Tsunami deposits in Santiago Island (Cape Verde archipelago) as possible evidence of a massive flank failure of Fogos volcano

Massive flank failures of volcanic edifices generate tsunami waves. These low-frequency but high magnitude hazards remain poorly documented because of the scarcity of observations. Offshore deposits are studied only by geophysical surveys and the failure rheologies are poorly constrained. Marine conglomerates found at unusually high elevations in Hawaii and in the Canary Islands were previously interpreted as being the result of giant tsunami waves generated by massive flank failures. This study focuses on the search for, and interpretation of, sedimentary evidence of a tsunami around the coastline of Santiago Island, which is located 55 km east of the active Fogo volcano. The Bordeira caldera in Fogo Island, which opens to the east, was formed by the Monte Amarelo flank collapse, and the active edifice is now nested in the failure's scar. The only evidence of a tsunami was found north of Tarrafal Bay. The deposits display many similarities with the tsunami conglomerate described in Hawaii and the Canary Islands: erosive contact with the substratum; rip-up clasts of paleo-soil; marine bioclasts never found in growth or live position; internal organisation into different facies with distinct characteristics of clast-size (up to boulders), sorting (from moderately to very poorly sorted) and clast-fabrics (landward or seaward orientations). This organisation suggests that the tsunami consisted of two main waves. The absence of tsunami deposits elsewhere on Santiago Island, coupled with the relatively low elevation of the conglomerate in Tarrafal (< 15 m), indicate retrogressive failures, rather than a single massive failure. The marine fauna in the tsunami conglomerate is not typical of warm fauna such as the Senegalese fauna (interglacial stages). The age of the tsunami is bracketed by the ages of a coral branch in the conglomerate (123.6 ± 3.9 ka U–Th age) and a post-collapse lava along the Bordeira caldera in Fogo (86 ± 3 ka K–Ar age).