A great thickness of evaporite strata were developed in the Ordovician in eastern Ordos BasinQuite a few researchers have proposed different views about their genesisThis paper presents another point of view in which the evaporites were deposited under the condition of the saline basin's periodical isolation from the open seaThis hypothesis is based on distribution patterns of the microfacies，profile architecture and geochemistry characters of the evaporitesThe idea of “reinfluxing and redissolving",is developed for explaining the origin of the thin black claylayers in thick saline rocks and the origin of various colors of the evaporitesThe lack of sylvitecarnallite layers is also consideredA new concept，“dissloving deposition bed"，is proposed to explain a special mud layer in evaporite strataThe microfacies of the Ordovician in this area show that some biobuildups including reefs or biomounds,grew on the elevated areas underwater along the eastern edge of the basinDuring the period of sea level falling，it acted as a barrier between the halite depression and the open sea that kept the halite depression isolated from the open sea，and brought it into the stage of evaporation drying to form halite bedsThe abrupt event of seawater reinfluxing，as a result of sealevel rising，could make part of the formerly formed halite bed dissolving(reinfluxing and redissolving) At the same time,the undissolved residues,dispersed in halites，gathered to form a bed(dissolving deposition bed),which may prevent the underlying halite from further dissolvationThe process of reinfluxing and redissolving may be the main reason for the lack of evidence of typical evaporation dryingThe sequence distribution style of carbonate and evaporites，weremainly controlled by regional sealevel change cycleCarbonates were formed during transgressive period or highstand period,while evaporites were formed during lowstand period

鄂尔多斯盆地东部奥陶系发育巨厚的蒸发岩地层，已有不少学者对其成因提出了不同的假设。本文通过对该区蒸发岩微相分布规律、剖面结构及地球化学等特征的分析，提出该区蒸发岩形成于成盐盆地与外海周期性隔绝的“干化蒸发”条件下的新认识。首次提出用“回灌重溶”机制来解释厚层石盐岩中的泥质薄夹层的成因、各种石盐岩颜色的成因及蒸发岩剖面中缺乏干化蒸发晚期阶段的钾镁盐层的原因，并由此提出 “溶积层”的概念。微相分析表明，在形成蒸发岩的盐坳盆地东部的水下古地形隆起上，发育有堤坝状的礁或生物丘等生物建隆，其在地形上的隆起构成了盐坳盆地与外海的障壁，在海平面下降期间可使盐坳与外海完全隔绝而进入干化蒸发阶段，从而形成了石盐沉积；由海平面上升所造成的突发性海水回灌事件又使先成的石盐层部分溶解（回灌重溶），并由于不溶残余物的富集成层（溶积层）而阻止了下伏石盐层的进一步溶解。这是该区蒸发岩地层中缺少典型干化证据的主要原因。本区奥陶系马家沟组碳酸盐岩与蒸发岩间互分布的地层层序结构特征，主要受控于区域海平面变化旋回，碳酸盐岩主要形成于海侵期和高水位期，蒸发岩则主要形成于低水位期。

