Astronomical time scale of the Paleogene lacustrine paleoclimate record from the Nanxiang Basin, central China

Palaeogeography, Palaeoclimatology, Palaeoecology 532 (2019) 109253

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Astronomical time scale of the Paleogene lacustrine paleoclimate record from the Nanxiang Basin, central China

T

Ke Xua, Honghan Chena, , Chunju Huangb, , James G. Oggb,c,f, Jingxiu Zhud, Sheqing Lind, Daoqing Yangd, Peng Zhaoe, Lingtao Konga ⁎

⁎⁎

a

Key Laboratory of Tectonics and Petroleum Resource, Ministry of Education, Faculty of Earth Resources, China University of Geosciences, Wuhan, Hubei 430074, China State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430074, China c State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan 610059, China d Geological Scientific Research Institute, Henan Oilfield Company of SINOPEC, Nanyang, Henan 473132, China e Exploration and Development Research Institute, HuaBei Oilfield Company of CNPC, Renqiu, Hebei 062552, China f Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA b

ARTICLE INFO

ABSTRACT

Keywords: Biyang Depression Cyclostratigraphy Source rocks Sediment accumulation rates Long-period ~1.2-Myr cycles

The chronostratigraphy of the Paleogene succession of the Nanxiang Basin, a petroliferous basin in central China with a predominantly lacustrine fill, remains controversial due to the lack of an accurate geological time scale. In this work, high-resolution gamma-ray (GR) logs from the B270 and BS1 boreholes from depocenters in the Biyang Depression of the Nanxiang Basin were used to analyze the cyclostratigraphy. Paleocene floodplain facies of red clastics and purple mudstone is conformably overlain by Eocene through Oligocene lacustrine facies of gray mudstone, oil shale, and clayey dolomite interbedded with siltstone-sandstone. The multitaper method of spectral analysis and Gaussian bandpass filtering were used to identify Milankovitch cycles. Based on the recognition of 405-kyr long-eccentricity cycles, the series was tuned to establish a 46-Myr floating astronomical time scale (ATS), which was then anchored to the Paleogene/Neogene boundary age of 23.03 Ma. The ATS enables the assignment of the ages to non-marine biozones, depositional facies and rifting episodes within the Nanxiang Basin. The subsidence and rates of sediment accumulation within the rift basin of the Nanxiang Basin has an inverse relationship to the convergence rates between Pacific plates and the eastern margin of Eurasia. The middle Eocene slowing of the convergence rate at ~50 Ma, coupled with the onset of the collision of the Indian and Eurasian plate, corresponds to the beginning of the climax stage of rifting, the development of deep lakes and the deposition of oil shales in the Nanxiang Basin. The organic-rich 3rd member of the Hetaoyuan Formation (Eh3) during this middle Eocene interval preserved a range of climatic cycles from annual varves to a ~1.2-Myr modulation of obliquity cycles, indicating the importance of climatic oscillations on the development of those hydrocarbon source rocks.

1. Introduction The Nanxiang Basin is a hydrocarbon-rich depression located at the Qinling-Dabie orogenic belt and the northern margin of the Yangtze platform in central China with petroleum resources of 5 × 108 tons (1 ton = 7.33 barrels) (Zhang et al., 1993; Lv, 2012; Shi et al., 2017b) (Fig. 1). During the Paleogene, a succession of continental clastic deposits of the Yuhuangding (Ey), Dacangfang (Ed), Hetaoyuan (Eh), and Liaozhuang (El) formations accumulated (Figs. 1, 2). However, their respective stratigraphic ages remain controversial. The Yuhuangding

Formation in the Liguanqiao basin was believed to have formed in the early to middle Eocene based on mammal fossil assemblage zones (Ma and Cheng, 1991). Hu et al. (2001) established a general age framework of Biyang Depression based on integrated litho-, seismo- and biostratigraphic correlations in the depression of Henan Oilfield by company researchers (Fig. 2); however, they only interpreted the formation boundary ages and did not provide details of these works. Yan (2008) aimed to establish a geochronological framework of the Paleogene series in the Nanyang basin by analyzing Characeae (green algae) fossil assemblage zones.

Correspondence to: H Chen, Key Laboratory of Tectonics and Petroleum Resource, Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (H. Chen), [email protected] (C. Huang). ⁎

https://doi.org/10.1016/j.palaeo.2019.109253 Received 13 January 2019; Received in revised form 23 June 2019; Accepted 25 June 2019 Available online 29 June 2019 0031-0182/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (A) Structural map of the Nanxiang Basin, showing location of the Biyang Depression (after Guo et al., 2018). (B) Structural map of the Biyang Depression showing locations of the study wells and cross-section X-X′ (after Dong et al., 2015). (C) Structural cross-section across the Biyang Depression based on seismic profiles (The seismic profiles were provided by Geological Scientific Research Institute, Henan Oilfield Company of SINOPEC). Burial depth is proxied by round-trip travel time. Abbreviations: Ey, Yuhuangding Formation; Ed, Dacangfang Formation; Eh3~Eh1, third to first members of Hetaoyuan Formation; El, Liaozhuang Formation; Nf, Fenghuang Formation; Qp, Pingyuan Formation.

However, this non-marine biostratigraphy can only provide qualitative chronological constraints, due to the large uncertainties on their calibration to international marine-based geologic stages. Magnetostratigraphy of the Cangfang section of the Dacangfang Formation (Ed) in the Liguanqiao basin was interpreted to span the interval of 45–39 Ma by Ma (2012); however, this paleomagnetic study of the single section did not span a regional stratigraphic boundary. Repetition of this field profile and experimental work by Peng (2017) came to a different conclusion that this Cangfang section spanned only 40–37 Ma. Yao et al. (2012) calculated the ages from the Hetaoyuan to Liaozhuang formations of the Biyang Depression based on interpreted Milankovitch cycles and using three base levels as anchor points (Fig. 2). Base level 1 was obtained by a correlation to the upper and lower interfaces of Member 4 of the Shahejie Formation in the Dongying sag; however, their chronostratigraphic framework for the Dongying sag was later contradicted in later research by Liu et al. (2018).

Base level 2 was based on a qualitative analysis of a non-marine paleontological assemblage, which was not suitable for accurate chronostratigraphy. Base level 3 was based on an assumed placement for the Neogene/Quaternary boundary. Furthermore, this work only interpreted the ages of two formations, rather than establishing a continuous Paleogene framework for the Nanxiang Basin. Therefore, overall, the existing geochronological framework of the Nanxiang Basin is of low resolution and with a high uncertainty. Astrochronology, based on the spectral identification of Earth's orbital periodicities (eccentricity, obliquity, and precession), provides a method to establish high-resolution time scales in successions that have continuous sedimentary records (Strasser et al., 2006; Hinnov and Hilgen, 2012; Huang, 2014). These Milankovitch cycles can be wellpreserved in terrestrial strata, especially those with lacustrine depositional systems, such as in the Eocene Green River Basin (Machlus et al., 2008), in the Late Cretaceous Songliao Basin (Wu et al., 2014), and in the Paleogene Bohai Bay Basin (Liu et al., 2018). Lacustrine 2

