Northwest China in this paper refers to the broad region bounded by the Helan Mo untains and Liupan Mountains to the east, and by the Kunlun Mountains to the south. Based on the study of stratigraphy and petrology of outcrop and well sect ions, according to single factor analysis and comprehensive mapping method, vari ous single factor maps of the Lower, Middle and Upper Ordovician and lithofacie s palaeogeography maps of the Early, Middle and Late Ordovician in Northwest Chi na were compiled. These lithofacies palaeogeography maps possess three character istics, i.e. quantification, multirank and multitype. Quantification means that the determination of each palaeogeographic unit is based on quantitative single factor data and maps. This makes palaeogeography develop to the quantitative sta ge. Multirank means that palaeogeographic units are classified into three ranks or more than three ranks. For instance, lands and seas are the first rank. In the seas, the platforms, basins and slopes are defined and belong to the second rank. In platf orms, the banks and tidal flats are further drawn out and belong to the third ra nk. Multitype means that the palaeogeographic units, especially the 2nd and 3rd rank units, can be further divided into different types on the basis of their ro ck types. For instance, platforms can be divided into carbonate rock platforms, clastic rock platforms, volcanic rock platforms ,etc.；basins can be divided int o mudstone basins, siliceous rock basins, clastic rock basins, volcanic rock bas ins, flysch basins, etc.；and banks can be divided into sparry grain banks (pene banks, embryonic banks), and limemud grain banks (penebanks, embryonic banks). T he framework of lithofacies paleogeography of the Ordovician in Northwest China can be summerized as following: There were both lands and seas. In seas there w ere platforms and basins distributed alternately. In platforms there were banks and flats. Between platforms and basins there were slopes. In the Early Ordovici an, the Cambrian transgression continued and became larger. In the Middle Ordovi cian, the transgression reached to its climax. In the Late Ordovician, the regre ssion appeared, and the refore the transgression and regression cycle of the Cam brian and Ordovician in Northwest China was completed.
Four third-order orthosequences have been recognised in the latest Cambrian to Early Ordovician Tremadocian ( from the Cordylodus intermedus to Serratognathus zones) successions in the Hunan and Hubei parts of the Yangtze platform. These four orthosequences could be well subcorrelated with each other in different depositional settings of the Yangtze platform, indicating four third-order transgression-regression cycles. Among them , the transgression systems tract in sequence 3 ( the lower and middle Paltodus deltifer Zone ) represents the maximum transgression event of the lower-order cycle of the Tremadocian, and the major regression events occurred during early Tremadocian Glyptoconus quadraplicatus Zone and late Tremadocian Serratognathus Zone ( middle and upper part ), respectively.Integrated sequence stratigraphy and facies analysis has improved our understanding of lithofacies palaeogeographical evolution in the region. At the beginning of the early Tremadocian ( C.lindstromi to lower C. angulatus Zone) the depositional facies from the northwestern Hubei to central Hunan change successively from tidal flat, tidal lagoon dolomite facies through restricted carbonate platform, open carbonate platform , platform margin shoal complex, to platform marginal slope and the black carbonaceous shale basin . During the early stage of Tremadocian (Glyptoconus quadraplicatus Zone ), relative sea level fall caused gradual regression from north to south, resulting in depositional environments varying from tidal flat, tidal lagoon dolomite facies, restricted carbonate platform, to platform margin shoal and outer shelf. During the middle Tremadocian, a new transgression resulted in onlapping of the bioclastic limestone and shale towards the Qingfeng-Xiangguang fault zone to the north. At this stage, the shallow-water carbonate domain evolved into an open carbonate platform, the mixed carbonate and siliciclastic zone evolved into a deep-water outer shelf, and the central Hunan region became a lower outer shelf. During the late Tremadocian, another relative sea level fall caused a further slow regression, and the open carbonate platform in Hubei and northwestern Hunan was developed with the bioclastic shoal of inner shelf and platform margin shoal complex. On these platform margin bioclastic shoals, sponge and bryozoan reefs were well developed. The mixed sedimentary realm evoled into an upper outer shelf, and the central Hunan area became a deep muddy outer shelf.
On the basis of integrated studies of seismic data, cores, well logs and palaeontological stratigraphy of the East Fukang Slope, Junggar Basin, six stratigraphic sequences or two tectonic sequences are recognized from the Jurassic System of the study area.Difference of the spatial distribution of the sequences and sedimentary facies indicates that, from the period of Badaowan Formation to the period of Xishanyao Formation, the main sedimentary provenance was in the north of the basin. The lower tectonic sequence was characterised by uplifting of the north and relatively balanced subsidence of the south; but from Toutunhe Age to Kalazha Age, the south area was uplifted and became the main sedimentary provenance, and thus the lake basin area decreased abruptly.In the study area, there are two sedimentary systems. The main one is an alluvial fanbraided riverdeltalake system, and the minor one is a meandering riverdatlalake system, both of which are developed along the short axis of the basin.From the early Jurassic Epoch to the middle Jurassic Epoch, Junggar Basin was dominated by warm humid climate, with two short hot and dry episodes in the Sangonghe and Toutunhe Period. In the late Jurassic stage, the climate became semiarid or arid, which had a great contribution to the decrease of the lake area.
