Research On The Late Triassic To Early Jurassic Sedimentary Environment And Paleogeography Of Shiwandashan Basin In Guangxi

As a foreland basin which is formed from the closure and orogency of theQinzhou residual trough between Yangtze Plate and Yunkai Plate during LateHercynian-Indosinian-Yanshanian period，the formation and evolution of Shiwanda-shan Basin is related with the Late Permain inversing-folding and Mesozoic tectonicreactivation of Middle-Late Paleozoic Qinzhou residual trough. As a result，theresearch of Mesozoic sedimentary environment is significant for the acquaintance oftectonic evolution in Basin and adjacent region.With the Late Triassic to Early Jurassic strata as research object and based on theprevious research achievement， this thesis uses theories and methods aboutPetrology，Sedimentology，Paleogeography，Sedimentary Geochemistry，SequenceStratigraphy etc. to analyze the geological characteristics of Shiwandashan Bainduring Late Triassic to Early Jurassic period，especially including the detailedstratigraphic division and correlation，the identification of sedimentary environment，the clarification of petrologic and paleogegraphyic features.The main research results are as follows：（1）The main rock types during Late Triassic to Early Jurassic period inShiwandashan Basin consist of zinzolin or maroom middle-fine sandstone，siltstoneand mudstone，which are followed by small amounts of medium-coarse sandstone.Fine-medium conglomerate is marginally found in local layers and regions. There aredefinite petrologic compositions which are variable from Pingdong Formation of LateTriassic to Baixing Formation of Early Jurassic. In addition， the petrologiccomposition of Fulongao Formation is different between the western and themiddle-eastern regions.（2）Detrital composition analysis shows that the main type of sandstone in LateTriassic to Early Jurassic strata is lithic quartz sandstone，which is followed by lithicsandstone and quartz sandstone. Quartz and rock debris（most of them are shallowmetamorphic）are the main grains of sandstone in Shiwandashan Basin and there is nofeldspar grain. The provenance analysis of clastic components indicates that thetectonic setting of provenance is mainly recycling orogeny，which has the nature ofcollision or seam belt and fold-overthrust belt.（3）In terms of sedimentary environment in Late Triassic to early Jurassic，thestudy area is given priority to fluvial facies and lacustrine facies，followed by alluvialfan deposits. Fluvial deposit includes two fundamental types：braided river depositand meandering river deposit，in which the former is the most common type while the latter is only found in some definite regions and stages. Along with low sinuositymeandering river facies in small amounts of layers， the main sedimentaryenvironments of Pingdong，Wangmen and Baixing formations are shore and shallowlake facies. The sedimentary environments of Fulongao formation are variable fromdifferent regions. The main sedimentary facies in the western region of Basin isfluvial facies，especially braided river deposit，while in the middle and estern regionof Basin，the main one is alluvial fan facies. Based on the analysis of REE and traceelements，this thesis finds that the deposits during Late Triassic to Early Jurassicperiod are as a whole terrestrial deposits，which are mostly oxydic （partiallyreductive）and have the same provence.（4）Two sequence boundaries are recognized in Late Triassic to Early Jurassicstrata. Specially，this two boundaries are respectively marked by the variegatedmudstone in the bottom Wangmen formation and the gray，ash black or celadonmudstone in the bottom Pingdong formation，which are deposited in deep waterenvironment. With this two boundaries and other petrologic，sedimentary andstratigraphic features as criterias for dividing，the whole set of strata is divided into2sequences and5system tracts.（5）With system tract as the mapping unit，this thesis uses the method ofsuperior sedimentary facies to compile lithofacies paleogeographic maps of differentperiods，especially including stages of SQ1-TST，SQ2-LST，SQ2-TST and SQ2-HST.The time of SQ1-TST is roughly equivalent to the period of bottom Pingdongformation，in which the depth of lake water increased rapidly，leading to theexpansion of lake basin and the transgression，occupying the whole scope of the studyarea. In SQ2-LST period，the lake basin is shrinking and filled mainly by braided riveralluvial fan deposits，besides small amounts of low sinuosity meandering river，shoreand shallow lake deposits. SQ2-TSTperiod is the time that lake basin expandedagain，with shore and shallow lake deposits as the mian deposits，which exists in thebelt of east-north belt. In SQ2-HST period，the lake basin has the most depth of waterand starts being filled by mainly low sinuosity meandering river，shore and shallowlake deposits，incorporating small amounts of alluvial fan deposits.