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Fig. 2. The general stratigraphic chart of the Biyang Depression in the Nanxiang Basin. The classification of the sequence stratigraphy is from Zhu et al., 2011. The Ostracoda, Characeae (green algae), palynology and vertebrate ranges are from Zhang et al. (1993). The interpretation of general ages is from Hu et al. (2001); and the two interpretations of an astronomical time scale (ATS) are by Yao et al. (2012) and by this paper. The rifting episodes are after Zhang and Xian (2004).

sedimentary successions of long-lived closed lake basins can provide continuous stable records of paleoclimate changes, thereby making them suitable for the establishment of an astronomical time scale. Stratigraphic studies within the Nanxiang Basin have interpreted paleoclimate, tectonic events and the conditions leading to hydrocarbon-source rocks (Zhang and Xian, 2004; Jiang et al., 2015; Li et al., 2017); however, the lack of an unambiguous high-resolution chronostratigraphic framework has precluded a reliable comparison of these researches to other petroliferous basins in East China. Therefore, we performed a cyclostratigraphic analysis of the entire column of the Paleogene lacustrine deposits in the Biyang Depression of the Nanxiang Basin by using natural gamma-ray (GR) logs and total organic carbon (TOC) data for spectral analysis. An additional ultrahigh-resolution examination was conducted on a 35 m interval of the varved organic-rich source rock of the third (Eh3) member of the Hetaoyuan Formation by handheld X-ray fluorescence (XRF)

spectrometry data to investigate the nature of the fine-scale cyclicity. The goals of this work are (1) to construct a high-resolution ATS for the full Paleogene succession of the Nanxiang Basin, (2) to apply the calibrated time scale to determine the ages of biozones, rifting episodes and paleoclimate stages within the basin, (3) to explore the relationship of paleoclimate and sediment-accumulation rates to regional tectonic events, and (4) to determine which astronomical signal influenced the hydrocarbon source rocks in the Eh3 Member of middle Eocene. 2. Geological background The Nanxiang Basin is a sedimentary rift basin that was superimposed on the pre-Paleozoic metamorphic rock basement of the Qinling-Dabie Mountains in eastern China during the late Yanshanian movement of Cretaceous and was infilled with Cenozoic strata. The main basin covers an area of ca. 1.7 × 104 km2 and is subdivided into 3

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Fig. 3. COCO analysis for each part of the GR series of the two boreholes: (1) El Formation, the correlation coefficient results show the optimal sedimentation rate at 11.5 cm/kyr, tested sedimentation rates range from 6 to 18 cm/kyr with a step of 0.1 cm/kyr; (2) Eh1 and Eh2 members, the correlation coefficient results show the optimal sedimentation rate at 14.3 cm/kyr, tested sedimentation rates range from 6 to 30 cm/kyr with a step of 0.1 cm/kyr; (3) Eh3 Member, the correlation coefficient results show the optimal sedimentation rate at 17.8 cm/kyr, tested sedimentation rates range from 10 to 30 cm/kyr with a step of 0.1 cm/kyr; (4) Ks-EyEd, 2-slice COCO analysis, the correlation coefficient results show the optimal sedimentation rate at 16.5 cm/kyr, tested sedimentation rates range from 6 to 30 cm/ kyr with a step of 0.1 cm/kyr. All of these sedimentation rates have H0 significance level lower than 1‰ and all 7 astronomical terms are used in the estimation. The number of Monte Carlo simulations is 2000. Linear interpolation is a 0.8 m sampling rate (the raw GR series data are over-sampled).

the Liguanqiao basin, Nanyang basin, Xiangzao depression, and Biyang Depression (Wang et al., 1987) (Fig. 1). The hydrocarbon-rich Biyang Depression in the eastern region of the Nanxiang Basin covers an area of about 1000 km2 and contains petroleum resources of 4 × 108 tons (Shi et al., 2017b). The depression developed as a rift with a northern slope, central depression and southern steep (Wang et al., 2001) (Fig. 1B). The rift and sedimentation history of the Biyang Depression has been divided into four episodes (Zhang and Xian, 2004) (Fig. 2):

(I). Early-initial stage Rifting Episode I spanning approximately the early Paleocene to early Eocene is represented by the Yuhuangding Formation (Ey). The Ey is dominated by red coarsegrained clastics of a floodplain facies. The base of the Ey Formation is at the boundary between the Cretaceous and the Paleogene, which is unconformable around the margins of the Biyang Depression but is considered to be conformable at its depocenter in the Zhaowa area where the BS1 borehole is located. (II). Late-initial stage Rifting Episode II in early Eocene coincides with 4

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Fig. 4. Cyclostratigraphy of the Liaozhuang (El) Formation in the B270 borehole. (A) Lithology, GR series, GR residual (after detrending by subtracting a 40% weighted average), ~54 m filter curve (passband is 0.0185 ± 0.0065 m−1) and ~14.3 m filter curve (passband is 0.07 ± 0.02 m−1) for the El Formation in depth domain. (B) 2π MTM (multitaper method) power spectrum and evolutive spectrum (sliding window is 80 m with a step of 1 m) for the untuned El GR depth series in depth domain. (C) GR residual, ~405-kyr and ~100-kyr filter curves throughout the El GR time-series in time main. (D) 2π MTM power spectrum and evolutive spectrum (sliding window is 600 kyr with a step of 1 kyr) for the tuned El GR time series in time domain.

the Dacangfang Formation (Ed). The Ed is mainly purple-red mudstone with sandstone and dark mudstone. (III). Climax stage Rifting Episode III during middle to late Eocene led to semi-deep to deep lacustrine deposits of the lowest two members (Eh3 to Eh2) of the Hetaoyuan Formation. The Eh3 Member consists of gray mudstone, muddy dolomite, dolomitic mudstone, oil shale, alkali ore and sandstone, and is the main source rock for hydrocarbons. The overlying Eh2 Member is beige shaly dolomite, gray dolomitic mudstone, dolomite and natural alkali. (IV). Late-stage Rifting Episode IV spanned approximately the late Eocene through the late Oligocene with the lacustrine deposits of the upper member (Eh1) of the Hetaoyuan Formation and the Liaozhuang Formation (El). The Eh1 Member consists of greenish mudstone, sandstone, oil shale and dolomite. The El Formation is brown-red mudstone, sandstone, and gypsum.

GR relationships has also been applied to analyze cyclostratigraphy of the fillings in other terrestrial and lacustrine basins (Liu et al., 2018). Therefore, GR logging was used as the main paleoclimate proxy for cyclostratigraphy analysis in this study. GR logs of the BS1 and B270 boreholes in the central area of the Biyang Depression (Fig. 1B) were selected for cyclostratigraphy analysis. The BS1 borehole, close to Zhaoao, is the deepest well in central China with a total depth of ~6005 m. It has well developed stratum of the Ey and Ed Formations overlying the upper strata of the Cretaceous Sigou Formation (Ks) (Fig. 2). The B270 borehole is adjacent to Anpeng and has a complete succession of the Eh and El Formations. The analysis intervals of the two wells are located in the main depocenters for the Ey-Ed period and Eh-El period, respectively; therefore, these intervals have high average rates of continuous sedimentation. These geological conditions are conducive to the cyclostratigraphy.

