Study On The Geological Structure And Superimposed Characteristics Of Sichuan Basin

The study of basin and orogenic belt is a hot issue of geologists'attention.The study of basin is also an important part of Earth system science.The Sichuan Basin located in the eastern margin of the Tibet and western part of the Yangtze Craton is an important tectonic unit in the Circum-Tibetan Plateau Basin-Range System and an petroleum-bearing superimposed basin which has undergone multi-cyclic tectonic evolution.Studying the internal structure of the Sichuan basin,the types of basins in different periods and the vertical configuration are of great significance to reserch the tectonic evolution of the Sichuan Basin and even the Yangtze craton,to speed up the exploration of marine carbonate rocks,to expand the new exploration areas and to further expand the scale of resources in the Sichuan basin.At present,there have been many achievements in understanding the geological structure of Sichuan Basin,but there are still some problems.The study of the basement structure mainly depends on the interpretation of the gravity,magnetic and electric data and lacks the constraints of the latest seismic profiles;the description of the sedimentary cover structrure needs to be refined with the improvement of the accuracy of the basin data;the properties of the prototype basins in different periods need to be further studied;the research on the influence of the basement on the development and evolution of the sedimentary cover is relatively weak and urgent to tackle key problems in order to improve the overall understanding of the basin.Therefore,under the guidance of basin dynamics theory,the paper aims at studying Sichuan Basin using the latest geological and geophysical data to make a detailed analysis of the textures,the prototypes in different periods and build the evolutionary sequence and superposition model of the basin,and discusse the dynamic mechanism of basin superposition evolution and the control effect of basement on the tectonic evolution of sedimentary cover.The main results and perceptions are as follows:?1?In the aspect of basement structure,seventeen basement faults with chessboard pattern were identified.And it is suggested that the basement of Sichuan basin has three vertical and three horizontal parts.Vertical parts consist of crystalline basement,fold basement and sedimentary basement,and horizontal parts consist of three different magnetic rock blocks?Emei-Chengdu-Santai,Luzhou-Chongqing-Kaijiang and Guangyuan-Nanjiang?.At the same time,the basement is divided into three secondary tectonic units?the middle Sichuan basement uplift zone,the western Sichuan basement depression zone and the eastern Sichuan basement depression zone?and seven tertiary tectonic units?southwest Sichuan,northwest Sichuan,south Sichuan,middle Sichuan,north Sichuan,southeast Sichuan and northeast Sichuan basement tectonic belt?.?2?With regard to the sedimentary cover structrure,five regional unconformities are identified.And five tectonic layers are divided.They are the Sinian-Silurian tectonic layer?Z-S?,the Middle Permain-Middle Traissic tectonic layer?P₂l-T₂l?,the member1-3 of the Upper Triassic Xujiahe Formation tectonic layer?T₃x¹-T₃x³?,the fourth member of the Upper Triassic Xujiahe Formation-Jurassic tectonic layer?T₃x⁴-J?and the Cretaceous-Quaternary tectonic layer?K-Q?.?3?On the evolution of the superposition of the prototype basins,it is proposed that the Sichuan Basin,which is in the crustal oscillation environment,are superimposed in the following order from the bottom to the bottom:marine rift cratonic basin?Z-S?,marine intra-craton depressed basin?D-T₂l?,downfaulted basin?T₃x¹-T₃x³?,continental depressed basin?T₃x⁴-J?,and foreland basin?K-Q?.?4?With regard to the control of the basement over the sedimentary cover:?1?It is considered that the basement structure and the deep faults of the basement control the present macroscopic structure pattern of the basin.The three-dimensional features of the basement in horizontal and the chessboard pattern basement fault system make the present basin have the characteristics of rhombic border,western depression,central uplift and strong deformation in the east.?2?The development of the Leshan-Longnusi paleo-uplift was controlled by the basement faults and basement activity.The northeast trend Huayingshan fault,the Longquan-Tongjiang fault and the northwest trend Houba-Pengan-Fengdu fault control the structure and distribution of Leshan-Longnusi paleo-uplift.The horst-graben system and the differential activities led to the Leshan-Longnusi paleo-uplift further uplift and denudation,and expand the scope of the paleo-uplift.?3?It is suggested that the phased activities of basement faults control the sedimentary pattern during the active period.In the late age of Early Cambrian,the Huayingshan fault,the Qiyueshan fault,the Houba-Pengan-Fengdu fault,the Nanjiang-Tongjiang-Kaijiang fault and the Leshan-Longnusi paleo-uplift controlled the depositional pattern of the Longwangmiao Formation in Sichuan Basin.In the Changxing Age of Late Permian,the Nanjiang-Tongjiang-Kaijiang fault and the Zhaohua-Biquan-Dazhou fault,the Houba-Peng'an-Fengdu fault and Suining-Hechuan fault were two opposite normal faults.Due to the different activities of the basement faults,the Kaijiang-Liangping ocean trough and Pengxi-Wusheng platform depression,which trend NW,were formed in the sedimentary cover and accompanied by the horsts,which resulted in the formation of the paleogeographic pattern of"three uplifts and two sags"in the late Permian and controlled the sedimentary pattern of this period.