New geochronological constraints for the Upper Cretaceous Nenjiang Formation in the Songliao Basin, NE China

Cretaceous Research 102 (2019) 160e169

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New geochronological constraints for the Upper Cretaceous Nenjiang Formation in the Songliao Basin, NE China Zhiqiang Yu a, b, c, Huaiyu He a, b, c, *, Chenglong Deng a, b, c, Dangpeng Xi d, Zuohuan Qin d, Xiaoqiao Wan d, Chengshan Wang d, Rixiang Zhu a, b, c a

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China Institutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029, China d China University of Geosciences, Beijing, 100083, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 October 2018 Received in revised form 28 April 2019 Accepted in revised form 17 May 2019 Available online 18 June 2019

The Nenjiang Formation in the Songliao Basin, northeastern China, is of great significance because it records a series of geological, geodynamical and paleoenvironmental events, such as lake transgression events, lake anoxic events, sea water incursion events, Cretaceous Normal Superchron, and important stratigraphic boundaries. Here we report a chronology for the non-marine Upper Cretaceous Nenjiang Formation based on secondary ion mass spectrometry (SIMS) UePb zircon analyses from the east borehole of the Cretaceous Continental Scientific Drilling and two outcrop sections, which are located in different structural provinces of the Songliao Basin. Stratigraphic correlation between the borehole sequence and the exposed outcrop sections is achieved by combining lithostratigraphy, biostratigraphy, and SIMS UePb zircon geochronology. Two bentonite layers are recognized in the Nenjiang Formation: the older occurs at the formational contact between the Yaojia and Nenjiang Formations; the younger occurs between Members 1 and 2 of the Nenjiang Formation. SIMS UePb zircon dating yields ages of 85.1 e85.2 Ma and 83.0e83.3 Ma, respectively, representing ages of the boundaries of the Yaojia/Nenjiang Formations and Members 1/2 of the Nenjiang Formation, as well as ages of lake transgression events and associated lake anoxic events, and two of the sea water incursion events. Furthermore, the age of Cretaceous Normal Superchron termination can be estimated at 82.5e82.8 Ma. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Cretaceous Songliao Basin Nenjiang Formation SIMS UePb zircon dating Geochronology

1. Introduction The Cretaceous is one of the warmest periods in Earth history that was characterized by much warmer global temperatures and high sea level (Skelton et al., 2003), although paleoclimatic changes in the Cretaceous marine sedimentary rocks have been well studied, our knowledge of Cretaceous terrestrial climate is limited due to fragmentary continental stratigraphic records (Friedrich et al., 2012; Gao et al., 2015). The Cretaceous continental sedimentary sequences in China offer a unique opportunity to investigate the nature of the terrestrial Cretaceous record and its correlation with global marine strata (Wang et al., 2013; Zhou and Wang, 2017). The Songliao Basin in northeastern China (Fig. 1) contains a nearly

* Corresponding author. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences Beijing, 100029, China. E-mail address: [email protected] (H. He). https://doi.org/10.1016/j.cretres.2019.05.006 0195-6671/© 2019 Elsevier Ltd. All rights reserved.

complete sequence of Cretaceous non-marine deposits intercalated with numerous layers of bentonite and tuffaceous clastic rocks. Therefore, the basinal record can be used to better constrain the ages for important geological and paleoenvironmental events. The Cretaceous Continental Scientific Drilling in the Songliao Basin (hereafter termed CCSD-SK) was conducted to better understand the nature of terrestrial processes and their relationships with global geological processes during the Cretaceous, including the north core (CCSD-SK-In) and the south core (CCSD-SK-Is) of CCSD-SK-I, and the east core (CCSD-SK-IIe) and the west core (CCSD-SK-IIw) of CCSD-SK-II (Wang et al., 2008; Wang et al., 2017) (Fig. 1). The terrestrial Upper Cretaceous Nenjiang Formation in the Songliao Basin is specifically interesting because it records a series of geological and paleoenvironmental events, including lake transgression events, lake anoxic events (Gao et al., 1994; Huang et al., 1998; Wu et al., 2013; Xi et al., 2016, 2018), sea water incursion events (Hou et al., 2000; Hu et al., 2015; Cao et al., 2016a),

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Fig. 1. (A) Schematic map showing the Songliao Basin and its adjacent regions. (B) Geological map of the Songliao Basin (modified after Feng et al. (2010)) as well as locations of the boreholes and outcrop sections mentioned in this paper (also see Table 1 for names of the boreholes and outcrop sections). (C) Stratigraphic cross sections through the Songliao Basin (AeA0 in Fig. 1b) (modified after Feng et al. (2010)). The black bars in the east borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-IIe) show the core intervals. Note that a summary of the symbols and abbreviations used in this paper is given in Table 1.

