An experimental mapping method by means of fossil mollusk faunas: The holocene thai paleogulf

In the present work, mollusk associations are used as a tool for the reconstruction of sea level changes in the Gulf of Thailand during the Holocene transgression event (Flandrian event). The study area comprises the Lower Central Plain of Bangkok and the Petchaburi coastal plain, where mollusk assemblages were sampled in several localities exposing the Holocene sedimentary successions. The detailed study of recovered material, described in a previous work currently inpress, led to the identification of molluscan associations, biofacies and single assemblages; each of these was attributed a restricted depth range consistent with the development of its mollusks. The 14C dating of shells allowed the separation of sampled sections into six intervals (timelines respectively at 9,000, 8,000, 6,000, 5,500, 5,000, and 4,000 years BP); each intersection between a timeline and a section was given a depth value within the range of the mollusk fauna sampled in that position. This resulted in a dataset of paleodepths for each time level and, subsequently, in the creation of six isobathymetric maps by interpolation. The maps, showing the evolution of Thai paleogulf from the first arrival of the sea near the present shoreline to the beginning of the regression, have a small scale validity only and are subject to uncertainties due to the approximations of the method; nevertheless, the reconstruction appears consistent with both paleogulf maps based on different data (Pleistocene-Holocene contact isopachs, pollen content analyses) and recent Quaternary geological surveys conducted on the Lower Central Plain.