Angular unconformity in Pennsylvanian strata from 3-D seismic interpretation,Goldsmith Field,West Texas

The Pennsylvanian unconformity,which is a detrital surface,separates the beds of the Permian-aged strata from the Lower Paleozoic in the Central Basin Platform.Seismic data interpretation indicates that the unconformity is an angular unconformity,overlying multiple normal faults,and accompanied with a thrust fault which maximizes the region's structural complexity.Additionally,the Pennsylvanian angular unconformity creates pinch-outs between the beds above and below.We computed the spectral decomposition and reflector convergence attributes and analyzed them to characterize the angular unconformity and faults.The spectral decomposition attribute divides the broadband seismic data into different spectral bands to resolve thin beds and show thickness variations.In contrast,the reflector convergence attribute highlights the location and direction of the pinch-outs as they dip south at angles between 2and 6.After reviewing findings from RGB blending of the spectrally decomposed frequencies along the Pennsylvanian unconformity,we observed channel-like features and multiple linear bands in addition to the faults and pinch-outs.It can be inferred that the identified linear bands could be the result of different lithologies associated with the tilting of the beds,and the faults may possibly influence hydrocarbon migration or act as a flow barrier to entrap hydrocarbon accumulation.The identification of this angular unconformity and the associated features in the study area are vital for the following reasons:1)the unconformity surface represents a natural stratigraphic boundary;2)the stratigraphic pinch-outs act as fluid flow connectivity boundaries;3)the areal extent of compartmentalized reservoirs'boundaries created by the angular unconformity are better defined;and 4)fault displacements are better understood when planning well locations as faults can be flow barriers,or permeability conduits,depending on facies heterogeneity and/or seal effectiveness of a fault,which can affect hydrocarbon production.The methodology utilized in this study is a further step in the characterization of reservoirs and can be used to expand our knowledge and obtain more information about the Goldsmith Field.

Dynamic Features of Angular Unconformity Formation——Extensional and Compressional Angular Unconformities

Angular unconformity is one of the most direct and strongest evidences for approving the tectonic movements of the earth's crust. Its dynamic genesis process has been understood to be mainly related to the compressional setting for a long time. Especially, in a detailed structural analysis for a specific region, when an angular unconformity is discovered people would regard it as the result of orogenic movements of a certain period or a certain episode and neglect the extensional facts. Based on a dialectical point of view of extension-compression, this paper has proved that angular unconformities can be formed not only in compressional settings, but also in extensional ones. Further more, their geological features are compared and he possible genetic mechanisms for angular unconformity under different dynamic settings are studied.

The characteristics of unconformity surface at the bottom of the Paleogene and its significance in hydrocarbon migration in the Sikeshu Sag of the Junggar Basin, Northwest China

The unconformity surface at the bottom of the Paleogene, located in the Sikeshu Sag of the Junggar Basin, Northwest China, is one of the most important hydrocarbon migration pathways and characterized by 3 layers of upper coarse clastic rock, lower weathering crust and leached zone. The upper coarse clastic rock displays features of higher density, lower SDT and gamma-ray logging while the weathering crust in the lower part displays opposite features. The formation water is of NaHCO 3 type but at lower mineralization degree. The QGF indices are generally between 2.19 and 3.77 and the GOI parameters vary from 1% to 5%. From the southeast to the northwest of the sag, the content of saturated hydrocarbon increases from 30.81% to 53.74% while that of non-hydrocarbon and asphaltene decreases. The Pr/nC 17 decreases from 0.65 to 0.47 while the Ph/nC 18 decreases from 0.66 to 0.27, and the content of benzo[c] carbazole declines while the benzo[a] carbazole amount and (alkyl carbazole)/(alkyl+benzo carbazole) ratio both increase. These revealed that the hydrocarbons migrated from the sag to the ramp region along the unconformity surface.

