Echo characters and sedimentary processes along a rifting continental margin, northeast of Taiwan

Abstract

The northeastern offshore of Taiwan, including the southern-most East China Sea continental margin, Ilan Shelf, Ilan Ridge and the western tip of the Okinawa Trough, is characterized by active rifting and an energetically complex hydrodynamic flow regime. In this study, sedimentary processes on the sea floor were inferred from regional mapping of 3.5 kHz echo characters. Eight distinct echo types were mapped, and based on echo type distribution, analysis of sediments and regional bathymetry, these were interpreted as deposits that had been formed under the influence of various local hydrodynamic processes. Different sedimentary processes, interpreted from the lithology and distribution pattern of sediments, were found to prevail on different physiographic provinces. In the southern East China Sea continental shelf margin, it is the outflow of Taiwan Strait Water and the on-shelf intrusion, upwelling and countercurrent induced by the impinging and turning of the Kuroshio Current that largely determine the distribution of sediments. On the narrow Ilan continental shelf, the deposition is mainly influenced by subaqueous deltaic and shallow marine processes. Over the rifting tip of the Okinawa Trough, including the Okinawa Trough Basin and its nearby slopes, the primary seafloor-shaping agents have been the mass-wasting processes and turbidity currents. Since the observed sediment data is in good consistency with other hydrographic data, the studies of transportation and deposition patterns of sediment can provide good constraints for the interpretation of physical oceanographic data.