Study On Sequence-lithofacies Paleogeography Of The Xujiahe Formation Of Upper Triassic In Northeast Of Sichuan Basin

This thesis used The Xujiahe Formation of Upper Triassic in northeast Sichuan basin(northeastern region) as the research object,and the principle of Sedimentology, Sequence Stratigraphy, Geochemistry, Paleontology,Geophysics and some other traditional geology theories were used as the instructions of this paper during the integrated researching of outcrops, cores,logging data,seismic data and slice.This article analyzed characteristics of rock, material source,the charaters of depositional system and sequence stratigraphy of The Xujiahe Formation in northeast Sichuan basin.On this basis analysis, we used the third-order sequences or tertiary sequence system tracts as the mapping unit to draw the sequence-based lithofacies paleogeography map with the charateristics of scientifically, isochronously, continuously and precisely.We explored sequence lithofacies paleogeography and evolution in this area also. Main contents and results are as follows:1. Based on previous studies and combined with this study, the Xujiahe Formation of the study area was divided into five lithologic:first member,second member,third member,fifth member of Xujiahe Formation;A large number of The thin section results show that the Xujiahe Formation in study area were developed 3 rock type:terrestrial sedimentary rocks, mixed sedimentary rocks and special categories, and summed up the class of 14 terrestrial sedimentary rocks;2.Based on the analysis of Component in the rock (sandstone debris, debris composition, gravel component) and combined with the characteristics of heavy mineral assemblages, we believed that Xujiahe Formation with characteristics of dual source:one is Longmen Shan region which is northwest to study area, the other is of the ancient Daba region which is northeast to study area. The evolution of Material source was closely related with the orogenic activity during the first member time to fifth member time of Xujiahe Formationt.With the construction of Longmen Mountain orogenic uplift,the study area was influenced by Longmen Shan orogenic belt more and more.3. By studying of surface profiles and drilling data analysis,and according to the different curve feature flags and sedimentary features:the color of rock,rock structures, sedimentary structures,fossils and so on,we identified three depositional systems group druing the sedimentary evolution of Xujiahe Formation in northeastern Sichuan: Continental System group,Sea-land transition system group and marine systems group of groups, including five depositional system; where 1) Continental System group:alluvial fan, river depositional system, Lacustrine depositional system,Lake-delta system; 2) sea-land transition system group:delta system; 3) group of marine systems:coastal depositional system,and summarizes two kinds of sediment mode characteristics:the transition of delta land to sea and continental alluvial fan -lake.4. according to outcrop and drilling cores,the outcrops and drilling is carried out a systematic stratigraphic division and contrast. After research on the types of sequence boundary and classification standard, six kinds of sequence boundaries are identified: Surface of unconformity, weathering crust, large-scale scour the bottom surface, intermittently exposed surfaces, lithologic facies transition and maximum surface (sea) flooding surface.Sequence stratigraphy division plan of Upper Triassic is setted, the Xujiahe Formation can be divided into 3 Structural sequences and 5 third-order sequences.5. we used the third-order sequences or tertiary sequence system tracts as the mapping unit to draw the sequence-based lithofacies paleogeography map,and drawed six sequence-based lithofacies-paleogeography maps(SQ,SQ2HST,SQ2TST. SQ3,SQ4,SQ5) that showed the evolution and features of regional lithofacies paleogeography. The results show that:in SQ1,study area through the marine terrestrial sedimentary environment and was influenced largely by the transgression;In SQ2, Sea completely withdrew from the study area that was continental sedimentary environment and a delta of sand deposition mainly;In SQ3, because of declining of orogenic tectonic activity surrounding the study area, source supply is limited, the study area deposit a thin shale with the sedimentary environment characteristics of shallow lakes.In SQ4, with the strengthening of basin margin orogenic tectonic activities, the study area showed the sedimentary characteristics of Alluvial Fan - Alluvial Delta from north to south;In SQ5, with the declining of basin margin orogenic tectonic activities, the sedimentary environment of the study area were much the same as SQ3 period.