Exploration Significance of Unconformity Structure on Subtle Pools

Vertical structure of an unconformity can be divided into three layers:basal conglomerate or transgressive sand,weathered clay layer and leached rock.When the weathered clay layer has a little thickness or limited distribution,the overlying and underlying strata will contact directly,and the lithology is often different.This lithologic difference causes different fluid transporting capacity,and it also

A STRATIGRAPHIC UNCONFORMITY SURFACE BETWEEN THE NEOGENE AND THE PALEOGENE IN THE JIYANG DEPRESSION,BOHAI BAY BASIN, EASTERN CHINA:GEOLOGIC CHARACTERISTICS, FORMATIONPROCESS AND GEODYNAMIC ORIGIN

Regional stratigraphic unconformity surfaces are commonly observed in both orogenic beltsand sedimentary basins. They have superficially simple features, but in fact contain abundant geo-logic and geodynamic in formation on basin evolution, which was rarely systematically studied.Based on the elaborate research on geologic characteristics of the stratigraphic unconformity sur-face between the Neogene and the Paleogene in the Jiyang depression, Bohai Bay basin, EasternChina, this paper reconstructed its formation process and paleotopography in the late Oligocene. ltis the most interesting that this stratigraphic uncon formity surface represents the transiton of theJiyang depression evolution from rifting extension to thermal down-warping and also implies an u-plift process during the transition. This paper thought it worthy to study the origin of thisstratigraphic uncohformity surface and its relation to the stage transition of basin evolution. ThenpossibIe influences of regional stress field and deep dynamic process variations on the stratigraphicunconforrnity surface formation were anaIyzed. The stratigraphic unconformity surface betweenthe Neogene and the Paleogene in the Jiyang depression should be the direct result of extremestretching of llthosphere and diapirism of hot upper mantle, which can also possibly explain the co-incidence of the unconformity surface formation with the stage transition of basin evolution. Fur-thermore. the influences of the stratigraphic unconformity surface formation on oil and gas accu-mulation, the erosion amount calculation, and the elaborate time structure recorded by thestratigraphic unconformity surface were discussed.

The role of the Mid-Cimmerian Unconformity on the quality of the underlying Skagerrak sandstone reservoir in the Kittiwake Field,central North Sea

The central aim of this paper is to address the role of unconformities in affecting reservoir quality.Do they facilitate diagenesis that leads to either enhanced or reduced porosity through dissolution or cementation?Or,do they have little effect?We have investigated the Late Triassic Skagerrak sandstone reservoir underlying the Mid-Cimmerian Unconformity in the Kittiwake Field,central North Sea.There is strong evidence for the development of secondary porosity through the dissolution of unstable silicate minerals,primarily feldspars.This includes the presence of oversized pores,partial dissolution of framework grains,and complete dissolution of grains leaving remnant grain margins and partially filled cores.This dissolution as a late-stage event is demonstrated by the complete lack of compaction effects on the secondary pores and diagenetic products despite present burial depths in excess of 3000 m.These observations,coupled with an absence of systematic trends linked to the unconformity surface in respects of reservoir porosity,feldspar amount and dissolution,and kaolinization,lead to the conclusion that there has been no effect of the Mid-Cimmerian Unconformity on reservoir quality in the Kittiwake Field.There is no evidence for leaching or cementation linked to meteoric water influx either shortly after deposition or following the uplift and exposure,which led to development of the Mid-Cimmerian Unconformity.Instead,we propose that the late-stage dissolution of feldspar and generation of secondary porosity are most likely related to the influx of organic acids and carbon dioxide generated either from thermogenic maturation of the source rock or from biodegradation of oil within the reservoir near the oil-water contact(OWC)transition zone.

