Nannoliths abundance and distribution in tsunami deposits: example from the December 26, 2004 tsunami in Lhok Nga (northwest Sumatra, Indonesia)

Abstract

Nannoliths are heterogeneous morphological suites of biogenic carbonate particles with silt-clay size dimensions. They are mainly composed of coccoliths, a suite of calcite complex structures produced by marine coccolithophores belonging to the Haptophyta microalgae division. Coccoliths can be found in marine and coastal facies, and they are used as a palaeoenvironmental proxy and a marine tracer in coastal sediments. Among microscopic biogenic structures commonly observed in tsunami deposits, diatoms are often used to determine the marine provenance and sediment source, but very few investigations about nannoliths were carried out so far. In this paper, we investigate the abundance, distribution and provenance of nannoliths observed in the 2004 tsunami deposits near Lhok Nga (northwest Sumatra, Indonesia) and their implications in terms of sediment sources and flow dynamics. The 2004 tsunami deposits display abundances in nannoliths that are significantly higher (1.0 x 104 to 7.2 x 104 nn/g) than other coastal deposits (4.5 x 103 nn/g), but lower than innershelf deposits (1.3 to 6.9 x 105 nn/g). Such enrichments in nannoliths could represent a tool to recognize palaeo-tsunami deposits. A remarkable characteristic of the Lhok Nga tsunami deposits is their nannolith coastal assemblages despite their relative impoverishment in clay content, which under normal marine hydrodynamic conditions would prevent nannoliths to settle. The abundance of nannoliths in tsunami deposits tends to decrease landward and upward, despite variations due to successive erosion/sedimentation phases by successive waves, and to topographical effects. When coupled with grain-size analyses, the study of vertical trends of nannolith abundances thus represents a complementary data for interpreting tsunami deposits.