, 1976). Finally, the recent discovery of a Roman villa, located 6 m bsl near the town of Saghiada seems to confirm this hypothesis.

Archaeological background

6The archaeological potential of northern Greece has been considered as significant over the last two decades (Hammond, 1967; Bailey, 1995; Riginos, 1996; Lespez, 2003; Berger, 2005; Christophilopoulou, 2005; Stefani and Vincenzi, 2005; Pross et al., 2006). Human occupation during the Late Glacial/Holocene transition is demonstrated by the existence of unequal distributed sites: however in Epirus, the Mesolithic and Neolithic background is still poorly known (Demoule et Perlès, 1993; Kotzakis, 2001; Perlès, 2001). The major research programs conducted by the US teams in the mid-1990s (Bailey et al., 1990; Besonen, 1997; Tartaron, 2004) focused on areas further south (Acheron delta and valleys of the Arachtos and Louros rivers). At this time, coastal areas had considerably evolved because of the post-glacial sea-level rise and reduced the dry lands and territories for human groups (Lambeck et Purcell, 2005). The reconstruction of coastal palaeogeographies allows a better understanding of the dynamics of the prehistoric settlements (Grammenos, 2003). In this case, the discovery of Mesolithic sites on the island of Corfu (Sordinas, 1970) and not in Epirus on the mainland suggests that the human groups were able to move to dry land in this area. For the Late Holocene, the archaeological sites of Mastilitsa and Pyrgos Raghiou can be considered as the most interesting. The structures of the former were dated from the Late Bronze Age. It is located on the top of the Mastilitsa hill at 45 m asl. Recent prospections made by the Ephoria of Igoumenitsa excavated some graves just below the hill, in the deltaic plain (Christophilopoulou, 2005; fig. 2). The later was held longer, since the Late Palaeolithic times. Then, since the archaic times, the site was occupied by fortifications. It is interesting to wonder, using cores and geophysical methods, what was the morphology of the plain at the time of occupation of these two strategic and well preserved sites.

Materials and methods

7A multi-method approach was preferred for this work combining fieldwork, laboratory analyses and spatial representation of the results. All data were integrated into a GIS. The geomorphological maps (non existent in Greece) used topographic maps of the Greek army (scale 1:50000) and the former USSR (scale 1:25000, 1:50000) and the geological maps of IGME (Institute of Geology and Mineral Exploitation). Bathymetric maps provided by the Italian Navy were also incorporated. Aerial photographs and remote sensing tools (Landsat ETM+ and SPOT imagery) allowed us to identify former courses of the river in the delta. Similarly, wetlands and marshes, sometimes below the mean sea level, have been mapped. The fieldwork consisted of the completion of geophysical profiles and core samples along several transects. Grain-size analyses were conducted at the Laboratoire de Géographie Physique (CNRS-UMR 8591, Meudon, France) and at the University of Harokopio (department of Geography, Athens, Greece). Geophysical profiles were conducted in collaboration with the National Technical University of Athens (section of mining engineering, Athens, Greece).

Geophysical prospections

8Geophysical surveys are particularly efficient in reconstructing the palaeotopographies near archaeological sites (Hecht, 2007; Siart et al., 2009). Three methods of geophysical surveys were used for this research: GPR, electric resistivity and seismic refraction profiles. Due to the high water content in the clays of the delta, the GPR obtained are not available. Seven profiles of electric resistivity were performed (electrode spacing: 2 m). Each profile is 200 m long. The results were incorporated into the GIS. Physical properties of sediments have not been extrapolated when they were not measured.

Drills in the plain (tab. 1)

9The grain size measurements were conducted both by a laser Coulter (grain-size fraction ≤2 mm) and manual measurements (sieving for grain-size fractions ≥2 mm) collected at 5 cm intervals on the cores S1, S2, S3, S5, S6, S7, S9, S10, S11 and S12 (1412 samples). Every sample was weighed and dried (24 h at 40 °C). Organic matter was removed (4 h with H2O2 treatment). Particles were then dispersed using 0.1% of sodium hexametaphosphate and left in dionised water for 4 h. Every sample was exposed 4 min to ultrasounds. The grain-size distribution was measured using a Coulter LS 230 laser (range 0.04-2000 μm). Due to the fine to very fine main grain sizes, measurements were conducted between 5% and 10% of obscuration and between 45% and 55% PIDS (Polarisation Intensity Differential Scattering). Several calculation models were used: Mie-Lorenz theory and Fraunhofer distribution for very fine particles (about 0.04 μm; Buurman et al., 1996; Blott et al., 2004). The grain-size analysis results only present two indexes: the mode and d10. These indexes are good indicators of the river competence (Arnaud-Fassetta, 2006). The mode reveals changes in the energy of deposition and the d10 corresponds to the grain size of the first 10% of the total amount of the particle analysed for every sample. Magnetic susceptibility measurements were conducted using a Bartington MS2B dual frequency sensor, at low and high frequencies. The magnetic parametres were directly measured on the cores (volumic susceptibility) and used the measurement setting of 0.1 (resolution 10-5 SI units; Dearin et al., 1996).

