CHARACTERISTICS OF BURIED PALEO-CHANNELS IN THE WESTERN SOUTH YELLOW SEA DURING THE LATE LAST GLACIATION

Original scientific paper Studies on the evolution of paleo-channels in coastal areas are important for submarine engineering construction and to reveal changes in the global paleoenvironment. Thus, to explore the characteristics of paleo-channels during the late Last Glaciation in the western South Yellow Sea, digital terrain analysis method and ArcGis river extraction function were employed, high-resolution shallow stratigraphic seismic profiles and core data were analysed, and river empirical formulas were used to determine river properties and river patterns. Results indicate that the ancient river system during the late Last Glaciation of the South Yellow Sea shelf is divided into paleo-Yellow (Huanghe) and paleo-Yangtze (Changjiang) Rivers. The paleo-channels near 33°N belong to the paleo-Yangtze River, and generally flow from east to northeast. The paleo-channels around 35°N and 123,5°E are part of the paleo-Yellow River. Compared with the paleo-Yellow River, the paleo-Yangtze River is prone to horizontal migration and has higher penetration depths and discharge. Based on the slope-width method, the paleo-Yellow River system can be considered mainly as a meandering river, whereas the paleo-Yangtze River system is largely a braided river. Remarkable differences are found between the paleo-Yangtze River and the paleo-Yellow River. The characteristics of buried paleo-channels during the late Last Glaciation can be useful in predicting the incident potential hazard of submarine engineering and in revealing the paleoenvironment changes in the South Yellow Sea shelf.


Introduction
Since the late Pleistocene era, global sea levels have experienced several fluctuations; the Yellow Sea and the East China Sea also went through several large-scale transgressions and regressions [1÷3].During the periods of low sea level, the rivers, deltas, lakes, marshes, and other landscape units developed upon the bare shelves of the Yellow Sea and the East China Sea.As the sea level rose, these units became buried sedimentary bodies.In sea areas with smaller deposition rates, the sedimentary bodies can preserve their original features to different extents, which can be identified from the shallow stratigraphic seismic profiles [4÷7].The buried sedimentary bodies are considered as adverse geological conditions for submarine engineering, because they may inflict direct harm or have potential effect on submarine engineering construction [8÷10].The buried paleochannels are also the product of climate and geological changes, and record abundant geological information during environment changes [11÷13].During the low sea level in the Last Glaciation, the Yellow (Huanghe) and the Yangtze (Changjiang) Rivers joined at the northern Jiangsu Province, and flowed into the Yellow Sea basin [14,15].This phenomenon significantly influenced the paleoenvironment of Yellow Sea.However, the characteristics of these paleo-channels on the shelf of the South Yellow Sea remain unclear.

State of the art
In the past two decades, considerable efforts have been exerted to study the buried channel systems on the continental shelf of the South Yellow Sea.Li et al. [16] discussed the distribution of buried paleo-channels and to which river system they belong.Gu et al. [17] identified the types, seismic stratigraphy, and sedimentary character of paleo-channel cross-section by conducting a synthetic study on seismic facies and core data.Kong et al. [3] revealed the seismic geomorphology of buried channel systems in the western South Huanghai Sea.Liu et al. [18] discussed the effects of buried channel systems on submarine engineering construction in the coastal and offshore areas of the Yellow River delta.Although several studies have been conducted, the flow path of paleo-Yellow River and paleo-Yangtze River remain controversial.Li et al. [16] proposed that the paleo-Yellow River system flowed from the northwest to the southeast during the late Pleistocene era, whereas the paleo-Yangtze River ran from the southwest to the northeast; they also argued that the two river systems converged in the mid-eastern regions in the South Yellow Sea.Yang et al. [19] suggested that the paleo-Yangtze River extended in the northern and eastern directions from Jianggang Region in Northern Jiangsu Province during the Last Glaciation.Gu et al. [17] viewed the paleo-Yellow River and paleo-Yangtze River in the Last Glaciation as two separate river systems in the north and the south.Previous studies have also suggested that buried paleo-channels of the Yellow River, the Yangtze River, and the Pearl River had been found in the Bohai Sea, the East China Sea, and the South China Sea, respectively.During the Last Glaciation, China's two largest rivers, the Yellow River and the Yangtze River, had converged at Northern Jiangsu and flowed into the South Yellow Sea for a long time.Paleo-deltas and buried paleo-channels were superposed upon the South Yellow Sea shelf beyond the shore of Northern Jiangsu.This complex situation is well recorded on a large number of the shallow stratigraphic seismic profiles [3,17,20,21].These paleochannels were formed from either the Yellow River or the Yangtze River.In terms of the origin or structure, these paleo-channels are more complex than those of the East China Sea and the South China Sea.However, relevant studies on these paleo-channels remain not enough.Thus, the distribution and characteristics of paleo-channels in the western South Yellow Sea during the late Last Glaciation were analysed in the present study.
The rest of this paper is organized as follows.Section 3 describes the data sources and processing methods.Section 4 presents the distribution of paleo-channels based on the interpretation of shallow stratigraphic seismic profiles, as well as the river patterns and paleochannel properties using river empirical formulas.Conclusions are provided in Section 5.

