Environment Evolution of the Western Coastal Plain in Taiwan Since the Last Glacial Maximum Period

In order to understand the depositional environment evolution and solve the impacts of eustacy, tectonic and sediment yield on western coastal plain of Taiwan since the last glacial maximum period, we use several borehores to reconstruct it. Based on the depositional environment, sequence stratigraphy and radiocarbon dating, significant environment and shoreline changes are recognized. (1) The western Taiwan is characterized by a subaerial environment and formed several incised valleys during the sea-level fall in the last glacial maximum period. (2) The environment experienced the sea-level rising with retrogradational deposition of the trangression system tracts and the shoreline was migrated into the incised valley and formed a drowned valley during 16,000~14,000 years ago. (3) The ongoing sea-level rise causes the valley filled and environment changed deepen into intertidal zone during 14,000~12,000 years ago. (4) Most areas occurred the deepest environment from shoreface to inner offshore during 8,500~8,000 years ago, and maximum flooding surface formed. But Tainan and Pingdong plain was experience the deepest environment earlier than 8,500~8,000 years ago. (5) After 8,500~8,000 years ago, the system occurred progradational deposition of the highstand system tracts. The sediments progradated to the seaward, hence, the environment was shallow from intertidal zone to fluvial environment and the shoreline was retreated and migrated to the westward to the recent appearance. The period of maximum flooding surface in different parts of the western plains are not the same. Based on the occurrence time of maximum flooding surface, it can be roughly divided into three types. Each type represents the complex interplay between eustacy, tectonics and sediment supply. In addition to the above three factors will control the depositional environment and shoreline changes, the ancient topography and slope will also affect the shoreline and environment evolution. Based on the vertical deformation rate rechecked in order to solve the impact on shoreline evolution during 6,000~4,000 years ago. The result shows the eastern part of coastal plain is uplift environment, causing shoreline move quickly into the western side of the subsidence coastal plain.