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Plio-Pleistocene magnetostratigraphy of northern Bohai Bay and its implications for tectonic events since ca. 2.0 Ma
Journal of Geodynamics 111 (2017) 1–14
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Plio-Pleistocene magnetostratigraphy of northern Bohai Bay and its implications for tectonic events since ca. 2.0 Ma ⁎
Qinmian Xua, , Guibang Yuana, Jilong Yanga, Houtian Xina, Liang Yib, Chenglong Dengb,c, a b c
MARK ⁎⁎
Tianjin Center, China Geological Survey, Tianjin 300170, China State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10029, China University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Northern bohai bay Plio-Pleistocene Magnetostratigraphy Tectonic events WNW-orientated tectonics
The sediments of Bohai Bay Basin in North China have recorded the processes of basin filling and structural evolution, which may have resulted from the destruction of the North China Craton during the late Mesozoic and early Cenozoic. However, the absence of a reliable chronostratigraphic framework for the sedimentary sequences in the basin has prevented a comprehensive understanding of these processes. In this study, we combine paleomagnetic and sedimentary analyses of the sediments from two new boreholes (NY05 and TZ02) from northern Bohai Bay to provide new insights into the sedimentary history and regional tectonic processes since the Pliocene. The main findings are as follows: (1) Magnetite and hematite are the main carriers of the characteristic remanent magnetization. (2) The boreholes record the Brunhes and Gauss normal chrons, and the Matuyama reversed chron. (3) Subsidence-related differences in the depths of the Matuyama/Brunhes (M/B) and Gauss/ Matuyama (G/M) boundaries, sediment accumulation rates, and the sedimentary environments of the different tectonic units, enable us to identify that tectonic movements started in the Olduvai normal subchron and the development of the WNW-orientated tectonic features were intensified. (4) In the Huanghua depression, comparative analysis of subsidence-related differences between western and northern Bohai Bay indicates that the subsidence of the northern Bohai Bay may have been superimposed on the WNW-orientated tectonic activity and faulting associated with the collision between the Indian and the Eurasian Plates, in the context of localized subsidence.
1. Introduction The Bohai Bay Basin in eastern China contains abundant information about the regional structural framework, processes of sedimentary basin infilling, and petroleum reservoirs (Allen et al., 1997; Ren et al., 2002; Qi and Yang, 2010; Yin, 2010; Li et al., 2012; Guo et al., 2015). The basin has also recorded information about structural processes possibly resulting from lithospheric thinning beneath the North China Craton during the late Mesozoic and the early Cenozoic (Li et al., 2012; Zhu et al., 2012). However, the absence of a reliable chronostratigraphic framework for the sedimentary sequences has precluded a comprehensive understanding of the processes of basin infilling, structural evolution and their relationships with regional tectonics, especially during the late Cenozoic. During the last two decades, numerous boreholes of PlioceneQuaternary age have been obtained from the Bohai Bay Basin and they offer an excellent opportunity to establish a regional
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chronostratigraphic framework by integrating magnetostratigraphic dating with other chronostratigraphic data (Yao et al., 2010; Yi et al., 2016). In this study, we combine new high-resolution magnetostratigraphic results from two borehole cores (NY05 and TZ02) with the previously-published magnetostratigraphy of other borehole cores from northern Bohai Bay, including BG10 (Yuan et al., 2014) and MT04 (Xu et al., 2014). Our aim is to explore the regional structural characteristics during the interval from the Pliocene to the Quaternary. 2. Geological setting and sampling 2.1. Geological setting The Bohai Bay Basin, located at the centre of the eastern block of the North China Craton (Li et al., 2012), is flanked by the Yanshan fold belt to the north, the Taihang fold belt to the west and the Tancheng-Lujiang Fault to the east (Fig. 1a). The basin consists of a series of depressions
Corresponding author. Corresponding author at: State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10029, China E-mail addresses: [email protected] (Q. Xu), [email protected] (C. Deng).
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http://dx.doi.org/10.1016/j.jog.2017.08.002 Received 16 September 2016; Received in revised form 17 August 2017; Accepted 22 August 2017 0264-3707/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Tectonic map of the North China Plain (a). Tectonic map of the coastal area of Bohai Bay and locations of boreholes (b). A: NW-trending structure, the Zhangjiakou-Penglai Fault zone; B: NE-trending structure; C: NE-trending structure, the Tancheng-Lujiang Fault zone. Red square: Ms > = 6.0. The base map data are from NASA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in the south. The area consists of a series of secondary sags and rises, including Jianhe sag, Laowangzhuang rise and Nanpu sag in the western part, and Matouying rise and Laoting sag in the eastern part. The formation and evolution of these sags and rises during the late Cenozoic
and uplifts separated by regional-scale faults (Qi and Yang, 2010) (Fig. 1a). Northern Bohai Bay is located within the northeastern Huanghua depression. The NEE-trending Ninghe-Changli Fault separates the Yanshan fold belt in the north from the Huanghua depression 2
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dependent magnetic susceptibilities (χ–T curves), isothermal remanent magnetization (IRM) acquisition curves and backfield curves, were made on representative samples from various sedimentary facies (Fig. 3). The rock magnetic measurements were conducted in the Paleomagnetism and Geochronology Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. χ–T curves were measured by continuous exposure of samples through temperature cycles from room temperature to 700 °C and back to room temperature in an argon atmosphere, using a KLY-3 Kappabridge with a CS-3 high-temperature furnace (Agico Ltd., Brno). IRM acquisition and its back-field demagnetization were measured using a MicroMag 3900 Vibrating Sample Magnetometer (Princeton Measurements Corp., USA). An IRM was imparted from 0 to 1.0 T (IRM1T, hereafter termed saturation IRM, SIRM), and then demagnetized in stepwise back-fields up to 1.0 T to obtain the coercivity of remanence (Bcr).
was controlled by a series of secondary faults (Fig. 1b). Cenozoic strata of the Huanghua depression are over 6000 m thick: ∼3000 m for the Paleogene, ∼2500 m for the Neogene, and ∼500 m for the Quaternary (Hebei Bureau of Geology and Mineral Resources, 1989; Editorial Committee of Petroleum Geology of Dagang Oil Field, 1993; Dong et al., 2010). The strata comprise alluvial, fluvial and lacustrine sediments, intercalated volcanic/volcaniclastic and neritic/ littoral sediments, and they are unevenly distributed across the depression (Hebei Bureau of Geology and Mineral Resources, 1989). Boreholes NY05 and TZ02 were drilled in Jianhe sag and Laoting sag, respectively (Fig. 1). The Quaternary strata of northern Bohai Bay are 320–490-m thick (Li and Wang, 1983; Xu et al., 2014; Yuan et al., 2014), and include four transgressive beds (Li et al., 1982). The sediments are mainly composed of the Luanhe River delta and alluvial fans deposited during varying intervals (Gao, 1981; Li et al., 1984; Wang et al., 2007).
3.2. Demagnetization of the natural remanent magnetization (NRM)
2.2. Lithology of the boreholes
Paleomagnetic measurements of the samples from borehole NY05 were made using a 2G Enterprises Model 755 cryogenic magnetometer installed in a field-free space (< 300 nT) in the Paleomagnetic Laboratory of Nanjing University. 391 fine-grained (clayey) samples were taken out from the plastic boxes and subjected to progressive thermal demagnetization up to a maximum temperature of 690 °C, at intervals of 25–50 °C below 585 °C and at 10–25 °C above 585 °C, using a Magnetic Measurements thermal demagnetizer (TD48) with a residual magnetic field of less than 10 nT. Subsequently, the remaining 26 coarse-grained (sandy) specimens were subjected to alternating field (AF) demagnetization, at peak fields up to 70 mT at intervals of 3–10 mT, using a Molspin demagnetizer. Paleomagnetic measurements of borehole TZ02 were made using a 2G Enterprises Model 760-R cryogenic magnetometer installed in a magnetically-shielded space (< 300 nT) in the Paleomagnetism and Geochronology Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. 368 specimens were subjected to progressive thermal demagnetization up to a maximum temperature of 690 °C, at intervals of 25–50 °C below 585 °C and at 10–25 °C above 585 °C, using a Magnetic Measurements thermal demagnetizer (TD48) with a residual magnetic field less than 10 nT. Some of the remaining 104 specimens were subjected to progressive thermal demagnetization, but the NRM became effectively zero at about 350–400 °C. The behaviors suggest that pyrrhotite and/or goethite may contribute to the NRM. Goethite, which has a high coercivity and low unblocking temperature (∼120 °C), cannot be removed at lower fields by AF demagnetization and can be easily removed by thermal demagnetization at low temperature (e.g., < 150 °C). Thus, the remaining 104 specimens were subjected to thermal demagnetization at 80 °C and 150 °C, followed by AF demagnetization at peak fields up to 70 mT at intervals of 3–10 mT, in order to eliminate the effect of goethite. The demagnetization results were evaluated using orthogonal diagrams (Zijderveld, 1967), and the principal components direction was
Borehole NY05 (N39°16.1′, E118°2.4′, 1.7 m a.s.l), in southern Fengnan District, Tangshan City, has a length of 550 m. The tilts of the borehole are 2° at 300-m depth, 4° at 400-m depth, 8° at 500-m depth, and 9° at 550-m depth. Borehole TZ02 (N39°19.7′, E118°5.3′, 0.5 m a.s.l) is in southern Laoting District, Tangshan City, and has a length of 550 m. The tilts of borehole TZ02 are 2° at 100-m depth, 5° at 200-m depth, 7° at 300-m depth, and 9° at 550-m depth. Based on sedimentary characteristics, together with changes in color and grain size, the sedimentary sequences of the two boreholes can be divided into 5 units (Tables 1 and 2). Note that borehole NY05 contains three intervals of marine sediments, at 2 − 16 m, 28 − 36 m and 56 − 58 m (Fig. 2a); and borehole TZ02 contains one interval of marine sediments at 2 − 14 m (Fig. 2b). 2.3. Sampling The cores were split longitudinally into two halves. Paleomagnetic samples from borehole NY05 were taken using plastic boxes of dimensions 2 cm × 2 cm × 2 cm. The sampling intervals in borehole NY05 were 0.5 m in clayey layers, and 1 m in sandy layers. The clayey samples were taken out from the plastic boxes and subjected to progressive thermal demagnetization in laboratory. Block samples from borehole TZ02 were taken in the field, and two paired specimens (8 cm3) were obtained from each sample. The sampling intervals in borehole TZ02 were 0.5 m in clayey layers. 3. Methods 3.1. Rock magnetic measurements To identify the remanence carriers and magnetic mineralogy of the sediments, rock magnetic measurements including temperatureTable 1 Lithology of borehole NY05. Unit
Lithology
Depth
#1
Yellowish-grey, yellowish-brown silt and fine-grained sand, interbedded with a small amount of yellow fine sand. Fragments of marine mollusk shell and cross bedding, current bedding and flaser bedding occur at 2–16 m, 33–36 m and 56–58 m. Several normal-graded sedimentary cycles with a thickness of 10–30 m, composed of olive-grey organic silt, sandy silt, silty sand and fine sand with horizontal bedding, cross bedding and occasional boulder clay. Yellowish-brown silt interbedded with grayish-yellow silty sand (content ∼25%). Several normal-graded sedimentary cycles with thicknesses of 5–10 m, composed of yellowish-brown, grayish-yellow silt (content ∼20%) and many calcareous concretions; yellowish-brown sandy silt and silty sand (content ∼15%); and olive-grey fine sand and fine sand (content ∼65%) and occasional gravel clasts and boulder clay. Olive-grey, grayish-yellow silt and sandy silt (content ∼75%) and occasional calcareous concretions, interbedded with fine and medium sand (content ∼25%) with cross bedding.
