Gran Roque Island partly submerged by a 2500-year-old tsunami, Los Roques Archipelago, Venezuela Leeward Antilles Islands

Abstract

Gran Roque Island (GRI), as the most northeastern salient of the Los Roques Archipelago, has been affected by several extreme-wave events (EWEs), as is recorded by several coastal boulder deposits (CBDs). The particular shape of its windward elevated coast and of its rather simple geology made of 2-end components: on one hand Mesozoic meta-igneous mafic rocks, and on the other Holocene sedimentary environments of typical active carbonate reef systems, allow differentiating a foreshore-lying CBD built by frequent storm and hurricanes from an inland CBD, which was deposited by a much larger and unusual EWE (tsunami). The former CBD is the source of the latter one. The source of this tsunami-transported CBD is unequivocal and forms and accumulates in the seashore of the Mal Viento Bay, the only opening of GRI to the North. This latter singularity also allows proposing that this 2500 yr cal BP tsunami must have generated in the northeastern Caribbean, because of the narrow opening to the North of this bay. A tsunamigenic earthquake about that time seems to have impacted, at least: a) Saint Thomas (Virgin Islands), b) lagoon Guichard in Saint Martin, c) 3 localities along the Atlantic coast of Martinique and d) Gran Roque Island (GRI). This pre-Current Era event now appears to have several run-ups at different localities. In the case of GRI, it has entrained blocks of diabase as heavy as 8.5-9 tons (volume of 3 m3) over a foreshore top of 2.2 m above msl, requiring a flow depth of 8.2 m. In a more realistic estimation, if the largest block is approximated to a 1-m-diameter, 3-m-long cylinder (volume of 2.35 m3 and ≈ 7 tons), a 6.5 m tsunami wave may have been sufficient to transport it inland by rolling/saltation.