The main sedimentary depocenter of the Ey-Ed interval in the Biyang Depression was in the Zhaowa area. The lacustrine depocenter began to move southeast towards the Anpeng area during the Eh3 period where it then stabilized through the Eh-El interval (Hu et al., 2009). The top of the El Formation is considered to be the contact between the Paleogene and Neogene, and is an unconformity surface throughout much of the Nanxiang Basin and around the margins of the Biyang Depression. However, this upper transition and therefore the Paleocene-Neogene boundary is considered to be nearly conformable within the depocenters of the El Formation where the B270 borehole is located (Hu et al., 2001; Li, 2008).

3.2. Total organic carbon and XRF measurements Total organic carbon (TOC) data of the BYHF1 borehole (2415–2451 m) were also collected with an average sampling interval of ~1 m and of the B270 borehole (2331–4275 m) with an average sampling interval of ~12 m. Handheld XRF spectrometry was used to perform elemental scanning on the core section of the BYHF1 borehole from 2415 to 2451.4 m with an average sampling interval of 0.22 cm. The TOC data and XRF data of the BYHF1 borehole were to investigate the meter-scale sedimentary cyclicity of varves. The TOC data of the B270 borehole were to study hydrocarbon-source rock response to the long-term astronomical signals.

3. Materials and methods

3.3. Cyclostratigraphy analysis methods

3.1. Gamma ray (GR) climatic proxy and logging

The GR series were resampled to uniform spacing using linear interpolation in the Acycle v1.1 software (Li et al., 2019). The series were then detrended using the Lowess method (William, 1979) in the Acycle v1.1 software (Li et al., 2019) to remove low-frequency long-term trends; thereby highlighting the higher frequency astronomical signals (Huang, 2014). The 2π multitaper method (MTM) of the SSA-MTM Toolkit (Ghil et al., 2002) was then applied to the series to obtain spectral analysis graphs using the robust red noise model reported at the 50%, 90%, 95% and 99% confidence levels for interpretation of spectral peak significance (Mann and Lees, 1996). Evolutive spectrum (Kodama and Hinnov, 2014) was based on the GR series to identify the changes in cycle frequencies due to variable sedimentation rate. The technique of COrrelation COefficient (COCO; Li et al., 2018a) is an objective method to determine optimal sedimentation rates that yield the time series that significantly fit astronomical models. This COCO method estimates the correlation coefficient between power spectra of a paleoclimate proxy series in the depth domain and an astronomical solution in the time domain, converting the proxy series from depth to time for a range of “test” sedimentation rates. The method does not require rigorous time control, provides a means to objectively determine the most optimal sedimentation rate. The most likely sedimentation rate corresponds to the highest correlation coefficient, the lowest significance level of the null hypothesis (H0) and largest total number of astronomical parameters (Li et al., 2018a; Li et al., 2019). Gaussian bandpass filtering was then performed using Acycle v1.1 software (Li et al., 2019) to extract the astronomical cycles (especially the eccentricity cycles) (Huang, 2014; Kodama and Hinnov, 2014). The

Quasi-periodic orbital variations influence the insolation distribution on the Earth's surface, thereby driving cyclical fluctuations of the climate system that can be recorded in the sedimentary successions (Strasser et al., 2006; Hinnov and Hilgen, 2012; Huang, 2014). Proxies of petrophysical parameters, magnetic properties, paleontology and isotopic indices can preserve periodic climate changes (Gong and Zhang, 2016). Natural gamma ray (GR) logging in sedimentary strata can be a paleoclimatic and paleoenvironmental proxy for cyclostratigraphy analysis in both marine and terrestrial strata. GR logging which relates to the amount of radionuclides of potassium, uranium and thorium in the rock, can reflect changes in clay minerals and organic matter. Warm and humid conditions can be related to increased chemical weathering and more stable rainfall, and therefore more inputs of clay minerals and organic matter that yield higher GR values. In contrast, landscapes undergoing cooler and more arid climate conditions have reduced vegetation and increased physical weathering; which results in pulses of coarse-grained clay-poor sediment being transported and left in braided stream deposits during the infrequent flooding (Vandenberghe, 2003, 2015; Bridgland and Westaway, 2008, 2014). During this cooler-arid climatic condition, there is a reduced clay influx relative to silt-sand and inorganic carbonate into the lacustrine basin; therefore, a lower GR value. This lacustrine cyclic-sedimentation model has been supported by the well-calibrated uppermost Cretaceous filling of the Songliao Basin (Wu et al., 2014), where higher eccentricity and the warmer climate correlates to relatively higher clay/sand ratios and therefore to high-gamma intervals. This general model of climate-clay6

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Fig. 5. Cyclostratigraphy of the upper two members (Eh2-Eh1) of the Hetaoyuan Formation in the B270 borehole. (A) Lithology, GR series, GR residual (after detrending by subtracting a 35% weighted average), ~59.9 m (passband is 0.0167 ± 0.0047 m−1) and ~15 m (passband is 0.0668 ± 0.0168 m−1) filter curves for the Eh2-Eh1 Members in depth domain. (B) 2π MTM power spectrum and evolutive spectrum (sliding window is 90 m with a step rate of 1 m) for the untuned Eh2-Eh1 GR depth series in depth domain. (C) GR residual, ~405 kyr and ~100 kyr filter curves throughout the Eh2-Eh1 GR time-series in time main. (D) 2π MTM power spectrum and evolutive spectrum (sliding window is 600 kyr with a step of 1 kyr) for the tuned Eh2-Eh1 GR time series in time domain.

405-kyr long-eccentricity cycle is the most stable orbital parameter of the past 250 Myr (Laskar et al., 2004; Hinnov and Hilgen, 2012), and thus can be used as the primary periodicity to calibrate geological time scales. These 405-kyr cycles have been given a numbered nomenclature relative to the present according to their predicted periodicity with the La2010d astronomical models of Laskar et al. (2011) to establish a highresolution astronomical time scale (Wu et al., 2011; Huang, 2014).

Interpreted 405-kyr cycles in the GR depth series were bandpass filtered using a ~54 m wavelength, which yielded 13 cycles. The interpreted 100-kyr cycles were bandpass filtered using a ~14.3 m wavelength to assign 53 cycles (Fig. 4A). The power spectrum of the long-eccentricity tuned El GR time-series (Fig. 4D) shows significant peaks at 2 Myr, 405 kyr (the tuned period), 175 kyr, 103 kyr and 58 kyr, with weaker peaks at 36 and 22.8 kyr. The evolutive spectrum indicates an approximate alignment of the ~405 kyr and ~100 kyr eccentricity terms. The sediment accumulation rates calculated for each 405-kyr interval throughout the El GR time-series varied between 0.097 and 0.14 m/kyr with an average of 0.12 m/kyr. The duration of the series based on the 405-kyr tuning was 5.9 myr.

