Evolution of Sedimentary Environments of the Middle Jiangsu Coast, South Yellow Sea Since Late MIS 3

Home , Baoying County, Geology of China, Huang Jiaxiang

Evolution of sedimentary environments of the middle Jiangsu coast, South Yellow Sea since late MIS 3

XIA Fei1,2,3, *ZHANG Yongzhan1,2, WANG Qiang4, YIN Yong1,2, Karl W. WEGMANN3, J. Paul LIU3

1. Department of Coastal Ocean Science, School of Geographic and Oceanographic Sciences, Nanjing Uni- versity, Nanjing 210093, China; 2. The Key Laboratory of Coastal and Island Development, Ministry of Education, Nanjing University, Nanjing 210093, China; 3. Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA; 4. Tianjin Institute of Geology and Mineral Resources, China Geological Survey Bureau, Tianjin 300170, China

Abstract: An evolutionary model of sedimentary environments since late Marine Isotope Stage 3 (late MIS 3, i.e., ca. 39 cal ka BP) along the middle Jiangsu coast is presented based upon a reinterpretation of core 07SR01, new correlations between adjacent published cores, and shallow seismic profiles recovered in the Xiyang tidal channel and adjacent northern sea areas. Geomorphology, sedimentology, radiocarbon dating and seismic and sequence stratigraphy are combined to confirm that environmental changes since late MIS 3 in the study area were controlled primarily by sea-level fluctuations, sediment discharge of paleo-rivers into the South Yellow Sea (SYS), and minor tectonic subsidence, all of which impacted the progression of regional geomorphic and sedimentary environments (i.e., coastal barrier island, freshwater lacustrine swamp, river floodplain, coastal marsh, tidal sand ridge, and tidal channel). This resulted in the formation of a fifth-order sequence stratigraphy, comprised of the parasequence of the late stage of the last interstadial (Para-Sq2), including the highstand and forced regressive wedge system tracts (HST and FRWST), and the parasequence of the postglacial period (Para-Sq1), including the transgressive and highstand system tracts (TST and HST). The tidal sand ridges likely began to develop during the postglacial transgression as sea-level rise covered the middle Jiangsu coast at ca. 9.0 cal ka BP. These initially submerged tidal sand ridges were constantly migrating until the southward migration of the Yel- low River mouth to the northern Jiangsu coast during AD 1128 to 1855. The paleo-Xiyang tidal channel that was determined by the paleo-tidal current field and significantly different

Received: 2012-07-28 Accepted: 2012-08-20 Foundation: National Basic Research Program of China (973 Program), No.2013CB956500; National Natural Science Foundation of China, Nos.40776023 & 40872107; Comprehensive Investigation and Assessment in Jiangsu Offshore Area, Nos.JS-908-01-05 & JS-908-01-101; Special Fund for Marine Scientific Research in the Pub- lic Interest, No.201005006; Special Fund for Land and Resources Research in the Public Interest, No.201011019; China State-Sponsored Postgraduate Study Aboard Program, No.2011619035 Author: Xia Fei, Ph.D. Candidate, specialized in coastal ocean geomorphology and sedimentology. E-mail: [email protected] *Corresponding author: Zhang Yongzhan, Ph.D. and Associate Professor, specialized in coastal ocean geomorphology and sedimentology. E-mail: [email protected]

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from the modern one, was in existence during the Holocene transgressive maxima and lasted until AD 1128. Following the capture of the Huaihe River in AD 1128 by the Yellow River, the paleo-Xiyang tidal channel was infilled with a large amount of river-derived sediments from AD 1128 to 1855, causing the emergence of some of the previously submerged tidal sand ridges. From AD 1855 to the present, the infilled paleo-Xiyang tidal channel has undergone scouring, resulting in its modern form. The modern Xiyang tidal channel continues to widen and deepen, due both to strong tidal current scouring and anthropogenic activities.

Keywords: Marine Isotope Stage 3 (MIS 3); South Yellow Sea; middle Jiangsu coast; tidal sand ridge; tidal channel; sedimentary environment; radiocarbon dating; seismic stratigraphy; sequence stratigraphy

1 Introduction Since Marine Isotope Stage 5 (MIS 5, i.e., 125 ka BP), the evolution of sedimentary environments along China’s continental shelves and coastal plains was primarily controlled by global sea-level cycles and sediments supplied by big paleo-rivers (Wang et al., 2006, 2007, 2012a, 2012b). The in-depth stratigraphic correlations between coastal plains and offshore areas are in progress (Wang and Tian, 1999), and a large body of research on offshore sediment cores, seismic stratigraphy, and high resolution analyses of sediment cores of coastal plains are also on going (Marsset et al., 1996; Zhang and Li, 1996; Li et al., 1998, 2000, 2001, 2006; Saito et al., 1998; Liu et al., 2000, 2007; Hori et al., 2001, 2002; Berné et al., 2002; Wang et al., 2002, 2012a; Yoo et al., 2002; Wellner and Bartek, 2003; Liu et al., 2004; Liu and Xia, 2004; Liu et al., 2007, 2009, 2010; Wang and Li, 2009). All these achievements promise to enrich our understanding of worldwide river-sea interactions while also further- ing our knowledge of sequence stratigraphic theory. The origin of the radial sand ridge field (RSRF) of the South Yellow Sea (SYS) off the northern Jiangsu coast is due to river-sea interaction (Wang et al., 2012a). The RSRF is a large sandy deposit on the western SYS to which some large- and medium-sized rivers de- liver abundant sediments both during the geological and current periods, such as the Chang- jiang River, Yellow River and Huaihe River. The RSRF also serves as a typical model for the land-sea interactions in the eastern China coasts (Figure 1). Xiyang, which is located in the northwest RSRF, is the largest tidal channel off the middle Jiangsu coast. Since the 1980s, many scholars have studied the marine geomorphology, sedimentology and Quaternary geology of the Xiyang tidal channel and its adjacent areas (Zhu and Xu, 1982; Cong et al., 1984; Ren et al., 1986; Zhang, 1992; Zhang et al., 1992; Wang et al., 1993, 1996, 1999, 2002, 2012a; Zhu and Gong, 1994; Wang et al., 1995a, 1995b, 1997, 1998; You et al., 1998; Zhu et al., 1999; Chen et al., 2007; Gao, 2009; Huang et al., 2009; Yin and Zhang, 2010; Liu et al., 2011; Ying et al., 2011; Wang et al., 2012). From these investigations it was recognized that the Xiyang tidal channel was caused by tidal current scouring and that it developed since the middle Holocene transgression, and its surface and subsurface sediments are genetically related to the subaqueous deltas of the paleo-Changjiang and paleo-Yellow Rivers, with the mud fraction perhaps influenced more by the paleo-Yellow River Delta (Wang et al., 1993, 1996, 1999, 2002, 2012a). The studies on sediment cores of coastal plains, tidal flats and tidal channels also have enriched the knowledge of the RSRF (Cong et al., 1984; Fu and Zhu, 1986; Zhang et al., 1992; Wang et al., 1993, 1996, 1999, 2002, 2012a; Wang et al., 1995a, 1995b, 1997, 1998; Zhu et al., 1999; Li et al., 2001; Yin and Zhang,

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Figure 1 Satellite remote sensing imagery (OrbView-2 SeaWiFS imagery, imaging time: 10/08/1997) of Jiangsu coast and adjacent regions, showing the location of main sediment cores (light blue dots) and shallow seismic profiles (A–A’, B–B’, C–C’, D–D’, E–E’) 2010). However, prior to this investigation, studies on the evolution of depositional environments within the Xiyang tidal channel since late MIS 3 (Cong et al., 1984; Zhang et al., 1992; Wang et al., 1993, 1996, 1999, 2002, 2012a; Wang et al., 1995a, 1995b, 1997, 1998; Li et al., 2001; Yin and Zhang, 2010) lacked the application of sequence stratigraphic theory. In this paper, we utilize the published data from core 07SR01 recovered from the Xiyang tidal channel (Yin and Zhang, 2010) and supplement many analyses of up-to-date laboratory testing data from core 07SR01, to make the in-depth correlations with shallow seismic profiles that cross the Xiyang channel, which were collected in 1994 and 2007, along with addi- tional correlations between core 07SR01 and adjacent published sediment cores and analyses of other referenced shallow seismic profiles. The aim of this paper is to reconstruct the evolution of sedimentary environments of the middle Jiangsu coast since late MIS 3, and also to update previous study results using multidisciplinary methods (e.g., geomorphology, sedimentology, radiocarbon dating, seismic and sequence stratigraphy), so as to promote further research endeavors into the nature of local and regional sea-level changes since MIS

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3, and land-sea interactions of this region and other subsiding coasts. The results presented here would also be favorable for Jiangsu Province to take advantage of this deep tidal channel to develop Dafeng Port on muddy coasts in a sensible way.

