Elsevier

Cretaceous Research

Volume 62, July 2016, Pages 172-182
Cretaceous Research

Late Cretaceous marine fossils and seawater incursion events in the Songliao Basin, NE China

https://doi.org/10.1016/j.cretres.2015.10.025Get rights and content

Highlights

  • Songliao Basin is the largest non-marine oil-bearing basin in China.

  • Presence of marine-related fossils found in the sediments of the Songliao Basin.

  • Evidence supports periodic seawater incursions into the palaeo Songliao Lake.

  • Seawater incursion events enhanced the preservation of organic matter.

Abstract

The Songliao Basin is the largest non-marine oil-bearing basin in China. Because of the absence of substantial evidence, the hypothesis of seawater incursion events into the Songliao Basin remains controversial. The presence of marine fossils could provide direct proof to support this supposition. Here, we report new discoveries of foraminifera, calcareous nannofossils, brackish dinoflagellates, and other marine and brackish-water fossils to support the suggestion of seawater incursion events in the Songliao Basin. Relatively abundant benthic and planktonic foraminifera, calcareous nannofossils, marine and brackish-water dinoflagellates, fish, and bivalves have been discovered in Members 1 and 2 of the Nenjiang Formation, a few foraminifera and brackish-water dinoflagellates have been found in the lower Qingshankou Formation, and just a few brackish-water bivalves have been found in the uppermost Qingshankou Fm. Based on the presence of marine molecular fossils and other evidence, we suggest that relatively large seawater incursion events occurred during the sedimentation of the lower Nenjiang Fm., relatively smaller seawater incursions occurred during the deposition of the lower Qingshankou Fm., and possibly a very small seawater incursion occurred during the sedimentation of the uppermost Qingshankou Fm. These seawater incursion events in the Songliao Basin were controlled by regional tectonic activity, evolution of the palaeo Songliao Lake, and global sea level change. These periodic seawater incursions brought marine biota into the palaeo Songliao Lake.

Introduction

The Cretaceous represents the warmest periods in the Mesozoic, with high sea level and oceanic anoxic events (Schlanger and Jenkyns, 1976, Haq et al., 1987; Skelton, 2003, Jenkyns, 2010, Hu et al., 2012). However, our knowledge of Cretaceous terrestrial climatic change remains weak, which is largely because of the fragmentary nature of the continental stratigraphic record (Wu et al., 2009, Wu et al., 2014). The Songliao Basin in northeast China is one of the largest Cretaceous continental rift basins in the world. The palaeo Songliao Lake was at its greatest extent in the Late Cretaceous, depositing continuous lacustrine sediments, which provide a unique opportunity to study the Cretaceous terrestrial biota, palaeoenvironment, and palaeoclimate. Two large transgressions occurred during the late Turonian (lower Qingshankou Formation) and late Santonian–early Campanian (lower Nenjiang Formation), depositing suites of dark mudstone, black shale, and oil shale (Huang et al., 1998, Feng et al., 2011). During the Late Cretaceous, the palaeo Songliao Lake was a large lake, which was not far from the western Pacific Ocean (Gao et al., 1992, Wang et al., 2013). Thus, it is possible that the Songliao Basin experienced seawater incursion events during these periods (Wang et al., 2001, Xi et al., 2011a, Xi et al., 2011b, Wang et al., 2013, Hu et al., 2015).

Since the 1950s, researchers have proposed that seawater incursion events occurred frequently in the palaeo Songliao Lake and that such events were closely related to the development of the oil-source rocks found within the basin (Hu et al., 2015). Evidence of seawater incursion events includes marine and brackish-water fish, bivalves, dinoflagellates, shark teeth, calcareous nannofossils, foraminifera (Gu, 1976, Zhang et al., 1977, Gao et al., 1992, Ye and Wei, 1996, Xi et al., 2011a), biological marker dinoflagellate steranes, 24-n-propyl and 24-isopropyl cholestanes (Hou et al., 2000, Feng et al., 2011, Hu et al., 2015), and stable sulphur isotopes and elements (Wang et al., 2001, Huang et al., 2013). However, fossil evidence supporting the hypothesis of seawater incursions remains poorly studied, and it is difficult to use geochemical parameters to distinguish non-marine and marine environments directly. Consequently, any suggestion regarding the occurrence of marine water incursions is doubted (Huang et al., 1999, Li et al., 2000, Zhang et al., 2014).

