Characteristics And Tectonic Significance Of Major Unconformities In Sichuan Basin

The unconformity surface defines the main stage of the evolution of a sedimentary basin.Only by correctly understanding the main unconformities in the basin can we better understand the evolution process of the basin.In the process of multi-stage variation of regional tectonic stress field,many unconformities have been formed in the sedimentary caprocks of Sichuan Basin.Although the predecessors have elaborated on the development characteristics of unconformity in Sichuan Basin,they have not systematically summarized and discussed the nature,structure and scale of unconformity in Sichuan Basin from the perspective of the whole basin.Using outcrop,drilling and regional seismic data,this paper describes and analyses in detail the structural properties,distribution range,time limit and denudation amount of 14 main unconformities in Sichuan Basin,and determines the structural type,development scale,regional denudation direction,denudation time limit and degree of unconformity.On this basis,the unconformity of Sichuan Basin is classified,and the tectonic sequence of Sichuan Basin is divided,and the tectonic evolution stage of Sichuan Basin is discussed.Then the unconformity and tectonic evolution of Caledonian-Hercynian period in the central-northwest Sichuan region,which has made important breakthroughs in oil and gas exploration and has been paid close attention to,are analyzed in detail in order to understand the shape of Sichuan Basin in depth.Evolution and petroleum exploration provide a reference.The unconformity between the second and third section of Dengying Formation formed by Tongwan Episode I tectonic movement mainly distributes in the west of Sichuan Basin,accounting for about 2/3 of the basin area,and mainly develops parallel unconformity.The micro-angle monoclinic unconformity develops in the middle of Sichuan Province,which only denudes the top stratum of Second Member of Dengying Formation and is NW-SE denudation.The unconformity of Cambrian and Sinian formed by Tongwan Episode II tectonic movement is complete.Basin distribution,structural types of unconformity include high-angle overlap-truncation unconformity,monoclinic unconformity of trough margin and parallel unconformity of out-trough area,which developed in the denudation zone of nearly north-south trending"stretching trough".The denudation amount of"stretching trough"area is huge,and the maximum denudation amount decreases from"stretching trough"area to outward denudation amount;The unconformity of Qiongzhusi Formation and Maidiping Formation is distributed in the whole basin.The Maidiping Formation is absent in most areas of the basin,and the structural types of unconformity is mainly parallel unconformity.The unconformity between Ordovician and Cambrian is mainly distributed in the paleo-uplift areas of central Sichuan and Northern Sichuan,accounting for about 1/2 of the basin area,and there are overlap unconformities and parallel unconformities with medium-low angles,and The denudation amount of Guangyuan area in northern Sichuan is the largest,and the directional denudation is NW-SE.;Silurian and Ordovician unconformities,which distribute in and around the basin,account for about 2/3 of the basin surface,develop medium-low angle overlap unconformity and parallel unconformity,and the denudation in Guangyuan area of northern Sichuan is the largest,and the denudation direction is from basin margin uplift area to Eastern Sichuan depression area;Early and Late Paleozoic unconformities,basin-wide distribution,and high angle development in central Sichuan uplift area,and Middle-angle monoclinic unconformity,parallel unconformity develops on the outer side,and the largest denudation area is the southern Longmenshan section in Western Sichuan,denuded to the second member of Dengying Formation;the unconformity of Permian and Carboniferous,the distribution of the whole basin,is unconformity overlapping with the movement of Guangxi in a large area of central Sichuan-western Sichuan,and parallel unconformity occurs in eastern Sichuan,which generally lacks the stratification and denudation of Upper Carboniferous and