Morphology, Hydrodynamics and Sediment Characteristics of the Changyun Sand Ridge offshore Western Taiwan

Abstract

The Changyun Ridge, a shelf sand ridge located off the western coast of Taiwan, is a tide-dominated sand body. This ridge is defined by the 50-m isobath and consists of two smaller ridges. The eastern Changyun Ridge is about 65 km long and 15 km wide and approximately parallel to the western shoreline of Taiwan. The western Changyun Ridge has a length of 53 km and a maximum width of 26 km, trending northwest-southeast normal to the Taiwan western coast. The eastern ridge has morphologic characteristics similar to those of typical linear sand ridges. Tidal currents show distinct bidirectional patterns with speeds exceeding 100 cm s^(-1) strong enough to move sandy sediments. Large to very large sand waves occur on the western Changyun Ridge and sand waves seldom appear on the eastern Changyun Ridge, implying that the former is presently active while the latter becomes inactive or moribund. The configuration and distribution of these two ridges further suggest that the ridges developed sequentially from near-shore to offshore, presumably in response to a westward progressive decrease in velocity and change of flow direction of the north-flowing tidal currents. Following the transgression beginning about 15000 years ago, the sea waters from the South China Sea began to flow over the sea floor between the Penghu Islands and Taiwan and the paleo-tidal currents finally excavated the sea bed into a channel now known as the Penghu Channel. Sands and mud are winnowed out but gravel and shell fragments have lagged behind in the Penghu Channel and only medium to fine-grained sands and mud carried by the strong tidal currents to the Changyun Ridge and beyond. Mainly medium to coarse-grained sands have accumulated on the Changyun Ridge to form sand banks. At the northern outlet of the Penghu Channel, the shoaling topography of the Changyun Ridge has caused a decrease in tidal current velocity and the subsequent deposition of sands, forming the present Changyun sand ridge. The formation of the eastern and western Changyun sand ridges is probably affected mostly by the tidal current patterns in the areas north of the Penghu Channel.