Magnetostratigraphy of two deep boreholes in southwestern Bohai Bay: Tectonic implications and constraints on the ages of volcanic layers

Accepted Manuscript Magnetostratigraphy of two deep boreholes in the southwestern Bohai Bay: Its tectonic implications and constraints on ages of volcanic layers Qinmian Xu, Jilong Yang, Yunzhuang Hu, Guibang Yuan, Chenglong Deng PII:

S1871-1014(16)30179-0

DOI:

10.1016/j.quageo.2017.08.006

Reference:

QUAGEO 866

To appear in:

Quaternary Geochronology

Received Date: 23 November 2016 Revised Date:

23 February 2017

Accepted Date: 25 August 2017

Please cite this article as: Xu, Q., Yang, J., Hu, Y., Yuan, G., Deng, C., Magnetostratigraphy of two deep boreholes in the southwestern Bohai Bay: Its tectonic implications and constraints on ages of volcanic layers, Quaternary Geochronology (2017), doi: 10.1016/j.quageo.2017.08.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Magnetostratigraphy of two deep boreholes in the southwestern

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Bohai Bay: its tectonic implications and constraints on ages of

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volcanic layers

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Qinmian Xu a,*, Jilong Yang a, Yunzhuang Hu a, Guibang Yuan a, Chenglong Deng b,c,

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Tianjin Center, China geological Survey, Tianjin 300170, China

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b

State Key Laboratory of Lithospheric Evolution, Institute of Geology and

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Geophysics, Chinese Academy of Sciences, Beijing 10029, China c

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University of Chinese Academy of Sciences, Beijing 100049, China

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* Corresponding authors.

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E-mail address: [email protected] (Q. Xu).

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ABSTRACT

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Bohai Bay Basin recorded the processes of basin filling and structural evolution,

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possibly resulting from the destruction of the North China Craton during the late

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Mesozoic

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chronostratigraphic framework of sedimentary sequences in this basin has precluded a

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better understanding of this critical evolution, especially in southwestern Bohai Bay,

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there are two Quaternary volcanoes named Dashan and Xiaoshan respectively. In this

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study, by combining paleomagnetic study with sedimentary analysis on two drilled

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boreholes (CK3 and G4) from the southwestern Bohai Bay Basin, new insights into

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and

the

early

Cenozoic.

However,

the

absence

of

reliable

ACCEPTED MANUSCRIPT regional tectonic process and geological problems since the Pliocene are obtained.

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The main results are as follows. (1) Magnetite and hematite were identified as the

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main carrier of the characteristic remanent magnetization in these sedimentary

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sequences, and the hematite was the main component. (2) These two borehole

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sedimentary sequences recorded the Brunhes, Olduvai and Gauss normal chrons, and

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the Matuyama and Gilbert reverse chron. (3) The subsidence differences of the M/B

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and G/M boundary of these boreholes along the Bohai Bay distributed in different

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structure units may reflect that the northern Bohai bay is the regional subsidence

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center and the WNW-orientated structures of the basin has been strengthened in

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Quaternary. (4) The tendency of elevations of three marine layers is similar to the

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M/B and G/M boundary along the Bohai Bay. With the Constraints of the

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magnetostratigraphies of these boreholes, the ages of the second and third marine

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layers belong to Marine Oxygen Isotope (MIS) 3 and MIS5, respectively. The

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WNW-orientated structures and “super interglacials” have formed the transgressions

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in the Huanghua depression. (5) In CK3 borehole, the ages of four volcanic layers

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distributing the depth of 13.14-16.18 m, 33.37-48.02 m, 145.8-154.05 m and

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222.16-233.80 m, are 10-18 Ka BP, 80-90 Ka BP, 2.2 Ma and 3.1 Ma, respectively. In

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G4 boreholes, the age of volcanic layer distributing the depth of 46.4-55.8 m is

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0.33-0.55 Ma. Contrastive analysis of the subsidence differences between the NEE

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and NE-orientated uplifts and the depressions in western and southern Bohai bay, the

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subduction of the Pacific plate mainly controlled the structure evolution in the vertical

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during the Quaternary.

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Keywords: Southwestern Bohai Bay; Late Cenozoic; Magnetostratigraphy; Volcanic

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layers

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1. Introduction

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The Bohai Bay Basin in eastern China contains a plethora of evidence for

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structural framework, sedimentary basin filling, and reservoirs of oil fields (Allen et

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al., 1997; Ren et al., 2002; Qi and Yang, 2010; Yin, 2010; Li et al., 2012; Guo et al.,

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2015). In tectonics, the Bohai Bay Basin has recorded structural processes, possibly

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resulting from lithospheric thinning beneath the North China Craton during the late

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Mesozoic and the early Cenozoic (Li et al., 2012; Zhu et al., 2012). But now the North

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China Plain is belong to the tectonic activity area (Deng et al., 2003), especially in

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southwestern Bohai Bay there are two Quaternary volcanoes named Dashan and

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Xiaoshan respectively. The magmas of these two volcanoes may come from

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asthenosphere, but have relatively independent volcanic structures based on

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petrological and geochemical features (Yu et al., 2015). So they have different timings

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of volcanic activity, the K-Ar ages dating on volcanic rocks of Dashan are 0.55-0.86

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Ma (Chen and Peng, 1985) and 0.33-0.86 Ma (Wang, et al., 1988), the ages of

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Xiaoshan have been still in a debate, some claimed the age is 41ka based on the

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Electron Spin Resonance (ESR) method (Yin et al., 2013), and others argued 10-15 ka

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BP based on

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beds have been distributed in Quaternary strum nearby the Xiaoshan volcano (Shao et

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al., 1983). It is important to date the volcanic layers for understanding the neotectonic

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C dating and stratigraphic relations (Hu et al., 2014)。Four volcanic

ACCEPTED MANUSCRIPT activities along the Bohai Bay. Magnetostratigraphy of borehole CK3 has been built

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by alternating field (AF) demagnetization (Hu et al., 2014), but the result is not so

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satisfied because the remanence is not completely cleaned by AF demagnetization. So

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we present new results of high-resolution magnetostratigraphic dating of two

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boreholes sedimentary sequences (CK3 and G4) from the southwestern Bohai Bay by

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progressive thermal demagnetization, and hope to date volcanic layers based on late

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Cenozoic magnetostratigraphy.

