Sedimentation characteristics on the Andaman shelf, Thailand: unraveling extreme events

The 2004 Indian Ocean tsunami had an impact on the entire coastline of Southeast Asia. As one of several recent tsunamis, it provides unique opportunities to study the geomarine effects of such events in detail. The Andaman shelf (Thailand) was chosen to evaluate offshore tsunami deposits. Its coastline was strongly influenced by the 2004 tsunami, while the infrequent occurrence of strong storms and negligible fluvial discharge increase the preservation potential of tsunami deposits on the seafloor. In total, 60 sediment cores and 156 seafloor sediment samples from water depth between 5-65 m, as well as 10 onshore sediment samples and supporting hydroacoustic data were available for this study. Investigations of shallow seismic data, sedimentary structures, grain size composition, physical sediment properties, element composition and Ti/Ca log-ratio as well as 210Pb and AMS14C dating revealed general information about the seafloor sediment distribution and its stratigraphy on the Andaman Sea shelf during the Holocene period. In the study area, the seafloor is dominated by coarse sediment (sand and boulders with coral rubbles). Partly, small incised channels are observed down to 30 m water depth. This structure has been interpreted as a remnant of an old reef platform, active during times of lower sea level. Especially within these channels, finer grained sediment, including event deposits, were trapped. Sediment cores retrieved from shallow water down to approx. 16 m revealed three different types of event deposits that are interpreted as tsunami, storm and flash-flood deposits. Thus, the dataset allowed to discriminate and to compare recent storm and tsunami deposits. The most recent event deposits were found in core depths between 0 and 40 cm, and were interpreted as being tsunami deposits. These tsunami layers have a thickness about 12-30 cm, and are comprised of several distinct units that include sedimentary features as lamination, fining and coarsening upward, massive structure, erosional of both upper and lower contacts, terrigenous and anthropogenic components, and clasts. The different units represent different hydrodynamic conditions (probably run-up and backwash phase). In core depths between 5 and 82 cm, a second type of event layer has been found. These layers are 1-10 cm thick and mainly contain sharp and erosional lower contacts, partly massive and partly show slight lamination. These are assigned to storm deposits. Accordingly, the prominent difference between tsunami and storm deposits is the presence of clasts, terrigenous materials, anthropogenic artifacts and mud content. A third kind of event layers found in core depths between 12 and 92 cm are considered to be result of small scale events such as flash floods during summer monsoon period. These deposits have a thickness of 2-13 cm, and are mainly composed of clayey silt with few sand layers, which are commonly laminated and generally fining upward with transitional boundaries