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Xunlian Wang, China University of Geosciences (Beijing) (China)
Haijun Zhang, China University of Geosciences (Beijing) (China)
Lei Wang, China University of Geosciences (Beijing) (China)
Shihong Zhang, China University of Geosciences (Beijing) (China)
Guoying Xia, Tianjin Institute of Geology and Mineral Resources (China)
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An integrated study is carried out on biostratigraphy, sequence stratigraphy, rock magnetic stratigraphy, and event stratigraphy of the Carboniferous-Permian boundary (CPB) section at Xikou, Zhen'an County, Shaanxi Province, a typical section of the marine deposits in China. Four biozones, Streptognathodus elegantulus Zone, S. elongatus Zone, S. gracilis Zone, and S. isolatus Zone from below upwards, are established around the CPB. Consequently the CPB is set at the base of the S. isolatus Zone, which is 3.3 m lower than the boundary defined by the base of Pseudofusulina krotowi sphaeroidea-Dunbarinella Zone. Twelve 4th-order sequences are recognized in the deposits ranging from the Upper Carboniferous Xiaoyao'an to the Lower Permian Longlinian, which constitute 5 3rd-order sequences. Except the base of Xiayangjiahe'an all bases of the stages, including the bases of Xiaoyao'an, Shangyangjiahe'an, Fanjiahe'an, Yakou'an, and Longlinian, locate either on the key surfaces of sequences and sea-level changes or around them. This fact implies a close relationship between the key surface of the 3rd-order sequence and the chronostratigraphy boundary. A statistically evident correlation between magnetic susceptibility (MS) and carbonate microfacies as well as sequence indicates that sea-level change is one of the principal factors controlling the MS variations in carbonates. In addition, almost all elevated MS values occur around stratigraphic boundaries, which suggest that the MS may be considered as one of the important indices for stratigraphic classification and correlation.
Two distinct radiation events of fusulinid fauna are recognized in Xikou area from the Upper Carboniferous to the Lower Permian. The first took place at the base of Occidentoschwagerina alpina-O. postgallowayi Zone, i.e. the base of Xiayangjiahe'an, and the second at the base of Mccloudia regularis-Paraschwagerina fragosa-Robustoschwagerina xiaodushanica Zone, i.e. the base of Fanjiahe'an. The important bioevent, sudden appearance and rapid propagation of Rugose corals with third order or higher than third order septa in the Late Carboniferous-Early Permian, occurs in the lower part of Pseudofusulina urdalensi Zone.
The integrated approach with biostratigraphy, sequence stratigraphy, rock magnetic stratigraphy, and event stratigraphy of the CPB section at Xikou reveals that the records of most geological events occur on relatively accordant horizons. The records of all kinds of geological events exhibit the best accordance at the base of the Yakou'an, second at the bases of Longlinian and Fanjiahe'an. Unlike the 3 boundaries mentioned above, the record of the CPB defined by the first appearance of the S. isolatus is not easily recognized by other means because neither is it an evolutionary surface nor is there an obvious MS variation as well as sea-level change. Therefore, the CPB defined by the first appearance of the S. isolatus could not be used easily in the practice work.
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