Sedimentary evolution of the Holocene subaqueous clinoform off the Shandong Peninsula in the Yellow Sea
Introduction
The Yellow Sea lies on a shallow, semi-enclosed, low-gradient shelf between the Chinese mainland and the Korean Peninsula (Fig. 1). It connects with the Bohai Sea to the northwest and the East China Sea to the south, and together the three seas form a vast marginal sea in the northwestern Pacific. A prominent subaqueous clinoform with a maximum thickness of 30 m was first noted in the 1980s to extend southward around the eastern tip of the Shandong Peninsula in the western Yellow Sea (Milliman et al., 1987), some 350 km east of the modern Yellow River mouth, which is on the western coast of the Bohai Sea. Milliman et al., 1987, Milliman et al., 1989 suggested that it is located along the escape route via which the Yellow River sediment has been transported from the Bohai Sea to the southern Yellow Sea during the last 5000 yr. Alexander et al. (1991) called the clinoform the Shandong subaqueous delta, on the basis of its similarity to the subaqueous delta system offshore of the mouth of the Amazon River, and speculated that it had formed between 6200 and 4060 yr BP, based on 14C dates of samples from a vibrocore with a total length of less than 3 m. More recent work has shown that the clinoform is also distributed along the northern side of the Shandong Peninsula (Cheng et al., 2001, Liu et al., 2002, Liu et al., 2004), and, based on high-resolution seismic profiles, Liu et al., 2002, Liu et al., 2004 identified proximal and distal phases of clinoform development in terms of the distance between the Yellow River mouth and the clinoform during the postglacial transgression. Moreover, they proposed that the underlying proximal and overlying distal sequences were formed between ∼ 11 and 9.2 cal kyr BP and after 9.2 cal kyr BP, respectively, in accordance with a presumed stepwise, postglacial sea-level rise. This debate on the origin and evolution of the subaqueous clinoform around the Shandong Peninsula arose largely because of the lack of deep boreholes through the clinoform.
During our geological survey conducted in the Yellow Sea in 2003, two boreholes up to 70 m long were drilled in the subaqueous clinoform around the Shandong Peninsula, and more than 1700 km of high-resolution seismic profiles were measured across it. In this paper, we illustrate the stratigraphic structure, lithologic characteristics, mineral compositions, and the age of the clinoform using the shallow-seismic profiles and the cores, and provide insight into its distinctive sedimentary evolution under a complex interplay of eustatic sea-level changes during the postglacial period and locally restricted processes of sediment supply affected by climate, river-mouth shifting, and depositional hydrodynamics.
Section snippets
General physiographic features and regional oceanography
The Yellow Sea is a broad, relatively shallow epicontinental sea above a flat, tectonically stable shelf (Fig. 1). It is separated from the Bohai Sea to the west by the Bohai Strait, and from the East China Sea to the south by a line connecting the north edge of the Yangtze River mouth with Cheju Island. The Shandong Peninsula separates the South from the North Yellow Sea (NYS). Water depths in the NYS are generally less than 60 m, and they deepen progressively southward and southeastward in
Materials and methods
To characterize the seafloor morphology and subsurface stratigraphic architecture as well as to ensure that cores were retrieved from suitable locations in the shallow-water area around the Shandong Peninsula, over 1700 km of high-resolution seismic profiles were acquired (Fig. 3) in May and June 2003- with a 400- to 500-J SBP/AAE sparker system fired at 1-s intervals, and records were filtered between 500 and 5000 Hz and limited to a vertical penetration of 100 ms two-way travel time. The
Seismic stratigraphy
Both the E–W and S–N seismic profiles in the study area show a sigmoidal clinoform morphology, which thins offshore to less than 1 m (Fig. 4, Fig. 5, Fig. 6). With a maximum thickness of 40 m, the clinoform wraps around the eastern end of the Shandong Peninsula and extends offshore beyond long. 123°30′E and lat. 38°30′N (Fig. 6(a)–(c); Liu et al., 2002). A close inspection of the isopach map of the clinoform (Fig. 7(a)) shows that the thickest part, demarcated by the 30-m isopachs, is 20–50 km
Depositional ages and sediment accumulation rates
Fig. 11 displays the age-depth plots (sediment accumulation curves) for the two cores, along with sea-level curves of the last 14 kyr for the East China Sea/Yellow Sea (Liu et al., 2004). The sediment accumulation curves neglect any sediment compaction effects. The postglacial sea-level rise reached its highest point around 6500 cal yr BP in the western Pacific region, and during the subsequent highstand, sea level has fallen to the present level (Nakada and Lambeck, 1989).
On the basis of the 14
Conclusions
With its thickest part decoupled from the shoreline, the sigmoidal clinoform off the eastern Shandong Peninsula has a maximum thickness of 40 m and generally thins seaward to less than 1 m. The clinoform is made up of three depositional units, DU 1, DU 2 and DU 3 in descending order, corresponding respectively to three seismic units, SU 1, SU 2 and SU 3. DU 3 consists of silt to sandy silt and shows a roughly fining-upward succession; it was deposited in a subtidal nearshore environment during
Acknowledgements
This study was funded jointly by the National Natural Science Foundation of China (Grant Nos. 40376018 and 90211022) and the National Basic Research Program (Grant Nos. 2005CB422304). We thank Dr. Zhengxin Chen, Tiehu Zhao, Xianghuai Kong, Liangyong Zhou, Yuan Liang, Shuli Wang, Yong Zhang, and Gang Hu for their help in the geological survey or in preparing this manuscript.
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