Geochemical Characteristics And Geological Significance Of The Wufeng And Lungmachi Formations Cherts In Chongqing Area, Sichuan Basin

The Wufeng and Lungmachi Formation were deposited from the the Late Ordovician Katian Stage to the Early Silurian Rhuddanian Stage in South China.This set of strata is widely distributed in Chongqing area of the Sichuan Basin.The lithology includes black shale,siliceous shale,cherts,bentonites occur,shell limestone and limestone.The authors systematically analyzed the black shale and organic matter enrichment of the Wufeng Formation and Lungmachi Formation,but relatively few studies have been conducted on silicalite.At present,there is controversy over the understanding of the siliceous source and sedimentary environment of the Wufeng and Lungmachi Formations.In this paper,the Late Ordovician to Early Silurian The Wufeng and Lungmachi Formation cherts in the Sichuan Basin are studied,combined with the regional tectonic setting,through the analysis of the petrology,geochemistry and paleontology of cherts,a comprehensive study was conducted on the siliceous source of cherts,the formation environment of cherts and their geological significance.In this study,a large number of graphitic fossils were found in the cherts of the Wufeng and Lungmachi Formations.Under the microscope,the rock is mainly composed of microcrystalline quartz.The siliceous organisms are mainly radiolarians,and a small number of spongy bone needles are found.XRF and icp-ms were used to study the geochemistry characteristics of 74 cherts samples in the study area.The results showed that the Si O2 content in the study area was 70.17%⁹5.75%?average is 82.09%?,Amount of rare earth elements?? REE?as a whole has changed a lot,from 14.66 ppm to 322.92ppm?average is 133.41ppm?.Through Al-Fe-Mn ? Fe/Ti-Al?Al+Fe+Mn?? Al₂O₃/TiO₂-Al/?Al+Fe+Mn??Fe₂O₃/TiO₂-Al₂O₃/?Al₂O₃+Fe₂O₃?and?La/Ce?N-Al₂O₃/?Al₂O₃+Fe₂O₃?figures,Combined with geological tectonic structure for comprehensive analysis,It is believed that during the sedimentary period of the Wufeng Formation,The influence of the Chinese Plate and the Yangtze Plate extrusion in the upper Yangtze region,The ancient uplift structure expands continuously,The sea level is rising,gradually,the semi-confined deep-water shelf sedimentary environment surrounded by the ancient uplift and platform was formed.Multiple transgressive effects are brought into a large number of terrigenous detrital materials.To create conditions for the formation of siliceous rocks;Along with the formation of the Leshan,the Longnv temple ancient uplift and the Xuefeng mountain ancient uplift during the sedimentary period of the Lungmachi Formation.Sea level rise,Water retention of oxygen,Land clastic input increased,Mainly for deep water shelf sedimentary environment,once again,conditions were created for the formation of organic and siliceous rocks.