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In addition, the absence of reliable chronostratigraphic framework of sedimentary

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sequences in this basin has precluded a better understanding of the processes of basin

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filling, structural evolution and their relationships with regional tectonics, especially

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in the late Cenozoic. During the past two decades, numerous boreholes of

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Pliocene-Quaternary age obtained in this basin offer a unique opportunity for

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establishing

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magnetostratigraphic dating and other chronostratigraphic data. In this study, we draw

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on our magnetostratigraphic results of the two cores, as well as previously published

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magnetostratigraphy of cores CQJ4 (Shi et al., 2009), BZ1 (Xiao et al, 2008), BZ2

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(Yao et al, 2010), G2 (Xiao et al, 2014), NY05 (Xu et al, 2016), BG10 (Yuan et al.,

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2014), MT04 (Xu et al, 2014) and TZ02 (Xu et al, 2016) along the Bohai Bay to

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explore the regional structural characteristics during the period from Pliocene to

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Quaternary.

regional

chronostratigraphical

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framework

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2. Geological setting and sampling

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2.1. Geological setting The Bohai Bay Basin, located at the center of the eastern block of the North

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China Craton (Li et al., 2012), is geologically surrounded by the Yanshan fold belt in

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the north, the Taihang fold belt in the west and the Tancheng-Lujiang Fault in the east

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(Fig. 1a). The basin consists of a series of depressions and uplifts separated by

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regional-scale faults (Qi and Yang, 2010) (Fig. 1a).

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The northern Bohai Bay is located within the northern Huanghua depression. The

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NEE-trending Ninghe-Changli Fault separates the Yanshan fold belt in the north from

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Huanghua depression in the south. This area consists of a series of secondary sags and

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rises, including Jianhe sag, Laowangzhuang rise and Nanpu sag in the western part,

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and Matouying rise and Laoting sag in the eastern part. These sags and rises are

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controlled by secondary faults in formation and evolution during the late Cenozoic

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(Fig. 1b). The western Bohai Bay is located within the middle Huanghua depression.

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The NE-trending Cangdong Fault separates the Cangxian uplift in the west from

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Huanghua depression in the east. This area consists of a series of secondary sags,

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including Beitang sag and Banqiao sag (Fig. 1b). The southwestern Bohai Bay is

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located within the Chengning uplift. The NEE-trending Yangerzhuang Fault separates

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the Huanghua depression in the north from Chengning uplift in the south (Fig. 1b).

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Cenozoic strata of the Huanghua depression are over 6000 m thick: ~3000 m for

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the Paleogene, ~2500 m for the Neogene, and ~500 m for the Quaternary; Cenozoic

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strata of the Cangxian uplift and Chengning uplift are similar over 2000 m thick:

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~1800 m for the Neogene, and ~200 m for the Quaternary (Hebei Bureau of Geology

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Oil Field, 1993; Yao et al, 2010). The strata comprise alluvial, fluvial and lacustrine

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sediments, intercalated volcanic/volcaniclastic and neritic/littoral sediments, are

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unevenly distributed across the depression (Hebei Bureau of Geology and Mineral

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Resources, 1989).

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The CK3 and G4 boreholes were drilled nearby the Xiaoshan volcano and Dashan

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volcano in Chengning uplift, respectively (Fig. 1b). Xiaoshan volcano located at the

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northeast ~4 km away from the Haixing country, Hebei province, is belong to Maar

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volcano, with ~2 km in diameter of crater and ~35 m in elevation of east side and the

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buried west side; and it consist of tuff in upper part and olivine basalt in lower part,

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which are mainly basaltic in composition. Dashan volcano with ~400 m in diameter of

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bottom and ~73 m in elevation is located at the northeast ~30 km away from the Wudi

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country, Shandong province; and it consist of volcanic agglomerate in upper part and

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alkali basalt in lower part, which are mainly nephelinte in composition.

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2.2. Lithology of the boreholes

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The CK3 borehole (N38°9.2', E117°32.5', 4.0 m a.s.l) is located at the southwest

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~2 km away from the west side of volcano crater, and has a length of 500 m (Fig. 2A).

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The obliquities of CK3 borehole is less 4° at 500 m depth. The G4 borehole (N38°2.2',

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E117°35.8', 4.5 m a.s.l) is located at the northeast ~4 km away from the volcanic cone,

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and has a length of 400 m (Fig. 2B). The obliquities of G4 borehole is less 4° at 400m

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depth. The two boreholes sediments are mainly consisted of yellowish-brown,

ACCEPTED MANUSCRIPT dark-brown silty clays, silts, sandy silts and silty sands, and a few fine-medium sands

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(15%). Note that three marine beds occur in the depth intervals of 3−11 m, 20-30 m

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and 52-59 m in CK3 and of 6-10 m, 22-31 m and 38-45 m in G4, respectively. The

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CK3 borehole include four volcanic layers: the depth of 13.14-16.18 m consisted of

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grayish olive basaltic tuffite (Fig. 2a), the depth of 33.37-48.02 m consisted of dark

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gray basaltic breccias, basaltic tuffite and basaltic volcaniclastic rocks (Fig. 2b, c), the

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depth of 145.8-154.05 m consisted of grayish green basaltic tuffite and basaltic

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breccias interbedded with grayish green silty clay (Fig. 2d, e), the depth of

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222.16-233.80 m consisted of grayish black and black basalt interbedded with basaltic

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tuff (Fig. 2f). The G4 borehole include only 1 volcanic layer consisted of basaltic

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tuffite with the depth of 46.4-55.8 m (Fig. 2g).

