Late Holocene Tsunami Imprint at the Entrance of the Ambrakian Gulf (NW Greece) Impacts Des Tsunamis Le Long Des Côtes Du Golfe Ambracien (Grèce Nord-Occidentale)
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N° 108 - 2007 43 Late Holocene tsunami imprint at the entrance of the Ambrakian gulf (NW Greece) Impacts des tsunamis le long des côtes du golfe Ambracien (Grèce nord-occidentale) Andreas VÖTT Helmut BRÜCKNER Matthias MAY Faculty of Geography Faculty of Geography Faculty of Geography Philipps-Universität Marburg Philipps-Universität Marburg Philipps-Universität Marburg Deutschhausstr. 10 Deutschhausstr. 10 Deutschhausstr. 10 D-35032 Marburg/Lahn, Germany D-35032 Marburg/Lahn, Germany D-35032 Marburg/Lahn, Germany Franziska LANG Svenja BROCKMÜLLER Department of Classical Archaeology Faculty of Geography Technische Universität Darmstadt Philipps-Universität Marburg El-Lissitzky-Str. 1 Deutschhausstr. 10 D-64287 Darmstadt, Germany D-35032 Marburg/Lahn, Germany Abstract - This paper presents geomorphological, sedimentological of the tsunami surge is estimated at 6 m. Further tsunami landfalls occurred and geoarchaeological evidence of multiple tsunami impacts on the Aktio during the last 700 or so years. The results document an extraordinarily high headland (NW Greece). Numerous vibracores revealed high energy event tsunami risk for the study area. deposits most of which were accumulated on terrestrial sites on top of erosional unconformities. The tsunami sediments, up to 3 m thick, mostly consist of Résumé - Cet article présente les impacts géomorphologiques, middle to coarse sand and include gravel and marine shell fragments. Both sédimentologiques et géoarchéologiques de plusieurs tsunamis qui ont affecté badly sorted and well sorted, laminated event deposits were observed. In le cap d’Aktio (golfe Ambracien, Grèce nord-occidentale). Nos carottages ont several places, an intersecting palaeosol unit documents repeated tsunamigenic mis en évidence des dépôts marins transgressifs, d’une épaisseur maximale influence after a phase of weathering and soil formation. Underwater studies de trois mètres, caractérisés par des sables grossiers, des graviers et des offshore Aktio headland showed that the Plaka palaeo beach ridges, including coquilles marines remaniées. Par endroit, un paléosol témoigne de deux its beachrock base, out of beachrock was destroyed by tsunamigenic wave évènements de haute énergie différents. Des prospections sous-marines ont activity. Dislocated beachrock fragments with a maximum diameter of 60 cm aussi mis en évidence le démantèlement de dalles de beachrock. Ces blocs, were encountered on Aktio headland up to 2.5 km to the west of the in-situ d’un diamètre maximal de 60 cm, ont été retrouvés jusqu’à 2,5 km de la ligne beachrock unit in elevations up to 3.15 m a.s.l. de rivage actuelle et jusqu’à plus de 3 mètres au-dessus du niveau marin A series of 14C-AMS dates and diagnostic ceramic fragments found in actuel. Par datation radiocarbone, nous mettons en évidence un tsunami vibracores allow to reconstruct several tsunami landfalls. Around 2870- important vers 2870-2350 cal BC et plusieurs autres évènements entre 1000 2350 cal BC, a major event hit the entire headland. Parts of the study area et 300 cal BC. Un tsunami exceptionnel a frappé le cap d’Aktio vers 840 cal. were probably affected by further tsunamis around 1000 cal BC and 300 cal AD. La hauteur atteinte par le tsunami est estimée à 6 m au dessus du niveau BC which are known to have hit the adjacent Lefkada coastal zone. A mega marin actuel. Ces résultats mettent en évidence l’importance des risques liés tsunami struck the Aktio headland around 840 cal AD. The minimum height aux tsunamis dans ce secteur littoral. 1 - Introduction coast near Lefkada were encountered during the course of interdisciplinary investigations on the Holocene evolution of The Ionian Sea and its coasts belong to the seismically most coastal Akarnania in northwestern Greece (VÖTT et al., 2006a, active regions of the Mediterranean and has been affected by 2006b, 2006c). Different types of interrelated tsunami deposits many earthquakes and associated tsunamis. The high tsunami were identified. They are part of a system of tsunami sediment risk has decisively been revealed by the increasing number of traps which seem to be unique for the eastern Mediterranean tsunami studies which have been carried out during the last (VÖTT et al., 2006d, 2007). two decades. By these studies, a variety of event deposits have This paper gives evidence for tsunami landfall at Aktio been identified and used to reconstruct tsunami landfall history headland at the entrance to the Ambrakian Gulf (NW during the Holocene (for instance DOMINEY-HOWES, 2002; Greece). The main objectives of our study are (i) to present DOMINEY-HOWES et al., 2000; KELLETAT AND SCHELLMANN, 2002; geomorphological, sedimentological and geoarchaeological MASTRONUZZI and SANSÒ, 2004; MINOURA et al., 2000; MORHANGE records of tsunamigenic activity, (ii) to establish a tsunami et al., 2006; REINHARDT et al., 2006; SCHEffERS, 2006a; STEFATOS geochronology, and (iii) to reconstruct the flow direction and et al., 2006). the flow intensity of palaeo tsunami waves in order to assess Geological records for multiple tsunami impacts on the the present tsunami risk of the area. 44 Fig. 1 - Topographic overview of the coastal zone between Preveza and Lefkada and detailed map of the Aktio headland showing vibracoring sites and location of underwater studies. Fig. 1 - Schéma topographique du secteur d’étude entre Preveza et Leucade. Croquis détaillé du cap Aktio. 