Lithofacies Palaeogeography Of The Maokou Period In SW Sichuan And Its Response To ELIP Event

The Permian strata in the Sichuan Basin are widely concerned by scholars at home and abroad due to their huge resources and unique paleogeographic pattern.In recent years,through extensive research on the Middle Permian in the Sichuan Basin,predecessors have obtained a large amount of research results in basin structure,diagenesis,reservoir,lithofacies,paleokarst and so on.However,the predecessors have some differences on the sedimentary pattern of the Maokou Formation in the Sichuan Basin.The research on the relationship between the Maokou period tectonic-sedimentary evolution and the Emeishan volcanic event in the southwestern Sichuan Basin is relatively lagging,which hinders the Sichuan Basin Understanding of the depositional pattern of the Permian Maokou Formation.This paper takes the Permian Maokou Formation and the Emeishan Large Igneous Rock(ELIP)eruption in the southwestern Sichuan area as the research objects,and discusses the relationship between the two.Through the theoretical guidance of sedimentology,lithofacies and paleogeography,combined with the field profile,drilling core,logging data and regional geological data in the area,the division plan of the Maokou Formation in the study area was determined,and the sedimentology of the Maokou Formation was studied,Paleontology,geophysics and other phase identification.On this basis,the sedimentary facies distribution characteristics were established using typical sections in the area and drilling to compare sedimentary characteristics,and the lithofacies palaeogeographic map of the Maokou Formation in the southwestern Sichuan Basin was drawn in sections.Finally,a comprehensive analysis of the control of the Emeishan volcanic event on the deposition of the Maokou Formation has established the Maokouian tectonic-sedimentary evolution model in the southwestern Sichuan.The study believes that the entire southwestern Sichuan area is a shallow-water carbonate platform sedimentary model,further divided into three types of sedimentary facies: open platform,limited platform,and continental shelf.The Mao 1st period was influenced by extensive transgression and deep magmatic activities.The whole is an open platform facies,with a continental shelf facies developed in local deep watermainly composed of marl and flint nodules limestone.With the decline of sea level,the land shed gradually shrinks,and the clastic beach develops,becoming an important shoal formation period;the Mao 4th period was affected by the crustal uplift before the large-scale eruption of ELIP,and the overall thickness was not large,and some areas even lacked Mao 4 segment.The early siliceous deposition of Maokou in the area is related to magmatic hydrothermal activity;the weathering crust deposit at the top of the Maokou Formation is the product of crustal uplift caused by magma eruption;the Sr isotope curve of the sample at the top of the typical drilling Maokou Formation in the southern part of the area records ELIP There is a correlation between the thickness of the Maokou Formation and the distribution of basalt.The basalt coverage effectively protects the underlying formation of the Maokou Formation.