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2.3. Sampling

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The cores were split into two halves. Block samples of two boreholes were taken

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in the field and two sets of sister specimens (2-cm cubic) for each sample was

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obtained. The sampling intervals are 0.5 m in silty layers and in volcanic layers.

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3. Methods

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3.1. Rock magnetic measurements

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In order to determine the remanence carriers and magnetic mineralogy of the

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sediments, rock magnetic measurements including high-temperature magnetic

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susceptibilities (χ–T curves), isothermal remanent magnetization (IRM) acquisition

ACCEPTED MANUSCRIPT curves and backfield curves of IRM were made on representative samples from

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various depths (Fig. 3). The rock magnetic measurements were conducted in the

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Paleomagnetism and Geochronology Laboratory of Institute of Geology and

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Geophysics, Chinese Academy of Sciences, Beijing.

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χ–T curves were measured by continuous exposure of samples through

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temperature cycles from room temperature to 700°C and back to room temperature in

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an argon atmosphere using a KLY-3 Kappabridge with a CS-3 high-temperature

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furnace (Agico Ltd., Brno).

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IRM acquisition and its back-field demagnetization were measured using a

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MicroMag 3900 Vibrating Sample Magnetometer (Princeton Measurements Corp.,

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USA). An IRM was imparted from 0 to 1.0 T (IRM1T, hereafter termed saturation

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IRM, SIRM), and then demagnetized in a stepwise backward-field up to 1.0 T to

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obtain the coercivity of remanence (Bcr).

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3.2. Demagnetization of the natural remanent magnetization (NRM)

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Paleomagnetic measurements of two boreholes were made using a 2G Enterprises

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Model 760-R cryogenic magnetometer installed in a magnetically shielded space

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(<300 nT) in the Paleomagnetism and Geochronology Laboratory of Institute of

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Geology and Geophysics, Chinese Academy of Sciences, Beijing. 571 specimens of

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CK3 borehole and 427 specimens of G4 borehole were subjected to progressive

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thermal demagnetization up to a maximum temperature of 690°C with 25−50°C

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interval below 585°C and 10−25°C above 585°C using a Magnetic Measurements

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thermal demagnetizer (TD48) with a residual magnetic field less than 10 nT. Demagnetization results were evaluated by orthogonal diagrams (Zijderveld,

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1967), and the principal components direction was computed by a ''least-squares

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fitting'' technique (Kirschvink, 1980). Representative demagnetization diagrams are

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shown in Figure 4. Only the magnetic inclination data were subsequently used to

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define the succession of magnetostratigraphic polarity because the magnetic

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declinations are arbitrary.

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4. Results

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4.1. Rock magnetic results

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4.1.1. χ–T curves

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χ–T curves are highly sensitive to mineralogical changes during thermal treatment,

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but such changes can provide useful information about magnetic mineral composition

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and magnetic grain size. It has been shown that natural sediments usually contain

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some thermally unstable components, such as iron-bearing clay minerals, maghemite

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and sulphides (e.g., pyrrhotite, pyrite, greigite), which can be traced by

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susceptibility changes (Roberts et al., 1995; Deng et al., 2001; Zhang et al., 2014).

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The χ–T curve of sample CK3-1 is characterized by a major decrease in magnetic

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susceptibility at about 585–600°C (Fig. 3a). This behavior indicates that nearly

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stoichiometric magnetite and partially-oxidized magnetite are the major contributors

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to the susceptibility. The two other samples, the large residual magnetic susceptibility

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above 585°C becomes effectively zero at ∼680°C (Figs. 3b, c), the Néel temperature

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of hematite, suggesting that hematite is not only present but also in great amount due

ACCEPTED MANUSCRIPT to its weakly magnetism (Liu et al., 2010). The significantly enhanced susceptibility

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after thermal treatment in some samples is mainly attributed to the neoformation of

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magnetite grains from the transformation of iron-containing silicates/clays (Deng et

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al., 2000, 2001) (Figs. 3a, b). Sample CK3-3 shows nearly reversible χ–T curves (Fig.

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4c), suggesting negligible neoformation of ferrimagnetic phases during thermal

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treatment. In the heating curve of sample CK3-1, there are a sharp increase of

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susceptibility at ~300°C and a pronounced hump between ~300°C and ~500°C, may

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be ascribed to the conversion of iron sulfides (e.g., pyrrhotite or pyrite) to magnetite.

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4.1.2. IRM acquisition curves and its back-field demagnetization curves The selected samples yield different IRM acquisition curves (Figs. 3d-f). All

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samples show a gradual rise below 0.1 T and relatively low values of IRM0.1T/SIRM

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(Figs. 3d-f), indicative of the existence of both low- and high-coercivity magnetic

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minerals. About 59%–77% of the SIRM was acquired below 0.3 T, also implying the

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existence of both low- and high-coercivity magnetic minerals. The remanence

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continued to be acquired above 0.3 T, which is generally considered to be the

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theoretical maximum coercivity of magnetite grains. S-ratio is the absolute value of

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IRM remaining after exposure to a reversed field of 0.3 T divided by SIRM (King and

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Channell, 1991). The relatively low values of S-ratio (generally less than 0.70) from

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all samples (Figs. 3d-f), especially samples CK3-2 and CK3-2 with very low value,

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suggest the high content of a high-coercivity magnetic phase. Evidence that the low-

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and high-coercivity minerals are respectively magnetite/pyrrhotite, goethite and

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hematite comes from the χ–T curves (Figs. 3a-c).

