Morphology and sedimentation of sand bodies in the tidal shelf sea of eastern Taiwan Strait

摘要

Changyun Ridge, located in the central-eastern part of the Taiwan Strait, consists of sand bodies: the eastern Changyun Sand Ridge (65 km long and 13 km wide), a sand shoal (53 km long and 22 km wide) and a sand sheet (60 km long, 40 km wide and 22 m in height). The eastern Changyun Sand Ridge and the sand sheet are oriented parallel to the coastline, but the sand shoal is perpendicular to the coast. The eastern Changyun Sand Ridge is oriented oblique to the narrow reciprocal tidal ellipses. The sand shoal and sand sheet are normal and oblique to the broad rotary tidal ellipses, respectively. Surface sediments of the Changyun Ridge are composed of very fine-to-coarse sands in a progressive seaward decrease in grain size. The configuration and spatial distribution of these three sand bodies with corresponding decrease in tidal speed and in grain size together suggest that the sand bodies developed sequentially from the near-shore to the offshore area, presumably in response to westward shifting of the north-flowing tidal currents. Sand waves occur widespreadly on the sand shoal and seldom appear on the eastern Changyun Sand Ridge. Occurrences of sand waves suggest that the sand shoal is considered to be active and the sand ridge to be moribund. Chirp sonar profiles reveal the internal structures of the sand shoal, showing a relatively flat basal reflector underlying the sand body. The thickness of the sand accumulation is about 22-25 m and it is firstly determined by the depth difference between the modern sea bed and the basal reflector to better describe morphology and document dimensions of these sand bodies. The presence of several short inclined reflectors dipping northeast and down-lapping the basal flat reflector indicate that sands are most likely transported from south to north for the sand shoal. The sand bodies of the Changyun Ridge rest upon a relative flat basal reflector, suggesting a mainly hydrodynamically controlled origin. The variations of the hydrodynamic conditions (mainly tidal currents) on the central-eastern Taiwan Strait shelf seem to be the primary control on sedimentary facies shift, morphological changes and sand ridge growth for the Changyun Ridge. The Changyun Ridge is ongoing to receive sands from the south in a transitional development stage. As long as the sand supply continues, and over a longer time period, the sand bodies eventually will develop into three individual linear sand ridges separated by swales oriented parallel to the peak currents.