0–75 m
#2 #3 #4
#5
3
75–277 m 277–362 m 362–490 m
490–550 m
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Table 2 Lithology of borehole TZ02. Unit
Lithology
Depth
#1 #2 #3
Yellowish-grey fine sand and fine-grained sand, interbedded with olive-grey silty mud, with occasional fragments of marine mollusk shells. Brownish-red fine and medium sand, with horizontal bedding, cross bedding and occasional boulder clay. Olive-grey fine sand and fine-grained sand with numerous gravel-sized clasts and boulder clay, interbedded with medium- thick-bedded silts (content 40%), and occasional carbonaceous and calcareous spots, with occasional fragments of marine mollusks shell at 70–73 m. Olive-grey silty mud and silt (content ∼70%) and occasional calcareous concretions and iron-manganese deposits, with occasional bivalve fragments, interbedded with fine sand with horizontal bedding, cross bedding and occasional boulder clay. Several normal-graded sedimentary cycles with thicknesses of 10–20 m, composed of brownish-yellow sandy silt interbedded with occasional grey silty mud (content ∼60%), silty sand with many iron-manganese concretions and reduced deposits (content ∼10%), fine and medium sand (content of 30%) with horizontal bedding and cross bedding.
0–22 m 22–45 m 45–227 m
#4 #5
computed using a least-squares fitting technique (Kirschvink, 1980). Representative demagnetization diagrams are shown in Fig. 4. Only the magnetic inclination data were used to define the succession of geomagnetic polarity intervals because the magnetic declinations are arbitrary.
227–480 m 480–550 m
4. Results 4.1. Rock magnetic measurements 4.1.1. χ–T curves χ–T curves are highly sensitive to mineralogical changes during thermal treatment and such changes can provide useful information about magnetic mineral composition. It has been shown that sediments usually contain some thermally unstable components, such as ironbearing clay minerals, maghemite and sulfides (e.g., pyrrhotite, pyrite, greigite), which can be identified using changes in magnetic
Fig. 2. Photographs of boreholes NY05 (a) and TZ02 (b) from northern Bohai bay.
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Fig. 3. χ–T curves, isothermal remanent magnetization (IRM) acquisition and back-field demagnetization curves of selected samples from different depth in boreholes NY05 and TZ02, northern Bohai Bay.
magnetism (Liu et al., 2010). The significantly-enhanced susceptibility after thermal treatment in some samples can be attributed mainly to the neoformation of magnetite grains from the transformation of ironcontaining silicates/clays (Deng et al., 2000, 2001) (Fig. 3a, b, d–f). Notably, the magnetic susceptibility values of the cooling curve of sample NY05-1 are much higher than those of the heating curve, with the room-temperature magnetic susceptibility increasing ∼80 times after the 700 °C heating/cooling cycle (Fig. 3a). This may be ascribed to the conversion of chalcopyrite to magnetite. Sample NY05-3 exhibits
susceptibility (Roberts et al., 1995; Deng et al., 2001; Zhang et al., 2014). All the χ–T curves are characterized by a major decrease in magnetic susceptibility at about 585–600 °C (Fig. 3). This indicates that nearly stoichiometric magnetite and partially-oxidized magnetite are the major contributors to the susceptibility. For some samples, the large residual magnetic susceptibility above 585 °C becomes effectively zero at ∼680 °C (Fig. 3a, c, e, f), the Néel temperature of hematite, indicating that hematite is present in abundance due to its weak 5
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Fig. 4. Orthogonal vector plots showing the results of thermal demagnetization of representative specimens from boreholes NY05 and TZ02, northern Bohai Bay. Circles and squares correspond to projections on the vertical and horizontal planes, respectively; NRM is the natural remanent magnetization.
almost reversible χ–T curves (Fig. 3c), indicating negligible neoformation of ferrimagnetic phases during thermal treatment. In the heating curves of some samples (Fig. 3a, b, f), the steady increase in susceptibility below ∼300 °C may be ascribed to the gradual unblocking of fine-grained (near the superparamagnetic/single-domain boundary) ferrimagnetic particles (Deng et al., 2005). The further drop in susceptibility between ∼300 °C and ∼450 °C is interpreted as the conversion of metastable maghemite to weakly magnetic hematite (Stacey and Banerjee, 1974). In the heating curve of sample TZ02-1
(Fig. 3d), the minor but steady increase in susceptibility from room temperature to ∼450 °C is interpreted as the behavior of coarse-grained (large pseudo-single domain or multidomain) magnetite. In the heating curve of sample TZ02-2 (Fig. 3e), there is a sharp increase in susceptibility at ∼300 °C and a pronounced hump between ∼300 °C and ∼500 °C, which may be ascribed to the conversion of pyrrhotite to magnetite.
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correlated with polarity intervals from the post-Jaramillo Matuyama chron (C1r.1r) to the late Gilbert chron (C2Ar) in the GPTS (Cande and Kent, 1995). Magnetozones R1 to R4 correspond to the Matuyama reversed chron, and magnetozones N2 and N4 correspond to the Jaramillo (C1r.1n) and Olduvai (C2n) normal subchrons, respectively. Magnetozones N5 to N7 correspond to the Gauss normal chron, and magnetozones R5 and R6 correspond to the Kaena (C2An.1r) and Mammoth (C2An.2r) reversed subchrons, respectively. Magnetozone R7 corresponds to the late Gilbert chron (C2Ar). This correlation places the Matuyama-Brunhes (M/B) boundary (Lower-Middle Pleistocene boundary), Gauss-Matuyama (G/M) boundary (Pliocene-Pleistocene boundary), and Gilbert-Gauss boundary (Gi/G) in borehole NY05 at the respective depths of 123.2 m, 361.9 m and 503.9 m, corrected for obliquity (Fig. 5). The bottom of the borehole is within the late Gilbert chron C2Ar.
4.1.2. IRM acquisition and back-field demagnetization curves The selected samples exhibit contrasting IRM acquisition curves (Fig. 3g–l). One sample shows a rapid increase up to 0.1 T and a relatively high value of IRM0.1T/SIRM (Fig. 3j), which indicates the dominance of magnetically soft components. Other samples show a gradual increase up to 0.1 T and relatively low values of IRM0.1T/SIRM (Fig. 3gI, k, l), indicative of the presence of both low- and high-coercivity magnetic minerals. About 75–96% of the SIRM was acquired below 0.3 T, also indicating the occurrence of both low- and high-coercivity magnetic minerals. The remanence continued to be acquired above 0.3 T, which is generally considered to be the theoretical maximum coercivity of magnetite grains. S-ratio is the absolute value of IRM remaining after exposure to a reversed field of 0.3 T divided by SIRM (King and Channell, 1991). The relatively low values of S-ratio (generally less than 0.70) from some samples (Fig. 3g, i, k) indicate the presence of a high-coercivity magnetic phase. Evidence that the lowand high-coercivity minerals are respectively magnetite/maghemite and hematite comes from the χ–T curves (Fig. 3a–f).
5.2. Correlation of the magnetozones in borehole TZ02 to the GPTS Borehole TZ02 is in the Luanhe River delta. One prominent marine bed, which formed during the Holocene (Liu et al., 2009; Yi et al., 2013; Wang et al., 2015; Xu et al., 2015), occurs in magnetozone N1, and indicates that this magnetozone can be unambiguously correlated with the Brunhes normal chron (C1n) (Fig. 6). Given this chronological constraint, magnetozones R1 to N5 can be correlated with polarity intervals from the post-Jaramillo Matuyama chron (C1r.1r) to the middle Gauss chron (C2An.2n) in the GPTS (Cande and Kent, 1995). Magnetozones R1 to R3 correspond to the Matuyama reversed chron, and magnetozones N2 and N3 correspond to the Jaramillo (C1r.1n) and Olduvai (C2n) normal subchrons, respectively. Magnetozones N4 to N5 correspond to the Gauss normal chron, and magnetozone R4 corresponds to the Kaena reversed subchron (C2An.1r). This correlation places the Matuyama-Brunhes (M/B) boundary (Lower-Middle Pleistocene boundary) and Gauss-Matuyama (G/M) boundary (Pliocene-Pleistocene boundary) in borehole TZ02 at the respective depths of 138.5 m and 435.3 m, corrected for obliquity (Fig. 6). The bottom of the borehole is within the middle Gauss chron (C2An.2n).
4.2. Paleomagnetic measurements Stepwise thermal, AF or hybrid demagnetization was successful in isolating the characteristic remanent magnetization (ChRM) after removal of a soft secondary component. Generally, a secondary magnetic component, probably of viscous origin, was present and removed by thermal demagnetization at 150–200 °C (Fig. 4b, c, h-k). For most specimens, the high-stability ChRM component was obtained between 250 °C and 585 °C (Fig. 4c and e), or between 20 mT and 60 mT (Fig. 4f and l). However, for some specimens, the high-stability ChRM component persisted up to 630 °C (Fig. 4k) or even to 680 °C (Fig. 4a, b, d, g–j). The NRMs of the samples fromdifferent sedimentary facies are carried by different magnetic minerals . For example, the NRM of the sediments is associated associate with magnetite at the depth of 35.4 m, with hematite at the depth of 40.4 m. Sedimentary facies of two samples are marine facies and floodplain facies respectively. In sum, both magnetite and hematite dominate the ChRM carriers of the marine and fluviolacustrine sediments in the Bohai Bay. At least four successive points in the orthogonal plots were used to calculate the ChRM directions (Kirschvink, 1980) and the maximum angular deviation (MAD) was generally less than 15°. After stepwise demagnetization, 289 specimens (69%) from borehole NY05, including 271 specimens after thermal demagnetization and 18 specimens after AF demagnetization, gave reliable ChRM directions; and 354 specimens (75%) from borehole TZ02, including 325 specimens after thermal demagnetization and 29 specimens after hybrid demagnetization, gave reliable ChRM directions. Following stepwise demagnetization, fourteen magnetozones are recognized in the borehole NY05 sequence: seven of normal polarity (N1 to N7) and seven of reversed polarity (R1 to R7) (Fig. 5). In addition, two short intervals of possible transitional field behavior, labeled e1 to e2, are recorded within magnetozone N1 and R7, respectively (Fig. 5). Nine magnetozones are recognized in borehole TZ02 sequence: five of normal polarity (N1 to N5) and four of reversed polarity (R1 to R4) (Fig. 6).
5.3. Plio-Pleistocene magnetostratigraphic framework Our new magnetostratigraphies for boreholes NY05 and TZ02 from the Jianhe and Laoting sags, respectively, combined with and the previously published magnetostratigraphies from boreholes BG10 and MT04 (Xu et al., 2014; Yuan et al., 2014) from the Nanpu sag and Matouying rise, respectively, enable us to establish a Plio-Pleistocene chronostratigraphic framework for northern Bohai Bay (Fig. 7). Three marine layers are distributed within the 0 − 120-m depth range along the coastal area of Bohai Bay. However, the ages of the intervals are strongly debated. For example, in the case of the second marine layer, an age corresponding to MIS 3 has been proposed based on radiocarbon (Liu et al., 2009) and OSL dating (Yan et al., 2006). However, ages corresponding to MIS 3-5 have been proposed based on a combination of OSL and paleomagnetic dating (Chen et al., 2012; Yi et al., 2013). Whichever dating method is applied, the three transgressive events all occur within the Brunhes normal chron with a lower boundary at the depth interval from 160 m to 120 m. However, the characteristics of the Matuyama chron in these four boreholes is slightly different. Specifically, there are three normal magnetozones in boreholes NY05 and BG10, but only two in boreholes MT04 and TZ02. Fine-grained lacustrine and floodplain sedimentation dominated during the Jaramillo normal subchron in boreholes NY05, BG10 and TZ02, indicating the continuous nature of these sedimentary records. The occurrence of sandy deposits overlying C1r.1n in borehole MT04 (Xu et al., 2014) suggests that erosion is probably responsible for the greater thickness of chron C1r.1r in borehole MT04 compared to boreholes BG10 and TZ02. During the Olduvai normal subchron,
5. Discussion 5.1. Correlation of the magnetozones in borehole NY05 to the geomagnetic polarity timescale (GPTS) Borehole NY05 is in a modern high tidal flat area. Three prominent marine beds that formed during the Late Pleistocene and Holocene (Liu et al., 2009; Yi et al., 2013; Wang et al., 2015; Xu et al., 2015) occur within magnetozone N1, indicating that this magnetozone can be unambiguously correlated with the Brunhes normal chron (C1n) (Fig. 5). Given this chronological constraint, magnetozones R1–R7 can be 7
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Fig. 5. Lithostratigraphy and magnetic polarity stratigraphy of borehole NY05 in northern Bohai Bay and correlation with the geomagnetic polarity timescale (Cande and Kent, 1995).
