Geochemical Characteristics and Sedimentary Environment of the Cambrian Carbonates in Zhongtiaoshan Area

The characteristics of sedimentary lithology in north China reveals that intertidal-subtidal deposition dominated the Xinji period of early Cambrian; while sea level was relatively stable during the Mantou period, large amount of subtidal oolitic shoals began to develop in the Maozhuang-Zhangxia period; in the early-middle Cambrian, Zhangxia period experienced the maximum transgression of this era; then with the fall of sea level, the tidal-facies dolostones were deposited during the late Cambrian. To discuss the redox environment of Cambrian deposition in north China, the Cambrian carbonate samples in Zhongtiaoshan mountain area are systematically collected, and geochemically analyzed. The geochemical research indicates that most δ¹³C isotope of these carbonates are between -1.23 and 0.34 and reflect the relative shallow sedimentary environment; the negative δ¹³C isotope during early Cambrian is changed step by step into a positive value during the middle of Cambrian, and become negative again in the end of the era, which demonstrate that the Cambrian sea level in this area had undergone a transition from shallow to deep, and then from deep to shallow. The maximum transgression took place in the mid-Cambrian Zhangxia period, which is consistent with the implications from lithological characteristics. The lower negative δ¹⁸O value shows that the Zhongtiaoshan area used to situate in relatively restricted waters. LREE/HREE ratios indicating enrichment of the light REE and deficiency of the heavy REE agrees with the Chondrite-normalized pattern. The δEu,δCe and La/Yb ratios suggest that the sea level during early and late Cambrian have both experienced several obvious fluctuations, which indicates that the redox environment of the sedimentary water body might occur repeatedly during the Cambrian carbonate deposition.