Microfaunal identification and AMS datings (tab. 2)

10Microfaunal fossils and shells are an excellent environmental marker (Ghilardi, 2007), particularly in changing environments such as deltas. The palaeontological identifications, using a binocular, were carried out after cleaning and drying of the shells. A first chronostratigraphic approach of the Holocene landscape changes in the delta was made possible by obtaining 3 AMS radiocarbon determinations, undertaken on peat and wood samples (core S8). These analyses were performed by BETA Analytic. 14C ages were then calibrated (Reimer and McCormac, 2002).

Tab. 1 – Location of the cores.
Tab. 1 – Localisation des carottages.

Core id

Absolute elevation (in m a.s.l.)

Latitude (WGS 84, UTM, 34N)

Longitude (WGS 84, UTM, 34N)

Depth (in m)

S1

2

39°36'60.3N

20°12'79.4E

11

S2

2

39°36'03.8N

20°13'50.5E

12

S3

0

39°32'14.9N

20°13'34.6E

12.4

S4

0

39°31'97.8N

20°13'76.3E

12.5

S5

1

39°32'18.3N

20°12'12.2E

10.5

S6

1

39°32'30.8N

20°12'39.1E

10.6

S7

1

39°32'41.2N

20°11'73.6E

11.3

S8

0

39°33'42.1N

20°14'21.6E

11.4

S9

13

39°34'85.5N

20°14'55.2E

6,5

S10

4

39°59'20.2N

20°22'85.5E

7

S11

3

39°59'16.5N

20°22'32.5E

10.5

S12

2.5

39°59'24.1N

20°21'56.3E

7.5

S13

5

39°57'02.9N

20°22'50.1E

9.5

S14

2

39°57'57.0N

20°20'27.0E

9

Tab. 2 – AMS radiocarbon results obtained on the core S8.
Tab. 2 – Datations radiocarbone obtenues par AMS sur le carottage S8.

Core

Depth b.s.l

Material

Methods

13C/12C

2 sigma calibration

S8

9.76

plant

AMS

-26

7440 cal. BP

S8

9.21

plant

AMS

-25.2

6630 cal. BP

S8

8.42

plant

AMS

-25.8

5730 cal. BP

Results

11According to the methodology we exposed and to several studies, the results dealing with geophysical prospections and grain-size analysis (fig. 3 and fig. 4) will be presented in a 'top-down' logical, focusing on geophysical transects scale first, and then on the core profiles scale.

12Transect 1 includes four resistivity profiles (TR1, TR2, TR3, TR4), for a total length of 800 m. The data show marked variations in resistivity values from the surface to -30/-40 m and provide thereby information about the palaeochannels of the Kalamas River. Several zones can be detected, with different resistivity values: the low values (R≤100 Ωm) correspond to the very fine-grained sediments, such as clays, sands and gravels. The higher resistivity zones (R≥100 Ωm) are more difficult to explain. They could correspond to buried archaeological structures (Siart et al., 2009) or to limestone blocks. Actually, Transect 1 is dominated by low resistivity values. However, changes in these low values must be noticed:

13- The TR2 profile show changes in a spectrum of low values, from the surface to a depth of 10 m. The values between 15-20 Ωm correspond to brawn clay. The vertical extent of this layer is not homogeneous: at the same depth, higher resistivity values (20-30 Ωm) correspond to sandy sediments (fig. 4). The very low resistivity of the sediments below 15 m (5-10 Ωm) could correspond (these layers were not collected by the core) to very fine clay (lacustrine or lagoonal infills).

14- The first meters of the TR3 profile correspond to the TR2 first values: clay deposits. From 6 m to 20 m depth, the resistivity values clearly indicate sandy sediments, with two areas of local high values (30-40 Ωm). The sediments obtained in the core S11 consist of gravels and coarse sands (from -6.6 m to -8.4 m). These two specific resistivity layers, associated with the grain-size analyses results, could correspond to Kalamas palaeochannels (fig. 4).

15- The TR4 profile presents very high resistivity values (75-300 Ωm) from 6 m to 25 m depth. The profile was made just below the Mastilitsa hill (fig. 3). Dense ceramic remains found on the surface of the field clearly indicate the presence of an ancient occupation. The high resistivity layers could then correspond to archaeological structures by the geometry (probably a buried part of the archaeological site). The core S12 was stopped on the top of this layer.

