Anatomy of a transgressive sediment-rich tide-dominated estuary: the paleo-Yangtze incised valley, China

Transgressive paleo-valley fill is crucial for understanding the coastal evolution under the sea-level rise. The post-glacial Yangtze valley-fill deposit provides an ideal place to study the geomorphological evolution and facies model of tide-dominated estuaries with abundant sediment supply. Utilizing the multi-proxy analysis (lithology, grain size, XRF core scanning, AMS ¹⁴C) of the four new cores, YD0901, YD0902, YD0903, and CX03, and accompanying collected borehole data, this paper systematically evaluated the morphological evolution and facies architecture of the paleo-Yangtze incised valley-fill deposits during the later Pleistocene to early Holocene. The paleo-Yangtze incised valley went through fluvial (before 14.6 ka), tidal river (14.6-13 ka), tide-dominated estuary (13-8 ka), and erosional shelf (after 8 ka), sequentially. The sedimentary facies recovered from cores reveal a tide-dominated estuary model, with a finning-seaward grain-size trend. The finning-upward tidal sand bars, shifting landward continuously during transgression (12-6.6 ka), are the unique feature constructed within the tide-dominated estuary, the result of the enormous sediment and freshwater supply pushing the bedload convergence zone overlapped with the turbidity maximum zone. The hypothesis that the Tidal sand bars (TSBs) were the predecessor for the shelf tidal sand ridges is suggested on account of the sedimentary features. The sediment-rich tide-dominated estuary migrated landward with the sea-level transgression, leaving the TSBs abandoned on the shelf. About 10 ka, the abandoned TSB deposits were resuspension and redeposition by the tide, prompting the construction of the tidal sand ridges. The fine content was redeposited at the early river mouth or the shelf mud patches, even Okinawa Trough.