2 Regional settings The Xiyang tidal channel is off the middle Jiangsu coast and located in the northwest RSRF. It is about 80 km long and 12 to 25 km wide and trends NNW-SSE. The west side of the Xiyang tidal channel is boarded by broad tidal flats and several small rivers empty into the SYS. The largest sandbank of the RSRF, Dongsha, is located along the southeast side of the Xiyang tidal channel. The Xiyang tidal channel is connected with the Pingtuyang seas to the north, and it is separated into two subchannels (i.e., western and eastern channels, Figure 2) by sand ridges named by Xiaoyinsha and Piaoersha (Wang et al., 2002). In this sea area, the

Figure 2 Location of the Xiyang tidal channel, main sediment cores and shallow seismic profiles (A–A’ and B–B’) Note: Inset maps (a) Regional study area location; (b) and with respect to the territory of China; (c) Distribution of bottom sediments in the study area (modified after Wang et al., 1993); ZBS: zonation boundary of sediments

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tidal regime, which is mainly dominated by the rotary tidal current system of the western SYS, is regular semidiurnal tide with an average tidal range of 3.56 m. The Xiyang tidal channel is controlled by strong alternating tidal currents, and the direction of the dominant tidal current is almost parallel to the coastline. The flood and ebb tidal currents are both moderately strong, with average velocities for the flood and ebb tidal currents off Wanggang of 1.9 and 1.8 m/s, respectively. The ebb tide is larger than the flood tide, and the transition time between flood and ebb tide is very short, enhancing suspension of fine-grained sediments and then it is difficult for sediments to deposit in the tidal channel and extremely favorable for maintenance of the deep tidal channel. The wave climate in the Xiyang tidal channel is not strong and primarily dominated by wind-generated waves. Throughout the year, the dominant wave direction is from the N, while the strongest waves approach from the NE. Wind-generated waves from the ESE, SE and E are pretty weak because of the shel- ter provided by offshore tidal sand ridges (Zhu and Gong, 1994; Wang et al., 2002). During the 1960s to 1990s, the Xiyang tidal channel became wider and deeper due to continuous tidal current scouring, especially along the more stable western channel (Zhu and Gong, 1994; You et al., 1998).

3 Materials and methods

3.1 Borehole drilling, documentation and subsampling

Core 07SR01 (33°15′50″N, 120°53′46″E) of Φ 71 mm was recovered from the western Xi- yang tidal channel of the RSRF in December, 2007 (Figure 2). The water depth at the core site was –15.40 m, which has been calibrated by the value of the tidal range during the borehole drilling. The total penetration depth of core 07SR01 is 36.10 m and its recovered length is 25.19 m, illustrating that the total core recovery is ca. 70%. Core 07SR01 was split lengthwise; and one half of it was preserved in the archives and the other half was used for the laboratory analyses that consisted of photographing the core, classifying sediment layers, recognizing and describing different types of sedimentary facies (e.g., colors based on the Munsell Rock Color Chart 2009, sedimentary textures and structures, contact relationships of sediment layers, macrofossils and concretions), and subsampling for subsequent laboratory tests.

3.2 Laboratory testing

Subsamples were generally taken at 10 cm intervals for grain-size analyses and 229 samples were measured with a Malvern Mastersizer 2000 laser particle size analyzer (Malvern In- struments Ltd., UK) after pretreating the samples with 10% H2O2, 0.1 N HCl and 0.5 N SHMP so as to remove organic and calcareous components and disperse these particles (OSP “908”, 2006) in the Key Laboratory of Coastal and Island Development (KLCID), Ministry of Education (MOE), Nanjing University (NJU). The sediment samples were classified based on the scheme proposed by Shepard (Shepard, 1954), and according to the formulas for graphic measures introduced by Folk & Ward (Folk and Ward, 1957), the grain-size parameters of sediments were calculated by use of the GRADISTAT software developed by Blott & Pye (Blott and Pye, 2001). Subsamples were generally taken at 20 cm intervals for bulk magnetic susceptibility

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analysis, and 104 samples were analyzed with a Bartington MS2 magnetic susceptibility system (Bartington Instruments Ltd., UK) at a lower frequency of 470 Hz after pretreating the samples with lower temperature (40℃) drying, dispersing and uniform mixing in the Laboratory of Earth Surface Process and Environment (LESPE), NJU. Subsequently the values of bulk magnetic susceptibility were converted to the values of mass magnetic susceptibility (Liu and Deng, 2009). Twelve samples of macrofossils (i.e., bivalves and gastropods) and 27 samples of foraminifers were analyzed in Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Nine samples of calcareous nannofossils were analyzed in the KLCID of MOE, NJU. The quantity calculation of calcareous nannofossils for one sample was based on ran- dom counting of all observable calcareous nannofossils across 110 microscope fields of view. Seventeen radiocarbon samples were analyzed in the Accelerator Mass Spectrometry (AMS) laboratory, Peking University using a compact 14C AMS system (PCAMS) manu- factured by the National Electrostatics Corporation (NEC), USA. Twenty five samples of detrital minerals, clay minerals and elemental geochemistry were analyzed in Nanjing Testing and Quality Supervision Center for Geological and Mineral Products, Ministry of Land and Resources, China. In addition, the samples of detrital and clay minerals were analyzed using an X’ Pert Pro X-ray diffractometer (XRD) by a semi- quantitative method; the samples of elemental geochemistry were analyzed in terms of whole sample content using a Philips PW2440 X-ray fluorescence spectrometer (XRF) and a WP1 one-meter plane grating spectrograph. Nine macroelements and chemical compounds

(i.e., SiO2, Ca, Fe, K2O, Mg, Na, Al2O3, P2O5 and MnO), and four microelements and chemical compounds (i.e., B, Ba, Sr and TiO2) were tested. 3.3 Data collection and correlations between seismic profiles and adjacent published boreholes

In the summer of 1994, more than 600 km of high-resolution shallow seismic profiles were acquired from the RSRF by KLCID of MOE, NJU using a GeoPulse Sub-bottom profiler (GeoAcoustics Ltd., UK). From those, we selected one profile that crosscuts the Xiaoyinsha tidal sand ridge and the western Xiyang tidal channel (Figure 5; Wang et al., 1996). We also collected four more referenced seismic profiles in the adjacent sea areas (Figures 6, 11–13) and more adjacent published cores (e.g., Dafeng: core Wanggang (Wang et al., 1993, 2002), a shallow stratigraphic profile in the Wanggang tidal flats that is comprised of 39 geological-engineering boreholes (Wang et al., 1995a, 1997); Dongtai: core G2 in Jianggang (Zhang et al., 1992; Wang et al., 1995b) and core PY-19 in Xincao (Cong et al., 1984)). In-depth correlations between shallow seismic profiles of the western Xiyang tidal channel, core 07SR01 and adjacent published cores were made with both global and regional sea-level curves since MIS 3 (Shackleton et al., 2000; Pahnke et al., 2003; Liu et al., 2004; Siddall et al., 2008) and the updated theory of sequence stratigraphy (Hunt and Gawthorpe, 2000).

4 Results

4.1 Sedimentary facies and chronostratigraphy of core 07SR01

Based on the comprehensive analyses of laboratory testing results of core 07SR01 (e.g., sediment colors and components, sedimentary textures and structures, grain size, magnetic

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susceptibility, macro (i.e., bivalves and gastropods) and micro (i.e., foraminifers and calcareous nannofossils) fossils, ratios of elemental geochemistry (i.e., values of Sr/Ba and

MgO/Al2O3)), five main types of sedimentary facies were re-identified and previous results (Yin and Zhang, 2010) were also updated. The characteristics of sedimentary facies and laboratory testing results will be described and discussed as follows (Figures 3 and 4).

Figure 3 Comprehensive column of core 07SR01

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Figure 4 Photos of representative sedimentary facies of core 07SR01 Note: (a) 8.05 to 8.35 m, clayey silt and fine sand layer (tidal sand ridge subfacies); (b) 15.51 to 15.81 m, shell fragment and sand layer (chenier?), underlain by coastal marsh peat below an erosion surface; (c) 17.72 to 18.02 m, coastal marsh peat that is composed of clayey silt and mixed with many shell fragment layers (coastal marsh facies); (d) 20.36 to 20.76 m, upper part, clayey silt layer that contains calcareous concretions, organic patches, shells and shell debris (coastal marsh facies) and lower part, clayey silt layer that contains calcareous concretions (river floodplain facies), separated by an erosion surface; (e) 24.17 to 24.47 m, clayey silt layer (freshwater lacustrine swamp facies), containing anomalous cracks caused by core splitting; (f) 31.47 to 31.77 m, fine sand and silty fine sand layer (barrier island facies)

