Near-Bottom Sedimentation Offshore Southwestern Taiwan from
Echo Character Study
Jui-kun Chiu, Char-Shine Liu* (Institute of Oceanography, National Taiwan
University)
(*Contact email address: csliu@ntu.edu.tw)
Abstract
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 area offshore
Southwestern Taiwan. These data were collected from 7 cruises of the Ocean
Researcher 1 from 2002 to 2007 in offshore Southwestern Taiwan area, with total
lengths of over 11,000 km. 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.
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 different 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 clear reflective echo characters in the
lower South China Sea continental slope, implies a more stable depositional
environment. On the contrary, the clear reflective 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 mostly hyperbolic, suggesting the sedimentary
environment is less depositional and many submarine landslides occurred here.
In addition, retrogressive failures have been observed in the passive South China
Sea continental margin near Taiwan. There are many shear planes observed on the
chirp sonar 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 passive continental margin.