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

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Abstract

Boulders disjoined from platform edges or accumulated in the nearshore are frequently transported and deposited inthe backshore as a consequence of tsunamis and high-magnitude storm waves. We investigate this process in theFavignana Island (Egadi Archipelago) coastal zone by integrating geological and morphological data with numericalhydrodynamic analysis and radiocarbon dating.Boulders and sockets are detected in proximity to the shorelines of Punta Faraglione and Punta Fanfalo, which arelocated in the NW and SE sectors of the Favignana Island, respectively. They are scattered on the rocky platform asisolated blocks or in small groups that, together, form a discontinuous berm. The latter is characterized by an imbricatestructure in which the A-axis of boulders steeply dips toward the sea. The boulders are composed of grainstones and marldeposits of Early-Middle Pleistocene age. Most of them are covered by marine biogenic incrustations. The bouldersexhibit a tabular shape with sharp broken edges. Their shape, length and width appear to be influenced by litho-structuralfeatures of the rocky platform. In fact, the thickness of bedding planes and fracture network are particularly of interestbecause weathering and wave action (i.e. hydraulic action, abrasion, attrition and solution) operate simultaneously inthese discontinuities to weaken them. The histogram of boulders’ thicknesses shows three main peaks that correlate withthe 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 tsunamiand storm wave height that satisfy the requirements to initiate the transport of a boulder. For each boulder mapped in thecoastal zone, two pre-transport conditions, which are submerged and joint bounded, are considered. In particular, weapplied 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 isat least one case in which the wave height required to start its transport is compatible with the significant wave heightsrecorded 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 coherentlysuggest that boulder deposits in the Favignana Island coastal zone are polyphasic.
Lingua originaleEnglish
Numero di pagine1
RivistaRENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA
Volume40
Stato di pubblicazionePublished - 2016

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