Cretaceous Normal Superchron (He et al., 2012; Deng et al., 2013), huge terrestrial petroleum source rocks (Wan et al., 2005), and important stratigraphic boundaries of Chinese terrestrial Cretaceous (Deng et al., 2013; Xi et al., 2019). Two large lake transgression events in the Songliao Basin occurred during the Late Cretaceous, with black shale and oil shale preserved in the lower Qingshankou Formation and the lower Nenjiang Formation, which were previously interpreted as lake transgression event 1 of the Middle Turonian age and lake transgression event 2 of the Late Santonian to Early Campanian time, respectively (Huang et al., 1998). Lake transgression event 2 is the largest transgression event in the Songliao Basin resulting in lake anoxic event 2 (Xi et al., 2011a). Meanwhile, the Songliao Basin experienced episodic sea water incursions. Several researchers suggested that the high lake level during the period of deposition of the lower Nenjiang Formation in the Songliao Basin may have been affected by changes in both regional and global sea level (Wagreich et al., 2014; Xi et al., 2016; Yang et al.,

2018), and may have a close relationship with the CenomanianTuronian oceanic anoxic event (Xi et al., 2018). However, the sea water incursion events for petroleum source rock deposition in the lower Nenjiang Formation remain controversial. An increasing number of studies suggest that the formation of a petroleum source rock in the lower Nenjiang Formation of the Songliao Basin relates to sea water incursions (Hou et al., 2000; Xi et al., 2011a; Hu et al., 2015). However, Jones et al. (2018) presented a model for water column stratification and petroleum source rock deposition in the upper Qingshankou Formation, which is independent of sea water incursions. Nevertheless, direct age constraints on the lower Nenjiang Formation was poorly known, thus limiting our understanding of the Late Cretaceous terrestrial processes and the correlation of the terrestrial records in the Songliao Basin with the global Cretaceous system. Here we report secondary ion mass spectrometry (SIMS) UePb zircon geochronology from the east borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-IIe), which is correlated to

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two outcrop sections, located in different structural provinces of the Upper Cretaceous Nenjiang Formation in the Songliao Basin. These data provide a direct constraint for the terrestrial Upper Cretaceous Nenjiang Formation, as well as ages of the lake transgression event and associated lake anoxic event, and sea water incursion events in the lower Nenjiang Formation. Besides, this study provides an age estimate for the termination of the Cretaceous Normal Superchron by combining our new SIMS UePb zircon geochronology and previously published magnetostratigraphy of the south borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-Is) (He et al., 2012). 2. Geological setting and sampling The Songliao Basin in northeastern China is a continental rift basin with the Late Paleozoic and Early Mesozoic consolidated crust as the basement (Fig. 1a) (Ren et al., 2002). A lot of data generated from regional geology and petroleum prospecting have demonstrated its subsidence and geothermal history, sedimentary facies, crustal underpinnings, and structural style (Song, 1997; Ren et al., 2002; Wei et al., 2010; Song et al., 2017). The tectonic evolution of the Songliao Basin can be divided into three phases: a rifting phase occurred during the latest Jurassic to the Early Cretaceous, a post-rift phase occurred during the Early Cretaceous to the middle Late Cretaceous, and a structural inversion phase occurred during the middle Late Cretaceous to the latest Cretaceous (Ren et al., 2002). The Songliao Basin is divided into six structural provinces (Fig. 1b): the north plunge zone, the central depression zone, the northeast uplift zone, the southeast uplift zone, the southwest uplift zone and the west slope zone. The Cretaceous strata in the Songliao Basin consist of nine lithologic formations (Fig. 1c, Table 1), including the Lower Cretaceous Shahezi Formation (K1s), Yingcheng Formation (K1y), Denglouku Formation (K1d), and the Upper Cretaceous Quantou Formation (K2q), Qingshankou Formation (K2qn), Yaojia Formation (K2y), Nenjiang Formation (K2n), Sifangtai (K2s) and Mingshui Formation (K2m). The Cretaceous strata rest unconformably over

