Sequence Stratigraphy and Relative Sea Level Changes in Sinian System of Yichang, Hubei Province

The study on outcrop sequence stratigraphy of the Sinian section in the Yangtze Gorges an important step in the comprehensive stratigraphy for the Sinian stratotype. In this paper, the base of the Nantuo tillite is selected as the bottom boundary of the Sinian, and the top of the Baimatuo Member of the Dengying Formation is taken as the top boundary. The Sinian lasted from 740 the to 570 Ma. According to the features of sequence boundary facies and the parasequence sets, two supersequences are recognized in the Sinian with six sequences. In this paper, the authors discuss the correlation of those sequences. On the basis of the subsidence rate and the thickness of decompressed deposits, the authors discuss the mechanism of tectonic and sedimentation of the Dengying Formation and the features of high-frequency rhythms as well as the 3rd-order sequences in the Yangtze Gorges.

Geochemical characteristics and exploration significance of ultra-deep Sinian oil and gas from Well Tashen 5,Tarim Basin,NW China

The Well Tashen 5(TS5),drilled and completed at a vertical depth of 9017 m in the Tabei Uplift of the Tarim Basin,NW China,is the deepest well in Asia.It has been producing both oil and gas from the Sinian at a depth of 8780e8840 m,also the deepest in Asia in terms of oil discovery.In this paper,the geochemical characteristics of Sinian oil and gas from the well were investigated and compared with those of Cambrian oil and gas discovered in the same basin.The oil samples,with Pr/Ph ratio of 0.78 and a whole oil carbon isotopic value of31.6‰,have geochemical characteristics similar to those of Ordovician oils from the No.1 fault in the North Shuntuoguole area(also named Shunbei area)and the Middle Cambrian oil from wells Zhongshen 1(ZS1)and Zhongshen 5(ZS5)of Tazhong Uplift.The maturity of light hydrocarbons,diamondoids and aromatic fractions all suggest an approximate maturity of 1.5%e1.7%Ro for the samples.The(4-+3-)methyldiamantane concentration of the samples is 113.5 mg/g,indicating intense cracking with a cracking degree of about 80%,which is consistent with the high bottom hole temperature(179℃).The Sinian gas samples are dry with a dryness coefficient of 0.97.The gas is a mixture of kerogen-cracking gas and oil-cracking gas and has Ro values ranging between 1.5%and 1.7%,and methane carbon isotopic values of41.6‰.Based on the equivalent vitrinite reflectance(R_(eqv)=1.51%e1.61%)and the thermal evolution of source rocks from the Cambrian Yu'ertusi Formation of the same well,it is proposed that the Sinian oil and gas be mainly sourced from the Cambrian Yu'ertusi Formation during the Himalayan period but probably also be joined by hydrocarbon of higher maturity that migrated from other source rocks in deeper formations.The discovery of Sinian oil and gas from Well TS5 suggests that the ancient ultra-deep strata in the northern Tarim Basin have the potential for finding volatile oil or condensate reservoirs.

Sinian gas sources and effectiveness of primary gas-bearing system in Sichuan Basin, SW China

Based on correlation between geochemical characteristics of Sinian and Cambrian source rocks and discovered gas reservoirs,paleoand the analysis on geological conditions of reservoir formation,the sources of natural gas in the Sinian of Sichuan Basin have been discussed to sort out the contribution of Sinian source rocks to the gas reservoirs and effectiveness of Sinian primary gas-bearing system.Through the analysis of natural gas composition,carbon and hydrogen isotopes and effectiveness of Sinian accumulation assemblages,it is concluded that:(1)The natural gas derived from the Sinian source rock is characterized by low ethane content,heavy ethane carbon isotope and light methane hydrogen isotope,and obviously different from the gas generated by the Cambrian source rock.(2)The gas reservoirs discovered in Sinian Dengying Formation are sourced by Sinian and Cambrian source rocks,and the Sinian source rock contributes different proportions to the gas in the 4th member and the 2nd member of the Dengying Formation,specifically,39%and 55%to the 4th member in marginal zone and intra-platform,54%and 68%to the 2th member in the marginal zone and intra-platform respectively.(3)The effectiveness of the Sinian primary gas-bearing system depends on the gas generating effectiveness of the source kitchen,reservoir and combination of gas accumulation elements.For high-over mature marine source rocks at the Ro of less than 3.5%,besides gas generated from the thermal cracking of liquid hydrocarbon,the kerogen still has some gas generation potential by thermal degradation.In addition,the Sinian microbial dolomite still preserves relatively good-quality reservoirs despite large burial depths,which match well with other basic conditions for gas accumulation in central Sichuan paleo-uplift,increasing the possibility of Sinian primary gas-bearing system.The research results confirm that the Sinian primary gas-bearing system is likely to form large-scale accumulation.