4. Results 4.1. Sedimentation rates for each formation from COCO analysis Based on the similar sedimentary characteristics or the tectonic “rift episodes” (Zhang and Xian, 2004; Hu et al., 2009) (Fig. 2), we divided the GR series of the two boreholes which have most complete representation of the formation into the following four intervals: (1) Liaozhuang (El) Formation (B270 borehole, 214–926 m); (2) Combined Eh1 and Eh2 Members of the Hetaoyuan Formation (B270 borehole, 926–2270 m); (3) Eh3 Member of the Hetaoyuan Form-ation (B270 borehole, 2270–4300 m); (4) Uppermost Sigou, Yuhuangding and Dacangfang formations (Ks-Ey-Ed) (BS1 borehole, 2857–5828 m). We carried out a refined COCO analysis for each interval using 2000 Monte Carlo simulations of a range of potential sedimentation rates (Fig. 3): 1) El Formation has only one main peak at 11.5 cm/kyr; 2) The combined Eh1 and Eh2 members have only one main peak at 14.3 cm/ kyr; 3) Eh3 Member has only one main peak at 17.8 cm/kyr; 4) Ks-Ey-Ed has only one main peak at 16.5 cm/kyr. All of these sedimentation rates have H0 significance level lower than 1‰ and all 7 astronomical terms are used in the estimation (Fig. 3), representing strong evidence of astronomically forced sedimentation process. The published estimates of average sedimentary rates for the El Formation (10 cm/kyr), Eh1 and Eh2 members (10 and 20 cm/myr, Eh3 Member (20 cm/kyr), and Ey-Ed formations (25 cm/ kyr) (Hu et al., 2001) are very similar to the COCO results, except for the merged Ks-Ey-Ed. Therefore, these optimal sedimentation rates from COCO are used as the main constraint on the set of separate cyclostratigraphy analyses for each formation or member. These are summarized from the uppermost formation (Liaozhung Fm.) to the lowest unit (Yuhuangding Fm.).

4.3. Eh2 and Eh1 members of the Hetaoyuan Formation (B270 borehole, 926–2270 m) The Eh1 Member in the B270 borehole consists of a thick (476 m) continuous sequence of dark-gray mudstone, oil shale, and gray dolomite sediments deposited in a shore to shallow lacustrine environment. The Eh2 Member consists of thick (868 m) continuous strata of darkgray mudstone, oil shale, clayey dolomite, and dolomite deposited in a saline lake environment. The GR value ranged from 70 to 340 API with an average and median value of 202 and 200 API, respectively. The evolutive power spectrum of the untuned Eh2-Eh1 GR depth series suggests variable sediment accumulation rates. There are clusters of potential prominent sedimentary cycles at wavelengths of 166, of 59.9, of 20-18.7-13, of 9.5-7-6.5-5.5, and of 3.9-3.6-2.9-2.7-2.3 m (Fig. 5B). The wavelength ratio of the clusters of 59.9 m: 20–9.5 m: 7–4.6 m: 3.9–2.3 m (ca. 60:17:7:3) is near the ~20:5:2:1 expected ratio of longeccentricity, short-eccentricity, obliquity and precession, i.e., 405:100:40:20 kyr. Therefore, the sedimentary strata are affected by astronomical forcing. The most likely sedimentary rate from COCO analysis of 0.143 m/kyr for the Eh2 and Eh1 members supports our interpretation of the identified wavelengths. Interpreted 405-kyr cycles in the GR depth series were bandpass filtered using the ~59.9 m wavelength to yield 24 cycles; and the interpreted 100-kyr cycles were bandpass filtered using the ~15 m wavelength to assign 96 cycles (Fig. 5A). The power spectrum of the long orbital eccentricity tuned Eh2-Eh1 GR time-series (Fig. 5D) shows significant peaks at 405 kyr (the tuned period), 101, 40, 23 and 18 kyr. The evolutive spectrum indicates 405kyr and ~100-kyr eccentricity cycles are relatively stable (Fig. 5D). The sediment accumulation rates calculated for each 405-kyr interval throughout the Eh2-Eh1 GR time-series varied between 0.1 and 0.18 m/ kyr with an average of 0.13 m/kyr. The duration of the series based on the 405-kyr tuning was 10.36 myr.

4.2. Liaozhuang (El) Formation (B270 borehole, 214–926 m) Samples of the El Formation in the B270 borehole consist of grayish shale and of gray-white pebbly sandstone deposited in a shallow lacustrine setting and in the upper and lower regions of the shore, respectively (Hu et al., 2001). GR values ranged from 90 to 290 API with an average and median value of 160 and 154 API, respectively. The evolutive power spectrum of the untuned El GR depth series suggests variable sediment accumulation rates. There are clusters of potential prominent sedimentary cycles at wavelengths of 254, of 63 and 51.6, of 22.8-19.3-14-11.2, of 9.1-8.0-7.1-5.2, and of 3.9-3.3-3.2-2.9 m (Fig. 4B).The wavelength ratio for the four clusters of 63–51.6 m: 22.8–11.2 m: 9.1–5.2 m: 3.9–2.5 m (ca. 57:17:7:3.5) is near the ~20:5:2:1 expected ratio of long-period orbital eccentricity, short-eccentricity, obliquity and precession cycles, i.e., 405:100:40:20 kyr. It is thus hypothesized that these sedimentary strata were affected by astronomical forcing. The most likely sedimentary rate from COCO analysis of 0.115 m/kyr in the El Formation support this interpretation of the identified wavelengths.

4.4. Eh3 Member of the Hetaoyuan Formation (B270 borehole, 2270–4300 m) The Eh3 Member in the B270 borehole consists of dark-gray mudstone, oil shale, dolomitic mudstone and fine sandstone deposited in semi-deep to deep lacustrine settings. The GR value ranged from 50 to 350 API, with an average and median value of 156.8 and 149.1 API, respectively. The evolutive power spectrum of the untuned Eh3 GR depth series suggests variable sediment accumulation rates. The power spectrum of the untuned Eh3 GR depth series showed prominent sedimentary cycles or clusters of potential cycles at wavelengths of 563, of 100 and 54, of 18.9 and 16.6, of 8.2-6.8-5.7, and of 4.5-4.2-4.0 m 8

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Fig. 6. Cyclostratigraphy of the lower member (Eh3) of the Hetaoyuan Formation in the B270 borehole. (A) Lithology, GR series, GR residual (after detrending by subtracting a 35% weighted average), ~100 m (passband is 0.01 ± 0.004 m−1) and ~24.4 m (passband is 0.041 ± 0.011 m−1) filter curves for the Eh3 Member in depth domain. (B) 2π MTM power spectrum and evolutive spectrum (sliding window is 135 m with a step of 1 m) for the untuned Eh3 GR depth series in depth domain. (C) GR residual, ~405 kyr and ~100 kyr filter curves throughout the Eh3 GR time-series in time main. (D) 2π MTM power spectrum and evolutive spectrum (sliding window is 600 kyr with a step of 1 kyr) for the tuned Eh3 GR time series in time domain.

(Fig. 6B). The wavelength ratio for the four clusters of 100–54 m: 18.9–16.6 m: 8.2–5.7 m: 4.5–4.0 m (ca. 77:18:7:4) is near the ~20:5:2:1 expected ratio of long-eccentricity, short-eccentricity, obliquity and precession, i.e., 405:100:40:20 kyr. It can thus be considered that sedimentary strata are affected by astronomical forcing. The most likely sedimentary rate from the COCO analysis of 0.178 m/kyr in the Eh3 Formation supports our interpretation of the identified wavelengths. The interpreted 405-kyr cycles in the GR depth series were bandpass filtered using the ~100 m wavelength to yield 25 cycles; and the interpreted 100-kyr cycles were bandpass filtered using the ~24.4 m wavelength to assign 98 cycles (Fig. 6A). The power spectrum of the long eccentricity tuned Eh3 GR timeseries (Fig. 6D) showed significant peaks at 2.1 Myr, 405 kyr (the tuned period), 101 kyr, 32 kyr, 28.5 kyr and 23.5 kyr, with weaker peaks at ~1 Myr, 137 kyr and 41 kyr. The evolutive spectrum indicated that 405-kyr and ~100-kyr eccentricity cycles were relatively stable. The sediment accumulation rates calculated for each 405-kyr interval throughout the Eh3 GR time-series varied between 0.14 and 0.27 m/kyr with an average of 0.196 m/kyr. The duration of the series based on the 405-kyr tuning was 10.3 Myr.

