Large Boulder Along the NE Maltese Coast: Tsunami or Storm Wave Deposits?

The accumulation of large boulders resulting from tsunamis or large waves is a phenomenon that has been observed in different areas along the Mediterranean Sea, mainly along the shoreline of the Messina Straits and the Apulian coasts. It is known through historical records that very few large earthquakes have originated close to the Maltese Islands. The seismicity which affects the Maltese coasts is mainly related to the Pantelleria Rift, a graben system which was active in Late Miocene till recent times. This rift system interrupts the shallow platform connecting Europe to Africa. The Maltese NE coast, mainly the area between Armier Bay and Il-Ponta ta' L-Ahrax is interspersed by large limestone boulders which are scattered on a large terrace which is gently sloping towards the sea. The deposit is about 100m wide and situated at an altitude ranging between 0m and 5m above sea level. This site probably represents the most suitable area for this type of deposition on the island given to the favourable morphological conditions. Most of the NW coast of Malta is characterised by cliffs that prevent the accumulation of wave deposits, while other sectors are characterised by the accumulation of large limestone boulders which have been deposited due to cliff retreat. This work aims to commence a preliminary survey of these deposits. Block sizes range between few and about 4m3, with the largest one being about 2m×2m×1m in size and with an estimate weight of 10 tonnes. Most of the blocks, mainly in the northernmost sector, are scattered individually while at the central parts of the study area one can find groups of few blocks. The largest boulders show typical coastal karst features (karstic pools, etc) similar to the coastal features found in the area. This suggests that the detachment area is close to the identified deposits. Further studies are foreseen, in particular to determine the nature of the deposits, period of detachment of the blocks and the possibility that the accumulation represents more than a single event (seismic or storm).