Regional unconformities and their controls on hydrocarbon accumulation in Sichuan Basin, sW China

Based on outcrop,seismic and drilling data,the main regional unconformities in the Sichuan Basin and their controls on hydrocarbon accumulation were systematically studied.Three findings are obtained.First,six regional stratigraphic unconformities are mainly developed in the Sichuan Basin,from the bottom up which are between pre-Sinian and Sinian,between Sinian and Cambrian,between pre-Permian and Permian,between middle and upper Permian,between middle and upper Triassic,and between Triassic and Jurassic.Especially,16 of 21l conventional(and tight)gas fields discovered are believed to have formed in relation to regional unconformities.Second,regional unconformity mainly controls hydrocarbon accumulation from five aspects:(1)The porosity and permeability of reservoirs under the unconformity are improved through weathering crust karstification to form large-scale karst reservoirs;(2)Good source-reservoir-caprock assemblage can form near the unconformity,which provides a basis for forming large gas field;(3)Regional unconformity may lead to stratigraphic pinch-out and rugged ancient landform,giving rise to a large area of stratigraphic and lithologic trap groups;(4)Regional unconformity provides a dominant channel for lateral migration of oil and gas;and(5)Regional unconformity is conducive to large-scale accumulation of oil and gas.Third,the areas related to regional unconformities are the exploration focus of large gas fields in the Sichuan Basin.The pre-Sinian is found with source rocks,reservoir rocks and other favorable conditions for the formation of large gas fields,and presents a large exploration potential.Thus,it is expected to be an important strategic replacement.

准噶尔盆地盆1井西凹陷石炭系火山岩凝析气藏的发现与勘探启示

准噶尔盆地石炭系火山岩油气藏是油气勘探的重点领域之一。根据录测井资料、地球化学分析数据及岩石薄片鉴定资料,结合地球物理方法,厘清了准噶尔盆地盆1井西凹陷石炭系火山岩油气成藏的主控因素,总结出深层火山岩气藏富集规律,明确了有利勘探方向。研究结果表明:(1)准噶尔盆地盆1井西凹陷风城组烃源岩厚度为100~300 m,面积约为5 400 km2,整体进入生凝析油—干气阶段,生气强度大于20×108m3/km2,为凹陷提供了丰富的天然气源。(2)研究区石炭系火山岩岩性复杂,爆发作用形成的安山质火山角砾岩受到风化淋滤作用,可形成物性较好的风化壳型储层。石炭系—二叠系大型不整合面和广泛发育的深大断裂是重要的输导体系,二叠系上乌尔禾组泥岩作为区域盖层,为凝析气成藏提供了保存条件,油藏主要分布在高部位,气藏分布于低部位。(3)通过“两宽一高”(宽方位、宽频带、高密度)技术,提高地震成像精度,联合时-频电磁技术(TFEM),实现了石炭系火山岩的精细刻画,为深层油气藏的勘探提供了有力支撑。石西16井的重大突破,证实了盆1井西凹陷石炭系火山岩具有巨大的勘探潜力。

云南开远市吉德金矿地质特征及成矿模式

吉德金矿矿体主要呈似层状、透镜状产于峨眉山玄武岩与下伏灰岩岩溶不整合面、玄武质火山碎屑岩夹层及次级构造破碎带中,有红土型和岩溶角砾岩型两种类型,金以微粒自然金为主,目前矿床规模较小、地质研究程度较低。通过总结成矿规律、构建成矿模式,对矿区及外围寻找红土型和岩溶角砾岩型金矿具有指导作用。

塔里木盆地西北部震旦系—寒武系不整合面成因——来自沉积学的证据

【目的】塔里木盆地西北部阿克苏地区广泛发育震旦系—寒武系平行不整合面,目前对其形成机制还存在争议。【方法】以沉积学方法为手段,基于上震旦统奇格布拉克组顶部的沉积相及其岩溶现象,通过详细的野外观察和镜下鉴定,对上述不整合面的成因开展研究。【结果】阿克苏地区奇格布拉克组顶部主要发育于台地边缘和斜坡环境。台地边缘包括颗粒滩和台缘礁,岩相为厚层—块状鲕粒白云岩、中—厚层状砂屑白云岩以及块状叠层石礁白云岩。斜坡环境包括的岩相有滑塌角砾白云岩、浊积岩以及薄层状粒泥白云岩。震旦纪晚期,阿克苏地区东部的沉积水体更深,岩溶发育程度也明显强于西部,垂向上以潜流带为主。此外,寒武纪早期沉积的玉尔吐斯组保持了与奇格布拉克组顶部相一致的沉积趋势,两者自西向东均表现为水体加深的地层序列。【结论】该不整合面由震旦纪末期台缘构造活动诱导的掀斜运动形成,导致阿克苏西部发生抬升,并形成了向东流动的大气淡水水文系统,使得东部台缘带岩溶作用强烈。该研究可为该区域震旦系白云岩储层的油气勘探提供参考。