5.4. Initiation of tectonic movements in the olduvai normal subchron
lacustrine strata with sandy particles occur in boreholes NY05, MT04 and TZ02, and lacustrine and floodplain facies in borehole BG10. In addition, the sedimentary characteristics during chron C2r are similar in all four cores, suggesting that the depositional environment and its duration were comparable along northern Bohai Bay during the interval from 1.8–2.6 Ma. In the lower parts, that is, prior to 2.6 Ma, fluvial deposits dominated in all four cores and the possibility of sedimentary hiatuses increases due to the relatively coarse-grained nature of the sediments. As a rough estimate, the basal ages of boreholes BG10, MT04, TZ02 and NY05 are 3.6 Ma, 3.2 Ma, 3.2 Ma and 3.9 Ma, respectively.
A synthesis of the magnetostratigraphies of the four borehole records with their sedimentary and acoustic characteristics enables us to characterize and date the sequence of tectonic events in the Huanghua depression in northern Bohai Bay during the late Cenozoic. Acoustic logging can be used to analyze the porosity of late Cenozoic sediments, and reflects the degree of compaction and consolidation. The results of acoustic logging reveal two distinct intervals with boundaries at 75–80 m. High amplitude variations, with values ranging from 500–700 μs/m, occur in the upper part, indicating a greater porosity and less consolidation; and low amplitude variations with values ranging from 300–500 μs/m values occur in the lower part, indicating a 8
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Fig. 6. Lithostratigraphy and magnetic polarity stratigraphy of borehole TZ02 in northern Bohai Bay and correlation with the geomagnetic polarity timescale (Cande and Kent, 1995).
these results, we infer the following sequence, in descending order of subsidence magnitude: Nanpu sag, Laoting sag, Jianhe sag and Matouying rise, and that it’s the depocenter shifted with time (Fig. 8). During the Pliocene, SARs varied across northern Bohai Bay, with values of 137 m/Ma, 120 m/Ma, 90 m/Ma and 168 m/Ma in the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag, respectively. This indicates that the WNW-orientated Laoting sag was the depocenter during this interval. From 1.95 − 2.58 Ma, the SAR values were 140 m/ Ma,109 m/Ma, 103 m/Ma and 161 m/Ma in the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag, respectively. This indicates during that during this interval the WNW-orientated Laoting sag remained the depocenter. The increased SARs of the WNW-orientated Matouying rise may indicate an enlargement of the depocenter. From 0.78–1.95 Ma,
high degree of consolidation. The effects of compaction and consolidation of the strata under this borderline at 75–80 m are basically similar, so the depths of the magnetozone boundaries and sediment accumulation rates (SARs) of different boreholes can be comparative analysis. The depths of the magnetozone boundaries in the studied sequences are as follows: in boreholes NY05, BG10, MT04 and TZ02 the Matuyama/Brunhes boundary is at 123.2 m, 157.8 m, 122.4 m and 138.5 m, respectively; the lower boundary of the Olduvai normal subchron is at 274.1 m, 409.1 m, 260.5 m and 332.8 m, respectively; and the Gauss/Matuyama boundary is at 361.9 m, 473.2 m, 327.2 m and 435.3 m, respectively. This indicates that the depth ranges of the magnetozones are roughly comparable in the four boreholes. From 9
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Fig. 7. Late Cenozoic magnetostratigraphic framework for various structural units across northern Bohai Bay. (a) and (b) are respectively the lithology and geomagnetic polarity record of borehole NY05 located in the NE-trending Jianhe sag (N39°16.1′, E118°2.4′, 1.7 m a.s.l). (c) and (d) are respectively the lithology and geomagnetic polarity record of borehole BG10 located in the Nanpu sag (N39°10.0′, E118°33.4′, 2.5 m a.s.l). (e) and (f) are respectively the lithology and geomagnetic polarity record of borehole MT04 located in the Matouying rise (N39°15.9′, E118°49.8′, 1.0 m a.s.l). (g) and (h) are respectively the lithology and geomagnetic polarity record of borehole TZ02 located in the Laoting sag (N39°19.7′, E118°5.3′, 0.5 m a.s.l). I, II, III and IV are the first, second, third and fourth marine layers, respectively. The third marine layer in borehole BG10 is divided into two sub-units, III-1 and III-2.
from northeastern part to middle-southern part in northern Bohao Bay. During 0 − 0.78 Ma, the SARs of the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag were 156 m/Ma, 205 m/Ma, 156 m/Ma and 177 m/Ma, respectively. This suggests that the depocenter had moved to the Nanpu sag. The Middle and Late Pleistocene sediments are less consolidated, and the SARs increased slightly, indicating that both
the differences between the SARs of the various structural sub-units increased significantly; the SAR values are 129 m/Ma, 211 m/Ma, 120 m/Ma and 167 m/Ma in the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag, respectively. The SARs of WNW-orientated units increased, especially in the Nanpu sag, which may indicate the occurrence of regional tectonic movements and the conversion of depocenter 10
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Fig. 8. Age-depth curve and acoustic velocity of different structural units across northern Bohai Bay. The dotted line is the acoustic velocity and the solid line is the age-depth curve.
chronostratigraphic framework for the coastal area of Bohai Bay in the Huanghua depression, and to elucidate the regional tectonic dynamics (Fig. 9). The depth of the Matuyama/Brunhes (M/B) boundary in borehole CQJ4 is shallower than in boreholes BZ1 and G2; all three boreholes are located in sags of the middle Huanghua depression. However, the depths of the three marine layers is basically similar in the three boreholes, therefore we believe that in borehole CQJ4 the base of the Jaramillo normal subchron may be the base of the Brunhes normal chron. In the centre of the Huanghua depression, the depths of the M/B and Gauss/Matuyama (G/M) boundaries are 99–104 m and 305–335 m, respectively. In Jianhe sag and the Matouying rise, in the northern Huanghua depression, the depths of M/B and G/M boundaries are 123 m and 327–362 m, respectively. In Nanpu sag and Laoting sag in the northern Huanghua depression, the depths of the M/B and G/M boundaries are 138–158 m and 435–473 m, respectively. The depth of the G/M boundary in the Matouying rise is similar to the depth in the centre of the Hunghua depression, but the subsidencerelated differences of the M/B boundary range from 20–25 m. The subsidence-related differences of the M/B and G/M boundaries between the Jianhe sag and the sags in the centre of the Hunghua depression range from 20–25 m and 27–57 m, respectively. This suggests that the northern Huanghua depression is the regional subsidence centre, and the WNW-orientated structures of the basin has intensified in the Quaternary. The subsidence-related extent of the WNW-orientated Laoting sag and Nanpu sag is significantly greater than that of the NEorientated Jianhe sag which is equivalent to the WNW-orientated Matouying rise. This also indicates that the WNW-orientated structures of the basin has intensified. The Cenozoic tectonic evolution of Bohai Bay Basin is generally considered to have been mainly affected by the collision of the Indian and the Eurasian Plates (Liu et al., 2004), as well as by the subduction of the Pacific Plate beneath the Eurasian Plate (Yin, 2010). The localized extension and opening of back-arc basins at 32–17 Ma (Yin, 2010) resulted in the development of the NEE- and NE-orientated depressions and uplifts in the Bohai Bay Basin (Qi et al., 2010). The depths of the magnetozone boundaries and marine layers of boreholes CQJ4, BZ1 and G2, from the middle of Huanghua depression, exhibit similar trends and therefore the structural dynamics of the region can be regarded as reflecting the subduction of the Pacific Plate beneath the Eurasian Plate. Based on the depth trends of the magnetostratigraphic boundaries and the marine layers in boreholes NY05, BG10, MT04 and TZ02, the extent of subsidence of the northern Huanghua depression has clearly been greater, especially the WNW-orientated structural units. Therefore, the
climate and tectonics affected the sedimentary processes in the study area. In addition, the depth of the Matuyama/Brunhes boundary shows that the depocenter was in the Nanpu sag, indicating that WNW-orientated tectonics played the major role, inheriting the subsidence framework of the early Quaternary. Prior to the Olduvai normal subchron, the sedimentary characteristics indicate the broad development of alternating floodplain and limnetic facies (Zhao et al., 2016), which may be consistent with the pattern of climatic change in the study area (Wang et al., 2002; Yang et al., 2006). From boreholes TZ02 to NY05, the lacustrine strata gradually decreased, the limnetic facies changed to a limnetic delta and the content of coarser deposits gradually decreased. These evidences indicates that the Laoting sag was the depocenter, which is consistent with the conclusions based on the SARs of the different units. From the Olduvai to the Jaramillo normal subchron, limnetic facies, and subaqueous channel deposits interbedded with occasional floodplain and fluvial facies, dominated in the four cores. Prior to the Jaramillo normal subchron, the sediments were fine-grained and the thickness of lacustrine sedimentary intervals increased, while after the Jaramillo subchron both the sediment grain-size and the thickness of the lacustrine intervals increased and the marine beds appeared. Since the Olduvai normal subchron, the sedimentary environment evident in the four cores changed from alternating floodplain and limnetic facies to limnetic facies interbedded with occasional floodplain facies. While the broad floodplain developed along western Bohai Bay (Yao et al., 2012). Therefore, we suggest that the changes in the sedimentary environment were caused by tectonic movements, rather than by climate changes. From the depths of the polarity chrons, the SARs of different tectonic units and the changes in the sedimentary environment during the Olduvai normal subchron, we can conclude that the tectonic movements commenced from the Olduvai normal subchron onwards, and the WNW-orientated tectonics played the major role. 5.5. Structural features and dynamics of the Huanghua depression during the Plio-Pleistocene Magnetostratigraphies for the coastal area of Bohai Bay are available for the following boreholes: Middle Huanghua depression, western Bohai Bay: CQJ4 in Banqiao sag (Shi et al., 2009), BZ1 in Banqiao sag (Xiao et al., 2008), and G2 in Beitang sag (Xiao et al., 2014). Northern Huanghua depression, northern Bohai Bay: BG10 in Nanpu sag (Yuan et al., 2014), and MT04 in Matouying rise (Xu et al., 2014). We have combined the results of this previous work with our new magnetostratigraphies for boreholes NY05 and TZ02 from Jianhe sag and Laoting sag respectively, northern Bohai Bay, to establish a Plio-Pleistocene 11
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Fig. 9. Late Cenozoic magnetostratigraphic framework for different structural units in the Huanghua depression. Boreholes CQJ4 and BZ1 are in Banqiao sag, borehole G2 in Beitang sag, borehole NY05 in Jianhe sag, borehole BG10 in Nanpu sag, borehole MT04 in Matouying rise, and borehole TZ02 in Laoting sag. I, II and III are the first, second and third marine layers, respectively. The third marine layer in borehole BG10 is divided into two subunits, III-1 and III-2.
intracontinental rifting and extension in North China during the Quaternary.
