Active, moribund and buried tidal sand ridges in the East China Sea and the Southern Yellow Sea

Abstract

Shallow seismic profiles and borehole data revealed active, moribund and buried tidal sand ridges in the East China Sea and the Southern Yellow Sea. The ridges consist of relatively clean, well-sorted sands with fairly uniform sequences. The microfossil assemblage exhibits a high diversity including both euryhaline and stenohaline species. The ridges are huge elongated sand bodies (several kilometres wide, 10–60 km long and up to 20 m thick) and occur in groups with a spacing of several to >10 km. Normally, they are parallel with each other, extending in the direction of the tidal currents. The field of tidal sand ridges on the East China Sea shelf covers an area of 57,000 km 2. Tidal sand ridges in the East China Sea and the Southern Yellow Sea were formed in estuarine and shallow shelf environments, where strong tidal currents reworked, transported and redeposited large amounts of relict sands from older deltaic and fluvial sediments. The development of tidal sand ridges in estuary-mouth areas is accompanied by a net sand transport from the shelf into the estuary. The evolution of tidal sand ridges is closely related to transgressions. Active tidal sand ridges are formed during sea-level rises. They show distinct morphology and active migrations in a lateral as well as in a landward direction. They are generally in equilibrium with present-day tidal processes and their distribution patterns are related to the flow field of the tidal currents. As the sea level rises further, these tidal sand ridges become moribund sand bodies with less distinct morphologies. Their upper surfaces are covered by a thin calcareous-rich layer with very high contents of foraminifera tests, reflecting a submarine hiatus condition on the shelf during the highstand period. During the subsequent steady fall in sea level, tidal sand ridges are often buried by fine-grained shelf and prodelta sediments. They have, therefore, a good chance of being preserved in stratigraphic records. The sequential buildup during a sea level rise (TR-tract) and a subsequent sea level fall (HS- and LS-tract) would consist of estuarine channel and tidal flat facies, tidal sand ridge facies, shelf mud facies, delta facies and fluvial facies in an ascending order. Tidally dominated estuary conditions may also occur when a major distributary is abandoned, such as at the Northern Branch of the modern Changjiang River mouth, where the accumulation of sand from an abandoned delta lobe and the development of tidal sand ridges have been observed. The observed features have important implications for the subsurface recognition of fossil tidal sand ridges, for the reconstruction of the regional palaeogeography and transgressional history and for the prediction of potential reservoir sandstones.