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.