16The transect 2 is located in the eastern part of the delta, near the village of Ragio (fig. 2). Two profiles (TR5, TR6) were made on the alluvial fan:

17- TR5 shows high resistivity values (40-150 Ωm, locally 300 Ωm). These values correspond to millimetric/centimetric gravels (fig. 4). The common brawn clay layer (15-20 Ωm) is almost non-existent in this area. The core was affected by this structure and ended at 6.5 m below the surface (core S9). The very high resistivity values (locally 300 Ωm) correspond to decimetric/metric blocks. According to some regional studies (Waters, 1994; Besonen, 1997), this profile probably highlights a Pleistocene alluvial fan (dates in progress).

18- The first five meters of the TR6 profile show usual low resistivity values (15-20 Ωm), which correspond to Holocene fine alluvial deposits. These deposits correspond to a recent deviation of the Kalamas course (before 1958; fig. 2). The meander formed at this place by the Kalamas River (also visible in the topography) was cut for agricultural activities and the course of the river was derived to the north. The underlying layers show high resistivity values, according to the profile TR5 and clearly indicate the spatial continuity of the Pleistocene alluvial fan.

19The transect 3 is located in the southern part of the delta, near the hill and the archaeological site of Pirgos Raghiou (fig. 2). Aerial photos and SPOT and Landsat ETM+ images show the existence of a palaeochannel in the vicinity of the hill (fig. 2). Only the core S6 was made in the palaeochannel. The electric profile is located just below the Pirgos Raghiou hill and show homogeneous very low values (5-20 Ωm) characterised by decantation deposits (this area is a continuous flooded area). The local high values (40-75 Ωm) correspond to the limestone of the hill.

Fig. 3 – Geoelectric profiles obtained along 3 transects in the delta.
Fig. 3 – Représentation des profils géoélectriques obtenus selon 3 transects dans le delta.

Fig. 3 – Geoelectric profiles obtained along 3 transects in the delta. Fig. 3 – Représentation des profils géoélectriques obtenus selon 3 transects dans le delta.

See image 3 for the geoelectric legend. Image 1 – 1: location of the first transect; 2: picture and orientation of the transect; 3: diagram block of the transect; 4: topographic profile of the transect. The cores S10, S11, S12 are represented. Image 2 – 1: location of the second transect; 2: diagram block of the transect; 3: topographic profile of the transect. Image 3 – 1: location of the transect; 2: topographic profile of the transect.
La légende des profils se trouve sur l’image 3. Image 1 – 1 : localisation du premier transect ; 2 : photographie de l’orientation du transect ; 3 : bloc diagramme ; 4 : profil topographique du transect. Image 2 – 1 : localisation du second transect ; 2 : bloc diagramme ; 3 : profil topographique du transect. Image 3 – 1 : localisation du troisième transect ; 2 : profil topographique du transect.

Fig. 4 – Sedimentological properties of the cores S10, S11, S12, S9 along the geoelectrical profiles.
Fig. 4 – Profils sédimentologiques des carottages S10, S11, S12, S9 localisés le long des profils géoélectriques.

Fig. 4 – Sedimentological properties of the cores S10, S11, S12, S9 along the geoelectrical profiles. Fig. 4 – Profils sédimentologiques des carottages S10, S11, S12, S9 localisés le long des profils géoélectriques.

1: brown clay, silt and fine sand; 2: bedded sand to coarse sand; 3: coarse sand; 4: coarse sand and gravel; 5: gravel; 6: limestone; 7: gravels to pebbles.
1 : argiles beiges, limons et sables fins ; 2 : sables fins-moyens lités et sables grossiers ; 3 : sables grossiers ; 4 : sables grossiers et graviers ; 5 : graviers ; 6 : calcaires ; 7 : graviers à galets.

Discussion

Subsurface morphology of the delta

20One of the most notable possibilities offered by the geophysical prospections concern the vertical differentiations in the types of deposits. Except for Transect 2 (on the alluvial fan), the resistivity values did not reveal marked differences in the subsurface layers. The vertical depth of investigation must be underlined. Preliminary studies (Kapsimalis et al, 2010) in the prodelta based on high-resolution seismic profiles reveal a Holocene thickness of almost 30-40 m. These results correspond to regional data obtained in the Acheron delta, in the plain of Thessaloniki (Besonen, 1997; Ghilardi, 2007) and in the delta of the rivers Arachtos/Louros (Kapsimalis et al., 2005). Unfortunately, the cores made along these profiles did not reach the point of change between terrestrial deposits of the last glacial period and the sediments provided by the last Holocene marine transgression. However, there is some evidence, showing that these changes may not clearly appear on the geoelectrical profiles because of the negligible differences in the grain-size properties. The longest Holocene stratigraphy was obtained in the core S8 (fig. 5).

