Geochemical Characteristics and Paleoenvironmental Significance of Lower Cambrian Maidiping and Qiongzhusi Formations in Southwestern Sichuan Basin:A case study of well JS1
[Objective]The Ediacaran-Cambrian,as a key transition period in geological history,saw a dramatic transformation of the Earth system in terms of tectonic evolution,climate fluctuations,marine environment and bio-logical community evolution. It is of great significance to study in depth the sediment sources,paleo-water depth,paleosalinity and redox conditions during this period.[Methods]Samples from the Maidiping Formation and the Qiongzhusi Formation of the well JS1 in the southwestern part of the Sichuan Basin were selected and analyzed in detail for total organic carbon (TOC),major elements,trace elements and rare earth elements (REE).[Results and Conclusions]The material sources of the Maidiping and Qiongzhusi Formations were found to be relatively consis-tent. The parent rock primarily consists of sedimentary rock and granite,and the material source area was created in the environment found at the continental margin. The Maidiping Formation contains more light rare earth elements (LREE) than heavy rare earth elements (HREE) ((La/Yb)N=6.05),slightly to the right,and the average δEu is slightly<1,indicating normal seawater deposition in a shallow water body that was not significantly affected by hydro-thermal action. The Qiongzhusi Formation has obvious LREE differentiation ((La/Yb)N=7.69),LREE enrichment,HREE depletion and an obvious right dip,with weak negative δCe and δEu anomalies,possibly related to intermit-tent hydrothermal activity. In addition,comparative analyses for different wells have shown that the water body during the sedimentary period of the Maidiping Formation was in an oxidizing environment,and the sedimentary system in the region ranges from tidal-flat to shallow-shelf facies. The Qiongzhusi Formation as a whole belongs to an anaerobic environment,with a reducing environment its base and the degree of anoxia decreasing upwards. The regional develop-ment was a shallow-to deepwater shelf sedimentary system.
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