The author initiated and adopted the single factor analysis and multifactor comprehensive mapping method as a new methodology of quantitative lithofacies palaeogeography in order to reconstruct quantitative lithofacies palaeogeography By meaning， single factors are the factors which can independently reflect some characteristics of sedimentary environments of a stratigraphic interval of a section in an area Presence or absence as well as high or low percentage of single factors can all independently and quantitatively reflect some characteristics of sedimentary environments of the interval of the section in the area, such as water depth, water energy, nature of the sedimentary environments, etc The thickness of the interval and its specific rock types, texture constituents, mineral composition, chemical composition, fossils and their ecological assemblages, etc, can all be taken as single factorsThe single factor analysis and multifactor comprehensive mapping method include three steps First, an indepth petrological and lithofacies study of each section, especially fundamental section, should be made in order to obtain various reliable firsthand qualitative and quantitative data, especially quantitative data, and to understand the characteristics of sedimentary environments of each interval of each section Second, according to the mapping units, from the quantitative data of each section, select those factors which can independently reflect the characteristics of sedimentary environments, ie the single factors Then the statistics of the percent contents of various single factors of each mapping unit of each section in the area are made Based on these percentage data, the various quantitative single factor maps, mainly isoline maps， are composed These single factor maps can quantitatively reflect the characteristics of sedimentary environments of the mapping unit of the area from different aspects This is the single factor analysis Third, through superimposition and synthesis of these quantitative single factor maps, combined with other quantitative and qualitative data of this mapping unit of the area, with the dross discarded and the essential selected, with the false eliminated and the true retained, the quantitative lithofacies palaeogeography map of the mapping unit of the area can be composed This is the multifactor comprehensive mappingThe core of this methodology is quantification That is, based on the quantitative single factor data of each section, starting from analysis of various quantitative single factor maps, through superimposition and comprehensive analysis and judgement, finally the quantitative lithofacies palaeogeography map can be composed In such palaeogeography map, the definition of each palaeogeographic unit is based on quantitative data and quantitative single factor maps Therefore, such lithofacies palaeogeography maps are no longer sketchy and qualitative, but quantitative instead Quantification is the most important character of the author's lithofacies palaeogeography maps This makes lithofacies palaeogeography map and lithofacies palaeogeogaphy develop into the quantitative stage This is an important development in palaeogeography

笔者倡导并持续采用单因素分析多因素综合作图法这一定量的岩相古地理学方法论。单因素是能独立地反映某地区、某地质时期、某沉积层段沉积环境某些特征的因素,它的有无或含量的多少均可独立地反映该地区、该层段沉积环境的某些特征,如沉积环境水体的深浅、能量大小、性质等。某沉积层段的厚度、岩石类型、结构组分、矿物成分、化学成分、化石及其生态组合等，均可作为单因素。单因素分析多因素综合作图可分三个步骤：第一，是对各剖面尤其是各基干剖面进行认真的地层学和定量岩石学研究，取得各种第一手的定性和定量资料，尤其是定量资料，了解各剖面各沉积层段的沉积环境特征。第二，在已取得的各剖面的定量资料中，按要求的作图单位，选择出那些能独立地反映其沉积环境特征的因素，即单因素；并把全区各剖面各作图单位的各种单因素的百分含量都统计出来，作出各种相应的单因素图，主要是等值线图。这些单因素图可以从不同的侧面定量地反映该地区该沉积层段的沉积环境。这就是单因素分析。第三，把这些定量的单因素图叠加起来，并结合该地区该沉积层段的其他定量和定性资料，去粗取精，去伪存真，全面分析，综合判断，即可编制出该地区该沉积层段的定量岩相古地理图。这就是多因素综合作图。这一方法论的核心是定量，即以各剖面的定量单因素资料为基础，从各单因素图的分析入手，再通过各单因素图的叠加和综合分析判断，最后作出定量的岩相古地理图。在这种岩相古地理图中，各古地理单元的确定都有确切的定量资料和单因素图为依据。定量是笔者等用单因素分析多因素综合作图法编制出的岩相古地理图的最大特色。这就使岩相古地理图和岩相古地理学发展到了定量的阶段。这在古地理学中是个重大的进展。

The Lower Palaeozoic in Ordos were identified with the comprehensive method of integrating the outcrop sections, well sections and seismic profiles,first integrating the synthetic seismograms and the vertical seismic profiles(VSP)of 11 wells. Totally 371 pieces and 15 683 kilometers of seismic profiles were processed and stratigraphically interpretated,which greatly upgraded the interpretation of the seismic profiles.Based on above achievements, the Lower Palaeozoic in Ordos were divided into five stratigraphical units, i.e. Lower and Middle Cambrian, Middle and Upper Cambrian and Lower Ordovician, Lower Majiagou Group, Upper Majiagou Group, and pingliang Formation and Beiguoshan Formation, and their isopach maps were compiled. These maps show that the palaeotectonic pattern of the former four ages was “one uplift and three depressions”,i.e. one middle uplift was surrounded by the western depression,southern depression and eastern depression, which together took the shape of “U”, and the palaeotectonic pattern of the last age was “one uplift and two depressions”, i.e. the huge Ordos uplift borders on the western depression and southern depression, which together took the shape of “L”. At the end of the Ordovician the Ordos and the whole North China Platform formed an unified uplift and the framework of uplifts alternating with depressions vanished.

