Search

Tsunami, storm and flash-flood event layers, which have been deposited over the last century on the shelf offshore Khao Lak (Thailand, Andaman Sea), are identified in sediment cores based on sedimentary structures, grain size compositions, Ti / Ca ratios and 210Pb activity. Individual offshore tsunami deposits are 12 to 30 cm in thickness and originate from the 2004 Indian Ocean Tsunami. They are characterized by (1) the appearance of sand layers enriched in shells and shell debris and (2) the appearance of mud and sand clasts. Storm deposits found in core depths between 5 and 82 cm could be attributed to recent storm events by using 210Pb profiles in conjunction with historical data of typhoons and tropical storms. Massive sand layers enriched in shells and shell debris characterize storm deposits. The last classified type of event layer represents reworked flash-flood deposits, which are characterized by a fining-upward sequence of muddy sediment. The most distinct difference between storm and tsunami deposits is the lack of mud and sand clasts, mud content and terrigenous material within storm deposits. Terrigenous material transported offshore during the tsunami backwash is therefore an important indicator to distinguish between storm and tsunami deposits in offshore environments.

We have investigated the benthic foraminiferal fauna from sediment event layers associated with the 2004 Indian Ocean tsunami and former storms, that have been retrieved in short sediment cores from offshore environments of the Andaman Sea, off Khao Lak, western Thailand. Species composition and test preservation of the benthic foraminiferal faunas exhibit pronounced changes across the studied sections and provide information on the depositional history of the tsunami layer, particularly on the source water depth of the displaced foraminiferal tests. In order to obtain accurate bathymetric information on sediment provenance, we have mapped the distribution of modern faunas in non-tsunamigenic surface sediments and created a calibration data set for the development of a transfer function. Our quantitative reconstructions revealed that the re-suspension of sediment particles by the tsunami wave was restricted to a maximum water depth of approximately 20 m. Similar values were obtained for former storm events, thus impeding an easy distinction of different high-energy events.

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 210Pb 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 210Pb 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.

The coastline of Khao Lak (Thailand) was heavily damaged by the 2004 Indian Ocean tsunami. Onshore tsunami deposits and satellite images, which show large amounts of sediment transported offshore, indicate that the seafloor was impacted by tsunami run-up and backwash. In this study, high-resolution maps of sediment distribution patterns and the geological development of the seafloor are presented. These maps are based on multibeam, side-scan sonar and seismic profiling surveys offshore Khao Lak. Paleoreefs, with associated boulder fields and sandy sediment dominate the inner continental shelf. Patches of fine-grained (silt to fine sand) sediments exist in water depths of less than 15 m. The sediment distribution pattern is stable between 2008 and 2010, apart from small shifts regarding the boundaries of the fine-grained sediment patches. In one sediment core and several grab samples an event layer was documented, situated below a cover of modern sediments which is only a few cm thick. The event-layer is traced down to 18 m water depth. It consists mostly of sand and contains compounds of terrigenous origin. It is interpreted as a 2004 Indian Ocean tsunami deposit. However, over large areas of the study-site, the impact of the tsunami is hardly identifiable by seafloor morphology or sediment distribution.