Methodology
Seismic and geological shallow drilling surveys (cores Nantong 1 [NT1] and Nantong 2 [NT2]) were conducted during the geological survey project on South Yellow Sea in 2002.Approximately 3000 km of highresolution shallow stratigraphic seismic profiles were acquired using the Delph seismic system for data acquisition (Figure 1).The thickness of the reflection in the profile was calculated using the acoustic velocity of 1650 m/s following Liu et al. [22] Interpretation of the seismic data was based on the recognition of minor seismic discontinuities by comparing seismic facies with lithofacies of cores NT1 and NT2.
Around 3000 km of shallow stratigraphic seismic profiles (Fig. 1) were used to study the buried paleochannels in the South Yellow Sea, and numerous paleochannels during the Last Glaciation were found by interpreting these profiles.The internal sediments in river cross-sections are characterized largely by chaotic and irregular reflections (Fig. 2).Mathematical statistics on the cross-section parameters of the paleo-channels during the Last Glaciation in seismic profiles were performed to investigate the river characteristics and their horizontal distribution.Hundreds of typical parameters of paleochannel cross-section were sorted out, such as azimuth of extended cross-section, cross-section trend (i.e., paleochannel direction), valley depth and valley width, etc.
Finally, a total of 93 clear, and typical cross-sections were selected (Tab.1), and those lying above the QS2-0 unconformity were continental strata in the late Last Glaciation.The various parameters of paleo-channel cross-sections are summarized in Table 1, of which the valley width refers to the width of valley shoulder, and the valley depth is the vertical distance from valley shoulder to valley bottom.Other parameters were calculated according to empirical formulas (1)÷( 6) [23].
In these formulas, f refers to the width-to-depth ratio of paleo-channel; c is the content of suspended matter; p is the river-bend curvature; s is the riverbed slope; l is the meander wavelength; q is the average discharge (m 3 /s); S v is the slope at the paleo-channel bottom; d is the valley depth; and w is the valley width.

Distribution of paleo-channels
Based on the paleotopography recovery and digital terrain analysis [15] and the location of paleo-channels reflected on the shallow stratigraphic seismic profiles, we extracted the horizontal distribution characteristics of paleo-channels in western South Yellow Sea during the late Last Glaciation.The results show that in the late Last Glaciation, three main paleo-channels had developed in the western South Yellow Sea: near 33°N, the ancient river system generally flowed from east to northeast; close to 35°N, the main stream of paleo-channels flowed from east to southeast; and near 123,5°E, the entire paleochannels flowed southeast (Fig. 3).
The results of provenance analysis of sediments from cores NT1 and NT2 [24÷26] are as follows: for Core NT2, the sediments from the upper section at 5,50÷19,40 m are mainly from the Yangtze River, which correspond to the continental sedimentation during periods of low sea level in the late Last Glaciation; for Core NT1, the sediments from the upper section at 7,70÷16,60 m are mainly from the Yellow River, which correspond to the continental sedimentation at the same period.
Based on the locations of the ancient river system and the data from the core sediments, the paleo-channels upon the South Yellow Sea shelf during the late Last Glaciation can be divided into paleo-Yangtze River and paleo-Yellow River (Figure 3).The paleo-channel close to 33°N is the paleo-Yangtze River system, which generally swept from east to northeast.The flow path of paleo-Yellow River might include the west (around 35°N) and north lines (around 123,5°E).In particular, near 35°N, the paleo-Yellow River system flowed from east to southeast, which affected the middle of the South Yellow Sea where Core NT1 locates.This river system then assembled with the north lines of the paleo-Yellow River system and turned to southeast direction.

