SEDIMENTARY CHARACTERISTICS AND EVOLUTIONARY HISTORY OF THE DISTAL PART OF THE TIDAL SAND RIDGES IN THE SOUTH YELLOW SEA, OFF THE JIANGSU COAST

The core SYS -0702 was recovered in the distal part of the tidal sand ridges in the South Yellow Sea, off the Jiangsu coast at a water depth of 32 m, and the uppermost 18.64 m of the core corresponds to the distal deposits of the sand ridges. Based on lithologic features, grain-size distribution pattern, AMS¹⁴C dating data and ²¹⁰Pb data of the core sediments, in combination with the high-resolution shallow seismic profile passing through the core site, this paper reveals the sedimentary characteristics of the distal sand ridges, formation time, and the influence of the old Yellow River on the sedimentation of the sand ridges when the river discharged into the South Yellow Sea from the Jiangsu coast during 1128 to 1855. The research results show that the distal sand ridges have been keeping active since about 2.5 cal.kaBP, and their internal reflections dip to the east and northeast downlapping onto the basal boundary of the sand ridges. The dip angles of the internal foresets are generally less than 0.2°, mostly convex in shape and tangent to the erosive surface of the sand ridges. The core section of the distal sand ridges roughly coarsens upward, and in the sandy sediments the saltation load is higher than 80% and the suspension load less than 20%, with a good sorting. During 1128-1855 when the old Yellow River entered the South Yellow Sea from the Jiangsu coast, sedimentation rates in the distal sand ridges increased significantly, from 0.28~0.77 cm/a before the river shifting up to 1.10 cm/a during the event. But the rate decreased to 0.23 cm/a after the old Yellow River going back to the Bohai sea in 1855. According to previous researches, the paleo-Yangtze river estuary and Dongtai were supposed to be the vertex of the radial tidal sand ridges in about 8 kaBP and 6 kaBP, respectively. The progradation of the sand ridges to the core site from the paleo-estuary during 8~2.5 kaBP and from Dongtai during 6~2.5 kaBP are about 61.7 m/a and 65.9 m/a, respectively. The progradation further towards the sea from the distal ridge, however, has reduced to about 4 m/a since about 2.5 cal.kaBP, which is considered to be caused by weakened tidal action and declined sediment supply in the deeper water.