Volcanostratigraphic Sequence And Tectonic-volcanic-basin Filling Evolution In The Faulted Period Of Songliao Basin

Songliao Basin is located in the eastern part of the Central Asian Orogenic Belt. Since the Paleozoic, the area has experienced the evolution from the Paleo-Asian Ocean tectonic regime to the Pacific tectonic regime. Due to the special geographical and structural location, the basin may experience the impact of different tectonic events in different periods. Because of the continuous sedimentary strata, sedimentary basin recorded formation and evolution history of itself as well as the process of structural transformation in the region. Volcanic rock is the surface expression of the deep dynamic mechanism of the earth, and it is an effective medium for studying the regional tectonic process. In recent years, abundant oil and gas resources have been found in the volcanic rocks of the Yingcheng Formation in Songliao basin. Under this background, the oil and gas exploration degree of volcanic rock is obviously improved, and have rich drilling, seismic, logging, geochronology, geochemistry data, which provides the conditions for the study of deep volcanic rocks in the basin. Stratigraphic and tectonic studies on the Mesozoic volcanic rocks, which can provide theoretical support for oil and gas exploration and deployment, and provide evidence for the Mesozoic tectonic evolution in the northeast of China. The volcanic rocks in the basin are characterized by multi activity centers, multiple stages, various types, complex compositions and so on.Due to the characteristics of multi activity centers, multiple stages, various types, complex compositions and so on, there are some differences in lithology characteristics, lithologic sequence, lithofacies assemblage of the volcanic rocks of Yingcheng Formation. Because the material source is from underground magma, and is controlled by the tectonic action, the volcanic formation sequence often has the phenomenon of diachronous and inversion of the lithostratlgraphic correlation. Coupled with the lack of fossil data in the volcanic rock, it is difficult to use the ancient biological characteristics to determine the volcanostratigraphic sequence. The se factors cause fine division and correlation of the volcanostratigraphic sequence to be very difficult. Xujiaweizi is the largest graben in the north of Songliao Basin. Because of the abundant oil and gas resources in the volcanic rocks of the Yingcheng Formation in Xujiaweizi graben, the predecessors have carried out extensive research on volcanic facies, volcanic edifices, volcanic reservoir, and geophysical recognition technology of volcanic rocks, and have achieved a series of good results. But there are still some problems in the division and correlation of volcanostratigraphic sequence, the formation and distribution mechanism of volcanostrata and the tectonic evolution of basin. Specific issues are as follows.(1) The study of volcanic formation sequence is lack of the theory of volcanostratigraphy.The formation of volcanostrata is more controlled by tectonic and magmatic activities, which is is different from sedimentary strata controlled by the subsidence of basin and the change of water level. Due to the lack of volcanostratigraphy theory, the present study in our country on volcanostratigraphic sequence is using the theory and method of sequence stratigraphy and sedimentology. Without consideration of the particularity of volcanostrata, the stratigraphic structure is not clear, which causes the difficulty of fine division and correlation of the volcanostratigraphic sequence.(2) The volcanostratigraphic sequence of Yingcheng Formation in Songliao Basin is not consistent.The sequence of Yingcheng Formation in the outcrop section and southern grabens in Songliao Basin is complete(First, Second, Third Member), which is not consistent with Xujiawei graben. The Second Member of Yingcheng Formation in Xujiaweizi graben is missing, and the reason for the inconsistency of the sequence is not clear. Due to The Second Member is a isochronous stratigraphic interface, and its missing makes it more difficult to divide and contrast volcanostratigraphic sequence.(3) There is a difference in the chronology of Yingcheng Formation volcanic rocks with different dating method.Due to the lack of fossil data in volcanic rocks, it is difficult to use the ancient biological age to determine the formation age of volcanostrata. The study of isotope chronology may be the only choice. But, there is a great difference in the results with different isotopic geochronology dating methods. The main range of the age of Yingcheng Formation volcanic rocks is 136-120 Ma with Ar-Ar and K-Ar methods, but it is 120-103 Ma when use zircon U-Pb method. The reason for the difference of the test results may be due to the difference of the methods, the lithology of the samples and the degree of weathering of rocks. In this case, the age of the samples will appear to be contradictory, which is difficult to provide a favorable basis for determining the volcanostratigraphic sequence.