Text S3. Lithostratigraphy and Facies Analysis
Lithostratigraphy is constructed based on an exposed profile (KhqS) at the Kuahuqiao archaeological site and two sediment cores, one from close to Kuahuqiao (KHQ-14) and one from Xiasun (XS-1603). Extensive shallow trenching up to the depths of 3–4 meters has been used as critical supplementary information to the profile and cores, particularly for the uppermost unit.
Coring was performed by professional scientific drilling company, with recovery rate being more than 95%. Cores were split by using Geo-Tec Core Splitter in the lab. Working-halves were described and sub-sampled for analyses. Archive-halves were photographed, XRF-scanned, and then stored in cold room. The KhqS profile was sampled continuously using sampling slots. The whole profile was treated in the same way as cores.
The principles of sedimentary facies analysis have been applied in the current study, particularly in the description of cores and profile. A sedimentary facies is a distinctive sediment unit that forms under certain conditions of sedimentation, reflecting a particular process or environment. The characteristics of the sediment unit come from the depositional environment and from the original composition. Facies analysis is the key to reconstruct sedimentary environments, which generally involves observation of lithology, colors, sedimentary structures, grain-size, mineralogy, digenesis, and fossil content.
The lithostratigraphy of the sediments across the three sample sites are sub-divided into four stratigraphic units (U1 to U4) based on facies analysis (Fig. 2).
U1 : A paleosol with thickness ranging from a meter to several meters, locally known as the First Hard Clay layer of Last Glacial age. It is widely distributed in coastal east China at depths ranging from 25 to 5 m below present-day OD. The lithology is stiff clay, with brownish to greenish grey color. Pedogenesis is evident, often exhibiting mottled texture, root structure and small carbonate nodules. This unit formed the paleo-surface of the regional landscape on which post-glacial transgression occurred.
U2 : Starting with rhythmitic mud and fine sand, typical of tidal flat deposition, which marks the onset of marine inundation in areas 25–30 m below the present OD. The upper part of U2 is composed of homogeneous, sticky, grey clay (Fig. S3a). Scattered foraminifera and plant debris can be observed. The upper U2 was deposited in a typical estuary.
U3 : Present at KhqS (-3.30 to -2.35 m OD) and XS-1603 (-2.50 to -2.25 m OD) is representative of the culture layer.
U4: The uppermost stratigraphic unit, present at all three sites. Extensive trenching has revealed its widespread presence all over the Qiantangjiang flood plain. It is yellowish-brownish, sandy silt, with relatively high (but varying) sand content and a comparatively high median grain size, with thickness ranging from 1 to 10 m. The unit overlies U2 and/or U3 unconformably. The most distinctive feature of U4 is its rhythmitic, but twisted, bedding structure (Fig. S3b, c, d & e). The contact of U4/U3 in KhqS and XS-1603, or U4/U2 in the case of KHQ-14, is sharp and erosional. Such sedimentary bedding and contacts are of deposition associated typically with turbulence, commonly occurring in flood deposits.