Characteristics of paleo-channels
Among the several thousand kilometres of highresolution shallow stratigraphic seismic profiles, more than 90 paleo-channel cross-sections with clear reflection characteristics were selected.We then recorded the coordinates of central points of the cross-sections and the morphological parameters of paleo-channels, including valley width and penetration depth.The relevant parameters (Tab. 1) were calculated based on the riverbed slope formula [27], and thus, the river patterns and river characteristics of paleo-channels were determined.

Paleo-channel property
The cross-section width and depth and other parameters of the paleo-Yellow River system and the paleo-Yangtze River system are listed in Tab. 1, including width-to-depth ratio, river-bend curvature, and content of suspended matter (Note: No. 1 ÷ No. 47 and No. 48 ÷ No. 93 are the characteristic parameters of the cross-sections of paleo-Yellow River and paleo-Yangtze River, respectively).
(1) Width-to-depth ratio (f) indicates the stability of paleo-channels during their development.The stability of the paleo-channel decreased with the increase of width-todepth ratio.In this study, the average width-to-depth ratio of the paleo-Yellow River cross-section was 22,0, whereas that of the paleo-Yangtze River cross-section reached 39,9.
(2) Penetration depth, or the so-called river depth, is the vertical distance from the valley shoulder to valley bottom.The average penetration depth of the paleo-Yellow River was 12,26 m, while the maximum was 25 m.The average penetration depth of the paleo-Yangtze River was 42,96 m, with the maximum reaching 93,5 m.The paleo-Yangtze River had an obviously higher penetration depth.
(3) The discharge of paleo-channels can be calculated according to the empirical formula (5) summarized from modern discharge, i.e., w , f , , q lg 42853 2 lg 13327 , where f is the width-to-depth ratio of river, w is the valley width, and q is the discharge (m 3 /s).As shown in Table 3, the average width of the paleo-Yellow River was 270 m, and the average discharge q was 460 m 3 /s.The average width of the paleo-Yangtze River was 650 m, and the average discharge q was 730 m 3 /s.This finding indicates that the discharge of the paleo-Yangtze River is higher than that of the paleo-Yellow River.

