Taiwan Strait: An ocean‐current‐dominated shallow‐water setting with a river‐fed detached muddy contourite deposit

ABSTRACT Contourites (contour‐current deposits) are commonly associated with deep‐water environments, but this study documents a shallow‐water, muddy contourite drift in the centre of Taiwan Strait. The drift body (water depths of 30 to 70 m, 220 km long, 50 to 70 km wide, 0 to 30 m thick), with its long axis approximately colinear with the Taiwan Strait Current, is flanked on both sides by moats. Compositional data show that the drift is composed of sediment carried northward by the Taiwan Strait Current from the Choshui River draining Taiwan. The coast‐normal Changyun Ridge directly offshore of the Choshui River splits the Taiwan Strait Current into two branches that occupy the moats. Oblique cross‐drift flow from west to east contributes to drift growth. The dispersal system shows a down‐current fining from sands on the Changyun Ridge ( ca 100 km long, 25 to 45 km wide) to the fine‐grained elongate drift; these together represent the asymmetrical subaqueous portion of the Choshui delta. Cores show that the drift is <8 kyr old, with sedimentation rates up to 7.8 m/ka. It coarsens upward during the initial phase of drift growth, perhaps due to an increase in current strength, with smaller‐scale textural bedding attributed to fluctuating current strengths or variations in wave energy depending on the location. Seismic data show that the southern end of the drift is migrating westward, due to elongation of the Changyun Ridge, whereas the middle and distal end are migrating eastward because of cross‐drift flow. Erosion by the eastern branch of the Taiwan Strait Current detached the drift from the coeval coastal mud belt. The influence of tidal currents is small because of the destructive interference of tidal waves entering from the two ends, creating weak rotary tidal currents over the drift. Wave energy is small within the Strait but larger waves arriving from the East China Sea are thought to be responsible for upward coarsening of the more exposed northern end. Taiwan Strait serves as a starting point for developing a facies model for ocean‐current‐dominated straits and seaways.