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Article

A Paleo-Changjiang Delta Complex on the East China Sea Shelf Formed Some 30 ka Ago (at the MIS 3 Stage)

by
Chen Dai
1,
Shu Gao
1,*,
Yongzhan Zhang
1,
Fei Xia
2 and
Dandan Wang
3
1
The Key Laboratory of Coast & Island Development of Ministry of Education, Department of Coastal Ocean Science, School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China
2
School of Geography, Jiangsu Second Normal University, Nanjing 211200, China
3
School of Information Engineering, Zhejiang Ocean University, Zhoushan 316022, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(11), 2035; https://doi.org/10.3390/jmse11112035
Submission received: 2 September 2023 / Revised: 13 October 2023 / Accepted: 16 October 2023 / Published: 24 October 2023
(This article belongs to the Section Coastal Engineering)
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Abstract

:
The distribution patterns of the paleo-Changjiang River deltas formed during the MIS 3 period before the last glacial maximum (LGM) contain important information about the deltaic deposits in response to climate and sea-level changes. However, so far, they are still poorly understood. Here, we reconstruct the stratigraphic and chronological framework of the deltaic system based on the analysis of 1835 km of high-resolution seismic profiles obtained from the northern East China Sea, together with a comparison with the research results from four existing boreholes and three groups of published seismic profiles from the study area. Within the strata from MIS 5 to MIS 1 that are preserved on the northern shelf of the East China Sea, we discovered a large-scale paleodeltaic complex formed during the MIS 3 period. During MIS 3, sea level dropped slowly; the paleodelta formed was large in scale and was distributed between water depths of 40 and 150 m. It is now directly exposed at the seabed at a depth of more than 100 m. The paleodelta had extended from the northwest to the southeast, with maximum thickness of the deposits around 55 m. The extensions in the east–west and south–north directions both exceeded 300 km, with a total area around 1.35 × 105 km2. Since the late Pleistocene, the tectonic structure beneath the East China Sea has been relatively stable. As such, sea-level fluctuations, sediment supply, and original topography were the main controlling factors for the development of the delta.

1. Introduction

The distribution of deltaic deposits is influenced by the rate and magnitude of relative sea-level change [1]. During the Quaternary, the climate fluctuated frequently and sea level changed greatly many times, which led to the frequent alternation of sea and land environment on the continental shelf, and the paleodeltas deposited in different periods may be preserved on the shelf [1,2]. Late Pleistocene paleodeltas have been documented on continental shelves worldwide, such as those associated with the East China Sea (ECS), the Yellow Sea, Bengal Bay and the Sunda region in Asia, and the Gulf of Mexico and the New Jersey shelf waters in North America [3,4,5,6,7,8].
In the sedimentary record of a paleodelta, a falling stage systems tract (FSST) is formed by forced regression [9]. Due to the rapid fall in sea level, where there was a large supply of terrigenous clastic materials, the sedimentary system advances rapidly. Therefore, the FSST may be dominated by progradational deltas, which appear as wedge-shaped and S-shaped foreset seismic facies. During the Marine Isotope Stage 3 (MIS 3), paleodelta deposits were formed on the continental shelf with the process of sea-level decline. These paleodeltas belonged to FSST deposits and could be preserved on the continental shelf [10,11,12,13,14].
The ECS shelf is one of the widest, low slope continental shelves in the world, with an average width of around 400 km (with a maximum of 600 km), an average slope of less than 0.028% and water depth of between 20 and 140 m [15,16] (Figure 1). Sea-level rise or fall by 101 to 102 m would cause the coastline to advance or retreat by 102 km. Since the late Pleistocene, the ECS shelf, especially the outer shelf, has frequently alternated between terrestrial and marine strata due to sea-level fluctuations [1,15]. During the last glacial maximum (LGM), sea level in the ECS dropped about 130~150 m, and most of the continental shelf was exposed to air [17]. A large amount of terrigenous sediments derived from large rivers (i.e., Yangtze and Yellow Rivers) accumulated on the shelf, with multiple periods of stacked paleodeltas on the ECS shelf being present [18,19,20]. Therefore, it is necessary to study the spatial distribution and characteristics of the late Pleistocene paleodeltaic complex, which can not only provide important information for the study of the environmental evolution of the ECS shelf since the late Pleistocene, but also enrich the understanding of the development model of the Yangtze River delta and predict its future evolutionary trend.
Over the past three decades, many studies on sedimentary environments and sequence stratigraphy of the ECS shelf since the late Pleistocene have been carried out [16,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. It is assumed that during the last glacial period, along with the decline in sea level, the paleoestuary had extended to the mid-outer continental shelf and there had formed a multistage accretion delta complex [19,21,26,27,30,31]. In the past, the study of the delta on the ECS shelf since the late Pleistocene mainly focused on the Holocene Yangtze River delta. According to data from drilling, dating, and shallow profile, a relatively complete and reliable stratigraphic framework and spatial distribution pattern of the Holocene delta has been established [21,22,23]. Compared with the modern Yangtze River delta developed since Holocene, the paleodeltas in late Pleistocene were still poorly studied. Based on seismic profiles and boreholes, some buried paleodeltaic deposits [19,24,25,26] and paleochannels have been found over the northern shelf of the ECS [26,27]. Although there have been studies on the occurrence of the paleo-Yangtze delta deposits [4,19,24,25,29,30,31,32,33,34,35], a clear, complete distribution pattern is required.
In this paper, we utilize seismic profiles totaling 1835 km in length obtained from the ECS shelf, and make the in-depth correlations with the data from adjacent published boreholes and seismic profiles. The aim of this is to establish the stratigraphic framework of the ECS shelf since Marine Isotope Stage (MIS 5) using the principle of sequence stratigraphy. On this basis, we analyzed the chronology and spatial distribution of the residual paleodeltas in the ECS, and discuss the influence of sea-level change, river shifts, sediment supply, and topography on the evolution of the delta in the ECS shelf since the late Pleistocene.

