Sea-Level Stands and Holocene Geomorphological Evolution of the Northern Deltaic Margin of Amvrakikos Gulf

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Sea-level stands and Holocene geomorphological evolution of the northern deltaic margin of Amvrakikos Gulf...

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Sea-level stands and Holocene geomorphological evolution of the northern deltaic margin of Amvrakikos Gulf (western Greece)

Serafim E. Poulos, Vasilios Kapsimalis, Christos Tziavos , Petros Pavlakis, George Leivaditis, Athens, and Michael Collins, Southampton

with 10 figures and 4 tables

Summary. The present contribution aims to identify the various sea-level stands in association with deltaic sedimentation processes, within Amvrakikos Gulf (western Greece); the latter is a semi- enclosed and relatively shallow (<65 m) embayment, separated from the open Ionian Sea by a rather narrow and shallow (< 10 m) passage. Furthermore, this investigation has been based, primarily, upon the identification of different sedimentary environments within sedimentary cores, radiocarbon dating of characteristic stratigraphic horizons; and, secondarily, upon subaerial and subaqueous geological and geomorphological characteristics as well as archeological evidence. On the basis of the above information, the Holocene evolution of the northern deltaic margin of the Amvrakikos Gulf is attributed to the active interaction between sea level rise, regional tectonism, riverine sediment supply and differential rates of the sedimentation. The Holocene formation and evolution of the northern deltaic margin of the Amvrakikos Gulf is attributed primarily to the deltaic progradation of the River Arachthos, whilst the action of River Louros has been restricted only to its northwestern part. Although a regional curve of sea level rise for the Holocene period could not be constructed with accuracy, due to the above-mentioned complexity of the operating processes, it is most likely that in the beginning of the Holocene period (earlier than 10 000 years BP) sea water from the open Ionian Sea had already entered into the Gulf, occupying water depths 35-40 m below present mean sea level. Following, sea level was rising at a rate of 0.5+0.02 cm per year up to ca 2000 years BP. Within the last 2000 years, sea level has fluctuated within a range of 1-2 m.

Zusammenfassung. Diese Arbeit hat zum Ziel, anhand der Ergebnisse der Deltasedimentation im Amvrakikos Golf (W. Griechenland) die verschiedenen Meeresspiegelniveaus zu identifizieren. Amvra- kikos Golf ist ein halbgeschlossenes Meer mit einerTiefe von weniger als 65 m. DieVerbindung zum offe- nen lonischen Meer kommt durch eine Enge mit einer Tiefe von ~ 10 m zustande. Im Rahmen dieses Studiums der Amvrakikos Gegend wurden Sedimentkerne untersucht, 14C Altersbestimmungen durch- geführt, geologische und geomorphologische Beobachtungen gemacht und archaologische Funde be- rücksichtigt. Die Ergebnisse aus diesen Versuchen führten zu einem synthetischen Bild der Evolution des Gebietes während des Holozäns. Diese Evolution hing mit den Meeresspiegelschwankungen, der lokalen Tektonik und der fluvialen Sedimentlieferung zusammen. Die Gegend wurde überwiegend vom Fluss Arachthos beeinflusst. Eine lokale Meeresspiegelstandskurve fur das Holozän konnte nicht mit Genauig- keit rekonstruiert werden. Auf jeden Fall beweisen die Daten aus dieser Arbeit, dass vor 10000 Jahren marines Milieu über dem Amvrakikos Golf herrschte. Der Meeresspiegelanstieg von diesem Zeitpunkt bis vor 2000 Jahren betrug 0,5+0,02 cm/Jahr. Während der letzten 2000 Jahre schwankte der Meeresspie- gel in einem Spektrum von 1-2 m.