4.1.1 Section 1 (0 to 15.77 m): Littoral and neritic facies (intertidal and subtidal zones) The topmost part of Section 1 (0 to 0.12 m) is attributed to tidal channel lag deposits and contains intact or fragmentary yellowish shells, calcareous cements and a small grain of quartziferous gravel (2.2×0.9×0.6 cm). Combining the following analyses of shallow seismic profiles, this section could be divided into two subsections with different subfacies (i.e., tidal channel infilling and reworking subfacies and tidal sand ridge subfacies) from top to bottom, which mainly shows that yellowish brown silty sand layers with thin interbedded grayish brown clay, have a great deal of well-developed wavy, lenticular, flaser and cross bedding, mixing with plenty of black charcoal fragments and some shell fragments. For the bottom of this section (15.60 to 15.77 m), a thick, hardened sand layer with a large number of fragmentary shells was observed. This interval is near totally comprised of incomplete yellowish brown bivalves and gastropods and sands with shell fragments, and also contains a few intact shells with obvious imprints of weathering. For the bivalves, there are Corbicula sp. and Corbula sp., and for the gastropods, there are Assiminea violacea and Natica sp. The bottom contacts with the underlying layer of coastal marsh peat via an obvious erosion surface that is likely a chenier deposit or the bottom layer of a tidal channel. Although the foraminiferal abundances of this section are relatively high, the species assemblage is relatively monotonous and the species are only attributed to the hyaline group with several rep- resentatives (i.e., Ammonia beccarii, A. pauciloculata major, Nonion grateloupi, N. schwa- geri, Cribrononion poeyanum, C. frigidum, Rotaliids), all of which are the common benthic

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species in the northern Jiangsu coastal zone, except for three samples from which one specimen of planktonic foraminifers was found per sample. One single calcareous nannofos- sil (i.e., Gephyrocapsa oceanica) was found in a sample with a burial depth of 15.55 m. The values of mass magnetic susceptibility range primarily between 27×10-8 and 50×10-8 m3/kg with obvious fluctuations. The Sr/Ba values range between 0.35 and 0.45, and the values of

MgO/Al2O3 (m-value) are between 12.6 and 14.1, all of which exhibit minor variations. 4.1.2 Section 2 (15.77 to 20.50 m): Coastal marsh facies Section 2 is primarily comprised of olive-gray and olive-black silty clay and clayey silt with multilayer shell fragments mixed with sands, few intact shells and hardened clay lumps, but no obvious stratification. This section contacts with underlying strata via an obvious erosion surface. The organic matter in core sediments gradually increases from bottom to top, and the color of the sediments gradually changes from dark yellowish-brown through olive-gray to olive-black, which illustrates that the coastal marsh has undergone peatification. Carbo- naceous silt and clay have progressively transitioned into peat. There are 22 interbeds comprised of shell-fragment-mixed sands with a few calcareous concretions, usually 8 cm apart from each other, between 16.70 and 18.95 m. The amount of intact shells is relatively high and a lot of shell fragments have changed colors owing to the post-deposition diagenesis. For the bivalves, there are Corbicula leana, fragments of C. sp., Potamocorbula amurensis and fragments of P. sp. and Ostrea sp. Gastropods include Polinices (Glossaulax) ampla, Umbonium sp., Odostomia sp., Natica tigrina, Turbonilla (Turbonilla) nonlinearis, T. (T.) sp., Dorsanum sp., Parafossarulus sp. and Assiminea sp. Several olive-gray elliptic sand patches were recognized between 19.82 and 20.16 m, likely due to sediment reworking and redeposition. The bottom of this section (20.16 to 20.50 m) contains many incomplete shells (e.g., bivalves: Potamocorbula amurensis and Barbatia parallelogramma; gastropods: Parafossarulus striatulus), fragments of cemented sands (e.g., 3.3×2 cm) and two strips of black organic-rich clay. All of the macrofossils found in this section indicate a mixing of freshwater, brackish and saltwater species. Except for one sample at the bottom without fo- raminifer, all the other five samples have just two species of foraminifers (i.e., Pseudoro- talia schroeteriana and Ammonia annectens), indicating that the species assemblage from this section is relatively monotonous, while the abundances are relatively high. Although P. schroeteriana and A. annectens are both typical shallow marine and stenohaline species, the characteristics of these buried foraminifers are very similar to the ones in coastal lagoon environments (GIDO et al., 1985). However, P. schroeteriana is a typical species in the late Pleistocene transgressive strata, and has not yet been reported from the regular Holocene strata (Wang et al., 1981). The mass of these two species is relatively high; therefore, they could be more easily sorted and buried in the Holocene transgressive strata during the processes of sediment reworking, transportation and redeposition. The appearance of these two species in the coastal marsh strata is an indication of reworking and redeposition of older foraminifers in dynamic coastal environments and cannot indicate the seawater depth. The values of mass magnetic susceptibility of Section 2 range primarily between 6.5×10-8 and 14×10-8 m3/kg with minor fluctuations. Except for one sample from the bottom showing an abnormal value, the other Sr/Ba values range between 0.21 and 0.28, and the values of

MgO/Al2O3 (m-value) are between 7.0 and 7.9, all of which also exhibit minimal fluctua-

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tions. Compared with the overlying strata of littoral and neritic facies (intertidal and subtidal zones), it is obvious that all of the values of mass magnetic susceptibility, Sr/Ba and

MgO/Al2O3 (m-value) of this section largely decreased. The mass magnetic susceptibility in this section illustrates a clear boundary that indicates a sudden change of sedimentary environments at the top. Considering that the values of Sr/Ba should increase with the increase of seawater depth and distance away from coasts and then could indicate the increase of marineness of sedimentary environments (Yin et al., 2007; Xu et al., 2009), and the values of MgO/Al2O3 (m-value) are sensitive to the changes of salinities of seawater (Zhang, 1988; i.e., m<1 for low salinity waters; 1

MgO/Al2O3 (m-value) are all between 8.8 and 9.4 with minimal fluctuations. This section, interpreted as river floodplain deposits, together with the underlying section which is interpreted as freshwater lacustrine swamp deposits, are both attributed to the stiff mud layers which formed during the Last Glacial Maxima (LGM). However, partial values of Sr/Ba and

MgO/Al2O3 (m-value) in this section are higher than the overlying section that is interpreted as coastal marsh deposits. This phenomenon is very likely due to the reworking, transportation and redeposition of sediments from the partially disintegrated marine strata of the SYS continental shelf, which was caused by paleostorms during lower eustatic sea levels of the LGM (Wang, 1995; Zhao, 1995). This section contains a few marine or estuarine shell fragments and calcareous nannofossils, which coincides with core T9 (located in Libao town, Haian, Jiangsu), from which marine-shell-fragment-mixed sand layers were also found in the stiff muds comprising the top of the late Pleistocene strata (Zhao et al., 1997). 4.1.4 Section 4 (21.67 to 26.65 m): Freshwater lacustrine swamp facies Section 4 is almost entirely formed of stiff mud deposits, except for the bottom (26.00 to 26.65 m) that is comprised of softer and mottled silt and clay. This section has a transitional lower contact with the underlying strata. The upper portion of the stiff mud deposits is comprised of yellowish-brown clay and silt, and the lower portion is comprised of light

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olive-gray, grayish-brown and dark yellowish-green clay and silt. Some of the stiff mud layers are mixed with very fine sands and have horizontal and small wavy bedding. A single calcareous gravel (1.1×0.9 cm) was found between 21.75 and 21.79 m. This interval contains a large amount of bivalve and gastropod fragments (e.g., Parafossarulus striatulus, Gyraulus albus, Nassarius (Phrontis) caelatulus, Potamocorbula amurensis), indicating a mixed burial of freshwater, brackish and saltwater species that accumulated together following reworking and redeposition that are attributed to paleostorm activities. This kind of phenomenon was also identified from core T9. The bottom of this section (26.00 to 26.65 m) is mixed with multicolored clay and silt (e.g., olive-gray, olive-black, and dark yellowish-brown) that is obviously different from the overlying stiff mud layers; hence it probably indicates one type of sedimentary environment that experiences alternating oxidation and reducing conditions due to frequent exposure and submergence caused by sea-level fall with some fluctuations during late MIS 3. Foraminifers are either absent or sparse (e.g., Pseu- dorotalia schroeteriana, Ammonia beccarii, A. pauciloculata major) in the three analyzed samples of this section. The values of mass magnetic susceptibility range primarily between 8.8×10-8 and 11.8×10-8 m3/kg with minimal fluctuations. The Sr/Ba values are between 0.22 and 0.36, and the values of MgO/Al2O3 (m-value) are between 5.3 and 9.3, exhibiting a bit larger fluctuations compared with the overlying strata. These ratios are close to, or a little bit smaller than the values of Section 2 (coastal marsh facies). Based on the above analyses, this section is interpreted as freshwater lacustrine swamp deposits, reflecting the processes of the fluctuating regression and the drying of enclosed water bodies, along with the impact of paleostorms that might have been favorable for the redeposition of a few marine microfossils. 4.1.5 Section 5 (26.65 to 36.10 m): Coastal barrier island facies Section 5 is mainly comprised of yellowish- and grayish-brown sands and sandy silt, with reverse grading and some horizontal and cross bedding. At the bottom, this section has some olive-gray and olive-black clay and silt. Near the top of this section, the sediment grain sizes decreased slightly. Between 26.66 and 27.41 m, there are two large, irregular calcareous concretions. Some vertical burrows filled with very fine sands, and a few gastropod fragments (e.g., Assiminea sp.) were found between 30.10 and 30.29 m. Bellow 32.45 m, burrows filled with silt, elongated and soybean-like mud patches are common, as are intact marine shells and shell fragments. Except for two samples near the bottom, foraminifers of littoral and neritic facies (intertidal and subtidal zones), similar to those observed in Section 1, were found in all the other seven samples. Most importantly, a species of warm water foraminifers that is only associated with the late Pleistocene transgressive strata (i.e., Astero- rotalia subtrispinosa; Wang et al., 1981), and a few porcellaneous foraminifers were found in this section. Except for two samples of the top (26.77 and 27.07 m), calcareous nannofossils (i.e., Emiliania sp., E. huxleyi, Gephyrocapsa oceanica, G. sp., Calcidiscus leptoporus, Helicosphaera carteri) were found in all the other five samples between 27.18 and 33.67 m, but the largest abundance of them per sample is only 10. The values of mass magnetic susceptibility range primarily between 31×10-8 and 42×10-8 m3/kg with obvious fluctuations, illustrating an obvious boundary at the top of this section interpreted to represent a sudden change in sedimentary environments. The Sr/Ba values are between 0.25 and 0.30, and the values of MgO/Al2O3 (m-value) are between 5.0 and 7.6, both with only minor fluctuations.

