Sedimentary Characteristics Of The Upper Triassic Xujiahe Formation In The Southern Of Longmenshan Foreland Basin

Longmenshan foreland basin is located in the west of Sichuan basin.The late Triassic is an important period of tectonic uplift of Longmenshan.The basin changed from Marine facies to continental facies during this period.Based on field outcrop,drilling core and rock slice,on the basis of detailed observations,combined with the analysis of test results,the existing in materials research,to study the stratigraphic sequence division,plane distribution of sedimentary facies and the analysis of the provenance of the late Triassic strata in the study area were carried out.Combined with the development and distribution of seismolites in the basin,to study the basin evolution process in the southern part of the Longmenshan foreland basin in the late Triassic,which lays a foundation for the further study of the Longmenshan foreland basin.Come to the following conclusion:(1)Through the study of sedimentological markers,logging curve markers and seismic profile markers in field profile and drilling,and the comprehensive of a large number of regional stratigraphic sequence division data,based on the theory of sedimentology and sequence stratigraphy,to study systematically the sequence of Xujiahe formation in the southern part of Longmenshan foreland basin,and divide three structural sequences(TS1~TS3)and six third-order sequences(SQ1~SQ6).The structural sequence TS1 is composed of sequence SQ1,which is equivalent to the first member of xujiahe formation,including maantang formation and xiaotangzi formation.Gray limestone with gray mudstone is developed at the bottom,and gray and black mudstone is developed at the top.The structural sequence TS2 is composed of sequence SQ2 and SQ3.The sequence SQ2 is equivalent to the second member of Xujiahe formation and mainly develops gray medium-fine sandstone.The lithology of the sequence SQ3 is mainly gray-black mud,shale and siltstone.The structural sequence TS3 is composed of sequence SQ4,SQ5 and SQ6,which is equivalent to the 4th to 6th member of Xujiahe formation.The sequence SQ4 mainly develops light gray medium-fine sandstone with a small amount of siltstone and mudstone.The lithology of SQ5 and SQ6 is mainly dark gray fine-siltstone and grayish-black mudstone.(2)By means of rock flake analysis,quartz cathode luminescence and detrital zircon U-Pb dating,it is believed that the provenance of the Xujiahe formation in the study area is relatively loaded.In the TS1 sequence sedimentation period,the Dickinson triangle plot shows that the provenance area is mainly within the craton,and the zircons dating from 1970~1720 Ma are mainly from the southern part of the north China plate and the northern part of the kang-dian paleocontinent.In the TS2 tectonic sequence deposition,the projection point of Dickinson triangle shows that the provenance area is mainly recyclic orogenic belt,and the cycle type is mainly quartz recyclic.Zircon samples were mainly from 1900~1720 Ma and 300~250Ma,and the provenance area was transformed into songpan-ganzi fold belt and qinling orogenic belt.In TS3 structural sequence sedimentary period,Dickinson triangle bit displayed as provenance mainly for the cycle of orogenic belts,cycle type quartz cycle and transition cycle again,and again a few cuttings cycle again,in sequence SQ4 sedimentary period,main peak age is 310 ~ 210 Ma,1050 ~ 900 Ma and 470 ~ 380 Ma,sequence SQ6 sedimentary period,the main peak of 880 ~ 730 Ma and 1970 ~ 1720 Ma age,mainly from songpan-ganzi fold belt and longmenshan orogenic belt to provide source.(3)According to the characteristics of color,paleontology,sedimentary structure,particle size and lithofacies characteristics in Xujiahe formation,the study area is divided into barrier coastal facies,Marine facies delta,lacustrine facies delta and lacustrine facies.In the sedimentary period of Maantang formation in the SQ1 sequence,there are mainly reef flat facies,lagoon facies and tidal flat facies.In the sedimentary period of Xiaotangzi formation,there are tide flat facies,lagoon facies,shallow sea shelf facies and Marine delta facies.Sequence SQ2 and SQ3 are dominated by braided river delta facies and lakeside subfacies.Braided river deltas were mainly developed in sequence SQ4,and shore-shallow lake subfacies and delta front subfacies were mainly developed in sequence SQ5 and SQ6.There is a large amount of seismite in the strata of Xujiahe formation in the southern segment of Longmenshan foreland basin.Seismolite was first developed in the sedimentary period of Xiaotangzi formation,and mainly developed in the first,second,third and fifth member of the Xujiahe formation.(4)Combined with sedimentary facies and source analysis results shows that the late Triassic period of Longmenshan orogenic belt in the Xiaotangzi formation had begun to tectonic uplift,the structural sequence sedimentary period of TS2 continue to rise,and part above the water,in the structural sedimentary period of TS3 completely above the water,purely Marine strata sedimentary basin into continental strata.