In search of the inland Carnian Pluvial Event: Middle–Upper Triassic transition profile and U–Pb isotopic dating in the Yanchang Formation in Ordos Basin, China

Abstract

The Carnian Pluvial Event (CPE) in the early Late Triassic is a global climate change. The CPE is not only recorded in the ocean but also on the continent. It has been well‐documented in the marine environments in Tethys and boreal areas. However, it is less reported for the terrestrial. As a global precipitation event, the CPE on the supercontinent Pangaea is related directly to the sudden increase in argillaceous transportation in the marine carbonate platform. Here, we report a chronological study of the Yunmeng profile in south‐eastern Ordos Basin, China where the continuous lacustrine deposits developed with several tuff interlayers. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) U–Pb isotopic dating of zircons from three bentonite samples show weighted mean ages of 234.1 ± 2.4 Ma, 234.8 ± 2.1 Ma, and 236.0 ± 1.7 Ma, respectively. Based on the review of isotopic dating, sedimentary and palaeontological characteristics of the Triassic Yanchang Formation, the Middle and Upper Triassic boundary is located at the bottom of Chang 7 oil layer (YC7) in the Yanchang Formation at the Yunmeng profile in the Ordos Basin. Therefore, the Yunmeng profile provides a potential case for understanding the inland CPE and might link the terrestrial and oceanic systems in the Carnian.

Vegetation changes and climate shift during the latest Ladinian to the early Carnian: Palynological evidence from the Yanchang Formation, Ordos Basin, China

“…The deposition of Triassic sediments in the Ordos Basin can be divided into two cycles (IGCAGS, 1980;PCOC, 1992): 1) the Lower and Middle Triassic cycles, including the Liujiagou, Heshanggou, and Ermaying formations, which are widespread throughout the entire North China Craton; 2) the Upper Triassic cycle composed of the Yanchang Formation (sensu lato) comprising alluvial, fluvial, deltaic, and lacustrine sedimentary rocks, approximately 1,000-1,300 m in thickness. The Yanchang Formation (Pan, 1934) or Group (ECSDC, 2000) can be divided into five members (Y1-Y5) (IGCAGS, 1980) and 10 oil layers or members (YC1-YC10), based on the depositional sequences (S1-S5), regional and local indicator beds (K0-K9), and bentonite beds (B0-B6) (PCOC, 1992;Pang et al, 2010;Yang et al, 2017;Deng et al, 2018;Zhang et al, 2019;Sun et al, 2020). The Chang 7 oil layer (member) of the Yanchang Formation (YC7) is one of the best layers of source rocks in the Ordos Basin (PCOC, 1992), shale oil being the most important fossil energy source in China and elsewhere (Jin et al, 2019;Liu et al, 2022).…”

“…The shale has also been proposed for the division of Triassic strata and their geological age. Its presence was assigned to the uppermost Tongchuan Formation (IGCAGS, 1980;Tong et al, 2019) or the lowest Yongping Formation (Li et al, 2016), while its age was assigned to either the Middle (IGCAGS, 1980;Deng et al, 2018) or Late Triassic (Li et al, 2016;Tong et al, 2019;Sun et al, 2020). However, the Yanchang Formation (sensu lato) is widely used (Bureau of Geology and Mineral Resources of Shaanxi Province, 1998) in geological references (Table 1).…”

A Late Triassic vegetation record from the Huangshanjie Formation, Junggar Basin, China: possible evidence for the Carnian Pluvial Episode

“…The pollen Aulisporites astigmosus, a presumed hygrophytic taxon (Roghi 2004), is conspicuous in its absence from our dataset, and indeed from other Late Triassic palynological records of North China. This species is a distinctive marker for the CPE across western Laurasia (Fijałkowska-Mader et al 2021 and references therein), but has not been found in the Junggar (IGCAGS AND IGXBGMR 1986;Jiang et al 2018), Tarim (Liu 2003) or Ordos (Qu 1980;Sun et al 2019) basins. This discrepancy likely reflects floral provincialism across Laurasia at this time, giving rise to distinct palynofloras (e.g.…”

A giant lacustrine flood‐related turbidite system in the Triassic Ordos Basin, China: Sedimentary processes and depositional architecture

“…From the chronological studies in the Ordos Basin (Wang et al, 2014;Deng et al, 2018;Liu et al, 2018;Chen et al, 2019;Zhang et al, 2019), the YC 7 times correspond to the Ladinian-Carnian age (Fig. 2A), which was characterized by increased global humidity (Trotter et al, 2015;Bernardi et al, 2018;Dal Corso et al, 2018, 2020Sun et al, 2019).…”