Sedimentary history of the western Bohai coastal plain since the late Pliocene: Implications on tectonic, climatic and sea-level changes

Journal of Asian Earth Sciences 54–55 (2012) 192–202

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Sedimentary history of the western Bohai coastal plain since the late Pliocene: Implications on tectonic, climatic and sea-level changes Zhengquan Yao a,b,⇑, Zhengtang Guo c, Guoqiao Xiao d, Qiang Wang e, Xuefa Shi a, Xianyan Wang f a

Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi’An 710075, China c Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China d Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China e Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China f School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210093, China b

a r t i c l e

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Article history: Received 13 August 2011 Received in revised form 16 February 2012 Accepted 13 April 2012 Available online 25 April 2012 Keywords: Sedimentology Sea-level Tectonic Climate Bohai Basin Late Cenozoic

a b s t r a c t Thick Cenozoic deposits from the western Bohai coastal plain, a tectonic-subsiding region, provide the potential to study the relations between sedimentary environments and tectonic, climatic and sea-level changes. However, sedimentary history of this region extending to the whole Quaternary, as well as their links to tectonic, climatic and sea-level changes are still poorly understood, mainly because of the lack of long-term records with well-constrained chronology. In this study, we present an integrated record based on sedimentology and proxies (grain-size and color reflectance) of a 203.6 m core recovered from the western Bohai coastal plain near Tianjin. The core was chronologically well constrained using paleomagnetic and optically stimulated luminescence dating methods. The results show that from the late Pliocene (3.3 Ma) to the late Pleistocene (0.10 Ma), the study region was mainly dominated by fluvial setting, and the extensive incursion of sea water into this region began during the last interglacial period (0.10 Ma). The sedimentology and the color index suggest that tectonic subsidence of the Bohai Basin during the Plio-Quaternary must have played a significant role in controlling the sedimentary environments in this region. The changes in base-level because of sea-level fluctuations during the Quaternary influenced the fluvial development greatly and led to the alternations of coarse crevasse splay/channel and finer floodplain deposits in the core sequence. In addition, climatic changes since the late Pliocene also have had significant effects on the sedimentary settings in the Bohai coastal plain by influencing the fluvial process with a series of mechanisms. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Coastal zones are transitions between continents and oceans, and are vital to the understanding of continent–ocean interaction related to tectonic, climatic and sea-level changes (McMillan, 2002; Limarino et al., 2006; Juhász et al., 2007). The Bohai Sea is a semi-closed marginal sea in eastern China. The study area is very sensitive to the Quaternary sea-level fluctuations because of very flat geomorphology and low altitude (less than 4 m). Large thickness of sediments has been accumulated in the western Bohai coastal plain because of the continuous tectonic subsidence since the Cenozoic era (Allen et al., 1997). The continuous subsidence would be favorable for the preservation of sediments, which ⇑ Corresponding author at: First Institute of Oceanography, State Oceanic Administration, 6 Xianxialing Road, Laoshan District, Qingdao 266061, China. Tel.: +86 532 88961651. E-mail address: [email protected] (Z. Yao). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.04.013

contain the information of climatic and sea-level changes. Therefore, the thick deposits in the western Bohai coastal plain may provide us an opportunity to study the relations between sedimentary environments and tectonic, climatic and sea-level changes. During the past several decades, most studies on the western Bohai coastal plain have primarily concentrated on sea-level changes and shoreline evolution (Zhao et al., 1979; Wang and Li, 1983; Yang and Xie, 1984), strata architecture of the Bohai coastal plain and marine transgression events (Yang et al., 1979; Wang et al., 1981, 2008b), as well as palaeochannel evolution and its relations to tectonic movements and climatic change (Xu et al., 1996a,b), most of which only span the late Quaternary. However, comprehensive studies on the lithology, sedimentary facies and environmental evolution of the western Bohai coastal plain extending to the whole Quaternary are sparse mainly because of the lack of long-term records with well-constrained chronology. Although Chen et al. (2008) recently reported Quaternary stratigraphy in the Bohai coastal plain mainly based on lithology and microfossils, detailed

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sedimentary facies analyses and associated sedimentological index, the issues of links between depositional environments and tectonic, climatic and sea-level changes were not addressed, which is the focus of the current research. In this study, a 203.6 m core (BZ2) was recovered near Tianjin west of the Bohai Bay. The chronology of the core was well defined using paleomagnetic and optically stimulated luminescence (OSL) methods, which yielded a basal age of 3.3 Ma (Chen et al., 2008; Yao et al., 2010). We present the first detailed and integrated sedimentological study of the BZ2 core spanning the late Pliocene–Quaternary. The specific objectives of the current paper were (1) to perform detailed facies analyses and establish the sedimentary history of the study area and (2) to tentatively discuss the sedimentary history of the western Bohai coastal plain since the late Pliocene as well as its relation to tectonic, climatic and sea-level changes.

2. Geological setting

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The Bohai Sea is an inland sea of China that connects to the Yellow Sea via the Bohai strait (Fig. 1a). The main rivers flowing through the Bohai coastal plain are the Yellow River, the Luanhe River, the Haihe River and the Liaohe River, which originate from the Tibetan Plateau, Taihang Mountain and Yanshan Mountain (Fig. 1). Among them, the Yellow River is the largest and is the main source of sediments in this region. The flat plain slopes