The Holocene pre-transgressive surface

21The pre-transgressive surface was identified in 4 cores (S2, S4, S7, S8). Three AMS radiocarbon dates allow us a first pre-transgressive reconstruction (tab. 2 and fig. 5). The core S8 is characterised by the alternation of peat and dark organic layers with marine or lagoonal deposits. The first phase (from the bottom to -9.84 m) is characterised by very fine organic blue clay. According to the grain size, it clearly appears that these sediments are of lacustrine origin. The low values of the magnetic parametres are due to the geology of the basin: mainly limestone and flyschs, with very low magnetic properties. The microfaunal analyses revealed the presence of Planorbis planorbis. This specie usually lives in fresh-water marshes in the Balkans and was not yet known in Northern Greece. The spatial extent of this lake is not known. Cores S3-S4 also recorded these lacustrine deposits. The second phase (from -9.84 m to -8.4 m) is characterised by an alternation of four dark organic layers including silty sand and wood remains, with high magnetic parametres. These events are dated from 7440±160 cal. BP to 5730±120 cal. BP. The changes in the magnetic-susceptibility curve reflect physical changes in the depositional environment, in relation with the climate or the land use during the Neolithics (Bloemendal, 1982; David et al., 1998; Weninger et al. 2006; Ghilardi et al., 2010). These changes could correspond to changes in the extension of the lake.

Fig. 5 – Sedimentological properties of the core S8.
Fig. 5 – Propriétés sédimentologiques du carottage S8.

Fig. 5 – Sedimentological properties of the core S8. Fig. 5 – Propriétés sédimentologiques du carottage S8.

1: brown clay, silt and fine sand; 2: peat; 3: sand; 4: dark organic layers; 5: silty clay.
1 : argiles beiges, limons et sables fins ; 2 : niveaux tourbeux ; 3 : sables ; 4 : niveaux organiques sombres ; 5 : argiles limoneuses.

Deltaic environments along the two core-transects (S1-S2, S3-S4-S7-S8)

22Without radiocarbon dates enough, an exact chronostratigraphy of the cores in the delta is still not possible. But the results obtained both by the grain-size analyses, the magnetic susceptibility and the microfaunal identifications allow a first overview of the Holocene evolution of the delta in order to compare these data with other Mediterranean studies. The discussed results were obtained on the 6 deepest cores (S1, S2, S3, S4, S7, S8) and along 2 transects: S1-S2 (fig. 2 and fig. 6) and S3-S4-S7-S8 (fig. 2 and fig. 7). Five sedimentary units were recorded in the cores and reflect several deltaic environments:

23- In the lower part of the cores S2-S7, the lowermost unit is composed of homogenous fine sediments (blue clay and silts), without microfaunal remains. This unit is interpreted as pro-delta deposits, as known in Greece (Ghilardi, 2007).

24- Above these pro-delta sediments, the second sedimentary unit mainly consists of clay deposits (grey clay). Microfaunal analyses revealed the presence of marine gastropods (Bittium reticulanum, Cerastoderma edule, rare remains of Ostreidae sp.). The species, associated to the sedimentary unit, allow us to interpret this unit as calm marine deposits (Marriner et al., 2008). Radiocarbon datings indicate that the marine transgression appeared after 5730 cal. BP. The exact period of the marine incursion is still unknown.

25- The third sedimentary unit consists of silts and sandy sediments with macrofaunal shells (Ostreidae sp., Cyclope neritea, Cerastoderma glaucum). These deposits do not appear in the core S8. According to the grain-size analyses and faunal identifications, this unit is interpreted as lagoonal deposits. The timing of this period is still unknown and also not easy to date.

26- Above the lagoonal deposits occurs a thick dark organic layer (peat). This layer occurs in the cores S3-S4-S8 and respectively reaches 1.8 m in the core S3, 2.4 m in the core S4 and 2.1 m in the core S8. Magnetic parametres recorded in the core S8 are very low. These deposits were only found in the isolated part of the delta and clearly reflect isolated dynamics. The thickness of this unit and its stratigraphic succession may correspond to the one studied in the Thessaloniki plain (Ghilardi et al., 2010).

27- Above these peat deposits, the fifth unit consists of massive brown clay including some silty and sandy events and was found in every core, without any evidence of microfaunal shells. This unit clearly corresponds to the fluvial sedimentation of Kalamas River (Arnaud-Fassetta, 2006). At least two buried palaeochannels were discovered through geophysical prospections. Another palaeochannel was discovered by studying the SPOT and Landsat ETM+ (with combination of spectral bands 2, 3, 6) images. It is located in the southern part of the delta, near the archaeological site of Pirgos Raghiou. The core S6 (fig. 3) highlighted this palaeochannel.