2. Regional Setting

The East China Sea (ECS) is a marginal sea in the Western Pacific, consisting of its shelf areas and the Okinawa Trough, covering an area of approximately 7.5 × 105 km2. The ECS shelf represents one of the widest and smoothest continental shelves in the world, 350–600 km wide with an average slope of less than 0.028% [16,17]. In terms of the sedimentary and topographical characteristics, the ECS shelf is roughly divided into two parts, an inner shelf and an outer shelf, separated by the 50 m isobath [36]. The surficial sediments of the inner shelf are mainly composed of fine-grained clayey silt and silty clay derived from modern terrigenous sources. The outer shelf is located between 60 and 200 m in depth, and the surface sediments are mainly composed of coarse gravel and sandy sediments derived from Late Pleistocene terrigenous clastics [15,36]. The ECS shelf depression belt is the main tectonic unit of the ECS shelf. It is dominated by slow subsidence, which is conducive to the deposition and preservation of materials. The deposition rate on the ECS shelf basin in the Quaternary is about 300 m/Ma [18]. Rivers, such as the Yangtze and Qiantang, carry abundant sediment discharge onto the ECS shelf. The modern Yangtze River, which is the main material source for the ECS, can transport 500 million tons of sediment per year [37]. Most of the sediments carried by the Yangtze River accumulated in the estuary area, forming the modern Yangtze River subaqueous delta [38]. The ECS shelf is also affected by the Pacific tidal system and has an irregular semidiurnal tide with strong reciprocation along a NW–SE orientation. A series of tidal sand ridges have thus developed extensively on the ECS shelf since the Holocene transgression [15].

3. Materials and Methods

3.1. Seismic Data

On the ECS shelf, 7 shallow seismic profiles were acquired during 2014–2017, with a total length of 1835 km (Figure 2 and Table 1). The shallow seismic survey was carried out using a GeoPulse sub-bottom profiler (GeoAcoustics, Norfolk, UK). Navigation was accomplished using a differential global positioning system (Trimble, SPS351, accurate to within 1 m), and the seismic system was towed at a speed less than 5 knots. The Geopluse seismic reflection data were collected using a 100–300 J pulse energy, a 300–7000 Hz frequency band, and 100–200 ms acquisition time. Two-way travel time (TWT) from seismic sections were converted to sediment thicknesses using an acoustic velocity of 1500 m/s, and stratum images of 40–110 m thickness below the seabed were obtained. In the past few decades, many examples of shallow seismic profiles have been carried out on the ECS shelf [19,25,26,27,28,35,39]. By combing the distribution range of seismic profiling, this paper selected three groups of published seismic profiles (for their locations, see Figure 2) to help analyze the spatial distribution range and characteristics of the paleodelta on the ECS shelf.

3.2. Borehole Data

Prior to this study, the stratigraphy has been studied using large amounts of borehole data [4,29,33]. Four published boreholes were selected with their information listed in Table 2 (locations are shown in Figure 2). These boreholes were close to the seismic sections and can help interpret the seismic profiles with detailed lithology, micropaleontology, and dating. Based on the regional sea-level-change curve [40] and the principle of sequence stratigraphy, the stratigraphic age framework for the ECS shelf could be established by comparing seismic profiles with boreholes.