用野外露头剖面、钻井剖面和地震剖面相结合的综合研究方法，首先用人工合成记录结合11口井的VSP资料，对鄂尔多斯下古生界进行层位标定。在此基础上，对鄂尔多斯全区下古生界371条共长15 683 km的地震剖面进行了重新处理和地层解释，这大大提高了解释精度。以此为基础，把鄂尔多斯下古生界划分为5个地层单位，即寒武系中下部、寒武系中上部及冶里组和亮甲山组、马家沟群下部、马家沟群上部、平凉组和背锅山组，并作出了它们的等厚图。从这些等厚图可以看出：前4个地质时期的古构造格局为“一隆三凹”，即西缘凹陷、南缘凹陷和东缘凹陷(或东部凹陷)呈“U”形从西南东三面包围着一个中部隆起。最后一个地质时期的古构造格局为“一隆二凹”，即西缘凹陷和南缘凹陷呈“L”形镶嵌在一个巨大的鄂尔多斯隆起的西缘和南缘。到奥陶纪末，鄂尔多斯全区以及整个华北地台均变为隆起区，从而结束了鄂尔多斯隆起和凹陷并存的历史。

The Ordovician in Ordos was identified with a comprehensive method of integrating the outcrop sections, well sections and seismic profiles. Totally 213 pie ces and 8317 km of seismic profiles were disposed and stratigraphically interpre tated, which greatly upgraded the interpretation of the seismic profiles. Based on above achievements, the Ordovician in Ordos was divided into 10 stratigraphi cal units, i.e. Yeli Formation, Liangjiashan Formation, Majiagou Formation Ⅰ, Majiagou Formation Ⅱ, Majiagou Formation Ⅲ, Majiagou Formation Ⅳ,  Majiagou Formation Ⅴ, Majiagou Formation Ⅵ, Pingliang Formation and Beiguoshan Formation.Their isopach maps were compiled. These maps show that the palaeotectonics of Yeli Age and Liangjiashan Age was in the style of one uplift and three depressions, i.e. o ne middle uplift was surrounded by western depression, southern depression and ea stern depression, which took the shape of “U". The palaeotectonics of Majiagou Age Ⅰ to Majiagou Age Ⅵ shaped up one uplift and three depressions, although the shape, size and location of uplifts and depressions were changeable. The palaeo tectonics of Pingliang Age was in the shape of one uplift and two depressions, i.e. the enormous Ordos uplift was edged with the western depression and southern depression, which took the shape of “L". The  palaeotectonics of Beiguoshan Age was in the shape of one u plift and one depression, i.e. the residual southwestern depression was located at the southwestern corner of the more enormous Ordos uplift. At the end of Ordov ician, the Ordos and the whole North China Platform came into an unified uplift and the framework of uplifts cooperated with depressions vanished.