Geological characteristics of the southern segment of the Late Sinian–Early Cambrian Deyang-Anyue rift trough in Sichuan Basin, SW China

Based on the latest drilling, seismic and field outcrop data, the geological characteristics(e.g. strata, development and sedimentary evolution) of the southern segment of the Late Sinian–Early Cambrian Deyang–Anyue rift trough in the Sichuan Basin are analyzed. First, the strata in the southern segment are complete. The first to second members of Dengying Formation(Deng 1 + Deng 2) are found with relatively stable thickness(400–550 m), and the third to fourth members(Deng 3+ Deng 4) show great thickness difference between the marginal trough and the inner trough, which is up to 250 m. The Cambrian Maidiping Formation and Qiongzhusi Formation in southern Sichuan Basin are relatively thin, with the thickness changing greatly and frequently. Second, the Deyang–Anyue rift trough extended southward during the Deng 4 period, affecting southern Sichuan Basin. Compared to the middle and northern segments of the rift trough, the southern segment is generally wide, gentle and shallow, with multiple steps, and alternating uplifts and sags, which are distributed in finger shape. Third, the Deng 1 + Deng 2 in southern Sichuan Basin records the dominance of carbonate platform and unobvious sedimentary differentiation, and the Deng 4 exhibits obvious sedimentary differentiation, namely, basin–slope–secondary slope–slope–secondary slope–platform margin–restricted platform, from the inner trough to the marginal trough. Fourth, the rift trough in southern Sichuan Basin has evolved in four stages: stabilization of Deng 1–Deng 2, initialization of Deng 3–Deng 4, filling of Maidiping–Qiongzhusi, and extinction of Canglangpu Formation.

Structural features and exploration targets of platform margins in Sinian Dengying Formation in Deyang-Anyue Rift, Sichuan Basin, SW China

Based on the seismic, logging, drilling and other data, the distribution, structural types and mound-shoal hydrocarbon accumulation characteristics of platform margins of the Sinian Dengying Formation in the Deyang-Anyue Rift and its periphery were analyzed. Four types of platform margins are developed in the Dengying Formation, i.e., single-stage fault-controlled platform margin, multi-stage fault-controlled platform margin, gentle slope platform margin, and overlapping platform margin. In the Gaoshiti West-Weiyuan East area, single-stage fault controlled platform margins are developed in the Deng 2 Member, which trend in nearly NEE direction and are shielded by faults and mudstones, forming fault-controlled–lithologic traps. In the Lezhi-Penglai area, independent and multi-stage fault controlled platform margins are developed in the Deng 2 Member, which trend in NE direction and are controlled by synsedimentary faults;the mound-shoal complexes are aggraded and built on the hanging walls of the faults, and they are shielded by tight intertidal belts and the Lower Cambrian source rocks in multiple directions, forming fault-controlled–lithologic and other composite traps. In the Weiyuan-Ziyang area, gentle slope platform margins are developed in the Deng 2 Member, which trend in NW direction;the mound-shoal complexes are mostly thin interbeds as continuous bands and shielded by tight intertidal belts in the updip direction, forming lithologic traps. In the Gaoshiti-Moxi-Yanting area, overlapping platform margins are developed in the Deng 2 and Deng 4 members;the mound-shoal complexes are aggraded and overlaid to create platform margin buildup with a huge thickness and sealed by tight intertidal belts and the Lower Cambrian mudstones in the updip direction, forming large-scale lithologic traps on the north slope of the Central Sichuan Paleouplift. To summarize, the mound-shoal complexes on the platform margins in the Dengying Formation in the Penglai-Zhongjiang area, Moxi North-Yanting area and Weiyuan-Ziyang area are large in scale, with estimated resources of 1.58×1012 m3, and they will be the key targets for the future exploration of the Dengying Formation in the Sichuan Basin.