basement rocks consisting of the middle Jurassic granites and Paleozoic strata (Wu et al., 2001); the Cretaceous section is unconformably overlain by Cenozoic strata (Fig. 1c). The CCSD-SK-IIe borehole (46140 N, 125 210 E) is located in the central depression zone of the Songliao Basin (Fig. 1b). The 4000 m long core consists of, from oldest to youngest, Huoshiling Formation (J3h), Shahezi Formation (K1s), Yingcheng Formation (K1y), Denglouku Formation (K1d), upper part of Yaojia Formation (K2y2þ3), and lower part of Nenjiang Formation (K2n) (Fig. 1c). Here we focus on the upper Yaojia Formation (K2y2þ3) and the lower Nenjiang Formation (K2n1 and K2n2). A brief description of biostratigraphy and magnetostratigraphy of the upper Yaojia Formation and lower Nenjiang Formation is shown in Fig. 2. The Yaojia and Nenjiang Formations were deposited during a late post-rift phase (Feng et al., 2010). The Yaojia Formation occurs at the depth interval of 12601247 m in the CCSD-SK-IIe borehole and is composed of gray, green, and brown mudstone and green siltstone facies deposited in deltaic and shallow lacustrine environments (Gao et al., 2017; Wang et al., 2017). The lower Members 1 and 2 of the Nenjiang Formation, occur at a depth range of 12471080 m and consist principally of lacustrine mudstones intercalated with oil-rich shales deposited in shallow lacustrine and deep lacustrine environments. The Nenjiang Formation reached is greatest subaerial extent of 2  106 km2 during Members 1 and 2 of the formation. Two bentonite layers were sampled for SIMS UePb dating, occurring at depths of 1145 m (sample S1145, at the boundary of Members 1/2 of the Nenjiang Formation) and 1236 m (sample S1236, 9 m above the boundary of the Yaojia/Nenjiang Formations) (Fig. 3b). The outcrop sections investigated as part of this study are located in the southeast uplift zone of the Songliao Basin (Fig. 1). The Yaojiachezhan section (44 470 N, 125 530 E) consists of Members 2 and 3 of the Yaojia Formation and Member 1 of the Nenjiang Formation, and lithologically dominated by siltstones and mudstones, which has yielded abundant fossils including ostracods, spinicaudatans, pollens and spores (Fig. 2) (Li et al., 2009a; Xi et al., 2009). The ostracod fossils from Member 1 of the Nenjiang

Table 1 Symbols and acronyms of geological events, stratigraphic description, borehole and outcrop sections, and radiometric dating methods as used in the paper. Symbols/Abbreviations

Description

CA-ID-TIMS CCSD-SK-IIe CCSD-SK-IIw CCSD-SK-In CCSD-SK-Is CNS HJG J3h K1d K1q K1s K1y K2m K2n K2n1 K2n2 K2qn K2s K2y K2y2þ3 LAE LTE SIMS SWIE YJCZ YWC

chemical abrasion isotope dilution thermal-ionization mass spectrometry east borehole of the Cretaceous Continental Scientific Drilling west borehole of the Cretaceous Continental Scientific Drilling north borehole of the Cretaceous Continental Scientific Drilling south borehole of the Cretaceous Continental Scientific Drilling Cretaceous Normal Superchron Houjingou section Huoshiling Formation Denglouku Formation Quantou Formation Shahezi Formation Yingcheng Formation Mingshui Formation Nenjiang Formation Member 1 of the Nenjiang Formation Member 2 of the Nenjiang Formation Qingshankou Formation Sifangtai Formation Yaojia Formation Members 2 and 3 of the Yaojia Formation lake anoxic event lake transgression event secondary ion mass spectrometry sea water incursion Yaojiachezhan section Yuewangcheng section

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Fig. 2. Chronostratigraphy, lithostratigraphy, magnetostratigraphy (He et al., 2012) and biostratigraphy of the upper Yaojia Formation (K2y2þ3) (Xi et al., 2009; Li et al., 2009a; Wan et al., 2013) and lower Nenjiang Formation (K2n1 and K2n2) (Xi et al., 2009, 2011a, 2011b; Wan et al., 2013; Shi et al., 2019).

Formation in this section can be divided into two zones: the Cypridea anonyma-Candona fabiforma zone and the Cypridea gracilaCypridea gunsulinensis zone (Xi et al., 2009), which can be used for correlation with other drilling borehole sequences and outcrop sections. The occurrence of zonal index fossil spinicaudatan, Halysestheria yui (Li et al., 2004, 2009a), and black mudstones marked the boundary of the Yaojia/Nenjiang Formations (Fig. 3a). One bentonite layer 3.5 m above the boundary of the Yaojia/Nenjiang Formations was sampled for SIMS UePb dating (sample YJ-39 in Fig. 3a). The second, Yuewangcheng section (44 520 N, 125 300 E), consists of Members 1 and 2 of the Nenjiang Formation, and is dominated by black mudstones and oil-rich shales, which has yielded abundant fossils, including ostracods, spinicaudatans, foraminiferas, plants, fishes and fish teeth (Fig. 2) (Shi et al., 2019). The ostracod fossils in this section can be divided into three zones: the Lycopterocypris triangularis-Cypridea spiniferusa zone (K2n1), the Periacanthella portentosa-Ilyocyprimorpha netchaeva zone and the Candoniella amabila-Mongolocypris magna zone (K2n2). At this section, one bentonite layer crops out 1.5 m above the boundary of Members 1/2 of the Nenjiang Formation was sampled for SIMS UePb dating (sample 15YWC in Fig. 3c). The borehole and the outcrop sections were stratigraphically correlated litho- and biostratigraphically (Fig. 3). Two stratigraphic marker layers occurred just above the boundary of the Yaojia/ Nenjiang Formations (labeled BeB0 in Fig. 3a and 3b) and the boundary of Members 1/2 of the Nenjiang Formation (labeled A-A0 in Fig. 3b and 3c), respectively. The sedimentary sequences of CCSD-SK-IIe (Fig. 3b) and CCSD-SK-Is boreholes (Wang et al., 2008) is well correlated by the marker layers at the bottom of Member 2 of the Nenjiang Formation (that is, A in Fig. 3b), which consists of lacustrine black shales at the depth interval of 1136e1145 m for CCSD-SK-IIe borehole (Fig. 3b) and a depth interval of 1012e1021 m for CCSD-SK-Is borehole (see Fig. 6 of Deng et al., 2013). Besides, both CCSD-SK-IIe and CCSD-SK-Is boreholes are located in the central depression units of the Songliao Basin (Fig. 1b). The total length of Member 1 of the Nenjiang Formation in CCSD-SK-Is borehole is 103.44 m (Wang et al., 2008), while that in the CCSDSK-IIe borehole is 105 m (Wang et al., 2017), implying the similar depositional processes. The marker layer A (Fig. 3b) in the lower part of Member 2 of the Nenjiang Formation of CCSD-SK-IIe and

