There is no evidence for a great unconformity of over 300 Ma between the Mesoproterozoic Xiamaling Formation and the “Neoproterozoic” Longshan Formation in the North China Craton, China
image: (a) Stratigraphy and age of the Meso-Neoproterozoic in the Yanshan-Liaoning area; (b) regional geological sketch and location of the study area; (c) figure denoting enlarged section from the study area denoted by red box. From east to west, the sections are: 1, Liujiang section, Qinhuangdao, Hebei Province (Teng and Sun, 1999; Xu, 2015); 2, Zhaozhuang section, Tangshan, Hebei Province (Chen, 1981); 3, Kuancheng section, Hebei Province (measured section); 4, Jixian section, Tianjin (Tian et al., 1990); 5, Miyun section, Beijing (measured section); 6, Longshan section, Changping, Beijing (Qi et al., 1999; Zhang et al., 2020); 7, Xishan section, Beijing (Qiao, 1976; Zhang C et al., 2017); 8, Zhaojiashan (and Liangjiazhuang) section, Huailai, Hebei Province (measured section); 9, Xiahuayuan section, Xuanhua, Hebei Province (Song and Zhang, 1983). view more
Credit: ©Science China Press
Recently, the journal SCIENCE CHINA Earth Sciences published a research by Professor Hongwei KUANG, Dr Nan PENG and Professor Yongqing LIU from the Institute of Geology, Chinese Academy of Geological Sciences. Based on multidisciplinary evidence from the sedimentary sequences and the continuity, the youngest detrital zircon age spectra, and geochemical indicators of the transition between the Xiamaling and the Longshan formations proved there probably no "a great sedimentary hiatus" or " a great unconformity" for more than 300 Ma between the Xiamaling and the overlying Longshan Formation in the North China Craton. They argue that the Xiamaling Formation and the Longshan-Jingeryu formations might be deposited continuously in the late Mesoproterozoic. On this basis, the paleogeography of the Xiamaling-Longshan-Jingeryu formations in Yanliao area of the North China Craton has been reconstructed.
Field geological records show that the pebbly sandstone at the bottom of the Longshan Formation does not have the properties of basal conglomerate. The Xiamaling and the Longshan formations are composed of multiple normal sedimentary cycles with coarse to fine sandstone/mudstone of different thicknesses. In ascending order, they develop a continuous clastic rock sedimentary sequence composed of a shallow sea, shore, and tidal flats from the Xiamaling Formation-Longshan Formation-Jing’eryu Formation.
The youngest detrital zircon age peaks of the Longshan Formation and the Jing’eryu Formation are both older than 1.6 Ga, lacking provenance information for the volcanic magmatic events prior to deposition of the Longshan Formation. This is highly consistent with the detrital zircon age spectra of the underlying Mesoproterozoic sedimentary rocks, which indicates that they share the same provenance, which further indicates that the Xiamaling Formation and its lower strata are not the provenance of the Longshan Formation.
From the top of the Xiamaling Formation to the lower part of the Longshan Formation, the δ7Li displays a rising trend, while Li content decreased. CIA and other series of weathering indices decreased, K2O/Al2O3 decreased, Na2O/ Al2O3 increased, and FeO/Fe2O3 decreased. Li/Al, V/Cr, and V/(V+Ni) demonstrated a slight increase or decrease in the interface. The ratios of Ce/Ce*, Eu/Eu*, Rb/Sr, and U/Th showed a pronounced change at transition interface. Ce/Ce* is positive, Eu/Eu* is negative, Rb/Sr and Al/Si indicated a pronounced decrease, with an increase of U/Th. These geochemical indices confirm that there is no weathering crust between Xiamaling Formation and Longshan Formation, and that the intensity of continental weathering gradually weakens.
Although the existence of large unconformity between the Xiamaling and Longshan formations needs more precise multidisciplinary evidence, especially the geochronological data, at present, this "great unconformity" has not been supported by basin evolution, sedimentary infilling, sedimentary and tectonic environment and geochemistry, etc. Therefore, this study suggested that the Longshan and Jingeryu formations should be regarded as conformable or continuous sedimentary products with Xiamaling Formation, and together placed at the upper part of the “Undefined System”. Meanwhile, this study provides new evidence for further revealing the evolution process of the North China Craton and the global paleogeography, paleoenvironment and paleotectonic evolution during the late Mesoproterozoic.
See the article:
Is there a great unconformity between Xiamaling and Longshan formations in the North China Craton?