structural dynamics of the region reflect the effects of localized subsidence superimposed on the development of WNW-orientated faults. The WNW-orientated fault system associated with the collision between the Indian and the Eurasian Plates may be linked with Cenozoic 12
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English abstract). Acta Geogr. Sin. 3, 303–314. Guo, L.L., Li, S.Z., Suo, Y.H., Ji, Y.T., Dai, L.M., Yu, S., Liu, X., Somerville, I.D., 2015. Experimental study and active tectonics on the Zhangjiakou-Penglai fault zone across North China. J. Asian Earth Sci. 144 (Part 1), 18–27. Hebei Bureau of Geology and Mineral Resources,, 1989. Regional Geological Annals of Hebei Province. Geological Publishing House, Beijing and Tianjin Beijing, pp. 590–616. King, J.W., Channell, J.E.T., 1991. Sedimentary magnetism, environmental magnetism and magnetostratigraphy. Rev. Geophys. 29 (Suppl. 1), 358–370. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. R. Astron. Soc. 62, 699–718. Li, H.M., Wang, J.D., 1983. Palaeomagnetic study on drill core from northern Bohai coastal plain (in Chinese with English abstract). Geochimica 12, 196–204. Li, Y.F., Gao, S.M., An, F.T., 1982. A preliminary study of the quaternary marine strata and its paleogeographic significance in the Luanhe delta region (in Chinese with English abstract). Oceanol. Limnol. Sin. 13, 433–439. Li, C.X., Chen, G., Wang, C.G., Zhang, Y.M., 1984. On the Luanhe river alluvial fan-delta complex (in Chinese with English abstract). Acta Petrolei Sin. 4, 27–36. Li, S.Z., Zhao, G.C., Dai, L.M., Zhou, L.H., Liu, X., Suo, Y.H., Santosh, M., 2012. Cenozoic faulting of the Bohai Bay Basin and its bearing on the destruction of the eastern North China Craton. J. Asian Earth Sci. 47, 80–93. Liu, M., Cui, X.J., Liu, F.T., 2004. Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo-Asian collision? Tectonophysics 393, 29–42. Liu, J., Saito, Y., Wang, H., Zhou, L., Yang, Z., 2009. Stratigraphic development during the late pleistocene and holocene offshore of the yellow river delta, Bohai sea. J. Asian Earth Sci. 36, 318–331. Liu, C.C., Deng, C.L., Liu, Q.S., Zheng, L.T., Wang, W., Xu, X.M., Huang, S., Yuan, B.Y., 2010. Mineral magnetism to probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China. Geophys. J. Int. 181, 1395–1410. Qi, J.F., Yang, Q., 2010. Cenozoic structural deformation and dynamic processes of the Bohai Bay basin province, China. Mar. Petrol. Geol. 4, 757–771. Ren, J., Tamaki, K., Li, S., Zhang, J., 2002. Late Mesozoic and Cenozoic rifting and its dynamic setting in Eastern China and adjacent areas. Tectonophysics 344, 175–205. Roberts, A.P., Cui, Y., Verosub, K.L., 1995. Wasp-waisted hysteresis loops: mineral magnetic characteristics and discrimination of components in mixed magnetic systems. J. Geophys. Res. 100, 17909–17924. Shi, L.F., Zhai, Z.M., Wang, Q., Zhang, X.B., Yang, Z.Y., 2009. Geochronological study on transgression layers of the CQJ4 borehole at dagang area in Tianjin, China (in chinese with english abstract). Geol. Rev. 55, 375–384. Stacey, F.D., Banerjee, S.K., 1974. The Physical Principles of Rock Magnetism. Elsevier, Amsterdam, pp. 195. Wang, S.M., Wu, X.H., Zhang, Z.K., Jiang, F.C., Xue, B., Tong, G.B., Tian, G.Q., 2002. Sedimentary records of environmental evolution in the Sanmen Lake basin and the Yellow River running through the Sanmenxia Gorge eastward into the sea. Sci. China (Ser. D Earth Sci.) 45, 595–608. Wang, Y., Fu, G.H., Zhang, Y.Z., 2007. River-sea interactive sedimentations and plain morphological evolution (in Chinese with English abstract). Quat. Sci. 5, 674–689. Wang, F., Li, J., Chen, Y., Fang, J., Zong, Y., Shang, Z., Wang, H., 2015. The record ofmidHolocene maximumlandward marine transgression in the west coast of Bohai Bay, China. Mar. Geol. 359, 89–95. Xiao, G.Q., Guo, Z.T., Chen, Y.K., Yao, Z.Q., Shao, Y.X., Wang, X.L., Hao, Q.Z., Lu, Y.C., 2008. Magnetostratigraphy of BZ1 borehole in west coast of bohai Bay，Northern China. Quat. Sci. 28, 909–916. Xiao, G.Q., Yang, J.L., Zhao, C.R., Wang, Q., Xu, Q.M., Hu, Y.Z., Qin, Y.F., Li, J., Xiao, G.Q., 2014. Magnetostratigraphy of drill hole G2 in the Tianjin coastal area and its tectonic significance. Geol. Bull. China 33, 1642–1650. Xu, Q.M., Yuan, G.B., Qin, Y.F., Yang, J.L., Yang, J.Q., 2014. Magnetostratigraphy and discussion of coupling relationship between tectonic movement and climate change of MT04 borehole in southern Luanhe River (in Chinese with English abstract). Quat. Sci. 34, 540–552. Xu, Q.M., Yang, J.L., Yuan, G.B., Chu, Z.X., Zhang, Z.Z., 2015. Stratigraphic sequence and episodes of the ancient Huanghe Delta along the southwestern Bohai Bay since the LGM. Mar. Geol. 367, 69–82. Yan, Y.Z., Wang, H., Li, F.L., Li, J.F., Zhao, C.R., Lin, F., 2006. Sedimentary environment and sea-level fluctuations revealed by Borehole BQ1 on the West Coast of the Bohai bay, China (in chinese with english abstract). Geol. Bull. China 3, 357–382. Yang, S.Y., Li, C.X., Cai, J.G., 2006. Geochemical compositions of core sediments in eastern China: implication for Late Cenozoic palaeoenvironmental changes. Palaeogeogr. Palaeoclimatol. Palaeoecol. 229, 287–302. Yao, Z.Q., Xiao, G.Q., Wu, H.B., Liu, W.G., Chen, Y.K., 2010. Plio-pleistocene vegetation changes in the North China Plain: magnetostratigraphy, oxygen and carbon isotopic composition of pedogenic carbonates. Palaeogeogr. Palaeoclimatol. Palaeoecol. 297, 502–510. Yi, L., Lai, Z.P., Yu, H.J., Xu, X.Y., Su, Q., Yao, J., Wang, X.L., Shi, X.F., 2013. Chronologies of sedimentary changes in the south Bohai Sea, China: constraints from luminescence and radiocarbon dating. Boreas 42, 267–284. Yi, L., Deng, C.L., Tian, L.Z., Xu, X.Y., Jiang, X.Y., Qiang, X.K., Qin, H.F., Ge, J.Y., Chen, G.Q., Su, Q., Chen, Y.P., Shi, X.F., Xie, Q., Yu, H.J., Zhu, R.X., 2016. Plio-Pleistocene evolution of Bohai Basin (East Asia): demise of Bohai paleolake and transition to marine environment. Sci. Rep. 6, 29403. http://dx.doi.org/10.1038/srep29403. Yin, A., 2010. Cenozoic tectonic evolution of Asia: a preliminary synthesis. Tectonophysics 488, 293–325. Yuan, G.B., Xu, Q.M., Wang, Y., Yang, J.L., Qin, Y.F., Du, D., 2014. Magnetostratigraphy and geology significance of Bg10 Borehole in Northern Coast of Bohai Bay (in Chinese with eEnglish abstract). Acta Geol. Sin. 88, 285–298. Zhang, Y., Guo, Z.T., Deng, C.L., Zhang, S.Q., Wu, H.B., Zhang, C.X., Ge, J.Y., Zhao, D.A.,
6. Conclusions Paleomagnetic studies combined with sedimentary analysis were conducted on two new boreholes from northern Bohai Bay to elucidate the regional tectonic processes since the Pliocene. The main conclusions are as follows. (1) Magnetite and hematite are the main carriers of the characteristic remanent magnetization in the studied sedimentary sequences. Magnetostratigraphic results indicate that the deposits recorded the Brunhes and Gauss normal chrons, together with the successive reversed polarity intervals of the Matuyama chron. (2) The depths of the Matuyama/Brunhes boundary, the base of the Olduvai normal subchron, and the Gauss/Matuyama boundary, are greatest in the Nanpu sag, followed by the Laoting sag, Jianhe sag and Matouying rise. The sediment accumulations rates (SARs) of the different units suggest that the WNW-orientated Laoting sag was the depocenter at ∼2.58 Ma, the WNW-orientated Nanpu sag was the depocenter during 2.58–1.95 Ma, and the structural features have inherited the subsidence framework since the early Quaternary. Since the Olduvai normal subchron, the sedimentary environment, as revealed by the four cores, changed from alternating floodplain and limnetic facies to limnetic facies interbedded with occasional intervals of floodplain facies. From the depths of the different magnetozone boundaries, the SARs of the different tectonic units and changes in sedimentary environment in the Olduvai normal subchron, we conclude that tectonic movement started in the Olduvai normal subchron and the development of the WNW-orientated tectonic features were intensified. (3) The subsidence of the central part of the Huanghua depression may be associated with the subduction of the Pacific Plate beneath the Eurasian Plate, and the subsidence of the northern part may have been superimposed on the WNW-orientated tectonic activity and faulting associated with the collision between the Indian and the Eurasian Plates, in the context of localized subsidence. Acknowledgements This study was supported by the China Geological Survey Project (grants 12120114007801, 1212011120746 and DD2016042). CD acknowledges additional support from the National Natural Science Foundation of China (grants 41690112 and 41621004). 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. 8, 951–972. Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095. Chen, Y.S., Wang, H., Pei, Y.D., Tian, L.Z., Li, J.F., Shang, Z.W., 2012. Division and its geological significance of the late Quaternary marine sedimentary beds in the west coast of Bohai bay, China (in Chinese with English abstract). J. Jilin Univ. (Earth Sci. Ed.) 3, 747–759. Deng, C.L., Zhu, R.X., Verosub, K.L., Singer, M.J., Yuan, B.Y., 2000. Paleoclimatic significance of the temperature-dependent susceptibility of Holocene Loess along a NWSE transect in the Chinese Loess Plateau. Geophys. Res. Lett. 27, 3715–3718. Deng, C.L., Zhu, R.X., Jackson, M.J., Verosub, K.L., Singer, M.J., 2001. Variability of the temperature-dependent susceptibility of the Holocene eolian deposits in the Chinese loess plateau: a pedogenesis indicator. Phys. Chem. Earth Part A 26, 873–878. Deng, C.L., Vidic, N.J., Verosub, K.L., Singer, M.J., Liu, Q.S., Shaw, J., Zhu, R.X., 2005. Mineral magnetic variation of the Jiaodao Chinese loess/paleosol sequence and its bearing on long-term climatic variability. J. Geophys. Res. 110, B03103. http://dx. doi.org/10.1029/2004JB003451. Dong, Y.X., Xiao, L., Zhou, H.M., Wang, C.Z., Zheng, J.P., Zhang, N., Xia, W.C., Ma, Q., Du, J.X., Zhao, Z.X., Huang, H.X., 2010. The Tertiary evolution of the prolific Nanpu Sag of Bohai Bay Basin, China: constraints from volcanic records and tectono-stratigraphic sequences. Geol. Soc. Am. Bull. 122, 609–626. Editorial Committee of Petroleum Geology of Dagang Oil Field, 1993. Petroleum Geology of China (volume 4) (in Chinese): Dagang Oil Field. Petroleum industry press, Beijing, pp. 83–114. Gao, S.M., 1981. Facies and sedimentary model of the Luan river delta (in Chinese with
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Journal of Geodynamics 111 (2017) 1–14
Q. Xu et al. Qin, L., Song, Y., Zhu, R.X., 2014. The use of fire at Zhoukoudian: evidence from magnetic susceptibility and color measurements. Chin. Sci. Bull. 59, 1013–1020. Zhao, L.L., Xu, Q.M., Yang, J.L., Yuan, G.B., Guo, J.J., 2016. Sedimentary evolution of BGl0 borehole in Northern Coast of Bohai Bay during late cenozoic (in Chinese with English abstract). Quat. Sci. 36, 196–207.
Zhu, R.X., Xu, Y.G., Zhu, G., Zhang, H.F., Xia, Q.K., Zheng, T.Y., 2012. Destruction of the north China craton. Sci. China Earth Sci. 55, 1565–1587. Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks: analysis of results. In: Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Paleomagnetism. Elsevier, New York, pp. 254–286.
