Analysis Of Sedimentary Environment In The Early Cretaceous In The Jinggu Region,West Yunnan,China

The Jinggu area is located in the northwest corner of the Simao Basin and is an important area for the evolution of the Sanjiang Tethys.There are many opinions on the palaeogeography of the region.In this paper,we have measured profiles and collected samples in the field and conducted identification and analysis and plotted maps indoor.Combined with previous research data,and theories and methods of petrology,sedimentology,paleontology,and tectonic geology,we began the study of the sedimentary environment and evolution of the Jingxing Formation and Nanxin Formation in the Early Cretaceous by signs of sedimentary facies,which is of great significance for exploring potential minerals in the area and restoring the evolution of the Sanjiang Tethys.In this paper,signs of sedimentary facies include rock association characteristics,rock texture,section structure,sedimentary structure,paleo-biotic sign and so on.Through measured profiles,rock association,section structure and sedimentary structure were summed up and the corresponding sedimentary environments were identified through the identification of paleontological fossils and the textures of the rock were studied by particle size analysis.The sedimentary facies can be divided on the basis of above signs.The Jingxing Formation defines two sedimentary facies of the lacustrine facies and the delta facies and three subfacies of shallow lake,deep lake,and delta front,and four macro facies of underwater distributary channel,channel mouth bar,distal bar,and delta-front sheet sands.The Nanxin Formation defines three sedimentary facies of lake facies,delta facies,and fluvial facies and five subfacies of shallow lake,delta front,river channel,embankment,and alluvial plain,and six micro facies of channel mouth bar,distal bar,delta-front sheet sands,channel lag,natural embankment and flooding.Based on the tectonic background and provenance analysis of the study area,the evolution of sedimentary facies and its controlling factors have been further explored.In the Early Cretaceous,under the tectonical background of arc-arc collisions,the Simao block sinked and remained stable periodically,resulting in the continuous evolution of the shallow lacustrine facies and the delta front facies.With the addition of new volcanic arc sources,the fluvial facies and the delta front facies mutually alternated at the time of Nanxin Formation.The sedimentary model of Jingxing Formation presents two co-rotating deltas that flow into the same lake,and the sedimentary model of Nanxin Formation presents four deltas that enter the lake,of which the last three were on an alluvial plain.