Following recent tsunamis, most studies have focused on the onshore deposits, while the offshore backwash deposits, crucial for a better understanding of the hydrodynamic processes during such events and offering an opportunity for sedimentary archives of past tsunamis, have mostly been omitted. Here, we present a unique sedimentary record of the backwash from two historical tsunamis sampled in a sheltered bay in American Samoa, namely the 2009 South Pacific Tsunami and the 1960 Great Chilean Earthquake Tsunami. Although not always concomitant with a marked grain size change, backwash deposits are identified by terrestrial geochemical and mineralogical signatures, associated with basal soft sediment micro-deformations. These micro-deformations, including asymmetric flame structures, are described for the first time in historic shallow marine backwash deposits and lead us to propose an improved depositional mechanism for tsunami backflow based on hyperpycnal currents. Moreover, this study brings a potential new criterion to the proxy toolkit for identifying tsunami backwash deposits, namely the basal soft sediment micro-deformations. We suggest that further studies focus on these micro-deformations in order to test the representability of this criterion for tsunami backwash deposits. Sheltered shallow marine environments in areas repeatedly impacted by tsunamis have a higher potential for the reconstruction of paleo-tsunami catalogs and should be preferentially investigated for coastal risk assessment. Over the last two decades, interest in tsunami-related research has increased significantly, with peaks in the number of published articles following the 2004 Indian Ocean Tsunami (IOT), the 2009 South Pacific Tsunami (SPT) and the 2011 Tohoku-Oki Tsunami (TOT) 1. However, most studies have focused on onshore deposits, with only a few tackling the issue of backwash depositional processes 2-16. Unlike onshore deposits, offshore deposits are not subject to subaerial erosion and less to anthropic reworking, but can be altered by waves, currents, mixing and bioturbation 1,16. Most of the studies of historic tsunami backwash were carried out in open beach environments following the 2004 IOT and 2011 TOT 5,13,14,16-18. However, such environments have a poor preservation potential due to their exposure to waves. In contrast, sheltered bays may provide a higher preservation potential due to less reworking by waves. Thus, the choice of the study zone is key when looking for marine backwash deposits. We suggest that in shallow marine sheltered environments characterized by a low hydrodynamic setting, a complete and uninterrupted record is more likely to be preserved. Most studies of backwash deposits are based on grain size, geochemical data and microfossils 1. Shallow marine tsunami deposits are usually characterized by an increase of the mean grain size within usually fine marine mud, due to the inclusion of coarse terrestrial sediment originating from the onshore-inundated or beach areas 5,6,12,15,16 , often accompanied by...
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 aggradational 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 14C 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 highstand, 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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