1 Introduction

Relative sea level history varies considerably around the world during the last glacial hemicycle (Newman et al. 1989 and Pirazzoli 1991) due to local, regional and global processes operating on different time and space scales. Hence, the determination of a single sea level curve of global applicability is impossible, according to Pirazzoli (1993). Furthermore, coastal landscapes represent a sensitive interface for environmental changes, being the product of the interaction between terrestrial, marine and atmospheric processes. In active tectonic regions, e.g. Greece, coastal evolution within Holocene and, in particular, that of deltaic coasts and associated shorelines is gov- erned by eustatic sea level oscillations, tectonic displacements and sediment infill. Evidence for sea level changes is provided by a variety of archeological and geological indica- tors. The former refer to archeological structures and to their relative position with respect to shoreline, whilst the latter include a number of forms, such as submerged terrestrial and/or lagoonal vegetation and sedimentary phases, distinctive seismic reflectors in shallow offshore sedimentary stratigraphy and notches, provided by marine solution. Sea level studies based upon archeological evidence have taken place for several locations along the Greek coastline, by Flemming (1978), Jacobsen & Farrand (1987), and van Andel (1987). Likewise, investigations based upon geological evidence have been provided by Kraft et al (1977), Kraft & Rapp (1975), Kraft et al. (1980), Zangger (1991), Marzolff (1981), van Andel & Lianos (1983), Flemming (1978), Thommeret et al. (1981), Pirazzoli et al. (1994), Fouache (1999), and Dini et al. (2000). Moreover, in the case of the Aegean Sea, Lambeck (1996), on the basis of a mathematical model that describes the combined behaviour of eustatic change and crustal rebound, has described sea level change and shoreline migration for the past 20 000 years. Greece, as part of the eastern Mediterranean, is a region of active tectonism and one of the most rapidly deforming continental areas of the Earth (Jackson 1994). Recent coastal zone changes of the Aegean region have been attributed to paroxysmic phases of actual neotectonic evolution, related to differential (in space and time) movements of small tectonic blocks (Mourtzas 1990). Examples of relative sea level changes due to neotectonic activity have been identified in several locations within the southern (primarily) Aegean Sea (e.g. Peloponnese, Crete) by Mourtzas & Marinos (1994), Kelletat et al. (1976), van Andel (1987), Keraudren & Sorel (1987), and Collier et al. (1992). Sediment infill in coastal waters can cause vertical movements, due to changes in the surface loads; this is a process analogous to glacial unloading. Therefore, in Greece sediment accumula- tion in shallow bays and gulfs is comparatively limited to have an important loading effect over time-scales of 10,000 or so years (Lambeck 1996). However, increased sediment fluxes associated with deltaic evolution have been attributed locally to the build-up of new land surface and the migration of shorelines to seawards. A characteristic example for such a deltaic progradation in Greece relates to inner Thermaikos Gulf (NW Aegean Sea). Here, the deltaic plain of the Axios, Aliakmon and Gallikos rivers has prograded more than 20 km within historic times; caused the ancient city of Pella (the capital of ancient Macedonia) to change from a coastal city and harbour, to become a continental city (Stuck 1908). Similar examples of coastal progradation in historical times, related to riverine sediment influx for the Aegean region, have been reported by Zamani & Maroukian (1980), and Bruckner (1997). Sea-level stands and Holocene geomorphological evolution 127

The present contribution aims to identify the various sea-level stands within Holocene, in relation to deltaic morphological evolution of the northern margin of the Amvrakikos Gulf (western Greece). In addition, the curve of relative sea-level change of the Gulf (within the Holocene) is constructed; this has discussed in relation to neotectonic activity of the region and other published sea level curves.

2 The study area

Amvrakikos Gulf is located in the northwestern part of Greece (eastern Mediterranean Sea) and is a shallow (water depths < 65 m) marine embayment, communicating with the open Ionian Sea through a narrow (about 600 m wide) and shallow (< 10 m, water depth) channel (Fig. l). The Gulf is a late Plio-Quaternary extensional basin, produced by backarc extensional faulting in western onshore Greece (Fig. 2). The entire northern terrestrial margin of the Gulf consists of the deltaic deposits of the rivers Arachthos and Louros, which cover an area of some 350 km2 (Fig. 3). The physical geographical characteristics of the drainage basins of the rivers Arachthos and Louros, together with those of the deltaic plain, are presented in Table 1. Between the two rivers the most important, in terms of water and sediment fluxes, is the River Arachthos, as it drains a

Fig. 1. Location map of Amvrakikos Gulf. The starred points show archaeological sites. 128 Serafim E. Poulos et al.

Fig. 2. Geological map of Amvrakikos Gulf and its surrounding area. much larger area that is characterised by increased sediment loads (>7xl06 tonnes/yr); this is due to its erodible lithology and relatively higher rainfall levels (Table l). A detailed geomorphological study of the deltaic plateau has been given by Fouache (1999), on the basis of aerial photographs, detailed topographic maps and archeological evidence. According to this study, a rapid progradation of the deltaic coast occurred within historical times (especially in the case of the River Arachthos); this is characterised, further, by the eastward displacement of the mouth of the River Arachthos and the change of the route of River Louros, from the east to the west side of Vigla Hill (Fig. 3). Moreover, interpretation of 3.5 kHz shallow seismic profiles from the bed of the Gulf, by Poulos et al. (1995), has revealed that during the last (Wurm) glacial period those parts of the Gulf that lie today at water depths > 41 m (below present sea level) were a palaeo- lake, with the remainder of the bed of the Gulf being exposed to sub-aerial erosion. A detailed study for sea level changes in Amvrakikos Gulf, during the Holocene, has not been published. However, information regarding sea level stands for the open Ionian Sea originate from Lambeck (1996), who has stated that sea level was 44 m, 2—3 m and 1 m below to its present stage at 10000, 6000 and 2000 years BP, respectively. Other information regarding the relative sea level stands comes from Lefkas Island (Fig. 1), where archeological ruins of some 2500 years in age Sea-level stands and Holocene geomorphological evolution 129

Table 1. Physical—geographical characteristics of the Arachthos and Louros river systems.