Petrophysical properties and their influencing factors of carbonates in the fourth member of Sinian Dengying Formation, Sichuan Basin, SW China

Based on the measurements of petrological, petrophysical and elastic properties of the samples of different sedimentary facies in the fourth member of Sinian Dengying Formation (Deng 4 Member) in the Sichuan Basin, the diagenetic processes of reservoirs of different sedimentary facies and their controls on the petrophysical properties were discussed. The results show that cracks and mineral composition jointly control the petrophysical properties, and both are significantly influenced by sedimentary environment and diagenesis. The microbial dolomite of mound-shoal facies mainly experienced multi-stage dolomitization, penecontemporaneous dissolution, tectonic rupture and hydrothermal/organic acid dissolution processes, giving rise to cracks and dissolved pores. The grannular dolomite of inter-mound-shoal bottomland or dolomitic lagoon facies mainly underwent mechanical compaction, burial dolomitization and tectonic-hydrothermal action, creating cracks and intercrystalline pores. The diagenesis related to crack development increases the pressure- and saturation-dependent effects of samples, leading to significant decrease in the compressional wave impedance and Poisson's ratio. Dolomitization changes the properties of mineral particles, resulting in a Poisson's ratio close to dolomite. The muddy, siliceous and calcareous sediments in the low-energy environment lead to the decrease of impedance and the differential change of Poisson's ratio (significantly increased or decreased). The samples with both cracks and dissolved pores show high P-wave velocity dispersion characteristics, and the P-wave velocity dispersion of samples with only fractures or pores is the lowest.

华南埃迪卡拉(震旦)系年代地层划分方案及其层型剖面

依据华南峡东标准剖面的地球化学和多重地层划分对比资料,2014年全国地层委员会发布了埃迪卡拉(震旦)系年代地层划分暂定方案。该方案近年来已经受到来自生物地层学和同位素地质年代学研究所获得的新成果、新认识的挑战,迫切需要推出新的具体划分方案,建立新的阶一级层型剖面,作为华南乃至全国埃迪卡拉系的对比划分标准。为此,本文建议在埃迪卡拉纪以生物地层学为主确立年代地层单元的划分标准,提出在华南分别建立埃迪卡拉系上统和下统的顶阶层型剖面,即江川阶和瓮会阶。本文推荐了这两阶的相关顶、底界线层型剖面及标志化石、推测年龄和内部细分的化石带;其次初步设计了新的埃迪卡拉系、统、阶年代地层格架(地层柱),提议待定的上、下统及其余各阶名称,指明今后重点工作、细致研究的层型剖面和对比区域。

辽东半岛震旦系甘井子组叠层石生长机制研究

辽东半岛新元古界震旦系甘井子组叠层石出露完好,宏观形态完整,微观构造清晰,是研究新元古代微生物碳酸盐岩建造的理想对象,对恢复古环境和古生态研究具有重要意义。笔者等选择甘井子组地层为研究对象,根据岩性特征该组自上而下可分成3段。叠层石出露于下段的底部和上段的顶部,底部叠层石为透镜状,顶部叠层石呈大规模层状。下段叠层石呈现波纹状—柱状—丘状的形态组合,上段叠层石呈现波纹状—柱状—长柱状的形态组合,均为水深不断增加的结果。笔者等通过对比现代叠层石生长机制,确定甘井子组叠层石纹层的3层构造和形成顺序,分析硬质基底对于叠层石定殖和叠层石密集生长的控制,厘定环境变化从微观到宏观对于柱体形态和叠层石规模的影响,揭示了甘井子组叠层石的形成机制。