4. Results

4.1. Seismic Facies and Stratigraphic Framework

Based on the different termination patterns of seismic reflection events (i.e., onlap, downlap, truncation, toplap), and the parameters of seismic facies (i.e., inner reflection configuration, external geometry, reflection continuity, amplitude, frequency) [43,44], a total of seven major seismic boundaries (assigned to T1–T7 in descending order) and seven major seismic units (assigned to U1–U7 in descending order) were recognized. Upon this, a spatial framework of seismic strata for the ECS shelf is established.
Seismic unit U7 lies between unconformity T7 at the bottom and unconformity T6 at the top. The T7 interface, which is buried at a depth of 40–70 m below the seabed, is characterized by small fluctuation and good continuity (Figure 3 and Figure 4). The U7 seismic unit is along the seismic line FF1, and is characterized by a sheet external geometry, and continuous low-angle progradational inner interface configurations (Figure 3 and Figure 4). The reflection amplitude, frequency, and continuity of U7 are strong, high, and good, respectively. U7 is distributed along the southeast side of the outer ECS shelf, with a thickness between 15 and 30 m and a length of more than 150 km (Figure 3 and Figure 4).
Seismic unit U6 lies between unconformity T6 at the bottom and unconformity T4/5 at the top. The T6 interface is buried at depths of 50–60 m below the seabed and characterized by small fluctuations and medium continuity. The U6 seismic unit is along seismic lines FF1 and EE1, and features sheet-shape geometry and chaotic inner reflection configurations (i.e., progradation, parallel or subparallel, and cut–fill) (Figure 4, Figure 5 and Figure 6). The reflection amplitude, frequency, and continuity of U6 are strong, high, and poor, respectively. U6 is in unconformable contact with the underlying U7 unit. U6 is distributed over almost the whole ECS shelf, with a thickness of 10–35 m. Several paleoriver valleys or lakes were identified in U6 (Figure 3 and Figure 4).
Seismic unit U5 is situated between unconformity T5 at the bottom and unconformity T4 at the top. The T5 interface is buried at the depth of 50 m below the sea floor and is characterized by poor continuity and medium fluctuation. U5 features a lenticular geometry and high-angle progradational inner interface configurations with south or southwest dip direction. The U5 unit is scattered on the outer ECS shelf, with a thickness of 0–10 m (Figure 5).
Seismic unit U4 lies between unconformity T4 at the bottom and unconformity T3 at the top. The T4 interface is buried at depths 20–50 m below the seabed and is characterized by small fluctuations and good continuity. The U4 seismic unit is along the seismic lines EE1 and N3N4, and features a sheet geometry, with chaotic inner interface configurations on the west side and continuous low-angle progradational inner interface configurations at the east side (Figure 3, Figure 4 and Figure 5). The reflection amplitude, frequency, and continuity of U4 are strong, high, and good, respectively. U4 is in unconformable contact with the underlying U6 unit. U4 is distributed on the middle and outer ECS shelf, with a uniform thickness of 30 m (maximum 40 m) and a length of more than 250 km. It is worth noting that unit U4 cannot be identified in the profile FF1 (Figure 5).
Seismic unit U3 is located between unconformity T3 at the bottom and unconformity T2 at the top, with the T3 interface being buried at depths of 0–35 m below the seabed and characterized by large variations in morphology and a medium continuity. The U3 seismic unit features a channel- or basin-filling geometry, and chaotic inner reflection configurations (i.e., progradation, parallel or subparallel, and cut–fill). The reflection amplitude, frequency, and continuity of U3 are strong, medium, and poor, respectively. U3 is in unconformable contact with the underlying U4/U5 unit. U3 is distributed almost over the whole ECS shelf, with a thickness of 0–25 m, with a thick inner shelf and thin outer shelf. Several paleoriver valleys or lakes, with a width of 1–25 km and a depth of 5–15 m, were identified in U3 (Figure 4 and Figure 5).
Seismic unit U2 lies between unconformity T2 at the bottom and unconformity T1 at the top (Figure 4 and Figure 5). The T2 interface, buried at depths of 5–30 m below the seabed, is characterized by small fluctuations and medium continuity. The U2 seismic unit features a lenticular or banded geometry, high-angle progradation, and parallel to subparallel inner reflection configurations. The reflection amplitude, frequency, and continuity of U2 are strong, medium, and medium, respectively. U2 is in unconformable contact with the underlying U3 unit. U2 is distributed throughout the ECS shelf, with a thickness of 5–30 m and an average thickness of about 10 m. From the middle to the outer shelf, the U2 unit is dominated by tidal sand ridges. The tidal sand ridges, with a high-angle progradation inner reflection over different scales, have a width of 2–20 km and a height of 5–20 m.
Seismic unit U1 lies between unconformity T1 at the bottom and unconformity T0 (seabed) at the top (Figure 3 and Figure 6). The T1 interface, which is only identified on the western ECS shelf buried at depths of 0–40 m below the seabed, features small fluctuations and good continuity. The U1 seismic unit is along the seismic lines EE1 and FF1, and is characterized by a wedge-shaped geometry pinching out toward the sea, and continuous sigmoidal-progradation inner reflection configurations. The reflection amplitude, frequency, and continuity of U1 are strong, high, and good, respectively. U1 is in unconformable contact with the underlying U2 unit. U1 is only distributed on the inner ECS shelf, with a thickness ranging from 0 to 35 m and a depth of 5–60 m. It is interpreted as a seismic sequence of the modern Yangtze River subaqueous delta based on the seabed topography and unit morphology, and the upper part of the sequence was transformed into a tidal sand ridge by tidal currents.