用野外露头剖面、钻井剖面和地震剖面相结合的研究方法，对鄂尔多斯地区奥陶系213条共长8317 km的地震剖面行了重新处理和地层解释，这大大提高了解释精度。以此为基础，把鄂尔多斯奥陶系划分为10个地层单位，即冶里组、亮甲山组、马家沟一组、马家沟二组、马家沟三组、马家沟四组、马家沟五组、马家沟六组、平凉组和背锅山组，并绘制了它们的等厚图。从这些图可以看出，冶里期和亮甲山期的古构造格局为“一隆三凹”，即西缘凹陷、南缘凹陷和东缘凹陷呈“U”字形包围着一个中部隆起。到马家沟一期至六期，“一隆三凹”的古构造格局仍然基本上保持着，尽管其隆起和凹陷的形状、大小和位置均有所变化。到平凉期，古构造格局为“一隆二凹”，即西缘凹陷和南缘凹陷呈“L”形镶嵌在一个巨大的鄂尔多斯隆起的西缘和南缘。到背锅山期，古构造格局为“一隆一凹”，即一个残留的西南缘凹陷位于一个更加庞大的鄂尔多斯隆起的西南缘一隅。到奥陶纪末，鄂尔多斯地区以及整个的华北地台均变为隆起区，从而结束了鄂尔多斯地区隆起与凹陷并存的古构造格局。

On the basis of the drilling data of 16 wells, the 2D profiles of the seismic characteristic parameters (e.g. velocity) were built for a total of 5000 km seismic sections by applying a comprehensive calibration on these seismic data. Furthermore, using the variable velocity mapping method, the isopach map of the saliferous strata of the M56 stage, which are the main potash rich strata in the Ordovician northern Shaanxi salt basin, was preliminarily obtained, and the tectonic differentiation in this region was also discussed. In the M56 stage of Ordovician, the northern Shaanxi salt basin shows the tectonic pattern of “two depressions and one lifting”, and there are five salt sags with the thickness of more than 150 meters in this region. In this study, we found that the western salt sags showing NNE spreading are controlled by the syndepositonal faults in the west of the basin, and the east two salt sags is divided into two salt sub-swamps (the east and the west). All these observations above indicate that there exists many relatively active salt sub-sags that controlled by the syndepositonal faulting in the inner northern Shaanxi salt basin. These salt sags are good for gathering potassium materials, and provides enough accommodation space for the large marine potash formation. And more drilling works are needed to get further understanding.

本文以16口钻井资料为基础，通过井震联合标定解释，获取累计5000km二维地震剖面的速度等特征物性参数，利用变速成图方法，初步编制出陕北奥陶纪盐盆主要含钾层段――马五6亚段盐系等厚图，并进行相应的构造分异解析。在奥陶纪马五6亚期，陕北盐盆总体表现为“两坳一隆” 的构造格局，其中厚度大于150m的成盐凹陷主要有5个。西部盐坳的发现和东二盐凹被进一步分解为东、西2个次级盐洼，系本次研究取得的新进展和认识，表明陕北盐盆并非是一个“平底锅”，其内部存在多个受同沉积断裂控制的、相对活动的次级盐凹，这些次级盐凹将有利于成钾物质的汇聚，为可能的大规模海相钾盐成矿提供了足够的可容空间和有利的构造区位。

Through systematically analyzing the shape of the Central paleouplift, the strata cropped out at its core, and the dynamical mechanism and evolution characteristics of its formation and development according to recent exploration data and predecessors’research achievements, it was considered that the Central paleouplift in E’erduosi Basin was formed in Early Cambrian; the westtoeast compressive stress coming from Qilian trough and the southtonorth compressive stress coming from Qinling trough were the dynamical base of the formation and development of the Central paleouplift; and the paleouplift had successively suffered two developing stages of intraplatform uplift and platform edge uplift in the process of E’erduosi Basin’s expansion from rift deposit to carbonate platform deposit. Therefore the sustained development of the Central paleouplift had important control effect on the Palaeozoic sedimentary framework and early gas reservoir formation in E’erduosi Basin, and the paleouplift area and the slopes Surrounding it are still prospective for gas exploration at present.