generally eastward from an altitude of approximately 100 m above sea level (asl) in the west to less than 4 m asl in the east. The Cenozoic strata are dominated by fluvial deposits in the piedmont plain, alluvial and lacustrine deposits in the central plain and alluvial deposits with interbedded marine deposits in the littoral plain. The Bohai Basin is a Cenozoic extensional basin bordered by Taihang Mountain, Yanshan Mountain, the Liaodong and Shandong peninsulas (Fig. 1b). Its basement is divided into a series of alternating uplifts and depressions trending NNE and NE (Allen et al., 1997; Fig. 1b). Previous studies revealed that the Bohai Basin experienced episodic sub-rifting processes from the Eocene to the end of the Oligocene (Allen et al., 1997; Hu et al., 2001), and as a whole the basin began to subside in a post-rift phase of thermal subsidence that has lasted until the present day (Allen et al., 1997). The thickness of sediments in the Bohai coastal plain is variable with uplifts and depressions. For example, the Quaternary sediments in the Jizhong depression and the Huanghua depression (where great subsidence has taken place) are near 500 m thick, whereas those in the Cangzhou uplift are only 200–300 m thick (Wang et al., 2003). The study area is located at the northwestern part of the Bohai Basin (Fig. 1b). In this region, three formations, Gu’an, Yangliuqing and Ouzhuang, were deposited during the Pleistocene, with fluvial–lacustrine deposits in the Early–Mid Pleistocene and fluvial– marine deposits in the Late Pleistocene, according to Chen and Wu (1997). During the Holocene, Yangjiasi, Gaowan and Qiko

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Fig. 1. Simplified map of the study area (a), tectonic structures of the Bohai Basin (b; modified after Hu et al., 2001), the location of the BZ2 core site (solid circle) and the boring sites mentioned (solid triangle) in the text (c).

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formations were deposited. The fluvial–lacustrine deposits dominated the Early and Late Holocene, while the marine transgression happened in the Middle Holocene (Chen and Wu, 1997). The study area is within the East Asian monsoon zone, characterized by warm-moist summers and cool-dry winters (Zhang, 1991). The mean annual precipitation and temperature are 700 mm and 12.6 °C, respectively (Wu, 1992). July average temperature is 25.5 °C and January average temperature is 2.3 °C. Precipitation mainly falls in July and August, accounting for 70% of the annual rainfall (Zhang et al., 2000). 3. Materials and methods The BZ2 core (203.6 m) was recovered near the Bohai Bay (117°80 E, 39°10 N; Fig. 1) with a core-top elevation of 4 m. The core was recovered using a rotary drilling method with an average recovery of 89.3%. In the laboratory, the core was split into two sections, photographed, described and subsampled for analyses. Grain-size analysis using sieving and Malvern Mastersize laser particle sizer was performed for 412 samples at 0.5 m intervals throughout the core to characterize the sediment texture. Carbonate and organic material are the main cements of sediments and, in general, must be removed by chemical treatment. In the current study, we have used the chemical procedure introduced by Konert and Vandenberghe (1997). About 2 g sample was pre-treated with 15 ml 10% H2O2 to remove organic matter, with 15 ml 10% HCl to remove carbonates and finally with 10 ml 0.05 N ((NaPO3)6) to facilitate dispersion. The grain-size distribution of the samples was measured using a Malvern Mastersize laser particle sizer for the finer fraction (<0.8 mm), and wet-sieving for the coarser fraction (>0.8 mm). Textural statistical parameters (mean, sorting, skewness and kurtosis) were calculated using the method of moments (Friedman and Sanders, 1978). Micropaleontological analyses mostly at 50–100 cm intervals throughout the core were conducted (Chen et al., 2008). In the present study, we selected four samples in the representative layers to examine the foraminifera and ostracods to confirm the results. The abundance of the foraminifera and ostracods was calculated based on a dry sediment sample of 20 g. Sediment color is an important parameter to indicate the redox conditions of the environment. Therefore the color reflectance of the split surface of the sediments was measured using a Minolta CM-2002 hand-held spectrometer at 10 cm intervals. The results were represented as a three-dimensional color system Lab and as Lch coordinates. In the Lab color system, a (b) varies between red (yellow) and green (blue), with higher values suggesting red (yellow) color. In general, color index of a and b values are mainly linked to the presence of high-valence iron oxide and hydroxide (Deaton and Balsam, 1991), which are prone to be enriched in more oxidized environment, and therefore can be employed to indicate the redox conditions. The index of c in the Lch coordinates is defined as c = (a2 + b2)1/2, thus contain both the imprints of a and b indexes, with higher c value indicating more oxidized conditions. 4. Chronostratigraphic framework The chronostratigraphic framework of the BZ2 core was defined by paleomagnetic measurements (Yao et al., 2010) and OSL dating method (Chen et al., 2008). Stepwise thermal demagnetization up to 585 °C–680 °C and progressive demagnetization in alternating field up to 60 mT were performed on 461 samples. In most cases, an alternative demagnetization at 25 mT or thermal demagnetization at 300 °C led to an unambiguous polarity assignment. The obtained magnetozones are shown in Fig. 2 and are fairly correlated with the standard polarity timescale (Cande and Kent,

1995). Extrapolations by sedimentation rates based on adjacent polarity zones yielded a basal age of 3.3 Ma BP (Fig. 2). OSL dating was performed for the upper 30 m portion to constrain the chronology of younger deposits (Chen et al., 2008). The OSL results demonstrated that the upper 30 m portion of the BZ2 core was deposited during the last interglacial period (Chen et al., 2008).

