Abstract
One of the main concerns of deciphering tsunami sedimentary records along seashore is to link the emplaced layers with marine high energy events. Based on a combination of morphologic features, sedimentary figures, grain size characteristics, fossils content, microfossils assemblages, geochemical elements, heavy minerals presence; it is, in principle, possible to relate the sedimentary record to a tsunami event. However, experience shows that sometimes, in reason of a lack of any visible sedimentary features, it is hard to decide between a storm and a tsunami origin. To solve this issue, the authors have used the Anisotropy of Magnetic Susceptibility (AMS) to evidence the sediment fabric. The validity of the method for reconstructing flow direction has been proved when applied on sediments in the aftermath of a tsunami event, for which the behaviour was well documented (2004 IOT). We present herein an application of this method for a 56 cm thick paleo-deposit dated 4220 BP laying under the soil covered by the 2004 IOT, SE of Banda Aceh, North Sumatra. We analysed this homogenous deposit, lacking of any visible structure, using methods of classic sedimentology to confirm the occurrence of a high energy event. We then applied AMS technique that allowed the reconstruction of flow characteristics during sediment emplacement. We show that all the sequence was emplaced by uprush phases and that the local topography played a role on the re-orientation of a part of the uprush flow, creating strong reverse current. This particular behaviour was reported by eyewitnesses during the 2004 IOT event.
Highlights
The Anisotropy of Magnetic Susceptibility (AMS) of rocks and sediments is certainly one of the most versatile techniques in geology, as it finds usage from Archean rocks (e.g., Borradaile et al, 2012) to contemporary deposits (e.g., Wassmer et al, 2010; Wassmer and Gomez, 2011)
The method, known as AMS, based on a first inference that ferromagnetic minerals realign after a rock is subject to deformation, started with the early findings of Graham (1954) on sedimentary rocks and Balsley and Buddington (1960) who proved that the AMS could detect the fabric of minerals in orthogneiss and granites
Interpretation Related a) Flow direction is retrieved from the overall sediment fabric but previous research (Rusnak, 1957; Allen, 1964; Rees, 1968) show that depending on the flow velocity, the grains long axes will parallel or be normal to the flow direction prevailing during emplacement
Summary
The Anisotropy of Magnetic Susceptibility (AMS) of rocks and sediments is certainly one of the most versatile techniques in geology, as it finds usage from Archean rocks (e.g., Borradaile et al, 2012) to contemporary deposits (e.g., Wassmer et al, 2010; Wassmer and Gomez, 2011). The method has been extended to a variety of sedimentary environment, either consolidated material, e.g., the non-deformed Callovo-Oxfordian argillites of the Paris Basin, France (Esteban et al, 2006), the tectonically impacted marine clays of the Crotone basin, Italy (Macri et al, 2014), deep-sea sediments transport (Housen et al, 2014); or unconsolidated material, e.g., tsunami washover deposits in Indonesia (Wassmer et al, 2010; Wassmer and Gomez, 2011) and New Zealand (Kain et al, 2014), or lake sediments in China (Dong et al, 2013)
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