Large boulders along the south-eastern Ionian coast of Sicily: Storm or tsunami deposits?

Abstract

This study investigates the size, shape, position, pre-transport setting and long-axis orientation of 175 boulders found along the south-eastern Ionian coast of Sicily. These megaclasts were deposited by the sea or moved from the same rocky coast to the shore platform and distributed within 60 m of the shoreline. Most boulders are rectangular to ellipsoidal, with sharp, broken edges. They are calcarenite fragments up to 27 t in mass. Some of the boulders (up to 32 m from shore) were observed to have moved after strong winter storms occurring during December 2008–January 2009, whereas the positions of many boulders were unchanged. We use wave transport equations jointly with statistical analysis of boulders in order to determine the extreme events – geological or meteorological – responsible for these singular accumulations. Using the significant wave height and period of maximum observed storms and historical tsunamis (1693 and 1908 events) along the Ionian coast of Sicily, we have estimated the approximate transport distance of boulders by these waves at the coast. Results show that the largest storm waves were probably responsible for the current distribution of most boulders. However, since the size and horizontal displacement distance of some boulders scattered randomly along the shoreline at distance > 40 m cannot be explained by storm wave action, we suggest that they are likely deposited by tsunamis. Radiocarbon dating performed on three of these anomalous boulders, at a distance > 40 m from the shoreline, gave an age suggesting that two of them were likely deposited by the 1693 or 1908 tsunamis, whereas the third was emplaced by a tsunami that occurred after 650–930 A.D. We show that both wave processes (storm and tsunami) can deposit boulders. We find that the significant difference between storm and tsunami waves is not the breaking wave height but the wave period that influences attenuation, and then the flooding distance. Hence, if the boulder-transport limit of the largest storm waves in the past at the study area is estimated, then it is possible to recognize that the boulders deposited far beyond this limit, because of their size and shape, were not transported by storm waves, but could have been emplaced by waves with longer period, such as the tsunamis.