R. Arachthos R. Louros Geomorphology Catchment area (km2 ) 1894 785 Maximum elevation (m) 2000 2000

Deltaic plain area (km2) 350

Type of river mouth birdfoot cuspate Maximum water depths in front of river mouth (m) 35 65 Lithology of catchment area Clastic (flysch, alluvial) sediment (%) 78.2 32.5 Carbonate rocks (%) 19.3 66.0

Metamorphic rocks (%) 0.7 1.5

Igneous rocks (%) 1.8 0 Climate Climatic variation terrestrial Mediterranean to humid Continental Mean annual temperature (°C) 17.7 17.9 Mean annual precipitation (mm) 1085 925 Water(a) / Sediment fluxes Mean annual water discharge (m3/s) 70 19

Maximum monthly discharge (m3/s) 167 (Dec) 31 (Feb) Minimum monthly discharge (m3/s) 4 (Aug) 10 (Aug)

Mean annual suspended sediment flux (106 t) 7.31(b) 0.78 (c)

Maximum monthly suspended sediment flux (106 t) 19.4 (Nov) 3.03(c) Minimum monthly suspended sediment flux (106 t) 0.1 (Aug) 0.13(c)

(a): values abstracted from Therianos (1974) — (b): measured values, abstracted from Poulos & Chronis (1997) — (c): estimated values, from measured water discharge (see Poulos & Chronis, 1997).

were found at 3.5 m below present sea level (Negris 1904). Similarly, the presence of the castle of Fidocastro (dated 219 BC), nearby the northern coast of the Gulf (Fouache 1999), reveals that sea level was about to its present stage at least some hundred years before 2200 years BP. The difference between the two sites can be most probably attributed to differential tectonic movements. 130 Serafim E. Poulos et al.

Fig. 3. Positions of coring and stratigraphical transects, including locations/ features referred to in text.

3 Data collection and methodology

The present investigation is based upon the lithostratigraphic analyses of 12 short cores (less than 6 m in length) obtained with a vibrocorer, for the abstraction of longer sediment cores (up to 30 m, in length), 4 drillings were undertaken (their locations are shown in Fig. 3). Grain size analysis was carried out according to Folk (1974), at selected samples along the cores, to determine the percentages of sand, silt and clay. Furthermore, radiocarbon analysis (14C) was performed on selected samples of shell fragments and/or turf layers, in the laboratories of the ‘Beta Analytic’ in USA; these results are presented synoptically in Table 2. Additional information regarding the progradation of the deltaic plain, within historical times, was provided by the local archaeological sites and monuments (Table 3; for locations, see Fig. l), whilst recent changes (since 1945) are revealed in aerial photographs and other published information. Palaeo-environmental investigations and, in particular, the identification of the different aquatic environments (e.g. open sea, coastal, lagoonal and lacustrine) were based upon the identification of the various species of foraminiferida and ostracoda with the use of a electron micro- scope of the University of Delaware (Department of Geology). In Table 4, the various species of foraminiferida and ostracoda are listed that have been found to live in different aquatic environments of Amvrakikos Gulf are listed. Sea-level stands and Holocene geomorphological evolution 131

Table 2. Results from the radiocarbon analysis.

Core Position relating to the present Material Age (years BP) sea level (m) S-01 -4.55 - -4.70 Turf 2130±90 S-07 -2.55 - -2.65 Shells 3730±110

S-09 -4.40 - -4.55 Turf 1140±70

S-25 -4.35 - -4.40 Turf 3530±120 S-26 +0.10 - -0.10 Shells 1110±80

L-l -17.90 - -18.10 Shells 6910±100

L-2 -10.10 - -10.30 Shells 6090±120 L-3 -3.60 - -3.85 Shells 2670±90

L-3 -16.00 - -16.50 Shells 4470±70

L-3 -23.15 - -23.20 Shells 5770±120 L-4 -26.20 - -26.30 Shells 9700±150

Table 3. Prehistoric and early historical archaeological sites in the surrounding area of the Amvrakikos Gulf (for location, see Fig. l).

Location Time (years BP)

40000- 8000- 4600- 4000/3800- 3200- 3125- 2900- 2700- 8000 4600 4000/3800 3200 3125 2900 2700 2000 Arta + + + Despotiko + Feidokastro + Kamarina + Louros + + Nea Kerasous + + Nikopolis + Oropos + + Pantanassa + Pantokrator + Salaora + Stefani + Thesprotiko +

132 Serafim E. Poulos et al.

Table 4. Species of foraminiferida and ostracoda of different aquatic environments identified in core-samples from Avrakikos deltaic plain.