4.6. Uppermost Sigou, Yuhuangding and Dacangfang formations (Ks-EyEd) (BS1 borehole, 2857–5828 m) The Ks-Ed formations in the BS1 borehole consisted of a thick (2971 m) continuous sequence of purple mudstone and fine sandstone deposited in shore to shallow lake settings. The GR value ranged from 40 to 336 API with both the average and median values as 115 API. The evolutive power spectrum of the untuned Ks-Ed GR depth series suggests variable sediment accumulation rates. There are clusters of potential prominent sedimentary cycles at wavelengths of 148.5, 66.6 and 63.3, of 15.8 and 12.7, of 8.4–6.7-5.8-5.0, and perhaps at 3.4 and 2.8 m (Fig. 8B). The ratio of the wavelength clusters of 66–63.3 m: 15.8–12.7 m: 8.4–5.0 m: 3.4–2.8 m (ca. 64:14:6:3) is near the ~20:5:2:1 expected ratio of long-eccentricity, short-eccentricity, obliquity and precession, i.e., 405:100:40:20 kyr. The sedimentary strata were thus affected by astronomical forcing. The most likely sedimentary rate of 0.165 m/kyr in the Ks-Ed Formation supports this interpretation of the identified wavelengths. Interpreted 405-kyr cycles in the GR depth series were bandpass filtered using the ~63.3 m wavelength, which yielded 48 cycles; then interpreted 100-kyr cycles were bandpass filtered using the ~15.8 m wavelength to assign 198 cycles (Fig. 8A). The power spectrum of the long orbital eccentricity tuned Ks-Ed GR time-series (Fig. 8D) shows significant peaks at ~1.7 Myr, ~1 Myr, 405 kyr (the tuned period), 100 kyr, and 37 kyr, with weaker peaks at 45 kyr, 31 kyr and 23.4 kyr. The evolutive spectrum indicates an approximate alignment of ~405-kyr and ~100-kyr eccentricity terms. The sediment accumulation rates calculated for each 405-kyr interval throughout Ks-Ed GR time-series varied between 0.08 and 0.205 m/kyr with an average of 0.15 m/kyr. The duration of the series based on the 405-kyr tuning was 19.64 Myr.

4.5. Characteristics and origin of lacustrine sedimentary cycles in the Eh3 Member of the Hetaoyuan Formation Laminated shale deposits in semi-deep to deep lacustrine setting of the organic-rich Eh3 Member of the Hetaoyuan Formation were studied in the core of the BYHF1 well from 2415 to 2451.4 m. The average TOC content of the studied interval is 2.7%. Petrographic thin sections show horizontal bedding and superb cyclicity of alternating light- and darkcolored layers with an average thickness of 0.22 mm (Fig. 7A). The light-colored layers are composed of calcite, gypsum, quartz and other light minerals, whereas the dark layers are mainly organic-rich clayey sediments (Liu and Wang, 2013; Guo et al., 2018). According to the average sedimentary rate of approximately 0.20 m/kyr in the Eh3 Member derived from our cyclostratigraphy (Section 4.4), each 0.22mm cycle may represent an annual varve. Seasonal climate changes during the Eh3 period were conducive to the enrichment of organic matter, and the stratification of the water column in the saline lake basin helped to preserve organic matter thereby forming dark clayey laminated sediments. Seasonal dry climate conditions may have contributed to strong evaporation and increased water salinity, thereby allowing for the formation of light-colored minerals, including calcite, quartz, dolomite and gypsum. The laminated shale was thus mainly controlled by annual seasonal changes and the supply of terrigenous clastics. But in addition to these annual varves, there is also a larger-scale cyclicity. The studied interval has seven main oscillations in the GR, Mn, Ca and Fe series (Fig. 7D). These cycles have a range of thickness of 3 to 8.2 m and are representative of the cycle thickness throughout the entire Eh3 Member of the B270 drillcore (3.7 to 6.8 m; see Section 4.4). The power spectrum of the each of the untuned GR, Mn, Ca, and Fe series in depth domain shows similar prominent peaks at approximate 5.5 m (Fig. 7E–F). Considering the average sedimentary rate of 0.20 m/ kyr for the studied interval, ~5.5-m cycles may represent precession cycles.

5. Discussion 5.1. Hydrocarbon-source rock response to astronomical forcing The middle Eocene Eh3 Member is nearly coeval with Member 4 through the lower of Member 3 of Shahejie Formation (Es4-Es3l) (50.8–40.2 Ma; Liu et al., 2018) in the Dongying Depression in the Bohai Bay Basin, which is adjacent to the Nanxiang Basin. Both of these formation intervals have high-quality source rocks. The organic-rich source rocks of the Eh3 Member, the main hydrocarbon source rock of the Nanxiang Basin, was therefore chosen for an analysis of the variations of the total organic content (TOC) in the B270 borehole. The plot of TOC partly shows a long-term periodic cycle of approximately 200 m that is supported by more than 4 data points for each approximate cycle (Fig. 9), thereby satisfying the conditions for a cyclostratigraphy analysis (Weedon, 2003). The power spectrum of this untuned TOC series showed a prominent sedimentary cycle at a wavelength of 250 m (Fig. 9B). The 405-kyr long-eccentricity cycle in the Eh3 Member of the B270 borehole has a thickness of ~100 m. Therefore, the 250-m-thick cycles may correspond to ~1.2-Myr obliquity-modulation cycles. The interpreted ~1.2-Myr cycles were bandpass filtered ~250 m to assign ~8 cycles (Fig. 9A). The power spectrum of the long obliquity-tuned TOC time-series of the Eh3 Member (Fig. 9D) shows significant peaks at 1 Myr. These interpreted ~1.2-Myr cycles of tuned Eh3 Member TOC time-series are closely inphase with the filter of the modeled O/T (obliquity variance/total variance) of La2010d (Laskar et al., 2011); but did not correspond well with the filter of the modeled O/T of the older La2004 model (Laskar 10