CCSD-SK-Is boreholes, which consists of black mudstones interbedded with oil-rich shales, was unambiguously identified in the outcrop of the Yuewangcheng section (Fig. 3c). In addition, fossils of ostracods and spinicaudatans (Xi et al., 2009; Li et al., 2009a), occurred in the lowest part of the Nenjiang Formation. Besides, ostracod species, Cypridea anonyma, Candona fabiforma, Cypridea gracila, Cypridea gunsulinensis, were also identified in the lowest part of the Nenjiang Formation of CCSD-SK-IIe borehole (personal communication with Dangpeng Xi), CCSD-SK-Is borehole (Wan et al., 2013), and Yaojiachezhan section (Xi et al., 2009) (see BeB0 in Fig. 3a and 3b). 3. SIMS UePb zircon geochronology analytical methods We separated zircon grains from the bentonite sample for SIMS UePb dating. Four samples for UePb analysis were processed by conventional magnetic and density techniques to pick out zircon grains. Zircon grains, together with zircon standard Plesovice and Qinghu were mounted in epoxy mounts which were then polished to section the crystals for analysis. Cathodoluminescence images, transmitted and reflected light micrographs were obtained prior to SIMS analyses, and the mount was vacuum-coated with highpurity gold prior to analysis. Most crystals have oscillatory zoning under cathodoluminescence (Fig. 4), while a minority has inherited cores. All analyses were conducted using the Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. UeThePb ratios and absolute abundances were sovice (337Ma, Sla ma determined relative to the standard zircon Ple et al., 2008), analyses of which were interspersed with those of unknown grains, procedures similar to those described by Li et al. (2009b). Zircon standard Qinghu was used to monitor the measurement procedures and data quality. Non-radiogenic 204Pb was used to correct the measured compositions. A long-term uncertainty of 1.5% (1 RSD) for 206Pb/238U measurements of the standard zircons was propagated to the unknowns (Li et al., 2010). Corrections are sufficiently small to be insensitive to the choice of common Pb composition, and an average of present-day crustal composition (Stacey and Kramers, 1975) is used for the common Pb assuming that the common Pb is largely surface contamination

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Fig. 3. Stratigraphy of the east borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-IIe) (A) and its correlation to the outcrop sections in the Yaojiachezhan (B) and Yuewangcheng sections (C). Black arrows show locations of the bentonite samples with SIMS UePb chronology. A-A0 and BeB0 , marker layers.

introduced during sample preparation. Individual analyses’ uncertainties in data tables are reported at a 1s level. Data reduction was carried out using the Isoplot/Ex v. 2.49 program (Ludwig, 2001). The SIMS UePb analytical results are listed in the Appendix and are further plotted as UePb concordia diagram in Fig. 5. The summary of UePb ages is shown in Table 2. 4. Results Sample S1236 yields euhedral to subhedral zircons with a range of sizes from 40 to 200 mm in length. Zircon grains from this sample

are relatively transparent and colorless. Cathodoluminescence images reveal oscillatory zoning and an absence of inherited zircon cores (Fig. 4a). Seven spots of 47 analyses yield discordant UePb data as indicated by high common Pb. Two analyses (spot 45 and spot 24) have a relatively young age, interpreted as a result of Pb loss. The remaining thirty eight analyses obtained a concordia UePb age of 85.2 ± 0.5 Ma and a weighted mean age of 85.2 ± 0.5 Ma (MSWD ¼ 1.3) (Fig. 5a). Sample YJ-39 yields euhedral to subhedral zircons which are mostly fine-grained, range from 30 to 190 mm. Zircon grains from this sample are transparent and colorless, with internal

