Akio Ohira

Late Holocene Mangrove Habitat and Evolution of Coastal Lowlands in Southern Thailand.

Holocene landforms and sediments of the coastal lowlands around Satun and Songkhla Lake regions in southern Thailand are studied in relation to mangrove habitat. Landforms of the Satun lowland are net a simple geomorphic surface but the surfaces of Pleistocene and late Holocene. Present mangrove vegetation develops mainly en the Holecene surface in the southern part of Satun lowland. The area is almost the same as that covered with mangrove in the period of Holocene maximum transgression. Based on the fuceis and ages of the sediments in the Satun lowland, sedimentation rate of the lowland since 1000 yr BP is relatively rapid and the mangrove forest expanded rapidly in the period. On the other hand, most mangrove forest has already disappeared in the coastal lowland along Lake Songkhla. The thickness of the Holocene sediments is less than 5 meters in most. t places, and consists of peat layer and overlying muddy sediments. Based on the radiocarbon ages of the peat layers collected from the southern part of the coastal lowland, mangrove forest started to develop around 6700 yr BP and expanded rapidly in the region. This is because the surface of the Pleistocene sediments is. very flat and shallow. Seawater invaded and expanded in the region very quickly and formed a wide tidal plain along the coast of lagoon. This condition causes rapid expansion of mangrove forest along the Songkhla Lake. Based en these facts, it is clarified that the development of mangrove forests in the study areas is related to the evolution of the Holocene coastal landforms. and deposition of sediments in mid to late Holecence. Furthermore, landform evolution and sedimentation arc related to the landforms of the pre-Holocene surface.