Magnetostratigraphy of the Upper Cretaceous Nenjiang Formation in the Songliao Basin, northeast China: Implications for age constraints on terminating the Cretaceous Normal Superchron

Late Cretaceous stratigraphic division in the northwestern Songliao Basin, Northeast China: Evidence from palaeontology and palaeomagnetism

“…The magnetozone R1 corresponds to chron C33r in the Geomagnetic Polarity Timescale (GPTS) (Ogg, 2020). The sedimentation age from the upper Yaojia Formation to the lower Nengjiang Formation in borehole Zk09 is about 83–85 Ma, corroborating those of He et al (2012), Shen et al (2022) and Wu et al (2014).…”

“…Most samples' high‐stability ChRM components were separated between 20 and 60 mT or 200 and 550°C (Figure 6). However, some specimens required further thermal demagnetization up to 610°C to separate high‐stability ChRM components (Figure 6), indicating that greigite and/or magnetite dominated the remanence carriers (Shen et al, 2022). At least four (normally 6–10) straight demagnetization points above 15 mT create a linear trend toward the origin of the orthogonal vector demagnetization plots to determine the ChRM directions.…”

“…The First and Second members of the Nenjiang Formation produce dark grey mudstone and argillutite with sandy shale and oil shale interbeds in the basin's centre, interbedding argillutite with sandstone in the northern and western basin margins (HBGM, 1993). In the GPTS, the First and Second members of the Nenjiang Formation's Chrom corresponded to C34n, while the Third member to Chron C33r (Shen et al, 2022; Wu et al, 2014). According to the lithology of the Zk09 borehole, the strata at 41.0–88.0 m can be correlated to the First, Second, and Third members of the Nenjiang Formation (Figures 3 and 4).…”

“…This sedimentary cycle's strata have a total thickness ranging from 100 to 600 m, gradually thin westward and northward away from the basin's centre, and dip at a low angle toward the basin centre (Wang et al, 2016). Its age was proposed to range from Cenomanian, Turonian, and Coniacian to early Campanian (He et al, 2012; Shen et al, 2022; Wu et al, 2009; Wu et al, 2014). Episodic tectonism uplifted the basin and folded the strata during the structural depression stage (Wang et al, 2016).…”

“…The deposition thickness thinned westward between the Cretaceous Qingshankou Formation and Nenjiang Formation (Gao & Cai, 1997; Li et al, 2021), suggesting that the lake expanded rapidly to practically the whole basin (Feng et al, 2010; Wang et al, 2016). Palaeomagnetic, palaeontologic, and lithostratigraphic dating from the Central Depression of the Songliao Basin determined the chronological boundary of these Late Cretaceous lithostratigraphic units (He et al, 2012; Shen et al, 2022; Wang et al, 1995; Wu et al, 2014; Yu et al, 2019). The structural depression and inversion processes of the Songliao Basin created the Western Slope of the Songliao Basin (Feng et al, 2010; Wang et al, 2016).…”