5. Results 5.1. Sedimentary facies The results of micropaleontological analysis (Chen et al., 2008 and this study), grain size composition, and color reflectance are shown in Fig. 3. The photos and grainsize distribution of representative lithofacies are shown in Figs. 4 and 5. Nine sedimentary facies were recognized in the BZ2 core. They are named using the modified Miall0 s codes and listed with their characteristics and environmental interpretations referred to Miall (1978, 1984; Table 1). Facies Ch is characterized by very poorly sorted (2.6–2.8U) grayish-yellow (2.5Y 6/2) medium to coarse sand (Fig. 4a) with a mean grain size of 2.9–3.8U and a multimodal grain-size distribution (Table 2; Fig. 5a). It has large kurtosis values (3.1–4.1U) suggesting leptokurtic form of the grain-size distribution curve. Strong positive skewed grain-size distributions for Facies Ch can be observed (Table 2, Fig. 5a). This facies is generally 3–4 m thick and displays an erosional lower boundary (Fig. 4a) with a finingupward tendency (Supplementary figure; S-Fig. 1a). Sedimentary structures of unidirectional cross laminations can be observed in sand body (S-Fig. 1a). Mud clasts occasionally occur at the bottom of the sediments. The erosional lower boundary of Facies Ch, combined with its relative large thickness, fining-upward trend, the presence of unidirectional flow structures and the absence of marine fossils suggest that Facies Ch is commonly identified in fluvial deposits as originating from channel processes of medium to high energy (Miall, 1978; Desloges and Church, 1987; Nichols, 1999). Facies Sm consists of very poorly sorted (2.5–2.8U) massive grayish-yellow (2.5Y 6/2) fine to medium sand (Fig. 4b), with varying thickness from 0.5 to 2.0 m. The grain-size distribution of Facies Sm is similar to that of Facies Ch, except that Facies Sm has smaller skewness and kurtosis (Table 2; Fig. 5b). This facies is generally less than 2 m thick and displays sharp lower boundary with either fining- or coarsening-upward trends. It is encapsulated by finer clay/silt in the vertical profile (Supplementary figure; S-Fig. 1b and c). Considering the lesser thickness, characteristics of grain size and vertical sequences, these sand bodies are interpreted to be the high energetic flooding deposits resulted from crevasse process in fluvial environments. The fining-up trends are interpreted to represent crevasse-channel deposits, while coarsening-upward successions with transition to the underlying mud could be the upper crevasse splay deposits. Massive sand in fluvial environments is generated by post-sedimentary modification (Miall, 1996) and/or resulted from highly concentrated sediment-laden floods (Miall, 1996; Jo et al., 1997). Facies Fl consists of alternating beds/laminations of normally or inversely graded, grayish-yellow (2.5Y 6/2), brownish-gray (10YR 5/1) fine to medium sand and clay/silt (Fig. 4c and c0 ; S-Fig. 2a) containing freshwater bivalves, Lamprota sp. and Gyraulus albus. Sediments of Facies Fl are poorly sorted (1.6–2.8U), with a mean grain size of 4.2–5.6U (Table 2). They are bimodal with fine (6– 8U) and coarse (2–4U) fractions (Fig. 5c). The sandy deposits are resulted from high-velocity flow as the water initially spreads out over the floodplain, followed by rapid clayey/silty deposits as the flow velocity quickly fell (Reading, 1986; Miall, 1996). Therefore, Facies Fl is interpreted as periodic flooding deposits in a flood-

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Fig. 2. Lithology (FS: fine sand; MS: medium sand; CS: coarse sand) and magnetostratigraphy of the BZ2 core (Yao et al., 2010). Black bars indicate normal polarity and white reversed. The geomagnetic polarity timescale (GPTS) is from Cande and Kent (1995). The color of the log was to reflect the color of sediments largely and oxidation–reduction of sedimentary environment (oxidation: red, yellow and brown; reduction: gray, blue and green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

plain environment adjacent to a fluvial channel (Miall, 1978, 1996), or as a result of crevasse process. Facies Mcsr consists of poorly sorted (1.9–2.6U) massive dark olive-gray (2.5GY 3/1), dark greenish-gray (10GY 3/1–4/1) clay/silt (Fig. 4d) with a mean grain size of 6.5–8.0U (Table 2) containing freshwater bivalves (Lamprota sp. and G. albus) and abundant freshwater ostracods (Chen et al., 2008). The grain-size distribution is bimodal with the fine fraction overlapping the coarse fraction (Fig. 5d). This facies has negative skewness of 0.2 to 0.8 (Table 2; Fig. 5d). The color of the sediments is characteristic of reductive sedimentary environment (S-Fig. 2b). It is inferred that Facies Mcsr was deposited in poorly drained distal setting in a floodplain, where the energy of the water flow was low and large amounts of fine suspended particles were deposited. Facies Mcso resembles Facies Mcsr (e.g., grain-size distribution; Fig. 5e), except that Facies Mcso bears the color of oxidization, such as reddish-brown (2.5 YR 4/6), dark reddish-brown (5YR 3/2–3/6) and yellowish-brown (10YR 5/6–5/8) (Fig. 4e). Plant rootlets with iron encrustation are common in this facies. Freshwater bivalve,

such as Lamprota sp. and G. albus, and freshwater ostracods are present in this facies. Therefore, Facies Mcso is interpreted as distal floodplain deposits with well-drained conditions (Reading, 1986). Facies Mc consists of massive mottled bright brown (7.5YR 5/6–5/ 8) and reddish-brown (2.5YR 4/6) clayey silt/silt with occasional greenish-gray (10GY 5/1–6/1) strip clayey silt/silt (Fig. 4f) containing carbonate nodules ranging in 1–5 cm in diameter in the bottom (SFig. 2c). These reddish-brown sediments are characteristic of granular to prismatic structures (S-Fig. 2c). The grain-size distribution of Facies Mc is very similar to those of Facies Mscr and Msco (Table 2; Fig. 5f). Thin-section observations show the presence of clay illuvial features (Yao et al., 2010) in most of the reddish-brown and bright brown layers, confirming that they are paleosols. These characteristics indicate that Facies Mc is the paleosols in a floodplain. Facies Sfm consists of massive brownish-gray (10YR 4/1) silt, occasionally interbedding fine sand layer (Fig. 4g) containing abundant benthic foraminifers and marine ostracods (Chen et al., 2008). The grain-size distribution is bimodal, with the fine fraction overlapping the coarse fraction (Fig. 5g). It has relatively lower