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Plio-Pleistocene magnetostratigraphy of northern Bohai Bay and its implications for tectonic events since ca. 2.0 Ma ⁎
Qinmian Xua, , Guibang Yuana, Jilong Yanga, Houtian Xina, Liang Yib, Chenglong Dengb,c, a b c
MARK ⁎⁎
Tianjin Center, China Geological Survey, Tianjin 300170, China State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10029, China University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Northern bohai bay Plio-Pleistocene Magnetostratigraphy Tectonic events WNW-orientated tectonics
The sediments of Bohai Bay Basin in North China have recorded the processes of basin filling and structural evolution, which may have resulted from the destruction of the North China Craton during the late Mesozoic and early Cenozoic. However, the absence of a reliable chronostratigraphic framework for the sedimentary sequences in the basin has prevented a comprehensive understanding of these processes. In this study, we combine paleomagnetic and sedimentary analyses of the sediments from two new boreholes (NY05 and TZ02) from northern Bohai Bay to provide new insights into the sedimentary history and regional tectonic processes since the Pliocene. The main findings are as follows: (1) Magnetite and hematite are the main carriers of the characteristic remanent magnetization. (2) The boreholes record the Brunhes and Gauss normal chrons, and the Matuyama reversed chron. (3) Subsidence-related differences in the depths of the Matuyama/Brunhes (M/B) and Gauss/ Matuyama (G/M) boundaries, sediment accumulation rates, and the sedimentary environments of the different tectonic units, enable us to identify that tectonic movements started in the Olduvai normal subchron and the development of the WNW-orientated tectonic features were intensified. (4) In the Huanghua depression, comparative analysis of subsidence-related differences between western and northern Bohai Bay indicates that the subsidence of the northern Bohai Bay may have been superimposed on the WNW-orientated tectonic activity and faulting associated with the collision between the Indian and the Eurasian Plates, in the context of localized subsidence.
1. Introduction The Bohai Bay Basin in eastern China contains abundant information about the regional structural framework, processes of sedimentary basin infilling, and petroleum reservoirs (Allen et al., 1997; Ren et al., 2002; Qi and Yang, 2010; Yin, 2010; Li et al., 2012; Guo et al., 2015). The basin has also recorded information about structural processes possibly resulting from lithospheric thinning beneath the North China Craton during the late Mesozoic and the early Cenozoic (Li et al., 2012; Zhu et al., 2012). However, the absence of a reliable chronostratigraphic framework for the sedimentary sequences has precluded a comprehensive understanding of the processes of basin infilling, structural evolution and their relationships with regional tectonics, especially during the late Cenozoic. During the last two decades, numerous boreholes of PlioceneQuaternary age have been obtained from the Bohai Bay Basin and they offer an excellent opportunity to establish a regional
⁎
chronostratigraphic framework by integrating magnetostratigraphic dating with other chronostratigraphic data (Yao et al., 2010; Yi et al., 2016). In this study, we combine new high-resolution magnetostratigraphic results from two borehole cores (NY05 and TZ02) with the previously-published magnetostratigraphy of other borehole cores from northern Bohai Bay, including BG10 (Yuan et al., 2014) and MT04 (Xu et al., 2014). Our aim is to explore the regional structural characteristics during the interval from the Pliocene to the Quaternary. 2. Geological setting and sampling 2.1. Geological setting The Bohai Bay Basin, located at the centre of the eastern block of the North China Craton (Li et al., 2012), is flanked by the Yanshan fold belt to the north, the Taihang fold belt to the west and the Tancheng-Lujiang Fault to the east (Fig. 1a). The basin consists of a series of depressions
Corresponding author. Corresponding author at: State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10029, China E-mail addresses: [email protected] (Q. Xu), [email protected] (C. Deng).
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http://dx.doi.org/10.1016/j.jog.2017.08.002 Received 16 September 2016; Received in revised form 17 August 2017; Accepted 22 August 2017 0264-3707/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Tectonic map of the North China Plain (a). Tectonic map of the coastal area of Bohai Bay and locations of boreholes (b). A: NW-trending structure, the Zhangjiakou-Penglai Fault zone; B: NE-trending structure; C: NE-trending structure, the Tancheng-Lujiang Fault zone. Red square: Ms > = 6.0. The base map data are from NASA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in the south. The area consists of a series of secondary sags and rises, including Jianhe sag, Laowangzhuang rise and Nanpu sag in the western part, and Matouying rise and Laoting sag in the eastern part. The formation and evolution of these sags and rises during the late Cenozoic
and uplifts separated by regional-scale faults (Qi and Yang, 2010) (Fig. 1a). Northern Bohai Bay is located within the northeastern Huanghua depression. The NEE-trending Ninghe-Changli Fault separates the Yanshan fold belt in the north from the Huanghua depression 2
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dependent magnetic susceptibilities (χ–T curves), isothermal remanent magnetization (IRM) acquisition curves and backfield curves, were made on representative samples from various sedimentary facies (Fig. 3). The rock magnetic measurements were conducted in the Paleomagnetism and Geochronology Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. χ–T curves were measured by continuous exposure of samples through temperature cycles from room temperature to 700 °C and back to room temperature in an argon atmosphere, using a KLY-3 Kappabridge with a CS-3 high-temperature furnace (Agico Ltd., Brno). IRM acquisition and its back-field demagnetization were measured using a MicroMag 3900 Vibrating Sample Magnetometer (Princeton Measurements Corp., USA). An IRM was imparted from 0 to 1.0 T (IRM1T, hereafter termed saturation IRM, SIRM), and then demagnetized in stepwise back-fields up to 1.0 T to obtain the coercivity of remanence (Bcr).
was controlled by a series of secondary faults (Fig. 1b). Cenozoic strata of the Huanghua depression are over 6000 m thick: ∼3000 m for the Paleogene, ∼2500 m for the Neogene, and ∼500 m for the Quaternary (Hebei Bureau of Geology and Mineral Resources, 1989; Editorial Committee of Petroleum Geology of Dagang Oil Field, 1993; Dong et al., 2010). The strata comprise alluvial, fluvial and lacustrine sediments, intercalated volcanic/volcaniclastic and neritic/ littoral sediments, and they are unevenly distributed across the depression (Hebei Bureau of Geology and Mineral Resources, 1989). Boreholes NY05 and TZ02 were drilled in Jianhe sag and Laoting sag, respectively (Fig. 1). The Quaternary strata of northern Bohai Bay are 320–490-m thick (Li and Wang, 1983; Xu et al., 2014; Yuan et al., 2014), and include four transgressive beds (Li et al., 1982). The sediments are mainly composed of the Luanhe River delta and alluvial fans deposited during varying intervals (Gao, 1981; Li et al., 1984; Wang et al., 2007).
3.2. Demagnetization of the natural remanent magnetization (NRM)
2.2. Lithology of the boreholes
Paleomagnetic measurements of the samples from borehole NY05 were made using a 2G Enterprises Model 755 cryogenic magnetometer installed in a field-free space (< 300 nT) in the Paleomagnetic Laboratory of Nanjing University. 391 fine-grained (clayey) samples were taken out from the plastic boxes and subjected to progressive thermal demagnetization up to a maximum temperature of 690 °C, at intervals of 25–50 °C below 585 °C and at 10–25 °C above 585 °C, using a Magnetic Measurements thermal demagnetizer (TD48) with a residual magnetic field of less than 10 nT. Subsequently, the remaining 26 coarse-grained (sandy) specimens were subjected to alternating field (AF) demagnetization, at peak fields up to 70 mT at intervals of 3–10 mT, using a Molspin demagnetizer. Paleomagnetic measurements of borehole TZ02 were made using a 2G Enterprises Model 760-R cryogenic magnetometer installed in a magnetically-shielded space (< 300 nT) in the Paleomagnetism and Geochronology Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. 368 specimens were subjected to progressive thermal demagnetization up to a maximum temperature of 690 °C, at intervals of 25–50 °C below 585 °C and at 10–25 °C above 585 °C, using a Magnetic Measurements thermal demagnetizer (TD48) with a residual magnetic field less than 10 nT. Some of the remaining 104 specimens were subjected to progressive thermal demagnetization, but the NRM became effectively zero at about 350–400 °C. The behaviors suggest that pyrrhotite and/or goethite may contribute to the NRM. Goethite, which has a high coercivity and low unblocking temperature (∼120 °C), cannot be removed at lower fields by AF demagnetization and can be easily removed by thermal demagnetization at low temperature (e.g., < 150 °C). Thus, the remaining 104 specimens were subjected to thermal demagnetization at 80 °C and 150 °C, followed by AF demagnetization at peak fields up to 70 mT at intervals of 3–10 mT, in order to eliminate the effect of goethite. The demagnetization results were evaluated using orthogonal diagrams (Zijderveld, 1967), and the principal components direction was
Borehole NY05 (N39°16.1′, E118°2.4′, 1.7 m a.s.l), in southern Fengnan District, Tangshan City, has a length of 550 m. The tilts of the borehole are 2° at 300-m depth, 4° at 400-m depth, 8° at 500-m depth, and 9° at 550-m depth. Borehole TZ02 (N39°19.7′, E118°5.3′, 0.5 m a.s.l) is in southern Laoting District, Tangshan City, and has a length of 550 m. The tilts of borehole TZ02 are 2° at 100-m depth, 5° at 200-m depth, 7° at 300-m depth, and 9° at 550-m depth. Based on sedimentary characteristics, together with changes in color and grain size, the sedimentary sequences of the two boreholes can be divided into 5 units (Tables 1 and 2). Note that borehole NY05 contains three intervals of marine sediments, at 2 − 16 m, 28 − 36 m and 56 − 58 m (Fig. 2a); and borehole TZ02 contains one interval of marine sediments at 2 − 14 m (Fig. 2b). 2.3. Sampling The cores were split longitudinally into two halves. Paleomagnetic samples from borehole NY05 were taken using plastic boxes of dimensions 2 cm × 2 cm × 2 cm. The sampling intervals in borehole NY05 were 0.5 m in clayey layers, and 1 m in sandy layers. The clayey samples were taken out from the plastic boxes and subjected to progressive thermal demagnetization in laboratory. Block samples from borehole TZ02 were taken in the field, and two paired specimens (8 cm3) were obtained from each sample. The sampling intervals in borehole TZ02 were 0.5 m in clayey layers. 3. Methods 3.1. Rock magnetic measurements To identify the remanence carriers and magnetic mineralogy of the sediments, rock magnetic measurements including temperatureTable 1 Lithology of borehole NY05. Unit
Lithology
Depth
#1
Yellowish-grey, yellowish-brown silt and fine-grained sand, interbedded with a small amount of yellow fine sand. Fragments of marine mollusk shell and cross bedding, current bedding and flaser bedding occur at 2–16 m, 33–36 m and 56–58 m. Several normal-graded sedimentary cycles with a thickness of 10–30 m, composed of olive-grey organic silt, sandy silt, silty sand and fine sand with horizontal bedding, cross bedding and occasional boulder clay. Yellowish-brown silt interbedded with grayish-yellow silty sand (content ∼25%). Several normal-graded sedimentary cycles with thicknesses of 5–10 m, composed of yellowish-brown, grayish-yellow silt (content ∼20%) and many calcareous concretions; yellowish-brown sandy silt and silty sand (content ∼15%); and olive-grey fine sand and fine sand (content ∼65%) and occasional gravel clasts and boulder clay. Olive-grey, grayish-yellow silt and sandy silt (content ∼75%) and occasional calcareous concretions, interbedded with fine and medium sand (content ∼25%) with cross bedding.