鄂尔多斯盆地怀远运动期不整合类型及其分布特征

加里东早期的怀远运动是早古生代的重要构造运动,它在华北古陆主要表现为区域性不整合。基于野外露头、地质图、钻井与最新的地震资料,研究鄂尔多斯盆地怀远运动期不整合的标志、类型及其时空分布。结果表明:(1)鄂尔多斯盆地怀远运动不整合面附近发育砂砾岩、风化壳,测井曲线具高声波时差、高伽马、高微量元素(U、Th、K)含量、高钍铀比值及低电阻率、低密度特征,地震响应特征为强相位,表现为不整合面上、下波组振幅强度,频率、连续性存在明显差异,局部可见削截点反射。(2)怀远运动期鄂尔多斯盆地形成平行、低角度削截2种不整合,前者广泛分布于盆地及周缘,后者分布于盆内古隆起附近。(3)鄂尔多斯盆地及周缘受到怀远运动3幕活动影响,依次表现为:中寒武世张夏组沉积期盆地东南部最先发生抬升,而后向西北部推进;晚寒武世三山子组沉积末期,盆内古隆起发育并遭受剥蚀;晚寒武世末期—奥陶纪马家沟组沉积前,受全球海平面下降的影响,盆内马家沟组底部的碳同位素呈负漂。上述认识可为鄂尔多斯盆地下古生界油气勘探新领域研究提供参考依据。

塔里木盆地晚奥陶世至侏罗纪不整合面分布特征及成因

不整合研究对盆地演化、古地貌重建及油气勘探具有重要指导意义。受多期构造活动影响,塔里木盆地发育多种不整合。本文通过地震剖面资料分析,研究盆地晚奥陶世—侏罗纪形成的不整合面展布及结构特征,并探讨其动力学机制。研究结果表明:(1)塔里木盆地不整合结构类型主要有平行、削截、褶皱、断褶、叠加超覆、单一超覆及异岩不整合;(2)志留系底界发育断褶、褶皱、削截、叠加超覆、单一超覆及平行不整合,呈东西带状分布,在盆地中南部发育大范围角度不整合,阿瓦提–满加尔坳陷中北部发育平行不整合,塔北隆起发育小范围角度不整合;泥盆—石炭系底界主要发育平行、削截、断褶、单一超覆、叠加超覆及异岩不整合,平面分布具有以北部坳陷沉积中心连线为轴近似南北对称特点;三叠系底界不整合由北向南,发育叠加超覆不整合、低角度削截不整合、平行不整合,反映海西晚期盆地北缘南天山洋闭合造山运动;侏罗系底界发育褶皱、叠加超覆、削截及异岩不整合,整体以满加尔–英吉苏凹陷沉积中心连线为轴似对称分布;(3)不整合面分布特征反映出构造运动的差异性、古隆起的迁移及活动性,不整合对盆内油气成藏具有控制作用。

塔里木盆地顺北西部地区火成岩侵入体发育特征及其与断裂耦合关系

塔里木盆地顺北西部地区火成岩侵入体分布广泛,对地层和含油气系统改造作用明显,但该地区火成岩侵入体发育特征及其与深部走滑断裂带之间的耦合关系并不十分清楚。基于顺北西部地区最新二维和三维地震资料,精细刻画了火成岩侵入体形态特征,划分了火成岩侵入体不同类型及组合关系,判别了火成岩侵入体发育期次,确定了火成岩侵入体与深部走滑断裂耦合关系。结果表明:(1)顺北西部地区火成岩侵入体具有“一”字形、舌形、碟形以及半碟形几种形态类型。(2)单一侵入岩床通过相接、相连及错断组合形成岩床复合体。(3)火成岩侵入体主要分布于中-下奥陶统顶面至中-下泥盆统顶面之间,侵入岩床活动类型包括单期单层、单期多层和多期多层3类。(4)不整合面和断裂作为岩浆侵入的通道,控制了火成岩侵入体发育数量和规模,导致研究区南部和北部侵入作用存在明显差异。(5)研究区北部深部走滑断裂作为岩浆主要侵入通道,纵向侵入作用较强;南部岩浆以不整合面为主要侵入通道,横向侵入作用更强。