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Fig. 3. Lithology (FS: fine sand; MS: medium sand; CS: coarse sand), micropaleontology (foraminifers and ostracods; AF: abundance of foraminifers; AMO: abundance of marine ostracods; ATO: abundance of terrestrial ostracods; from Chen et al., 2008 and this study), grain-size (fractions of sand, silt and clay) and color reflectance (c value), facies and environment interpretations of the BZ2 core. Note that the decreases in paleosol layers (red layers in log) from the late Pliocene to Pleistocene. The OSL ages (solid circle) are referred to Chen et al. (2008). Dashed lines indicate the boundary of depositional units. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

skewness and kurtosis (Table 2; Fig. 5g) compared with other facies. Plant rootlets with iron encrustation in the massive silt (Fig. 4g) and freshwater bivalve (Lamprota sp.) indicate terrestrial environment, whereas the presence of benthic foraminifers and marine ostracods suggest marine-influenced environment. The mixture of the terrestrial and marine-related deposits suggests that Facies Sfm is deposited in environments, possibly estuary, influenced by both fluvial and marine processes (Boyd et al., 1992; Dalrymple et al., 1992). Facies Tdm consists of lenticular-bedded dark gray (N 3/0) muddy silt (Fig. 4h) containing abundant benthic foraminifers and marine ostracods (Chen et al., 2008). The sediments are highly bioturbated. Facies Tdx consists of dark gray (N 3/0) wavy-, lenticular- and flaser-bedded sand and silt (Fig. 4i). These two facies have similar bimodal patterns of grain-size distribution, whereas

Facies Tdx is coarser than Facies Tdm (Table 2; Fig. 5h and i). Mud drapes on the current ripples produce flaser bedding (Collinson and Thompson, 1989). The rhythmic sand–mud couplets forming lenticular, flaser and wavy bedding are common in tideinfluenced coastal environments (Dalrymple, 1992). Sand layers are deposited during ebb and flood currents, whereas mud drapes accumulate from suspension during or near tidal slack water (Dalrymple, 1992). In addition, Facies Tdm and Tdx characteristically contain brackish bivalve (Corbicula sp. and Crassostrea sp.), benthic foraminifers and marine ostracods which are unique in tidal deposits in the Bohai coastal plain (Wang et al., 2008a). It is therefore inferred that Facies Tdm and Tdx are tidal deposits in shallow marine environment (Chen et al., 2008). Facies Tdm might have deposited in a mud flat, whereas Tdx might have deposited in a mixed sand and mud flat.

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mud sand mud sand mud

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Mud clasts

Massive sand

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Erosional boundary mud sand

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Fig. 4. Photographs of representative sedimentary facies of the BZ2 core. The vertical length of each photo is 30 cm. The facies codes are listed in Table 1. (a) Core depth: 174.3–174.6 m; erosional boundary (in black dashed line) between medium to coarse sand and underlain fine floodplain deposits. (b) Core depth: 178.3–178.6 m; massive sand. (c) Core depth: 168.4–168.7 m; interbedding of sand and mud layers, photos c and c0 are same with different contrast. (d) Core depth: 111.7–112.0 m; massive clay/silt bearing reductive color. (e) Core depth: 3.10–3.40 m; massive clay/silt bearing oxidized color. (f) Core depth: 184.8–184.1 m; B/Bt horizon in a soil layer with granular structure (black arrow). (g) Core depth: 23.8–24.1 m; iron encrustation (black arrows). (h) Core depth: 7.3–7.6 m; massive clay/silt. (i) Core depth: 10.8–11.1 m; muddominant sand–mud couplets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5.2. Facies associations Two main facies associations–terrestrial association and marine association can be recognized on the basis of the occurrence patterns and vertical distribution of the different sedimentary facies described above (Table 3). Terrestrial association mainly consists of Facies Ch, Sm, Fl, Mcsr, Mcso and Mc, and indicates a fluvial system because all associated sedimentary facies point to the elements of a fluvial-related environment. Marine association consists of estuary (Sfm) and tidal flat (Tdm and Tdx).

5.3.1. Unit I (154.9–203.6 m) This unit mainly consists of cycles of fluvial channel and overbank deposits. The proportion of the fluvial-channel deposits is much lower compared with that of the floodplain deposits. The finer floodplain deposits are characteristic of reddish-brown (2.5 YR 4/6), yellowish-brown (10YR 5/6–5/8) and bright-brown (7.5YR 5/6–5/8) as indicated by higher c values (Fig. 3). The pedogenic characteristics are very common within the overbank deposits of this portion. No marine fossils were observed throughout this unit and terrestrial ostracods occurred occasionally. Paleomagnetic study (Yao et al., 2010) showed that Unit I was formed from 3.3 to 2.6 Ma.

5.3. Depositional units of the BZ2 core The core sediments can be divided into four depositional units (I, II, III and IV) from the bottom to the top of the core based on lithology, color reflectance, characteristics of vertical sequence and biofacies. Detailed characteristics of each unit are described in the following paragraphs.

5.3.2. Unit II (101.3–154.9 m) This unit is characterized by large thickness of dark olive gray (2.5GY 3/1) and dark greenish gray (10GY 3/1–4/1) finer floodplain deposits as indicated by lower c values and high abundance of mud fraction (Fig. 3). The coarse sandy flooding deposits resulted from

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Fig. 5. Textural cumulative and frequency curves of sedimentary facies identified at the BZ2 core. The facies codes are listed in Table 1.

Table 1 Characteristics of sedimentary facies of the BZ2 core. Facies code

Lithofacies

Lithology, sedimentary structures and fossils

Interpretations

Ch

Medium to coarse sand facies

Fluvial channel deposits

Sm

Fine to medium sand facies

Medium to coarse sand embedded in Facies Fl, Mcsr, Mcso and Mc; lower erosional boundary; no marine fossils Fine to medium sand; massive, parallel or cross laminations; no marine fossils

Fl

Alternating beds of sand and silt/clay facies

Alternating beds of sand and silt/clay facies; horizontal lamination; freshwater bivalve Lamprota sp. and G. albus

Mcsr

Massive clay/silt with reductive color facies

Dark olive gray (2.5GY 3/1), dark greenish-gray (10GY 3/1–4/1) clay/silt; massive; many freshwater ostracods and bivalve (Lamprota sp. and G. albus)

