Geochemical features and geological implications of the Lower Cambrian Qiongzhusi Formation in the central Yunnan region

Key words: 
• Central Yunnan /
• The Lower Cambrian Qiongzhusi Formation /
• Sedimentary environment /
• Weathering and paleo-climate /
• Provenance

Abstract: [Objectives] The lower Cambrian Qiongzhusi Formation in the western Yangtze Block is a good reservoir for shale gas, and it has been well targeted for shale gas exploration in the Sichuan Basin. On the contrary, the shale gas exploration in the central Yunnan is underestimated. To provide geological supports for shale gas exploration in the central Yunnan, this study focuses on siliciclastic rocks of the Lower Cambrian Qiongzhusi Formation, and investigates its sedimentary and diagenetic environment, source rocks, nature of weathering and paleo-climate, tectonic setting of source regions. [Methods] Targeted on the Yinmachi section of the Liangwangshan area, it conducts stratigraphic logging of the section, microscopic petrography and whole-rock geochemistry works on the Qiongzhusi Formation. [Results] The Qiongzhusi siliciclastic samples are mainly siltstones with low compositional maturity. All analysed siltstone samples yield U/Th ratios of 0.23-0.81, V/Cr ratios of 1.38-3.56, V/Sc ratios of 7.36-18.9, Ni/Co ratios of 2.34-4.85, Ba/Sr ratios of 9.04-16.2, La/Ce ratios of 0.48-0.57, (La/Yb)n ratios of 0.54-1.25 and δEu values of 0.62-0.67. [Conclusions] Integrated with published data of the Qiongzhusi siltstone and mudstone samples in central Yunnan, it concludes that the Qiongzhusi Formation in the central Yunnan was predominantly sourced from felsic magmatic rocks and siliceous sedimentary rocks, with minor mafic components. After chemical weathering and transportation, the detritus were deposited on an oxic shallow marine continental shelf, and were influenced by hydrothermal fluids during its sedimentation and diagenesis processes. Source regions of the Qiongzhusi detritus possibly include local Kangdian oldland and exotic Gondwana continents. During the early Cambrian, source regions (include the western Yangtze) were likely sitting on an active continental margin with a warm and humid weather condition.