Tempestite of Early Triassic Feixianguan Formation in Shangsi Section, Guanyuan: Are they extreme climatic event under megamonsoon system?

Key words: 
• Early Triassic /
• Feixianguan Formation /
• megamonsoon /
• Gaia effect

Abstract: Early Triassic Yangtze platform was located in the northeastern margin of Tethys. Based upon the large amount of geological evidence and simulation experiments, researchers confirmed that the Triassic climate system was megamonsoon that controlled the climate of whole Pangaea and surrounding sea. However abundant outcrops of Triassic strata in our country, whether contained some information implying monsoon climate? We found a set of wellpreserved tempestite in the Lower Triassic at Shangsi Section, Guangyuan city, northwest Sichuan, which developed in the lower part of the Feixianguan Formation. The sedimentary characteristics of Lower Triassic Tempestite was significant, including stormgenerated gravel, storm erosion structures, storm tear structures and bedding structures. Gravel was composed of micritic limestone, and might be divided into two types according to roundness. One contained brecciated, strip, axiolitic shape; the other presented flatgravel shape. Storm erosion structures included pocket and chrysanthemum shape formed by drainage and channel mode, erosion surface structure. Storm tear structures included messy mud gravel, torn sutureline breccia and truncated structure amongst lays. Bedding structures included hummocky cross bedding, graded bedding,blocky bedding ,homogeneous bedding and parallel bedding. According to sedimentary structures and lithology, three kinds of sequence of SaSbSc, SbSd, SaScSd(Sastranded gravel deposition, Sbgraded bedding, Schummocky crossstratification, Sdhomogeneous bedding) were recognized. The storm deposits occurred in the shallow sea and the formation, development and characteristics of the tempestites have important significance on paleoclimate research. We think that tempstite in Shangsi (and even whole UpperYangtze region) was controlled likely by the strong monsoon system (Megamonsoon) in Early Triassic, and the P/T boundary mass extinction event let tempestite be preserved below the setting that organism burrows were few. 