Vichai Chutakositkanon

Erosion and Deposition by the 2004 Indian Ocean Tsunami in Phuket and Phang-nga Provinces, Thailand

Abstract The devastating December 26, 2004, tsunami produced abundant geologic effects along the Andaman coast of Thailand. The tsunami inundated the numerous sandy beaches and flowed over the adjacent aeolian dunes. On some of the dunes, the tsunami scoured circular holes 10–30 cm in diameter, and in its waning phases, it coated the holes with mud. The tsunami locally deposited a sand sheet that ranged from 0–30 cm in thickness, with an average thickness of approximately 10 cm. Sedimentary structures within the sand sheet include ripples from inflow and outflow, graded bedding, parallel lamination, and double-layered deposits. Erosion, locally severe, affected sand beaches and tidal inlets. We use these erosional and depositional features to infer the main processes that acted during inundation from the tsunami.

Tectonic significance of detrital chromian spinels in the Permian Nam Duk Formation, central Thailand

Detrital chromian spinels in the Permian Nam Duk Formation, Thailand are examined to determine the petrological characteristics of eroded mafic-ultramafic rocks in the Phetchabun area. Detrital chromian spinels are found as one of accessory heavy minerals from turbiditic sandstones of the Nam Duk Formation. The detrital chromian spinels are generally deep brown to almost opaque in thin section due to their wide compositional variation, and are relatively small, about 20–190 microns across. They sometimes exhibit subhedral to euhedral shapes and rarely have inclusions. Thirty–five grains were chosen for microprobe analysis. They have relatively high Cr contents, the Cr#(=Cr/[Cr+Al] atomic ratio), being mostly higher than 0.50, and up to 0.83. TiO2 contents are generally low but variable from 0.02 to 1.16 wt%, and Fe3+ ratios are consistently low. The relatively high Cr# and low TiO2 contents of the Nam Duk detrital chromian spinels in the Permian sediments indicate their derivation from mafic-ultramafic rocks of are origin. The relatively small and euhedral to subhedral spinels with inclusions especially suggest their provenance from volcanic rocks. It is strongly inferred that igneous rocks, namely volcanics and peridotites, of arc origin were exposed in this region at the Permian at least. The Loei-Phetchabun-Ko Chang volcanic belt, interpreted to have been constructed by several magmatic episodes (Devonian, Permian, Triassic, and Late Tertiary), could be a good candidate for the source of detrital chromian spinels in the Nam Duk Formation.