Triassic Allochthonous Carbonate Blocks And Carbonate Debris Flows In Songpan Area And Its Palaeogeographical Implications

Allochthonous carbonate blocks are exotic biggish carbonate blocks that have an consolidated or semiconsolidated state appearing in normal sedimentary stratum by a long distance transporting and redepositing. Both allochthonous carbonate blocks and carbonate debris flows belong to resedimented carbonates. There are a lot of allochthonous carbonate blocks and carbonate debris flows formed time after time in Songpan area which is Southwest to Ganzi-Litang, Northwest to southern slope of East Kunlun, North to West Qinling, Southwest to Longmenshan. Two opposing views of tectonic genetic and sedimentary genetic had been debating continuously for allochthonous carbonate blocks. The research on allochthonous carbonate blocks and carbonate debris flows will help us reveal the visage of Paleozoic carbonate sediments and recognize further Songpan area basement and Triassic basin property.Allochthonous carbonates blocks in Runiange Formation, Xianglatai Formation, Heisi Formation and Xinduqiao Formation in the hinterland of Songpan area and carbonate debris flows in West Qinling are the research objects of this paper. Determining the original age of resedimented carbonates by fossil, determining the original environments of resedimented carbonates by microfacies analysis, determining the resedimented environments by research on host stratum, its sedimentary features and paleographic implications have been studied systematically.Allochthonous carbonate blocks in Luhuo-Daofu and Chuanzhusi region have a syn-sedimentary relation with host stratum. Allochthonous carbonate blocks appeared in volcanic rock or clastic rock. Contacts between isolated blocks and volcanic rock often have a roasted band and have been metamorphosed into marble. Allochthonous carbonate blocks in clastic rock often were parallel to host stratum with a toplap in block rock top. Allochthonous carbonate blocks often accompanied by carbonate debris flows and never changed normal stratum sequence. The age of allochthonous carbonate blocks in Luhuo-Daofu and Chuanzhusi region is from Late Devonian to Triassic. The origin genetic environments of allochthonous carbonate blocks were restricted sea, open sea, platform-shallow, platform-edge reef and other carbonate platform environments. Geochemistry and sedimentology analysis shows that allochthonous carbonate blocks in Luhuo-Daofu and Chuanzhusi region were the results of syn-sedimentary rift and reconstructed by tectonism later.There is a uniform and static basement with carbonate platform sediments-covered from Ltea Paleozoic to Triassic in the hinterland of Songpan area. Research area existed in a sustaining extension rifting environment of passive continental margin. Carbonate platforms were destroyed many times. The extension increased gradually from north to south in research with a series of grabens and hosts. From north to south, there were North Qingling platform, Bailongjiang platform, Motianling Platform, Ruoergai hidden platform and Luhuo-Daofu isolated platform in Anisian. Luhuo-Daofu carbonate platform deposition ended in Carnian.