Mcso

Massive clay/silt with oxidized color facies Massive clayey silt/silt with carbonate nodules occasionally facies Massive silt, occasionally fine sand with both freshwater and brackish fossils facies Lenticular-bedded muddy silt facies

Dark reddish-brown (5YR 3/2–3/6) and yellowish-brown (10YR 5/6–5/8) clay/ silt; massive; many freshwater ostracods and bivalve (Lamprota sp. and G. albus) Mottled bright brown (7.5YR 5/6–5/8) and reddish-brown (2.5YR 4/6) clayey silt/ silt; Massive Silt or fine-grained sand; massive and parallel lamination; many benthic foraminifera and marine ostracods; containing freshwater bivalve Dark gray (N 3/0) muddy silt, Lenticular bedding; abundant benthic foraminifers and marine ostracods; Corbicula sp. Dark gray (N 3/0) alternating beds of sand and mud; Wavy- and flaser-bedding; abundant benthic foraminifers and marine ostracods; Corbicula sp.

Mc Sfm Tdm Tdx

Wavy- and flaser-bedded alternating beds of sand and mud facies

the crevasse splay process are common in the unit. Similar to unit I, this unit was free of marine fossils, suggesting a terrestrial dominant setting. The soil layers in this unit decreased greatly and coarse crevasse splay deposits increased compared with those in unit I. Fluvial-channel deposits, however, rarely have been observed in this interval. According to the paleomagnetic results

Crevasse-channel/crevasse splay deposits Proximal floodplain deposits; cyclic flood deposits Distal floodplain deposits with poorly drained conditions Distal floodplain deposits with well drained conditions Paleosol in a floodplain; Pedogenic characteristics Estuarine deposits Tidal flat (mud flat) Tidal flat (mixed flat)

(Yao et al., 2010), this unit covers the time interval of 2.6– 1.5 Ma (Fig. 3). 5.3.3. Unit III (24.3–101.3 m) This unit is characteristic of increased frequency of coarse crevasse splay deposits with high content of sand (Fig. 3). The most

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Z. Yao et al. / Journal of Asian Earth Sciences 54–55 (2012) 192–202 Table 2 Statistical parameters of grain-size distribution of sedimentary facies from the BZ2 core. Sedimentary facies

Sand (%)

Silt (%)

Clay (%)

Mean (U)

Sorting (U)

Skewness (U)

Kurtosis (U)

Ch Sm Fl Mcsr Mcso Mc Sfm Tdm Tdx

66–74 37–57 22–68 1–10 1–20 0.1–19 9–23 0.3–6 3–24

18–24 30–42 18–65 46–75 53–77 47–73 50–70 62–77 58–72

7–11 13–23 7–21 21–52 20–46 20–52 21–32 18–37 15–33

2.9–3.8 4.4–5.6 4.2–5.6 6.5–8.0 5.9–7.8 6.4–7.9 6.3–6.8 6.2–7.5 5.7–7.2

2.6–2.8 2.5–2.8 1.6–2.8 1.9–2.6 2.0–2.5 1.8–2.6 2.1–2.6 1.7–2.4 1.9–2.3

1.1–1.4 0.3–1.0 0.6–1.6 0.2 to 0.8 0.3–1.2 0.0–0.8 0.3–0.5 0.5–1.3 0.4–1.1

3.1–4.1 2.1–3.2 2.3–4.9 2.4–3.6 2.6–3.8 2.3–3.6 2.4–3.4 2.8–4.2 2.7–4.0

Table 3 Facies associations of the BZ2 core. Facies associations

Sedimentary environment

Elements (sedimentary facies)

Terrestrial (fluvial)

Channel Floodplain Crevasse splay

Ch, Sm Fl, Mcso, Mcsr, Mc Sm

Marine

Estuary Tidal flat

Sfm Tdx Tdm

different characteristics from unit II are oxidized floodplain deposits and abundant terrestrial ostracods. Furthermore, some foraminifers can be observed at the core depth of 40–55 m. The proportion of soil layers increased compared with unit II, but was much lower than that in unit I (Fig. 3). This unit is expected to be deposited from 1.5 to 0.13 Ma based on the paleomagnetic results (Figs. 2 and 3; Yao et al., 2010) and one OSL age of 122.2 ± 10.3 ka at the depth of 28.96 m (Fig. 3; Chen et al., 2008). 5.3.4. Unit IV (0–24.3 m) In this unit, the abundance of foraminifers and marine ostracods increased greatly, suggesting a marine-influenced environment. The alternation of the marine and terrestrial deposits are dominant, identified as Facies Sfm and Tdm/Tdx interbedding with floodplain deposits (Fig. 3). The proportion of sand is much lower and displays high fluctuations in this unit (Fig. 3). OSL dating data at the core depth of 20.39 m and 24.23 m are 88.7 ± 6.0 ka and 92.0 ± 6.4 ka (Chen et al., 2008), indicating that this unit was deposited since the last interglacial period (0.10 Ma). 6. Discussion 6.1. Sedimentary history of the western Bohai coastal plain Conceptual models for a fluvial distributary system were established in previous studies (Friend, 1978; Nichols, 1989; Kelly and Olsen, 1993) and have been developed recently by Nichols and Fisher (2007). According to Nichols and Fisher (2007), the deposits of a fluvial system can be divided into three parts from upper reaches to lower reaches: proximal facies, medial facies and distal facies. The proximal facies is characterized by the coarsest deposits, and no associated fine-grained floodplain deposits are preserved. The medial facies is marked by an increase in the proportion of fine-grained floodplain facies. The distal facies is also characterized by the high proportion of floodplain facies, however, with a prominent feature that much more sand are present as thin sheet deposits within the fine-grained deposits (Nichols and Fisher, 2007).