依据近期勘探资料，结合前人研究成果，通过对中央古隆起的形态、核部出露地层、形成发育的力源机制及其演化特征进行系统分析后认为：鄂尔多斯盆地中央古隆起形成于早寒武世，来自祁连海槽由西向东的挤压应力和秦岭海槽由南向北的挤压应力是中央古隆起形成和发育的动力学基础。在鄂尔多斯盆地由裂谷沉积发展为碳酸盐岩台地的过程中，中央古隆起先后经历了台内隆起和台缘隆起两个阶段。因此，中央古隆起的持续发育对鄂尔多斯盆地古生界沉积格局和天然气的早期成藏具有重要的控制作用，指出古隆起区及其周缘斜坡带，目前仍具有较大的勘探潜力。

A set of marine carbonate rocks interbedded with clastic rock was deposited in the Early Paleozoic in the Ordos area，with huge thickness and various sedimentary types.According to characteristics of sedimentary sequence，petrology，sedimentary structure and palaeontology，four depositional systems can be distinguished in the Lower Paleozoic in the Ordos including coast，carbonate rock platform，platform margin and deepwater slope-trough，which can be subdivided into several subfacies and microfacies.For each facies，its sedimentary characteristics，spatial and temporal distribution were summarized.Based on these studies，the lithofacies palaeogeography maps of the Lower Paleozoic in the Ordos were compiled by each age.The Early Cambrian only formed a set of mud-sand flat and mud-dolostone flat sediments around the southwestern margin of the Ordos land.Followed the marine transgression spread in the Middle Cambrian，the central-eastern part of the Ordos was gradually transformed from sand-mud flat to restricted platform-open platform sediments，and developed platform edge to deepwater trough sediments in the western margin.Since the Late Cambrian marine regression，the central-eastern part of the Ordos was changed to restrictive platform dolostone flat sediments，but the deepwater slope-trough facies was still developed in the western margin.To the Early Ordovician，the main body of the Ordos became a uniform palaeoland with mud-dolostone flat and dolomitic-limestone flat sediments around its southeastern margin.Then a massive transgression in the Middle Ordovician led to an extensive shallow epicontinental deposits in the Ordos.The Northern Shaanxi Depression formed a gypsum salt lake，and developed restrictive platform-open platform sediments outside it.At the late Middle Ordovician，the southwestern margin began to develop platform foreslope-deepwater trough facies sediments.The Late Ordovician Caledonian Movement uplifted the main Ordos as a land，and formed a narrow sea continuing deposit in the southwestern margin of the Ordos.The sedimentary facies was quickly changed from carbonate rock platform to deepwater slope-trough，and developed some reefs in the southern margin of Ordos，indicating the epicontinental marginal rimmed platform deposit.

鄂尔多斯地区早古生代沉积了一套以海相碳酸盐岩夹碎屑岩为主的沉积建造，沉积厚度巨大，沉积类型多样，可以划分为海岸沉积、碳酸盐岩台地沉积、台地边缘沉积和深水斜坡—海槽沉积4大沉积类型以及数个亚相和微相，并对各个沉积相带的沉积特征和时空展布进行了归纳和总结。在此基础上，编制了早古生代岩相古地理图1套。岩相古地理研究表明，早寒武世仅在鄂尔多斯西南缘环陆发育了一套泥砂坪、泥云坪沉积；中寒武世海侵扩大，广大的鄂尔多斯中东部逐渐由砂泥坪发展为局限—开阔台地沉积，在其西缘发展为台地边缘—深水海槽沉积；晚寒武世开始海退，中东部主体演变为局限台地云坪沉积，而西侧的深水斜坡—海槽则继续发育。早奥陶世鄂尔多斯主体为一古陆，仅在东南缘形成环陆泥云坪和云灰坪相沉积；中奥陶世的大规模海侵使鄂尔多斯大部地区形成了广阔的浅水陆表海沉积，在陕北凹陷发育膏盐湖沉积，向外依次发育局限台地和开阔台地沉积，中奥陶世晚期西南侧开始发育台地前缘斜坡—深水海槽沉积；晚奥陶世的加里东运动使鄂尔多斯整体抬升为陆，仅在西南缘的狭窄海域接受沉积，由碳酸盐岩台地很快过渡为深水斜坡—海槽沉积，并在南缘发育生物礁建造，形成陆缘海型镶边台地沉积。