Fresh waters (river and/or lacustrine) Foraminiferida Ostracoda Candona albicans, Cadona neglecta, candona sp, Cypris spp, Ilyocypris gibba, Lineo- cypris sp, freshwater ostracod, lineocypris sp, aurila speyeri, aurila convexa Lagoon Foraminifera Ammonia becarii, Trochamina sp, Elphidium complanatum, Elphidium sp 1 Ostracoda Cyprideis torosa, Loxoconcha elliptica, Aurila woodwardii, Loxoconcha sp. Coastal marine Foraminifera Basslerites berchoni, Cibicides lobatulus, Elphidium sp. 2, Elphidium crispum, Eggerer- ella scabra, Millionela sidebottoni, Quinqueloculina oblonga, Quinqueloculina poeyana, Quinqueloculina milletti, Quinqueloculina seminulum, Quinqueloculina sp, Propontocypris pirifera, Rosallina globularis, Trilocullina sp, Valvulinerina compla- nata Ostracoda Callistocythere pallida, Semicytherura sp, Leptocythere rara, Xestoleberis commuis, Xestoleberis dispar

4 Results and Discussion

4.1 Lithostratigrapbic analyses of the cores The results of the lithostratigraphic analyses of both the long (drilling) and short (vibro) cores are presented schematically in Figs. 4 and 5, respectively. For the investigation of deltaic plain evolution within Holocene and with the use of these cores, three stratigraphic sections have been constructed (A, B and C) that trend almost parallel to each other, in a W-E direction (for locations, see Fig. 3). The upper sedimentary sequences of Section A (Fig. 6) consists of the fluvial and deltaic deposits of the rivers Arachthos (central and eastern part) and Louros (western part). The thickness of these alluvial deposits at the eastern part of the Gulf exceeds 40 m, i.e. between +20 m and —20 m, with respect to present mean sea level (pmsl). In contrast, the thickness of the western alluvial deposits does not exceed 10 m, lying between +5 m and —5 m, in relation to pmsl. This difference is attributed to the much higher sediment fluxes of the River Arachthos in relation to those of the River Louros (see Table l). In general, fluvial and/or deltaic deposits of the River Louros rest on top of about 25 m of lagoonal deposits. Among these lagoonal deposits, in the region of Agios Spyridon (core L-III), there is a layer (approx. 1.5 m in thickness) of coastal (marine) deposits. Radiocarbon analysis of shell fragments of this layer (3.7 m below pmsl) has shown that it has been deposited before 2670 (±90) years BP; the presence of such a layer indicates a more rapid transgression, which may be related to the climatic optimum (ca 2500 BP). Furthermore, two other radiocarbon analyses at depths of 16.25 ±0.25 m and 23.20 m below pmsl, indicate the pre- Sea-level stands and Holocene geomorphological evolution 133

Fig. 4. Lithology and sedimentary facies of the long cores: L-1 to L-IV (for locations see Fig. 3). 134 Serafim E. Poulos et al.

Fig. 5. Lithology and sedimentary facies of the short cores: S-01 to S-28 (for locations see Fig. 3). Sea-level stands and Holocene geomorphological evolution 135

Fig. 6. Stratigraphical transect A (for location, see Fig. 3). sence of sea level at those depths at 4470 (±70) and 5770 (±120) years BP, respectively. The persis- tence of the lagoonal character of these deposits indicates a gradual increase in sea level (at a rate of 0.5-0.7 cm/yr) and a relatively low fluvial sediment influx. Therefore, during the middle Holo- cene, the mouth of River Louros could be located at the area of Agios Spyridon (Core L-III), whilst the area to the west of the Vigla hill might have been an extended lagoon. Therefore, the deltaic deposits at the bottom of the Core L-III should have been formed before the aforementioned sea transgression, when the mouth of River Louros was located more to the south. 136 Serafim E. Poulos et al.

