Sedimentological and geoarchaeological evidence of multiple tsunamigenic imprint on the Bay of Palairos-Pogonia (Akarnania, NW Greece)

Abstract

This paper presents evidence of multiple tsunami impact on the Bay of Palairos-Pogonia, NW Greece, during the Holocene based on detailed geo-scientific studies. Altogether, 41 vibracores were drilled to detect high-energy influence in the stratigraphical record. Layers of coarse-grained allochthonous marine deposits were found intersecting autochthonous fine-grained back beach sediments in the Palairos coastal plain, on top of beach or marly bedrock units at Pogonia beach and along the Pogonia cliff section. High-energy deposits are associated with specific sedimentary structures such as fining upward sequences, rip up-clasts, basal erosional contact, bi- to multimodal grain size distribution, lamination, and co-existence of both well rounded and angular, sharply edged components, all of them typical of tsunami impact. Macro- and microfaunal analyses revealed tsunami-related input of numerous ex situ marine species and mostly angular shell debris into quiescent fresh- to brackish water environments by high-energy and long-distance transport. Geophysical methods helped to find subsurface structures and to seize the lateral extent of tsunami layers. Ca–Fe ratios derived from XRF measurements were used to trace tsunami influence inland. Most important is the widespread spatial distribution of the tsunamites with a lateral extent between 70 m at the Pogonia cliff section, 200 m at Pogonia beach, and 1 km in the central Palairos plain, and overall tendencies of fining and thinning towards inland. Highest elevations of tsunamites were found around Pogonia village at ca. 7.5 m a.s.l. Vibracore data give evidence that the overall coastal evolution has been strongly affected, locally even controlled by multiple high-energy impact. A conspicuous geoarchaeological tsunamite, up to 1.5 m thick, was found at Pogonia cliff consisting of mixed marine (sand, pebbles, bio-erosively altered stones, shell debris, foraminifers) and terrigenous material (abundant ceramic fragments, bones, building material). It is directly associated with the submerged remains of an ancient mole which was destroyed and its building material partly dislocated by the same event. Dating of tsunamites was accomplished by archaeological age determination of diagnostic sherds and by radiocarbon and luminescence dating techniques. Strong tsunami events were dated to the beginning of the 6th, the mid-5th, the 4th/3rd and the late 2nd millennia BC as well as to the 4th/3rd century BC. On a regional scale, the recurrence interval for large events is ca. 500–1000 years. At least twice during the late Holocene, the Palairos coastal plain was flooded quasi-contemporaneously from both the northern and southern side.