2 - Topography and geotectonic setting coastal area with maximum elevations of 6 m above present sea level (m a.s.l.) and has a slightly undulating surface. The Aktio headland, almost 7 km long and 4 km wide, is located Phoukias sand spit which extends into the Bay of Aghios in the coastal zone between Preveza and Lefkada (Fig. 1). It Nikolaos and the Plaka are the most striking geomorphological is bordered by the Ionian Sea to the west, by the Ambrakian features of the present-day coast. The Plaka represents the Gulf to the east, and by the Strait of Aktio/Preveza, only 600 m partly submerged ruins of a former strandline and is entirely wide and 8 m deep, to the north. The headland is a low lying made up of beachrock (VON SEIDLITZ, 1927), up to 6-10 m thick. 4 The formation of the Plaka beachrock started in the 5th in the study area was 18 m below ground surface (m b.s.). millennium BC at the latest closing off the lagoons of Lefkada and Vibracores were analyzed in the field using geomorphological Aghios Nikolaos from the open Ionian Sea (VÖTT et al., 2006d). and sedimentological methods. Colour, grain size, texture, The shallow waters of the Lagoon of Saltini cover the central macrofossil fragments and carbonate content were determined parts of the Aktio lowlands. In the north, the spit of Aktio bears for each sediment layer. Macrofaunal remains were used to the sparse archaeological remains of the ancient sanctuary elucidate the sedimentary environment. of Apollo which was used by the Akarnanian people at least Sediment samples were taken for geochemical analyses during Hellenistic times (MURRAY, 1982). of parameters such as electrical conductivity, pH-value, According to PAscHOS et al. (1991) and IGME (1996), the Aktio loss on ignition, content of orthophosphate, as well as of headland is largely made up of Holocene coastal sediments the concentration of (earth-) alkaline and heavy metals. and swamp deposits. The Preveza area and parts of the hills of Geochemical values were helpful to detect facies changes Stoupas (Fig. 1) consist of Pleistocene sand units and Pliocene (VÖTT et al., 2003). to Pleistocene marls, sandstones and conglomerates of a Flysch- Detailed geomorphological surveys were conducted on land like facies. In contrast, the Plaghia Peninsula, the hills between and under water by means of scuba diving along transects Aghios Nikolaos and Vonitsa as well as the hills east of Aghios across the submerged Plaka beachrock remains. Underwater Thomas near Preveza are composed of limestone, partly findings of dislocated beachrock blocks and rubble ridges brecciated, and dolomite of Triassic to Cretaceous age. Parts of were measured, photographed, and sampled. A high-resolution the underground are characterized by thick Triassic evaporitic differential GPS system (Leica SR 530) was used to determine units which have considerable influence on tectonodynamic the position and elevation of vibracoring sites. (GALONOPOULOS and EKONOMIDES, 1973; LAIGLE et al., 2004) and The geochronological framework of the study is based on coastal evolution (UNDERHILL, 1988). (i) 14C-AMS analyses of organic material and shells made out The Ambrakian Gulf is a tectonic basin which has been of marine carbonate as well as on (ii) the age determination of formed by NE-SW directed crustal compression and NNW- diagnostic ceramic fragments encountered in vibracore profiles SSE trending extension (CLEws, 1989). The main triggering and during field surveys. Concerning samples from marine factors are pull-apart dynamics caused by the rapid southwest- environments, the palaeo-reservoir effect may differ from the ward movement of the Aegean microplate (DOUTSOS and modern one and has possibly been variable through time and KOKKALAS, 2001). Moreover, northwestern Greece underwent space (GEYH, 2005). This is why we refrained from determining a 40° clockwise rotation during 15-8 Ma and a 20° clockwise the modern reservoir effect and corrected all marine samples rotation after 4 Ma (VAN HINsbERGEN et al., 2005). Northern for a mean marine reservoir age of 402 years (ΔR =� 0; REIMER Akarnania even shows a 90° clockwise rotation since Oligo- and MCCORMAC, 2002). Calibration was carried out using the Miocene times (BROADLEY et al., 2004). Both rifting and Calib 5.0.2 software. rotation are responsible for the opening and spreading of the Ambrakian Gulf. The northernmost section of the Hellenic Arc lies not far 4 - Event deposits at Aktio headland offshore Aktio headland. As part of a multiple plate junction, the right-lateral strike slip Cefalonia transform fault (CF) and We present sedimentological and geomorphological evidence the Lefkada fault (LF) induce the high seismo-tectonic activity of multiple tsunami impacts encountered in 15 selected of the area (COCARD et al., 1999; LOUVARI et al., 1999).