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The values of mass magnetic susceptibility of this section are similar to Section 1, but the values of Sr/Ba and MgO/Al2O3 (m-value) are obviously smaller than Section 1. This is very likely due to the coarser grain sizes of sediments of this section which would increase the “grain size effect” that could impact the contents of geochemical elements (Zhang et al., 2005). The shallow seismic profile (Figure 5) shows that there is a low angle reflection wave-group that is inclined offshore in the shoreward side of this seismic profile. Based on the above analyses, the sediments from Section 5 are interpreted as representing a coastal barrier island during the high sea-level period of the late MIS 3 transgression, and the foraminifers (Asterorotalia subtrispinosa) found in this section could indicate the nature of sedimentary environments dependably. Considering that the basal peat underlying the Holocene transgressive unit is the stable and characteristic layer (Wang and Lv, 1995; Wang and Li, 2009), only four of 17 14C ages were adopted to reconstruct the chronostratigraphic framework of core 07SR01. The other 13 14C ages that are not in stratigraphic order were likely disturbed during sediment reworking and redeposition (Table 1). The materials dated from the bottom of Section 5 (34.52 to 34.57 m and 34.12 to 34.22 m) were recognized as in situ buried marine shells that yielded ages of 39681±477 and 39626±219 cal a BP, respectively, suggesting that the coastal barrier island of Section 5 developed during the high sea-level period of late MIS 3. A 14C age dated by organic carbon from sandy silt at the middle of Section 3 (21.11 to 21.16 m) is 26800

Table 1 14C dating results of core 07SR01 Lab No. Sampling No. Burial depth (m) Dating material 14C age (a BP) Comment BA090468 07SR01-02-C 0.00–0.12 Mollusk shell 3920±35 Old shell redeposition BA090469 07SR01-07-C 0.80–0.81 Charcoal fra. >43000 Old organic carbon redeposition BA090470 07SR01-17-C 3.43–3.45 Silty clay 32940±130 Old organic carbon mixed BA090472 07SR01-33-C 9.88–9.90 Charcoal fra. >43000 Old organic carbon redeposition BA090473 07SR01-41-C 12.90–12.91 Charcoal fra. >43000 Old organic carbon redeposition BA090474 07SR01-51-C 15.6–15.77 Mollusk shell 35495±140 Old shell redeposition BA090475 07SR01-54-C 16.74–16.78 Mollusk shell >43000 Old shell redeposition BA101050 07SR01-sup.1 17.20–17.25 Saltwater peat 8820±40 In situ early Holocene basal peat BA101051 07SR01-sup.2 17.84–17.87 Saltwater peat 19890±80 Old organic carbon mixed BA090476 07SR01-56-C 18.10–18.13 Mollusk shell 42185±320 Old shell redeposition BA090477 07SR01-58-C 18.94–18.96 Mollusk shell >43000 Old shell redeposition BA090478 07SR01-62-C 20.20 Mollusk shell 42645±615 Old shell redeposition BA101052 07SR01-sup.3 21.11–21.16 Sandy silt 22510±100 In situ buried organic carbon BA101053 07SR01-sup.4 21.13 Calcareous c. 34060±320 Underwater dead carbon mixed BA090479 07SR01-73-C 23.20–23.26 Mollusk shell >43000 Old shell redeposition BA090480 07SR01-109-C 34.12–34.22 Mollusk shell 34480±140 In situ buried shell BA090481 07SR01-112-C 34.52–34.57 Mollusk shell 34535±460 In situ buried shell Note: (1) The value of 5,568 a was adopted as the half life of 14C and the calculated 14C ages indicate ages before AD 1950; (2) The adopted 14C ages except for BA101052 were corrected for the regional marine reservoir effect first by use of a surface ocean radiocarbon reservoir age (i.e., R=220 a) after the website database: http://radiocarbon.LDEO. colum- bia.edu, and then calibrated to calendar ages by using “Fairbanks 0107 calibration program” (Fairbanks et al., 2005); (3) The 14C calendar ages of BA090481, BA090480, BA101052 and BA101050 are 39681±477, 39626±219, 26800±163 and 9547±22 cal a BP, respectively; (4) Charcoal fra.: Charcoal fragment; Calcareous c.: Calcareous concretion

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±163 cal a BP, illustrating that the study area was dominated by river floodplain environments during the interval of dramatic sea-level fall at the end of late MIS 3. The materials dated from the top of Section 2 (17.20 to 17.25 m) were recognized as pure peat (organic carbon) that were collected from the middle of a thick carbonaceous silt and clay layer that is not affected by shell-fragment-mixed sands and supposed to be without redeposited old organic carbon. They yielded a reliable 14C age of 9547±22 cal a BP, which illustrates that the coastal marsh of Section 2 is equivalent to the basal peat that began to develop before the entire early Holocene transgression along the middle Jiangsu coast.

4.2 Seismic stratigraphy and sequence stratigraphy of the Xiyang tidal channel

4.2.1 Seismic stratigraphy of the Xiyang tidal channel Based on the different termination patterns of seismic reflection events (i.e., onlap, downlap, truncation, toplap), the parameters of seismic facies (i.e., inner reflection configuration; external geometry; reflection continuity, amplitude and frequency) and the further correlation between the sedimentary facies of core 07SR01 and the shallow seismic profiles of the western Xiyang tidal channel, three major seismic boundaries (i.e., T1 to T3) and five major seismic units (i.e., U1 to U5) were ascertained. These results could update the previous knowledge (Yin and Zhang, 2010). The same interpretation methods were also applied to the referenced shallow seismic profiles in the adjacent sea areas. The seismic stratigraphy of the Xiyang tidal channel will be described and discussed from top to bottom in the following paragraph (Figure 5). U1: As a result of the diversity of modern submarine topography of the Xiyang tidal channel, Seismic Unit 1 (i.e., U1) could be further divided into two sub-units (i.e., U1a and U1b). U1a is buried between ca. 0 and 4 m below the seabed. It is characterized by a sheet drape external geometry, and parallel and sub-parallel inner reflection configurations. The reflection amplitude, frequency and continuity of U1a are large, low and good, respectively. The contact relationship between U1a and underlying Seismic Unit 2 (i.e., U2) is unconformable and the contact boundary (i.e., T1) is rugged. U1a is correlated to the layer between 0 and 3.88 m of core 07SR01 and thus interpreted as a layer of tidal channel infilling and reworking subfacies. U1b is buried between ca. 0 and 8 m below the seabed. It is characterized by a mounded external geometry and progradational inner reflection configurations with the major dip direction of west by south. The reflection amplitude, frequency and continuity of U1b are relatively large, relatively low and medium, respectively. The contact relationship between U1b and underlying U2 is also unconformable and the contact boundary (i.e., T1) is more rugged. Although U1b is not located at the position from which core 07SR01 was recovered, U1b is interpreted as a layer of modern tidal sand ridge subfacies according to the current submarine topography which shows that it is a modern tidal sand ridge named by Xiaoyinsha (Wang et al., 2002). U2: U2 is buried between ca. 4 and 17 m below the seabed. It is characterized by a sheet external geometry and progradational inner reflection configurations with some divergent configurations. All the major apparent dip directions, which are east by north but with different angles, of the progradational reflection configurations, are accordant with each other in U2. Cross-superimposition of multiperiod progradational reflection configurations is a

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d reinterpreted tation (lower) in the western Xiyang tidal channel (modified an strata infected by the second reflection wave) ., 1996; Seis. stra. infected by the SRW: Seismic ., 1996; Seis. stra. infected by the SRW: et al upper) and its interpre shallow seismic profile (A–A’, Traverse

