Sedimentary Characteristics Analysis And Significance Of The Alluvial Fan Form Upper Jurassic,northern Section Of Longmen Mountain

The Mesozoic tectonic uplift of the eastern edge of Longman Mountains in the Tibetan Plateau is a research hotspot. According to the special characteristics of the multiple stacking of episodic tectonic movement, the alluvial fans deposited in the front of Longman Mountains has been regarded as a research hotspot. Most of the previous studies have only been concentrated on alluvial fans studies on the the early stage(Late Triassic-early Jurassic) and late stage(Cretaceous – Cenozoic), but those studies on the alluvial fans formed in the late Jurassic Lianhuakou Formation where the climate change is the most distinct and sedimentary facies mutate are relatively weak, the main reasons for which is the joint action of the climate and tectonic function, which lead to the restricted recognition of the basin-mountain coupling evolution. This article has a concrete analysis of the sedimentary facies and sedimentary environment based on the alluvial fans of the northern Longman Mountains in the late Jurassic Lianhuakou Formation double-controlled by the climate and structure. At first, asystematic petrographic analysis of the profile is conducted in the open field and then the different ingredients of the gravels are recognized and the paleocurrent direction is measured to have a comprehensive analysis of the sedimentary features and the origin of the alluvial fans in the Lianhuakou Formation.This research comes to following understandings:(1)Utilising lithofacies analysis method and Sedimentary facies pattern, nine kinds of gravels lithofacies are recognized from the Lianhuakou Formation first,which are rich arenaceous heterobase conglomerates(Gmd), massive particle supporting conglomerates(Gcd), irectional particle-ordered bedding conglomerates(Gco), bottom-erosed positive article-ordered conglomerates(Ghg),sand lens conglomerates(Gmg), groove-appearanced interleaving bedding conglomerates(Gt), intercalary strata sand conglomerates(Gst), diagonal bedding conglomerates(Go), parallel bedding conglomerates(Ghi); five kinds of sndstone facies are recognized, which are trough cross bedding sandstones(St), massivesandstones(Sm), evenly bedding sandstones(Sh), bottom-erosed sedimentary sandstones(Sh), diagonal bedding sandstones(So); nine kinds of fine sedimentary facies are recognized, which are fine laminate mudstones(Fl), massive(tabular)mudstone-siltstones(Fm), parallel bedding mudstone-siltstones(Fh), fine laminate mudstone-siltstones(Fg), bioturbation mudstone-siltstones(Fb), grey greenish mudstone-siltstones(Ms), calcificated mud-gravel mudstones(Cg), carbonate rocks(Cm), paleosol(P); seven kinds of main sedimentary formation are sediment gravity flow(SG), the braided river deposition(BF), fluvial sedimentary(CH),floodplain flooded plains(OB), sheetflood deposition(SF), natural levee(MF), shallow lakes facies(LD). Based on this, ten kinds of rockassemblages are divided:①-③complex composition massive gravels(PC):typeⅠ、typeⅡ、typeⅢ;④gravels and mudstones bands(CM); ⑤sandstones and gravels CS; ⑥gravels, sandstones and mudstones CSM;⑦ mudstones and complex composition gravels MC;⑧sandstones and mudstones SM;⑨ mudstones and gravels lens MS;⑩mudstones and marl MCa.Finally, the sedimentary environment in the Lianhuakou Formation in the research area are divided into four kinds: alluvial fan sedimentary environment, braided river sedimentary environment, anastomosing river sedimentary environment, shallow lake sedimentary environment.(2)The pleocurrent direction and inheritance characteristics of the gravels in the Lianhuakou Formation is found. By measuring and counting the paleowater direction of the sedimentary structure of the paleowater direction in the research area and comparing the paleowater direction characteristics since late Triassic in the research area, the initial judgement is that the paleocurrent direction in the Lianhuakou Formation is mainly SE direction( 140°~160°),and the inference is that the paleowater direction from late Triassic to Early Cretaceous in the northern part of the Longman Mountains has a distinct whole inheritance mainly in SE direction(130°~170°).(3)The composition of the gravels and fragmentary materials in the Lianhuakou Formation and the lithological characters in its provenance are defined. Utililising the statistics of the systematic gravels composition in the open field, conventional microscopes and cathode luminescence to analyse the composition of gravels and fragmentary materials, the compositions of gravels and fragmentary materials are consistent. There exists distinct diaparities in district in the gravel composition space:① near the Jin Zi Mountain in Guangyuan, the composition of gravels are mainly quartzite and quartz sandstones, containing 67%~89% of quartzose conglomerates,not morn than 30% of the limestone gravel composition at most, and sporadic dolomite gravels; ② near An county, the composition of the gravels are mainly quartz sandstones, limestones and dolomite, containing 90%(52% in average)of limestone gravels at most, as much as 23% of dolomite gravels composition which is distinctly higher than that in other areas. At the same time, the material source of the gravels in the Lianhuakou Formation is initially deduced based on the characteristics of petrology and biological fossil: ① limestone gravels are mainly from the carboniferous-permian in Longman Mountains(mainly in permian);② quartz sandstone gravels are all calcificated quartz sandstones, cathode luminescence light color characteristics show that part of the calcificated quartz sandstones have a high similarity with the devonian quartz sandstones. After that, the statistics of the fragment composition show that: in the gravels in the profile of Jin Zi Mountain, the general content of quartz is more than 30%(averagely 20%~29%), that of feldspar is 5.7%~16.3%and that of rock fragments is 37.7%~74.4%,more than 60% averagely;② in the gravels in Qing Linkou area, the content of quartz is stably 23%~25%, that of feldspar is 2.3%~9%and that of rock fragments is 65.6%~72.6%;③ in the gravels in An county, the content of quartz is 26%~45%,that of feldspar is 0.7%~2.8%and that of rock fragments is 53%~81.9%. Based on the difference in the composition of gravels and sandy rock fragments and the types of quartz origins and the inheritance of paleocurrent, the whole conclusion is that alluvial fan sedimentary material distribution is controlled by the flow of the paleocurrent, with a corresponding relationship between isolated point source.(4)After Sichuan Basin changed from black layers to red layers in late Jurassic,from fine gravels in Suining Formation to crude gravels in Lianhuakou Formatiom,sediments show the mutation of lithofacies and sedimentary environment in the same climate settings, which should be drived by the strong tectonic movement in Longman Mountain. Comparing the alluvial fans in Baitianba Formation deposited in black layers phase and the sedimentary sequences of alluvial fans in the Lianhuakou Formation deposited in red layers phase, it shows that, in the similar active tectonic background, the sedimentary sequence of cycles in the Lianhuakou Formation is distinctly controlled district climate. It furthere shows that in the depositional system research in the research area, the double factors of structre and climate should be concentrated and it is inadvisable to invade or ignore the climate factors.(5)The formation mechanism of alluvial fans in the Lianhuakou period in northern Longman Mountains is understood as a combined control by three factors oftectonics, climate and palaeocurrent water system. That is, under the background of tectonic activities of Longman Mountains, the tectonic uplift driven source region, the deposition of space, the spatial distribution of sediments controlled by the palaeocurrent water system and the alternative climate change by continuous drought and plenty rainfall recharge affect multiphase composite fan bodies whose gravel-mud sedimentary sequence is a cycle superposition and whose fan body gravel layers focuses on vertical superposition with a weaker lateral extension.