0–75 m
#2 #3 #4
#5
3
75–277 m 277–362 m 362–490 m
490–550 m
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Table 2 Lithology of borehole TZ02. Unit
Lithology
Depth
#1 #2 #3
Yellowish-grey fine sand and fine-grained sand, interbedded with olive-grey silty mud, with occasional fragments of marine mollusk shells. Brownish-red fine and medium sand, with horizontal bedding, cross bedding and occasional boulder clay. Olive-grey fine sand and fine-grained sand with numerous gravel-sized clasts and boulder clay, interbedded with medium- thick-bedded silts (content 40%), and occasional carbonaceous and calcareous spots, with occasional fragments of marine mollusks shell at 70–73 m. Olive-grey silty mud and silt (content ∼70%) and occasional calcareous concretions and iron-manganese deposits, with occasional bivalve fragments, interbedded with fine sand with horizontal bedding, cross bedding and occasional boulder clay. Several normal-graded sedimentary cycles with thicknesses of 10–20 m, composed of brownish-yellow sandy silt interbedded with occasional grey silty mud (content ∼60%), silty sand with many iron-manganese concretions and reduced deposits (content ∼10%), fine and medium sand (content of 30%) with horizontal bedding and cross bedding.
0–22 m 22–45 m 45–227 m
#4 #5
computed using a least-squares fitting technique (Kirschvink, 1980). Representative demagnetization diagrams are shown in Fig. 4. Only the magnetic inclination data were used to define the succession of geomagnetic polarity intervals because the magnetic declinations are arbitrary.
227–480 m 480–550 m
4. Results 4.1. Rock magnetic measurements 4.1.1. χ–T curves χ–T curves are highly sensitive to mineralogical changes during thermal treatment and such changes can provide useful information about magnetic mineral composition. It has been shown that sediments usually contain some thermally unstable components, such as ironbearing clay minerals, maghemite and sulfides (e.g., pyrrhotite, pyrite, greigite), which can be identified using changes in magnetic
Fig. 2. Photographs of boreholes NY05 (a) and TZ02 (b) from northern Bohai bay.
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Fig. 3. χ–T curves, isothermal remanent magnetization (IRM) acquisition and back-field demagnetization curves of selected samples from different depth in boreholes NY05 and TZ02, northern Bohai Bay.
magnetism (Liu et al., 2010). The significantly-enhanced susceptibility after thermal treatment in some samples can be attributed mainly to the neoformation of magnetite grains from the transformation of ironcontaining silicates/clays (Deng et al., 2000, 2001) (Fig. 3a, b, d–f). Notably, the magnetic susceptibility values of the cooling curve of sample NY05-1 are much higher than those of the heating curve, with the room-temperature magnetic susceptibility increasing ∼80 times after the 700 °C heating/cooling cycle (Fig. 3a). This may be ascribed to the conversion of chalcopyrite to magnetite. Sample NY05-3 exhibits
susceptibility (Roberts et al., 1995; Deng et al., 2001; Zhang et al., 2014). All the χ–T curves are characterized by a major decrease in magnetic susceptibility at about 585–600 °C (Fig. 3). This indicates that nearly stoichiometric magnetite and partially-oxidized magnetite are the major contributors to the susceptibility. For some samples, the large residual magnetic susceptibility above 585 °C becomes effectively zero at ∼680 °C (Fig. 3a, c, e, f), the Néel temperature of hematite, indicating that hematite is present in abundance due to its weak 5
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Fig. 4. Orthogonal vector plots showing the results of thermal demagnetization of representative specimens from boreholes NY05 and TZ02, northern Bohai Bay. Circles and squares correspond to projections on the vertical and horizontal planes, respectively; NRM is the natural remanent magnetization.
almost reversible χ–T curves (Fig. 3c), indicating negligible neoformation of ferrimagnetic phases during thermal treatment. In the heating curves of some samples (Fig. 3a, b, f), the steady increase in susceptibility below ∼300 °C may be ascribed to the gradual unblocking of fine-grained (near the superparamagnetic/single-domain boundary) ferrimagnetic particles (Deng et al., 2005). The further drop in susceptibility between ∼300 °C and ∼450 °C is interpreted as the conversion of metastable maghemite to weakly magnetic hematite (Stacey and Banerjee, 1974). In the heating curve of sample TZ02-1
(Fig. 3d), the minor but steady increase in susceptibility from room temperature to ∼450 °C is interpreted as the behavior of coarse-grained (large pseudo-single domain or multidomain) magnetite. In the heating curve of sample TZ02-2 (Fig. 3e), there is a sharp increase in susceptibility at ∼300 °C and a pronounced hump between ∼300 °C and ∼500 °C, which may be ascribed to the conversion of pyrrhotite to magnetite.
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correlated with polarity intervals from the post-Jaramillo Matuyama chron (C1r.1r) to the late Gilbert chron (C2Ar) in the GPTS (Cande and Kent, 1995). Magnetozones R1 to R4 correspond to the Matuyama reversed chron, and magnetozones N2 and N4 correspond to the Jaramillo (C1r.1n) and Olduvai (C2n) normal subchrons, respectively. Magnetozones N5 to N7 correspond to the Gauss normal chron, and magnetozones R5 and R6 correspond to the Kaena (C2An.1r) and Mammoth (C2An.2r) reversed subchrons, respectively. Magnetozone R7 corresponds to the late Gilbert chron (C2Ar). This correlation places the Matuyama-Brunhes (M/B) boundary (Lower-Middle Pleistocene boundary), Gauss-Matuyama (G/M) boundary (Pliocene-Pleistocene boundary), and Gilbert-Gauss boundary (Gi/G) in borehole NY05 at the respective depths of 123.2 m, 361.9 m and 503.9 m, corrected for obliquity (Fig. 5). The bottom of the borehole is within the late Gilbert chron C2Ar.
4.1.2. IRM acquisition and back-field demagnetization curves The selected samples exhibit contrasting IRM acquisition curves (Fig. 3g–l). One sample shows a rapid increase up to 0.1 T and a relatively high value of IRM0.1T/SIRM (Fig. 3j), which indicates the dominance of magnetically soft components. Other samples show a gradual increase up to 0.1 T and relatively low values of IRM0.1T/SIRM (Fig. 3gI, k, l), indicative of the presence of both low- and high-coercivity magnetic minerals. About 75–96% of the SIRM was acquired below 0.3 T, also indicating the occurrence of both low- and high-coercivity magnetic minerals. The remanence continued to be acquired above 0.3 T, which is generally considered to be the theoretical maximum coercivity of magnetite grains. S-ratio is the absolute value of IRM remaining after exposure to a reversed field of 0.3 T divided by SIRM (King and Channell, 1991). The relatively low values of S-ratio (generally less than 0.70) from some samples (Fig. 3g, i, k) indicate the presence of a high-coercivity magnetic phase. Evidence that the lowand high-coercivity minerals are respectively magnetite/maghemite and hematite comes from the χ–T curves (Fig. 3a–f).
5.2. Correlation of the magnetozones in borehole TZ02 to the GPTS Borehole TZ02 is in the Luanhe River delta. One prominent marine bed, which formed during the Holocene (Liu et al., 2009; Yi et al., 2013; Wang et al., 2015; Xu et al., 2015), occurs in magnetozone N1, and indicates that this magnetozone can be unambiguously correlated with the Brunhes normal chron (C1n) (Fig. 6). Given this chronological constraint, magnetozones R1 to N5 can be correlated with polarity intervals from the post-Jaramillo Matuyama chron (C1r.1r) to the middle Gauss chron (C2An.2n) in the GPTS (Cande and Kent, 1995). Magnetozones R1 to R3 correspond to the Matuyama reversed chron, and magnetozones N2 and N3 correspond to the Jaramillo (C1r.1n) and Olduvai (C2n) normal subchrons, respectively. Magnetozones N4 to N5 correspond to the Gauss normal chron, and magnetozone R4 corresponds to the Kaena reversed subchron (C2An.1r). This correlation places the Matuyama-Brunhes (M/B) boundary (Lower-Middle Pleistocene boundary) and Gauss-Matuyama (G/M) boundary (Pliocene-Pleistocene boundary) in borehole TZ02 at the respective depths of 138.5 m and 435.3 m, corrected for obliquity (Fig. 6). The bottom of the borehole is within the middle Gauss chron (C2An.2n).
4.2. Paleomagnetic measurements Stepwise thermal, AF or hybrid demagnetization was successful in isolating the characteristic remanent magnetization (ChRM) after removal of a soft secondary component. Generally, a secondary magnetic component, probably of viscous origin, was present and removed by thermal demagnetization at 150–200 °C (Fig. 4b, c, h-k). For most specimens, the high-stability ChRM component was obtained between 250 °C and 585 °C (Fig. 4c and e), or between 20 mT and 60 mT (Fig. 4f and l). However, for some specimens, the high-stability ChRM component persisted up to 630 °C (Fig. 4k) or even to 680 °C (Fig. 4a, b, d, g–j). The NRMs of the samples fromdifferent sedimentary facies are carried by different magnetic minerals . For example, the NRM of the sediments is associated associate with magnetite at the depth of 35.4 m, with hematite at the depth of 40.4 m. Sedimentary facies of two samples are marine facies and floodplain facies respectively. In sum, both magnetite and hematite dominate the ChRM carriers of the marine and fluviolacustrine sediments in the Bohai Bay. At least four successive points in the orthogonal plots were used to calculate the ChRM directions (Kirschvink, 1980) and the maximum angular deviation (MAD) was generally less than 15°. After stepwise demagnetization, 289 specimens (69%) from borehole NY05, including 271 specimens after thermal demagnetization and 18 specimens after AF demagnetization, gave reliable ChRM directions; and 354 specimens (75%) from borehole TZ02, including 325 specimens after thermal demagnetization and 29 specimens after hybrid demagnetization, gave reliable ChRM directions. Following stepwise demagnetization, fourteen magnetozones are recognized in the borehole NY05 sequence: seven of normal polarity (N1 to N7) and seven of reversed polarity (R1 to R7) (Fig. 5). In addition, two short intervals of possible transitional field behavior, labeled e1 to e2, are recorded within magnetozone N1 and R7, respectively (Fig. 5). Nine magnetozones are recognized in borehole TZ02 sequence: five of normal polarity (N1 to N5) and four of reversed polarity (R1 to R4) (Fig. 6).
5.3. Plio-Pleistocene magnetostratigraphic framework Our new magnetostratigraphies for boreholes NY05 and TZ02 from the Jianhe and Laoting sags, respectively, combined with and the previously published magnetostratigraphies from boreholes BG10 and MT04 (Xu et al., 2014; Yuan et al., 2014) from the Nanpu sag and Matouying rise, respectively, enable us to establish a Plio-Pleistocene chronostratigraphic framework for northern Bohai Bay (Fig. 7). Three marine layers are distributed within the 0 − 120-m depth range along the coastal area of Bohai Bay. However, the ages of the intervals are strongly debated. For example, in the case of the second marine layer, an age corresponding to MIS 3 has been proposed based on radiocarbon (Liu et al., 2009) and OSL dating (Yan et al., 2006). However, ages corresponding to MIS 3-5 have been proposed based on a combination of OSL and paleomagnetic dating (Chen et al., 2012; Yi et al., 2013). Whichever dating method is applied, the three transgressive events all occur within the Brunhes normal chron with a lower boundary at the depth interval from 160 m to 120 m. However, the characteristics of the Matuyama chron in these four boreholes is slightly different. Specifically, there are three normal magnetozones in boreholes NY05 and BG10, but only two in boreholes MT04 and TZ02. Fine-grained lacustrine and floodplain sedimentation dominated during the Jaramillo normal subchron in boreholes NY05, BG10 and TZ02, indicating the continuous nature of these sedimentary records. The occurrence of sandy deposits overlying C1r.1n in borehole MT04 (Xu et al., 2014) suggests that erosion is probably responsible for the greater thickness of chron C1r.1r in borehole MT04 compared to boreholes BG10 and TZ02. During the Olduvai normal subchron,
5. Discussion 5.1. Correlation of the magnetozones in borehole NY05 to the geomagnetic polarity timescale (GPTS) Borehole NY05 is in a modern high tidal flat area. Three prominent marine beds that formed during the Late Pleistocene and Holocene (Liu et al., 2009; Yi et al., 2013; Wang et al., 2015; Xu et al., 2015) occur within magnetozone N1, indicating that this magnetozone can be unambiguously correlated with the Brunhes normal chron (C1n) (Fig. 5). Given this chronological constraint, magnetozones R1–R7 can be 7
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Fig. 5. Lithostratigraphy and magnetic polarity stratigraphy of borehole NY05 in northern Bohai Bay and correlation with the geomagnetic polarity timescale (Cande and Kent, 1995).