Figure 5 Figure after Wang after Wang

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commonly observed phenomenon in U2, but partial shapes are complicated, or they could not be recognized well due to the interference waves. The reflection amplitude, frequency and continuity of U2 are medium to relatively large, relatively high and medium, respectively. The contact relationships between U2 and overlying U1 and underlying Seismic Unit 3 (i.e., U3) are both unconformable, and the top boundary of U2 (i.e., T1) shows a toplap and the bottom boundary of U2 (i.e., T2) shows a downlap in terms of the termination patterns of seismic reflection events. Both of the two contact boundaries are a little rugged. U2 is correlated to the layer between 3.88 and 15.77 m of core 07SR01 and then interpreted as a layer of buried tidal sand ridge subfacies. This seismic unit is pretty typical at the locations of 1728, 7821 and 9874 m in the seismic profile (Figure 5). U3 and U4: Seismic Unit 3 and 4 (i.e., U3 and U4) are buried between ca. 17 and 28 m below the seabed. They are characterized by a sheet drape external geometry, and parallel and sub-parallel inner reflection configurations. The reflection amplitude, frequency and continuity of U3 and U4 are medium to large, medium, and medium to good, respectively. The top and bottom boundaries of these two seismic units are both unconformable with the overlying U2 and underlying Seismic Unit 5 (i.e., U5), respectively (i.e., T2 and T3), but the bottom boundaries of some sections are not clear enough. U3 and U4 are correlated to the layer between 15.77 and 26.65 m of core 07SR01, which incorporates coastal marsh, river floodplain and freshwater lacustrine swamp facies. Because the resolutions of shallow seismic profiles which were recovered by the GeoPulse Sub-bottom profiler are limited under normal working conditions, it is usually difficult to distinguish the clear differences among the seismic reflection configurations for sediment layers of which thickness are less than 5 m. Because of this, the seismic boundary between the sediment layers of river floodplain facies and freshwater lacustrine swamp facies is very challenging to identify in U4. The seismic boundary between U3 and U4 is also not very clear, but the seismic profile which is adjacent to core 07SR01 shows that the reflection amplitude and frequency gradually be- come larger and higher, respectively, from bottom to top (Figure 5). It appears as though these changes are related to the transition in sedimentary environments. Compared with the river floodplain and freshwater lacustrine swamp, both of which are relatively lower-energy environments, the coastal marsh is likely influenced directly by strong coastal dynamics (e.g., storms, rapid sea-level rise) and thus it will display some differences in terms of seismic reflection received from it. U5: The top boundary of U5 is buried between ca. 28 and 32 m below the seabed and the contact relationship between U5 and overlying U4 is unconformable (i.e., T3). The seismic waves did not penetrate the bottom boundary of this unit, and many seismic reflection events are indistinct, hence little detailed information could be identified from this unit. Partial sections show chaotic reflection configurations, and the reflection amplitude is relatively large with obvious variations while the reflection continuity is medium to bad. Some seismic wave groups representing low-angle seaward dipping strata were recognized on the onshore side of the seismic profile (Figure 5). U5 is correlated to the layer between 26.65 and 36.10 m of core 07SR01 and thus is interpreted as a layer of coastal barrier island facies. 4.2.2 Fifth-order sequence stratigraphic framework of the middle Jiangsu coast Based on the sedimentary facies and chronostratigraphy of core 07SR01, and the seismic

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stratigraphy of the Xiyang tidal channel, a fifth-order sequence stratigraphic framework of the middle Jiangsu coast was established (Figure 3). During the low sea-level period of MIS 2, two major geomorphic units (i.e., incised paleovalleys and paleointerfluves) developed on the North Jiangsu Plain and the Changjiang River Delta (Li et al., 1998, 2000, 2001, 2006). According to the updated viewpoints proposed by Hunt and Gawthorpe (2000) about the traditional “Exxon sequence stratigraphic model”, the parasequence boundary (PSB) of the paleointerfluve is usually the top boundary of the stiff mud layer (i.e., paleosol), and the PSB of the incised paleovalley, which is the erosion surface resulting from the river incision caused by the eustatic fall, is usually the side wall and bottom of the incised paleovalley (Li et al., 2006; Wang and Li, 2009; Li et al., 2010; Yang et al., 2010). Considering that the stiff mud could be recognized from the lower part of core 07SR01 and Seismic Unit 3 and 4 developed steadily and universally in this area, the top boundary of the layer of river floodplain facies could be regarded as the PSB. Below the PSB, it is the late stage of the last interstadial parasequence (Para-Sq2) and above the PSB, it is the postglacial parasequence (Para-Sq1). After further differentiating the boundaries between the system tracts of parase- quences, the top boundary of the layer of coastal barrier island facies could be regarded as the boundary between the highstand and forced regressive wedge system tracts (HST and FRWST) in Para-Sq2. If correct, then the layer of coastal barrier island facies belongs to the HST, and the layers of river floodplain and freshwater lacustrine swamp facies belong to the FRWST; the buried position of the maximum benthic foraminiferal abundance of the layer of tidal sand ridge subfacies could then be regarded as the maximum flooding surface (MFS) in Para-Sq1, while the layer between the PSB and the MFS belongs to the transgressive system tract (TST), including the whole coastal marsh facies and the lower half of the buried tidal sand ridge subfacies, and the layer between the MFS and the modern seabed belongs to the HST. Three scenarios with respect to the identification of the boundary between Para-Sq2 and Para-Sq1 observed in the seismic profiles of the study area and adjacent regions were identified and discussed as follows via the further correlations between the previously acquired seismic data in the RSRF (Wang et al., 1996). Scenario 1: For the areas that were incised by paleo-river channels that developed during the low sea-level period of MIS 2, or paleo-tidal channels formed during the postglacial transgression, the stiff mud layers are missing. The position of U3 and U4 is characterized by the reflection configurations of chaotic fill and the shapes of paleo-river channels and -tidal channels are identifiable in these units. Scenario 2: For the areas in which layers of coastal marsh facies are dominant (i.e., large thickness and not incised by paleo-river channels or -tidal channels; as this paper), the seismic reflection configurations of the PSB are characterized by no obvious unconformity and there are only a few differences between U3 and U4 in terms of the reflection amplitude and frequency. Scenario 3: For the areas in which layers of coastal marsh facies are poorly developed and not incised by paleo-river channels and -tidal channels, the reflection configurations of the PSB are characterized by a typical unconformity, and U2 downlap directly onto either U4, or where U3 is only a thin layer that is difficult to identify from the seismic profiles. In addition, there are a few deep scour holes whose maximum depth is greater than 40 m in the modern tidal channels that have strong tidal currents (e.g., the Xiyang tidal channel, Figure 6), where the stiff mud layers have been exposed to the bottom of tidal channels. In such

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instances, U1 through U4 are often absent or only a part of U4 still remains, and the PSB is difficult to identify in the seismic profiles.

Figure 6 Vertical shallow seismic profile (B–B’) and its interpretation in the western Xiyang tidal channel

5 Discussion

5.1 Correlations between core 07SR01 and adjacent published cores based on the fifth-order sequence stratigraphic framework of the middle Jiangsu coast

During the past 30 years, the research into Quaternary transgressive records and the stratigraphic development of China’s coastal plains and continental shelves has mainly focused on the three major transgressions since MIS 5. Meanwhile, Quaternary geologists began to use the principles of sequence stratigraphy and the records of global sea-level changes to analyze the formation and distribution of transgressive strata while developing sequence stratigraphic models for China’s continental shelves, costal zones, and deltaic plains (Wang and Lv, 1995; Zhang and Li, 1996; Li et al., 1998, 2000, 2001, 2006; Saito et al., 1998; Berné et al., 2002; Hori et al., 2002; Yoo et al., 2002; Wellner and Bartek, 2003; Liu et al., 2004; Yang et al., 2004; Li et al., 2005; He et al., 2006; Wang and Li, 2009; Li et al., 2010; Yang et al., 2010). Consequently, based on the above fifth-order sequence stratigraphic framework of the study area, this paper selected well-studied adjacent cores that have 14C ages (e.g., Dafeng: core Wanggang (Wang et al., 1993, 2002), core G34, G15, G21 and G39 in Wanggang (Wang et al., 1995a, 1997); Dongtai: core G2 in Jianggang (Zhang et al., 1992; Wang et al., 1995b), core PY-19 in Xincao (Cong et al., 1984)) to correlate with core 07SR01 thoroughly and made further analyses via combining other valuable cores. The new results presented here will update previous knowledge of the late Quaternary stratigraphy of the study area (Cong et al., 1984; Fu and Zhu, 1986; Zhang et al., 1992; Wang et al., 1993, 1996, 1999, 2002, 2012a; Wang et al., 1995a, 1995b, 1997, 1998; Li et al., 1998, 2001, 2006; Zhu et al., 1999; Yang et al., 2000, 2002; Yin and Zhang, 2010). From the results of correlations between core 07SR01 and adjacent important cores (Fig- ures 7 and 8), it can be seen that the stiff mud layer could be identified from both of core G2 and PY-19. Core G2 did not penetrate the stiff mud layer and core PY-19 has revealed the

