MARINE TSUNAMI DEPOSITS: NEW EXAMPLES FROM MESSINIAN OFFSHORE SEDIMENTS

Marine tsunamigenic deposits are almost unknown in the geological record. A few examples are tied to major impact events because impact ejecta are mixed with clastic debris within a single, but complex, event deposit. New examples of probable tsunamigenic deposits were recognized in offshore Lago Mare deposits in Italy and Spain. Three tsunamigenic layers occur in the late Messinian San Donato-Colombacci (SC) formation of the Corvi section near Ancona, Italy. The SC represents an offshore environment, remaining below storm wave base for about 170ky during the Lago Mare period. The three layers are intercalated in homogenous fine-grained sediments, characterized by abundant thin, likely storm-triggered turbidites. Each of the three layers consists of several graded and cross-bedded sub-layers decreasing upwards in grainsize and thickness. The sub-layers are channelized, downcutting into SC and each other, indicating rapid deposition. Common components are pebble-sized black cherts, and numerous bivalves, likely derived from a coast further west, but absent from surrounding sediments. The channel axes and the measured current directions point to a source area to the NW. The three layers are remarkably continuous, and can be traced over 500m in the coastal cliffs. These layers were earlier interpreted as stormlayers, unusual turbidites, or fluvial flood density currents (1). Problematic interpretations, whereas deposition of the sub-layers by individual surges of a single tsunami, each separated by >½ hour, provide an elegant explanation. Tsunamis are best described as tidal surges, rather than waves, leading to a succession of strong, mostly backwash high-density currents, likely to leave a record in offshore environments. The second example is from the Feos formation in the Nijar basin, south Spain (2). Intercalated in fine-grained offshore deposits are hundreds of couplets and triplets of graded clastic layers, often associated with individual tsunami surges. The deposition of these layers is related to seismic activity along the nearby Serrata fault, because underlying fissures, opened by Rayleigh-waves of earthquakes, are filled with the coarsest material from the basal layer of the couplets/triplets. 1. Roveri et al. Geoacta 2005. 2. Fortuin, A.R. and Krijgsman, W. (2003). Sed. Geol. 160, 213-242.