4.2. Chronostratigraphic Frameworks

Based on the characteristics of seismic units in seismic profiles, seven seismic units identified on the ECS shelf can be divided into three types of seismic facies: fluvial–lacustrine facies, subaqueous delta–neritic facies, and tidal sand-ridge facies. In this study, four boreholes, SFK-1, DZQ-4, SHD-1, and EY02-1, on the ECS shelf were selected to help reveal the sedimentary sequence and provide chronostratigraphy for the seismic units observed in the seismic profiles (Figure 7). All these boreholes were tested and analyzed for their lithology, micropaleontology, and chronology, and have in-depth discussion on sedimentary facies, sedimentary environment, and age [33,35]. According to the lithology and sedimentary characteristics of the different boreholes, a joint borehole section was constructed. Through comparison, it is found that the reflection interface of the shallow seismic profile has a good corresponding relationship with the lithological interface of adjacent boreholes, and can be tracked continuously. Based on AMS 14C and OSL dates in boreholes SFK-1 and SHD-1, a chronological framework of the ECS shelf is established.
OSL dates at a depth of around 160–170 m below sea level in SFK-1 and SHD-1 are about 91 ± 10 and 108.6 ± 11.4 ka BP, respectively (Figure 8) [33,35]. According to the oxygen isotope curve, the dividing age between MIS 6 and MIS 5 is 128 ka BP. Therefore, the seismic profiles and borehole data mainly reveal the stratigraphic structure and sedimentary environment evolution of the ECS shelf since about 128 ka BP (MIS 5); the unconformity T7 is the bottom interface of MIS 5. The sedimentary strata of MIS 5, MIS 4, MIS 3, MIS 2, and MIS 1, which correspond to the seismic strata U7, U6, U5+U4, U3, and U2+U1, respectively, are relatively intact on the ECS shelf.
Seismic unit U7: In borehole SFK-1, unit U7 is correlated to the layer between about 55 and 82 m, which incorporates neritic sea, prodelta, tidal sand ridge, tidal flat, and littoral facies upward. The sediment in the lower part is mainly composed of gray silt and sand, and the upper part is mainly gray-black fine sand with interbedded silt. Deep-water species of foraminifera dominate the assemblages on the outer shelf but gradually decrease upward, while euryhaline shallow-water species increase [33]. The OSL date at a depth of around 165 m below sea level reveals an age of 91 ± 10 Cal ka BP. In borehole SHD-1, U7 is correlated to the layer between 63.6 and 123 m, which incorporates estuary, delta, and neritic sea facies upward. The sediments in the lower part are mainly composed of interbedded silt and sand, and the upper part is mainly black-gray medium-fine sand with relatively coarse grain size. The OSL date of the sediment at a depth of around 159 m below sea level is 108.6 ± 11.4 ka BP [33]. Synthesis of these dates indicates that seismic unit U7 would have been deposited during MIS 5. The largest transgression occurred in MIS 5 during the late Pleistocene; sea level began to rise in early MIS 5; by MIS5.5, it was 3~5 m higher than modern sea level [45,46], and it then began to decline. The ECS shelf experienced neritic, estuary, delta, and littoral sedimentary environments. Seismic unit U7 mainly represents a paleodelta formed by the river advancing seaward when the sea level dropped.
Seismic unit U6: In borehole SFK-1, unit U6 is correlated to the layer between about 36 and 55 m, which incorporates tidal flat, estuary, and tidal flat facies upward. The sediment in the lower part is mainly composed of gray-black fine sand, which becomes upward-coarsening, mixed with numerous shell fragments, remnants of biological activities, and occasionally yellow-brown mud gravel. The upper part is mainly composed of abundant gray to gray-black lenticular bedding and flattened bedding. The biological disturbance is significant, and there are numerous biological burrow relics. Black shell sand is developed on the top. The foraminiferal assemblage is mainly cold-water species [33]. In borehole SHD-1, U6 is correlated to the layer between 47 and 63.9 m, which incorporates neritic sea and littoral facies upward. Two OSL dates at the top and bottom of this layer both indicated an age of about 74.1 ka BP [35]. In borehole EY02-1, U6 is correlated to the layer between 44.6 and 66 m in the borehole, which incorporates neritic sea and estuary facies upward [41], and the foraminiferal assemblages from 44.6 to 53.47 m reflects estuary deposits. Borehole DZQ-4 did not fully penetrate the U6 layer, which occurs between 39.0 and 51.65 m in the borehole. The sedimentary facies here represents littoral facies [4,25]. Therefore, the seismic unit U6 should have been formed during MIS 4. Due to the rapid drop in sea level, the sedimentary environment changed from sea to land, and the shelf was dominated by river, littoral, and tidal-flat sedimentary environments.