准噶尔盆地西部坳陷带二叠系构造-地层层序与盆地演化

玛湖凹陷、沙湾凹陷和盆1井西凹陷是准噶尔盆地西部三大富烃凹陷,通过对深钻井、测井和深反射地震剖面资料的分析,揭示出这三大凹陷在早二叠世隶属于同一个坳陷,据此厘定了准噶尔盆地西部坳陷带的范围。在西部坳陷带内的二叠系—上三叠统共识别出3套区域性不整合面,依据不整合面划分了3套构造层,分别是下二叠统构造层、中二叠统构造层和上二叠统—三叠系构造层。在此基础上,运用断层相关褶皱理论和平衡剖面技术进行精细构造解析和构造演化分析,并结合前人对于准噶尔盆地热史和火山岩地球化学特征的分析,重点解析西部坳陷带二叠纪—三叠纪的构造演化过程,认为二叠纪—三叠纪西部坳陷带共经历了3期构造演化,分别是早二叠世伸展裂陷阶段(早二叠世末期挤压)、中二叠世弱伸展拗陷阶段和晚二叠世—三叠纪挤压隆升阶段,且西部坳陷带的构造演化与西准噶尔和准噶尔地块的逆时针旋转在时空上相耦合。该研究成果对于揭示准噶尔盆地二叠纪盆地性质及二叠纪以来的陆内演化过程具有重要的意义。

云南新平佐思孔铅锌矿矿床不整合面成矿作用

佐思孔铅锌矿是位于康滇地轴、扬子准地台西缘的多金属矿床,该区区域构造运动强烈、岩浆活动频繁、构造行迹复杂,与“大红山式”铁、铜矿形成具有一定成因联系,具有火山喷流-沉积相带边缘相特征。受区域多期次构造运动的影响,“大红山式”红山组、曼岗河组第三岩性段剥蚀殆尽,并形成三叠系不整合面。不整合面角砾岩主要为含绢云母、白云母石英砂质细砾岩及砾质粗粒岩屑砂岩,铁富集主要以褐铁矿形式出现。

辽宁新元古代各阶段升降界面的再认识

【研究目的】辽宁地区位于华北板块东北缘,传统观点认为华北板块自吕梁-四堡运动之后直至古生代末期,一直处于构造稳定期,没有受到明显的构造运动影响。近年来研究结果表明,辽宁地区新元古代时期构造运动相对较为频繁。前人根据大连地区1/25万区调(2001)确认的风化夷平面,将辽宁新元古代时期的构造运动统称为晋宁运动,并划分为四个次一级构造运动,分别为万福升降运动(相当芹峪升降运动)、驼山升降运动、茶叶沟升降运动和大连升降运动,其中驼山升降运动和茶叶沟升降运动的划分定义存在不准确性,其命名有待商榷,需进一步研究探讨。【研究方法】本文以辽宁地区的新元古代地层为研究对象,以区域地质调查资料为基础,结合同位素测年资料对辽宁省新元古代升降运动进行了重新划分与定义。【研究结果】通过区域对比研究划分出晋宁运动和大连运动两个旋回和七个次一级的升降运动,并将其对应于三次冰期、两次间冰期以及冰期前和冰期后。七个次一级的降运动分别命名为芹峪升降、殷屯升降、大孤山升降、太子河升降、复州升降、南关岭升降和大连升降。殷屯升降、大孤山升降、太子河升降、复州升降和南关岭升降等五个次一级构造运动,本次统称之为晋宁运动,代表新元代早期的构造运动;新元古代中晚期(约890~541 Ma),辽宁地区缺失原兴民村组之上地层,代表新一期构造运动抬升的产物,本次将该期运动称之为大连运动,与西北的柯坪运动和南方的澄江运动基本相当。芹峪升降界面标志着四堡运动的结束和晋宁运动的开始;青白口系周家崴子组和王家坦子组之间的沉积间断称为南关岭升降,南关岭升降界面标志着晋宁运动的结束和大连运动的开始;大连升降界面(约541 Ma)的确认,标志着新元古代大连运动的结束和早古生代兴凯运动的开始。【结论】辽宁地区新元古代早期构造运动较为频繁,存在三次冰碛作用和多处平行不整合界面,可划分为晋宁运动和大连运动两个旋回以及七个次一级的升降运动,为区域地层研究对比提供重要参考资料。