华夏地块中部震旦系区域对比及其大地构造意义:来自碎屑锆石年代学证据

华夏地块中部震旦纪沉积地层广泛分布,由于岩性较为单一、受后期构造及岩浆作用影响,缺乏顶、底齐全且具有标志层位的剖面,其区域地层对比一直存在争议。本文在粤北和粤东地区对震旦纪沉积地层开展路线剖面调查和碎屑物源研究,结果表明:该地区震旦纪沉积岩是以一套浅变质砂岩、粉砂岩为主,间夹硅质岩的深海-半深海沉积,顶、底以硅质较高的层位为标志;碎屑锆石U-Pb定年结果显示其物源具有~1000 Ma及650~600 Ma两个显著峰值,同时有较为复杂的组成,是典型的“华夏型”物源;根据岩性及碎屑物源特征,华夏地块中部震旦纪沉积地层可以与华夏地块其他地区及华南板块部分区域同时期沉积地层进行对比,指示震旦纪期间华南板块中部不存在宽阔的华南洋,属于统一的沉积盆地。

四川盆地PS8井区灯影组四段丘滩体储层特征及成因机制

四川盆地震旦系灯影组四段是近期的重点勘探层位,PS8井灯影组四段试气获得高产,展示出灯影组四段天然气勘探的巨大潜力。基于钻井岩心、岩屑、薄片、分析化验和测井等资料,对PS8井区灯四段丘滩体储层特征及成因机制开展研究。结果表明:①PS8井区灯影组四段从裂陷槽向台内具有逐渐增厚的趋势,储集层岩性主要为藻凝块白云岩、藻叠层白云岩、砂屑白云岩;储集空间以溶蚀孔洞及裂缝为主,柱塞样孔隙度为0.86%~6.59%,平均3.58%,渗透率为0.00085~14.75mD,平均1.71mD,全直径孔隙度为1.15%~8.42%,平均3.84%,渗透率为0.003~16.8mD,平均2.16mD,属于低孔低渗型储层;②台缘带发育的微生物丘滩沉积是PS8井区灯影组四段优质储层发育的物质基础,表生期风化壳岩溶和埋藏期热液溶蚀作用是PS8井区灯影组四段储层大量溶蚀孔洞发育的关键因素。研究成果落实了研究区灯影组四段丘滩体储层特征,并明确了储层的成因机制,为后期储层发育分布预测提供了理论支撑,同时为评价方案的部署提供了地质依据。

Formation conditions of Sinian–Cambrian large lithologic gas reservoirs in the north slope area of central Sichuan Basin,SW China

Based on analyses of characteristics,hydrocarbon charging history and geological conditions for the formation of Sinian-Cambrian reservoirs in the north slope area of central Sichuan paleo-uplift,the natural gas origin,accumulation evolution,accumulation pattern and formation conditions of large lithologic gas reservoirs have been investigated.Through comprehensive analyses of natural gas composition,carbon and hydrogen isotopic compositions,fluid inclusions,reservoir bitumen,and geological conditions such as lithofacies paleogeography and beach body characterization,it is concluded that:(1)The natural gas in the Sinian-Cambrian of the north slope area is mainly oil cracking gas,and different contribution ratios of multiple sets of source rocks lead to different geochemical characteristics of natural gas in different reservoirs.(2)Although the both Sinian and Cambrian gas reservoirs in this area are lithologic gas reservoirs under monocline background,the former has normal-pressure and the latter has high-pressure.There are three types of source-reservoir-caprock combinations:single source with lower generation and upper reservoir,double sources with lower generation and upper reservoir or with side source and lateral reservoir,double sources with lower generation and upper reservoir or with upper generation and lower reservoir.The Permian-Triassic is the main generation period of oil,Early-Middle Jurassic is the main generation period of oil cracking gas and wet gas,and Late Jurassic-Cretaceous is the main generation period of dry gas.(3)The Sinian-Cambrian system of the north slope area has two favorable conditions for formation of large lithologic gas reservoirs,one is that the large scale beach facies reservoirs are located in the range of ancient oil reservoirs or near the source rocks,which is conducive to the"in-situ"large-scale accumulation of cracked gas in the paleo-oil reservoirs,the other is that the large scale mound-beach complex reservoirs and sealing layers of inter beach tight zones match effectively to form large lithologic traps under the slope background.The research results confirm that the north slope area has large multi-layer lithologic gas reservoirs with more than one trillion cubic meters of natural gas resources and great exploration potential.