Seismic unit U5+U4: In borehole SFK-1, unit U4 is correlated to the layer between 24.8 and 36 m, which incorporates littoral and neritic sea facies. The sediment is composed of uniform gray-black fine sand, and the benthic foraminiferal assemblage is mainly dominated by shallow-water and warm-water species on the typical inner shelf. A sample of benthic foraminifera obtained at a depth of 28.8 m yielded an AMS 14C age of 41,000 ± 400 a BP [33]. In borehole DZQ-4, unit U4 is correlated to the layer between 16.0 and 39.0 m [25], which mainly incorporates neritic, prodelta, and delta-front facies. The sediment is composed of clayey silt and silt sand. In borehole SHD-1, unit U4 is correlated to the layer between 12.9 and 47 m, which incorporates neritic and delta facies. The sediment is mainly composed of clay and fine sand. The benthic foraminiferal assemblages are dominated by shallow-water species on the inner shelf, and the pollen concentration is high. The OSL date in the lower part is 63.1 ± 7 ka BP, and the AMS 14C date in the upper part is 43.5 Cal ka BP [35]. In borehole EY02-1, unit U4 is correlated to the layer between 15.2 and 44.6 m, which incorporates neritic and delta facies [41]. The sediment is composed of fine sand and muddy silt interbeds, and the benthic foraminifera assemblage is mainly cold-water species [42]. Therefore, we believe that seismic unit U4 should have been formed during MIS 3, which is a prograding delta formed during the period of slow sea-level decline. Unit U5 was not identified in any of the boreholes. According to the seismic unit profile, unit U5 is a typical tidal ridge. Tidal ridges were mainly developed on the ECS shelf during the transgression when the sea level rose [47]. U5 is located between seismic units U6 and U4 (Figure 5). During early MIS 3, the climate warmed and the ECS shelf developed small-scale transgression. Thus, it is inferred that the U5 unit consists of tidal ridges formed during the early stage of MIS 3 transgression, and is scattered on the continental shelf. Afterward, sea level decreased and the sand ridges were buried by the delta U4. In summary, we believe that the U5 and U4 units likely formed during MIS 3.
Seismic unit U3: In borehole SFK-1, unit U3 is correlated to the layer between 5.2 and 24.8 m [47], which incorporates littoral, fluvial, and estuary facies upward. The sediment in the lower part is composed of yellow-brown-gray sandy deposits, and it is enriched in both planktonic foraminifera and benthic foraminifera. The OSL dates indicate that it should have been formed during LGM (18 ka BP). The sediment in the upper part is interbedded mud and sand. The foraminiferal assemblages are dominated by cold-water species and their abundance decreased upward. The traditional 14C and OSL dates in the upper part are about 11–18 ka BP, corresponding to late MIS 2 deglaciation [32,33,47]. In borehole SHD-1, unit U3 is correlated to the layer between 6.3 and 12.9 m, which incorporates land facies, and the sediment is composed of gray medium-fine sand. No foraminifera were found in this layer, and there was a certain amount of pollen. The OSL dates in this layer indicate an age of about 22.5 ka BP [35]. In borehole DZQ-4, unit U3 is correlated to the layer between 11.0 and 16.0 m [25], which incorporates estuarine and fluvial facies. The sediment is composed of silty sand, and pollen indicated a dry and cold environment, probably formed around LGM. In borehole EY02-1, U3 is correlated to the layer between 11.40 and 15.20 m [42], the sediment is mainly composed of fine sand, and the foraminiferal assemblages reflected a littoral environment. Therefore, seismic unit U3 should be formed during MIS 2. Sea level declined rapidly during MIS 2 on the ECS, and sea level dropped to −140 m at the LGM (18 ka BP) [15]. The entire ECS shelf was exposed as land, with rivers and estuaries developed.
Seismic unit U2: In borehole SFK-1, unit U2 is correlated to the layer between 0 and 5.2 m, which mainly incorporates tidal-ridge facies, which formed during early MIS 1 [33]. In borehole SHD-1, unit U2 is correlated to the layer between 0 and 6.3 m, mainly composed of neritic sea sediments. In borehole DZQ-4, it corresponds to the sedimentary layer between 0 and 11.1 m, which incorporates tidal-ridge facies. In borehole EY02-1, unit U2 is correlated to the layer between 0 and 11.40 m. The bottom AMS 14C date indicated an age of about 10 ka BP, and this layer is interpreted as tidal ridge formed during MIS 1 transgression [42]. Therefore, U2 is a neritic sea or tidal-ridge deposit formed during MIS 1 transgression.
Seismic unit U1: Due to the deep-water depth in the mid-outer shelf and the lack of sediment supply, the high sea-level deposits on the tidal ridges are only tens of centimeters thick [19], which can hardly be identified in the seismic profile. Therefore, borehole ECS-0702 in the inner shelf was used to analyze the deposition environment. In borehole ECS-0702, unit U1 is correlated to the layer between 0 and 19.6 m (Figure 2 and Figure 9), which mainly incorporates neritic and prodelta facies. The sediment is composed of muddy silt, and the benthic foraminifera and ostracod assemblages are dominated by euryhaline and neritic species. The bottom AMS 14C date indicated an age of 7.25 Cal ka BP [16]. Unit U1 is interpreted as the subaqueous delta of the modern Yangtze River deposited during the high sea level in the Holocene.

