Sedimentary Characteristics And Provenance Analysis Of The Middle Part Of Simao Basin In Jurassic,Western Yunnan

The Simao Basin is between the Lancang tectonic belt and the Ailaoshan tectonic belt,and its sedimentary evolution was controlled by both of these tectonic belts.The sedimentary evolution of the basin,especially since the Mesozoic,has always been the focus of attention.So,as the research object,the Jurassic sedimentary strata in the basin are researched by petrological methods,including field investigation and sample testing.Using previous conclusion and sedimentary theories,the sedimentary environment and basin evolution were studied in this paper.The basin evolution in this area were explored by using the tectonic evolution characteristics.Through the investigation of the Jurassic in the middle of the basin,it is considered that the Jurassic strata are formed by the Yangjiang Formation,the Huakaizuo Formation,and the Bazhulu Formation from the bottom to the top.The Yangjiang Formation and the Bazhulu Formation are mainly distributed in the middle-east in this basin.The Jurassic rocks are mainly purple-red,and the rock combination are dominated by mudstone,silty mudstone,muddy siltstone and lithic sandstone,with a small amount of volcanic rocks in the area.There are the horizontal bedding and parallel bedding in the Yangjiang Formation and the Bazhulu Formation,additionally,oblique bedding in the Huakaizuo Formation.Further,in the Bazhulu Formation,there are binary structure and sad lamination.Use the statistical analysis of particle size,the vertical comparison of detrital composition and material source analysis,it is suggest that the origin of the materials in Jurassic strata are the west volcanic arc of the orogenic belt at the southwestern margin of the basin,but are the Ailaoshan orogenic belt at the eastern margin of the basin.In summary,during the early Jurassic,this basin was basically completed regression,and the tectonic was stable,with the sediments in rivers and lakes.Obviously,in the Yangjiang Formation,the micro-facies of the point bar,the crevasse fan and the natural levee has been developed.Otherwise,the volcanic rocks are sandwiched in the mudstone at the western region,which indicate that there are small-scale volcanic activities in this area.In the middle Jurassic,the lower part of the Huakaizuo Formation is a braided river sediment,and evolve upwards into a braided river delta plain and delta front deposit.The upper part of the HuakaiZuo Formation generally shows a delta front or semi-deep lake deposit,which indicate that there is a strong accumulation process.Then the environment has been rapidly changed into the delta front or semi-deep lake.In the upper Jurassic,the Bazhulu Formation is the semi-deep lake deposit.The evolution characteristics of the sedimentary indicate that the middle section of the basin during the Jurassic period has underwent a transgression process,and the surface sea has remained in some areas,and the coastline has moved westward.In general,in the early period,the basin has experienced Paleo-ocean basin formation,subduction and back-arc basin formation.In the medium period,collision has been occurred,and the foreland basin has been formed,which is caused by continental continuous extrusion.In the end,the basin has been faded away,then current landscape was formed by thrust nappe and Strike slip.