A geological record of multiple Pleistocene tsunami inundations in an oceanic island: The case of Maio, Cape Verde

Abstract

In the Central Atlantic archipelagos – the Canaries, Cape Verde, Madeira and the Azores – tsunami hazard is often regarded as low, when compared with other extreme wave events such as hurricanes and storms. The geological record of many of these islands, however, suggests that tsunami hazard may be underestimated, notwithstanding being lower than in areas adjacent to subduction zones, such as the margins of the Pacific and Indian oceans. Moreover, tsunamis in oceanic islands are generally triggered by local large‐scale volcanic flank collapses, for which little is known about their frequency, making it difficult to estimate the probability of a new occurrence. Part of the problem lies in the fact that tsunami deposits are usually difficult to date, and few islands in the world exhibit evidence for repeated tsunami inundation on a protracted timescale. This study reports on the presence of abundant tsunami deposits (conglomerates and sandstones) on Maio Island (Cape Verde) and discusses their stratigraphy, sedimentological characteristics, probable age and tsunamigenic source. Observations indicate that four distinct inundation events of variable magnitude took place during the Pleistocene. One of the tsunami deposits yielded a high‐confidence U/Th age of 78·8 ± 0·9 ka, which overlaps within error with the 73 ± 7 ka age proposed for Fogo volcano's flank collapse, an event known to have had a significant tsunami impact on nearby Santiago Island. This shows that the Fogo tsunami also impacted Maio, resulting in runups in excess of 60 m above coeval sea‐level at ca 120 km from the source. Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano in Santo Antão Island, yielded lower‐confidence ages of 479 to 390 ka and 360 to 304 ka. A younger deposit (<78 ka) remains undated. In summary, the geological record of Maio exhibits well‐preserved evidence of repeated tsunami inundation, reinforcing the notion that tsunami hazard is not so low at volcanic archipelagos featuring prominent and highly‐active volcanoes such as in Cape Verde.

Revisiting the tsunamigenic volcanic flank collapse of Fogo Island in the Cape Verdes, offshore West Africa

“…This estimate corresponds well with previous bathymetric-based estimates, which ranged between 80 and 160 km 3 (Le Bas et al 2007;Madeira et al 2008;Masson et al 2008). Tsunami deposits found on the nearby islands of Santiago and Maio indicate that the Monte Amarelo flank-collapse was tsunamigenic, with the resulting tsunami achieving a run-up in excess of 270 m above coeval sea level on Santiago (Ramalho et al 2015;Madeira et al 2019). The exact age of the collapse, however, is the topic of ongoing debate.…”

“…Consequently, the distribution of the debris avalanche deposits, and of any additional slope instabilities in the area, are still poorly constrained. As Fogo's volcanic flank-collapse likely triggered a megatsunami with a recognizable impact on the adjacent islands' coastlines (Paris et al 2011(Paris et al , 2018Ramalho et al 2015;Madeira et al 2019), a full characterization of this landslide and its related volume is crucial for improving numerical models of tsunami generation, propagation and inundation, and for constraining the hazard potential associated with large, tsunamigenic volcanic flank-collapses.…”

Tsunami deposits: Present knowledge and future challenges

“…One of the dated conglomerates roughly coincides with the Fogo volcano mega‐collapse (78.8 ± 0.9 ka). Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano on Santo Antão Island, yielded lower confidence ages of 479 to 390 ka and 360 to 304 ka (Madeira et al ., ). A younger deposit (<78 ka) remains unattributed to a specific generation mechanism (Madeira et al ., ).…”

“…New evidence of tsunami-derived conglomerates and sandstones on Maio Island (Cape Verde) was presented by Madeira et al (2020). An association with four distinct Pleistocene tsunami inundation events of variable magnitude was established based on ages obtained through U/Th dating.…”

“…Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano on Santo Antão Island, yielded lower confidence ages of 479 to 390 ka and 360 to 304 ka (Madeira et al, 2020). A younger deposit (<78 ka) remains unattributed to a specific generation mechanism (Madeira et al, 2020). Abad et al (2020) described coastal boulder deposits on a Late Pleistocene marine terrace in Cisne Bay (Atacama, northern Chile).…”