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Fig. 4. Cathodoluminescence images of representative zircons from four samples in this study. See Fig. 3 for positions of the samples. Ellipses on the analyzed zircon grains show the positions of UePb analytical sites, with corresponding 206Pb/238U age.

oscillatory zonation visible under cathodoluminescence (Fig. 4b). Thirty one analyses of 31 zircons were obtained on separate zircon grains were obtained performed. Thirty analyses yield a concordia age 85.0 ± 0.5 Ma and the weighted mean age of 85.1 ± 0.6 Ma (MSWD ¼ 1.4) (Fig. 5b). Spot 24 yields an age of 261.5 Ma, older than ages of most zircon grains of this sample suggesting a xenocryst from the basement of the Songliao Basin (Wu et al., 2001). Sample S1145 yields euhedral to subhedral zircons which are mostly fine-grained, range from 30 to 190 mm. Zircon grains from this sample are transparent and colorless, with internal oscillatory zonation visible under cathodoluminescence (Fig. 4c). For 40 of 42 analyses yield a concordia UePb age calculated at 83.4 ± 0.5 Ma. Weighted mean age is calculated at 83.3 ± 0.6 Ma (MSWD ¼ 1.5) (Fig. 5c). Spot 24 yields an older age of 94.4 Ma, which may be a detrital zircon from the Qingshankou Formation (He et al., 2012). Spot 26 yields a 206Pb/238U age of 150 Ma and is thought to be a xenocrystic basement grain on the basis of similarly aged basement rocks in the Songliao Basin (Wu et al., 2001). Sample 15YWC yields euhedral to subhedral zircons which are mostly fine-grained, range from 30 to 200 mm. Zircon grains from this sample are transparent and colorless, with oscillatory zonation visible under cathodoluminescence images (Fig. 4d). Forty-five zircon grains were analyzed from the bentonite sample 15YWC, for 40 of the 45 analyses, 206Pb/238U ratios agree internally within the analytical precision, a concordia UePb age of 83.3 ± 0.5 Ma and a weighted mean age of 83.0 ± 0.5 Ma (MSWD ¼ 1.18) (Fig. 5d). Four analyses (spots 02, 03, 24 and 26) yield significantly large UePb age error as indicated by high common Pb. Spot 22 yields a 206Pb/238U age of 90.2 Ma, most likely a detrital zircon from the Qingshankou Formation (He et al., 2012).

5. Discussion 5.1. Geochronologic framework of lower part of the Nenjiang Formation Early geochronologic studies on the CCSD-SK-I were based on an integrated stratigraphy of the Upper Cretaceous in the Songliao Basin (He et al., 2012; Deng et al., 2013; Wu et al., 2013; Wan et al., 2013; Xi et al., 2018). However, direct age constraints on the boundaries of the Yaojia/Nenjiang Formations and Members 1/2 of the Nenjiang Formation remain poorly known (He et al., 2012; Xi et al., 2018). The age of the boundary of the Yaojia/Nenjiang Formations was estimated to be 84.487 Ma and 84.673 Ma, respectively, by magnetostratigraphy (Deng et al., 2013) and cyclostratigraphy (Wu et al., 2013) of the CCSD-SK-Is borehole sequence. Most recently, Xi et al. (2018) reported a weighted mean 206Pb/238U age of 85.6 ± 0.6 Ma from SIMS UePb zircon dating (Fig. 6) for a bentonite bed from X1-4 borehole in the central depression zone of the Songliao Basin (Fig. 1b), which is 9.88 m above the boundary of the Yaojia/Nenjiang Formations (see Fig. 2 of Xi et al. (2018)). Sample S1236, collected 9 m above the boundary of the Yaojia/ Nenjiang Formations, and sample YJ-39, 3.5 m above the boundary of the Yaojia/Nenjiang Formations yield ages of 85.2 ± 0.5 Ma and 85.1 ± 0.6 Ma, respectively. These ages are indistinguishable with previous age from X1-4 borehole (Xi et al., 2018). Therefore, the age of the boundary of the Yaojia/Nenjiang Formations is 85.1e85.2 Ma, considering analytical uncertainties (Fig. 6). Sample 1019 from a bentonite bed in the CCSD-SK-Is borehole is 2 m above the boundary of Members 1/2 of the Nenjiang Formation and yields a weighted mean 206Pb/238U age of 83.7 ± 0.8 Ma (see

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Fig. 5. UePb concordia diagram showing analytical data for zircons from four samples obtained from four bentonite layers in Nenjiang Formation. See Fig. 1b and Fig. 3 for locations and positions of the samples, respectively. Data-point error ellipses are 2s level.