The Cretaceous Continental Scientific Drilling project in the Songliao Basin (CCSD-SKI) offers a unique opportunity to investigate the nature of the non-marine Upper Cretaceous palaeoenvironment (Gao et al., 2008, Gao et al., 2009a, Gao et al., 2009b, Wang, 2013). The sedimentary sequence has been studied in two boreholes (SKI south and north): the lower 959.55 m from the south core (SKI(s)), and the upper 1636.72 m from the north core (SKI(n)) (Fig. 1). The south core comprises the upper Quantou (K2q), Qingshankou (K2qn), Yaojia (K2y), and lower Nenjiang formations (K2n), while the north core comprises the middle to upper Nenjiang (K2n), Sifangtai (K2s), and Mingshui formations (K2m). Since the first discovery of foraminifera in the Lower K2n SKI(s) (Xi et al., 2011a, Xi et al., 2011b), relatively abundant foraminifera, calcareous nannofossils, and brackish-water dinoflagellates have been found more recently in the SKI(s) and SKI(n).

In this study, based on the first report of foraminifera in the Lower K2n of SKI(s) (Xi et al., 2011a, Xi et al., 2011b), we report new discoveries of foraminifera, calcareous nannofossils, brackish-water dinoflagellates, and other brackish-water fossils, together with previously discovered marine and brackish-water fossils, to discuss the likelihood of seawater incursion events of the Songliao Basin. This will help us better understand the seawater incursion events, palaeoenvironment, and formation of the hydrocarbon source rocks within this basin.

Section snippets

Geological setting

The Songliao Basin is a Mesozoic–Cenozoic intracratonic basin in northeastern China. The modern fault-bounded Songliao Basin is about 750-km long and 300–370-km wide, with a total area of 26 × 104 km2. It is one of China's most prolific oil- and gas-producing sedimentary basins and it contains a complete Cretaceous sequence of terrestrial sediments (Gao et al., 1994, Huang et al., 1998, Wan et al., 2013). The Sk1 boreholes are situated in the Qijia-Gulong Depression, which is one of the major

Studied material

Samples for the study of the foraminifera and ostracods were collected on average at 1-m intervals. Samples of 100 g dry weight were dissolved in deionised water for several weeks prior to sieving through a 200-μm sieve. Foraminifera were picked from the samples under a low-power binocular microscope and stored on micropalaeontological specimen slides. Scanning electron microscope images of the foraminifera were taken at the China University of Geosciences (Beijing).

Samples from the lower K2n

Foraminifera

Relatively abundant foraminiferal tests were identified from Member 1 (K2n1) and lower Member 2 (K2n2) of the Nenjiang Fm. About nine species of eight genera were identified from K2n1 and lower K2n2 of SKI(s), including benthic calcareous foraminifera: Gavelinella sp., Anomalinoides sp., Haplophragmoides sp., and Pullenia sp., benthic agglutinated foraminifera: Karrerulina hokkaidoana, Clavulinoides sp., and planktonic foraminifera: Archaeoglobigerina blowi, Archaeoglobigerina cretacea, and

Distribution and ages of the brackish-water fossils in the Songliao Basin

Brackish-water and sea-related fossils have been discovered in the K2qn1 and lower K2qn2, uppermost K2qn2+3, and K2n1+2 (Fig. 5). Most of the brackish-water fossils were found in the K2n1+2; a few brackish-water fossils were found in the K2qn1 and lowermost K2qn2, and just a few possibly brackish-water fossils were discovered in the uppermost K2qn2+3, e.g., bivalves, ostracods, and the biomarker 24-isopropyl cholestane (1320 m of SKI(s)). The distribution of 24-isopropyl cholestane has good

Conclusion

Discovery of relatively abundant benthic and planktonic foraminifera, nanofossils, brackish-water algae, fish, and bivalves in the K2qn1, lowermost K2qn2, uppermost K2qn3, and K2n1+2 provide evidence supporting the hypothesis of periodic seawater incursions into the Songliao Basin. These seawater incursion events into the Songliao Basin were controlled by regional tectonic activity, evolution of the palaeo Songliao Lake, and global sea level change. Furthermore, because of these periodic

Acknowledgements

This study was financially supported in part by the fund of the National Basic Research Program of China (973 Program, No. 2012CB822002), National Natural Science Foundation of China (41302008, 41172037, 41372110, 41172015), State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (123111), State Key Laboratory of Biogeology and Environmental Geology (GBL21314) and Beijing Higher Education Young Elite Teacher Project (YETP0665). Thanks to

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