Zircon U–Pb Dating of Tuff Layers in the Middle–Upper Triassic Strata of Successions and Sedimentation of Chang 7 Member in the Ordos Basin, China

Abstract

During the Late Triassic, the southern margin of the Ordos Basin was influenced by the Indosinian Movement, in response to closure of the Paleo-Tethys Ocean. Thinly bedded tuff layers are exposed near the base of the Middle–Upper Triassic formations (Chang 8/7/6 members) in the Ordos Basin. Tuff samples from drilling core (well An-35) and outcrop (Motiangou section) samples in the Yanchang Formation were studied by the detailed field work. Igneous zircon crystals were picked and analyzed by the instruments of cathodoluminescence and laser ablation inductively coupled plasma mass spectrometery (LA-ICP-MS). The timing of volcanic activity was determined by means of high-precision zircon age. Combining tectonic setting and paleogeographic reconstruct, the ages of tuff layers indicate that volcanic activities occurred at three peaks (241.6 ± 1.1 Ma, 240.2 ± 1.1 Ma, and 233.5 ± 1.2 Ma) from the top of Chang 8 Member to the Chang 7 Member. The results implied that the volcanic activities lasted from Ladinian to Carnian, as well as define the sedimentary duration of the Chang 7 Member with a deposition rate of 1.34 cm/ka.

Volcanically Driven Terrestrial Environmental Perturbations during the Carnian Pluvial Episode in the Eastern Tethys

“…The Chang 8 Member is witnessed to be part of the Middle Triassic by tuff zircon U-Pb dating (241.6 ± 1.1 Ma) at the Motiangou outcrop section in the Tongchuan region (Lu et al, 2022), which is close (450 m south) to the Yishicun section in this study. In palynological biostratigraphy, the Chang 8 1 and Chang 7 3 submembers correspond to the Ladinian and Carnian stages, respectively, as indicated by the presence of Lundbladispora watangensis-Taeniaesporites combinatus and Lundbladispora communis-Discisporites granulus assemblages Furthermore, the age results for Chang 7 may vary owing to different study locations and analysis methods (i.e., LA-ICP-MS, TIMS, and SHRIMP) (Jin et al, 2021).…”

“…2). The deposition of the Chang 7 Member is linked to the Middle-Late Triassic, based on zircon U-Pb dating of the tuff and astronomical data (Lu et al, 2022;Li and Yang, 2023). The Chang 7 Member is classified into sub-lithological units, such as Chang 7 3 , Chang 7 2 , and Chang 7 1 , which are the focus of this study.…”

“…6b, c). Recent studies (e.g., Zhang W et al, 2017;Lu et al, 2022;Li and Yang, 2023) have demonstrated that the lower part of Chang 7 3 dates to the Ladinian and the middle-upper parts of Chang 7 3 , Chang 7 2 , and Chang 7 1 belong to the Carnian stage based on astrochronology and zircon U-Pb dating. Sun et al (2020) proposed that the Ladinian-Carnian boundary (LCB) is located at the base of the Chang 7 3 submember in the Yunmeng section.…”

“…The granitoids evolved in a continental arc context associated with Mianlue Ocean subduction between the South China Block and the South Qinling Belt (Wang et al, 2013). Therefore, the Middle-Late Triassic tuffs and bentonites are interbedded within the Chang 7 oil shale of the Ordos Basin, probably because of simultaneous tectonic-magmatic processes in the Qinling orogenic belt (Qiu et al, 2014;Xu et al, 2019;Lu et al, 2022;Li and Yang, 2023).…”