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4.2. Paleomagnetic results Stepwise thermal was capable of isolating the characteristic remanent

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magnetization (ChRM) after removal of soft secondary component of magnetization.

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Generally, a secondary magnetic component, probably of viscous origin, was present

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and removed by thermal demagnetization at 150–200°C. For some specimens, the

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high-stability ChRM component was obtained between 250°C and 610°C (Figs. 4c).

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However, for some specimens, the high-stability ChRM component persists up to

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680°C (Fig. 4a,e) or even to 690°C (Figs. 4b, f). These behaviors indicate that both

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magnetite and hematite dominate the ChRM carriers in the Bohai Bay Basin

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fluviolacustrine sediments. The high-stability ChRM component of the basalt sample

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CK3-3 was obtained between 300°C and 580°C (Figs. 4c), which indicate that

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magnetite dominate the ChRM carriers in the volcanics.

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At least four successive points at the orthogonal plots were used to calculate the

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direction of ChRMs (Kirschvink, 1980) and maximum angular deviation (MAD) was

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generally smaller than 15°. After stepwise demagnetization, 208 specimens (36%) in

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CK3 borehole, gave reliable ChRM directions; and 218 specimens (51%) in G4

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borehole, gave reliable ChRM directions.

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Inclination was used for magnetostratigraphy because the core is unoriented. As

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discussed previously, the ChRMs of the studied core were carried by hematite and

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magnetite. Histograms of inclinations are shown in Fig. 6. For the CK3 borehole,

ACCEPTED MANUSCRIPT there exist minor differences in inclinations between the hematite (36.4°±16.8°, n=98,

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see Fig. 5a) and magnetite (41.4°±19.8°, n=110, see Fig. 6a) remanences, the latter of

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which are very close to the average inclinations for all the measured specimens

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(39.0°±18.6°, n=208, see Fig. 5a). For the G4 borehole, there also exist minor

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differences in inclinations between the hematite (35.1°±15.5°, n=105, see Fig. 5c) and

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magnetite (41.1°±18.3°, n=113, see Fig. 5c) remanences, the latter of which are very

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close to the average inclinations for all the measured specimens (37.8°±17.3°, n=218,

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see Fig. 5c). The accepted 208 inclinations of CK3 and 218 inclinations of G4 are

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shown in Fig. 5b, d with 5 intervals. Generally, both positive and negative inclinations

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follow normal distribution. The mean normal inclination of CK3 is 40.9° and that of

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reversed inclination is -36.6°, the mean normal inclination of G4 is 41.6°and that of

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reversed inclination is -35.8°.The inclination difference between the normal and

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reversed inclinations are all less than 5 that suggests they are antipodal.

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Following stepwise demagnetization, fourteen magnetozones are recognized in

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the CK3 borehole sequence: seven with normal polarity (N1 to N7) and seven with

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reverse polarity (R1 to R7) (Fig. 6). In addition, one short intervals of possible

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transitional field behavior, labeled e1, is recorded within magnetozone R1 (Fig. 6).

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Fifteen magnetozones are recognized in the G4 borehole sequence: eight with normal

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polarity (N1 to N8) and seven with reverse polarity (R1 to R7) (Fig. 6).

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5. Discussion

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5.1. Correlations of the recognized magnetozones in CK3 to geomagnetic polarity

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timescale (GPTS) The CK3 borehole is located in the Holocene ancient Huanghe Delta (Xu et al,

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2015). Three prominent marine beds formed during the Late Pleistocene and

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Holocene (Liu et al, 2009; Yi et al., 2013; Wang et al., 2015) occurs in magnetozone

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N1, suggesting that this magnetozone can be unambiguously correlated with the

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Brunhes normal chron (C1n) (Fig. 6). Given this chronologic constraint,

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magnetozones R1 to R7 determined for the CK3 borehole can be readily correlated

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with polarity intervals from the post-Jaramillo Matuyama chron (C1r.1r) to the C3Ar

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chron in the GPTS (Cande and Kent, 1995). Magnetozones R1 to R2 correspond to

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the Matuyama reverse chron, among which magnetozones N2 correspond to the

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Olduvai (C2n) normal subchrons. Magnetozones N3 to N5 correspond to the Gauss

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normal chron, among which magnetozones R3 and R4 correspond to the Kaena

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(C2An.1r) and Mammoth (C2An.2r) reverse subchrons, respectively. Magnetozones

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R5 to R6 correspond to the Gilbert reverse chron, among which magnetozone N6

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correspond to the C3n normal subchrons. Magnetozone N7 corresponds to the C3An

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normal chron, Magnetozone R7 corresponds to the late Gilbert chron (C3Ar).

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This correlation puts the Matuyama-Brunhes (M/B) boundary (Lower-Middle

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Pleistocene boundary), Gauss-Matuyama (G/M) boundary (Pliocene-Pleistocene

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boundary), Gilbert-Gauss boundary (Gi/G), Gilbert-C3An boundary and C3An-C3Ar

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in the CK3 borehole at 85.4 m, 176.4 m, 289.7 m, 400.2 m and 480.3 m, respectively

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(Fig. 6). The bottom of the borehole is within the C3Ar reverse chron.

