Sedimentary and geochronological studies on tuff levels from Triassic strata in the Nanzhao Basin, North Qinling Orogenic Belt, and their geological significance

Abstract

The Ordos Basin is the second largest oil‐ and gas‐bearing sedimentary basin in China. This basin formed during the Triassic. At present, the location of the basin margins is still highly debated in the literature, especially its eastern and southern margins. The Triassic Yanchang Formation extensively outcrops in the Ordos Basin, and its distribution can provide a significant constraint on the basin extension. In this study, we focus on the measurement and correlation of the Middle–Late Triassic sedimentary strata from Nanzhao Basin, which is located to the south‐east of the present‐day Ordos Basin. The results of this study indicate that the sedimentary sequence of Triassic strata from Taizishan Formation and Taishanmiao Formation in the Nanzhao Basin is comparable with that of the Yanchang Formation in the Ordos Basin. Tuff intercalations in clastic rocks from Taizishan Formation in the Nanzhao Basin were studied with zircon chronology and Hf isotope analysis. The results show that the tuff sample has concord U–Pb age of 234.6 ± 1.5 Ma, which is considered as the boundary age between Middle and Late Triassic with a similar age in respect to that of the Yanchang Formation. The TDM2 values for the studied sample range from 1.0 to 1.4 Ga with the peak at 1.2 Ga, which is also similar to TDM2 values for tuffs encountered in the Yanchang Formation. On the other hand, the corresponding εHf(t) values of tuff in Nanzhao Basin range from −1.1 to +4.2 with an average of 1.2 and peak at 2, which are different with respect to those of the Yanchang Formation. According to the geochronological data, the tuffs in Nanzhao Basin might be mainly derived from a depleted‐mantle source from Late Meso‐Proterozoic period, and contaminated by a continental Early–Middle Meso‐Proterozoic crust. Therefore, we suggest that in Middle–Late Triassic, the southeastern margin of the Ordos Basin may have extended to Nanzhao area and the different εHf(t) values between tuffs in Nanzhao Basin and present Ordos Basin may be the result of scissor‐type collision of North China Block and South China Block during the Triassic.

Development characteristics of deepwater gravity flows and tight sandstone reservoir characteristics in Chang 6 in the Huaqing area, Ordos Basin, China

“…Qinling has experienced a complex and long process of plate orogeny, which mainly included three tectonic evolutionary stages. The sedimentary period of Chang 6 3 in the Yanchang Formation corresponds to the plate tectonic stage of the Neoproterozoic‐Middle‐Late Triassic main orogenic period (Liu, Han, et al, 2021a, Liu, Qiu, et al, 2021b). The specific manifestation was ocean‐continental subduction to continental‐continental subduction collision orogeny from the late Palaeozoic to the early Mesozoic and the middle and late Indosinian periods (Dang et al, 2018; Liu, Han, et al, 2021a, Liu, Qiu, et al, 2021b).…”

“…The supply of sediment sources has an extremely important influence on the distribution and reservoir characteristics of sand bodies in continental basins (Liu, Han, et al, 2021a; Liu, Qiu, et al, 2021b; Ye et al, 2022). Although previous researchers have had different opinions on the provenance direction of the Chang 6 3 reservoir in the Huaqing area, it is generally believed that the provenance mainly came from the southwest, northeast and north (Li et al, 2009; Liao, Li, et al, 2013a; Liao, Zhu, et al, 2013b; Sun et al, 2017).…”

“…T A B L E 6 Various types of reservoir microheterogeneity evaluation parameters Formation corresponds to the plate tectonic stage of the Neoproterozoic-Middle-Late Triassic main orogenic period (Liu, Han, et al, 2021a, Liu, Qiu, et al, 2021b. The specific manifestation was ocean-continental subduction to continental-continental subduction collision orogeny from the late Palaeozoic to the early Mesozoic and the middle and late Indosinian periods (Dang et al, 2018;Liu, Han, et al, 2021a, Liu, Qiu, et al, 2021b. Previous researchers studied the formation of gravity flows in the study area and the uplift process of the Qinling Mountains from zircon dating (Wang et al, 2017), core structure, development thickness of tuffaceous matter, distribution mode and continuity (Chen et al, 2019;Fu et al, 2019).…”

Triassic temporal and spatial migration of the provenance along the South Ordos Basin: Insights into the tectonic evolution of Eastern Palaeo‐Tethys Ocean

“…The ages of interbedded tuffs are from Liu et al. (2021), Sun, Li, et al. (2020), Zhao, Zheng, et al.…”

“…Late Triassic strata are essentially missing from the eastern NCB and the QB (Liu, Kuang, et al., 2015; Meng et al., 2019). The Triassic succession conformably overlies Permian strata in the Luoyi Basin (Figure 3; Li & Huang, 2013), but unconformably overlies the Proterozoic basement in the Nanzhao Basin (Figure 3; Liu et al., 2021). Therefore, the Luoyi Basin and SOB have similar continuities and lithologies and are generally studied as one basin, whereas the Nanzhao Basin is generally considered to be an intermontane basin.…”