Section B (Fig. 7) is divided by the Vigla Hill into two sub-sections, with different physio- graphic characteristics. The eastern subsection is characterised by the deltaic deposits of the River Arachthos; these now reach smaller altitudes (up to +12 m), when compared to those of the previous section (A). Radiocarbon dating of a turf layer 4.4 m below pmsl (S-25) has shown that, at 3530 (±120) years BP, the area closely to Vigla Hill was not a terrestrial part of the deltaic plain; this situation continued up to the present day, as they covered by lagoonal and flood plain deposits. Along Core L-II, the deltaic prism reaches a depth of 7 m below pmsl, this is followed by 4 m of lagoonal/coastal deposits. Below these deposits, a swampy layer of 3 m rests on top of older deltaic plain deposits. Dating of shell fragments from 10.2 m below pmsl has shown that the lower lagoonal/coastal sequence was deposited around 6090 (±120) years BP; this indicates that

Fig. 7. Stratigraphical transect B (for location, see Fig. 3). Sea-level stands and Holocene geomorphological evolution 137 marine transgression has reached this region before 6100 years BP, whilst the absence of marine deposits indicates the dominance of the fluvial/deltaic processes. Finally, the deepest deposits of terrestrial material (possibly deltaic) may be attributed to a previous (at least before 6100 years BP) low stand of sea level, during which the mouth of the River Arachthos would be located more to the south. The surface topography of the western sub-section is rather flat with altitudes <1 m (Fig. 7); this, in association with the absence of deltaic deposits between the position of Vigla Hill (Core L-IV) and the modern route of River Louros, not only at the surface but also down through the stratigraphic section, reveals the dominance of marine and lacustrine conditions, as a conse- quence of River Louros’ rather small sediment influx. The two lobes of fluvial deposits, the upper one (from +0.5 m to -3.2 m relative to pmsl) consisting of flood-plain deposits, and the lower one (from -4.5 m to -9 m) consisting of deltaic deposits and having in between them a marshy (semi- salty) layer of lacustrine sediment, indicate different stages of the mouth of River Louros. Thus, the lower deltaic lobe has been formed when the river mouth was close by; the surface lobe following a diversion of the river route and relocation of its mouth to another area (possibly closer to its present position). The reminder of the stratigraphic column consists of semi-salty marshy deposits (with some riverine influence) up to a depth of 32 m (relatively to pmls), with the exception of two layers that are characterised by marine/coastal deposits; these are found between 20-22 m and 24-30 m. Radiocarbon dating of shell fragments from the depth of 26 m has shown that they have been deposited at 9700 (±150) years BP. This date provides the first indication, if not abso- lute proof, that the sea entered Amvrakikos Gulf ca 10000 years BP. Furthermore, the deltaic deposits found at 32.5 m, can be attributed, most probably, to a chronostratigraphic period prior to the last sea transgression, when the River Louros had his mouth at > 32 m below pmsl. The relief along Section C (Fig. 8) is rather smooth, with the deltaic deposits of River Arachthos dominating its central and eastern part; where, elevations do not exceed the 5 m. At the western end of this section, the current valley of River Louros is present, having on its eastern side the deposits of Tsoukalio Lagoon. Along Core (ll), the long core closest to the present shoreline, the modern fluvial deposits reach a depth of about 8.5 m, lying on top of a stratigraphic sequence (from 8.5 m down to 21.2 m) that consists mostly of sediments deposited in a lagoonal (salty) and/or swampy conditions. Radiocarbon dating on shell fragments from a depth of 18 m (relative to the pmsl) has shown a date of 6910 (±100) years BP. This fact in association with the presence of two distinctive layers of marine (coastal) deposits between the depths of 12.2—13.7 m and 15-16 m, indicate that sea transition should have commenced ca 7000 years BP. Furthermore, un- der this sequence (below the 21.2 m) older deposits of terrestrial (possibly deltaic) material exist, indicating an earlier depositional environment associated with a much lower stage of water level in Amvrakikos Gulf. The fluvial deposits observed in the region of Core S-07 should have been deposited some 3000 years later, as shown by the dating (3730 (±110) years BP) on shell fragments of lagoonal deposits abstracted from a depth of 2.6 m (below pmsl). The presence of marine (coastal) deposits below the lagoonal ones in Core S-07, together with the chronological inter- comparison of those two cores, indicates that sea transgression in this area (between Core L-I and S-07) was at rate of 0.5 cm/yr. The latter figure might be smaller if the sediment compaction effect, especially along the longer core L-I, is taken into account. However, a value of approximately 0.5 cm/yr is similar to that observed in the long Core L-III (within the Agios Spyridon area). 138 Serafim E. Poulos et al.

Fig. 8. Stratigraphical Transect C (for location, see Fig. 3).

