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Shallow marine sediment record of tsunamis : analysis of the sediment-fill of the bays of Tutuila (American Samoa) and backwash deposits of the 2009 South Pacific Tsunami

Abstract : Following recent destructive tsunamis, especially the 2004 Indian Ocean Tsunami and the 2011 Tohoku-Oki Tsunami, and their worldwide coverage, there has been a growing interest in tsunami research amongst the scientific community. However, most of the studies dealing with geological evidence of past tsunamis focus on onshore deposits, leaving aside marine deposits despite their potential for recording of the backwash phase. Moreover, the few studies on tsunami marine deposits were carried out in open and dynamic shallow marine environments, which are not favorable for long-time preservation. In this study, we focus on sheltered bays of Tutuila (American Samoa) frequently impacted by tsunamis, supposed to offer ideal preservation potential, to gain knowledge on tsunami backwash deposits. The sediment-fill of Pago Pago Bay was first examined. The internal architecture and sediment facies show that the bay infilling was emplaced during the last 12 000 years through the last sea-level rise and highstand. The upper bay-fill consists of alternations between low-energy and high-energy sediment units, interpreted as stacked tsunami backwash deposits, emplaced during the last millenaries. Within the uppermost meter-thick silty unit, backwash deposits emplaced following the 2009 South Pacific Tsunami and the 1960 Great Chilean Earthquake Tsunami were identified based on geochemical, mineralogical and microstructural signatures. Basal microstructural features give evidence that those tsunami backflows behave as hyperpycnal currents. Finally, backwash deposits of four recent tsunamis were identified in bays located along the north shore of Tutuila, including the 2009 South Pacific Tsunami, the 1960 Great Chilean Earthquake Tsunami or the 1957 Aleutian Islands Tsunami, the 1917 Tonga Trench Tsunami and an older tsunami never reported in American Samoa, most likely the 1868 South American Tsunami. Backwash deposits emplaced by the 1868 South American Tsunami would represent the first marine geological evidence of this tsunami. As a whole, this study shows the great potential of sheltered shallow marine environments for tsunami backwash archiving, with at least four tsunami backwash deposits identified for the last 150 years in the bays of Tutuila. In addition, this work provides new identification criteria for tsunami backwash deposits, particularly in comparison with flash-flood deposits.
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Brieuc Riou. Shallow marine sediment record of tsunamis : analysis of the sediment-fill of the bays of Tutuila (American Samoa) and backwash deposits of the 2009 South Pacific Tsunami. Earth Sciences. Université de La Rochelle, 2019. English. ⟨NNT : 2019LAROS012⟩. ⟨tel-02511060⟩

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