River pattern
The river patterns of the paleo-channels in the western South Yellow Sea were determined by the slopewidth method [27].The primary parameters are discharge, slope, and valley width.Slope can reflect the sediment transportation characteristics of a river.Gravel riverbed is dominated by bed load transportation, whereas sandy riverbed is dominated by suspended load transportation.Under the same conditions, both the strength of bed load transportation and the slope of the braided river are higher than those of the meandering river.Valley width reflects the erosion and circulation distribution pattern of river.Given a constant discharge, the wider river tends to form a braided river, whereas the narrower one is more likely to form a meandering river [27].The judgment formula based on slope-width method is composed of the empirical formulas (1), ( 2) and (3).
Using the formulas (1), ( 2) and ( 3), the suspended sand content, river-bend curvature, and vertical slope of the paleo-Yellow River and the paleo-Yangtze River were calculated (Tab.1).Figs. 4 and 5 were plotted according to slope and valley width of the two paleo-channels.The boundary between meandering river and braided river is .The results show that the gravel riverbed dominated the paleo-Yellow River, where the braided river accounted for 40 % and the meandering river for 60 %.Sandy riverbed dominated the paleo-Yangtze River, where the braided river accounted for 59,6 % and the meandering river for 40,4 %.
A considerable number of studies have shown that the Yangtze River and the Yellow River, as the two largest rivers in China, have significant differences in terms of geological background, hydrology conditions, geomorphology, and sedimentary characteristics.The Yangtze River is characterized by more water, less sand, fine material, more stable, and contains extremely diverse rock types in the basin.On the contrary, the Yellow River has less water, more sand, fine material, easy swing, and relatively simple rock types within the basin.These differences result in the obvious distinction in pattern and property between the two rivers.In the late period of the Last Glaciation, the paleo-Yangtze River had higher width-to-depth ratio at river cross-section than the paleo-Yellow River.Therefore, the riverbed location of paleo-Yangtze River is unstable and has the characteristics of lateral migration.This finding is similar to research findings on buried ancient channels in the continental shelf area out of the mouth of the Yangtze River [27].According to the judgment of the slope-width method, the paleo-Yellow River system is dominated mainly by meandering rivers.This result is consistent with the results of a study on paleo-channel in the western South Yellow Sea (near the abandoned Yellow River Delta) [16].However, the paleo-Yangtze River system is composed of braided rivers.This conclusion agrees well with the conclusions obtained by previous research on buried paleo-channels in the inner continental shelf out of the Yangtze River [27].Interpretation of the high-resolution seismic profiles indicates that the two paleo-channel systems developed during the Last Glaciation on the shelf of the western South Yellow Sea.Using the estimated paleo-hydrologic parameters and the slope-width method, the properties of paleo-channels are determined.The main conclusions are summarized as follows: During the late Last Glaciation, numerous paleochannels developed upon the western South Yellow Sea shelf.According to their locations and core sediments, the paleo-channels are divided into paleo-Yangtze River system and paleo-Yellow River system.The paleochannels near 33°N belong to the paleo-Yangtze River system, which generally swept from east to northeast.And the paleo-channels close to 35°N and 123,5°E are considered to be the two flow paths of paleo-Yellow River system.
Compared with the paleo-Yellow River, the paleo-Yangtze River is prone to horizontal migration and has higher penetration depth and discharge.The bottom of paleo-Yellow River is steeper than that of the paleo-Yangtze River.The average discharge of the paleo-Yellow River is 460 m 3 /s, and that of the paleo-Yangtze River is 730 m 3 /s.Therefore, the discharge of the paleo-Yangtze River is higher than that of the paleo-Yellow River.
By employing the slope-width method, the paleo-Yellow River system is determined to be composed mainly of meandering rivers with gravel riverbed, whereas the paleo-Yangtze River system is dominated by braided rivers with sandy riverbed.
Abundant oil and gas resources are found on the continental shelf of the South Yellow Sea, indicating its significant development potential.The buried paleochannels are widely distributed in this area as geologic factors of the unstable seafloor and as parts of the latent disastrous types.These buried paleo-channels have a significant effect on the development of oil and gas resources.The results obtained in this study provide basic geological data of submarine engineering on the Southern Yellow Sea shelf.The details of the effects of the buried paleo-channels on submarine engineering should be considered in future extensions of this study.

Figure 1
Figure 1 Location of the research area

Figure 2
Figure 2 Reflecting interface of sub-bottom profile QS0: Seabed reflecting interface; QS1: Marine strata bottom interface during the Holocene stage; QS2-0: Continental strata bottom interface during the late Last Glaciation; QS2-1: Marine strata bottom interface during the Last Glaciation; QS2-2: Continental strata bottom interface during the early Last Glaciation.

Figure 3
Figure 3 Distribution of paleo-channels in western South Yellow Sea during the Last Glaciation

Figure 4 Figure 5
Figure 4 Judgment of riverbed type of the paleo-Yellow River based on slope-width method

Table 1
Characteristic parameters of paleo-channel cross-sections

Table 1
Characteristic parameters of paleo-channel cross-sections (continuation)