Formation Mechanism Of Ooids And Their Environmental Significance In Late Permian To Early Triassic,South China

The widespread ooids after the end-Permian mass extinction(EPME)are considered to be a special compenstive carbonate form when biotic skeletons dramatically decreased,which are rich in geological information.Based on comprehensive collation and analysis of numerous previous data,field survey and systematic sampling,and indoor tests including microscope,X-ray diffraction,SEM,stable isotopes and trace elements analyses,we clarified the temporal and spatial distribution of ooids from the end of late Permian to early Triassic in south China,as well as their microscopic petrological and geochemical characteristics,and explored their formation mechanism and environmental significance.The results of this paper show that during the end of late Permian to early Triassic,ooids in South China mainly deposited in 4 periods: Griesbachian,early and middle Dienerian,middle and late Smithian-early Spathian and late Spathian,but there are also obvious regional differences in the distribution of ooids in different periods.At the same time,the size of ooids increased significantly during the Griesbachian,middle Dienerian,middle and late Smithian and late Spathian.The episodic and widespread distribution and size variation of these ooids may have a close relationship with the extremely unstable environment at that time.The distribution and microscopic characteristics of the bright and dark laminae in early Triassic concentric ooids show that the distribution of these laminae is closely related to the formation process of the ooids,and the bright laminae are composed of relatively coarse calcites with good crystallines,while the dark laminae consist of micronsize calcites with abundant microbe-relative structures(filament and eps,etc).Therefore,it is suggested that the bright laminae are formed by inorganic chemical precipitation during suspension stage,while the dark laminae are formed by microbial organomineralization.A formation model of ooid by alternation of inorganic chemical precipitation and microbial organomineralization is eatablished.The results of study on the distribution and size variation of the early Triassic ooids on the Great Bank of Guizhou(GBG)indicate that the hydrodynamic force is one of the important factors affecting the distribution and size variation of ooids.From the interior towards the margin of the GBG,the hydrodynamic force gradually increased,resulting that the oolite beds thicken and become coarser.In addition,the development of oolitic margin shoals also affected the architecture of the isolated carbonate platform.Due to the barrier effect of the oolitic margin shoals,the hydrodynamic force within the platform became more weakened,so that the ooids’ distribution is limited and their sizes significantly decreased during Dienerian.On the contrary,the slope increasingly steepened with the development of oolitic margin shoals,leading to the frequent collapse of the margin,so that the shallow-water ooids on the margin could be transported into slope and basin by the gravity debris flow.The lower Triassic Daye formation at Kangjiaping section of Cili(Hunan)consists of several types of rocks,containing a large number of ooids,and the size of ooids changes rapidly.Through the petrological and geochemical studies,the depositional environments of different types of rocks(including the terrestrial material injection,dry and wet climate and water salinity,etc.)has been recoveried.It is believed that the Daye Formation in this section was a sedimentary sequence affected by the alternating wet and dry climates and the relative sea-level fluctuations,and the sedimentary model of early Triassic ooids under the joint control of alternating wet and dry climate and relative sealevel fluctuations was established.Ooids are mainly deposited in the turbulent and oxygen-rich shallow water with high salinity under arid climate conditions.The supersaturation state of seawater with respect to calcium carbonate is the main reason for the widespread distribution and significant increasement in grain size of ooids.After the EPME,several extinctions and many extreme environmental events broke out globally.A comparison of these extreme environmental events with the temporal and spatial distribution of ooids during the end of Permian to early Triassic in the south China revealed that there is an obvious correspondence between them.The episodic and widespread ooids during this period are the sedimentary response of the coupling of extreme environmental events such as rapid sea-level fluctuation,abnormal high temperature,strong continental weathering and oceanic anoxic event,which has a signigicant indications for the extremely special paleo-ocean environment at that time.