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Fig. 7. (A) Detailed core image spanning 80 cm and thin-section photos of the BYHF1 well from 2432.8 to 2432.0 m (Member Eh3). Each red vertical line in the B′ and C′ images represent a full cyclic lamination with the blue vertical lines indicating the component dark and a lighter layer. The black numbers represent the thickness of a full cyclic lamination. (D) Geochemical data for a depth interval 2415 to 2451.4 m in organic-rich Eh3 Member of the Hetaoyuan Formation within BYHF1 borehole exhibiting superb laminations and cyclicity. GR, Mn, Ca and Fe series (These series were detrended by subtracting a 35% weighted average) and ~5.5 m (passband is 0.18 ± 0.02 m−1) filter curves (the red dash line in each box). The location of the core image of Fig. 4 is shown by the blue bar at 2432 m to the right of the lithologic column. GR = natural gamma ray log. Elemental concentrations (Mn, Ca, Fe) are based on XRF scanning (units = counts). TOC = total organic carbon. (E) 2π MTM (multitaper method) power spectrum for the untuned interval GR series (the blue line) and Ca series (the purple line) in depth domain. (F) 2π MTM (multitaper method) power spectrum for the untuned interval Mn series (the orange line) and Fe series (the green line) in depth domain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. Cyclostratigraphy of the uppermost Sigou, Yuhuangding and Dacangfang formations (Ks-Ey-Ed) in the BS1 borehole. (A) Lithology, GR series, GR residual (after detrending by subtracting a 15% weighted average), ~63.3 m (passband is 0.0158 ± 0.0038 m−1) and ~15.8 m (passband is 0.0634 ± 0.0164 m−1) filter curves for the Ks-Ed Formations in depth domain. (B) 2π MTM power spectrum and evolutive spectrum (sliding window is 100 m with a step of 1 m) for the untuned Ks-Ed GR depth series in depth domain. (C) GR residual, ~405 kyr and ~100 kyr filter curves throughout Ks-Ed GR time-series in time main. (D) 2π MTM power spectrum and evolutive spectrum (sliding window is 600 kyr with a step of 1 kyr) for the tuned Ks-Ed GR time series in time domain.

et al., 2004) throughout the Eh3 period (Fig. 9C). Long-period (1–2 Myr) obliquity-forced continental reservoir changes had a significant impact on global sea-level variations and groundwater fluctuations during nonglacial periods (Li et al., 2018b). Lower water storage in a terrestrial basin correlates with high sea level and decreased obliquity forcing in both marine records and the La2010d astronomical solution (Li et al., 2018b), and vice versa. This may involve the hypothesis of groundwater-driven eustasy. The modeled O/T of La2010d shows generally good agreement with ~1.2-Myr filter curves of the published long-term sea-level change curves, including Miller et al. (2005) and Kominz et al. (2008) (Fig. 9C). During the late-middle Eocene interval of 43.4 Ma to 39.3 Ma, the ~1.2-Myr cycles of TOC time-series have the closest match to the filter of the modeled O/T of La2004 and La2010d. Within the corresponding depth interval (2270–3080 m), the average TOC content was more than 2 wt% and high-quality source rocks developed that had relatively high sediment accumulation rates. High values of ~1.2 Myr cycles of tuned Eh3 Member TOC time-series corresponded with high values of the Modeled O/T of La2010 and high values of water masses. Maybe the high water depth affect the development of high-quality source rocks. During the older early-middle Eocene interval from 49.6 to 43.4 Ma, the ~1.2-Myr cycles of TOC time-series are slightly unsynchronized relative to the phase of the filter of the modeled O/T of La2010d and partly antiphased with the filter of the modeled O/T of La2004. Within the corresponding depth interval (3080–4300 m), the average TOC content was only near 1 wt% and medium-quality source rocks developed with relatively low sediment accumulation rates (Fig. 9A). Although high values of ~1.2 Myr cycles of tuned Eh3 Member TOC time-series corresponded with high values of water masses, the interval did not have high-quality source rocks. This may be influenced by factors other than water depth, such as preservation conditions, paleoproductivity and so on. Overall, ~1.2-Myr obliquity cycles appear to have had an important influence on the development of source rocks, and it may affect the development of high-quality source rocks by controlling water depth.

durations of 405 kyr for long-eccentricity cycles, 131, 125 and 95 kyr for short-eccentricity cycles, 40 and 38 kyr for obliquity cycles, and 23 and 22 kyr for precession cycles, as well as a ~2.3 Myr ultra-long-eccentricity cycle (Fig. 10C). The entire ATS encompasses 46 Myr if there are no major hiatuses, of which about 3 Myr is in the uppermost Cretaceous and 43 Myr in the Paleogene. The set of Paleogene formations are generally considered to encompass the interval from the Cretaceous/Paleogene boundary at 66.0 Ma in GTS2012 (Geologic Time Scale 2012; Gradstein et al., 2012) to the Paleogene/Neogene boundary at 23.0 Ma, which is a similar span of 43 Myr. Owing to lack of marine biostratigraphy, radiometric dating or reliable paleomagnetic data directly on the strata in the Nanxiang Basin, it is not possible to get precise anchors to establish the absolute ATS; but the agreement of the computed cyclostratigraphic span with the duration for the Paleogene seems to warrant the simple assumption of equivalence. Indeed, the top of the El Formation, corresponding to contact between the Paleogene and Neogene, is considered to be nearly conformable within its depocenter where the B270 borehole is located, even though it is an unconformity near the margins of the Biyang Depression (Hu et al., 2001; Li, 2008). For convenience, we assumed that there was no denudation in the B270 borehole at this depocenter location and adopted 23.03 Ma for the top of the preserved El Formation as our age control point on the floating ATS. The base of the Yuhuangding (Ey) Formation at the boundary between the Cretaceous and the Paleogene is considered to be conformable at its depocenter where the BS1 borehole is located; therefore, the Cretaceous/Paleogene boundary of 66 Ma was used as a verification point for this floating ATS. Relative to the selected upper anchor point, our entire ATS extends from the Paleogene/Neogene boundary of 23.03 Ma through the latest Cretaceous at 69.24 Ma; thereby spanning the 57th to 171st 405-kyr eccentricity cycle (E57−E171) of the La2010d orbital record. The basal boundary of the Ey Formation corresponds to the 163.5th 405-kyr eccentricity cycle, the age of which in our ATS (66.24 Ma) closely matches the age of the Cretaceous/Paleogene boundary in the GTS2012 (66.0 Ma). These results also correspond well with the La2010d eccentricity solution through the entire Paleogene (Fig. 10A).

5.2. Astronomical time scale (ATS) for the Paleogene formations of Nanxiang Basin

5.3. Age calibration of Paleogene terrestrial biozones of central East China

The 405-kyr tuned GR time-series of the sequences were merged to establish a composite chronological framework (Fig. 10). The floating ATS of the Paleogene succession of the Nanxiang Basin spans 43.2 Myr. Previously, Yao et al. (2012) had studied the cyclostratigraphy from the Eh Formation to the El Formation. Therefore, our ATS both partly verifies and extends this previous study. The composite GR time-series was tuned to the 405-kyr eccentricity bandpass cycles for preparing a power spectrum in the time domain (Fig. 10B). In addition to the assigned 405-kyr long-eccentricity peak, this time-domain spectra displays: (1) 131 and 100 kyr short-eccentricity peaks; (2) a ~1.2-Myr ultra-long obliquity peak in addition to a 40 kyr peak within the obliquity band; and (3) 23, 21 and 20.2 kyr peaks within the precession band. In addition, there is a peak at ~2 Myr that may correspond to the ~2.25 Myr ultra-long eccentricity modulation cycle that has been seen in some other studies (e.g., Liu et al., 2018). These astronomical cycles are present above the 95% confidence level and compare well with the La2010d astronomical models (Laskar et al., 2011) (Fig. 10B and C). That La2010d ETP is a synthetic time series constructed from the sum of the normalized eccentricity (E), obliquity (T) and precession (P) (Wu et al., 2013; Shi et al., 2017a), and includes the astronomical solutions for Paleogene that indicates