Fig. 6 – Sedimentological facies of the cores S1-S2 (transect) based on faunal and grain-size analyses.
Fig. 6 – Faciès successifs des carottages S1-S2 (transect) à partir des études malacologiques et sédimentologiques.

Fig. 6 – Sedimentological facies of the cores S1-S2 (transect) based on faunal and grain-size analyses. Fig. 6 – Faciès successifs des carottages S1-S2 (transect) à partir des études malacologiques et sédimentologiques.

F: fluvial sediments; L: lagoonal deposits; M: marine to shallow marine deposits; P.D.: pro-delta.
F : dépôts fluviatiles ; L : dépôts lagunaires ; M : dépôts marins ; P.D. : dépôts prodeltaïques.

Fig. 7 – Sedimentological facies of the cores S7, S3, S4, S8 (transect) based on faunal and grain-size analyses.
Fig. 7 : Faciès successifs des carottages S7, S3, S4, S8 (transect) à partir des études malacologiques et sédimentologiques.

Fig. 7 – Sedimentological facies of the cores S7, S3, S4, S8 (transect) based on faunal and grain-size analyses. Fig. 7 : Faciès successifs des carottages S7, S3, S4, S8 (transect) à partir des études malacologiques et sédimentologiques.

F: fluvial deposits; L: lagoonal deposits; M: marine to shallow marine deposits; P.D.: prodeltaic deposits; La: lacustrine deposits.
F : dépôts fluviatiles ; L : dépôts lagunaires ; M : dépôts marins ; P.D. : dépôts prodeltaïques ; La : dépôts lacustres.

Conclusions

28The first results presented and discussed in this article highlight the potential of the area for palaeoenvironmental studies:

29- Geoelectrical measurements, conducted along three transects emphasise the vertical structure of the sediments on nearly 30 m. The general low resistivity results indicate fine to very fine sediments. According to several geophysical studies conducted in the prodelta, the Pleistocene/Holocene transition may not have been reached or may not been as clear as in the regional deltas. By combining both geophysical and sedimentological approach, at least two palaeochannels (radiocarbon measurements in progress) have been highlighted (fig. 3 and fig. 4) near the archaeological site of Mastilitsa. The geoelectric profile (TR4), characterised by very high resistivity values, may have highlighted the extension of the archaeological site. On the contrary, the Pleistocene/Holocene transition is clear along the second profile (TR5, TR6; fig. 3).

30- Analyses of the cores S1-S2 and S7-S3-S4-S8 highlight the complete Holocene environmental suites. The pre-transgressive environments (dated in the core S8; fig. 5) suggest the presence of a fresh-water lake (cores S3-S4-S8) (fig. 7). The extension of this lake is still unknown. Cores S3-S4-S8 are located on the eastern margins of the delta. The presence of the lake probably is the result of a huge influx of fresh water at the Late Glacial/Holocene transition, as observed in the plain of Thessaloniki (Ghilardi et al., 2010). The regression of the lake and the peat deposits coincide with the 'Rapid Climate Change' 8.2 ka event. Then, a classical deltaic progradation is identified, regressing eastward to westward. Based on grain size and microfaunal identifications, four major facies (prodeltaic, marine to shallow marine, lagoonal, fluviatile) were identified.

31- The exact chronostratigraphy of the 14 cores in the delta is still not possible (radiocarbon measurements in progress) but the first results, obtained both by geophysical/sedimentological analyses, confirm the interest for palaeoenvironmental studies in the Kalamas delta. Further studies (ostracods and foraminifers) combined with new 14C measurements will precise the Holocene sea-level rise in the area and will be compared to regional studies.

Thanks to IGME and Irene Zananiri for providing the drilling authorisation (2009-2010). The analyses presented in this article were conducted in the Laboratoire de Géographie Physique (UMR CNRS 8591), directed by C. Kuzucuoglu. Particularly, the teaching qualities of J.-P. Degeai were appreciated. This study was possible by the financial and bibliographical support of the French School of Athens. The authors are grateful to S. Brousse, A. Creuzieux, A. Gonnet, J.-B. Léger, R. Perrier, A.-M. Desiderio for their help in coring, B. Chabrol for the pictures and M. Ghilardi for the scientific advices and his friendship all along this study. We also thank the municipality of Igoumenitsa and the Ephoria of Thesprotia for supporting our researches. Finally, C. Ferreira, P. Kossmann, A. Perrier, M. Perron, B. Taviot and D. Vandarakis for their help in Greece. We also thank the journal referees for their helpful and interested reading, corrections and comments, which led to improvements in the paper.