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5.2. Correlations of the recognized magnetozones in G4 to GPTS The G4 borehole is also located in the Holocene ancient Huanghe Delta (Xu et al,

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2015). Three prominent marine beds formed during the Late Pleistocene and

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Holocene (Liu et al, 2009; Yi et al., 2013; Wang et al., 2015) also occurs in

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magnetozone N1, suggesting that this magnetozone can be unambiguously correlated

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with the Brunhes normal chron (C1n) (Fig. 6). Given this chronologic constraint,

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magnetozones R1 to N8 determined for the G4 borehole can be readily correlated with

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polarity intervals from the post-Jaramillo Matuyama chron (C1r.1r) to the middle

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Gilbert chron (C3n.4n) in the GPTS (Cande and Kent, 1995). Magnetozones R1 to R2

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correspond to the Matuyama reverse chron, among which magnetozones N2

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correspond to the Olduvai (C2n) normal subchrons. Magnetozones N3 to N6

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correspond to the Gauss normal chron, among which magnetozones R3-R4 and R5

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maybe correspond to the Kaena (C2An.1r) and Mammoth (C2An.2r) reverse

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subchrons, respectively. Magnetozones R6 to N8 correspond to the Gilbert reverse

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chron, among which magnetozone R6 correspond to the C2Ar reverse subchrons, and

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magnetozones N7 to N8 correspond to the C3n normal subchrons.

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This correlation puts the M/B boundary (Lower-Middle Pleistocene boundary),

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G/M boundary (Pliocene-Pleistocene boundary) and Gi/G boundary in the G4

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borehole at 75.7 m, 183.7 m and 290.5 m, respectively (Fig. 6). The bottom of the

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borehole is within the C3n normal chron.

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5.3. Late Cenozoic magnetostratigraphic framework of the coastal area of Bohai Bay

ACCEPTED MANUSCRIPT The combination of our new magnetostratigraphies of the CK3 and G4 boreholes

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from the Chening uplift, southwestern Bohai bay, and previously published

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magnetostratigraphies of the CQJ4 (Shi et al., 2009), BZ1 (Xiao et al, 2008) and G2

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(Xiao et al, 2014) boreholes from the middle Huanghua depression and

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magnetostratigraphy of BZ2 (Yao et al, 2010) borehole form the Cangxian uplift,

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western Bohaibay, and magnetostratigraphies of the NY05, BG10, MT04 and TZ02

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(Xu et al, 2016) boreholes from the northern Huanghua depression, northern Bohai

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Bay, has led to the establishment of a late Cenozoic chronostratigraphic framework

320

for the coast of Bohai Bay (Fig. 7) (Table 1).

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Three marine layers distributing 0−120 m depth range along the coastal area of

322

the Bohai Bay have occurred within the Brunhes normal chron, with a lower boundary

323

at the elevation interval from -97 m to -41 m. The elevations of three marine layers

324

are the highest in uplifts, and gradually decrease form south to north in the Huanghua

325

depression (Fig. 7).

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The elevation of M/B boundary in CQJ4 boreholes located in Banqiao sag is

327

higher than its in CK3 and G4, which is against regional geological rules. So we think

328

that the bottom of the Jaramillo normal subchron maybe the bottom of Brunhes

329

normal chron. If the fourth normal subchron correlate to the C2An.1n, the sixth

330

normal subchron correlate to the C2An.3n, the changing tendency of marine layers,

331

M/B, G/M and Gi/G boundary will be more aligned with regional geological rules.

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332

Comparing magnetostratigraphies of CK3, G4 and BZ1 boreholes located in

333

different uplifts, the changing tendency of elevations of the M/B and G/M boundary

ACCEPTED MANUSCRIPT have relative uniformity. While the elevations of the G/M and Gi/G boundary in CK3

335

and G4 boreholes, CQJ4, G2 and NY05 boreholes located in depression, have the

336

gradually decreased tendency. So the magnetostratigraphies of CK3 and G4 boreholes

337

should be credible.

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334

In the Chengning uplift and Cangxian uplift, the elevations of the M/B and G/M

339

boundary are -53−-81m and -160−-180m. In the middle of Huanghua depression

340

included of Banqiao sag and Beitang sag, the elevations of the M/B and G/M

341

boundary are -96−-101m and -301−-334m.

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In some sags of northern Huanghua depression included of Jianhe sag, Nanpu sag

343

and Laoting sag, the elevations of the M/B and G/M boundary are -122−-160m and

344

-360−-474m. In Matouying rise of northern Huanghua depression, the elevations of

345

the M/B and G/M boundary are -122m and -325m.

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The subsidence differences of the M/B and G/M boundary between the

347

Chengniang uplift, Cangxian uplift and the middle of Huanghua depression are

348

10-50m and 120-170m respectively, which inherited Cenozoic tectonic characteristics,

349

which may represent the impacts from the subduction of the Pacific plate and

350

expansion of the Sea of Japan (Yin, 2010). The subsidence differences of the M/B and

351

G/M boundary between the middle and north of Huanghua depression are 20-60m and

352

30-170m respectively, which reflect that the northern Huanghua depression is the

353

regional subsidence center and the WNW-orientated structures of the basin

354

strengthened in Quaternary. The subsidence extent of the WNW-orientated Laoting

355

sag and Nanpu sag have obviously greater than the NE-orientated Jianhe sag that

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ACCEPTED MANUSCRIPT 356

being equivalent to the WNW-orientated Matouying rise, which also reflect that

357

WNW-orientated structures of the basin strengthened.

358

5.4. Constraints to the ages of the marine layers

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359

Although sedimentary sequences of these three marine events were comparable

361

with each other, their ages have been still in a hot debate. Taking the second marine

362

layer for example, some claimed the age belongs to MIS3 based on the radiocarbon

363

method (Liu et al., 2009) and OSL dating (Yan et al., 2006), and others argued

364

MIS3-5 by combining OSL dating and paleomagnetic results (Chen et al., 2012; Yi et

365

al., 2013, 2015).