The Plio-Pleistocene sedimentary history in the western Bohai coastal plain is governed by the occurrence of three major sedimentary processes responsible for the faces identified in the BZ2 core: Fluvial, Estuarine and Tidal flat. The vertical stacking pattern of facies observed in the BZ2 core shows that the late Pliocene– Quaternary succession consists of mainly alluvial deposits in the lower part (203.6–25.0 m) and alternation of marine and alluvial deposits in the upper 25.0 m portion (Fig. 3). The changes of the sedimentary environment are described for the four stages as follows. 6.1.1. Late Pliocene to Early Pleistocene (3.3–2.6 Ma) During this period, large thickness of oxidized floodplain deposits suggest oxidizing condition and lower water table during the drying up of the floodplain. Frequent pedogenesis pointed to periods of subaerial exposure and relatively low rates of sediment accumulation. These findings, combined with the fact that present geomorphology of the study area is quite flat and low in altitude (less than 4 m), suggest the fluvial channel is more like a meandering type (Reading, 1986) and keeps relatively stable during this period, a condition favorable for the conservation of thick finer floodplain deposits and the formation of soils. The characteristics and pattern of deposits are very similar to the medial facies described by Nichols and Fisher (2007). Thus, the study site was likely located at the middle overbank area beyond the fluvial channel during this period and was far away from the shoreline. 6.1.2. Early Pleistocene (2.6–1.5 Ma) From 2.6 to 1.5 Ma, the sedimentary environments in the study area were characteristic of reducing, waterlogged conditions (Reading, 1986; Nichols, 1999), as indicated by large thickness of reductive-color deposits. Meanwhile, the coarse flooding deposits due to the crevasse process are common and the conditions were not be suitable for the pedogenesis any more. These characteristics imply that the BZ2 site might have been located at the lowland overbank area and was close to the shoreline during the early Pleistocene, according to the model proposed by Nichols and Fisher (2007). Under these circumstances, the fluvial channel would be less confined and the channel avulsion took place frequently on the one hand. On the other hand, the lowland area would be favorable for the preservation of large thickness of reductive-color deposits. 6.1.3. Early to Late Pleistocene (1.5–0.10 Ma) During this period, the study area was influenced by frequent alternations of crevasse splay and oxidized fluvial floodplain deposits. The presence of abundant terrestrial ostracods, companying with less foraminifers in the core depth 40–55 m may suggest a large change in sedimentary environment. In view of very lower abundance of foraminifers, which might be transported by storm and strong winds (Wu, 1993), we speculate that the study area was still terrestrial-dominant but with minor marine influence during sea-level highstands. These characteristics demonstrate

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that the BZ2 site might have been located close to the shoreline beyond the lowland overbank area during the time interval of 1.5– 0.10 Ma according to the model (Nichols and Fisher, 2007). 6.1.4. Since the late Pleistocene (since 0.10 Ma) The study area was dominated by alternating marine and fluvial deposits since 0.10 Ma. The marine transgression patterns revealed in the BZ2 core since the last interglacial period is largely consistent with previous study in this region (BQ1, BQ2, HH4, TN1 and TS6 in Fig. 1c; Wang et al., 1981, 1987; Wang and Li, 1983; Yan et al., 2006; Wang et al., 2008b; Shao et al., 2010). These findings imply abrupt changes in sedimentary environment in the western Bohai coastal plain since the last interglacial period. During this period, the BZ2 site might be quite close to the shoreline and easily influenced by sea-level fluctuations (Lambeck et al., 2002). 6.2. Sedimentary history of the western Bohai coastal plain and implications on tectonic, climatic and sea-level changes Recent studies have shown that climate (Vandenberghe, 2003; Briant et al., 2005; Amorosi et al., 2008; Tedesco et al., 2011) and tectonics (Hickson et al., 2005) can be generally regarded as the major controlling factors of fluvial architecture. For fluvial systems close to coastal area, sea-level fluctuations have been thought as another important factor influencing the sedimentary architecture because of marine water incursion, as well as the changes in the base level of fluvial systems (Amorosi et al., 1999; Komatsubara, 2004; Liu et al., 2009; Tedesco et al., 2011). The study site was located on an active tectonic-controlled coastal setting during the Pliocene–Quaternary period. Thus, the sedimentary history of the western Bohai coastal plain is tightly linked with tectonic, climatic and sea-level changes. Previous studies demonstrated that the Bohai Basin experienced episodic sub-rifting processes from the Eocene until the end of the Oligocene (Allen et al., 1997; Hu et al., 2001), and as a whole the basin began to subside and lasted until the present day (Allen et al., 1997). Ye et al. (1985) suggested that during the Quaternary, the subsidence rate of the Bohai Basin increased. The facies architecture and sedimentary characteristics of the BZ2 core reveal that tectonics probably exerted an important control in the sedimentary history of the western Bohai coastal plain. Quaternary climate is characterized by glacial and interglacial alternations (Shackleton et al., 1984, 1995), which led to sea-level oscillations (Lambeck et al., 2002). In addition to the glacial–interglacial climate, the relative sea-level change can also be influenced by glacial isostatic adjustment and tectonic movement (Milne and Mitrovia, 2008). The study area, however, is far from the ice-sheet influenced region and the glacial-isostatic adjustment might be neglectable. Since the early Pleistocene, global sea-level has become gradually lower as indicated by the marine oxygen isotopes record (Lambeck et al., 2002). However, the influence of marine on the sediments began at 1.5 Ma, although its influence was very limited. The intensive marine transgression did not occur until the last interglacial period, and alluvial deposits dominated the late Pliocene to late Pleistocene strata. These findings suggest that tectonic subsidence must have played an important role in the deposits of marine transgressions. Continuous decrease in the relief in the study area that resulted from the tectonic subsidence since the early Pleistocene has increasingly promoted sea-water incursion since the late Pliocene. Therefore, the marine transgressions in this region were resulted from the combined effects of sea-level rise during the interglacial period and tectonic subsidence. Changes in base-level of fluvial system have great influence on the fluvial development (Blum and Törnqvist, 2000). In case of rivers in the coastal plain, sea level generally acts as the base-level.