The western end of the section is occupied by the fluvial deposits of the River Louros (Fig. 8), whilst the absence of Arachthos' deltaic deposits around Tsoukalio, over the last 2-3 thousands years, indicates an eastward displacement of its mouth. The thickness of fluvial/deltaic deposits does not extend deeper than 5—6 m, as the deepest part of the Core S-09 reveals a prodeltaic sedimentation (clayey material) in a lagoonal and/or shallow marine environment. Moreover, radiocarbon analyses of a turf layer (a marshy deposit) found in Core 9 at a depth of 4.5 m (below pmsl), has been dated 1140 (±70) years BP.This layer is covered by deltaic deposits, indicating that ca 1200 years BP the River Louros was present in this area, followed possibly another route farther to the west and having its mouth some 5 km to the north of its present location. It is inter- esting to note also that swampy deposits, associated with lacustrine (mostly) conditions, are observed also at the same depth at all the cores (S-08, L-IV, S-27) located to the west of Vigla Hill, with the exception of core S-26, where swampy deposits are observed at the level of the present sea stage; this is despite the fact that all have been deposited ca 1100 years BP and are covered by 3.6 m of riverine deposits. This observation provides evidence of a general submergence of the whole of the basin, located to the west of Vigla Hill; this is in response to local neotectonic activity and the natural compaction of the recently accumulated fluviatile deposits. Possibly, the Sea-level stands and Holocene geomorphological evolution 139 change in the route of River Louros can be related to such tectonic activity, as a faulting zone trending W-E is the northern boundary of the Amvrakikos deltaic plain (see Fig. 2). Core S-01, taken from the barrier island that encloses the Tsoukalio Lagoon, includes also a turf layer at 4.6 m (relatively to present msl) which has been dated 2130 (±90) years BP; this is covered by marine (from -4.5 m to -3.2 m) and lagoonal (from —3.2 m to +0.2 m) deposits. This information supports our interpretation that: (a) the deltaic deposits of River Arachthos reached this area around 2000 years BP (fluvial deposits form the base of Core S-07); (b) the River Arachthos shifted its route eastwards after this period (2000 years BP), depriving this region from his sediment fluxes; and, (c) this area has undergone subsidence, as part of the larger basin located to the west of Vigla Hill (as mentioned previously).

4.2 Holocene evolution of the northern (deltaic) margin of Amvrakikos Gulf At the beginning of the Flandrian transgression, the basin of Amvrakikos Gulf was isolated from the open Ionian Sea. When the sea level exceeded -50 m, relative to the present msl, the sea water started to enter the Gulf through the passage between Aktio and Preveza (Fig.l); this is in accordance to the founding of lagoonal deposits between —50 m and —20 m, overlain by coastal marine deposits (from -25 m to -18 m) across the Preveza Strait (Tziavos 1996). The same author has pointed out that, possibly, the water intrusion has taken place in smaller depths (<50 m), as the initial thickness of these lagoonal deposits would have been larger (by some meters) from what it is observed today, due to their natural compaction; the latter might have enhanced by the accumulated above coastal/marine sediment. Therefore, when the sea level was ca 45 m (±a few meters) the salty Ionian waters entered the gulf; this, in terms of time period, is attributed to 10—11 thousands years BP (Lambeck 1996). Therefore, at the beginning of the Holocene period within the Gulf, only the parts below the 40 m relatively to pmsl were covered by brackish and, later, by sea water. The mouths of the rivers Louros and Arachthos were located many kilometers to the south of their present positions. In Fig. 9a, the palaeogeographic reconstruction of the northern margin of the Gulf is provided schematically, for 10 000 years BP; this is based upon the stratigraphic description of the cores and published information concerning the sea bed sediments (Poulos et al. 1996). At this period of time, marine transgression within the gulf moved the deltaic shoreline northwards, whilst active river mouths are present at locations A, B and C; the first two (A and B) may attributed to the River Arachthos tributaries, whilst the third (C) to the River Louros. Further, within this area (core L-IV) the oldest (9700 years BP) marine deposits has been identified at a depth of 26.5 m relative to pmsl; however, the lower part of these marine deposits is at 29.5 m. These depths may be reduced, if the processes of natural compaction and tectonic subsidence (as discussed previously) are taken into account. The coastline retreat continued; ca 7000 years BP it was about 300 m to the north of Logarou Lagoon (Core L-l). Before 6000 years, Amvrakikos Gulf presented its maximum expansion (see Fig. 9b). During this period: (a) the coastline is estimated to be about 6.5 km, to the north of Mytika (including the location of Core L-III) and some 13.5 km north to Koronisia (close by the location of Core L-II); (b) the mouth of River Louros has moved to the NE (location A), whilst locations B and C indicate the seaward limit of the deltaic prism and, possibly, two mouths of River Arachthos; and (c) 140 Serafim E. Poulos et al.