Late Paleozoic Sequence-Stratigraphic Frameworks and Sea Level Changes in Dianqiangui Basin and Its Adjacent Areas with Systematic Revision of Regional Unconformities

The formation process of the Dianqiangui basin, a special basin, occurred after the Caledonian orogeny, in the south of Guizhou, the west of Guangxi and the southeast of Yunnan, experienced three periods: it began in the Devonian, persisted in the Carboniferous, and became fiercer in the Permian. Controlled by syndepositional fault-zones, varieties of isolated carbonate platforms, large and small, were developed in the background of a deep-water basin, namely, an inter-platform ditch. And a special paleogeographical Late Paleozoic pattern marked by “platform-basin-hill-trough” was produced in both the Dianqiangui basin and its adjacent areas. Affected by regional tectonic activities and the global changes in the sea level, the platform carbonates and coal measures superimposed each other cyclically on the attached platform. The reef-building on the isolated platform and the margin of the attached platform corresponds to the development of the shale succession in the deep-water basin. All of these elementary characteristics reflect a regular and sophisticated filling succession of the Dianqiangui basin, a result of the dual controls of the regionally tectonic activities and the eustacy. Based on the two elementary features of the third-order sequences, i.e. the regularity of sedimentary-facies succession in space and the simultaneity of environmental changes in time, 25 third-order sequences could be discerned in the Upper Paleozoic strata in the Dianqiangui basin and its adjacent areas. On the basis of the two kinds of facies-changing surfaces and the two kinds of diachronisms in stratigraphic records, the regional Late Paleozoic sequence-stratigraphic framework in the Dianqiangui basin and its adjacent areas can be established. There are two types of facies-changing surfaces and two types of diachronisms in stratigraphic records: the static type, a result of the change in sedimentary facies in space, and the dynamic type, a result of the change in time. These two types of facies-changing surfaces led to the generation of the two types of diachronisms: the diachronism of facies-changing surfaces that was formed by the static facies-changing surfaces, and the diachronism of punctuated surfaces that was formed by the dynamic facies-changing surfaces. The two types of facies-changing surfaces and the two types of diachronisms in stratigraphic records are the key to the establishment of the sequence-stratigraphic framework. The sequence boundaries could be divided geologically into four types: tectonic unconformity, sedimentary unconformity, drowned unconformity and their correlative surfaces. All of these four types can be further grouped into exposed punctuated surfaces and deepened punctuated surfaces. The tectonic unconformity is similar to Type Ⅰ sequence boundary, and the sedimentary unconformity is similar to Type Ⅱ sequence boundary defined by Vail et al.. In terms of sequence stratigraphy, the tectonic unconformities of the Ziyun movement, the Qiangui epeirogeny and the Dongwu revolution as well as the drowned unconformity in the transitional period from the Permian to the Triassic can be systematically defined and their geological characteristics are briefly presented.

