JOURNAL OF SEDIMENTARY RESEARCH,VOL. 72, NO. 6, NOVEMBER, 2002, P. 884–897 Copyright  2002, SEPM (Society for Sedimentary Geology) 1527-1404/02/072-884/$03.00 EVOLUTION OF THE COASTAL DEPOSITIONAL SYSTEMS OF THE CHANGJIANG (YANGTZE) RIVER IN RESPONSE TO LATE PLEISTOCENE–HOLOCENE SEA-LEVEL CHANGES KAZUAKI HORI 1 , YOSHIKI SAITO 2 , QUANHONG ZHAO 3 , AND PINXIAN WANG 3 1 Japan Society for the Promotion of Science, c/o MRE, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8567, Japan 2 MRE, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan 3 Laboratory of Marine Geology, Tongji University, Shanghai, 200092 P.R. China e-mail: k-hori@aist.go.jp or yoshiki.saito@aist.go.jp ABSTRACT: The paleo-Changjiang (Yangtze) incised-valley fills, ap- proximately 80–90 m thick, provide an opportunity to document the evolution of coastal depositional systems with large sediment supply in response to late Pleistocene–Holocene sea-level fluctuations on time scales of 10 3 to 10 4 years. The sedimentary facies of the incised-valley fills record three main depositional systems: fluvial, tide-dominated es- tuary, and tide-dominated delta. Radiocarbon ages for the incised-val- ley fills suggest that these depositional systems developed before about 11 ka, between 11 and 8 ka, and after approximately 8 ka, respectively. By applying sequence-stratigraphic concepts, the evolution of the de- positional systems can be divided into three systems tracts—a lowstand systems tract (LST), a transgressive systems tract (TST), and a high- stand systems tract (HST). Sea-level changes on a 10 4 -year time scale controlled the basic architecture of the sequence of the incised-valley fill. On the other hand, sea-level changes on a millennial time scale af- fected the stacking pattern of the systems tracts. In particular, the continuous sea-level rise with episodic rapid rises during the last de- glaciation affected the stacking pattern of the TST, which is charac- terized by a combination of aggradation and backstepping. The aggra- dation of fluvial and estuarine systems was dominant and the shoreline migrated only gradually landward under the relatively slow rise in sea level, and a very rapid sea-level rise around 12 and 10 ka caused the system to migrate abruptly landward. Unlike the transgressive estuarine phase, the stacking pattern of the regressive tide-dominated delta (HST), which developed within the al- most filled incised valley and on the surrounding interfluve zones, was characterized by seaward progradation with clinoform architecture. It was initiated with aggradational and progradational stacking about 8 ka during the last phase of decelerated sea-level rise, and was followed by a progradational phase after the highest sea level about 6 ka. INTRODUCTION Coastal depositional systems evolve actively through transgressive–re- gressive cycles, which are controlled principally by relative sea-level changes and by sediment supply (Curray 1964; Vail et al. 1977; Boyd et al. 1992). The application of sequence-stratigraphic concepts provides a schematic, qualitative model for strata formation in response to sea-level changes (Vail 1987; Van Wagoner et al. 1988). This model has been widely applied to incised-valley fills in response to the sea-level change since the Last Glacial Maximum (LGM), and shows that transgressive systems tracts constitute major parts of the incised-valley fills (e.g., Allen and Posamentier 1993; Dalrymple and Zaitlin 1994; Nichol et al. 1994, 1996; Roy 1994; Saito 1995; Zhang and Li 1996; Heap and Nichol 1997; Lessa et al. 1998). Proposed valley-fill models appear to indicate that rates of Holocene sea- level rise greatly exceed rates of sediment supply (e.g., Allen and Posa- mentier 1993; Thomas and Anderson 1994). Some studies show that the transgressive systems tracts of such incised- valley fills consist of parasequences (Thomas and Anderson 1994; Nichol et al. 1996). These parasequences are regarded to be the result of periods of sea-level standstill during a rapid rise of sea level, and results of ‘‘zig- zag’’ changes of the shoreline. Each parasequence formed during the period of standstill, and its boundary (i.e., marine flooding surface) is related to a rapid rise of sea level. Therefore, the bulk of incised-valley fills are formed of regressive sediments, with transgressive periods only episodically ex- pressed. The relationship between high-frequency sea-level fluctuations on a millennial time scale and sediment facies, depositional systems, and their stacking is one of the key issues in the improved comprehension of par- asequence formation in coastal environments. The Changjiang (Yangtze) River, one of the world’s largest rivers in terms of sediment load, has filled the incised valley with 80- to 90-m-thick deposits since the Last Glacial Maximum (LGM) (Li et al. 2000). This thick and widely distributed incised-valley fill makes it possible to carry out high-resolution analyses to investigate the detailed responses to high- frequency sea-level changes of coastal depositional systems developed in the river-mouth area, particularly during rise of sea level after the LGM. Moreover, the highest sea-level position above that of the present occurred about 6 ka in the study area, and since then sea level has gradually fallen to its present level. This high sea level is due to the influence of hydro- isostasy (Pirazzoli 1996). Therefore, the sea-level changes in the study area allow us to analyze a more complete succession consisting of two obvious phases: sea-level rise and stable/falling (highstand) sea level. The purpose of this study is to discuss the evolution of the paleo-Chang- jiang incised-valley systems in response to the sea-level changes on time scales of 10 3 to 10 4 years since the LGM, and to develop a sequence stratigraphic model of this incised-valley fill. This study is an extension of our previous work, which described the sedimentary facies and architecture of the transgressive, tide-dominated paleo-Changjiang estuary system (Hori et al. 2001b) and the regressive, tide-dominated Changjiang delta system (Hori et al. 2001a; Hori et al. 2002). REGIONAL SETTING The Changjiang River The Changjiang River has a catchment area of approximately 1.8  10 6 km 2 (Fig. 1A). The river originates on the Tibetan Plateau and empties into the East China Sea near Shanghai with a fall height of about 6800 m. The river discharges 980 km 3 /yr of water (Milliman and Meade 1983). Approximately 70% of the annual discharge occurs during the flood season (May to October). Peak discharge reaches 4–5  10 4 m 3 /s in the wet summer months, and minimum discharge of approximately 1  10 4 m 3 /s occurs in the dry winter months (Li et al. 1983; Chen 1998). The mean annual suspended sediment load of the river is 4.8  10 8 t/yr (Milliman and Meade 1983). The sediment discharge during the flood season consti- tutes about 87% of the annual sediment load. Most of the suspended sed- iments are composed of silt and clay. Oceanography Mean tidal range near the river mouth is approximately 2.7 m, and the maximum tidal range approaches 4.6 m (Shen et al. 1988). The mean and maximum wave heights within the river mouth are 0.9 and 6.2 m, respec- tively (Zhu et al. 1988). The waves are primarily wind-driven waves and, secondarily, swell-driven waves. The wind-driven waves approach the coast