Fig. 9. Paleogeographical reconstruction of the complex delta plain of the rivers Arachthos and Louros. (a) 10000 years BP; (b): 6000 years BP; (c): 2500 BP; and (d) 1788 AD. the present hills of Salaora, Vigla and Koronisia were islands. Following this period, it seems that sea level rise was slower, whilst the coastline started to prograde southwards in the areas of active river mouths. Thus, around 3700 years BP the mouth of River Arachthos was south to the Vigla Hill and, during the period of 1600-1100 years BP the Rhodia lagoon was formed. During this period, the area to the east of Vigla Hill (Cores S-25 and S-10) was a lagoon and/or a shallow coastal environment, whilst the mouth of River Louros was located close to the Nea Kerasous. After the end of the Bronze Age (3600-3100 years BP) and within classical and Hellenistic times (2500-2000 years BP), this River Arachthos had managed to reach the area to the SW of the Sal- aora Hill, contributing, therefore, to the formation of the Tsoukalio Lagoon. During this period, the River Louros moved its mouth to the north of Vigla Hill, in the region of Strongyli (core L-IV) (location C, Fig. 9c). After this period (around 2000 years BP), the River Arachthos started to shift its mouth from the Salaora area (location B) towards Paleompouka (location A). Evidence for the eastward shifting of the mouth of River Arachthos and its temporarily transfer to the Sea-level stands and Holocene geomorphological evolution 141

Koronisia area is provided also by the Castle of Fidokastro (the commercial port for the ancient city of Amvrakia during classical times); this is located today at a distance of about 650 m from the coast. Besides, all the archaeological monuments, aged up to 3000 years BP, have been found at the periphery of the active deltaic plain (see Fig. l). A new diversion of River Louros took place within Roman times (800-900 years AD); during these, its main channel moved from the south area of Core S-26 to the north of it, reaching its present position. Furthermore, a sketch map published by Barbie du Bocage (1788) shows that the route of River Louros has not changed up until today and its mouth has not prograded; this is in comparison with the River Arachthos that temporarily discharged at 2 or 3 different positions (Fig. 9d). Further, the “barrier islands” separating theTsoukalio and Logarou Lagoons from the open Gulf, seems to be present and almost unchanged since the end of the 18th century. In addition, the commercial road (constructed in ancient times) connecting Koronisia-Fidokastro and Arta was in use up to the end of the 17th century A.D. (Fig. 9d). Today, active progradation of River Arachthos has been replaced by retreat, following the construction of hydroelectric dams that retain most of the fluvial sediment load (Poulos et al. 1997).

4.3 Relative sea level fluctuations

Around 10000 years BP, the sea entered the basin of Amvrakikos Gulf from the strait between Aktio and Preveza, which was about 45 m below present mean sea level (Tziavos 1996). The first indication for the sea stand within the Gulf comes from the Core IV, where that lagoonal/ coastal deposits are at 26 m before pmsl, dated 9700 (±150) years BP. Moreover, this sequence extends down to 32 m, where there is the boundary between those and earlier deltaic deposits. Therefore, the sea was present some hundred years earlier (ca 10000) at a depth of about 32 m. A similar interface was found at a depth of approximately 18 m below pmsl, in Core L-I and dated at 6910 (±100) years BP, in the case of Core II, this was found at a depth of approx. 10 m below pmsl and was dated at 6090 (±120) years BP. From radiocarbon dating on shell fragments from the lagoonal environment within Core L-III, sea level was revealed to be higher, by a few metres; it was at 23 m, in 5770 (±120) years BP and at 16 m in 4470 (±70) years BP. In Core S-07, there is evidence of the presence of sea level at 2.5 m during and prior to 3530 (±120) years BP; in the case of Core S-25, at a similar date (3530±120 years BP), lagoonal deposits were found at 4.5 m. Similarly, in Core L-III and at a depth of 3.7 m, shell fragments from lagoonal deposits were dated at 2670 (±90) years BP. The last pieces of information may not be accurate in terms of sea level stand, as they originate from lagoons isolated to the open sea by barrier islands related to, firstly, the River Arachthos and, secondly, to the River Louros deltaic progradation. Thus, during this period, sea level could be a few metres higher, whilst differential compaction may not be excluded. A turf layer (of marshy environment) at 0 m relative to present sea level stand, in Core S-26 and dated 1110 (±80) years BP, indicates that sea level was ±0.1 m relative to its present level; in Core S-09, a similar turf layer from 4.5 m depth was dated at 1140 (±70) years BP; the latter has been found within lacustrine deposits, indicating subsidence. Finally, the presence of Fido- castro Castle at 2200 years BP, close to the shore, indicates the proximity of the sea; its level would be 1-2 m lower during the classical period. 142 Serafim E. Poulos et al.

Fig. 10. Evolution of sea level within the Holocene, deduced on the basis of: (a) the findings of the present investigation (solid line); and, (b) the Lambeck’s (1996) model (dotted line).