The ATS provides time constraints for the biozones, tectonic episodes and paleoclimatic evolution stages in the Nanxiang Basin and can be used to compare to the stratigraphic history of other nearby basins and to regional tectonic events. Paleontological correlations play an important role in the stratigraphic division of the Nanxiang Basin. The most crucial fossil groups in correlations are the Ostracoda, Characeae and palynology (Figs. 2, 11). Previous studies divided the Paleogene succession in the Biyang Depression into Paleocene, Eocene and Oligocene stages based on changes in the Characeae (green algae) assemblages (Zhang et al., 1993; Yan, 2008) (Figs. 2, 11). Although paleontological data have been widely used in the Nanxiang Basin, biozones lack absolute age constraints; therefore, our ATS can provide relatively reliable ages for these biozones (Fig. 11). For example, our age framework assigns the time span of the Charites producta-Croftiella piriformis Zone (as currently placed within the El Formation and Eh1 Member) as 32.99–23.03 Ma; which is consistent with its general Oligocene estimate (Yan, 2008), but provides a potentially more precise placement. The Eocene/Oligocene boundary was assigned as the base of the Eh1 Member based on Grovesichara sinensis/Stephanochara globula (Zhang 13

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Fig. 9. Long-term cycles of the Total Organic Carbon (TOC) variations of the lower member (Eh3) of the Hetaoyuan Formation in the B270 borehole. (A) TOC series, detrended TOC series (after detrending by subtracting a 35% weighted average), ~250 m (passband is 0.004 ± 0.001 m−1) filter curves and sediment accumulation rate of the Eh3 Member in depth domain. (B) 2π MTM power spectrum of the detrended TOC series of Eh3 Member in depth domain. (C) TOC series and ~1.2-Myr filter curves of the B270 borehole in Eh3 Member compared to the filter of the modeled O/T (obliquity variance/total variance) of La2004 and La2010d models (Laskar et al., 2004, 2011), Sea-level change curve (brown) of Miller et al. (2005) and ~1.2-Myr filter curve (passband is 0.83 ± 0.22 cycle/Myr; green), and sea-level change curve (brown) of Kominz et al. (2008) and ~1.2-Myr filter curve (passband is 0.83 ± 0.2 cycle/Myr; green); (D) 2π MTM power spectrum of the tuned TOC series of Eh3 in time domain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 14

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Fig. 10. (A) Astronomical time scale (ATS) for the composite El-Ks succession (the B270 and BS1 boreholes) of the Biyang Depression. The right column overlays the sediment accumulation rates (m/kyr for each 405-kyr cycle), the rifting episodes in the Nanxiang Basin, and convergence rates between the Pacific and Eurasia margin (modified from Northrup et al., 1995) (km/Myr; green line – note that the scale increases to left in order to highlight the inverse relationship to the sediment accumulation rates). (B) 2π MTM power spectrum of the composite GR time-series after tuning to 405-kyr eccentricity cycles; (C) 2π MTM power spectrum of the ETP based on the La2010d model (Laskar et al., 2011). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

et al., 1993; Yan, 2008). This agrees with our assignment of the 81.5th 405-kyr eccentricity cycle of this study to this level, which has an age of 32.99 Ma that is within 1 Myr of the age of the Eocene/Oligocene boundary in the GTS2012 (33.9 Ma). The Paleocene/Eocene boundary within the Ey Formation had been assigned based on Gyrogona qianjiangica/Grovesichara changzhouensis (Zhang et al., 1993; Yan, 2008). This level corresponds to the 138.4th 405-kyr eccentricity cycle of this study, which has an age of 56.05 Ma that is identical to the age of the Paleocene/Eocene boundary in the GTS2012 (56.0 Ma).

conclusion of the Paleocene at ~23.03 Ma, the apparent timing and span of each Rifting Episode as currently assigned and numbered within each separate basin seems to be quite inconsistent. There is a general correlation between the rifting stages of the Nanxiang Basin and the Pacific-Eurasian convergence rate (Fig. 10A). The ages for the interpreted tectonic episodes that affected the Nanxiang Basin (Zhang and Xian, 2004) were calibrated based on our ATS (Figs. 10A, 11). Early-initial Rifting Episode I (Ey Formation) extended from about 66 to 54 Ma; Late-initial Episode II (Ed Formation) was a brief interval from 54 to 50 Ma. Most of these initial rifting stages coincided with a rapid rate of convergence of the Pacific plates and the earliest phase of rifting of the future Sea of Japan and other extensional basins along the eastern margin of China. The ‘climax stage’ Episode III (Eh3 through Eh2 Members) spanned 50 to 33 Ma. The Late-stage Rifting Episode IV (Eh1 Member through El Formation) was from 33 to 23 Ma. Our age framework revises the onset of ‘climax stage’ Episode III to 50 Ma from the previously poorly constrained and much younger estimate of 42.7–36.5 Ma by Yao et al. (2012), but falls within the broad estimate of 50–38.6 Ma by Hu et al. (2001). This event coincided with a significant slowing of the Pacific convergence rate, with the onset of more rapid subsidence in the basin (Table 1) and with the associated development of semi-deep lacustrine deposits that included oil shales and other future hydrocarbon source rocks. There is a significant midEocene tectonic event or unconformity that began at ~50 Ma in some of the continental rift basins of East China (Wang et al., 2010), although the nomenclature varies. This ~50 Ma event is the onset of Rifting Episode II of the Bohai Bay Basin (Liu et al., 2018) and the onset of Rifting Episode III of the Nanxiang Basin. We suggest that this general ~50 Ma Episode may be related to the change in the convergence direction of the Pacific plate with the Eurasian margin. At ~53–47 Ma, the drift direction of the Pacific plate over the Hawaiian hotspot changed by 45° from a NNW direction to NW to create the EmperorHawaii bend (O'Connor et al., 2013). The convergence rates between Pacific and Eurasia decreased from ~75 km/Myr prior to 50 Ma to ~40 km/Myr at ~40 Ma (Northrup et al., 1995) (Fig. 10A). This change of Pacific convergence direction is synchronous with many other tectonic events in the Pacific and its adjacent areas (Zhou and Sun, 2017). The onset of the main collision of the Indian Plate with the Eurasian Plate was also at ~50 Ma (Yin and Harrison, 2000; Ding et al., 2016). Therefore, the combination of the change in the convergence direction of the Pacific plate with East China, the slowing of that rate of convergence, and perhaps far-field stresses caused by the collision of the India Plate with Eurasia may have been the major causes of the interpreted tectonic response and development of major rift-valley lakes within the Nanxiang Basin at ~50 Ma.