Table 2 Summary of UePb ages. Sample S1236 YJ-39 S1145 15YWC

Location 

0

(46 14 (44 470 (46 140 (44 520

N, N, N, N,



0

125 21 125 530 125 210 125 300

E) E) E) E)

Formation (Member)

206

Pb/238U date

K2n1 K2n1 K2n2 K2n2

85.2 85.1 83.3 83.0

± ± ± ±

0.5 0.6 0.6 0.5

MSWD

n

1.3 1.4 1.5 1.18

38 30 40 40

Note: MSWD: mean square of weighted deviates; n: number of analyses included in the calculated weighted mean date.

Fig. s 8e10 of He et al. (2012) and Fig. 6 of this study). The age of the boundary of Members 1/2 of the Nenjiang Formation was defined to be 83.7 Ma by Deng et al. (2013). Subsequently, Wang et al. (2016) obtained a weighted mean 206Pb/238U age of 83.269 ± 0.063 Ma using chemical abrasion isotope dilution thermal-ionization mass spectrometry (CA-ID-TIMS) method for Sample 1019 mentioned above (see Fig. 2 of Wang et al. (2016)). The bentonite layer at 1145 m depth of the CCSD-SK-IIe borehole and just at the boundary of Members 1/2 of the Nenjiang Formation yields a weighted mean 206Pb/238U age of 83.3 ± 0.6 Ma from SIMS UePb zircon dating (see sample S1145 in Fig. 3b). The bentonite layer in Yuewangcheng section and 1.5 m above the boundary of

Members 1/2 of the Nenjiang Formation yields a weighted mean Pb/238U age of 83.0 ± 0.5 Ma (see sample 15YWC in Fig. 3c). Those new ages from SIMS UePb zircon dating are well consistent with that of Wang et al. (2016) from CA-ID-TIMS method. Thus, the age of the boundary of Members 1/2 of the Nenjiang Formation was set to 83.0e83.3 Ma (Fig. 6).

206

5.2. Age constraints on the geological events recorded in the lower Nenjiang Formation Two large lake transgression events occurred in the Songliao Basin, including lake transgression event 1 (LTE1) in the lower

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Fig. 6. Integrated stratigraphy, geochronology and paleoenvironmental events from the upper Yaojia Formation to lower Nenjiang Formation (AeD) Lithostratigraphy and magnetostratigraphy of the south borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-Is) (He et al., 2012), which were used for the reference section of this study. (E) Age data of the two bentonite layers, respectively in the lower parts of Member 1 (K2n1) and Member 2 (K2n2). (F) Paleoenvironmental events. Two lake transgression events (LTE2a and LTE2b) and associated lake anoxic events (LAE2a and LAE2b) are from Xi et al. (2011a). Seawater incursion events (SWIEs) are from Hu et al. (2015) and Cao et al. (2016a).

Qingshankou Formation, which is of Middle Turonian age, and lake transgression event 2 (LTE2) in the lower Nenjiang Formation, which is of Late SantonianeEarly Campanian age (Huang et al., 1998; Xi et al., 2011a). Recently, Jones et al. (2018) measured the initial osmium ratios, elemental concentrations and bulk organic carbon isotopes of the CCSD-SK-Is borehole, and then they correlate a notable d13C excursion in the upper Qingshankou Formation of TuronianeConiacian age in the Songliao Basin to the CenomanianTuronian ocean anoxic event in the Western Interior Basin of North America. The lake transgression event 2 in the Songliao Basin was thought to be the largest lake transgression event in the region, with dark mudstones, black shales and oil-rich shales preserved in the lower Nenjiang Formation (Huang et al., 1998). Xi et al. (2011a) obtained high total organic carbon contents, low pristane/phytane ratios from black shales and oil-rich shales of the lower Nenjiang Formation of the Yaojichezhan and Houjingou outcrop sections

(Fig. 1b), implying an anoxic depositional environment. They recognized two high lake level in Members 1 and 2 of the Nenjiang Formation. During the deposition of Members 1 and 2 of the Nenjiang Formation, the lake level of the Songliao Basin increased and a deep-lake environment was formed with bottom waters being oxygen depleted. In the meantime, as the lake deepened bottom conditions changed from oxic to anoxic, and providing favorable conditions for petroleum source rock accumulation (Xi et al., 2011a). The largest lake transgression event resulted in a large-scale lake anoxic event. Thus, lake anoxic event 2 resulted from lake transgression event 2 in the lower Nenjiang Formation can be divided into two stages: lake anoxic event 2a/lake transgression event 2a during deposition of lower Member 1 of the Nenjiang Formation, and lake anoxic event 2b/lake transgression event 2b during deposition of lower Member 2 of the Nenjiang Formation (Fig. 6) (Xi et al., 2011a, 2016).