The well-known gravity flow sediments were formed by the debris, sliding and turbidity flows in Late Jurassic-Early Cretaceous in Shalagang mining area of Jomzi basin in southern Tibet, which consisted of the slope clastic rocks and interbeds of siliceous rocks and limestones. Three kind lithofacies assemblages of upper slope, lower slope and submarine fan emerged in the slope. The upper slope was characterized by all kinds of sliding sediments and lower slope by all kinds of debris flow sediments. The submarine fan that had completely inner, middle and outer fan can be divided into progressive and regressive sequence, which were composed of sandstone bodies formed by turbidity flow.
The Lower Ordovician in Xingshan consists of the Nanjinguang Formation,Fenxiang Formation, Honghuayuan Formation, Dawan Formation and Guniutan Formation.The rock types include limemud limestones, bioclast limestones, oolites, intraclast limestones and dolomites, indicating a typical set of epeiric sea carbonate platform or submerged platform deposits. Grainstones are the main part of the section, among which the oolites, intraclast limestones and most bioclast limestones are cemented by sparry calcite. The grain size distribution and parameters of oolites and intraclastic graistones show that those grains had been transported and agitated during their process of formation. From the beginning of Xilingxia Formation deposition to the end of Guniutan Formation deposition, the sedimentary environments evolved from the tidal flats of restricted platform to open platform and shelf. The tidal flats include dolomitic flat, limemud flat and dolomiticlimemud flat, whereas the open platform mainly consists of grain banks and reefs.
Various magnetic minerals and each other, s transformation phenomena exist in carbonate rocks .Magnetic minerals produced in the each stage of the sedimentation evolusion process of the rocks carry information about the palaeoenvironment in their occurrence,thus they become direct objects of study in sedimentary palaeogeography and environment magnetism.Magnetic minerals derive primary remanent magnetization from the geomagnetic field during their diagenesis.It is prereguisite for gaining the reliable palaeomagnetic data,which are used for working out polar wander paths,drawing up palaeogeography maps with nonpresent boundaries and working the palaeogeographic reconstruction,correctly to identify them and to distinguish remanent magnetism components of all kinds.Thus,they become the major objects of study in palaeomagnetism and tectonic palaeogeography.In the epigenesis stage,magnetic minerals into which is usually translated by the primitive ones are often caused by tectonic movement or by the magmatism and the fluid medium activity relating to it .New magnetic minerals carry some information about remagnetized remanent magnetization .Thus,they should similarly be the major objects of study in palaeomagnetism and tectonic palaeogeography. We will offer more reliable and abundant palaeomagnetism data to people for working the palaeogeographic reconstruction if we collect more samples relative to the tectonic event and the thermal event and advance the minerageny study for magnetic minerals.It follows that the tight relation between magnetic minerals and the study on palaeogeography reguires our close attention though palaeogeography,a highly comprehensive branch of geoscience, has evolved into several branches.
The conglomerates developed in the Middle-Upper Devonian of the Fengxian-Shanyang area，Shaanxi Province, include mud conglomerates, dolostone conglomerates, ore conglomerates, and albite conglomerates. These conglomerates are closely associated with hydrothermal sedimentary bodies in space. Their petrochemical compositions were also affected by hot water activity. The petrological and petrochemical characteristics indicate that the formation of these conglomerates was controlled by growing faults during the metallogenic epoch. These growing faults not only resulted in the formation of the deep water basin where the conglomerates were deposited as a result of deep water mass flows, but also carrying heat flows for the mineralisation of these conglomerates. The lead-zinc deposits are products of the synsedimentary hydrothermal sedimentation. Through studying petrology and petrochemistry (including isotope dating) of mud conglomerates in the Chigou formation (D2ch), the author suggests that the hydrothermal ore deposit series (siderite-silver-multi-metal-gold deposits) in the Daxigou formation (D2d) are of natures of secondary hydrothermal sedimentation.