On the basis of the above synthesis the evolution of sea level within Holocene is shown schematically in Fig. 10 (solid line). This line does not represent the actual fluctuation of sea level, as the processes of natural compaction of the sedimentary column of the various cores and the neotectonic activity have not been taken into consideration; the latter is expected to be important, due to the geotectonical structure of the Gulf. The 12 m difference in sea level, between 6910 (±100) years BP (Core L-l) and 6090 (±120) years BP (Core L-III) may attributed either to a dramatic reduction in sea level between 7000 and 6000 years BP and/or to neotectonic activity. Such events as a sudden lowering of sea level within the Holocene have never been identified in the case of the eastern Mediterranean Sea, together with the fact that the rate of sea level rise seems to be unchanged prior to 6900 years BP and after 6100 years BP, leads to the conclusion that the whole Core L-III has been moved tectonically downwards, in relation to Core L-II, by about 12 m. This is in accordance with sub-bottom seismic data that reveals the presence of active faults (upper Pleistocene to Holocene), with displacements in the order of 12-15 m. Further- Sea-level stands and Holocene geomorphological evolution 143 more, it seems likely also that the eastward shift of the active mouth of the River Arachthos, after 2500 years BP, can be associated with neotectonic activity and/or increased subsidence of the eastern part of the deltaic plateau. Further, it is known from archeological data that the eastern Mediterranean basin has been affected by intensive tectonism within the upper Holocene (for a period of 2000-3000 years), having its paroxysmic phase ca 1530 (±40) years BP (Pirazzoli 1986). Furthermore, seismic activity from historical times to present either in the nearby Ionian region and/or in the Gulf region (the last earthquake shocks of a magnitude >6 Richter took place in 1966 and 1967 (Papazachos & Papazachou 2003) is supportive evidence of the aforementioned neotectonic activity. Therefore, in order to investigate the neotectonic activity of the Gulf, Lambeck's (1996) curve has plotted, against the sea level curve deduced from the findings of the present investigation (Fig. 10). From the comparison between these two lines, it may be deduced that almost the entire northern margin of the Gulf has subsided by 3—4 m. Moreover, Core L-II, which is located to the east of the extension of the deformed zone, presents evidence of additional lowering (in total, 8-10 m). The maximum subsidence, revealed by Core L-III (see above), may be explained by the fact that its location is surrounded by active faults (see Fig. 2). Finally, if we assume that Core L-IV has subsided also by 2-4 m, then at 9700 (±150) years BP the sea level was at 28—29 m below present mean sea level. This means that sea level was at a level of 28-29 m about 500±250 years, after the first entrance of sea water in the Gulf (between 10 000 and 10 500 years BP) at a water depth of around 45 ±5 m.

5 Conclusions

Holocene evolution of the northern deltaic margin of the Amvrakikos Gulf is the result of active interaction between sea level rise, regional tectonism, riverine sediment supply and to differential rate of compaction of the sedimentary column. Each of these processes fluctuates over time (within the Holocene) and in space (with the eastern part associated with a higher rate of subsidence). Although a regional sea level curve for the Holocene period could not be produced with accuracy, due to various limitations (as discussed previously), it is most likely that at the beginning of the Holocene period (earlier than 10 000 years BP), sea water from the open Ionian Sea had already entered into the Gulf, occupying present water depths 30-35 m below present mean sea level. Subsequently, sea level was continuing to rise up to ca 2000 years BP, at a rate of 0.5 ±0.02 cm/yr, reaching almost its present level. Over the last 2000 years, sea level fluctuations would be in the order of 1—2 m. In terms of sediment infill, the formation of the northern deltaic margin of the Amvrakikos Gulf can be attributed primarily to the deltaic progradation of the River Arachthos; the action of the River Louros has been restricted only to the northwest part of the Holocene plain. Thus, the River Arachthos the middle Bronze age (ca 3700 years BP) has reached the region west to Salaora Hill and has remained there until to the classical times; then it started to move its mouth eastwards to its current position, where it has been presently active over the last 2 centu- ries. 144 Serafim E. Poulos et al.

Acknowledgements

The authors thank Dr. C. Anagnostou and Dr. S. Stavrakakis for their significant assistance in the field work. We appreciate also the help of Mr. D. Filippas and Mr. A. Morfis (Hellenic Center for Marine Research) for grain-size analyses. Finally, Dr. S.E. Poulos thanks the Special Account for Research Grants of the National & Kapodistrian University of Athens for the economical support during the preparation of this manuscript (Grant No. 70/4/4523).

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Addresses of the authors: S.E. Poulos and G. Leivaditis, University of Athens, Faculty of Geology & Geo Environment, Department of Geography-Climatology, Panepistimioupoli, Zografou 157 84, Athens, Greece. V. Kapsimalis, Ch. Tziavos and P. Pavlakis, Hellenic Centre for Marine Research, Institute of Oceanogra- phy, P.O. Box 712,190 13 Anavyssos, Attica, Greece. M. Collins, School of Ocean & Earth Science, University of Southampton, SOC, European Way, South- ampton SO14 3ZH, United Kingdom.

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