New geochronological constraints for the Upper Cretaceous Nenjiang Formation in the Songliao Basin, NE China
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 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, Xi et al., 2018), sea water incursion events (Hou et al., 2000, Hu et al., 2015, Cao et al., 2016a), 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 Cenomanian-Turonian 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) U–Pb zircon geochronology from the east borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-IIe), which is correlated to 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 U–Pb zircon geochronology and previously published magnetostratigraphy of the south borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-Is) (He et al., 2012).
Section snippets
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
SIMS U–Pb zircon geochronology analytical methods
We separated zircon grains from the bentonite sample for SIMS U–Pb dating. Four samples for U–Pb 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
Results
Sample S1236 yields euhedral to subhedral zircons with a range of sizes from 40 to 200 μm 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 U–Pb 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
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
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 U–Pb zircon data for intercalated bentonite layers indicate that the bentonite bed
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 U–Pb analyses. This research used
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