The fossil mollusc faunas: their bearing on the Holocene evolution of the Lower Central Plain of Bangkok (Thailand)

For the first time, fossil molluscan assemblages are analyzed in order to reconstruct the evolution of the Northern Gulf of Thailand area during Holocene. The marine sediments (belonging to Bangkok Clay Formation) of the Lower Central Plain of Bangkok and the coastal plain of Phetchaburi were sampled in 16 localities, where the Holocene sequence was at least partly exposed. The samples yielded fossil shells whose 14C ages have proved to range from about 9000 to 2000 years BP. A statistical treatment of abundance data returned four major groups, namely the Dendostrea rosacea association (intertidal mud), the Corbula fortisulcata - Mactra luzonica association (shallow infralittoral sandy mud), the Nuculana mauritiana - Timoclea scabra association (shallow infralittoral sand spit) and the Timoclea scabra - Arcopagia pudica association (infralittoral mud); two of them, i.e. C. fortisulcata - M. luzonica and N. mauritiana - T. scabra associations, were furtherly analyzed in terms of subfacies, each accounting for specific environmental conditions. These data allowed both a stratigraphic correlation along N-S and W-E transects covering all the Holocene basin and the creation of a series of digitalized maps showing the evolution of the Thai paleogulf from about 9000 to 4000 years BP. The results show that the Flandrian transgression event brought on Thai plains a shallow sea, not exceeding 10m in depth; the coastline was marked by wide tidal flats, occupied in the upper part by mangrove forests. The northern limit of clearly marine conditions seems to place well south of the ancient capital Ayutthaya, which therefore could have been a fluvial harbor on the lower course of Chao Phraya River rather than a marine one at some some time during its history.