Formation and destruction processes of upper Sinian oil-gas pools in the Dingshan-Lintanchang structural belt, southeast Sichuan Basin, China

The lower Cambrian Niutitang Formation hydrocarbon source rocks at the Dingshan- Lintanchang structure in the southeast Sichuan Basin were of medium-good quality with two excellent hydrocarbon-generating centers developed in the periphery areas, with a possibility of forming a medium to large-sized oil-gas field. Good reservoir rocks were the upper Sinian （Dengying Formation） dolomites. The mudstone in the lower Cambrian Niutitang Formation with a good sealing capacity was the cap rock. The widely occurring bitumen in the Dengying Formation indicates that a paleo oil pool was once formed in the study area. The first stage of paleo oil pool formation was maturation of the lower Cambrian source rocks during the late Ordovician. Hydrocarbon generation from the lower Cambrian source rocks stopped due to the Devonian-Carboniferous uplifting. The lower Cambrian source rocks then restarted generation of large quantities of hydrocarbons after deposition of the middle Permian sediments. This was the second stage of the paleo oil pool formation. The oil in the paleo oil pool began to crack during the late Triassic and a paleo gas pool was formed. This paleo gas pool was destroyed during the Yanshan-Himalayan folding, uplifting and denudation. Bitumen can be widely seen in the Dengying Formation in wells and outcrops in the Sichuan Basin and its periphery areas. This provides strong evidence that the Dengying Formation in the Sichuan Basin and its periphery areas was once an ultra-large structural-lithologic oil-gas field, which was damaged during the Yanshan-Himalayan period.

Hydrothermal Dolomite in the Upper Sinian (Upper Proterozoic) Dengying Formation, East Sichuan Basin, China

Hydrothermal Dolomite （HTD） is present in the Upper Sinian （Upper Proterozoic） Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while STSr/86Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in S7Sr/S6Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270-320℃. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are： （1） dolomite host rock →sphalerite-galena-barite-fluorite; （2） dolomite host rock →saddle dolomite →quartz; （3） dolomite host rock →saddle dolomite→bitumen; （4） dolomite host rock →saddle dolomite →barite. The mean chemical composition of the host dolomite matrix and HTD didn＇t change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation ＆ expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement. The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian and Cambrian and were generated mainly during karstification. Hydrothermal alteration enhanced the reservoir property of the Dengying Formation dolomites with 3%-5% increase in porosity. No agreement has been reached why zebra dolomite occurs only in the Upper Sinian strata, which would indicate that HTD mineralization occurred during two different periods, each of them related to major extensional tectonic event. The early one related to the Xingkai taphrogenesis （Z2-C1） and the later one to the Emei taphrogenesis （D2-T2）. But, all the data from saddle dolomite suggest that the predominant crystallization occurred during the latter event.

Distribution Features of the Nanhua-Sinian Rifts and their Significance to Hydrocarbon Accumulation in the Tarim Basin

On the basis of reprocessing 34 new two-dimensional spliced long sections(20,191 km) in the Tarim Basin, the deep structure features of the Tarim Basin were analyzed through interpreting 30,451 km of two-dimensional seismic data and compiling basic maps. Seismic interpretation and geological analysis conclude that the Nanhua-Sinian strata are a set of rift-depression depositional systems according to their tectonic and depositional features. The rift valley formed in the Nanhua Period, and the transformation became weaker during the late Sinian Period, which eventually turned into depression. From bottom to top, the deposited strata include mafic igneous, tillite, mudstone, and dolomite. Three major depocenters developed inside this basin during the rift stage and are distributed in the eastern Tarim Basin, the Awati area, and the southwestern Tarim Basin. Among them, the rift in the eastern Tarim Basin strikes in the near east-west direction on the plane and coincides with the aeromagnetic anomaly belt. This represents a strong magnetic zone formed by upwelling basic volcanic rock along high, steep normal faults of the Nanhua Period. Controlled by the tectonic background, two types of sedimentary systems were developed in the rift stage and depression stage, showing two types of sequence features in the Sinian depositional stage. The Nanhua System appears as a wedge-shaped formation, with its bottom in unconformable contact with the base. The rifting event has a strong influence on the current tectonic units in the Tarim Basin, and affects the distribution of source rock in the Yuertus Formation and reservoir beds in the Xiaoerbulake Formation in Lower Cambrian, as well as the gypseous cap rock in Middle Cambrian. The distribution features of the rifts have important and realistic significance for determining the direction of oil and gas exploration in the deep strata of the Tarim Basin. Comprehensive analysis suggests that the Tazhong region is the most favorable zone, and the Kalpin-Bachu region is the optimal potential zone for exploring sub-salt oil and gas in deep Cambrian strata.