The initial indication of the Shahejie Formation estuarine environment of the Dongpu Depression was the deposition of seismites and the formation of the marine waterrelated sedimentary structures such as synaeresis. These evidences suggest that the Dongpu Depression has experienced tectonic subsidence and occasional incursion of marine water during the Tertiary.At the beginning of the middle part of the division 4 in the Shahejie Formation, another important event took place; that is, a mass death of fresh water Ostracodes. At the same time, marine trace fossils such as Ophiomorpha and Tigillites started to occur. This substitution suggests that fresh water of the ephemeral lake was replaced by saline water of the estuarine at that time. In the late sedimentary period of the middle part of the division 4 of the Shahejie Formation, coastal high energy environments, such as tidal channels and tidal sand bars, were developed, forming oolitic sandstones, shelly sandstones and trace fossils Arenicloites. Over this period, three environmental units, including distributary channel (terrestrial portion), estuarine point bar (mixed energy portion), tidal channel and tidal bar (marine portion), have been developed in the section and the combination of these environments have recorded periodical sea level changes.There are 10 beds containing fresh water trace fossils, plant traces or paleosols (in Kai well 31), and 12 beds containing marine trace fossils or body fossils as well as marine sediments (Kai well 31, New Hu well 4) in sections of the Shahejie Formation. These facts imply that more than ten periods of sea level changes occurred during deposition of the Shahejie Formation.There are some evidences showing that the fluvial channel might have incised into the banks along palaeovalley between the Lanliao Fault and the Neihuan uplift and have produced an incised valley during sea level fall. During the early period of sea level rise, the accommodation space in the incised valley has been fully or partially filled with sandstones of distributary channel, fluvial point bar and estuarine point bar environments, which constitute the incised valley reservoir type in the Dongpu Depression. During the middle period of sea level rise the tidal channel and tidal bar sandstones have developed in this area, which constitute the shelf onlap reservoir type
It is well known that clay minerals are the important factor that controls sandstone reservoir properties. This paper discussed clay mineral genesis of sandstone reservoir in Tarim and Shaanxi-Gansu-Ningxia basin. Study on the relationship of reservoir properties with content, occurrence and crystalline form of clay minerals was also carried out. The results show that the influence of clay mineral on the sandstone properties associates with sandstone maturation. The influence is significant on high maturation sandstone, and it is slight on lowmaturation sandstone. The high maturation sandstone properties degrade with the increase of clay mineral content, especially, permeability, and it is enhanced by the increase of kaolinite and fillingstate clay mineral content. The results also show the influence of three illinite crystalline forms on sandstone properties is different. Sheet like illinite is favourable to sandstone properties, while fibrous and hair form are not.
The Mesozoic-Cenozoic in the Qaidam Basin contain abundant microfossils, especially in Ostracoda among which Cetacella、Cypridea、Austrocypris、Hemicyprinotus、Cyprideis and Ilyocypris inermis are index fossils for specific stratigraphical sequences since the Late Jurassic. The extinction or appearance of these Ostracoda always occurred in a short time interval, and can be correlated locally and globally, so they are of importance in bioeventstratigraphy. This article listed six such biological events in Qaidam Basin and discussed their causes. The extinction of Cetacella reflected temperature decrease at global scale and changes of palaeoclimate zones during the latest Jurassic and the earliest Cretaceous; The rapid decline of Cypridea in the earliest Cretaceous may be a result of the strikes of small planets; at the latest middle Eocene Austrocypris became extinct, and at Oligocene Hemicyprinotus disappeared. These events related to the changes of the geological conditions at that time. The explosion of Cyprideis at the middleMiocene was because the rise of Qinghai-Xizang Plateau which resulted in the dry climate and development of salt lakes. The rapid decrease of Ostracoda in the Late Pleistocene resulted from the last glacial stage in which the global climate became extremely cold.
The principal part of the Middle-Late Proterozoic Yanshan Rift Basin is located in Hebei and Liaoning provinces of North China. An obvious rifting of the Yanshan Basin occurred in the early stage of the Middle Proterozoic, named Changchengian about 1800-1400Ma. Sedimentary fills of the basin during this period are earliest deposits within the covering strata after formation of the crystalline basement of the Archaeozoic-Early Proterozoic.The Changchengian of the early Middle Proterozoic mainly consists of five Formations, namely, Changzhougou, Chuanlinggou, Tuanshanzi, Dahongyu and Gaoyuzhuang Formations.During the Changchengian the palaeotectonic activities of the basin controlled the basin fillings in four ways, that is, synsedimentary faulting, structural tilting, and intent subsidence in basin axis and equilibrium subsidence in entire basin. In particular, the synsedimentary faulting, characterized by abrupt or pulsing activities, controls the distribution of main facies belts of the basin. During the intense periods of the basin, the synsedimentary faulting also caused a series of event depositions, such as palaeo-seismic records, carbonate slumping, clastic turbidites, and volcanism-induced volcanic turbidite deposits. Analyses of palaeocurrent, stratigraphic framework, and special facies belts show that the main axis of the basin is along NE direction. The outlet position, towards open sea, of the intracontinental rift basin probably was located in the north part of the Sino-Korea Paraplatform and the basin infilling during the Changchengian was not obviously affected with the E-W direction (from Chicheng to Beipiao) faulting.