(4) The relationship among tectonic-volcanic-basin filling is not clear in Xujiaweizi graben.The stratigraphic sequence is the basis for the study of Geology. If the stratigraphic sequence division and correlation is not correct, this may cause a certain degree of misleading for studying the possible relationship among the volcanic rocks distribution, faulting stages, volcano and tectonic activities. In the Xujiaweizi graben, distribution of 3 NNW-trending main faults is characterized by belts in west-east, and distribution of volcanic rocks is characterized by blocks in south-north. There is a certain degree of coordination in distribution of main faults and volcanic rocks. Previous studies showed that the Xudong fault occurred large-scale dextral strike slip shear in the period of Yingcheng Formation, but volcanic rocks distribution has not been affected by the strike slip shearing effect. There is a mismatching with faults activities and volcanic rocks distribution.(5) The relationship between the regional dynamic background and the tectonic evolution of the basin is not clear enough.Previous studies showed that the Songliao Basin is formed in continuous extensional background controlled by Paleo-Pacific tectonic regime / Mongolia Okhotsk tectonic regime / two regime work together. In recent years, some scholars suggest that a strong compression had occoured in the Songliao Basin at the end of Shahezi Formation period, which did not cause enough attention in previous studies. Stretching and compression alternate recorded in sedimentary basins may indicate it undergoes the influence of different tectonic regimes in the formation and evolution and have different dynamic background.In this doctoral thesis, in view of the above problems, the following 5 main contents were studied detailedly by taking the volcanostrata in outcrop section and Xujiaweizi graben as the research object, based on cross section, core sections/cuttings, well-logs of boreholes, 3-D seismic, and volcanic rocks chronology data.(1) Summary lithology and sequence features, geophysical characteristics, plane distribution of the Second Member, Yingcheng Formation(2) Fine division and correlation of the volcanostratigraphic sequence of Yingcheng Formation in the Xujiaweizi graben by taking the Second Member as a marker bed.(3) To carry out the geochronology research on volcanic rocks of Yingcheng Formation, to provide the basis for stratigraphic division and correlation.(4) Based on the results of stratigraphic sequence division and correlation, to study the relationship between tectonic activities and volcano, to summary basin filling law and control mechanism.(5) Based on the tectonic evolution history section, to carry out tectonic evolution research of faulted basin, to discusses dynamic background of the tectonic evolution stages. The specific research process, methods and results are as follows.1. Characteristics, origin, identification and stratigraphic significance of interlayer in the volcanostrta of Yingcheng Formation, Songliao Basin.The stratigraphic sequence of Yingcheng Formation is the basic and key to volcanic oil and gas exploration in Songliao Basin. The Second Member, Yingcheng Formation is the sedimentary interlayer and have a series of characteristics of a certain thickness, easy to identify in seismic profile, and the meaning of a isochronous stratigraphic interface. The stratigraphic sequence of Yingcheng Formation can be divided when use the sedimentary layers(the Second Member, Yingcheng Formation) as a marker bed. Based on cross section, core sections/cuttings, well-logs of boreholes, and 3-D seismic data, we compare the cross section at the basin margin and boreholes in Xujiaweizi garben to analysis whether the Second Member, Yingcheng Formation is widespread in Songliao Basin and summary lithology and sequence features, geophysical characteristics, plane distribution of the Second Member, Yingcheng Formation.Through stratigraphic correlation we consider that the Second Member, Yingcheng Formation is indeed widespread in Songliao Basin, and its age can be well constrained to Albian with a chronology age ranging from 110 Ma to 108 Ma. There was enough time to form the sedimentary bed during 2-3 Ma volcanic eruption interval. The main lithology features of the Second Member, Yingcheng Formation is rich in tuffaceous and contain coal seams occasionally, and the complete lithologic sequence from the bottom showing "coarse-fine-coarse" variation. There are two different types of the Second Member, Yingcheng Formation. One type is mainly influenced by the thermal subsidence after the volcanic eruption, and another type is mainly controlled by the palaeogeomorphology. The stratigraphic sequence of Yingcheng Formation can be divided to three member when use the sedimentary layers(the Second Member, Yingcheng Formation) as a marker bed, the First Member and the Third Member are volcanic, and the former Forth Member should be divided to two parts, the lower part of fine sand-mud layers belong to the Second Member, and the upper part of conglomerate layers should be a part of Delouku Formation.2. Volcanostratigraphic sequence of Yingcheng Formation in Xujiaweizi graben, Songliao Basin.Clear characteristics of stratigraphic sequence are the basis of oil and gas exploration and development and subsequent related scientific research. Guided by the theory of volcanostratigraphy, made the Second Member of Yingcheng Formation as the mark layer, based on the characteristics of volcanic eruption stages revealed by outcrop sections of the basin margin, based on 121 boreholes and related cores, cuttings, logging, and well seismic combination strata correlation, volcanostratigraphic sequence of Yingcheng Formation in Xujiaweizi graben are divided into 3 Members and 6 volcanic eruption stages. The First Member and Third