Ambiguous correlation of precisely dated coral detritus with the tsunamis of 1861 and 1907 at Simeulue Island, Aceh Province, Indonesia

Precise U–Th dates from coral detritus in two pre-2004 tsunami deposits on Simeulue Island in Aceh Province allow us to correlate the deposits with historically documented tsunamis in the recent few centuries, but because of potential discordance between the death dates of the corals and deposition of the sand layers, ambiguity in this correlation remains. Pits at coastal lowland sites exposed sand layers beneath the 2004 tsunami deposit at Busung and Naibos on southern Simeulue Island. The layers share sedimentological characteristics with the deposit of the 2004 tsunami, and are interpreted as pre-2004 tsunami deposits. Historical accounts document earthquakes and tsunamis in 1907 and 1861 and suggest that the 1907 tsunami was larger locally than any others historically. Nonetheless, U–Th analyses of coral boulders in the younger of two pre-2004 deposits at Busung and in the lone deposit at Naibos yielded dates of death that overlap with 1861, but there were no tsunami layers that could be directly dated to 1907. The younger pre-2004 sand deposit can be attributed to both the 1861 and 1907 events, if the dated corals were killed by uplift in 1861 and subsequently entrained and deposited by the 1907 tsunami. A piece of coral in the older of the two pre-2004 sand layers at Busung dated to around AD 1783, and was deposited by an unknown tsunami that occurred after AD 1783, or possibly by the 1861 tsunami. The nearly equatorial latitude of the study sites minimizes potential for geological confusion between tsunami and storm. Our results show the difficulty in matching known events and geological records when using limiting maximum ages from allochthonous fossils, even with high precision radiometric dates.