Depositional characteristics of the paleo-gulf off northeastern Taiwan since the Last Glacial Maximum

Sediment types, grain size parameters and fragmental minerals of the 356 surface sediment samples from the paleo-gulf located off the northeastern Taiwan and surrounding shelves are studied for dynamic environments and origins. The evolution of the Holocene sedimentary dynamic environments of the gulf could be clearly divided into three phases. During the last glacial maximum (LGM), turbidity currents and landslides dominated the edge of the paleo-gulf (B-3). A great amount of terrigenous sediments rapidly poured into the Gulf. Seabed and underlying strata were eroded and two gravel belts formed as turbidity channels, and left behind poorly-sorted and inadequately reworked sediments. Tides, waves and cross-shelf currents, especially strengthened by the East Asian Winter Monsoon (EAWM), severely washed and eroded the edge and mouthof the B-3, where formed the sediments with high contents of sand and quartz, quartz/feldspar ratios, and little biodetritus; During the last deglaciation, owing to the periodic sea-level rises and significant coast retreat, turbidity currents weakened, and tidal currents became the major dynamic force on the shelf. Tidal bottom stress reworked the shelf sediments. Differences in sediment compositions, types, contents of quartz, feldspar, lithic fragments and biodetritus, quartz/feldspar ratios between(B-2)the western shelf pebbly-gravelly sand area (B-1) and the northern shelf pebbly sand-pebbly muddy sand area (B-2) reflect the difference in sea bottom topography which control the strength of tides and waves in addition to the speed of horizontal transgression; In the subsequent highstand period, the influence of Kurosio Current (KC) in the study area was increased. In addition to seafloor erosion, its bottom current also hindered the deposition of modern terrestrial sediments, and thus left behind coarse sediments in B. The deposition of fine-grained sediments was partly attributed to the weakening of the bottom current in A as well as local tides and waves. For the origin of the paleo-gulf, turbidity currents of the LGM certainly played an important role. They eroded the bottom deposits and strata, deepened the depressed geomorphology under the influence of the previous NW-trending fault activity; Moreover, strong hydrodynamics such as tide-wave system, the KC of subsequent periods prevented fine-grained sediments deposition in the B-3, and finally retained its depressed geomorphology.