Depositional Environment Of The Late Triassic Daqiaodi Formation, Hongni Basin, Panzhihua

Using the basic method of sedimentology，and all kinds of collected geologicdocuments， based on the help of the research achievements acquired by thepredecessors，with the outcrop and by the means of test analysis of samples，this paperfirst analyses the sedimentary facies and coal-forming condition in the Late TriassicDaqiaodi formation，Hongni Basin in Panzhihua，and then predicts the favorable areasfor coal prospecting.With the outcrop and by the means of test analysis of samples，The sedimentaryfacies has been classified into alluvial fan，braided stream，meandering stream anddelta facies which can be further divided into nine sub-facies and thirteen microfacies.From the vertical， the first segment of Daqiaodi formation mainly developmeandering stream facies，while the second segment predominated by braided streamfacies，and alluvial fan is the major facies of the third segment；In the early Daqiaodistage，the studied area mainly develops meandering stream facies，delta facies onlydeveloped in the west of Hongguo area. While in middle Daqiaodi stage，the studiedarea mainly develops braided stream facies，the meandering depositional environmentgradually extends to the west. In the late Daqiaodi stage，the alluvial fan developsvery well in the studied area. Based on above studies with the consideration of thetectonic evolution，the paleogeography of the Late Triassic Daqiaodi formation ofHongni Basin was reconstructed，indicating that the coal-accumulating center migratewesterly and northwesterly.Through the research of coal-forming condition of all kinds of depositionalenvironment developed in the studied area，the first segment of Daqiaodi formationdevelops meandering river facies，of which swamp is the favorable coal seam formingmicro-facies，and the thickness and the number of layers of coal seam decreaseupwards and downwards. Laterally near the Hongni area，coal seams much moredeveloped and become thicker. From the depositional environment point of view，flood plain of braided stream and swamp of meandering stream are the most potential coal-accumulating environments.