Depositional System And Coal-accumulating Of The Upper Triassic Daqiaodi Formation, Southern Baoding, Panzhihua

With the help of the research achievements acquired by the predecessors, based on the outcrop and drills data, and by the means of test analysis of samples, this paper first analyses the depostional facies and coal-accumulating in the upper Triassic Daqiaodi formation, southern Baoding , Panzhihua and then predicts the favorable areas for coal prospecting.The depostional facies of upper Triassic Daqiaodi formation has been studied in detail. The depostional facies has been classified into alluvial fan, braided stream, meandering stream and delta facies which can be further divided into eight sub-facies and eleven microfacies. Number one to five members of Daqiaodi formation mainly develop meandering stream facies while number six to eight members predominated by meandering stream and braided stream facies, and alluvial fan is the major facies of number nine to eleven members.Through the analysis of thin sections and SEM, it can be concluded that Daqiaodi formation mainly undergone diagenesis of compaction, cementation and dissolution, and the present diagenetic stage is in early period of B to A.In the early Daqiaodi stage, the studied area mainly develops fluvial sedimentation while the mining area in Baoding and Daqing syncline develop delta and lacustrine facies. In the middle Daqiaodi stage, the fluvial depositional environment gradually extends to Daqing syncline. In the late Daqiaodi stage, the alluvial fan develops well in the studied area, and shifted extensively to Daqing syncline. Based on above studies with the consideration of the tectonic evolution, the paleogeography of the upper Triassic Daqiaodi formation of southern Baoding area was reconstructed, indicating that the coal-accumulating center migrate westerly and northwesterly.Coal-accumulating potential was principally controlled by depositional environments. In the studied area, number seven member of Daqiao formation develops meandering river facies, of which swamp is the favorable coal seam forming micro-facies, and the thickness and the number of layers of coal seam decrease upwards and downwards. Laterally near the Goubian area, coal seams much more develop 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.