The Characteristics Of Oolitic Shoal Of Platform Edge Of The Early Feixianguan Formation Triassic In Nanjiang-Tongjiang Area

During the deposition period of the Lower Triassic Feixianguan Formation,there are a number of rows of platform marginal belts dominated by the oolitic beach deposits including migration type and accretion type.The temporal-spatial evolution of oolitic beach and its response to the type and particle size changes in the platform structure are still to be revealed.This study is based on the platform margin of the Feixianguan Formation in the Nanjiang-Tongjiang region of northeastern Sichuan Province.It focuses on the comparison of oolitic beach inside the platform,combines previous data,and based on field outcrops and indoor laboratory analysis.The systematic analysis of the sequence stratigraphy of the depositional marginal zone of the Feixianguan Formation,the spatial-temporal distribution of the depositional system,and the spatial evolution of the grain beach,grain types,led to the following main conclusions and understanding:(1)The types of sedimentary facies and spatial distribution of the Feixianguan Formation in the Nanjiang-Tongjiang area were analyzed.Five kinds of sedimentary facies have been identified,such as evaporative platform,restricted platform,platform margin,platform margin slope,and shelf,and further divided into several subfacies.(2)Based on field outcrops and indoor comprehensive analysis,the Feixianguan Formation in the Nanjiang-Tongjiang region of northeastern Sichuan can be divided into two three-level sequences: SQ1 and SQ2,all of which consist of transgressive system tract and high system tract.The geomorphological differences at different depositional stages were significant.During the SQ1 period,the landform slopes in the platform marginal slopes to shelf,and platform showed undulating topography,and the characteristics of gradual relief of the landforms corresponded with each other,forming a platform with significant upward thickening and overall vertical deposition.During the SQ2 period,the undulation of the landforms was not obvious,and a stable confined restricted-evaporative platform was formed.In the early stage of SQ2,there were eastward migration characteristics of the oolitic beach.(3)Oolites are mainly developed in the platform margin of SQ1 stage,and are also visible in the intra-platform shoal of SQ2 stage.Through the observation ofindoor microscope,combined with the differences in the degree of later transformation,the comprehensive classification of the ooid types in the Feixianguan Formation of Nanjiang-Tongjiang Region was conducted,and two types of ooids were identified including the deposition period oolites and the later modified oolites.The later modified oolites were obviously affected by the later diagenesis,while the oolites in the sedimentary period basically had the structure of sedimentarypenecontemporaneous period.During the deposition period,oolites can be identified as true oolites,superficial oolites,concentric oolites,and compound oolites.The later modified oolites include dolomitic oolites,negative oolites,irregular oolites,recrystallization oolites,single-crystal oolites,and algal oolites.Besides,the vertical evolution of oolite types had been discussed.(4)A systematic particle size analysis study was conducted on the oolites samples,and the different types of particle size curves were divided and the hydrodynamic condition indications were discussed.The frequency distribution curve of oolites identified three forms: single peak,double peak,and multi-peak;the cumulative relative frequency curves were linear,two-segment,and multi-segment;the two-segment type included the left and right types;the multi-segment lines were at least 4 or more segments.Different frequency distribution curves correspond to different types of particle size probability curves.Overall,the single peak samples are dominated by jumps and suspensions.The overall reflection of hydrodynamic conditions is stable,while the multi-peak samples can be seen as rolling,jumping,and floating,reflecting the dynamic conditions is very unstable.On this basis,the vertical changes in the grain size characteristics of the grains were analyzed.