Tsunami deposits related to flank collapse in oceanic volcanoes: The Agaete Valley evidence, Gran Canaria, Canary Islands

Enigmatic marine conglomerates are attached at 41–188 m asl to the walls of the valley of Agaete, on the northwest coast of Gran Canaria (Canary Islands). They are formed by heterogeneous, angular to rounded heterometric volcanic clasts (roundness and maximal size decreasing with altitude), and fossils (rhodolites and marine shells), never found in growth position and often broken. The deposits are internally stratified into several layers, most of them showing very poor sorting, matrix-supported and reverse grading. They present lenticular morphologies with poor lateral continuity in transversal and longitudinal sections. Slopes show values and orientations similar to those of the relief of the substratum to which they seem to adapt. Although they show clear evidence of erosive contact with the substratum (rip up clasts), they do not tend to form horizontal terraces. Soft materials (soils and colluviums) are preserved in the contact with the substratum in outcrops with deposit slopes of up to 15°. The age of the deposits is constrained between 1.75 Ma and 32 ka. Their altitude and slope distributions are not related to Pleistocene interglacial sea level changes, storm deposits or isostatic movements. All the above suggests that the Agaete marine deposits were generated by tsunami waves, the most probable source being a flank failure, at least nine major such events having occurred in the Canary Islands during the Pleistocene. The Güímar sector collapse (east coast of Tenerife,< 0.83 Ma, > 30 km3) is the closest possible source for the tsunami and the sole flank failure that is directed towards another island in the Canaries.