Study On Sequence Stratigraphy And Evolution Of Paleogeography For Carboniferous-Zisongian Stage Permian In Northeast Of Yunnan And Adjacent Area

The research area in this paper is located in the junction of northeast Yunnan and western Guizhou, in the subtriangularstructure formed by Xiaojiang Fault, Ziyunyadu Fault and Mileshizong Fault.The main goal of this paper is to conduct the integrated stratigraphic study on the sequence stratum of northeast Yunnan and its surrounding sedimentary strata mainly with Carboniferousmarine carbonatite, establish and perfect the stratigraphic division system suitable for the whole area, and sets up the curvilinear relationship between the chronological framework of Carboniferous-Permian Lower Maping Formation and sea level change cycle curve relationship, so as to deeply understand the characteristics of Carboniferous-Early Permian Epochstratumdevelopment and the spatial distribution pattern of sedimentary system.According to the above research, combined with the geochemical methods, the Carboniferous-Early Permian sequence lithofacies paleogeographic map in northeast Yunnan was prepared based on the chronostratigraphic framework, further revealing the sedimentary evolution process and characteristics in the research area, and providing the theoretical basis for the future geological researchin southwestern margin of the Yangtze terrain.The main achievements and new understanding aresummarized into the following five aspects.(1) Rearrange andre-divide the lithostratigraphy in the research area and summarize the stratum diachronismrules.The research area is located in the southwestern margin of the Yangtze terrain, the junction of northeast Yunnan and west Guizhou.The Carboniferous-Permian low series in the research area and the surrounding areas develops completely with vast areaand longresearch history, leading to two different stratigraphic classification systems in this area. This paper refers to theprevious research results, studies 22 field profilescarefully, re-determine the Carboniferous stratigraphic unit in the area and establish a unified stratigraphic classification scheme. According to the latest international stratigraphic classification scheme, the Carboniferous is transformed from the three series (upper, middle and lower series) to the two series (upper and lower series). Gelaohe Formation in the originalCarboniferous Yanguan Formation is classified into the Devonian, and the originalCarboniferous Maping Formation is thecross-system stratum.The Carboniferous-Permian low series is divided into six stratigraphic units, namely: Tangbagou Formation, Wanshoushan Formation, Datang Formation, Baizuo Formation, Weining Formation and Maping Formation.Study the stratum diachronism in the research area, and summarize the Carboniferous-Permiandiachronism rules. Theupward diachronous phenomenon occurs in the basin margin for Jiusi Stage and Shangsi Stagestratum, while the downward diachronous phenomenon occurs in the middle of basin for Dewuan Stage and Luosuan Stage stratum.(2) Analyze the petrologic feature of each stratum, microfacies features under microscope, test result of sedimentary geochemistry, distribution strataand distribution characteristics.In the research area, conduct the carbonatitemicroelement analysis, and restore the paleoenvironment using the test results:(1) through the Sr/Cu ratio method research, it is found that the environment is gradually dry and hot in Huizearea, the environment ishot first and then humid in Yulu area, and that in Maoping changes more obviously, hot and dry first and then humid and warm; (2) through the Sr/Ba ratio method research, it is found that Huize and Yulu areas are the salt water environment, and the watersalinity in Maoping areachanges greatly, which is the brackish water and brackish waterenvironment, and the water salinity increases significantly during certain periods; (3) through the V/(V+Ni) ratio method and U/Th ratio method, analyze the redox conditions in the research area, and it is found that the research area is the oxygen deficientenvironment (anaerobic environment). The oxygen content in water in Weining Ageis low, and the oxygen content in water decreases firstand then slightly increases, and it can be predicted that the sea level rises first and then fell. Theresearch holds that the area inCarboniferous-early Permian is the sedimentary environment under the reducing condition.Obtain theswamp facies, open platform facies, restricted platform facies, shallow facies within platform and platform margin slope faciesin different ages, and identify the trench facies and bioherm facies on the basis of previous research, supplementing and perfecting the previous research achievements.At the same time, define the distribution characteristics and rules of sedimentary facies belt in the research area.-inYanguan Stage age, the sea level fell, most of the area was exposed, and only the southeasternand central areas deposited, mainly the open platform-restricted platform facies; in Jiusi Stage period, under the influence of river, the terrigenous detrital supply was adequate, so it was mainly the swamp facies and restricted platform faciessedimentation; in Shangsi Stage and Dewuan Stage period, the sea level started to rise again, and it was mainly the open platform facies, restricted platform facies, trench facies and epiplatform slope facies sedimentation. The basalt existed on the top of Shangsi Stage stratum from Weining to Shuicheng, so it was speculated that the age was the tectonic extension period, and this phenomenon led to the formation of Weining trench; in Huashibanian Stage-Dalan Stage, the sea levelwent up and down, and the sedimentary facies in the area wassimilar to that in Shangsi Stage and Dewuan Stage, but the number of biological speciesincreased, the bioclastic limestone and oolitic limestone significantly increased, the bioherm faciessedimentation appeared, and the dolomite and dolomization appeared on the top, so it was speculated that the sea level was lower than that in the previous cycle. In Xiaodushan Stage-Zisong Stage period, the sea level fell, the bioherm facies and platform-inter-shallow developed more than before.