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 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.

Tsunami deposits: Present knowledge and future challenges

“…Moreover, not only internal sedimentary structures but also the bedforms of a tsunami deposit can be used to estimate tsunami flow conditions. However, most tsunami deposit studies have focused on sedimentary structures observed in tsunami deposit cross‐sections, and only a few descriptions of tsunami depositional bedforms have been reported (Fujiwara et al ., , ; Fujiwara & Tanigawa, ; Falvard & Paris, ; Riou et al ., ). Sedimentary structures are often difficult to identify in tsunami deposits since depositional processes commonly form massive deposits.…”

“…These units indicated a changeover in the nature of the sedimentation between homogeneous silt units and coarse coral and Halimeda debris units pinching out seaward. The size and shape of these coral fragments, combined with the highly sheltered morphology of Pago Pago Bay (Samoa), revealed the tsunami‐induced nature of these deposits (Riou et al ., ). Evidence of backwash deposits caused by the 2009 South Pacific Tsunami was found as a result of X‐ray fluorescence (XRF) data while there was no difference detected in grain size.…”

“…There are few descriptions of marine, and particularly subtidal tsunami deposits (e.g. Bussert & Aberhan, ; Tamura et al ., ; Riou et al ., ; Smedile et al ., ). Little is known about offshore tsunami deposits, although they have the potential to improve palaeo‐tsunami reconstructions, especially in areas with limited preservation of onshore materials (Goodman‐Tchernov & Austin, ; Tamura et al ., ; Riou et al ., ).…”

“…Nevertheless, the spatial extension of the correspondent signature is usually more restricted due to channelling effects and to variations in local bathymetry, (e.g. Dawson, ; Dawson, ; Dawson & Stewart, ; Tamura et al ., ; Riou et al ., ; Smedile et al ., ). It is also important to note that the sedimentological differences between the onshore and offshore imprints of the backwash phase are still poorly studied.…”

“…In offshore tsunami deposits, Riou et al . (), based on seismic profiles and sediment cores, identified eight sedimentary units overlying an eroded volcanic bedrock. These units indicated a changeover in the nature of the sedimentation between homogeneous silt units and coarse coral and Halimeda debris units pinching out seaward.…”