Member are mainly volcanic rocks, and the Second Member is mainly sedimentary rocks. Thickness ratio of each Member bottom-up is 8:1:5. The First and Third Member each contain 3 volcanic eruption stages. The stage of the First Member bottom-up is intermediate-basic lava stage, acidic pyroclastics stage, and massive acid lava stage, and the thickness ratio of these 3 stages is about 1: 2: 3. The stage of the Third Member bottom-up is acidic pyroclastics stage, massive intermediate-basic lava stage, and acidic lava-pyroclastics stage, and the thickness ratio of these 3 stages is about 1: 2: 2. Stratigraphic sequence of Yincheng Formation in each area of Xujiaweizi graben are different. The strata thickness of the paleo uplift area is thin, but the sequence is relatively complete. The strata thickness of the paleo depression in is large, but the lack of a volcanic rock, but there is a lack of stratigraphic sequence.3. Volcanostratigraphic chronology of Yingcheng Formation in Xujiaweizi graben, Songliao Basin.The volcanostratigraphic sequence is often diachronous or reversal, therefore, study on chronology is more significance for judging volcano stratigraphic sequence of key strata on the basis of clarifying of volcanostratigraphic sequence. The purposes include that analysing of the geological age of each volcanic eruption stage, approaching start and stop time of volcano activity, determining the volcanostratigraphic sequence, and providing accurate chronological evidences for the division and correlation of volcanostrata of Yingcheng Formation in Xujiaweizi graben.Selected 14 volcanic core and cutting samples with clear sequences and obvious features to analyze ages with the method of zircon U-Pb LA-ICP-MS dating. The weighted age of the volcanic rock samples and the results of single zircon age show that the age of the Yingcheng Formation can be well const rained from Aptian to Albian with a chronology age ranging from 120 Ma to 105 Ma. The age of the First Member range from 120 Ma to 110 Ma, the age of the Second Member range from 110 Ma to 109 Ma, and the age of the Third Member range from 109 Ma to 105 Ma. The results show that the sequence of the volcanostrata is consistent with the age of the volcano strata. The volcanostratigraphic division scheme is reasonable.4. The characteristics and controlling mechanism of tectonic-volcanic-basin filling.The volcanic rocks of Yingcheng Formation are rich in oil and gas resources, which is the main target of deep oil and gas exploration in Songliao basin. To study the characteristics and controlling mechanism of the formation and distribution of volcanostrata is of great significance to oil and gas exploration and development. Based on the fine division and correlation of volcanostratigraphic sequence(Formation, Member and stage), to make the thickness maps of 3 members and 6 stages. Through the structural evolution history sections, analyzing the characteristics, active stages, and structural properties(extension, strike slip and the mode of controlling fault depression) of the main faults, revealling the temporal and spatial relationship between the fault/paleolandform and the thickness and distribution of the volcanostrata, and exploring the mechanism of the formation and distribution of volcanostrata of Yingcheng Formation in Xujiaweizi graben.Detailed description of the stratigraphic sequence reveals the temporal and spatial evolution of volcanostrata in different stages. The distribution of volcanostrata of Yingcheng Formation is consistent with that of the main faults, and all of which are NNW. The volcanostrata of the First Member present contiguous, and mainly distributed in the southeast of graben. The volcanostrata of the Third Member present separate block, and mainly distributed in the west and north of graben. Each volcanic eruption stages in the same member from early to late show the eruption scale increased, the distribution scope expanded, and thickness increased gradually. The active stages of the main fault were analyzed through tectonic evolution history sections. The activity of Xu Zhong and Xu Dong strike slip faults is obvious in the period of the First Member, and all faults are active in the period of the Third Member. The superposition maps of volcanic rock thickness distribution of every eruptive stages of Yingcheng Formation and main faults reveal clearly that the strata of form early to late migrate from southwest to the northeast. The formation thickness center of every eruptive stages are located near the Xu Zhong fault and southern branch of Xu Dong fault. The formation thickness center of the Third Member of Yingcheng Formation is located near the Xu Xi fault and northern branch of Xu Dong fault. The main faults are the magma upwelling channels and present strike slip shear.Comprehensive analysis of the property of faults characteristics, tectonic evolution history of baisn and distribution characteristics of volcanostrata, the formation and distribution of volcanostrata are obviously controlled by faults and paleolandform formed in the late stage of Shahezi Formation. The control effects of the faults on the formation and distribution of volcanostrata are shown as the following. 1 The faults are the magma upwelling channels, and the maximum thickness centers of the volcanostrata are usually located near the faults. 2The temporal and spatial variation of the faults activities control the temporal and spatial migration of the volcanic eruption centers. 