Estimation of the amount of erosion at unconformities in the last stage of the Eocene Sanduo period in the Subei Basin,China

Strata erosion is a widespread phenomenon in sedimentary basins. The generation, migration, and accumulation of hydrocarbon is influenced by the scale of erosion, so estimating the amount of erosion is essential in the analysis of oil and gas bearing basins. According to the geological features in the Subei Basin and the actual data, using the integrated method, we estimated the level of erosion at the unconformities caused by the Sanduo event. By using the mudstone interval transit time method and the vitrinite reflectance method on data from typical wells, it can be concluded that the Gaoyou, Jinhu, and Hongze depressions suffered strong strata erosion from the late Eocene to Oligocene, and the total strata erosion thickness was 300–1,100 m. Different tectonic units in the same depression have extremely uneven erosion intensity: the low convex regions have the maximum erosion thickness, amounting to 800–1,100 m; the slope regions have an erosion thickness of generally 600–800 m; the erosion thickness of the slope-hollow transition zone is 300–500 m. For the whole basin, we used the strata thickness trend analysis method combined with the interval transit time and vitrinite reflectance methods to estimate the erosion thickness in the Sanduo period. The results show that the most severe erosion of the Sanduo event in the Subei Basin is between 1,000 m to 1,200 m, mainly located in depressions around the Jianhu Uplift; the deep hollow area has the least erosion, generally about 300–600 m, and the erosion in the slope area is about 600–900 m. Compared with the northern part, the southern part has relatively little erosion. It is also proved that the Sanduo movement has heterogeneous intensity, and the western region has greater intensity than the eastern region.

Kinematics of syn-tectonic unconformities and implications for the tectonic evolution of the Hala'alat Mountains at the northwestern margin of the Junggar Basin,Central Asian Orogenic Belt

The Hala＇alat Mountains are located at the transition between the West Junggar and the Junggar Basin. In this area, rocks are Carboniferous, with younger strata above them that have been identified through well data and high-resolution 3D seismic profiles. Among these strata, seven unconformities are observed and distributed at the bases of： the Permian Jiamuhe Formation, the Permian Fengcheng Formation, the Triassic Baikouquan Formation, the Jurassic Badaowan Formation, the Jurassic Xishanyao Formation, the Cretaceous Tugulu Group and the Paleogene. On the basis of balanced sections, these unconformities are determined to have been formed by erosion of uplifts or rotated fault blocks primarily during the Mesozoic and Cenozoic. In conjunction with the currently understood tectonic background of the sur- rounding areas, the following conclusions are proposed： the unconformities at the bases of the Permian Jiamuhe and Fengcheng formations are most likely related to the subduction and closure of the Junggar Ocean during the late Carboniferous-early Permian; the unconformities at the bases of the Triassic Baikoucluan and Jurassic Badaowan formations are closely related to the late Permian Triassic Durbut sinistral slip fault; the unconformities at the bases of the middle Jurassic Xisbanyao Formation and Cretaceous Tugulu Group may be related to reactivation of the Durbut dextral slip fault in the late Jurassic -early Cretaceous, and the unconformity that gives rise to the widely observed absence of the upper Cretaceous in the northern Junggar Basin may be closely related to large scale uplift. All of these geological phenomena indicate that the West Junggar was not calm in the Mesozoic and Cenozoic and that it experienced at least four periods of tectonic movement.

Geological characteristics and tectonic signifcance of unconformities in Mesoproterozoic successions in the northern margin of the North China Block

Several stratigraphic breaks and unconformities exist in the Mesoproterozoic successions in the northern margin of the North China Block. Geologic characters and spatial distributions of five of these un- conformities, which have resulted from different geological processes, have been studied. The uncon- formity beneath the Dahongyu Formation is interpreted as a breakup unconformity, representing the time of transition from continental rift to passive continental margin. The unconformities beneath the Gaoyuzhuang and the Yangzhuang formations are considered to be the consequence of regional eustatic fluctuations, leading to the exposure of highlands in passive margins during low sea-level stands and transgressive deposition on coastal regions during high sea-level stands. The unconformity atop the Tieling Formation might be caused by uplift due to contractional deformation in a back-arc setting, whereas the uplift after the deposition of the Xiamaling Formation might be attributed to a continental collision event. It is assumed that the occurrences of these unconformities in the Mesoproterozoic successions in the northern margin of the North China Block had a close bearing on the assemblage and breakup of the Columbia and Rodinia supercontinents.