The boulder berm of Punta Saguerra (Taranto, Italy): a morphological imprint of the Rossano Calabro tsunami of April 24, 1836?

Abstract

An extended berm of calcarenitic boulders is recognisable at Punta Saguerra, few kilometres south of Taranto (Apulia, Italy) while isolated boulders are sparse in other near localities. The berm is at 2–5 m above present sea level (a.p.s.l), on a rocky headland gently sloping toward the sea; it is separated from the coastline by a large terrace. A detailed study of its stratigraphy and its morphology has been performed in order to define its depositional mechanism; in particular, integrated DGPS and Laser Scanner surveys have provided precise details of each boulder: position, size and distance from the shoreline. The accumulation is constitute of boulders up to 30 tons, which locally are arranged in rows of embricated patterns. The surfaces of the biggest boulders are characterised by biogenic encrustations and by solution potholes that indicate their intertidal/adlittoral/spray zone provenience. Based on direct observations of each boulder (size, shape, weight and long axis azimuth), together with hydrodynamic equations it is possible to hypothesize the extreme event—geodynamic or meteorological— which was responsible for this singular accumulation. AMS age determinations on Vermetid sp. sampled on boulder surfaces and chronicle suggest that the accumulation may be attributed to the tsunami generated by the strong earthquake that occurred on April 24, 1836, the epicentre of which has been localised near Rossano Calabro, along the Ionian coast of northern Calabria.