Facies architecture and depositional model of a macrotidal incised-valley succession (Qiantang River estuary, eastern China), and differences from other macrotidal systems.

Filling of the modern coastal-zone portion of the Qiantang River (QR) in eastern China was initiated by marine inundation during the transgression after the Last Glacial Maximum and has continued during the Holocene sea-level stillstand. The early part of the fi ll is transgressive in character, while the younger part is regressive. This paper deals with the sedimentary facies, surfaces, architecture, and depositional model of the QR incised-valley fi ll based on the detailed analysis of the newly drilled core SE2 and its correlation with more than 800 boreholes. The incised-valley deposits are grouped into fi ve stacked facies: amalgamated channel, floodplain and channel, paleo-estuary, offshore shallow marine, and present-day estuary. A paleo-estuary facies had never been identifi ed before, making these observations novel. This facies is characterized by a sedimentary succession and sediment distribution that are distinct from those of the present-day estuary because of a change in sediment supply from the sea fl oor beyond the estuary mouth. It also contains large numbers of tidal-channel sand bodies that are signifi cant reservoirs for shallow biogenic gas. Since the last glaciation, there have been three stages in the development of the QR incised-valley fi ll sequence: (1) a formation stage associated with sea-level fall, (2) a fi lling stage during the early transgression with rapid sea-level rise, and (3) a burial stage corresponding to the slowing of sea-level rise and the onset of progradation. This fill consists of four longitudinal segments, each of which is distinguished by a distinct stratigraphic succession and different degrees of marine and fluvial influence. The basal erosion surface and sidewalls of the incised valley, the top of stiff clay on the interfl uves, and the top of falling-stage fluvial-terrace deposits compose the sequence boundary. The initial flooding surface and maximum flooding surface are located with in the amalgamated channel and offshore shallow-marine sediments, respectively. These observations indicate that: (1) relative sea-level changes determine the stratal stacking patterns, but local environmental factors, such as physical processes, accommodation, sediment supply, and coastal confi guration control the nature of the facies, surfaces, and sediment-distribution patterns; and (2) the tripartite facies organization and wave ravinement surfaces typifying wavedominated or mixed-energy (wave and tide) incised-valley fi lls are absent in the macrotidal QR succession, and the tidal ravinement and erosion surfaces of the macrotidal QR incised valley are more extensive and numerous than those of the wave-dominated or mixed-energy succession. [ABSTRACT FROM AUTHOR]