4.3. Sedimentary Evolution on the East China Sea since MIS 5

The results showed that strata of the ECS shelf were well preserved during the last glacial period, and three obvious transgressions (MIS 5, MIS 3, and MIS 1) and two large-scale regressions (MIS 4 and MIS 2) were discovered. Influenced by sea-level fluctuation, the sea strata and land strata in the northern shelf of the ECS changed alternately. Since MIS 5, the sedimentary environment evolution of the ECS shelf was, in turn, neritic and delta deposits during MIS 5; estuary, littoral, and tidal-flat deposits during MIS 4; neritic and delta deposits during MIS 3; estuary and littoral deposits during MIS 2; and tidal sand-ridge deposits during MIS 1.

5. Discussion

5.1. Spatial Distribution Pattern and Thickness of the Paleodelta during MIS 3

Since MIS 5, the ECS shelf has experienced three significant transgressions and two large-scale regressions. Two delta complexes are founded on the ECS shelf, which are the U7 and U4 sedimentary units from bottom to top (Figure 3). They are mainly distributed on the mid-outer ECS shelf and formed during MIS 5 and MIS 3, respectively. The most notable feature of the delta units is the obvious parallel and subparallel progradational reflection extending a long distance into the marine region, which is interpreted as a progressive (regressive) delta.
The two deltas were also identified in the seismic profiles and boreholes obtained from previous research [19,25,34,35] (Figure 7). Climate fluctuations were relatively small during MIS 3, and their influence on the ECS shelf was less than that during MIS 5. However, due to its long duration, the paleoriver extended relatively far toward the sea, and a large-scale paleodelta complex accumulated on the ECS shelf [4]. Niu [36] divided the distribution range of paleodelta (?~126° E, 30° N~32°20′ N) based on the differences in geomorphological features and physical, biological, and geochemical processes in the ECS sedimentary area, and believed that the paleodelta was distributed in a fan-shaped arc trending NEE with the center at 31°45′ and 122°20′. The seismic records from a joint Sino–French study in 1996 revealed in detail the existence of two stages of paleodeltas in the middle-outer ECS shelf in the Late Pleistocene, and identified the local spatial extent of paleodelta deposits (123.8° E~126.4° E, 28.6° N~30.5° N) [18,24]. The late Pleistocene Yangtze River delta was also found in the seismic profiles near the slope break line on the outer margin of the ECS shelf. The area of delta interpreted in the section is about 7500 km2 (126.75–127.7° E, 29.7–30.7° N) [39]. Chen [35] analyzed the seismic profiles of the outer shelf in the northern ECS, and obtained the spatial distribution and thickness of the paleodelta deposits in the outer shelf and shelf break area (126–127.66° E, 28.8–31° N).
Combining the seismic profiles in this paper and previous research, the complete spatial distribution and thickness of the U4 paleodelta formed during MIS 3 were obtained (Figure 10a). The U4 paleodelta was distributed at water depths of 40~150 m, and developed from northwest to southeast with a thickness of 0~55 m (Figure 10b) and a length of more than 300 km (123.82–127.36 °E, 28.58–32.40 °N) in both east–west and south–north directions. The area of the paleodelta is about 1.35 × 105 km2. The elevation of the paleodelta declined from northwest to southeast, and its outline is consistent with the modern seabed topography of the area (Figure 1). The water depth at the front edge of the delta ranges between 130 and 50 m, and the delta is directly exposed on the seabed at the depth of 100 m [18,24,34]. The west side of the paleodelta is roughly bounded by a water depth of 40 m, and the seismic profiles of the ECS shelf at a water depth of less than 40 m often show chaotic reflection structures. The top interface (T3) of the delta is an erosion interface with incised valleys, indicating that the delta underwent erosion by the river in the later period.
During the MIS 2 stage, sea level of the ECS shelf dropped rapidly, and the river was incised. The deep-cut valley of the Yangtze River in the inner shelf during the LGM Period could reach a depth of 60–80 m [20], with a maximum width of over 330 km [27]; it is speculated that the paleodelta sediments developed on the inner shelf in the early period of MIS 3 suffered from erosion by channel incision and were not preserved.

5.2. Controlling Factors in Delta Development during MIS 3

Since the late Pleistocene, sea level in the ECS has fluctuated more than 100 m. As an area with frequent sea–land interactions, stratigraphic development and environmental evolution on the ECS shelf were particularly sensitive to sea-level changes. Although subsidence, sediment compaction, and erosion may reduce the top surface elevation of the delta to some extent, the paleowater depths of the delta can be approximated as the paleoelevations of sea level during delta deposition process considering the relatively stable tectonic setting in the ECS, the relatively thin sediment thickness since MIS 2, and no obvious erosion except for some incised valleys [16]. During MIS 3 (from 60 to 25 cal ka BP), sea level fluctuated downward from −60 to −90 m [40] (Figure 11), and the paleowater depth of the top interface (T3) of the U4 paleodelta was about −60 to −105 m, which is consistent with sea level during MIS 3. A relatively stable sea surface is conducive to the development of deltas. During MIS 3, sea level on the ECS presented characteristic low-amplitude fluctuations, and the rate of sea-surface descent was significantly slowed [19,24,25,27,34]; thus, it could fully support the development of a paleodelta complex.
According to the scale and geographical location of the U4 paleodelta, the river forming the delta would have been a large river from the west continent. Based on the data from boreholes and profiles, combined with the sea-level-change curve, and through the analysis of paleochannel and provenance, it is considered that U4 is probably the paleo-Yangtze River delta [15,28]. A warm and wet climatic condition is beneficial for rivers to deliver sufficient sediments to coastal areas and quickly accumulate into deltas. During MIS 3, the climate in eastern China was warm and wet [48,49], and paleorivers discharged abundant sediments to the ECS shelf [27]; hence, the U4 delta grew rapidly across the shelf. During the latter part of MIS 3, the climate began to gradually become cold and dry, but the river incision was strong, causing erosion in the upper part of the paleodelta (Figure 5 and Figure 7).
According to the seismic profiles and boreholes, the top interface (T4) of the U6 strata on the south-side profile FF1 is about 70–90 m below present sea level (bpsl) (Figure 6), while the interface T4 on the north-side profile EE1 is about 125–130 m bpsl, indicating that the ECS shelf topography showed a trend of high in the south and low in the north (Figure 5 and Figure 7). A large number of paleo-Yangtze River channels during the LGM have also been discovered in this area; the Yangtze River mainly entered the lowland plains on the northeast of shelf through the Yangtze Depression [27,28,38]. Therefore, when sea level fell during MIS 3, the rivers converged in the low-lying northern part of the mid-outer ECS shelf, and the sediment carried by the rivers accumulated here to form a paleodelta (Figure 11). According to the borehole section (Figure 8), the northeast part of the outer ECS shelf was relatively flat during the MIS 3, and wandering network river systems were widely developed [18,50].