5.4. Response of Nanxiang Basin sedimentation to paleoclimate trends and regional tectonic events 5.4.1. Paleoclimate trends From the late Paleocene to the early Eocene, there was rapid global warming with a peak excursion at the PETM (Paleocene-Eocene Thermal Maximum) when global ocean and land surface temperatures increased by 4–8 °C (Norris and Röhl, 1999; Tripati and Elderfield, 2005). This warming continued until the late Eocene followed by a rapid global cooling into the early Oligocene icehouse realm when global ocean and land surface temperatures decreased by 2–4 °C and Antarctica developed its first major ice sheet of the Cenozoic (Barker et al., 2007; Zachos et al., 2008; Liu et al., 2009; Pusz et al., 2011; Hren et al., 2013). These global climate events have been recorded in the Nanxiang Basin. For example, Zhu et al. (2010) and Chen et al. (2014) found evidence of the PETM in the Ey1 Member. The middle to late Eocene Eh3 through Eh2 members in the B270 borehole are characterized by dark mudstone, oil shales and dolomite, reflecting a deep lacustrine setting under a humid warm paleoclimate. The assemblages of Characeae differ greatly across the interpreted assignment of the Oligocene/Eocene boundary (Zhang et al., 1993; Yan, 2008). The Eh1 Member through the El Formation in the B270 borehole developed purple-celadon mudstone and coarse clastics, suggesting a semi-arid and perhaps cooler paleoclimate through the Oligocene. 5.4.2. Basin subsidence and regional tectonic events The two wells in this study are located in the main depocenters for the Ey-Ed and the Eh-El formations; therefore, the calculated sediment accumulation rates could reflect the general subsidence rates in the basin. We used Pearson product-moment correlation coefficient analysis (Derrick et al., 1994) of these sediment accumulation rates and the convergence rates of Pacific–Eurasia (Northrup et al., 1995) to determine the optimal correlation (Table 1). The sediment accumulation rates are significantly negatively correlated with the convergence rates of Pacific–Eurasia, indicating that a slower convergence rate corresponds to a faster sediment accumulation rate within the subsiding central rift (Table 1). Assuming that, for the most part, the accumulation in these depocenters kept pace with basin subsidence, then the subsidence history of the Nanxiang Basin was significantly negatively influenced by the rates of subduction of the Pacific Plate along the eastern margin of Eurasia. In the current widespread model of the tectonic evolution of East China, there are 4 main tectonic episodes during the Paleogene that affected many of the basins. Although the interpreted beginning of the first Rifting Episode is the beginning of the Paleocene at ~66 Ma and the termination the last Rifting Episode is considered to be at the

6. Conclusions Natural GR logging of the Paleogene sedimentary strata in boreholes in depocenters of the Biyang Depression of the Nanxiang Basin were used as a paleoclimate proxy to analyze climate cycles. A floating astronomical time scale (ATS) spanning the entire Paleogene Period was constructed by tuning to 405-kyr long-eccentricity cycles. The ATS has a good correspondence with the La2010d eccentricity solution through the Paleogene. The ATS enables a geochronological framework of the Nanxiang Basin for calibrating the ages of biozones, of major changes in depositional environments and of episodes of rifting. Our ATS indicates that the change at ~50 Ma during middle Eocene in the direction and 16

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Fig. 11. Summary of the general stratigraphic chart of the Biyang Depression in the Nanxiang Basin. The Biozone column and the Rift episodes were rescaled to this study's time scale. The Biozones ranges are from Yan (2008). The rifting episodes are after Zhang and Xian (2004).

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Table 1 Mean sediment accumulation rates and the convergence rates between the Pacific plates and the eastern margin of Eurasia (rounded from Northrup et al., 1995).a Interval (Ma) Convergence rates (km/Myr) Sediment accumulation rate (m/ kyr)

68.5–53 78 0.15

53–39.5 38 0.185

39.5–28.5 77 0.13

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28.5–23.03 90 0.12

a The total number of samples used in this analysis is 116; r values = −0.58687 are significant at 99% confidence level (p-value was 4.41 × 10–12); the sediment accumulation rates and the convergence rates are significant negative correlations.

the slowing in the rate of Pacific convergence with East China, coupled with the simultaneous onset of the collision of India Plate with Eurasia, corresponds to the onset of an important tectonic rifting stage and development of deep lakes in the Nanxiang Basin. The Eh3 source rocks deposited in those middle Eocene lakes recorded a full suite of cycles from annual varves to ultra-long-period ~1.2-Myr modulation of obliquity. These climatic cycles during the middle Eocene super-greenhouse interval had an important influence on the development of highquality source rocks by controlling water depth and circulation. Acknowledgments We are grateful for the help from Mingsong Li during the writing process. We express our gratitude to the SINOPEC Henan Oilfield for their core samples and data. This research was funded by the National Natural Science Foundation of China (Grant No: 41730421 and 41772029) and the 13th Five-Year Plan's Major Science and Technology Programs of the SINOPEC (Grant No. ZDP1705). This work was also supported by the Programme of Introducing Talents of Discipline to Universities (No. B14031, B08030), and the Natural Science Foundation for Distinguished Young Scholars of Hubei Province of China (2016CFA051), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGCJ1703; CUGQYZX1705), and the Fundamental Research Funds for National Universities, China University of Geosciences (Wuhan). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.palaeo.2019.109253. References Barker, P.F., Diekmann, B., Escutia, C., 2007. Onset of Cenozoic Antarctic glaciation. Deep-Sea Res. II 54 (21), 2293–2307. Bridgland, D.R., Westaway, R., 2008. Climatically controlled river terrace staircases: a worldwide Quaternary phenomenon. Geomorphology 98, 285–315. Bridgland, D.R., Westaway, R., 2014. Quaternary fluvial archives and landscape evolution: a global synthesis. Proc. Geol. Assoc. 125, 600–629. Chen, Z., Wang, X., Hu, J., Yang, S., Zhu, M., Dong, X., Tang, Z., Peng, P., Ding, Z., 2014. Structure of the carbon isotope excursion in a high-resolution lacustrine Paleocene–Eocene Thermal Maximum record from central China. Earth Planet. Sci. Lett. 408, 331–340. Derrick, T.R., Bates, B.T., Dufek, J.S., 1994. Evaluation of time-series data sets using the Pearson product-moment correlation coefficient. Med. Sci. Sports Exerc. 26 (7), 919. Ding, H., Zhang, Z., Dong, X., Tian, Z., Xiang, H., Mu, H., Gou, Z., Shui, X., Li, W., Mao, L., 2016. Early Eocene (c. 50 Ma) collision of the Indian and Asian continents: constraints from the North Himalayan metamorphic rocks, southeastern Tibet. Earth Planet. Sci. Lett. 435, 64–73. Dong, T., He, S., Liu, G., Hou, Y., Harris, N.B., 2015. Geochemistry and correlation of crude oils from reservoirs and source rocks in southern Biyang Sag, Nanxiang Basin, China. Org. Geochem. 80, 18–34. Ghil, M., Allen, M.R., Dettinger, M.D., Ide, K., Kondrashov, D., Mann, M.E., Robertson, A.W., Saunders, A., Tian, Y., Varadi, F., Yiou, P., 2002. Advanced spectral methods for climatic time series. Rev. Geophys. 40 (1), 3–1-3-41. Gong, Y., Zhang, K., 2016. In: China University of (Ed.), Stratigraphic Fundamentals and Frontiers. Geoscience Press, Wuhan, pp. 266–292 (in Chinese). Gradstein, F.M., Ogg, J.G., Schmitz, M., 2012. In: Ogg, G. (Ed.), The Geologic Time Scale.

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