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Annexe

Version française abrégée

Malgré la fluctuation de refroidissement du Younger Dryas, le réchauffement climatique entre 15000 BP et 6000 BP s’est traduit par une remontée rapide du niveau marin (respectivement de -120 m à -10 m par rapport au niveau marin actuel). La quasi totalité des zones littorales fréquentées entre le Paléolithique supérieur et le Néolithique se trouvent aujourd’hui submergées (parfois sur des distances considérables) voire fossilisées sous les alluvions de deltas progradants ou des cordons littoraux. Ces derniers se mettent progressivement en place à partir du moment où, la vitesse de transgression post-glaciaire ralentissant et l’Homme ouvrant de plus en plus le milieu végétal, l’érosion et le transport sédimentaire nourrissent de plus en plus les cours d’eau et la dérive littorale des sédiments. Depuis plusieurs décennies, les deltas et les plaines côtières de Grèce ont fait l’objet de nombreuses études paléoenvironnementales qui ont permis de mieux comprendre les rythmes de cette remontée du niveau marin post-glaciaire. Situés à l’interface entre mer et continent, ces milieux en constante évolution se prêtent particulièrement bien aux études paléoenvironnementales en lien avec les données archéologiques.

Le delta du Kalamas est le plus petit delta (65 km²) d’Épire (Grèce du NW) et n’a fait l’objet d’aucune étude paléoenvironnementale. Les grandes étapes de son édification sont encore méconnues. Cet article présente les résultats obtenus sur l’évolution holocène du delta à travers deux approches complémentaires : l’une fondée sur des prospections géophysiques, l’autre sur la réalisation de carottages. Les résultats sont encourageants et permettent une première comparaison avec les scénarios établis régionalement pour d’autres deltas.

La première partie de l’étude a consisté à réaliser des profils géophysiques. Trois méthodes de prospection ont été privilégiées : la résistivité électrique, la sismique réfraction et l’utilisation du GPR (Ground Penetrating Radar). Les données de terrain ont été récoltées selon 3 transects (fig. 2 et fig. 3) comprenant en tout 7 profils. L’organisation verticale des sédiments a ainsi pu être abordée sur une profondeur d’une trentaine de mètres. Le transect 1 (fig. 3), réalisé à proximité du site de Mastilitsa (Âge du Bronze), a mis en évidence la présence probable de plusieurs paléochenaux (TR2, TR3) à différentes profondeurs (de -5 m à -10 m) qui se traduisent par une résistivité électrique plus importante. Le profil TR4 a mis en évidence l’existence de blocs grossièrement alignés dont les mesures indiquent qu’il peut s’agir de calcaires. L’hypothèse de la continuité du site archéologique de Mastilitsa peut être envisagée. Le transect 2 a permis de mettre en évidence le cône pléistocène du Kalamas, caractérisé par une forte résistivité électrique (sables grossiers, graviers et galets ; fig. 4). Le troisième transect, réalisé à proximité du site archéologique de la colline de Pirgos Raghiou, n’a mis en évidence qu’une sédimentation très fine sur toute la profondeur du sondage. Les changements dans la sédimentation Pléistocène/Holocène dans la plaine deltaïque ne sont pas clairement visibles sur ces profils de résistivité électrique car ils sont situés au-delà des 30 m d’investigation. L’analyse granulométrique des carottages S10-S11-S12 (fig. 4) montre une correspondance avec les sondages électriques, permettant ainsi de conforter l’hypothèse de la mise en évidence de paléochenaux entre -5 m et -10 m de profondeur. De même, l’étude du sondage S9 confirme que les hautes valeurs de résistivité obtenues dans le profil TR5 à partir d’une profondeur d’1 m sont bien dues à la présence de graviers et de galets à faible profondeur, marquant ainsi la limite entre les dépôts pléistocènes et holocènes.

La seconde partie de l’étude présente les analyses de laboratoire effectuées sur une partie des 14 carottages. L’identification des différents faciès a été possible à partir des analyses granulométriques et malacologiques. Les résultats présentés concernent 2 transects matérialisés par les carottages S1-S2 et S7-S3-S4-S8 (fig. 6 et fig. 7). La présentation de ces carottages a été mise en avant car ce sont les seuls qui ont atteint et/ou dépassé les niveaux marins holocènes, permettant ainsi des scénarios de reconstitution de l’espace à l’échelle de l’Holocène. Cinq faciès ont été identifiés et une première approche chronologique a pu être tentée par le biais de 3 datations 14C obtenues sur le carottage S8. La surface pré-transgressive, identifiée dans les carottages S3-S4 et S8, se caractérise par la présence de dépôts lacustres (antérieurs à 7440 cal. BP ; fig. 5). L’incursion marine est postérieure à 5730 cal. BP. Les dépôts marins ont été observés sur les 6 carottages et sont caractérisés par des dépôts argileux riches en malacofaune. Progressivement, la sédimentation marine laisse la place à un type de dépôt caractéristique de conditions lagunaires, plus sableux. Ces dépôts ne sont pas présents dans le carottage S8 qui est isolé dans la partie la plus occidentale du delta. Enfin, la sédimentation fluviale superficielle est caractérisée par des dépôts argileux, limoneux et sableux bruns qui renvoient aux différentes dynamiques fluviales au cours de l’Holocène récent. Cette succession d’environnements correspond aux scénarios déjà envisagés régionalement en Grèce du Nord. Les datations obtenues permettent d’ouvrir des perspectives d’étude intéressantes sur les changements d’environnements survenus au début de l’Holocène, en lien avec les fluctuations climatiques (événement 8200 BP notamment).