6. Conclusions

Stratigraphic reflection interfaces (T1–T7) and seismic units (U1–U7) have been identified from the seismic profile data across the East China Sea shelf. A comparison of seismic units with borehole sedimentary strata reveals the stratigraphic characteristics and sedimentary sequence since MIS 5. These data demonstrate the presence of two paleodeltas deposited in MIS 5 and MIS 3, respectively. During MIS 3, sea level dropped slowly; the paleodelta formed was large in scale and distributed within a water depth of 40 and 150 m. It is now directly exposed at the seabed at a depth of more than 100 m. The paleodelta had extended from the northwest to the southeast, with a maximum thickness of deposits of around 55 m. The extensions in the east–west and south–north directions both exceeded 300 km, with a total area of around 1.35 × 105 km2. The upper part of the delta was partly eroded, forming incised valleys as sea level dropped during the LGM. Sea-level fluctuations, sediment discharge of paleorivers, and original topography were the main controlling factors for the delta development. During the MIS 3 period, sea level was relatively stable and dropped slowly, and the warm and wet climate supplied a large quantity of sediment. The bed elevation of the shelf in the East China Sea was relatively high in the south and low in the north, causing the delta complex to concentrate in the northern part of the East China Sea shelf.

Author Contributions

Conceptualization, C.D. and S.G.; methodology, C.D.; validation, C.D., S.G., Y.Z. and F.X.; formal analysis, C.D.; investigation, C.D. and Y.Z.; resources, C.D. and Y.Z.; data curation, C.D.; writing—original draft preparation, C.D.; writing—review and editing, C.D., S.G., Y.Z, F.X. and D.W.; visualization, C.D.; supervision, S.G.; project administration, S.G. and Y.Z; funding acquisition, S.G. and Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was jointly funded by National Basic Research Program of China (Grant No. 2013CB956500) and National Natural Science Foundation of China (Grant No. 41901107).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We thank Yunkun Sun, Zixing Li, Li Wang, and Junfei Xia for participating in the fieldwork and the analysis of shallow seismic profiles. We thank Yangyang Zhao for proofreading the manuscript. We greatly appreciate the constructive reviews of anonymous reviewers for improving the final manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Jmse 11 02035 g001
Figure 1. Schematic map showing the bathymetry and regional circulation pattern in the East China Sea and adjacent areas during winter (modified after [16]). The dashed square indicates the study area. YSWC, Yellow Sea Warm Current; TWC, Taiwan Warm Current; YSCC, Yellow Sea Coastal Current; SKCC, South Korean Coastal Current; SSCC, South Shandong Coastal Current; NJCC, North Jiangsu Coastal Current; CDW, Changjiang Diluted Water; ECSCC, East China Sea Coastal Current.
Figure 1. Schematic map showing the bathymetry and regional circulation pattern in the East China Sea and adjacent areas during winter (modified after [16]). The dashed square indicates the study area. YSWC, Yellow Sea Warm Current; TWC, Taiwan Warm Current; YSCC, Yellow Sea Coastal Current; SKCC, South Korean Coastal Current; SSCC, South Shandong Coastal Current; NJCC, North Jiangsu Coastal Current; CDW, Changjiang Diluted Water; ECSCC, East China Sea Coastal Current.
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Figure 2. Location of shallow seismic profiles and boreholes collected on the ECS shelf (red solid lines are seismic profiles modified after [19]; green solid lines are seismic profiles modified after [26]; blue solid lines are seismic profiles modified after [35]).
Figure 2. Location of shallow seismic profiles and boreholes collected on the ECS shelf (red solid lines are seismic profiles modified after [19]; green solid lines are seismic profiles modified after [26]; blue solid lines are seismic profiles modified after [35]).
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Figure 3. Fence diagram generated from the interpretation of seismic data from the ECS shelf.
Figure 3. Fence diagram generated from the interpretation of seismic data from the ECS shelf.
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Figure 4. Part of seismic profile FF1 (a) and part of the seismic profile from [18] (b,c), with their stratigraphic interpretations (for location, see Figure 2).
Figure 4. Part of seismic profile FF1 (a) and part of the seismic profile from [18] (b,c), with their stratigraphic interpretations (for location, see Figure 2).
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Figure 5. Part of seismic profile EE1 (a), part of seismic profile N3N4 (b), and part of seismic profile N4N5 (c), with their stratigraphic interpretations (for location, see Figure 2).
Figure 5. Part of seismic profile EE1 (a), part of seismic profile N3N4 (b), and part of seismic profile N4N5 (c), with their stratigraphic interpretations (for location, see Figure 2).
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Figure 6. Part of seismic profile FF1 (a) and part of seismic profile M3M4 (b), with their stratigraphic interpretations (for location, see Figure 2).