5.4. Initiation of tectonic movements in the olduvai normal subchron
lacustrine strata with sandy particles occur in boreholes NY05, MT04 and TZ02, and lacustrine and floodplain facies in borehole BG10. In addition, the sedimentary characteristics during chron C2r are similar in all four cores, suggesting that the depositional environment and its duration were comparable along northern Bohai Bay during the interval from 1.8–2.6 Ma. In the lower parts, that is, prior to 2.6 Ma, fluvial deposits dominated in all four cores and the possibility of sedimentary hiatuses increases due to the relatively coarse-grained nature of the sediments. As a rough estimate, the basal ages of boreholes BG10, MT04, TZ02 and NY05 are 3.6 Ma, 3.2 Ma, 3.2 Ma and 3.9 Ma, respectively.
A synthesis of the magnetostratigraphies of the four borehole records with their sedimentary and acoustic characteristics enables us to characterize and date the sequence of tectonic events in the Huanghua depression in northern Bohai Bay during the late Cenozoic. Acoustic logging can be used to analyze the porosity of late Cenozoic sediments, and reflects the degree of compaction and consolidation. The results of acoustic logging reveal two distinct intervals with boundaries at 75–80 m. High amplitude variations, with values ranging from 500–700 μs/m, occur in the upper part, indicating a greater porosity and less consolidation; and low amplitude variations with values ranging from 300–500 μs/m values occur in the lower part, indicating a 8
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Fig. 6. Lithostratigraphy and magnetic polarity stratigraphy of borehole TZ02 in northern Bohai Bay and correlation with the geomagnetic polarity timescale (Cande and Kent, 1995).
these results, we infer the following sequence, in descending order of subsidence magnitude: Nanpu sag, Laoting sag, Jianhe sag and Matouying rise, and that it’s the depocenter shifted with time (Fig. 8). During the Pliocene, SARs varied across northern Bohai Bay, with values of 137 m/Ma, 120 m/Ma, 90 m/Ma and 168 m/Ma in the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag, respectively. This indicates that the WNW-orientated Laoting sag was the depocenter during this interval. From 1.95 − 2.58 Ma, the SAR values were 140 m/ Ma,109 m/Ma, 103 m/Ma and 161 m/Ma in the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag, respectively. This indicates during that during this interval the WNW-orientated Laoting sag remained the depocenter. The increased SARs of the WNW-orientated Matouying rise may indicate an enlargement of the depocenter. From 0.78–1.95 Ma,
high degree of consolidation. The effects of compaction and consolidation of the strata under this borderline at 75–80 m are basically similar, so the depths of the magnetozone boundaries and sediment accumulation rates (SARs) of different boreholes can be comparative analysis. The depths of the magnetozone boundaries in the studied sequences are as follows: in boreholes NY05, BG10, MT04 and TZ02 the Matuyama/Brunhes boundary is at 123.2 m, 157.8 m, 122.4 m and 138.5 m, respectively; the lower boundary of the Olduvai normal subchron is at 274.1 m, 409.1 m, 260.5 m and 332.8 m, respectively; and the Gauss/Matuyama boundary is at 361.9 m, 473.2 m, 327.2 m and 435.3 m, respectively. This indicates that the depth ranges of the magnetozones are roughly comparable in the four boreholes. From 9
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Fig. 7. Late Cenozoic magnetostratigraphic framework for various structural units across northern Bohai Bay. (a) and (b) are respectively the lithology and geomagnetic polarity record of borehole NY05 located in the NE-trending Jianhe sag (N39°16.1′, E118°2.4′, 1.7 m a.s.l). (c) and (d) are respectively the lithology and geomagnetic polarity record of borehole BG10 located in the Nanpu sag (N39°10.0′, E118°33.4′, 2.5 m a.s.l). (e) and (f) are respectively the lithology and geomagnetic polarity record of borehole MT04 located in the Matouying rise (N39°15.9′, E118°49.8′, 1.0 m a.s.l). (g) and (h) are respectively the lithology and geomagnetic polarity record of borehole TZ02 located in the Laoting sag (N39°19.7′, E118°5.3′, 0.5 m a.s.l). I, II, III and IV are the first, second, third and fourth marine layers, respectively. The third marine layer in borehole BG10 is divided into two sub-units, III-1 and III-2.
from northeastern part to middle-southern part in northern Bohao Bay. During 0 − 0.78 Ma, the SARs of the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag were 156 m/Ma, 205 m/Ma, 156 m/Ma and 177 m/Ma, respectively. This suggests that the depocenter had moved to the Nanpu sag. The Middle and Late Pleistocene sediments are less consolidated, and the SARs increased slightly, indicating that both
the differences between the SARs of the various structural sub-units increased significantly; the SAR values are 129 m/Ma, 211 m/Ma, 120 m/Ma and 167 m/Ma in the Jianhe sag, Nanpu sag, Matouying rise and Laoting sag, respectively. The SARs of WNW-orientated units increased, especially in the Nanpu sag, which may indicate the occurrence of regional tectonic movements and the conversion of depocenter 10
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Fig. 8. Age-depth curve and acoustic velocity of different structural units across northern Bohai Bay. The dotted line is the acoustic velocity and the solid line is the age-depth curve.
chronostratigraphic framework for the coastal area of Bohai Bay in the Huanghua depression, and to elucidate the regional tectonic dynamics (Fig. 9). The depth of the Matuyama/Brunhes (M/B) boundary in borehole CQJ4 is shallower than in boreholes BZ1 and G2; all three boreholes are located in sags of the middle Huanghua depression. However, the depths of the three marine layers is basically similar in the three boreholes, therefore we believe that in borehole CQJ4 the base of the Jaramillo normal subchron may be the base of the Brunhes normal chron. In the centre of the Huanghua depression, the depths of the M/B and Gauss/Matuyama (G/M) boundaries are 99–104 m and 305–335 m, respectively. In Jianhe sag and the Matouying rise, in the northern Huanghua depression, the depths of M/B and G/M boundaries are 123 m and 327–362 m, respectively. In Nanpu sag and Laoting sag in the northern Huanghua depression, the depths of the M/B and G/M boundaries are 138–158 m and 435–473 m, respectively. The depth of the G/M boundary in the Matouying rise is similar to the depth in the centre of the Hunghua depression, but the subsidencerelated differences of the M/B boundary range from 20–25 m. The subsidence-related differences of the M/B and G/M boundaries between the Jianhe sag and the sags in the centre of the Hunghua depression range from 20–25 m and 27–57 m, respectively. This suggests that the northern Huanghua depression is the regional subsidence centre, and the WNW-orientated structures of the basin has intensified in the Quaternary. The subsidence-related extent of the WNW-orientated Laoting sag and Nanpu sag is significantly greater than that of the NEorientated Jianhe sag which is equivalent to the WNW-orientated Matouying rise. This also indicates that the WNW-orientated structures of the basin has intensified. The Cenozoic tectonic evolution of Bohai Bay Basin is generally considered to have been mainly affected by the collision of the Indian and the Eurasian Plates (Liu et al., 2004), as well as by the subduction of the Pacific Plate beneath the Eurasian Plate (Yin, 2010). The localized extension and opening of back-arc basins at 32–17 Ma (Yin, 2010) resulted in the development of the NEE- and NE-orientated depressions and uplifts in the Bohai Bay Basin (Qi et al., 2010). The depths of the magnetozone boundaries and marine layers of boreholes CQJ4, BZ1 and G2, from the middle of Huanghua depression, exhibit similar trends and therefore the structural dynamics of the region can be regarded as reflecting the subduction of the Pacific Plate beneath the Eurasian Plate. Based on the depth trends of the magnetostratigraphic boundaries and the marine layers in boreholes NY05, BG10, MT04 and TZ02, the extent of subsidence of the northern Huanghua depression has clearly been greater, especially the WNW-orientated structural units. Therefore, the
climate and tectonics affected the sedimentary processes in the study area. In addition, the depth of the Matuyama/Brunhes boundary shows that the depocenter was in the Nanpu sag, indicating that WNW-orientated tectonics played the major role, inheriting the subsidence framework of the early Quaternary. Prior to the Olduvai normal subchron, the sedimentary characteristics indicate the broad development of alternating floodplain and limnetic facies (Zhao et al., 2016), which may be consistent with the pattern of climatic change in the study area (Wang et al., 2002; Yang et al., 2006). From boreholes TZ02 to NY05, the lacustrine strata gradually decreased, the limnetic facies changed to a limnetic delta and the content of coarser deposits gradually decreased. These evidences indicates that the Laoting sag was the depocenter, which is consistent with the conclusions based on the SARs of the different units. From the Olduvai to the Jaramillo normal subchron, limnetic facies, and subaqueous channel deposits interbedded with occasional floodplain and fluvial facies, dominated in the four cores. Prior to the Jaramillo normal subchron, the sediments were fine-grained and the thickness of lacustrine sedimentary intervals increased, while after the Jaramillo subchron both the sediment grain-size and the thickness of the lacustrine intervals increased and the marine beds appeared. Since the Olduvai normal subchron, the sedimentary environment evident in the four cores changed from alternating floodplain and limnetic facies to limnetic facies interbedded with occasional floodplain facies. While the broad floodplain developed along western Bohai Bay (Yao et al., 2012). Therefore, we suggest that the changes in the sedimentary environment were caused by tectonic movements, rather than by climate changes. From the depths of the polarity chrons, the SARs of different tectonic units and the changes in the sedimentary environment during the Olduvai normal subchron, we can conclude that the tectonic movements commenced from the Olduvai normal subchron onwards, and the WNW-orientated tectonics played the major role. 5.5. Structural features and dynamics of the Huanghua depression during the Plio-Pleistocene Magnetostratigraphies for the coastal area of Bohai Bay are available for the following boreholes: Middle Huanghua depression, western Bohai Bay: CQJ4 in Banqiao sag (Shi et al., 2009), BZ1 in Banqiao sag (Xiao et al., 2008), and G2 in Beitang sag (Xiao et al., 2014). Northern Huanghua depression, northern Bohai Bay: BG10 in Nanpu sag (Yuan et al., 2014), and MT04 in Matouying rise (Xu et al., 2014). We have combined the results of this previous work with our new magnetostratigraphies for boreholes NY05 and TZ02 from Jianhe sag and Laoting sag respectively, northern Bohai Bay, to establish a Plio-Pleistocene 11
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Fig. 9. Late Cenozoic magnetostratigraphic framework for different structural units in the Huanghua depression. Boreholes CQJ4 and BZ1 are in Banqiao sag, borehole G2 in Beitang sag, borehole NY05 in Jianhe sag, borehole BG10 in Nanpu sag, borehole MT04 in Matouying rise, and borehole TZ02 in Laoting sag. I, II and III are the first, second and third marine layers, respectively. The third marine layer in borehole BG10 is divided into two subunits, III-1 and III-2.
intracontinental rifting and extension in North China during the Quaternary.