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Figure 7 Sedimentary strata and 14C ages revealed by a great profile of tidal flats in Wanggang, Dafeng, North Jiangsu (modified and reinterpreted after Wang et al., 1997)

Figure 8 Stratigraphic correlations between core 07SR01, seismic units and adjacent published boreholes. Note: Core PY-19 is a hydrogeological core with a length of 329.27 m. The cited paper (Cong et al., 1984) just studied its upper part with a length of 125 m, and only reported the classification of transgressive and continental strata and identification of the lithology but without subfacies, hence core PY-19 is not listed in Figure 8 but discussed in the rele- vant paragraphs

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stratigraphic structures since late MIS 3, but the stiff mud layer could not be recognized from core Wanggang, G34, G15, G21 and G39. Wang et al. (1995b) analyzed four 14C ages of core G2 and considered that except for the 14C age of a calcareous concretion buried in the top of the stiff mud layer, the other three 14C ages that were recovered from layers of estuary and littoral facies and tidal sand ridge and channel facies, are all unreliable, probably due to mixing of dating materials with older organic carbon that was reworked and redeposited by strong tidal currents off the Jiangsu coast. Those analyses illustrate again that because of the possibilities of reworking, redeposition and mixing of sediments used for dating, not every 14C age from coastal cores is dependable and caution should be exercised in their adoption for reconstructing sea-level change history (Wang and Lv, 1995; Li et al., 1998). Wang et al. (1995a, 1997) also evaluated 14C ages of core G15, G21 and G39 and selected four 14C ages to reconstruct the time sequence of a shallow stratigraphic profile in the Wanggang tidal flats. They considered that the 14C age of 33700±4000 a BP from layer D could indicate the formation time of the supratidal deposits, and the 14C ages of 22900±1100 and 21400±1400 a BP from layer D could indicate the formation time of the intertidal deposits. However, those interpretations pose some problems: (1) The progression of sea-level changes since late MIS 3 (Zhao, 1987; Shackleton et al., 2000; Pahnke et al., 2003; Xin et al., 2006; Siddall et al., 2008; Li et al., 2009), the 14C age of the paleo-Yellow River Delta (26820±2380 a BP) in the central SYS (Li et al., 1998) and the 14C age of the regressive peat (34000±2000 a BP) overlying the second transgressive layer of core PY-19 (Cong et al., 1984), all of which do not support that tidal flats developed in the Wanggang area at ca. 23 ka BP; (2) Those three 14C ages adopted to determine the formation time of the supratidal and intertidal deposits at the bottom, are from core G21 and G39, which are 800 m apart from each other, and if those deposits belong to the same sedimentary unit illustrated by Figure 7, the 14C age which is located at the middle position of the other two 14C ages of ca. 22 ka BP, should not be older than 23 ka BP, hence there exists the possibility of the inversion of those three 14C ages; (3) The correlations between cores and seismic profiles indicate that stiff mud layers of the LGM and coastal marsh deposits of the postglacial period represent the stable, and widespread depositional environments in this area. Thus, the missing stiff mud layers in the Wanggang tidal flats might have been eroded away by the incision of local small paleovalleys, while the supratidal and intertidal deposits identified from their bottom might be sediments that infilled these valleys. Consequently, in layer D, the 14C ages between 20 and 40 ka BP indicate the inversion of 14C ages, and the sand and gravel layers are probably more deeply buried. Based on considerations of the above analyses and the sedimentary characteristics of layer D, we propose a reinterpretation of layer D from the Wanggang tidal flats as littoral tidal flat deposits of the postglacial period. Radiocarbon ages with errors greater than 1 ka could not be adopted as the basis for determining the true time of formation of these deposits. In addition, the 14C age of 29200±1200 a BP that was dated from inorganic carbon from core G21 is located at layer C, and the 14C age of 13530±370 a BP that was dated from a calcareous concretion recovered from core Longgang in western Yancheng, is located at the same position with a depth of 10 to 10.5 m (Cao et al., 1988). These two 14C ages are both too old, and also illustrate that reworking, redeposition and mixing processes in tidal sand ridge environments are widespread and active. Another 14C age (4550±150 a BP) that was

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obtained from organic carbon of layer C in core G15, is close in age to a dated sample of shell debris (4290±150 a BP) from the same position in core Wanggang. We believe that both are reliable ages; however, the possibility of reworking, redeposition and mixing of older sediments remains. Inversions of 14C ages exist commonly in sediment cores of the RSRF (e.g., core 07SR01; 07SR04 (Wang et al., 2012a); G2, G21, G39; Sancang (T15) (Yang et al., 2000); RD03A, RD03B, RD03C (Yin et al., 2007); Sanmin (Wang et al., 2002)) and they are consistent with the results from other tidal sand ridges of the Bohai Sea, Yellow Sea and East China Sea (Liu and Xia, 2004), demonstrating that this is not an uncommon phenomenon of dynamic tidal sand ridge systems. From the above analyses of 14C ages and sedimentary strata in the RSRF, it is evident that the sequence stratigraphic framework based on seismic stratigraphy and the parasequence boundary controlled by sea-level changes, could provide the foundation for regional correlations between cores, while reconstructed stratigraphic sections will not be mired by the existence of questionable 14C ages. Consequently, the correlations between core 07SR01 and adjacent important cores in the study area are illustrated by Figure 8. In addition, the layers of coastal barrier island facies, river floodplain and freshwater lacustrine swamp facies, coastal marsh facies, tidal sand ridge subfacies and tidal channel infilling and reworking subfacies in core 07SR01 could be correlated to the layers of the second transgression (33.6 to 44.8 m), continental facies A (16.6 to 33.6 m), 12 to 16.6 m, 6 to 12 m, and 0 to 6 m in core PY-19, respectively. Meanwhile, the continental strata A is relatively thick and several fluvial sequences that not well developed could be identified from silt and fine sand layers. They are river branch deposits developed during the regression of MIS 2. Overlying the continental strata A, they are river floodplain deposits (16.6 to 19.6 m) which were reworked into the stiff mud layer (Cong et al., 1984). The layers of tidal sand ridge subfacies that could be identified from many cores in this area, are actually alternated by fine sand and silt beds, being one of the sedimentary characteristics of tidal sand ridges (Wang et al., 1995a, 1997; Yin et al., 2007).

5.2 Formation process of the sequence stratigraphy of the middle Jiangsu coast since late MIS 3

The special warm-humid climate in China during the late stage of the last interstadial of the last glaciation (i.e., MIS 3a, ca. 44 to 25 ka BP), has a strong relationship with the higher July insolation in the mid and low latitudes during this period, which is mainly due to the changes in the Earth’s precessional cycle (Shi et al., 2001; Shi and Yu, 2003; Yang et al., 2004; Yu et al., 2007; Shi and Zhao, 2009). The higher July insolation caused temperature rises and precipitation increases in the monsoon and westerly areas of China, which resulted in the partial melting of glaciers and concomitant transgressions with fluctuations within a certain range that happened along the continental shelves of East China. The increased ter- restrial biomass (vegetation) had a positive feedback with both temperature and humidity (Shi and Yu, 2003; Shi and Zhao, 2009; Li et al., 2009). The middle Jiangsu coast is located in the North Jiangsu-South Yellow Sea subsiding belt and as such would be influenced obviously by Quaternary transgressions (Ren et al., 1986). Coastal barrier island deposits which belong to the HST of Para-Sq2, developed in this area at ca. 39 cal ka BP when sea level was relatively high (i.e., ca. –40 m, Figure 9), as revealed by more than 9.45 m of strata

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in core 07SR01. During the interval from ca. 39 to 26 ka BP, core BY-1, which is located in Baoying county, west North Jiangsu Plain and 120 km distant from the present Jiangsu coastline, also recorded the transgressive event as revealed by ca. 7 m of tidal flats and lagoon facies strata (Zhang et al., 2010).

