Sedimentary deposits left by the 2004 Indian Ocean tsunami on the inner continental shelf offshore of Khao Lak, Andaman Sea (Thailand)

Abstract

Tsunami waves leave sedimentary signatures both onshore and offshore, although the latter are hardly known. The objective of the present study is to provide new evidence for the 2004 Indian Ocean tsunami deposits left on the inner continental shelf of the Andaman Sea (Thailand) and to identify diagnostic sedimentological and geochemical properties of these deposits. Based on extensive seafloor mapping, three sediment cores were selected for study and were analysed for their sedimentary structures, grain size composition, chemical elemental composition, physical properties and 210 Pb activity. Sediment cores retrieved from shallow water (9-15 m) within 7.5 km off the shore revealed distinct event layers, which were interpreted as being tsunami deposits. These 20-25 cm thick deposits were already covered with post-tsunami marine sediments. They were composed of several units, marine sand layers alternating with poorly sorted mud with terrigenous and anthropogenic components, representing different hydrodynamic conditions (probably during run-up and backwash phase). These sedimentological observations were supported by geochemical and physical data and were confirmed using 210 Pb dating. A sediment core taken from a depth of 57 m at a distance of 25 km offshore did not reveal clear event deposits. Comparisons with available data from offshore tsunami deposits showed that there is no single set of signatures that could be applied to identify this kind of deposits.

Internal structure of event layers preserved on the Andaman Sea continental shelf, Thailand: tsunami vs. storm and flash-flood deposits

“…Regular reworking and redeposition from suspension are indicated by the frequent oc- currence of fining-upward sequences, showing a distinct absence of bioturbation. This indicates episodically high accumulation rates (Wheatcroft and Drake, 2003), supported by the tsunami layers buried to depths > 20 cm in some cores (this study; Sakuna et al, 2012). Notable is the frequent occurrence of thin sand layers less than 1 cm in thickness with sharp upper and lower boundaries within the laminated core sections in core 051207-31.…”

“…History of nine tropical storms and typhoons that approached within a 180-nautical-mile radius of Phuket during the 52-year period . Modified from the Typhoon Havens Handbook (Brand, 2009 (Di Geronimo et al, 2009;Sugawara et al, 2009;Feldens et al, 2012;Sakuna et al, 2012;Milker et al, 2013). Due to the reworking of shelf sediments and the re-establishment of the coastline Grzelak et al, 2009) following the tsunami, the remaining offshore tsunami deposits have been chiefly found in locally sheltered positions adjacent to granitic outcrops, within an incised channel system and in areas of locally higher sediment accumulation rates Sakuna et al, 2012;Milker et al, 2013).…”

“…Those clasts are widely used as a sedimentological proxy to identify tsunami deposits and are interpreted as relating to hyperpycnal tsunami backwash flows (Goff et al, 2004;Le Roux and Vargas, 2005;Morton et al, 2007;Goodman-Tchernov et al, 2009;Sakuna et al, 2012). Clasts are found in the uppermost part of the tsunami sequence and include material transported during the backwash, as shown by foraminifera composition (Milker et al, 2013) and the presence of laterites and grass Sakuna et al, 2012). The Ti / Ca ratios in this facies type are not noticeably different from the inferred flash floods deposits, which have to originate from onshore sediment as well (Fig.…”

“…However, while the tsunami's impact increases with decreasing water depth towards the coastline, only very few offshore tsunami deposits have been reported until now. Therefore, it is not surprising that the described offshore deposits of historical tsunami -not considering inferred tsunami paleorecords on geological timescales (Le Roux and Vargas, 2005;Fujiwara and Kamataki, 2007;Spiske et al, 2014) -are highly variable in thickness, texture and structure (e.g., van den Bergh et al, 2003;Abrantes et al, 2008;Goodman-Tchernov et al, 2009;Paris et al, 2010;Smedile et al, 2011;Sakuna et al, 2012;Milker et al, 2013). Reported offshore tsunami deposits range from centimeters to 1 m in thickness while spanning grain sizes from mud to boulders, including terrigenous and marine sediments over one or several layers.…”

Recent studies on the Silurian of the western part of Ukraine

“…Lists of sedimentary features characteristic of palaeotsunami deposits have been presented by some authors (e.g. Sakuna et al 2012); however, none of them can be treated as a single indicator of a tsunami origin, as in many cases the tsunami deposits showed no distinct characteristics, which allow them to be distinguished from other sediments (Shanmugam 2006). The aim of the referred studies was to describe a new type of palaeotsunami deposit and to add new arguments to the discussion on the sedimentological features of ancient tsunamis.…”