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Brackish and marine fossils were identified from the lower Nenjiang Formation of CCSD-SK-Is borehole, including planktonic foraminifera (Xi et al., 2011b), brackish algae (Wan et al., 2013), calcareous nannofossils (Cao et al., 2016b), suggesting depositional environments with a marine influence. Hu et al. (2015) and Cao et al. (2016a) obtained a branch of organic geochemical proxies from black shales in lower part of the Nenjiang Formation from CCSD-SK-Is borehole and Houjingou section (Fig. 1), such as total organic carbon, d13C and specific marine biological markers. Their findings suggested that two episodes of high relative lake level occurred in deposition of the Nenjiang Formation related to lake transgression event 2a/lake anoxic event 2a and lake transgression event 2b/lake anoxic event 2b. They further recognized strong periodic sea water incursion events in lower part of the Nenjiang Formation (Fig. 6f). Our new ages of the lower Nenjiang Formation provide better age constraints on the lake transgression event 2a/ lake anoxic event 2a and lake transgression event 2b/lake anoxic event 2b as well as two of sea water incursion events. Four ages we obtained from the CCSD-SK-IIe borehole and natural outcrop sections are 85.1e85.2 Ma and 83.0e83.3 Ma, respectively, which represent the beginning of the lake transgression event 2a/lake anoxic event 2a, and lake transgression event 2b/lake anoxic event 2b (Fig. 6). The termination of the Cretaceous Normal Superchron (correlating to the C34n-C33r geomagnetic reversal) correlates with the Santonian-Campanian boundary (Ogg et al., 2008; Ogg and Hinnov, 2012). Previous high-resolution magnetostratigraphy and SIMS UePb geochronology (He et al., 2012) indicate that the geomagnetic reversal was recorded in the lower Member 2 of the Nenjiang Formation of CCSD-SK-Is borehole and was 33.05 m above the boundary of Members 1/2 of the Nenjiang Formation. Furthermore, the age was estimated to be 83.4 Ma (He et al., 2012). Later, Wang et al. (2016) revised the age of termination of the Cretaceous Normal Superchron to be 83.07 ± 0.15 Ma based on high-precision CA-ID-TIMS UePb dating for the same bentonite samples used by He et al. (2012) and astronomical calibration. In this study, bentonite samples S1145 in CCSD-SK-IIe borehole and 15YWC in Yuewangcheng section (Fig. 3), which correspond to bentonite sample S1019 in CCSD-SK-Is borehole, yield SIMS UePb ages of 83.0e83.3 Ma (Fig. 6); and bentonite samples S1236 in CCSD-SK-IIe borehole and YJ-39 in Yaojiachezhan section (Fig. 3) yield SIMS UePb ages of 85.1e85.2 Ma (Fig. 6). With the use of an averaged sediment accumulation rate for Member 1 of the Nenjiang Formation of CCSD-SK-Is borehole, the age of Cretaceous Normal Superchron termination can be estimated at 82.5e82.8 Ma. Taking into account the SIMS UePb date total uncertainty (~1%), the new age of 82.5e82.8 Ma is consistent with previous age estimates for the Cretaceous Normal Superchron termination, further providing a reference age for the Santonian-Campanian boundary. However, additional magnetostratigraphic studies from drilling boreholes and outcrop sections are needed to more precisely constrain the age of Cretaceous Normal Superchron termination. 6. Conclusions We have radiometrically examined the CCSD-SK-IIe borehole and outcrops of Yaojiachezhan and Yuewangcheng sections. Four bentonite samples, two in the CCSD-SK-IIe borehole, one in the Yaojiachezhan section and one in the Yuewangcheng section occur close to important stratigraphic boundaries (that is, the boundary of the Yaojia/Nenjiang Formations and the boundary of Members 1/ 2 of the Nenjiang Formation). New SIMS UePb zircon data for intercalated bentonite layers indicate that the bentonite bed in the black oil-rich shales of the lowest part of Member 1 of the Nenjiang Formation was deposited at 85.1e85.2 Ma, and that the bentonite

