Near-Bottom Sedimentation Processes in the Taiwan Strait and Offshore Southwestern Taiwan from Echo Character Studies

Chirp sonar profile data have been analyzed and compiled in this study to map the echo character distribution of the sea floor sediments in the Taiwan Strait and in the area offshore Southwestern Taiwan. These chirp sonar profile data were collected from 12 cruises of the Ocean Researcher 1 and Ocean Researcher 2 from 2002 to 2007, with total lengths of over 2,000 km in the Taiwan Strait and over 11,000 km in offshore Southwestern Taiwan area. By integrating the echo character studies with sea floor topography and results of previously published seismic and coring studies, sea floor sedimentary processes and possible sediment transport mechanisms in the study area can be established. ased on the different echo characters, three types of echo character patterns (flat, mound and irregular) and seven sub-types have been recognized in the Taiwan Strait. In general, those patterns are distributed in several NE-SW trending long and narrow zones, parallel to the M2 axis of the local tidal current ellipses, suggesting that sediment transport in the Taiwan Strait is strongly influenced by the tidal current. Offshore southwestern Taiwan, four types of echo character patterns (distinct, indistinct, hyperbolic and irregular) and eight sub-types have been identified. The echo character distribution patterns reveal that there are district sedimentary processes in the upper and lower continental slopes of the passive South China Sea continental margin, and in the active orogenic margin of the Kaoping slope area. The upper continental slope of the South China Sea continental margin presents hyperbolic echo characters which suggests that the submarine landslides is the dominative processes here, while most chirp profiles show distinct echo characters in the lower South China Sea continental slope, implies a more stable depositional environment. On the contrary, the distinct echo character patterns are prevailing in the upper Kaoping Slope, reflecting a depositional environment here due to the huge influx of the orogenic sediments from Taiwan. In the lower Kaoping Slope, sea floor echo characters appear to be most hyperbolic, suggesting the sedimentary environment is less depositional and many submarine landslides occurred here.n the Taiwan Strait, the chirp sonar data show that a mud belt extending northward from the north end of the Penghu Channel along the western flank of the Yun-Chang Ridge for over 100 km. The westward dipping reflectors and acoustic blanking caused by shallow gas in the chirp sonar profiles suggest that the mud should come from Taiwan. In the northern Taiwan Strait, the rugged sea-floor in the Kuayin Depression and the presence of a 20-m thick sand ridge suggests that there is a tidal deposition system here which plays a key role in transporting fine grain sediments from the Taiwan Strait eastward to the Okinawa Trough.n the passive South China Sea continental margin near Taiwan, retrogressive failures have been observed. There are many shear planes on the sub-bottom profiles where the submarine slides occurred and the sediment layers near the shear plane were stretched, bended and thinned. We suggest that the retrogressive failure processes are mainly caused by the loading of the Taiwan mountain belt as this process is well developed on the South China Sea continental slope near Taiwan while is less developed westward along the continental margin. n the active margin of the Kaoping Slope, folds and thrusts are the dominative structure features which form a series of submarine ridges and intra-slope basins. In the upper Kaoping Slope region, the “fill and spill” processes are observed, and echo character patterns also support this process here. n summary, this study has demonstrated that sea floor echo characters can be used to identify the sediment distribution patterns and to reveal the sedimentary processes involved. In the northern Taiwan Strait, the existence of a tidal deposition system, named the “Kuayin Depression and North-Taiwan Sand Ridge system”, has been suggested which is the driving force for transporting fine grain sediments from Taiwan Strait to deep sea in a short period. In the passive continental margin offshore Southwestern Taiwan, sea floor erosion caused by submarine slides is widespread on the upper South China Sea continental slope, while the lower South China Sea continental slope is a stable depositional environment. In contrast, huge amount of sediments are deposited on the upper Kaoping Slope, while rugged sea floor topography and submarine landslides are the main features in the lower Kaoping Slope.