Chaotic conglomerates from Santiago Island (Cabo Verde): a review and insights into the proximal impacts of collapse-triggered megatsunamis

The Cabo Verde Archipelago holds a remarkable sedimentary record of tsunami inundations, as highlighted by recent finds on Santiago and Maio Islands. Santiago, in particular, constitutes an exceptional site to study in detail the proximal impacts of the megatsunami(s) triggered by the well-known catastrophic flank collapse of Fogo volcano (~60 km to the west of this island) and one of the most active ocean island volcanoes in the Atlantic. Previous studies identified and documented deposits – fields of megaclasts and chaotic conglomerates on northern Santiago – which were attributed to the impact of this megatsunami(s); moreover, the pioneer use of cosmogenic ³He geochronology on basaltic megaclasts quarried/displaced by the event bracketed its occurrence within the 65-84 ka time interval. Here we present the results of a recent study conducted within the remit of the project UNTIeD, which combined detailed field surveys and U-Th disequilibrium geochronology to review and further document the tsunamigenic conglomerates of Santiago and gain additional insights into their formative event(s). We can confirm the presence of tsunami conglomerates on several sectors of the island, chiefly in the north and southeast of the island. Furthermore, on the northern sector, our study suggests the presence of two distinct sets of deposits, of differing ages, as corroborated by U-Th geochronology on corals entrained in the conglomerates. The older set of deposits is restricted to 60–100 m in elevation and yielded coral ages at the upper limit of U-Th techniques (200-450 ka); its origin is still uncertain. The younger and most widespread chaotic deposit can be found from 0 to 100 m in elevation, is poorly consolidated, and mostly mantles a topography partially carved on the older deposit. Coral ages are very widespread from ~58 to >400 ka (as commonly observed in tsunami deposits) but with the higher-confidence younger ages clustering around 58-65 ka, in agreement with the youngest cosmogenic ages. This suggests a more constricted timing for Fogo's (main) megatsunami at around 58-65 ka, in close agreement with recent studies at Fogo. A distinct and younger deposit, of Holocene age, can only be found at low elevations in Nossa Senhora da Luz Bay and likely represents a local event possibly triggered by a small submarine landslide. Taken together, these finds not only provide a better time constraint and insights on the impact of Fogo's megatsunami but reinforce the notion that the Cabo Verde Islands have been impacted by multiple tsunamis in the last 500 ka.

This work was supported by project PTDC/CTA-GEO/28588/2017 - LISBOA-01-0145-FEDER-028588 UNTIeD, co-funded by the ERDF through POR Lisboa 2020 and FCT, and by projects IF/01641/2015 MEGAWAVE and FCT/UIDB/50019/2020 - IDL, also funded by FCT.