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The elevation of the bottom first marine layer in these boreholes are -7 m, -5 m,

367

-11 m, -9 m, -18 m, -14 m, -16 m, -17 m, -9 m and -20m from CK3 to TZ02 boreholes

368

in order, respectively (Fig. 7). The elevation of the bottom second marine layer in

369

these boreholes are -26 m, -27 m, -19 m, -27 m, -31 m, -31 m, -35 m and -42 m from

370

CK3 to BG10 boreholes in order, respectively (Fig. 7). The elevation of the bottom

371

third marine layer in these boreholes are -55 m, -41 m, -48 m, -55 m, -59 m, -67 m,

372

-73 m and -97 m from CK3 to BG10 boreholes in order, respectively (Fig. 7). The

373

elevations of three marine layers all decrease from south to north, and reflect that the

374

northern Bohai Bay is the subsidence center, which is similar to the tendency of the

375

M/B and G/M boundary. Although the extent and types of three transgressions have

376

been different in different areas in late Quaternary along the Bohai Bay, we also think

377

that the three marine layers have respective relative unified ages.

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366

ACCEPTED MANUSCRIPT According on credible radiocarbon dates, the age of the first marine layer belongs

379

to Holocene (Liu, et al, 2009; Xu et al., 2015). Although there have the debate about

380

the age of the second marine layer, but they all admit that a transgression has happed

381

in MIS3. With the Constraints of the magnetostratigraphies of boreholes along the

382

Bohai Bay, we believe that the age of the second marine layer belongs to MIS3, and

383

the age of the third marine layer belongs to MIS5.

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378

With the sea level decreasing in Cenozoic (Miller et al., 2005), but the

385

transgressions have been in Quaternary in Huanghua depression. Structural

386

subsidence may provide the huge sedimentation space. The dynamical effects

387

attributed to the Indo-Euarasin collision controlled the dynamics of the North China in

388

the horizontal after ~8 Ma (An et al., 2011). So the WNW-orientated structures may

389

play the first-order role. In “super interglacials” during the Quaternary, such as MIS31,

390

MIS11c and MIS5e, the extreme warm conditions had been relative to West Antarctic

391

Ice Sheet retreats (Melles et al., 2015). The transgressions also happened in the “super

392

interglacials” in the Huanghua depression. The WNW-orientated structures and “super

393

interglacials” have formed the transgressions in the Huanghua depression.

394

5.5. Ages estimation of volcanic layers

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SC

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On the basis of the paleomagnetic data, the recognized magnetozones of the

396

studied two boreholes have been correlated with GPTS (Cande and Kent, 1995), and

397

the ages of second and third marine layers constrained by magnetostratigraphies along

398

the Bohai Bay can belongs to MIS3 and MIS5, respectively.

399

In CK3 borehole, four volcanic layers have been named the first to forth volcanic

ACCEPTED MANUSCRIPT layers from up to bottom in order. The first volcanic layer with the depth of

401

13.14-16.18m occurs just below the basal peat of the first marine layer, and above the

402

terrestrial hard mud layer formed during the Last Glacial Maximum (Xu et al., 2015).

403

So the age of the first volcanic layer can be estimated to 10-18 Ka BP. The second

404

volcanic layer with the depth of 33.37-48.02m occurs below the terrestrial hard mud

405

layer underlied the second marine layer, and above the third marine layer. The age of

406

terrestrial hard mud layer underlied the second marine layer maybe belong to MIS4

407

(Xu et al., 2011). So the age of the second volcanic layer can be estimated to the late

408

MIS5, maybe 80-90 Ka BP. The third volcanic layer with the depth of 145.8-154.05m

409

occurs in the C2r reverse subchron. Giver that the duration of C2r is about 631, 000 a

410

- between the termination of the Gauss normal chron (2, 581, 000 a) and the onset of

411

the Olduvai normal subchron (1, 950, 000 a) - the interpolated age of the third

412

volcanic layer is about 2.2 Ma based on an averaged rate of sediment deposition. The

413

forth volcanic layer with the depth of 222.16-233.80 m occurs in the C2An.1r, so its

414

age can be estimated to 3.1Ma. The 40Ar/39Ar age of the forth volcanic layer is 14 Ma

415

(Hu et al., 2014), which disobey regional geological rules. So the magmas maybe mix

416

the older components.

SC

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417

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400

In G4 borehole, the volcanic layer with the depth of 46.4-55.8 m occurs in

418

Brunhes normal chron. Givern that the duration of C1n is about 780, 000 a - between

419

now and the onset of the Brunches normal chron (780, 000 a) - the interpolated age of

420

the volcanic layer is about 0.57 Ma based on an averaged rate of sediment deposition.

421

The overlying strata of the volcanic layer are the third marine layer. So the age of

ACCEPTED MANUSCRIPT volcanic layer in G4 borehole can be about estimated to 0.12-0.57 Ma based on the

423

stratigraphic sequence. While the K-Ar ages dating on volcanic rocks of Dashan are

424

0.55-0.86 Ma (Chen et al, 1985) and 0.33-0.86 Ma (Wang et al, 1988). So we believe

425

that the age of volcanic layer in G4 borehole is 0.33-0.55 Ma, which means that the

426

times are the active phase of volcanic activities in Dashan areas.

RI PT

422

The bottom-to-up effects attributed to the subduction of the Pacific plate is the

428

dominant factor controlling the dynamics of the North China during ~8-23 Ma (An et

429

al., 2011). The volcanic layers in CK3 and G4 boreholes also derived from the slab

430

upwelling. The subsidence differences between the NEE and NE-orientated the uplifts

431

and depressions in western and southern Bohai bay also represented the impacts from

432

the subduction of the Pacific plate. So the subduction of the Pacific plate mainly

433

controlled the structure evolution in the vertical during the Quaternary.

