Holocene regression and the tidal radial sand ridge system formation in the Jiangsu coastal zone, east China

Abstract

Abstract A radial tidal sand ridge system (RTSRS) spans more than 20 000 km 2 in the southwest Yellow Sea, is fan-shaped and stretches from Qianggang toward the sea. The marine RTSRS is located in a convergent–divergent tidal current field (CDTCF). Numerous drilling holes reveal an area of 3000 km 2 containing tidal sand strata in the Jiangsu coastal plain on land, adjacent to the marine RTSRS, which stretches eastward from Chengdong and is also a fan-shaped with a central angle of 130°. The tidal sand unit lies on a transgressive coastal barrier-lagoon unit with an erosional surface between them, and the latter unit is in turn lying on late Pleistocene stiff mud. The tidal sand unit is overlain by the littoral–tidal-flat unit and together they constitute the Holocene regressive succession. Paleo-current study reveals that a paleo-current pattern similar to the modern marine CDTCF was present in the Holocene sub-aerial tidal sand strata. Therefore, the sub-aerial tidal sand bodies are deduced to be an ancient RTSRS that was formed during the Holocene regression, and gradually exposed and changed into land as the Changjiang and Huanghe deltas prograded. Both sub-aerial and marine RTSRS should be formed in the same tidal depositional environment but in different development stages. Thus, the radial tidal sand ridges in southwestern Yellow Sea should have formed not during the Holocene transgression, as most previous studies have inferred, but in the process of Holocene regression. Mineralogical analyses demonstrate that the sediment source of the sub-aerial RTSRS initially came from the Changjiang and later predominantly from the Huanghe. Sea-floor erosion by tidal currents may also provide sandy material to form the RTSRS. Drilling cores and ground-penetrating profiles show that the paleo-valleys in the study area are much smaller than the present distributaries of the Changjiang River. Thus, the presumption of direct sand supply by a large river to form the RTSRS through its apical area is cannot be supported. After the sea level reached the present high stand, the Jiangsu coastline has been prograding seaward as the Changjiang Delta expanded and this progradation was accelerated when the Huanghe debouched in the study area. The nearshore part of RTSRS has been gradually changing into land while the entire RTSRS extends seawards. In the process of the development, the RTSRS apex changed from Chengdong to Qianggang. At present, the coastline is still moving seaward while the outer part of the RTSRS suffers from some erosion owing to reduction of sediment supply from both the Changjiang Delta and the abandoned Huanghe Delta.