从古生物-古地理研究分析,研究华北地块的古构造位置是分析早古生代全球古地理格局的一个重要问题。然而,有关华北地块早古生代的构造位置及其与邻近地块相对位置关系的

Lower Ordovician Majiagou Formation is composed of six members in which Ma1,Ma3 and Ma5 are evaporate including dolomite, anhydrite and salt and Ma2, Ma4, Ma6 are limestone with dolomite in Ordos Basin. Majiagou Formation is formed by cyclic sedimentation by intermeshed evaporite and carbonate. Upwarps around the basin and a downwarp in the centre of the basin constitute the EarlyOrdovician Palaeogeomorphologic pattern. Upwarps as underwater barriers led to exchange limitation between lagoon and exterior sea and weakness of lagoon hydrodynamic force during high sea level stage. During low sea level stage, upwarps resulted in the entire isolation between lagoon and exterior sea. Owing to the strong evaporation, lagoon level was greatly lower than exterior sea level .So lagoon margin flat was partly exposed to atmosphere and the deep sag in the centre of the basin was covered by concentrated bittern.  〓〓Analysis of single well and well tie sedimentary facies demonstrates the depositional setting from Ma1 and Ma2 member is composed of two integrating cycles from transgression to regression due to sea water invasion from the east and southeast of the basin. Ma3 member is mainly composed of regressive deposition owing to the rapid extensive transgression in the early Ma3 stage. North China sea, Qilian oceanic trough and Qinling oceanic trough formed a united basin and the centre uplift zone was developed to a platform margin , the west of the centre uplift zone was developed to a basin and a slope and the east of the centre uplift zone was developed to a broad lagoon during the biggest sea water invasion in Ma4 stage. Ordos Basin was isolated from exterior sea and the salty lagoon was changed to a salt lake gradually during Ma5 stage owing to sea level descend and barrier of the centre uplift zone. In Ma6 stage the Basin was changed to a broad lagoon. 〓〓Based on the pioneer research of paleostructure, paleogeography, paleohydrodynamic as well as paleoclimate, a new model barrierlagoon sedimentary model of Majiagou Formation is developed. The model includes three phases during transgression to high sea level (phaseⅠ), carbonate deposits in broad barrierlagoon, and mixed carbonate /evaporate succession in concentrated lagoon during regression (phaseⅡ), and anhydrite and salt in saline during low sea level( phaseⅢ). 〓〓And two main types of carbonate reservoir including the dolomite reservoir in relation to karstification and the crystallization dolomite reservoir in connection with dolomitization are discovered in this paper. The authors further analyze the carbonate reservoir distribution law within the mentioned model, conclude that beachflat of platform edge of the model(phase Ⅰ)and lagoon margin flat of phaseⅡ are favorable for reservoir growth and point out that the reservoir of Majiagou Formation is karst type and is mainly distributed on the top of Ma5,Ma3 and Ma1 Formation. Anhydrite and salt of phase Ⅲ are important seal rock.

依据鄂尔多斯盆地古构造古地理背景、古水动力以及古气候条件，结合单井沉积相分析建立了马家沟组障壁潟湖沉积相模式，该模式分三个阶段即与海侵期相对应的以灰岩沉积为主的障壁潟湖沉积阶段（阶段Ⅰ）、与海退相对应的以灰岩夹蒸发岩沉积为主的障壁咸化潟湖沉积阶段（阶段Ⅱ）和与低海平面期对应的以膏岩盐沉积为主的障壁膏盐湖沉积阶段（阶段Ⅲ）。文章进一步分析并总结了在上述模式下各阶段储层的发育规律，认为阶段Ⅰ中的障壁滩相和阶段Ⅱ中的障壁滩及潟湖边缘坪有利于储层的发育，并指出了马家沟组储层主要为岩溶型储层，分布于马一、马三和马五的顶部。