Evolution of the coastal depositional systems of the Changjiang (Yangtze) river in response to late pleistocene-holocene seal-level changes

Abstract

The paleo-Changjiang (Yangtze) incised-valley fills, approximately 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 estuary, and tide-dominated delta. Radiocarbon ages for the incised-valley 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 depositional 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 affected the stacking pattern of the systems tracts. In particular, the continuous sea-level rise with episodic rapid rises during the last deglaciation affected the stacking pattern of the TST, which is characterized by a combination of aggradation and backstepping. The aggradation 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 almost 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.