Lower Permian;the unconformity between the Middle and Upper Permian occupies about 4/5 of the basin area,forming three strong denudation zones in the basin.In the basin,the Kaijiang paleo-uplift area has the largest denudation amount,and the parallel unconformity generally develops with the denudation direction SW-NE;The unconformity between the Middle and Upper Triassic,the distribution of the whole basin,the development of high-medium angle overlap unconformity,overlap-truncation unconformity and parallel unconformity,Luzhou paleo-uplift area has the largest denudation,denudation to the third Member of Jialingjiang Formation;the unconformity of the fourth and third members of Xujiahe Formation mainly develops in Western Sichuan basin,with middle-low angle overlap unconformity,parallel unconformity and progressive syntectonic unconformity developing only at the top of the third member;the unconformity of Triassic and Jurassic mainly develops in Western Sichuan and Northern Sichuan.In this area,low-angle monoclinic unconformity and parallel unconformity are developed,with the largest denudation in Jiange area and NW-SE denudation direction;The unconformity between Cretaceous and Jurassic is mainly developed in Western Sichuan,and the eastern Sichuan has been uplifted to land erosion.The unconformity between upper and lower Cretaceous is mainly distributed in Western Sichuan and southern Sichuan,accounting for about 1/2 of the basin area.The unconformity of Neogene-Quaternary and underlying strata mainly distributes in Chengdu Basin.The unconformity structure type is mainly angular unconformity,and its distribution area is less than 1/3 of the basin area.Based on the classification criteria of unconformity in different sedimentary basins at home and abroad,a preliminary classification criterion of unconformity in Sichuan Basin is drawn up,and all unconformities in Sichuan Basin are divided into three levels.The first-order unconformities include Cambrian/Sinian,Upper Paleozoic/Lower Paleozoic,Upper Triassic/Middle Triassic;the second-order unconformities include Z₂dn³/Z₂dn²,Qiongzhusi Formation/Maidiping Formation,Ordovician/Cambrian,Silurian/Ordovician,Permian/Carboniferous,Jurassic/Triassic,Cretaceous/Jurassic,Upper Cretaceous/Lower Cretaceous;and the third-order unconformities include:The unconformity of Upper Permian/Middle Permian,T₃x⁴/T₃x³,Dayi conglomerate-Quaternary and underlying strata.Based on the study of unconformity characteristics of main basins in Sichuan,the tectonic sequence of Sichuan Basin is divided and the tectonic evolution stage of Sichuan Basin is discussed.The tectonic sequence of Sichuan basin is divided into 6first-order tectonic sequences and 12 second-order tectonic sequences.The first-order tectonic sequence includes:Ts₁?AnZ??Ts₂?Z??Ts₃??-S??Ts₄?D-T₂??Ts₅?T₃x-E??Ts₆?N-Q?,second-order tecttectonic sequence includes:Ts_2-1?Z₁l-Z₂dn²??Ts_2-2?Z₂dn³-Z₂dn⁴??Ts_3-1??₁m??Ts_3-2??₁q-?₁x??Ts_3-3?O₁t-O₃w??Ts_3-4?S₁l-S₂h??Ts_4-1?D-C??Ts_4-2?P₂??Ts_4-3?P₃-T₂??Ts_5-1(T₃x^1-6)?Ts_5-2?J??Ts_5-3?K-E?.Sichuan Basin has undergone eight stages of evolution since the Ediacaran Period.During the evolution of the basin,there are different basin characteristics in different stages,namely,intracratonic rift stage?Z?,intracratonic depression stage??-O₂?,foreland depression stage?O₃-S?,marginal rift of craton depression stage?D-C?,intracratonic riftstage?P??intracratonic depression stage?T₁-T₂??foreland basin stage?T₃-E?and intracontinental depression stage?N-Q?.There are mainly eight unconformities in the Caledonian-Hercynian period in the central-northwestern Sichuan region,including Z₂dn³/Z₂dn²??/Z??₁q/?₁m?O₁t/??S₁l/O?Upper Paleozoic/Lower Paleozoic and P₃w/P₂m.According to the characteristics of unconformity development,denudation degree and stratum thickness and zone changes during Caledonian-Hercynian period,it is inferred that the tectonic evolution process of this period is as follows:during the Caledonian-Hercynian period,the central-northwest Sichuan region underwent four tectonic evolutionary stages,including the overall differential uplift-fall stage?Z-?₁m?,uplift-depression stage??₁q-S?,continuous uplift stage?D-C?and Stretching-sextension Stage?P?.