Generally, a relative sea-level rise could create accommodation space and lead to aggradation of fluvial deposits. Meanwhile, the location of the aggradation–incision boundary migrated toward upper reaches of the river under constant discharge and sediment supply. During the time interval of 3.3–2.6 Ma, the site of BZ2 core was far from the shoreline, and the amplitude of sea-level changes were small (Lambeck et al., 2002). Under this circumstance, the fluvial system developed mostly dominated by incision process. Thus less crevasse process can be expected, favoring the pedogenesis in the floodplain. Following the Quaternary, the distance between core site and shoreline was shortening due to continuous tectonic subsidence and increased sea-level fluctuations (Lambeck et al., 2002). Thus the fluvial process was sensitive to changes in base-level resulted from sea-level oscillations, leading to the alternation of crevasse splay and finer floodplain deposits during this period. The crevasse splay tends to be deposited during high sea-level (base-level) stand and finer floodplain deposits were dominant in sea-level lowstand. In addition to the tectonic factor, climate change also has important effects on the fluvial process via a series of direct and indirect mechanisms (Vandenberghe and Maddy, 2001; Vandenberghe, 2003). Precipitation is regarded as the most important and direct climatic parameter. The intensity and the seasonal variation of precipitation are important in influencing the fluvial process (Vandenberghe, 2003) by modulating the changes in discharge, especially in monsoonal regions (Jain and Tandon, 2003). The study area is within the East Asian monsoon zone and the monsoon precipitation is important for the hydrological cycles in the Bohai coastal plain. All rivers in this region are characterized by typical seasonal changes, which rise suddenly when it rains and fall or even dry up quickly after rain (Xu et al., 1996b). Therefore, the precipitation in this monsoonal region is highly variable during cold-dry periods but relatively stable during warm-humid periods, which controls the capability of sediment discharge (Xu et al., 1996b). Fluvial development is also affected by other indirect factors, such as available sediment supply and the corresponding soil erosion, vegetation coverage, catchment properties, and so forth (Thornes, 1990; Jain and Tandon, 2003; Vandenberghe, 2003; Dosseto et al., 2010), which are partially dependent on or amplified by climate. Among these factors, available sediment supply is regarded as extremely important in the fluvial process (Millar, 2000; Huisink et al., 2002), which can be affected by vegetation coverage. Vandenberghe (2003) suggested that vegetation controls the susceptibility of surface sediments to erosion by runoff, and its presence or absence is quite important in stimulating river incision, abandon or aggradation by intercepting rainfall and by modifying soil infiltration capacity (Vandenberghe, 2003). Furthermore, the high diversity of river patterns may be attributed to the high variability of vegetation density and character (Vandenberghe, 1995, 2001; Vandenberghe and Woo, 2002). During the late Pliocene, the relatively stable warm-humid climate in eastern China that resulted from intense Asian summer monsoon (Yang et al., 2006) led to good and extensive vegetation coverage in this region (Fan et al., 2009). Under this condition, the rivers might have had limited sediment supply as a result of extended vegetation coverage (Nador et al., 2003). Meanwhile, during the stable climatic conditions in the late Pliocene, that is, weak glacial and interglacial fluctuations, the relatively stable precipitation concentration (Fan et al., 2009) led to less variability of changes in discharge. In this case, the rivers are characterized by approximately balanced lateral or downstream sediment accretion and erosion (Nador et al., 2003). Thus, the fluvial channel was relatively stable and avulsion rarely happened, favoring the formation of large thickness of fine-grained deposits with oxidizing color. These findings are also supported by the widespread soil within the fine-grained floodplain deposits. Soils are well developed under

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warm-humid climatic conditions. The most important factor, however, controls the soil development in fluvial environments is no or very low sediment aggradation rates (Reading, 1986; Nichols, 1999) because pedogenesis rarely happens in frequent channel migration/ avulsion and high aggrading floodplain environments. Following the Quaternary, global climate generally changed into cold and dry, and was characterized by alternations of glacial and interglacial (Shackleton et al., 1984, 1995). These alternations would have led to large amplitudes of changes in precipitation, and consequently in hydrological cycles in the Bohai coastal plain. Furthermore, the vegetation coverage in northern China during the Quaternary was sparse (Fan et al., 2009) because of increased colddry climate, as indicated by soil carbonate stable isotope records from the same core (Yao et al., 2010). The vegetation coverage displayed more frequent changes in alternations of trees and grass/ shrub compared with the late Pliocene (Fan et al., 2009). Thus, large amounts of sediments from the catchments could be easily eroded by rivers and transported to the deposition area. All these factors might have led to the frequent flooding events, consequently, poorly drained pond deposits associated with crevasse splay were widespread in the western Bohai coastal plain, as recorded in the BZ2 core deposits. Furthermore, the abrupt decreases in paleosol within the fluvial deposits have partly confirmed the above inference. However, the fluvial response to climate change is very complicated and essentially nonlinear and may vary over different temporal and spatial scales (Jain and Tandon, 2003; Vandenberghe, 2003). It is should be noted that many unconformities can be observed throughout the BZ2 sequence (Fig. 3). These unconformities were mostly the boundary between Crevasse splay/fluvial channel deposits characteristic of medium-coarse sand and finer floodplain deposits (Fig. 3). The unconformities always indicate the existence of hiatus in sedimentary sequence more or less. Paleomagnetic study (Yao et al., 2010) and OSL dating (Chen et al., 2008) showed that the upper 50 m portion of the BZ2 core was dated to 780 ka, whereas the upper 30 m of the core covered the late Pleistocene period. Therefore, significant hiatus must be existed from the Middle Pleistocene to the Late Pleistocene. The potential hiatus might be located at the position of unconformities at the depth of 31.3 m and 45.6 m in the core, corresponding to 200 ka and 550 ka respectively (Yao et al., 2010). 7. Conclusions Detailed sedimentological study, grain size and color reflectance records of the BZ2 core demonstrated that the western Bohai coastal plain was dominated by fluvial setting from the late Pliocene (3.3 Ma) to late Pleistocene (0.10 Ma). Alternating fluvial and marine deposits were present in the Bohai coastal plain since the late Pleistocene. The architecture and characteristics of the fluvial deposits from 3.3 to 0.10 Ma and the occurrence of marine transgression since the last interglacial period imply that tectonic subsidence in the Bohai Basin during the Quaternary must have played a key role in controlling the depositional system in this region. Changes in base level of fluvial system due to sea-level fluctuations affected the fluvial development significantly during the Quaternary. Besides, climate changes since the late Pliocene also have had significant effects on the sedimentary environment by modulating the fluvial process via changes in discharge, available sediment supply, soil erosion, vegetation coverage, etc. Acknowledgments We would like to thank Editors Bor-ming Jahn, L. Jennifer and two anonymous reviewers for constructive comments on the