Figure 9 Correlations between global sea-level changes and depositional sequences along the middle Jiangsu coast during MIS 3 to LGM (These two global sea-level curves were modified after Shackleton et al., 2000, Pahnke et al., 2003 and Siddall et al., 2008)

Following the end of MIS 3, MIS 2 began at ca. 25 ka BP with the initiation of dramatic global cooling. The polar ice sheets advanced very quickly and sea level began to fall rapidly. A group of four buried beach ridges indicates that paleo-coastlines that formed during a transient pause of sea-level fall at ca. 23.7 ka BP, developed on the outer shelf of the East China Sea (Zhao, 1987). At the LGM (i.e., ca. 23 to 19 cal ka BP), sea level of the SYS fell to ca. –120 m (Figure 10) and the study area was completely subaerial. Deposits of freshwater lacustrine swamp and river floodplain, belonging to the FRWST of Para-Sq2, developed successively to a stratal thickness of 6.15 m, as revealed by core 07SR01 (Figure 9). Fur- thermore, due to the strong deflation by paleostorms on China’s continental shelves during the LGM, marine strata partially disintegrated, and sediments were dispersed by increased wind activities. Some of those reworked marine sediments were redeposited in the study area, becoming an important sediment source (Wang, 1995; Zhao, 1995). The top of river floodplain strata, which exhibits a hiatus or partial absence as a result of long-term subaerial exposure and reworking by coastal dynamics during the postglacial period, is regarded as a PSB that is widespread in the North Jiangsu Plain, the Changjiang River Delta Plain and even the entire continental shelf of China Seas. The deglacial period began at ca. 15 cal ka BP as global climate warmed. The step-like and rapid rise of postglacial sea level occurred as a result of global meltwater pulses (Fair- banks, 1989; Liu et al., 2004). The TST which overlaps the FRWST and whose bottom is

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Figure 10 Correlations between relative sea-level changes in the western Pacific (SYS, ECS, and SCS) and depositional sequences in the middle Jiangsu coast since the LGM (ECS: East China Sea; SCS: South China Sea; The relative sea-level curve was modified after Liu et al., 2004) marked by a transgressive surface is found extensively across the continental shelves during the postglacial transgression (Yang et al., 2004; Li et al., 2005) when Para-Sq1 began to form (Figure 10). As the global climate ameliorated, the rate of sea-level rise slowed down and ultimately stagnated (between –42 and –38 m) during 11.4 to 9.6 cal ka BP (Figure 10). During this interval, the modern-day coastal plains and inner continental shelves were covered by coastal marsh and related deposits (Wang and Lv, 1995). For example, core T12 which is located in Yannan, Rudong county of Jiangsu, reveals that the study area was influenced by the postglacial transgression at ca. 12 cal ka BP (Li et al., 1998); the thickness of coastal marsh deposits belonging to the TST of Para-Sq1 in core 07SR01 is up to 4.73 m, while the bottom of coastal marsh deposits which recorded the evident storm sedimentation is a typical transgressive surface, and it probably formed during MWP-1B (Meltwater Pulse 1-B, 11.8–11.4 cal ka BP), which was inferred from the sea-level curve (Figure 10). Follow- ing MWP-1B, sea level stagnated between –42 and –38 m for a short time (i.e., ca. 1.8 ka, Figure 10) and the influences of marine waters in the study area became more evident. Core 07SR01 recorded more than 22 layers of shell fragments mixed with sands that indicate a higher occurrence frequency of storm sedimentation during this period. The coastal marsh underwent peatification when the postglacial climate improved obviously, as recorded by the undecomposed organic residues of plants gradually accreted to form peat layers. Around 9.5 to 9.2 cal ka BP, sea level rose rapidly again in response to MWP-1C, jumping from –36 to 14 –16 m (Figure 10). The C age of peat (9547±22 cal a BP) located in the upper coastal marsh deposits of core 07SR01 (17.20–17.25 m) also indicates that cheniers developed and their burial was in response to this rapid sea-level rise. At the same time, coastal marshes were quickly inundated by more saline waters. From then on, the study area was totally covered by seawater and became littoral. Before the mid-Holocene sea-level highstand, sea level rose rapidly once more, jumping to +2 to +3 m, in response to MWP-1D (ca. 8–7 cal ka

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BP, Figure 10). During this period, water depths in the study area increased to ca. –30 m. Because, active tidal sand ridges usually appear on continental shelves whose water depths are less than –60 m (Liu and Xia, 2004), the study area therefore possessed the depth conditions necessary to support the development of tidal sand ridges. During the early and middle MIS 3 (ca. 60–40 ka BP) and early Holocene (ca. 9.6–8.5 ka BP), the paleo-Yellow River emptied itself into the SYS off the northern Jiangsu coast, constructing several paleo-Yellow River Deltas (illustrated by seismic data, Figures 11–13) (Wang, 1982, 1990; Milliman et al., 1987, 1989; Liu et al., 2004; Xue et al., 2004; Yu et al., 2005; Liu et al., 2010; Wang et al., 2012b). These unconsolidated deltaic sediments were in part reworked and transported during the postglacial transgression. Numerical modeling of the paleo-tidal current field illustrates that the radial nearshore paleo-tidal current field gradually developed in the SYS off the northern Jiangsu coast during 10–7 ka BP, when strong alternating tidal currents in an approximate S-N direction were already very typical in the Xiyang area (Lin et al., 2000; Zhu and Chang, 2001; Wang et al., 2002). The numerical modeling results indicate that dynamic conditions necessary for the development of radial tidal sand ridges were established by the early Holocene. The radial paleo-tidal current field reworked and transported sediments of the paleo-Yellow River Delta that were distributed between 0 and –40 m to the nearshore area off the northern Jiangsu coast. This was an important sediment source for the early development of the northern RSRF, probably adding some sediments of different origins.

Figure 11 Seismic profile of the MIS 3 paleo-Yellow River Delta (?) and its interpretation in the Haizhou Bay, South Yellow Sea (C–C’, modified and reinterpreted after Tao, 2009)

The content of carbonate minerals in the Yellow River-derived sediments is usually much higher than that from the Changjiang River, thus the content of carbonate minerals in core sediments could serve as a proxy of the Yellow River’s impact on the study area (Xiong et al., 2003). Zhang (2010) showed that the average content of carbonate minerals in core 07SR01 is relatively low, but the contents of carbonate minerals (i.e., dolomite, vaterite and

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Figure 12 Seismic profile of the MIS 3 paleo-Yellow River Delta (?) and its interpretation off the Haizhou Bay, South Yellow Sea (D–D’, modified and reinterpreted after Li et al., 1998)

Figure 13 Seismic profile of the early Holocene paleo-Yellow River Delta and its interpretation off the Aban- doned Yellow River Delta along the northern Jiangsu coast (E–E’, provided by Dr. John D. Milliman, Virginia Insti- tute of Marine Science, USA) calcite) in eight samples above the depth of 16.14 m show an obvious increase, while the layer of tidal sand ridge subfacies is just right located in the depth between 3.88 and 15.6 m. Therefore, the change in carbonate mineral abundance in core 07SR01 confirms that the paleo-Yellow River Delta deposits were an important sediment source for the early development of the northern RSRF during the Holocene. Sediment source identification results from the postglacial period of core Tongshang (Nanyang town, Dafeng), Dongtao (Daqiao town, Dafeng) and Sancang (T15) (Sancang town, Dongtai) all illustrate that the Changjiang River-derived sediments just appear in the southern Jiangsu coast; however, the important influences of the Yellow River-derived sediments are more prominent in the northern Ji- angsu coast, while sediments from core Tongshang are almost all derived from the Yellow River during the postglacial period (Yang et al., 2000, 2002).

XIA Fei et al.: Evolution of sedimentary environments of the middle Jiangsu coast since late MIS 3 907

As previously discussed, inversions of 14C ages in tidal sand ridge strata of the RSRF are believed to result from the reworking, redeposition and mixing of older sediments via strong coastal dynamics, the selection scheme of materials for radiocarbon dating, etc. However, according to correlations between sediment cores and seismic profiles, the 14C ages of peat that have reliable stratigraphic positions and correlations between sea-level changes and depositional sequences in the middle Jiangsu coast since the LGM (Figure 10), it could be inferred that tidal sand ridges initiated as the study area became littoral during the postglacial transgression (ca. 9.0 cal ka BP). After the mid-Holocene sea-level highstand (ca. 6.5 cal ka BP), a stable sea level and dynamic conditions were both favorable for the extensive development of radial tidal sand ridges. However, between the early Holocene (ca. 8.5 ka BP) and AD 1128, the Yellow River did not shift its river course considerably as it flowed into the SYS for a long time (Xue et al., 2004). During this period, the Changjiang River-derived sediments were concentrated along the areas south of Jianggang (Li and Fan, 2009). Under the circumstances, in the Xiyang area, tidal sand ridges were submerged with constant adjustments in deeper waters that resulted from the insufficiency of direct sediment supply by rivers and the active reworking by marine dynamics even though the water depth during that period (within –30 m) was favorable for sand ridge formation. Also during the early Holo- cene, the paleo-tidal current field gave rise to the paleo-Xiyang tidal channel that is significantly different from the modern one, which is consistent with previous knowledge based on archaeological data and historical documents (Zhang et al., 1992). The upper buried tidal sand ridges probably contain residual sand ridges derived from the subaqueous delta of the Abandoned Yellow River during AD 1128 to 1855, but they could not be identified based on seismic profiles and 14C ages from cores. The buried position of the maximum benthic foraminiferal abundance in buried tidal sand ridge strata is regarded as the MFS. Then the upper and lower parts of the buried sand ridge layer belong to the HST and TST of Para-Sq1, respectively. Furthermore, the landward extent of tidal sand ridges developed during the mid-Holocene was likely greater than the present RSRF, as its apex could reach to Dongtai (Zhao et al., 1997; Li et al., 2001). During AD 1128 to 1855, the Yellow River captured the Huaihe River, entering the SYS off the northern Jiangsu coast. The subaqueous delta of the Abandoned Yellow River developed and the paleo-Xiyang tidal channel was infilled along with emersions of some submerged tidal sand ridges during this period due to the high sediment input. According to historical records, the paleo-Xiyang tidal channel still existed before the entire capture of the Huaihe River by the Yellow River (AD 1494) as it was utilized as an official sea-route between Haimen and Yancheng during the early Ming Dynasty (ca. AD 1368). However, this marine shipping route was soon discarded, suggesting that this tidal channel was gradually infilled (Zhang et al., 1992). However, since AD 1855 when the Yellow River shifted its river course northward and flowed into the Bohai Sea, the direct supply of the Yellow River-derived sediments off the northern Jiangsu coast has been cut off and marine dynamics have dominated in the study area again. Since then the infilled paleo-Xiyang tidal channel began to undergo strong tidal scouring in concert with its southward extension and depth increase, resulting in its modern form. The tidal channel infilling and reworking deposits revealed by core 07SR01 are composed of 3.88 m thick yellowish fine sediments. Analyses of sediments in adjacent published cores indicate that these sediments have a close relation-