Figure 6. Part of seismic profile FF1 (a) and part of seismic profile M3M4 (b), with their stratigraphic interpretations (for location, see Figure 2).
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Figure 7. Borehole section and stratigraphic correlation associated with (a) borehole SFK-1, (b) DZQ-4, (c) SHD-1, and (d) EY02-1. SFK-1, DZQ-4, EY02-1, and SHD-1 are modified after [33,29,41,35] respectively. Core locations and the shallow seismic profiles are shown in Figure 2.
Figure 7. Borehole section and stratigraphic correlation associated with (a) borehole SFK-1, (b) DZQ-4, (c) SHD-1, and (d) EY02-1. SFK-1, DZQ-4, EY02-1, and SHD-1 are modified after [33,29,41,35] respectively. Core locations and the shallow seismic profiles are shown in Figure 2.
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Figure 8. Borehole section and stratigraphic correlation among boreholes SFK-1, DZQ-4, SHD-1, and EY02-1, together with the age date data. SFK-1, DZQ-4, EY02-1, and SHD-1 are modified after [33,29,41,35] respectively. Core locations are shown in Figure 2.
Figure 8. Borehole section and stratigraphic correlation among boreholes SFK-1, DZQ-4, SHD-1, and EY02-1, together with the age date data. SFK-1, DZQ-4, EY02-1, and SHD-1 are modified after [33,29,41,35] respectively. Core locations are shown in Figure 2.
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Figure 9. Correlation between the stratigraphic units of borehole ECS-0702 (modified after [43]) and seismic profile (location shown in Figure 2 and Figure 6).
Figure 9. Correlation between the stratigraphic units of borehole ECS-0702 (modified after [43]) and seismic profile (location shown in Figure 2 and Figure 6).
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Figure 10. Geometric features of the paleodelta: (a) Distribution of deltaic deposits during MIS 3 in the ECS shelf, indicated by the shaded area (lines with purple dots present the MIS 3 paleodelta found in the seismic profiles). (b) Isopach map (in meters) of the MIS 3 paleodelta on the ECS shelf.
Figure 10. Geometric features of the paleodelta: (a) Distribution of deltaic deposits during MIS 3 in the ECS shelf, indicated by the shaded area (lines with purple dots present the MIS 3 paleodelta found in the seismic profiles). (b) Isopach map (in meters) of the MIS 3 paleodelta on the ECS shelf.
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Figure 11. Schematic diagram illustrating the evolution of the paleodelta complex in comparison with global relative sea-level changes since MIS 5 (modified from [40]) and the sedimentary sequence on the ECS shelf. The colors in the figure represent the corresponding seismic units, respectively.
Figure 11. Schematic diagram illustrating the evolution of the paleodelta complex in comparison with global relative sea-level changes since MIS 5 (modified from [40]) and the sedimentary sequence on the ECS shelf. The colors in the figure represent the corresponding seismic units, respectively.
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Table 1. Basic information of the seismic survey on the ECS shelf, from 2014 to 2017.
Table 1. Basic information of the seismic survey on the ECS shelf, from 2014 to 2017.
Seismic
Line		Start and End PointWater Depth (m)Longitude (E)Latitude (N)Length (km)
EE1E14.4122.030°31.952°497.2
E1100126.877°30.832°
DD1D17.5121.696°32.426°273.6
D159.6124.506°32.790°
N3N4N455124.741°31.329°157.5
N347124.291°32.770°
M2M3M341.8123.590°31.605°115.5
M240123.372°32.655°
FF1F6.5122.199°31.229°505.5
F1118125.988°28.101°
N5N4N455124.736°31.319°170.1
N567123.404°30.221°
M4M3M341.8123.551°31.553°115.4
M431122.694°30.823°
Table 2. Location, water depth, and length of boreholes collected on the ECS shelf (locations are shown in Figure 2).
Table 2. Location, water depth, and length of boreholes collected on the ECS shelf (locations are shown in Figure 2).
Core No.Longitude
(E)		Latitude
(°N)		Water
Depth (m)		Core
Length (m)		Reference
SHD-1126.05°29.92°84.1150.4[33,35]
SFK-1125.25°29.05°88.382.90[33]
DZQ4125.36°29.41°88.751.65[4,2529]
EY02-1126.57°30.73°8070.2[41,42]
ECS070231°122.67°2235.6[16]
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Dai, C.; Gao, S.; Zhang, Y.; Xia, F.; Wang, D. A Paleo-Changjiang Delta Complex on the East China Sea Shelf Formed Some 30 ka Ago (at the MIS 3 Stage). J. Mar. Sci. Eng. 2023, 11, 2035. https://doi.org/10.3390/jmse11112035

AMA Style

Dai C, Gao S, Zhang Y, Xia F, Wang D. A Paleo-Changjiang Delta Complex on the East China Sea Shelf Formed Some 30 ka Ago (at the MIS 3 Stage). Journal of Marine Science and Engineering. 2023; 11(11):2035. https://doi.org/10.3390/jmse11112035

Chicago/Turabian Style

Dai, Chen, Shu Gao, Yongzhan Zhang, Fei Xia, and Dandan Wang. 2023. "A Paleo-Changjiang Delta Complex on the East China Sea Shelf Formed Some 30 ka Ago (at the MIS 3 Stage)" Journal of Marine Science and Engineering 11, no. 11: 2035. https://doi.org/10.3390/jmse11112035

APA Style

Dai, C., Gao, S., Zhang, Y., Xia, F., & Wang, D. (2023). A Paleo-Changjiang Delta Complex on the East China Sea Shelf Formed Some 30 ka Ago (at the MIS 3 Stage). Journal of Marine Science and Engineering, 11(11), 2035. https://doi.org/10.3390/jmse11112035

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