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This study focuses on sedimentary research at the subaqueous Changjiang (Yangtze River) delta, based on five high-resolution seismic profiles and seven borehole cores with accurate AMS 14 C datings. Three distinct seismic units were identified from the seismic profiles according to seismic reflection characteristics, and five sedimentary facies were recognized from borehole cores. These facies constituted a fining upward sedimentary sequence in relation to postglacial sea-level transgression. Three sequence surfaces (sequence boundary (SB), transgressive surface (TS), and maximum flooding surface (MFS)) demarcate the boundaries between early transgressive system tract (E-TST), late transgressive system tract (L-TST), early highstand system tract (E-HST) and late highstand system tract (L-HST), which constitute the sixth order sequence. These system tracts were developed coevally with postglacial sea-level rise. E-TST (~ 19–12 ka BP) corresponds to an incised-valley infilling in the early stages of postglacial transgression whereas L-TST (~ 12–7.5 ka BP) was formed during the last stage of postglacial transgression. The progradational structure of L-TST reflected in seismic profiles is possibly related to the intensification of the East Asian summer monsoon. E-HST (~ 7.5–2 ka BP) was deposited in response to the highstand after maximum postglacial transgression was reached, while L-HST (~ 2 ka BP–present) was initiated by accelerated progradation of the Changjiang delta. [ABSTRACT FROM AUTHOR]

This study focuses on sedimentary environmental changes offshore of Hangzhou Bay, East China, since the Late Quaternary. AMS C ages from core CJK10, lithologies, distribution of foraminifera, heavy minerals, and S and Cl elements show a fluvial terrace environment during ∼23.2-11.0 cal ka BP; a littoral to tidal-flat environment during 11.0-10.2 cal ka BP; and a shallow marine environment with a relatively low sedimentation rate (0.1-0.22 cm/a) since 4.3 cal ka BP. High depositional rates (∼1.6 cm/a) from 10.9 to 10.2 cal ka BP resulted from sufficient accommodation space created by rapid sea level rise from −44 m to −33 m, from high sediment delivery by local rivers, and effective trapping of sediments by tidal-flat vegetation. The rate of sea level rise was variable; relatively high from 10.9 to 10.6 cal ka BP (2.1 cm/a), and lower since 10.6 cal ka BP (1.2 cm/a). The Changjiang alongshore current crossed the Hangzhou Bay to form the mud wedge on the inner shelf of the East China Sea later than 9.4 cal ka BP. The CJK10 site was a tide-dominated shelf environment and experienced erosion from approximately 9.4-9.2 cal ka BP to 4.3 cal ka BP. The depositional hiatus was caused by the Changjiang alongshore current, which was relatively weak during 9.4-7.5 cal ka BP and increased in strength during ∼7.5-4 cal ka BP. From ∼4.3 cal ka BP, a large amount of sediment from the Changjiang River was partly deposited on the continental shelf of Hangzhou Bay with some transported southward. Therefore, this study clarifies the history of Changjiang-derived sediment dispersal and deposition, although a detailed record of the changes in the Changjiang alongshore current since 4.3 cal ka BP is difficult to obtain because of the scarcity of evidence. [ABSTRACT FROM AUTHOR]

The sedimentary facies of the subaqueous Changjiang (Yangtze) River delta since the late Pleistocene was studied based on lithology and foraminifera analysis for two boreholes, CJK07 and CJK11, along with C dating. Four sedimentary facies were identified, namely fluvial, tidal flat, offshore, and prodelta facies. The fluvial sedimentary facies is comprised of fluvial channel lag deposits, fluvial point bar deposits, and floodplain deposits, showing a fining-upward sequence in general with no benthic foraminifera. A layer of stiff clay overlies the fluvial deposits in core CJK07, indicating a long-term exposure environment during the Last Glacial Maximum (LGM). During the postglacial sea-level rise around 13-7.5 cal ka BP, the tidal flat facies was deposited in core CJK11, characterized by abundant silt-clay couplets. Euryhaline species dominate the subtidal flat foraminiferal assemblages, while almost no foraminifera was found in the intertidal flat. The offshore environment was the major sedimentary environment when the sea level reached its highest level around 7.5 cal ka BP, with a maximum accumulation rate of 10 mm/a found in core CJK11. Prodelta sediments have been deposited in core CJK11 since ∼3 cal ka BP, after the formation of the Changjiang River delta. The difference in sedimentary facies between core CJK07 and CJK11 is due to their location: core CJK07 was in an interfluve while core CJK11 was in an incised valley during the LGM. Furthermore, AMS 14 C dating of core CJK07 shows poor chronological order, indicating that the sediments were reworked by strong tidal currents and that sediment deposited since ∼7.7 cal ka BP in core CJK07 was eroded away by modern hydrodynamic forces caused by the southward shift of the Changjiang River delta depocenter. [ABSTRACT FROM AUTHOR]

Seismic profiles integrated with borehole cores from offshore of the Yangtze River Delta characterize three distinctive Holocene deposition units in correspondence to estuarine, shallow marine and prodelta facies respectively (Units Ⅰ to Ⅲ). Sedimentary processes and evolution of three units in response to sea-level rise and Yangtze delta development was studied. Estuarine facies occurred as a cluster of mud ridges (unit Ⅰ) was considered to be formed during the later stage of the period ∼10-8 cal ka BP with the prevalence of tidal erosion processes. At the same time however, the area south of the ridge field experienced rapid deposition, possibly due to shelter effects of the Zhoushan Archipelago. Unit Ⅱ, characterized by homogeneous mud composition, rapid accumulation rates and a seaward dipping structure, indicated a gravity flow deposition most likely related to the wave and/or tidal current induced fluid mudflows during the period of ∼ 8-5 cal ka BP. Intrusion of a mud diapir during ∼5-3 cal ka BP related to gravity subsidence localized near the river mouth, lead to deformation of unit Ⅱ. During the ∼3 ka to present, the delta advanced to the study area, depositing subaqueous clinoform (unit Ⅲ). Meanwhile, erosional processes dominated by tidal currents were promoted by the southeastward progradation of the delta. Sediments deposited in the north of the area were transformed into tidal sand ridges. We predict that the delta will likely reduce in area with a further progradation in the recent future. [ABSTRACT FROM AUTHOR]