Cyclic steps along the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea

摘要

Large-scale step-like features within the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea are investigated by integrating high-resolution multibeam bathymetric data and multichannel seismic profi les. These step-like features, ranging from 1.2 to 10.0 km in wavelength and 5.4-80.9 m in wave height, are mostly interpreted as cyclic steps formed by turbidity currents flowing through the canyons, based on their characteristic step-like morphology, in-train alignment, large wavelengths and aspect ratios (ratio of wavelength to wave height), and typical upstream-sloping backset bedding, among others. A train of 19 continuous steps delineated along the thalweg of the South Taiwan Shoal canyon measures up to 100 km and may be the longest ever reported. Nine short trains of scours identified on a terrace of the South Taiwan Shoal canyon are oriented parallel to the distributaries draining over the terrace and roughly perpendicular to the main canyon thalweg, indicating a complicated flow pattern within the canyon valley. Two trains of scours separated by an intracanyon high in the steeper middle reach of the West Penghu canyon are interpreted as transitional bed forms between antidunes and cyclic steps, which develop downstream into a train of five net-depositional cyclic steps with typical backset bedding in the gentler-sloping lower reach of the canyon. Average slope gradients for the canyon reaches with cyclic steps range from 0.26 degrees to 1.24 degrees. Along each thalweg step train, a slope break is identified to separate the neterosional cyclic steps in the steeper upstream segment from the net-depositional ones in the gentler downstream segment. Rough estimations indicate that the paleoflows are 100 to 300 m thick with maximum velocities of up to 10 m s(-1). The estimated flow depths match well with those inferred from geomorphologic analysis. Estimated paleodischarges of similar to 7-23 x 10(5) m(3) s(-1) are equivalent to ten times the discharge of the modern Amazon River.