structural dynamics of the region reflect the effects of localized subsidence superimposed on the development of WNW-orientated faults. The WNW-orientated fault system associated with the collision between the Indian and the Eurasian Plates may be linked with Cenozoic 12
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English abstract). Acta Geogr. Sin. 3, 303–314. Guo, L.L., Li, S.Z., Suo, Y.H., Ji, Y.T., Dai, L.M., Yu, S., Liu, X., Somerville, I.D., 2015. Experimental study and active tectonics on the Zhangjiakou-Penglai fault zone across North China. J. Asian Earth Sci. 144 (Part 1), 18–27. Hebei Bureau of Geology and Mineral Resources,, 1989. Regional Geological Annals of Hebei Province. Geological Publishing House, Beijing and Tianjin Beijing, pp. 590–616. King, J.W., Channell, J.E.T., 1991. Sedimentary magnetism, environmental magnetism and magnetostratigraphy. Rev. Geophys. 29 (Suppl. 1), 358–370. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. R. Astron. Soc. 62, 699–718. Li, H.M., Wang, J.D., 1983. Palaeomagnetic study on drill core from northern Bohai coastal plain (in Chinese with English abstract). Geochimica 12, 196–204. Li, Y.F., Gao, S.M., An, F.T., 1982. A preliminary study of the quaternary marine strata and its paleogeographic significance in the Luanhe delta region (in Chinese with English abstract). Oceanol. Limnol. Sin. 13, 433–439. Li, C.X., Chen, G., Wang, C.G., Zhang, Y.M., 1984. On the Luanhe river alluvial fan-delta complex (in Chinese with English abstract). Acta Petrolei Sin. 4, 27–36. Li, S.Z., Zhao, G.C., Dai, L.M., Zhou, L.H., Liu, X., Suo, Y.H., Santosh, M., 2012. Cenozoic faulting of the Bohai Bay Basin and its bearing on the destruction of the eastern North China Craton. J. Asian Earth Sci. 47, 80–93. Liu, M., Cui, X.J., Liu, F.T., 2004. Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo-Asian collision? Tectonophysics 393, 29–42. Liu, J., Saito, Y., Wang, H., Zhou, L., Yang, Z., 2009. Stratigraphic development during the late pleistocene and holocene offshore of the yellow river delta, Bohai sea. J. Asian Earth Sci. 36, 318–331. Liu, C.C., Deng, C.L., Liu, Q.S., Zheng, L.T., Wang, W., Xu, X.M., Huang, S., Yuan, B.Y., 2010. Mineral magnetism to probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China. Geophys. J. Int. 181, 1395–1410. Qi, J.F., Yang, Q., 2010. Cenozoic structural deformation and dynamic processes of the Bohai Bay basin province, China. Mar. Petrol. Geol. 4, 757–771. Ren, J., Tamaki, K., Li, S., Zhang, J., 2002. Late Mesozoic and Cenozoic rifting and its dynamic setting in Eastern China and adjacent areas. Tectonophysics 344, 175–205. Roberts, A.P., Cui, Y., Verosub, K.L., 1995. Wasp-waisted hysteresis loops: mineral magnetic characteristics and discrimination of components in mixed magnetic systems. J. Geophys. Res. 100, 17909–17924. Shi, L.F., Zhai, Z.M., Wang, Q., Zhang, X.B., Yang, Z.Y., 2009. Geochronological study on transgression layers of the CQJ4 borehole at dagang area in Tianjin, China (in chinese with english abstract). Geol. Rev. 55, 375–384. Stacey, F.D., Banerjee, S.K., 1974. The Physical Principles of Rock Magnetism. Elsevier, Amsterdam, pp. 195. Wang, S.M., Wu, X.H., Zhang, Z.K., Jiang, F.C., Xue, B., Tong, G.B., Tian, G.Q., 2002. Sedimentary records of environmental evolution in the Sanmen Lake basin and the Yellow River running through the Sanmenxia Gorge eastward into the sea. Sci. China (Ser. D Earth Sci.) 45, 595–608. Wang, Y., Fu, G.H., Zhang, Y.Z., 2007. River-sea interactive sedimentations and plain morphological evolution (in Chinese with English abstract). Quat. Sci. 5, 674–689. Wang, F., Li, J., Chen, Y., Fang, J., Zong, Y., Shang, Z., Wang, H., 2015. The record ofmidHolocene maximumlandward marine transgression in the west coast of Bohai Bay, China. Mar. Geol. 359, 89–95. Xiao, G.Q., Guo, Z.T., Chen, Y.K., Yao, Z.Q., Shao, Y.X., Wang, X.L., Hao, Q.Z., Lu, Y.C., 2008. Magnetostratigraphy of BZ1 borehole in west coast of bohai Bay，Northern China. Quat. Sci. 28, 909–916. Xiao, G.Q., Yang, J.L., Zhao, C.R., Wang, Q., Xu, Q.M., Hu, Y.Z., Qin, Y.F., Li, J., Xiao, G.Q., 2014. Magnetostratigraphy of drill hole G2 in the Tianjin coastal area and its tectonic significance. Geol. Bull. China 33, 1642–1650. Xu, Q.M., Yuan, G.B., Qin, Y.F., Yang, J.L., Yang, J.Q., 2014. Magnetostratigraphy and discussion of coupling relationship between tectonic movement and climate change of MT04 borehole in southern Luanhe River (in Chinese with English abstract). Quat. Sci. 34, 540–552. Xu, Q.M., Yang, J.L., Yuan, G.B., Chu, Z.X., Zhang, Z.Z., 2015. Stratigraphic sequence and episodes of the ancient Huanghe Delta along the southwestern Bohai Bay since the LGM. Mar. Geol. 367, 69–82. Yan, Y.Z., Wang, H., Li, F.L., Li, J.F., Zhao, C.R., Lin, F., 2006. Sedimentary environment and sea-level fluctuations revealed by Borehole BQ1 on the West Coast of the Bohai bay, China (in chinese with english abstract). Geol. Bull. China 3, 357–382. Yang, S.Y., Li, C.X., Cai, J.G., 2006. Geochemical compositions of core sediments in eastern China: implication for Late Cenozoic palaeoenvironmental changes. Palaeogeogr. Palaeoclimatol. Palaeoecol. 229, 287–302. Yao, Z.Q., Xiao, G.Q., Wu, H.B., Liu, W.G., Chen, Y.K., 2010. Plio-pleistocene vegetation changes in the North China Plain: magnetostratigraphy, oxygen and carbon isotopic composition of pedogenic carbonates. Palaeogeogr. Palaeoclimatol. Palaeoecol. 297, 502–510. Yi, L., Lai, Z.P., Yu, H.J., Xu, X.Y., Su, Q., Yao, J., Wang, X.L., Shi, X.F., 2013. Chronologies of sedimentary changes in the south Bohai Sea, China: constraints from luminescence and radiocarbon dating. Boreas 42, 267–284. Yi, L., Deng, C.L., Tian, L.Z., Xu, X.Y., Jiang, X.Y., Qiang, X.K., Qin, H.F., Ge, J.Y., Chen, G.Q., Su, Q., Chen, Y.P., Shi, X.F., Xie, Q., Yu, H.J., Zhu, R.X., 2016. Plio-Pleistocene evolution of Bohai Basin (East Asia): demise of Bohai paleolake and transition to marine environment. Sci. Rep. 6, 29403. http://dx.doi.org/10.1038/srep29403. Yin, A., 2010. Cenozoic tectonic evolution of Asia: a preliminary synthesis. Tectonophysics 488, 293–325. Yuan, G.B., Xu, Q.M., Wang, Y., Yang, J.L., Qin, Y.F., Du, D., 2014. Magnetostratigraphy and geology significance of Bg10 Borehole in Northern Coast of Bohai Bay (in Chinese with eEnglish abstract). Acta Geol. Sin. 88, 285–298. Zhang, Y., Guo, Z.T., Deng, C.L., Zhang, S.Q., Wu, H.B., Zhang, C.X., Ge, J.Y., Zhao, D.A.,
6. Conclusions Paleomagnetic studies combined with sedimentary analysis were conducted on two new boreholes from northern Bohai Bay to elucidate the regional tectonic processes since the Pliocene. The main conclusions are as follows. (1) Magnetite and hematite are the main carriers of the characteristic remanent magnetization in the studied sedimentary sequences. Magnetostratigraphic results indicate that the deposits recorded the Brunhes and Gauss normal chrons, together with the successive reversed polarity intervals of the Matuyama chron. (2) The depths of the Matuyama/Brunhes boundary, the base of the Olduvai normal subchron, and the Gauss/Matuyama boundary, are greatest in the Nanpu sag, followed by the Laoting sag, Jianhe sag and Matouying rise. The sediment accumulations rates (SARs) of the different units suggest that the WNW-orientated Laoting sag was the depocenter at ∼2.58 Ma, the WNW-orientated Nanpu sag was the depocenter during 2.58–1.95 Ma, and the structural features have inherited the subsidence framework since the early Quaternary. Since the Olduvai normal subchron, the sedimentary environment, as revealed by the four cores, changed from alternating floodplain and limnetic facies to limnetic facies interbedded with occasional intervals of floodplain facies. From the depths of the different magnetozone boundaries, the SARs of the different tectonic units and changes in sedimentary environment in the Olduvai normal subchron, we conclude that tectonic movement started in the Olduvai normal subchron and the development of the WNW-orientated tectonic features were intensified. (3) The subsidence of the central part of the Huanghua depression may be associated with the subduction of the Pacific Plate beneath the Eurasian Plate, and the subsidence of the northern part may have been superimposed on the WNW-orientated tectonic activity and faulting associated with the collision between the Indian and the Eurasian Plates, in the context of localized subsidence. Acknowledgements This study was supported by the China Geological Survey Project (grants 12120114007801, 1212011120746 and DD2016042). CD acknowledges additional support from the National Natural Science Foundation of China (grants 41690112 and 41621004). 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. 8, 951–972. Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095. Chen, Y.S., Wang, H., Pei, Y.D., Tian, L.Z., Li, J.F., Shang, Z.W., 2012. Division and its geological significance of the late Quaternary marine sedimentary beds in the west coast of Bohai bay, China (in Chinese with English abstract). J. Jilin Univ. (Earth Sci. Ed.) 3, 747–759. Deng, C.L., Zhu, R.X., Verosub, K.L., Singer, M.J., Yuan, B.Y., 2000. Paleoclimatic significance of the temperature-dependent susceptibility of Holocene Loess along a NWSE transect in the Chinese Loess Plateau. Geophys. Res. Lett. 27, 3715–3718. Deng, C.L., Zhu, R.X., Jackson, M.J., Verosub, K.L., Singer, M.J., 2001. Variability of the temperature-dependent susceptibility of the Holocene eolian deposits in the Chinese loess plateau: a pedogenesis indicator. Phys. Chem. Earth Part A 26, 873–878. Deng, C.L., Vidic, N.J., Verosub, K.L., Singer, M.J., Liu, Q.S., Shaw, J., Zhu, R.X., 2005. Mineral magnetic variation of the Jiaodao Chinese loess/paleosol sequence and its bearing on long-term climatic variability. J. Geophys. Res. 110, B03103. http://dx. doi.org/10.1029/2004JB003451. Dong, Y.X., Xiao, L., Zhou, H.M., Wang, C.Z., Zheng, J.P., Zhang, N., Xia, W.C., Ma, Q., Du, J.X., Zhao, Z.X., Huang, H.X., 2010. The Tertiary evolution of the prolific Nanpu Sag of Bohai Bay Basin, China: constraints from volcanic records and tectono-stratigraphic sequences. Geol. Soc. Am. Bull. 122, 609–626. Editorial Committee of Petroleum Geology of Dagang Oil Field, 1993. Petroleum Geology of China (volume 4) (in Chinese): Dagang Oil Field. Petroleum industry press, Beijing, pp. 83–114. Gao, S.M., 1981. Facies and sedimentary model of the Luan river delta (in Chinese with
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Q. Xu et al. Qin, L., Song, Y., Zhu, R.X., 2014. The use of fire at Zhoukoudian: evidence from magnetic susceptibility and color measurements. Chin. Sci. Bull. 59, 1013–1020. Zhao, L.L., Xu, Q.M., Yang, J.L., Yuan, G.B., Guo, J.J., 2016. Sedimentary evolution of BGl0 borehole in Northern Coast of Bohai Bay during late cenozoic (in Chinese with English abstract). Quat. Sci. 36, 196–207.
Zhu, R.X., Xu, Y.G., Zhu, G., Zhang, H.F., Xia, Q.K., Zheng, T.Y., 2012. Destruction of the north China craton. Sci. China Earth Sci. 55, 1565–1587. Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks: analysis of results. In: Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Paleomagnetism. Elsevier, New York, pp. 254–286.
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