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The sediment-fill of Pago Pago Bay (Tutuila Island, American Samoa): New insights on the sediment record of past tsunamis

Abstract : Extensive bathymetric and two-dimensional seismic surveys have been carried out and cores collected in Pago Pago Bay (Tutuila, American Samoa) in order to describe and gain a better understanding of the sediment fill of the bay, which was affected by the 2009 South Pacific Tsunami. Eight sedimentary units were identified over the volcanic bedrock. The basal transgressive unit displays retrograding onlaps towards the shore, whereas the overlying seven aggradational layers alternate between four draping units and three pinching out seaward units. 'Core to seismic' correlation reveals that draping units are composed of homogeneous silts, while pinching out units are dominated by very coarse coral fragments showing fresh cuts, mixed with Halimeda plates. The basal unit is attributed to transgressive sedimentation in response to flooding of the bay after the last glacial maximum, followed by the upper aggrada-tional units corresponding to highstand sedimentation. The changeovers in these upper units indicate an alternation between low-energy silt units and high-energy coral debris units interpreted as tsunami-induced deposits. The 14 C dating reveals that high-energy sedimentation units can last up to approximately 2000 years while low-energy sedimentation units can last up to approximately 1000 years. This alternation, deposited during the last high-stand, may be explained by cycles of tectonic activity and quiescence of the Tonga Trench subduction, which is the main source of tsunamigenic earthquakes impacting the Samoan archipelago. In the uppermost silt unit, only the geochemical signature of the terrestrial input of the 2009 SPT backwash deposits was detected between 7 cm and 9 cm depth. Hence, Pago Pago Bay offers a unique sediment record of Holocene bay-fill under the impact of past tsunamis intermittently during the last 7000 years.
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Brieuc Riou, Eric Chaumillon, Jean-Luc Schneider, Thierry Corrège, Catherine Chagué. The sediment-fill of Pago Pago Bay (Tutuila Island, American Samoa): New insights on the sediment record of past tsunamis. Sedimentology, Blackwell Publishing, In press, ⟨10.1111/sed.12574⟩. ⟨hal-02422133⟩

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