The Evolution Of Paleogeographic And Rift Trough During Sinian-early Cambrian In Sichuan Basin

The discovery of Anyue giant oil and gas field reveals that there exists a north-southward negative geological structure during Sinian to Early Cambrian in the Sichuan basin.However,there are still many controversies in terms of its areal extent,forming duration,structural properties and sedimentary filling characteristics.Based on the analysis of seismic,welling,core information and field outcrop investigation in combination with the stratigraphic unconformity,core lithology,well logging and rock geochemistry data in Sichuan Basin and its peripheral regions,the plane distribution and structural characteristics of the rift trough were carefully depicted,and its tectonic evolution as well as palaeogeographic features during Sinian to Early Cambrian are discussed in detail.The research results indicate that:(1)In the rift trough,the Dengying Formation is dominated by dark micritic dolomite,showing deep-water sedimentary characteristics.The thickness of the Maidiping formation is large and deposited Siliceous shale,black shale?microcrystalline dolomite?Siliceous carbonaceous shale?siltstone and phosphorite from bottom to top,belonging to slope-shelf deposits;(2)the rift trough,whose width in the north and south segments is wider than the middle segment,can be divided into 3 segments on the plane.and its boundaries are steep on the east side and relatively gentle on the west side.(3)Tongwan tectonization include 3 episodes characterized by regional uplift and crosion,forming 3 disconformitics.Affected by that movement,An intense karstification took place in dolomite of the Dengying formation in the carbonate platform area,however,its influence on the deep-water facies rift trough is limited.(4)The formation of the rift trough,which is characterized by early development in the north and late development in the south,is controlled by the inherited palaeogomorphology and regional extension activities.The evolution of the rift trough was primarily divided into 5 stages,i.e.,Z2 ds embryonic stage,Z2 dn developing stage,?1m formative stage,?1q filling stage,and ?1c shrinkage stage.(5)Affected by the development and evolution of rift trough,Sichuan Basin had uplift and concave Palaeogeographic pattern during Sinian to Early Cambrian.The rift not only controls the developments of the reef and shoal reservoirs on both sides of the rift,but also controls the distribution of high-quality source rock of the lower Cambrian.