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ship to the Abandoned Yellow River-derived sediments (Wang et al., 1995a, 1997). The corresponding seismic unit is just a thin layer (U1a) that is characterized by a sheet drape external geometry, and parallel and sub-parallel inner reflection configurations, indicating a depositional state that accommodates alternating tidal currents in the Xiyang channel. The infilling deposits have been scoured constantly by strong tidal currents after AD 1855, and the top of the residual infilling deposits was totally reworked and covered on the bottom of the Xiyang tidal channel. Therefore, it is not appropriate to interpret U1a as tidal channel infilling deposits as proposed by Yin and Zhang (2010). This kind of seismic reflection characteristics is also identified from outer areas of the northern RSRF (Liu et al., 2010) and tidal sand ridges of the middle continental shelf of the East China Sea (Liu and Xia, 2004). At present, accompanied by the retarded erosion and retreat of the Abandoned Yellow River mouth, sediment transport from the Abandoned Yellow River Delta has decreased accordingly and the modern Xiyang tidal channel continues to widen and deepen, due both to strong tidal current scouring and anthropogenic activities. The results of modern scouring and silting changes of the RSRF during 1979 to 2006 illustrate that the scouring depth of neighboring areas of core 07SR01 has reached to 0 to –3 m and in the western deep holes, it has exceeded –5 m (KLCID of MOE, NJU, 2009). The shallow seismic profile recovered in 2007 also illustrates that the depths of deep holes could exceed –40 m (Figure 6) and these sediments eroded from tidal channels could contribute to the slow deposition of modern tidal flats and adjacent tidal sand ridges. However, the tidal flat profiles show that supratidal zones continue to accumulate and broaden, while intertidal zones experience scouring, as scoured slopes appear along the nearby water’s edge so as to accommodate the dynamic conditions of the Xiyang tidal channel. The middle Jiangsu coast between downtown Yancheng and Dongtai is located in the relative uplift area between two sub-depressions (i.e., Yancheng-Funing and Dongtai depressions), and the tectonic subsidence of this area is smaller compared with the adjacent sub-depressions, as it is estimated to be only ca. 5 m during the past 40 ka (Yang, 1986; Lin, 2006). The coastal barrier island strata which is not penetrated by core 07SR01 is buried between 42.05 and 51.50 m, and after calibrating for tectonic subsidence (ca. 5 m) and mi- nor hydro-isostatic adjustment of the past 40 ka, and also considering that coastal barrier islands usually develop at sea level of ca. ±5 m (Wang and Zhu, 1989), then sea level derived from its burial depth is consistent with a reported late MIS 3 sea level of ca. –40 m. Additionally, this correlation result could validate the reliability of the sea-level curve of MIS 3 (Figure 9). Consequently, Para-Sq2 and Para-Sq1 identified from the study area are controlled by sea-level cycles since late MIS 3, changes of the sediment input from paleo-rivers, and to a less extent, tectonic subsidence. The study of sequence stratigraphy of the middle Jiangsu coast since MIS 5 is still at its preliminary stage and future in-depth study will depend on the high resolution analyses of boreholes and seismic stratigraphy, and the breakthrough in the study of eustatic sea-level changes, particularly MIS 5 and MIS 3. Accordingly, due to the double influences of big rivers and strong tidal currents in the study area, lower core recovery rate, limitations on recovery of seismic profiles in shallow waters, wider intervals between seismic tracklines and limitations on geochronological techniques, the viewpoint proposed in this paper is still quite preliminary and the above problems need to be solved gradually so as to enrich our knowledge of sequence stratigraphic theory.

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6 Conclusions (1) Stratigraphic and geochronologic data from core 07SR01 was combined with shallow seismic profiles that were recovered from the northern radial sand ridge field off the middle Jiangsu coast. Both data sets recorded environmental changes in the study area since late MIS 3 (ca. 39 cal ka BP) that were mainly controlled by sea-level fluctuations, sediment discharge of paleo-rivers into the South Yellow Sea, and minor tectonic subsidence, all of which impacted the progression of regional geomorphic and sedimentary environments (i.e., coastal barrier island, freshwater lacustrine swamp, river floodplain, coastal marsh, tidal sand ridge, and tidal channel). Based on the established sequence stratigraphic framework of the study area, the correlations between boreholes allow for increased insight into regional sedimentary strata and environmental changes. (2) A fifth-order sequence stratigraphic framework of the middle Jiangsu coast since late MIS 3 includes the late stage of the last interstadial parasequence (Para-Sq2) and the postglacial parasequence (Para-Sq1), between which the top boundary of river floodplain strata (the stiff mud layer), is regarded as the parasequence boundary. The highstand and forced regressive wedge system tracts of Para-Sq2, and the transgressive and highstand system tracts of Para-Sq1 were also identified. Based on the updated viewpoints proposed by Hunt and Gawthorpe (2000) about the traditional “Exxon sequence stratigraphic model”, the sequence identifications and divisions of system tracts in terms of the transitional zone between land and ocean of East China that was controlled by the fourth-order forced regression during MIS 5 to MIS 1, are of high maneuverability and realistic significance, which could modify the incoherence of the conceptual system of sequence stratigraphy. (3) Tidal sand ridges probably have begun to develop since the study area became littoral during the postglacial transgression (ca. 9.0 cal ka BP) and were submerged with constant adjustments in deeper waters until AD 1128. The paleo-Xiyang tidal channel which was determined by the paleo-tidal current field and is significantly different from the modern one, was in existence during the Holocene transgressive maxima and lasted until AD 1128. Dur- ing AD 1128 to 1855, the Yellow River captured the Huaihe River empting into the South Yellow Sea off the northern Jiangsu coast. The paleo-Xiyang tidal channel was infilled gradually with a large amount of river-derived sediments, causing the emergence of some of the previously submerged tidal sand ridges. From AD 1855 to the present, the infilled paleo-Xiyang tidal channel has undergone scouring, resulting in its modern form. The modern Xiyang tidal channel continues to widen and deepen, due both to strong tidal current scouring and anthropogenic activities. (4) There generally exist inversions of 14C ages in tidal sand ridge strata of the radial sand ridge field as a result of the reworking, redeposition and mixing of older sediments via strong coastal dynamics, the selection scheme of materials for radiocarbon dating, etc. This appears to be a typical characteristic of the sedimentary environments of tidal sand ridges. Owing to the strong deflation and transportation by paleostorms on China’s continental shelves during the LGM and the reworking by coastal dynamics during the postglacial period, older shells or other sources of organic carbon were also likely reworked and transported from their original place of deposition to other places, thus those 14C ages which were published but abandoned in this paper, can not indicate the reliable ages of deposits, how-

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ever, recorded the nature of paleo-environments of China’s continental shelves and coastal plains. Only the 14C ages of peat in coastal marshes that have reliable stratigraphic positions and were not influenced by event deposits, are reliable indicators of stratal ages and as tie points for relative sea-level determinations.

Acknowledgements This paper is derived from the first author’s Ph.D. thesis. The sedimentary facies analysis of core 07SR01 was supervised by Prof. Wang Ying. The identification of calcareous nannofossils of core 07SR01 was accomplished by Prof. Zou Xinqing. Prof. Gao Shu, Zhao Songling and Associate Prof. Han Zhiyong made many constructive comments on this paper. Two anonymous referees also gave a lot of valuable suggestions for the further revision. Dr. Li Gang and Mr. Huang Jiaxiang participated in the field work. Dr. Li Gang, Dr. Sun Zhuyou, Mr. Xu Liang, Miss. Zhang Ning, Mr. Ge Song, Miss. Gao Minqin, Miss. Liu Qun and Miss. Du Liqiong participated in the analysis of sedimentary facies and subsampling of core 07SR01. Dr. Sun Zhuyou and Miss. Zhang Ning also did some laboratory work of sample tests. We are extremely grateful to the above persons who helped us a lot for this study.

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