The Sedimentary Record Of Earliest Triassic Anachronistic Facies In Upper Yangtz Area

Various types of anachronistic facies including microbiolites, vermiculate limestone, ribbon limestone, laminated Limestone and flat-pebble conglomerate were developed in Upper Yangtz area during the P/T transforming interval. Microbiolites were found as dome shapes on outcrop and interior lamina occurred in the microbiolites,which were formed by wackestone and packstone, characteristicing for less organisms and monotonous species inside, such as, ostracodes, minitype proleg and tiny brachiopoda etc..Vermiculate limestone was underlied by the bioclastic limestone in the toppest layer of upper Permian Changxing formation, and the geological age of the vermiculate limestones was infered as late Permian. The vermiculate limestone mainly consisted of light-colored 'matrix' and dark-colored 'worm grains', the former contained higher component of calsite and lower component of mud than that of the latter. Vermiculate limestone could be divided into following three main types according to its distribution shape:(1) continuous line shaped, (2) intermittent line shaped, (3) strange and littery shape. The continuous line shaped one was formed first, the second was the intermittent line shaped vermiculate limestone and the strange and littery shape was the last one. It's thought that all the three types of vermiculate limestones were interrelated to their genesis, and the formation of these rocks were associated with sedimentary differentiation, diagenesis and the rework of the hydrodynamic force after thet were deposited.Ribbon limestone often occurred with interbedded micritic limestone or silty mudstones. The microscopic features showed that ribbon limestone was usually comprised by regular thin mud bed and irregular marl crumb. With the influence of compaction, the mud lamina often disrupted the carbonate crumb, which indicated that the formation of irregular calcareous crumb was resulted from the compaction. flat-pebble conglomerate was often found in P/T Stratigraphic profile in Guangyuan Shangsi and Qingchuan Dagouli. It usually co-occured with thin or lamina argillaceous limestone and calcilutite, and was often accompanied by the storm-generated sedimentary structures, such as hummocky cross bedding, chrysanthemum structure, base erosion structure, etc.. The formation of the flat-pebble conglomerate had experienced the following four stages:(1) the quick deposition of thin limestone-calculutite; (2) the deformation or the breaking of the carbonate-rich layer, (3) the further disruption to carbonate-rich layers by mud-rich layers, (4) the erosion and transportation of the former calcrudite by storm.The globle climate sharply changed after the P/T mass extinction. Anachronistic facies widely appeared during this period, which indicated the disorder and unusual atmospheric conditions of the marine environment.Microbiolites growing rapidly after the extinction demonstrated that the microbiolite could adapt to the degradation condition at that time, and had experienced survival-stable-prosperous evolutionary process. According to the petrological features and genetic mechanism of the vermiculate limestone, ribbon limestone, laminated limestone and flat-pebble conglomerate demonstrate, it was concluded that the marine chemical conditions and depositional setting were almost similar when they were deposited. Stratification was in good keeping of the primary deposit, which was the common character of the anachronistic facies, indicating the decrease of metazoon and the receding of its vertical disturbance. The synsedimentary cementation caused by the upturning of poor-oxygen oversaturated-seawater and by metabolism of microbiolite, was the other main factor for the formation and conserve of sedimentary structure of the anachronistic facies. The laminated limestone, thin-bedded pelmicrite, argillaceous limestone were widely covered by storm-generated brecciform limestone, flat-pebble conglomerate calcirudyte, hummocky cross-bedding siltstone, which reflected that the Tethys during this interval might experience the changes from the sluggish to the oscillating sea. The storm-generated sediments widely emerged around the world in the lower Triassic, which indicates that the atmospheric conditions were in an extremely abnormal state. The severe changes in atmospheric conditions might be correlated with the 'megamosoon'. The stagnant ocean circulation, the comprehensive anoxia in bottom water, the increase of atmospheric CO2 and H2S, enhancing of alkaline and salinity, along with the extensive synsedimentary cementation in seafloor, were the major conditions for the development of the anachronistic facies at the P-T boundary. On the other hand, the external factors such as violent megamonsoon and frequent storms result in the erosion and rework to the sediments, which finally resulted in the reoccuring of the abundant types of anachronistic facies at this time.After the mass extinction, feedback effecting to the earth system by barren biosphere got weaker. Due to the lack of effective restricting by biosphere, the chaotic earth system further deteriorated and finally arrived its extreme. As the representative of the abnormal deposits, anachronistic facies were just the response to an extreme natural environment. Therefore, this process that the biosphere adjusts the earth system was named as Gaia effection.