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Table des illustrations

Titre Fig. 1 – Location map of the area of study based on ASTER 30 m resolution DEM. Fig. 1 – Carte de localisation du terrain d’étude réalisée à partir des MNT ASTER (résolution: 30 m).
Légende The delta of the Kalamas River is located just near the Albanian boundary. The deltaic sediments are constrained by the altitudinal range 0-14 m. United Transverse Mercator WGS 84 coordinates.Le delta du Kalamas se trouve à proximité immédiate de la frontière avec l’Albanie. Les dépôts deltaïques sont situés à des altitudes comprises entre 0 et 14 m. Le système de coordonnées est exprimé en United Transverse Mercator WGS 84.
URL http://journals.openedition.org/geomorphologie/docannexe/image/9731/img-1.png
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Titre Fig. 2 – Geomorphological map of the delta including the archaeological settlements, the cores and the geophysical transects.Fig. 2 – Carte géomorphologique du delta comprenant les types de vestiges archéologiques ainsi que l’emplacement des carottages et des transects géophysiques.
Légende 1: 'new' Kalamas direction, since 1958 and the construction of the dam of Ragio; 2: 'old' Kalamas direction, before 1958; 3: channel abandoned after the construction of the dam and the incision of the river; 4: palaeochannels discovered with aerial photos and SPOT and LANDSAT ETM+ images; 5: swamps and marshes. The swamps are Natura 2000 zones; 6: Ionian Sea; 7: Holocene deltaic deposits; 8: Pleistocene alluvial deposits; 9: Late Pleistocene deposits; 10: alluvial fan; 11: calcareous clay; 12: flyschs; 13: limestone; 14: limestone; 15: breccia; 16: Late Palaeolithic site; 17: Neolithic site; 18: Bronze Age site; 19: Classical site; 20: Roman site; 21: Fortress; 22: probable buried harbour; 23: fault; 24: overlap; 25: core; 26: geophysical transect. 1 : nouveau cours du Kalamas depuis 1958 ; 2 : ancien cours du Kalamas avant 1958 ; 3 : méandre abandonné après la construction du barrage et l’incision du fleuve ; 4 : paléo-cours du Kalamas découverts grâce à l’étude des photographies aériennes et des images satellite SPOT et Landsat ETM+ ; 5 : marais et zones humides. Zones classées Natura 2000 ; 6 : mer Ionienne ; 7 : dépôts deltaïques d’âge holocène ; 8 : dépôts alluviaux pléistocènes ; 9 : dépôts de la fin du Pléistocène ; 10 : cône alluvial ; 11 : marnes ; 12 : flyschs ; 13 : calcaires ; 14 : calcaires ; 15 : brèches triasiques ; 16 : site du Paléolithique supérieur ; 17 : site néolithique ; 18 : site de l’Age du Bronze ; 19 : site d’époque Classique ; 20 : site d’époque romaine ; 21 : forteresse ; 22 : probable port enfoui ; 23 : faille ; 24 : chevauchement ; 25 : carottage ; 26 : transect géophysique.
URL http://journals.openedition.org/geomorphologie/docannexe/image/9731/img-2.png
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Titre Fig. 3 – Geoelectric profiles obtained along 3 transects in the delta. Fig. 3 – Représentation des profils géoélectriques obtenus selon 3 transects dans le delta.
Légende See image 3 for the geoelectric legend. Image 1 – 1: location of the first transect; 2: picture and orientation of the transect; 3: diagram block of the transect; 4: topographic profile of the transect. The cores S10, S11, S12 are represented. Image 2 – 1: location of the second transect; 2: diagram block of the transect; 3: topographic profile of the transect. Image 3 – 1: location of the transect; 2: topographic profile of the transect. La légende des profils se trouve sur l’image 3. Image 1 – 1 : localisation du premier transect ; 2 : photographie de l’orientation du transect ; 3 : bloc diagramme ; 4 : profil topographique du transect. Image 2 – 1 : localisation du second transect ; 2 : bloc diagramme ; 3 : profil topographique du transect. Image 3 – 1 : localisation du troisième transect ; 2 : profil topographique du transect.
URL http://journals.openedition.org/geomorphologie/docannexe/image/9731/img-3.png
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Titre Fig. 4 – Sedimentological properties of the cores S10, S11, S12, S9 along the geoelectrical profiles. Fig. 4 – Profils sédimentologiques des carottages S10, S11, S12, S9 localisés le long des profils géoélectriques.
Légende 1: brown clay, silt and fine sand; 2: bedded sand to coarse sand; 3: coarse sand; 4: coarse sand and gravel; 5: gravel; 6: limestone; 7: gravels to pebbles.1 : argiles beiges, limons et sables fins ; 2 : sables fins-moyens lités et sables grossiers ; 3 : sables grossiers ; 4 : sables grossiers et graviers ; 5 : graviers ; 6 : calcaires ; 7 : graviers à galets.
URL http://journals.openedition.org/geomorphologie/docannexe/image/9731/img-4.png
Fichier image/png, 635k
Titre Fig. 5 – Sedimentological properties of the core S8. Fig. 5 – Propriétés sédimentologiques du carottage S8.
Légende 1: brown clay, silt and fine sand; 2: peat; 3: sand; 4: dark organic layers; 5: silty clay.1 : argiles beiges, limons et sables fins ; 2 : niveaux tourbeux ; 3 : sables ; 4 : niveaux organiques sombres ; 5 : argiles limoneuses.
URL http://journals.openedition.org/geomorphologie/docannexe/image/9731/img-5.png
Fichier image/png, 309k
Titre Fig. 6 – Sedimentological facies of the cores S1-S2 (transect) based on faunal and grain-size analyses. Fig. 6 – Faciès successifs des carottages S1-S2 (transect) à partir des études malacologiques et sédimentologiques.
Légende F: fluvial sediments; L: lagoonal deposits; M: marine to shallow marine deposits; P.D.: pro-delta.F : dépôts fluviatiles ; L : dépôts lagunaires ; M : dépôts marins ; P.D. : dépôts prodeltaïques.
URL http://journals.openedition.org/geomorphologie/docannexe/image/9731/img-6.png
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Titre Fig. 7 – Sedimentological facies of the cores S7, S3, S4, S8 (transect) based on faunal and grain-size analyses. Fig. 7 : Faciès successifs des carottages S7, S3, S4, S8 (transect) à partir des études malacologiques et sédimentologiques.
Légende F: fluvial deposits; L: lagoonal deposits; M: marine to shallow marine deposits; P.D.: prodeltaic deposits; La: lacustrine deposits.F : dépôts fluviatiles ; L : dépôts lagunaires ; M : dépôts marins ; P.D. : dépôts prodeltaïques ; La : dépôts lacustres.
URL http://journals.openedition.org/geomorphologie/docannexe/image/9731/img-7.png
Fichier image/png, 246k
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Référence papier

