Depocenter shift and en-echelon shoal development in the pre-Holocene incised valley of the Yangtze Delta, China

This study establishes a Holocene sedimentation model for the incised valley of the Yangtze Delta based on key information taken from existing literature and new cores collected for this study. Distinctive sedimentary processes and mechanisms operating in the incised valley were revealed: 1) the significant role of a pre-Holocene incised valley on which the Holocene funnel-shaped bay was established in forming the topography of the present river mouth; 2) the backstepping migration of the depocenter in association with tidally-dominated sandy shoals developed in the Early-Middle Holocene, followed by fluvially-tidally dominated progradation during the Mid-Late Holocene; 3) the model of depocenter shift characterized by en-echelon sandy shoals in the incised valley, revealing stepwise migration from transgressive to retrogressive sedimentation in the Holocene, and 4) the dextral diversion of the river mouth controlled by strong tidal sedimentation in the valley and the Coriolis-driven tidal sedimentation off the valley throughout the Holocene. Our results further show that the sediment in the Holocene valley was characterized by an upward increase in foraminiferal abundance. This is a reversal of the usual pattern to the Holocene marine transgression, indicating a unique depositional setting in the valley, presumably due to higher turbidity and poor preservation caused by strong tidal currents.