434

6. Conclusions

TE D

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427

Paleomagnetic studies with sedimentary analysis were conducted on two

436

new-drilled boreholes from the southwestern Bohai Bay in order to probe into

437

regional tectonic processes and the distribution and ages of volcanic layers in

438

Xiaoshan and Dashan areas since the Pliocene. The main conclusions are as follows.

AC C

439

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435

(1) Magnetite and hematite were identified as the main carrier of the

440

characteristic remanent magnetization in these sedimentary sequences, and the

441

hematite was the main component. Magnetostratigraphic results show that these

442

deposits recorded the Brunhes, Olduvai, Gauss and C3An normal chrons, and the

443

successive reverse polarity portions of the intervening Matuyama and Gilbert chron in

ACCEPTED MANUSCRIPT CK3 borehole, the age of bottom within C3Ar reverse chron was about 6.8 Ma. In G4

445

borehole, Magnetostratigraphic results show that these deposits recorded the Brunhes,

446

Olduvai and Gauss normal chrons, and the successive reverse polarity portions of the

447

intervening Matuyama chron, the age of bottom within the C3n normal chron was

448

about 5.2 Ma.

RI PT

444

(2) The subsidence differences of the M/B and G/M boundary of these boreholes

450

along the Bohai Bay distributed in NE-orientated depressions and uplifts may

451

represent the impacts from the subduction of the Pacific plate and expansion of the

452

Sea of Japan. The subsidence differences of the M/B and G/M boundary of these

453

boreholes distributed in Huanghua depression reflect that the northern Huanghua

454

depression is the regional subsidence center and the WNW-orientated structures of the

455

basin strengthened in Quaternary. In northern Bohai Bay, the subsidence extent of the

456

WNW-orientated sags have obviously greater than the NE-orientated sag that being

457

equivalent to the WNW-orientated rise, which also reflect that WNW-orientated

458

structures of the basin strengthened.

EP

TE D

M AN U

SC

449

(3) The tendency of elevations of three marine layers is similar to the M/B and

460

G/M boundary. With the Constraints of the magnetostratigraphies along the Bohai Bay,

461

we believe that the age of the second and third marine layers belongs to MIS3 and

462

MIS5, respectively. The WNW-orientated structures and “super interglacials” have

463

formed the transgressions in the Huanghua depression.

AC C

459

464

(4) In CK3 borehole, the ages of four volcanic layers derived from the slab

465

upwelling distributing the depth of 13.14-16.18 m, 33.37-48.02 m, 145.8-154.05 m

ACCEPTED MANUSCRIPT and 222.16-233.80 m, and are 10-18 Ka BP, 80-90 Ka BP, 2.2 Ma and 3.1 Ma

467

respectively. In G4 boreholes, there is one volcanic layer distributing the depth of

468

46.4-55.8 m with the age of 0.33-0.55 Ma. Contrastive analysis the subsidence

469

differences between the NEE and NE-orientated the uplifts and depressions in western

470

and southern Bohai bay, the subduction of the Pacific plate mainly controlled the

471

structure evolution in the vertical during the Quaternary.

474 475

Acknowledgements

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This study was supported by the China Geological Survey Project (12120114007801, 1212011120170, 1212011120746 and 1212010610408).

476

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(Eds.),

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Runcorn,

Methods

in

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ACCEPTED MANUSCRIPT 626

Table captions

627

Table 1. Basic data and the testing specimens of boreholes used in this study.

628

Elevation is below present mean sea level.

630

RI PT

629

Figure captions

631

Fig. 1. Tectonic map of North China Plain (a), the tectonic and map of coastal area in

633

Bohai Bay and the location of boreholes (b). A: NW-trending structure,

634

Zhangjiakou-Penglai Fault zone, B: NE-trending structure, C: NE-trending

635

structure,Tancheng-Lujiang Fault zone. Red square: Ms>=6.0. The base map data was

636

from NASA.

M AN U

SC

632

TE D

637

Fig. 2. The location of CK3 (A) and G4 (B), and the photos of volcanic layers in CK3

639

and G4 boreholes. (a) basaltic tuffite, (b) basaltic tuffite, (c) basaltic volcaniclastic

640

rocks, (d ) basaltic tuffite, (e) silty clay and basaltic breccias, (f) basalt, are all in CK3

641

borehole. (g) basaltic tuffite is in G4 borehole.

AC C

642

EP

638

643

Fig. 3. Temperature-dependence of magnetic susceptibility, Isothermal remnant

644

magnetization (IRM) acquisition curves and its back-field demagnetization curves of

645

selected samples according on the different depth in Ck3 boreholes, southwestern

646

Bohai bay.

647

ACCEPTED MANUSCRIPT Fig. 4. Demagnetization results of representative specimens in CK3 and G4 boreholes,

649

southwestern Bohai bay. Every specimens consisting of orthogonal vector diagrams

650

and normalized decay curves of magnetization; triangle and square refer to projections

651

on the vertical and horizontal planes, respectively; NRM is the natural remanent

652

magnetization.

RI PT

648

653

Fig. 5. Histograms of inclinations (5° windows) of the measured specimens from the

655

CK3 and G4 borehole. (a, c) Black line meaning the specimens, red line meaning the

656

specimens with ChRMs carried by hematite and the blue line meaning the specimens

657

with ChRMs carried by magnetite. (b, d) Positive and negative inclinations denote

658

downward and upward pointing remanence vectors, respectively. (a, b) specimens

659

from the CK3 borehole, (c, d) specimens from the G4 borehole.

M AN U

TE D

660

SC

654

Fig. 6. Lithostratigraphy and magnetic polarity stratigraphy of CK3 and G4 boreholes

662

in southwestern Bohai bay and their correlations with the geomagnetic polarity

663

timescale (Cande and Kent, 1995).