(3) In the research area, five third-order sequenceswere identified at the bottom of Carboniferous-Permian, including SQ14, SQ15, SQ16, SQ18 and SQ19, equivalent to SQ14-SQ19 in 25 third-order sequences in Neopaleozoic.Under the influence of Qiangui epeirogeny firsttectonic uplift, sea level falling and weather denudation, SQ17 is missing.At the same time, the quartz gravel layer was identified: SQ14 and SQ15, SQ15 and SQ16 surface; the sedimentary facies changesurface: such as SQ18 and SQ19 surface; the ancient weathering crust:such as SQ16 and SQ18 surface and their relevant surface.Through the paleotological featuresand sea level change features, theCarboniferous-early Permian chronostratigraphic framework and sequence strati graphic framework of the research area are built; the sequence stratigraphic framework reflects the basic characteristics of third-order sequences-, the phase-sequenceordering in the space andthe synchronicity of environmental change in the time.These features are clearly reflected in the sequence stratummap and the corresponding sequence-based lithofacies-paleogeography map.(4) According to the sequence stratigraphic framework, combined with the research results in biological assemblagecharacteristics. sedimentary systemand system tract, draw five sequence-based lithofacies-paleogeography maps using the single-factor mapping and multi-factor analysis with third-order sequenceas the unit, and analyze the Carboniferous-early Permian sequence-based lithofacies-paleogeographyevolution:In the Yanguanian Stage period, the regression continued in the research area, and the majority of area was exposed above the sea level with small sedimentary area, mainly the open platform facies sedimentation along the northeast to southwest, the local restricted platform facies sedimentation in Honglujiao and Erguanying, and the ancient island in mina plant and Zhexingshang. Since the Jiusi Stage, the entire research area entered the wide-range and long-time transgression period, and the main sediment was the terrigenous detrital. The climate was humid in the period, which was very suitable for plant growth, especially the tall trees, so the coal seam was formed during this period in the research area.There was mainly the coastal marsh facies, mixed tidal flat facies and restricted platform faciessedimentation. The swamp facies was mainly distributed in the Maoping-Bailachang in the northeast of the research area, and Chunai in the southwest of the research area. The open platform facies in Yanguanian Stage period was transformed into the restricted platform facies with smaller area, and the rest area was the mixed tidal-flat facieswith one ancient island in Weining base. The ShangsiStage basically inherited the sedimentary pattern of Jiusi Stage, thesea level continued to expand to the northwest, the Kangtien ancient Iandand Yangtze ancient land area reduced, and the open platform was developed greatly. In Dewuan Stage-Huashibanian Stage period, the sea levels reached the highest level in Carboniferous-early Permianand the transgression was also the widest. Dueto the influence of tectonic movement, the submarine topography in the research area was complex:the sea water was gradually deeper from west to east, and the eastern deep water inter-platform basin was the deepest in the area. The inter-platform basin, platform facies and reef-shoal facieswere reflected. The sedimentary facies in the research area was distributed symmetrically with the deepwater inter-platform basin to the southeast of Weining-Shuicheng area as the center. Three facies belts, the restricted platform, open platform and platform margin slope, appeared from the ancient land to the inter-platform basin, and among the three facies belts, Zuji, Erguanying and Taoyuan were the isolated platformsedimentary environment. The regression stage from Dalan Stageto Zisongian Stage started from the Qiangui epeirogeny firsttectonic movement. Dueto the tectonic movement, the land began to rise, and the sealevel fell significantly. Under the influence of Qiangui epeirogeny second tectonic movement, the land rose obviously, the sedimentary environmentchanged greatly and the land-sea pattern alsochanged obviously, and the sedimentary environment was transformed from the marine faciesenvironment to the continental faciesenvironment.In Huashibanian Stage-Dalan Stageperiod, the distribution of sedimentary facies beltchanged little. The shallow facies appeared near the ancient land, Zuji and and Taoyuan ancient island disappeared, and the Erguanying isolated platform still existed; in Xiaodushan Stage-Zisongian Stage, the sea level fell, the area to the north of northern sea was exposed above the sea level.Kuangshanchang-Dazhuyuan, Erguanying, Chunai-Yangtianchong was the shallow facies, the inter-platform basin was disappeared into the open platform, and most areas in the research area was transformed into the restricted platform facies due to the sea level falling.(5) According to structure-controlled basin and basin-controlled phase theory, analyze the basin evolution combined with the paleogeography and tectonic evolution.The research area is dominated by Ziyun-Luodian-Nandan Fault, influencing the terrain and palaeoenvironment inside the basin.The fault zone wasin the obviousextensional depression in Carboniferous-early Permian period. The basin evolution in the research area is divided into two periods:the first one isthe extensional depression period. After the Devonian regression, the fault depression zone was extended again, and the earthcrustsubsidedagain. The research area was in the same tectonic zone, so it was also extended towards the north and west; as a result, the crustal extensional depression occurredin Weining area, forming the Weining trench. The basalt on the top of Datang Formation indicated that the tectonic zone was in the extensional periodat this moment, and due to the expanded transgression, the area was mainly composed of the restricted platform, open platform, platform-margin slopes and trench.The second one is the extensional stationary period.Under the influence of external structure and sea level falling, the fault trench was filled with the sediments, so the sedimentary area decreased, the trenchwas not obvious, and the shallow faciesand biohermfacies were developed, so the area was mainly composed of the restricted platform, open platform, platform-inter-shallow, bioherm and trench.