Holocene fan alluviation and terrace formation by repeated tsunami passage at Epitalio near Olympia (Alpheios River valley, Greece)
Vött, Andreas; Fischer, Peter; Röbke, Björn R.; Werner, Vera; Emde, Kurt; Finkler, Claudia; Hadler, Hanna; Handl, Mathias; Ntageretzis, Konstantin; Willershäuser, Timo
published: Dec 1, 2015
This paper presents, for the first time in the Mediterranean, geo-scientific evidence of fan al- luviation and terrace formation by repeated tsunami inundation since the mid-Holocene. Tsunami events revealed to be a dominating factor in the overall landscape evolution of the near-coast zone around the Gulf of Kyparissia. High-resolution multi-proxy palaeoenvironmental analyses were carried out based on vibracores, down to 16 m below surface, drilled along a transect in the Epitalio valley to the SE of the city of Pyrgos. We used sedimentological, micropalaeontological, mineralogical, geochemical and geochro- nological methods in order to detect and evaluate landscape changes. Vibracoring sites were selected after detailed geophysical prospection using electrical resistivity tomography measurements. The Epitalio valley, located approximately 4 km distant from the present coastline and only approximately 800 m long, is characterised by a large triangle-shaped fan. This fan ends at the distinctive step of the Olympia terrace, 5.5 m high, facing towards the lower Alpheios River valley. The prominent and active Alpheios Fault and Epitalio Fault zones directly run across the study area. The Epitalio fan stratigraphy revealed conspicuous coarse-grained layers out of gravel, coarse and medium sand and marine shell debris associated with basal erosional unconformities, fining upward of grain size and incorporated rip-up clasts out of underlying material. These high-energy layers were found repeatedly intersecting silt-dominated fluvio-limnic deposits. Microfossil analyses of high-energy layers revealed the presence of mixed lagoonal, shallow marine and deep water foraminifera, in parts strongly damaged. This allochthonous marine signal goes far beyond the (para-)autochthonous background signal of ubiquitous marine species which come from reworked bedrock units. Further evidence of high-energy impact from the seaside are the strong geomorphological discrepancy between the small Epitalio valley head and the large Epitalio fan together with lithostratigraphical characteristics and dimensions of the high-energy deposits, as well as mineralogical and geochemical fingerprints such as high Ca/Fe and Sr/K ratios. We conclude that the Epitalio valley was repeatedly inundated by tsunami waters across the Epitalio pass accumulating a large amount of tsunami inflow sediments in the form of the Epitalio fan. The Olympia terrace front was formed both during tsunami backflow along the Alpheios River depression line and, later, by Alpheios River floods. Isochronic tsunami signatures are known from several coastal archives in the closer and wider environs of the study area: We found that Epitalio tsunami events T0, T1 and T2 occurred at approximately 5300 – 5200 cal BC, 4350 – 4250 cal BC and during the 3rd millennium BC, respectively, as part of supra-regional tsunami events. Based on the study of reliable relative sea level markers, we detected vertical crust uplift bound to the foot walls of the Pheia, Alpheios and Epitalio Fault Zones in the range of at least 12 –13 m, in some parts even 20 m, during the past 6400 or so years. Tsunami flooding of the Epitalio valley and the formation of the Epitalio fan at least partially thus occurred at times when the elevation of the local topography was minimum 12 –13 m lower. Under such circumstances, the local tsunami hazard was even much higher than today.