(a) Stratigraphy and Age of the Meso-Neoproterozoic in the Yanshan-Liaoning Area
In a recent publication by Professor Hongwei Kuang, Dr. Nan Peng, and Professor Yongqing Liu from the Institute of Geology, Chinese Academy of Geological Sciences, new research regarding the stratigraphy and age of the Meso-Neoproterozoic in the Yanshan-Liaoning area was presented. This study incorporates multidisciplinary evidence from sedimentary sequences, detrital zircon age spectra, and geochemical indicators to investigate the transition between the Xiamaling and Longshan formations. The findings challenge previous notions of a significant sedimentary hiatus or unconformity between these formations, suggesting instead that they were deposited continuously during the late Mesoproterozoic. Furthermore, the researchers have reconstructed the paleogeography of the Xiamaling-Longshan-Jingeryu formations in the Yanliao area of the North China Craton based on these findings.
(b) Regional Geological Sketch and Location of the Study Area
To provide context for the study, a regional geological sketch and the location of the study area are depicted in Figure 1. The study area is denoted by a red box, and it encompasses several sections from east to west, including the Liujiang, Zhaozhuang, Kuancheng, Jixian, Miyun, Longshan, Xishan, Zhaojiashan (and Liangjiazhuang), and Xiahuayuan sections. These sections were either previously measured or obtained from literature sources. They provide valuable geological information that contributes to the overall understanding of the stratigraphy and age of the Meso-Neoproterozoic in the Yanshan-Liaoning area.
(c) Figure Denoting Enlarged Section from the Study Area Denoted by Red Box
Figure 2 displays an enlarged section of the study area denoted by the red box in Figure 1. This section provides a detailed view of the geological features and sedimentary sequences present in the area. It serves as a useful reference for analyzing the stratigraphic and age relationships between the Xiamaling and Longshan formations.
To investigate the sedimentary evolution and geological characteristics of the Xiamaling-Longshan-Jingeryu formations, various geochemical indicators were employed. These indicators include major and trace elements such as Al2O3, Na2O/Al2O3, Sr/Ba, Rb/Sr, FeO/Fe2O3, as well as the chemical index of weathering (CIW), plagioclase alteration index (PIA), and Li isotopes. The results indicate a weakening of chemical weathering during the transition between the Xiamaling and Longshan formations and the absence of a weathering crust. These findings contribute to our understanding of provenance, continental weathering, paleoclimate, and redox environment during that time period.
The field geological records reveal that the pebbly sandstone at the bottom of the Longshan Formation does not possess the characteristics of a basal conglomerate. The Xiamaling and Longshan formations exhibit multiple normal sedimentary cycles consisting of coarse to fine sandstone/mudstone layers of varying thicknesses. These cycles represent a continuous clastic rock sedimentary sequence that documents the development of a shallow sea, shore, and tidal flats from the Xiamaling Formation to the Longshan Formation and Jing’eryu Formation.
The detrital zircon age peaks of the Longshan Formation and Jing’eryu Formation are both older than 1.6 Ga, indicating a common provenance with the underlying Mesoproterozoic sedimentary rocks. This suggests that the Xiamaling Formation and its lower strata are not the source of sediment for the Longshan Formation. The sedimentary evolution and infilling response from strong subsidence to rift valley drift are evident in the depositional environment and stratigraphic evolution of the Xiamaling Formation to the Longshan Formation.
The geochemical indices further support these findings, indicating a gradual weakening of continental weathering and a significant change in the sedimentary environment at the transition interface. These observations align with the global-scale Columbia supercontinent rifting peak around 1.4 Ga during the Mesoproterozoic, leading to crustal extension and the formation of a narrow and deep aulacogen basin. The sedimentation patterns and relative sea levels recorded in this basin demonstrate a shift from deep and narrow to shallow and wide, with the largest rift area located in northwestern Hebei.
As the Longshan-Jing’eryu formations approached the end of their deposition, rifting may have begun in the east Liaoning rift valley. This resulted in a change in the paleogeographic pattern, with a shift towards higher elevations in the west (Yanliao) and lower elevations in the east (Shanhaiguan-east Liaoning) during the Diaoyutai-Nanfen stage. The Diaoyutai-Nanfen formations are believed to be contemporaneous with or slightly later than the Longshan-Jing’eryu formations. Subsequent uplift in the Yanliao areas halted deposition in the Jiao-Liao-XuHuai areas.
While the existence of a large unconformity between the Xiamaling and Longshan formations requires further precise geochronological data, the current research does not support the presence of this “great unconformity” based on basin evolution, sedimentary infilling, sedimentary and tectonic environment, and geochemical analysis. Instead, it suggests that the Longshan and Jingeryu formations should be considered continuous sedimentary products of the Xiamaling Formation. This study contributes to our understanding of the late Mesoproterozoic evolution of the North China Craton and provides insights into global paleogeography, paleoenvironment, and paleotectonic processes.
Reference:
Hongwei Kuang et al, Is there a great unconformity between Xiamaling and Longshan formations in the North China Craton?, Science China Earth Sciences (2023). DOI: 10.1007/s11430-022-1034-9
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