AC C

664

EP

661

665

Fig. 7. Late Cenozoic magnetostratigraphic frame in different structural units along

666

Bohai Bay. CK3 and G4 boreholes are located in the Chengning uplift, BZ2 in the

667

Cangxian uplift. CQJ4 and BZ2 are located in the Banqiao sag, G2 borehole in

668

Beitang sag, NY05 borehole in Jianhe sag, BG10 borehole in Nanpu sag, MT04

669

borehole in Matouying rise and TZ02 borehole in Laoting sag, which all belong to the

ACCEPTED MANUSCRIPT Huanghua depression. Blue areas represent marine layers, and I, II, III and Ⅳ

671

represent the first, second, third and fourth marine layers, respectively.. The third

672

marine beds can divide into III-1 and III-2 in BG10 borehole. Red areas represent

673

volcanic layers.

AC C

EP

TE D

M AN U

SC

RI PT

670

ACCEPTED MANUSCRIPT

structure unit

elevation

depth

susceptibility

paleomagnetism

thermal

alternating field

characteristic remanent

(m)

（m）

specimens

specimens

demagnetization

demagnetization

magnetization

magnetic

specimens

specimens

specimens

specimens

208

3

CK3

Chengning uplift

4.0

500

571

571

G4

Chengning uplift

4.5

400

427

427

Cangxin uplift

4

203

471

398

CQJ4

Banqiao sag

3

500

815

487

BZ1

Banqiao sag

2

204

417

G2

Beitang sag

3

1226

1423

NY05

Jianhe sag

1.7

550

1051

417

BG10

Nanpu sag

2.5

600

1361

760

MT04

Matouying rise

1

383

647

333

TZ02

Laoting sag

0.5

550

715

Rock

references

this study this study

73

445

Yao et al, 2010

328

450

Shi et al., 2009

290

127

373

Xiao et al., 2008

866

557

488

Xiao et al., 2014

391

26

289

6

Xu et al., 2016

608

152

516

11

Yuan et al, 2014

259

74

208

6

Xu et al., 2014

104

354

6

Xu et al., 2016

M AN U

218

TE D EP

472

AC C

BZ2

]

SC

borehole

RI PT

Table 1. Basic data and the testing specimens of boreholes used in this study. Elevation is below present mean sea level.

368

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

673

ACCEPTED MANUSCRIPT Fig. 1. Tectonic map of North China Plain (a), the tectonic and map of coastal area in

675

Bohai Bay and the location of boreholes (b). A: NW-trending structure,

676

Zhangjiakou-Penglai Fault zone, B: NE-trending structure, C: NE-trending

677

structure,Tancheng-Lujiang Fault zone. Red square: Ms>=6.0. The base map data was

678

from NASA.

RI PT

674

679

AC C

EP

TE D

M AN U

SC

680

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

681

Fig. 2. The location of CK3 (A) and G4 (B), and the photos of volcanic layers in CK3

683

and G4 boreholes. (a) basaltic tuffite, (b) basaltic tuffite, (c) basaltic volcaniclastic

684

rocks, (d ) basaltic tuffite, (e) silty clay and basaltic breccias, (f) basalt, are all in CK3

685

borehole. (g) basaltic tuffite is in G4 borehole. The base map data of A and B were

686

from Google Earth.

EP AC C

687

TE D

682

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

688

Fig. 3. Temperature-dependence of magnetic susceptibility, Isothermal remnant

690

magnetization (IRM) acquisition curves and its back-field demagnetization curves of

691

selected samples according on the different depth in Ck3 boreholes, southwestern

692

Bohai bay.

EP AC C

693

TE D

689

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

694

Fig. 4. Demagnetization results of representative specimens in CK3 and G4 boreholes,

696

southwestern Bohai bay. Every specimens consisting of orthogonal vector diagrams

697

and normalized decay curves of magnetization; round and square refer to projections

698

on the vertical and horizontal planes, respectively; NRM is the natural remanent

699

magnetization.

EP

AC C

700

TE D

695

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

TE D

701

Fig. 5. Histograms of inclinations (5° windows) of the measured specimens from the

703

CK3 and G4 borehole. (a, c) Black line meaning the specimens, red line meaning the

704

specimens with ChRMs carried by hematite and the blue line meaning the specimens

705

with ChRMs carried by magnetite. (b, d) Positive and negative inclinations denote

706

downward and upward pointing remanence vectors, respectively. (a, b) specimens

707

from the CK3 borehole, (c, d) specimens from the G4 borehole.

709

AC C

708

EP

702

710

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

711

Fig. 6. Lithostratigraphy and magnetic polarity stratigraphy of CK3 and G4 boreholes

712

in southwestern Bohai bay and their correlations with the geomagnetic polarity

713

timescale (Cande and Kent, 1995).

714

715

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 7. Late Cenozoic magnetostratigraphic frame in different structural units along

717

Bohai Bay. CK3 and G4 boreholes are located in the Chengning uplift, BZ2 in the

718

Cangxian uplift. CQJ4 and BZ2 are located in the Banqiao sag, G2 borehole in

719

Beitang sag, NY05 borehole in Jianhe sag, BG10 borehole in Nanpu sag, MT04

720

borehole in Matouying rise and TZ02 borehole in Laoting sag, which all belong to the

721

Huanghua depression. Blue areas represent marine layers, and I, II, III and Ⅳ

722

represent the first, second, third and fourth marine layers, respectively.. The third

AC C

716

ACCEPTED MANUSCRIPT marine beds can divide into III-1 and III-2 in BG10 borehole. Red areas represent

724

volcanic layers.

AC C

EP

TE D

M AN U

SC

RI PT

723