bed in the black oil-rich shales of the lowest part of Member 2 of the Nenjiang Formation was deposited at 83.0e83.3 Ma. The age of the boundary of the Yaojia/Nenjiang Formations is now more precisely recognized to be 85.1e85.2 Ma, representing the age of lake transgression event 2a/lake anoxic event 2a. The age of the boundary of Members 1/2 of the Nenjiang Formation is defined to be 83.0e83.3 Ma, representing the age of lake transgression event 2b/lake anoxic event 2b. Together, our data indicate that the age of Cretaceous Normal Superchron termination can be more precisely places between 82.5 and 82.8 Ma. Acknowledgements We thank the Editor Eduardo Koutsoukos, and three anonymous reviewers for their insightful comments and suggestions, to improve the manuscript. This work was supported by the Strategic Priority Program (B) of the Chinese Academy of Sciences (Grant No. XDB18030505), and the National Natural Science Foundation of China (Grant Nos. 41688103, 41425013, and 41790452). We thank Li Xianhua, Li Qiuli, Liu Yu, Tang Guoqiang, Ling Xiaoxiao, Li Jiao and Lu Kai for SIMS UePb analyses. This research used samples provided by China Geological Samples Center of Land and Resources. This paper is a contribution to the IGCP Project 679. References Cao, H.R., Hu, J.F., Peng, P.A., Xi, D.P., Tang, Y.J., Lei, Y., Shilling, A., 2016a. Paleoenvironmental reconstruction of the Late Santonian Songliao Paleo-lake. Palaeogeography, Palaeoclimatology, Palaeoecology 457, 290e303. https:// doi.org/10.1016/j.palaeo.2016.05.027. Cao, W.X., Xi, D.P., Huang, Q.H., Cheng, Y., Qu, H.Y., Gao, L.F., Wan, X.Q., 2016b. Seawater incursion event in Songliao Basin: New evidence from calcareous nannofossils of SK-1 (in Chinese, with English abstract). Geological Bulletin of China 35, 866e871. Deng, C.L., He, H.Y., Pan, Y.X., Zhu, R.X., 2013. Chronology of the terrestrial Upper Cretaceous in the Songliao Basin, northeast Asia. Palaeogeography, Palaeoclimatology, Palaeoecology 385, 44e54. https://doi.org/10.1016/ j.palaeo.2012.07.028. Feng, Z.Q., Jia, C.Z., Xie, X.N., Zhang, S., Feng, Z.H., Cross, T.A., 2010. Tectonostratigraphic units and stratigraphic sequences of the nonmarine Songliao basin, northeast China. Basin Research 22, 79e95. https://doi.org/10.1111/j.13652117.2009.00445.x. Friedrich, O., Norris, R.D., Erbacher, J., 2012. Evolution of middle to Late Cretaceous oceansda 55 my record of Earth's temperature and carbon cycle. Geology 40, 107e110. https://doi.org/10.1130/G32701.1. Gao, R.Q., Zhang, Y., Cui, T.C., 1994. Cretaceous Oil and Gas Strata of Songliao Basin (in Chinese, with English abstract). The Petroleum Industry Press, Beijing, pp. 1e373. Gao, Y., Ibarra, D.E., Wang, C.S., Caves, J.K., Chamberlain, C.P., Graham, S.A., Wu, H.C., 2015. Mid-latitude terrestrial climate of East Asia linked to global climate in the Late Cretaceous. Geology 43, 287e290. https://doi.org/10.1130/G36427.1. Gao, Y.F., Qu, X.J., Jiang, L.J., Wang, S.X., Wang, P.J., 2017. Lithology and stratigraphic interfaces prediction of the Continental Scientific Drilling Project of Cretaceous Songliao Basin (SK2) (in Chinese, with English abstract). Earth Science Frontiers 24, 242e256. He, H.Y., Deng, C.L., Wang, P.J., Pan, Y.X., Zhu, R.X., 2012. Toward age determination of the termination of the Cretaceous Normal Superchron. Geochemistry, Geophysics, Geosystems 13, Q02002. https://doi.org/10.1029/2011GC003901. Hou, D.J., Li, M.W., Huang, Q.H., 2000. Marine transgressional events in the gigantic freshwater lake Songliao: paleontological and geochemical evidence. Organic Geochemistry 31, 763e768. https://doi.org/10.1016/S0146-6380(00)00065-6. Hu, J.F., Peng, P.A., Liu, M.Y., Xi, D.P., Song, J.Z., Wan, X.Q., Wang, C.S., 2015. Seawater Incursion Events in a Cretaceous Paleo-lake Revealed by Specific Marine Biological Markers. Scientific Reports 5, 9508. https://doi.org/10.1038/srep09508. Huang, Q.H., Chen, C.R., Wang, P.Z., Han, M.X., Li, X.J., Wu, D.Q., 1998. The Late Cretaceous bio-evolution and Anoxic events in the ancient lake in the Songliao basin (In Chinese with English abstract). Acta Micropalaeontologica Sinica 15, 417e425. Jones, M.M., Ibarra, D.E., Gao, Y., Sageman, B.B., Selby, D., Chamberlain, C.P., Graham, S.A., 2018. Evaluating Late Cretaceous OAEs and the influence of marine incursions on organic carbon burial in an expansive East Asian paleo-lake. Earth and Planetary Science Letters 484, 41e52. https://doi.org/10.1016/ j.epsl.2017.11.046. Li, G., Chen, P.J., Wan, X.Q., Jiang, J.H., Liu, J.C., Yin, D.S., Yan, W., 2004. Stratotype of the basal boundary of the Nenjiang stage, Cretaceous (in Chinese with English Abstract). Journal of Stratigraphy 28, 297e300.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10. 1016/j.cretres.2019.05.006.