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manuscript and language revision. Special thanks also extend to Professor Huayu Lu from Nanjing University for assistance in the grainsize analysis and Professor Baoyin Yuan in the Institute of Geology and Geophysics, Chinese Academy of Sciences for helpful discussion. This work was jointly supported by the National Basic Research Program of China (973 Program; Grant No. 2010CB950200), the National Natural Science Foundation of China (Grant Nos. 41006039 and 40872107) and Project of State Oceanic Administration of China (Grant Nos. 2012317, 2011G21 and 90801-BC15). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jseaes.2012. 04.013. References Allen, M.B., Macdonald, D.I.M., Xun, Z., Vincent, S.J., Brouet-Menzies, C., 1997. Early Cenozoic two-phase extension and late Cenozoic thermal subsidence and inversion of the Bohai Basin, northern China. Mar. Petrol. Geol. 14, 951–972. Amorosi, A., Colalongo, M.L., Pasini, G., Preti, D., 1999. Sedimentary response to Late Quaternary sea-level changes in the Romagna coastal plain (northern Italy). Sedimentology 46, 99–121. Amorosi, A., Pavesi, M., Lucchi, M.R., Sarti, G., Piccin, A., 2008. Climatic signature of cyclic fluvial architecture from the Quaternary of the central Po Plain, Italy. Sediment. Geol. 209, 58–68. Blum, M.D., Törnqvist, T.E., 2000. Fluvial responses to climate and sea-level changes: a review and look forward. Sedimentology 47, 2–48. Boyd, R., Dalrymple, R., Zaitlin, B.A., 1992. Classification of clastic coastal depositional environments. Sediment. Geol. 80, 139–150. Briant, R., Bateman, M., Coope, G., Gibbard, P., 2005. Climatic control on Quaternary fluvial sedimentology of a Fenland Basin river, England. Sedimentology 52, 1397–1423. Cande, S.C., Kent, D.V., 1995. Revised calibration of geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095. Chen, J.B., Wu, T.S., 1997. Regional Stratigraphy of North China. China University of Geosciences Press, Beijing, p. 199. Chen, Y.K., Li, Z.H., Shao, Y.X., Wang, Z.S., Gao, W.P., Yang, X.L., 2008. Study on the Quaternary chronostratigraphic section in Tianjin Area. Seismol. Geol. 30, 383– 399 (in Chinese, with English Abstr.). Collinson, J.D., Thompson, D.B., 1989. Sedimentary Structures. Unwin Hyman, London, p. 207. Dalrymple, R.W., 1992. Tidal depositional systems. In: Walker, R.G., James, N.P. (Eds.), Facies Models-Response to Sea Level Change. Geological Association of Canada, pp. 195–218. Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models; conceptual basis and stratigraphic implications. J. Sediment. Res. 62, 1130–1146. Deaton, B.C., Balsam, W.L., 1991. Visible spectroscopy; a rapid method for determining hematite and goethite concentration in geological materials. J. Sediment. Res. 61, 628–632. Desloges, J.R., Church, M., 1987. Channel and floodplain facies in a wandering gravel-bed river. Recent Devlop Fluvial Sedimentol 9, 191–196. Dosseto, A., Hesse, P.P., Maher, K., Fryirs, K., Turner, S., 2010. Climatic and vegetation control on sediment dynamics during the last glacial cycle. Geology 38, 395– 398. Fan, S.X., Liu, H.K., Xu, J.M., Zheng, H.R., Zhao, H., Bi, Z.W., Yang, Z.J., Lin, F., Zhang, J., 2009. Palaeovegetation and environmental evolution in Hengshui district of Hebei province since 3.5 Ma BP. Geoscience 23, 75–81 (in Chinese, with English Abstr.). Friedman, G.M., Sanders, J.E., 1978. Principles of Sedimentology. John Wiley & Sons, New York, p. 792. Friend, P.F., 1978. Distinctive features of some ancient river systems. In: Miall, A.D. (Ed.), Fluvial Sedimentology. Canadian Society of Petroleum Geologists Memoir 5, pp. 531–542. Hickson, T.A., Sheets, B.A., Paola, C., Kelberer, M., 2005. Experimental test of tectonic controls on three-dimensional alluvial facies architecture. J. Sediment. Res. 75, 710–722. Hu, S., O’Sullivan, P.B., Raza, A., Kohn, B.P., 2001. Thermal history and tectonic subsidence of the Bohai Basin, northern China: a Cenozoic rifted and local pullapart basin. Phys. Earth Planet. Interi. 126, 221–235. Huisink, M., De Moor, J., Kasse, C., Virtanen, T., 2002. Factors influencing periglacial fluvial morphology in the northern European Russian tundra and taiga. Earth. Surf. Proc. Land 27, 1223–1235. Jain, M., Tandon, S.K., 2003. Fluvial response to Late Quaternary climate changes, western India. Quaternary Sci. Rev. 22, 2223–2235. Jo, H.R., Rhee, C.W., Chough, S.K., 1997. Distinctive characteristics of a streamflowdominated alluvial fan deposit: Sanghori area, Kyongsang Basin (Early Cretaceous), southeastern Korea. Sediment. Geol. 110, 51–59.

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