Boulder dynamics in the Favignana Island coastal zone (Egadi Archipelago, Central Mediterranean)

Abstract

Boulders disjoined from platform edges or accumulated in the nearshore are frequently transported and deposited in the backshore as a consequence of tsunamis and high-magnitude storm waves. We investigate this process in the Favignana Island (Egadi Archipelago) coastal zone by integrating geological and morphological data with numerical hydrodynamic analysis and radiocarbon dating. Boulders and sockets are detected in proximity to the shorelines of Punta Faraglione and Punta Fanfalo, which are located in the NW and SE sectors of the Favignana Island, respectively. They are scattered on the rocky platform as isolated blocks or in small groups that, together, form a discontinuous berm. The latter is characterized by an imbricate structure in which the A-axis of boulders steeply dips toward the sea. The boulders are composed of grainstones and marl deposits of Early-Middle Pleistocene age. Most of them are covered by marine biogenic incrustations. The boulders exhibit a tabular shape with sharp broken edges. Their shape, length and width appear to be influenced by litho-structural features of the rocky platform. In fact, the thickness of bedding planes and fracture network are particularly of interest because weathering and wave action (i.e. hydraulic action, abrasion, attrition and solution) operate simultaneously in these discontinuities to weaken them. The histogram of boulders’ thicknesses shows three main peaks that correlate with the thickness of the strata of coeval deposits cropping out close to the town of Favignana. Numerical hydrodynamic analysis is used in order to determinate both the minimum flow velocity and the tsunami and storm wave height that satisfy the requirements to initiate the transport of a boulder. For each boulder mapped in the coastal zone, two pre-transport conditions, which are submerged and joint bounded, are considered. In particular, we applied equations that differ for submerged blocks because of their initial transport mode (sliding, rolling/overturning, saltation/lifting). Hydrodynamic modeling indicates that among all of the considered scenarios for each boulder, there is at least one case in which the wave height required to start its transport is compatible with the significant wave heights recorded by the RON buoy of Mazara del Vallo. The historical data of tsunami and storms events, the results of the hydrodynamic equations, the radiocarbon dating, the dispersion of the A-axis direction, and the presence of small boulders with striae due to a fresh impact coherently suggest that boulder deposits in the Favignana Island coastal zone are polyphasic.