Antoine Chabrol, Georges Apostolopoulos, Kosmas Pavlopoulos, Eric Fouache et Charles Le Cœur, « The Holocene evolution of the Kalamas delta (northwestern Greece) derived from geophysical and sedimentological survey », Géomorphologie : relief, processus, environnement, vol. 18 - n° 1 | 2012, 45-58.

Référence électronique

Antoine Chabrol, Georges Apostolopoulos, Kosmas Pavlopoulos, Eric Fouache et Charles Le Cœur, « The Holocene evolution of the Kalamas delta (northwestern Greece) derived from geophysical and sedimentological survey », Géomorphologie : relief, processus, environnement [En ligne], vol. 18 - n° 1 | 2012, mis en ligne le 01 avril 2014, consulté le 15 décembre 2017. URL : http://journals.openedition.org/geomorphologie/9731 ; DOI : 10.4000/geomorphologie.9731

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Auteurs

Antoine Chabrol

Université Panthéon-Sorbonne (Paris 1) - UMR 8591 du CNRS - 1, place Aristide Briand - 92195 Meudon Cedex - France (antoine.chabrol@cnrs-bellevue.fr).

Georges Apostolopoulos

National Technical University of Athens - Section of Mining Engineering - Zographou Campus - 15780 Athens - Greece.

Kosmas Pavlopoulos

Harokopio University (Athens) - Department of Geography - 70, el. Venizelou Street - 17671 Athens - Greece.

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Eric Fouache

Université Paris-Sorbonne (Paris 4) - IUF - UMR 8185 du CNRS - France.

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Charles Le Cœur

Université Panthéon-Sorbonne (Paris 1) - UMR 8591 du CNRS - 1, place Aristide Briand - 92195 Meudon Cedex - France.

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