3 The attitude of volcanostrata is controlled by the attitude of the faults, and the formed the "eruption unconformity interface ". 4The strike slip fault has the function of shearing for the distribution of volcanostrata, which make a difference of strata thickness of on the both sides of fault. The control effects of the paleolandform on the formation and distribution of volcanostrata are shown as the following. 1The thickness of volcanostrata in the depression area is large, and the thickness of volcanostrata in the paleouplift area is thin. 2Paleouplift area obstructs the opposite flow of magma, resulting in the lack of volcanostrata in both sides of the paleouplift area, and the paleouplift area has become the overlapping area of the volcanic rocks.5. Tectonic evolution of Xujiaweizi graben and discussion on its dynamic background.Songliao Basin is located in the northeast of Mongolia-North China plate. The northern part of the basin is the Siberia plate, and the eastern part is the paleo-Pacific plate. Tectonic events of Mongolia Okhotsk Ocean closure and paleo-Pacific plate subducting to the eastern margin of Eurasian continent had occurred in the fault depression period(150-105 Ma) around the basin. Due to the special geographical and structural location, formation and evolution of the basin may experience the impact of different tectonic events in different periods. Due to the continuous deposition of sediments, sedimentary basin recorded formation and evolution history of itself as well as the process of structural transformation in the region.Based on the analysis of basin tectonic evolution history, quantizing formation extension rate of fault depression and dividing the tectonic evolution stages. Combined with tectonic attribute of each evolution stage(stretching, extrusion, intensity and the direction of strike slip), analyzing the tectonic stress characteristics of each tectonic stage. Combined with the characteristics of the plate movement and volcanic rocks, exploring the spatial and temporal evolution of regional tectonic system.Based on the tectonic evolution history sections, the formation extension rate of each stages were calculated. The rate is 18.6% in the period of Huoshiling Formation, 20% in the period of Shahezi Formation, and 0.5% in the period of Yingcheng Formation. The distribution of volcanostrata of Yingcheng Formation is consistent with that of the main faults, and all of which are NNW. The fracture activity showed the coexistence of NNE-SSW extension and NNW-SSE strike sliping in the period of Huoshiling-Shahezi Formation, however, NNE-SSW extension is the main. The fault depression suffered NNW-SSE extrusion at the end of Shahezi Formation, formed the fold uplift in central area and depression in south and north area. The fault activity was mainly NNW-SSE strike sliping and shearing in the period of Yingcheng Formation.According to the variation characteristics of the formation extensional rate in different periods and properties and active stages of main faults, the Xujiaweizi fault depression period is divided into 3 stages of tectonic evolution. Extensional period of Huoshiling-Shahezi Formation(late Late Jurassic-middle and late Early Cretaceous, 150-135 Ma), extrusion fold period at the end of Shahezi Formation(middle-late Early Cretaceous, 135-130 Ma), and strike sliping and shearing period of Yingcheng Formation(late Early Cretaceous, 130-105 Ma). The tectonic stress direction of each tectonic evolution stage is inferred from the tectonic characteristics. The coexistence of NNE-SSW extension and NNW-SSE extension, and NNE-SSW extension is the main direction in the extensional period. NNW-SSE squeeze in the squeeze fold period. NNW-SSE extension in the strike sliping and shearing period.The transformation of tectonic stress in the basin may be derived from the transformation of regional tectonic regime under different dynamic background. Comparing the magnitude and direction of regional tectonic stress in each tectonic evolution period of this study with spatial and temporal evolution characteristics of Mongolia Okhotsk tectonic domain and paleo-Pacific tectonic domain in the Mesozoic. Comparing spatial and temporal distribution characteristics between volcanic rocks(Huoshiling Formation in Late Jurassic-early Early Cretaceous and Yingcheng Formation in late Early Cretaceous)in fault depression and Mesozoic volcanic rocks in Northeast China. Combined with geochemical characteristics reveal that the Mesozoic volcanic rocks in Songliao basin are formed in extensional tectonic settings. Comprehensive analysis that tectonic evolution of Xujiaweizi fault depression is jointly determined by the movement direction and velocity of plate and interaction between two plates. Specifically, the extension and strike sliping of the fault depression in the period of Huoshiling-Shahezi Formation is formed under the joint influence of extension caused by the Mongolia Okhotsk Ocean closure and subduction of the paleo Pacific plate. Extrusion at the end of Shehezi Formation is formed under the extrusion background caused by steering and accelerated subduction of paleo Pacific plate. Strike sliping and shearing in the period of Yingcheng Formation is formed in the extensional setting that is caused by the subduction of the paleo Pacific plate. Influence of the Mongolia Okhotsk tectonic domain on Songliao Basin may tend to disappear in the middle-late Early Cretaceous(135-130 Ma).