Characteristics And Genetic Mechanism Of Soft-Sediment Deformation Structures Of The Middle Jurassic Hepingxiang Formation In Yanjiao Lead-Zinc Deposit,Southern Yunnan

The Yanjiao lead-zinc deposit in southern Yunnan is located at the southern end of the Simao Basin in the Lanping-Simao Mesozoic-Cenozoic Basin in the "Sanjiang" orogenic belt in southwestern China.The host rock is classified as the Middle Jurassic-Lower Cretaceous.Through the detailed geological cataloging of the core of the Yanjiao mining area,a large number of soft-sediment deformation structures(SSDs)of different types,scales and stages have been discovered in the Middle Jurassic Hepingxiang Formation in the mining area,and show that The close symbiosis(associated)relationship with the lead-zinc deposit(chemical)body is obviously different from the metallogenic characteristics of the classic sandstone type(SST)lead-zinc deposit.Since the Late Mesozoic,the Simao Basin has experienced multiple periods of tectonic activities.Especially since the Cenozoic,the India-Eurasia collision has caused strong intracontinental structural deformation in the region,which has led to people's understanding of the genesis of this type of lead-zinc deposit It is not clear,and at the same time,there are new differences in understanding of basin attributes,evolution mechanism and stratigraphic ownership.SSDs are mostly a series of congenital-quasi-contemporaneous soft deformed structures triggered by tectonic activities(earthquakes).Therefore,they are an important record carrier of basin attributes,sedimentary environment and evolution mechanism,and have become a hot spot in sedimentary basin research.For this reason,this article focuses on such special deformation structures in Yanjiao lead-zinc mining area,and conducts systematic investigation and observation research on them,clarifies its development characteristics,and explores its genetic mechanism.This study can provide a new perspective for understanding the genetic relationship between syn-sedimentary tectonic activities and basin evolution in the region,and at the same time provide a reliable basis for in-depth understanding of the genesis of lead-zinc deposits.This research mainly achieved the following results and understandings:(1)This work is based on the observation of 2 geological corridors on the surface of the field and the detailed recording of 16 boreholes.According to the lithological combination and sedimentary structure characteristics,the method of sedimentary cycle comparison,sedimentary facies changes and multi-factor control are used to control the Yanjiao mining area.The stratigraphic sequence of the Hepingxiang Formation has been re-divided,and at least 5 lithological sections can be divided from top to bottom: The fifth section mainly deposits a set of gray-white calcareous fine sandstones of delta plain subfacies,and intermittently developed purple-red siltstones and mudstones in the bays between distributary channels;In the fourth section,there are mainly purple-red mudstone,argillaceous siltstone and a small amount of fine sandstone in the delta front subfacies;The third section is a set of variegated silty mudstone and argillaceous siltstone of shallow lake subfacies,and is characterized by high organic matter content and frequent inclusions of blue-gray thin layered marl;The second segment is a set of sandy muddy deposits in the delta front subfacies;The first section developed shallow lake subfacies purple-red mudstone,silty mudstone,and occasionally thin layered fine sandstone.The vertical phase change represents a complete lake life cycle.The first,second,and third stages are the lacustrine system,and the fourth and fifth stages are the lake-shrinking system.(2)After a detailed description of the morphological characteristics of SSDs in the area,according to the classification scheme of Montenat et al.(2007)and T(?)r(?)and Pratt(2016),they are divided into plastic deformation structure,brittle-plastic hybrid deformation structure and brittle deformation.Plastic deformation structures mainly include syn-sedimentary microfolds,spiral bedding,liquefied sand and mud veins,landslide structures,load structures and flame structures;The brittle-plastic hybrid deformation structure is mainly manifested as a combination of microfolds and micro-faults or liquefied breccia,and locally developed pudding-like structures and sausage-like structures;The brittle deformation structures are mainly liquefied breccia,syn-sedimentary faults and seismic fractures.(3)Analysis of the deformation mechanism,development characteristics and the relationship with the depositional environment of SSDs suggests that liquefaction and fluidization are the main deformation mechanisms of SSDs.The factors that may trigger SSDs are eliminated one by one,and it is finally believed that strong earthquakes caused by tectonic activities are the main reason for the formation of SSDs in the region.A small number of slumped structures and loaded structures may be caused by the instability of sediments on the delta front slope or local sediment overload caused by earthquakes.According to Rodriguez-Pascua et al.(2000)on the relationship between the types of lacustrine SSDs and the earthquake magnitude,the earthquake intensity is greater than or equal to 5.5,and the strongest earthquake is above 7.5.It can be inferred from the horizontal distribution of SSDs that the tectonic activity center is located in the south-east direction of the study area.(4)SSDs can be used as an effective tool to reconstruct syn-sedimentary tectonic activities.The large-scale SSDs developed in the second member of the Hepingxiang Formation indicate that tectonic activities at this stage have caused the basin to sink and increase the accommodation space,and then the shallow lacustrine sedimentary system of the third member of the Hepingxiang Formation has developed.SSDs are not developed in the third and fourth stages,which means that there is no tectonic activity or weak tectonic activity during this period.The development of SSDs in the fifth stage indicates that large-scale tectonic activities have occurred again in the area,resulting in the rapid filling of the accommodating space of the basin,forming a large-scale proliferation sequence.Overall,it reflects the transformation mechanism of Simao Basin from a rift basin to an intracontinental depression basin during the Middle Jurassic.(5)The development of SSDs can effectively improve the porosity and permeability of rock formations,and is beneficial to the migration and enrichment of oil and gas and ore-forming fluids.Through detailed observation under the microscope,it is found that the mineralization enrichment and organic matter residues of the Yanjiao lead-zinc deposit are closely related to SSDs.Metal mineralization has the characteristics of syn-sedimentary mineralization and Seismites horizons are high-quality reservoirs for metal mineralization and oil and gas enrichment.