Formation and evolution of Sinian oil and gas pools in typical structures, Sichuan Basin, China

The only major breakthrough in the exploration of the Sinian of the Sichuan Basin has been the Weiyuan gas field. Taking the typical structures in the Sichuan Basin as examples, an apatite fission track simulation was applied to constrain the thermal evolutionary history of the source rocks in this study. Combined with trap formation and evolution, the formation, destruction, and accumulation history of the Sinian reservoirs were analyzed from a dynamic perspective. The Sinian reservoirs underwent several primary stages： the paleo-reservoir formation and destruction in the late Silurian, hydrocarbon recharge in the Permian-Triassic, the cracking of oil to gas and dissolved-gas in the late Triassic-Mid-Late Jurassic, and the exsolution and accumulation of dissolved-gas as a result of episodic uplift since the Late Cretaceous. The exsolution process of dissolved-gas is of great significance to the accumulation of natural gas. The formation of the Weiyuan gas field is also related to this process. The Sinian in the Sichuan Basin has a broad exploration prospect, and exploration targets focused on the Leshan-Longnüsi Paleo-uplift tectonic zone with weak influences on potential hydrocarbon reservoirs from the late tectonism and transformation.

Geochemical Indication of Sinian Bedded Siliceous Rocks in the Hunan-Guizhou-Guangxi Area and Their Environmental Significance

: In the Hunan-Guizhou-Guangxi area there have developed very thick bedded siliceous rocks of the late Sinian. The rocks have a fairly pure composition, with an average content of siliceous minerals exceeding 95%. They are relatively rich in Fe and Mn, and poor in Al, Ti and Mg. The Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn) and U/Th ratios and the Al-Fe-Mn and Fe-Mn-(Ni+Co+Cu)×10 triangle diagrams all show that they are hydrothermal sedimentary siliceous rocks. In the rocks the total amount of REEs is low, the δCe shows an obvious negative anomaly and the δEu a weak anomaly, and LREE>HREE, all indicating that they are products of hydrothermal processes. The δ30Si and δ18O values, as well as the formation temperature of the rocks all clearly show that the silica forming the rocks comes from hot water. Besides, analyses of the depositional environment of the rocks using the MnO/TiO2 ratio and the δCe and δ30Si values yield the same conclusion that they are formed in environments from continental marginal slopes of a bathyal environment to oceanic basins of a deep-sea environment.

Occurrences and Formation Mechanisms of Botryoidai Structures from the Sinian Dengying Formation,Sichuan Basin,China

ObjectiveThis study aims to characterize the occurrences and interior structural features of botryoidal structures from the Sinian Dengying Formation in the Sichuan Basin of Southwestern China, and to shed light on their formation mechanism.

Sedimentary and Geochemical Characteristics of Sinian Cap Carbonates in the Upper Yangtze Region

A global\|scale glaciation occurred at about 600 Ma ago. As a result, the Earth became the "Snowball Earth". The glaciation came to the end abruptly when atmospheric carbon dioxide increased to such an extent as to be about 350 times the modern level because of subaerial volcanic degassing. The rapid termination of glaciation would have led to warming of the Snowball Earth and extreme greenhouse conditions would have been created. The transfer of atmospheric carbon dioxide to oceans would give rise to the rapid precipitation of calcium carbonate in warm surface seawaters, thus forming the cap carbonate rocks as observed worldwide today.\; Regionally persisting, thin layers of carbonate rocks directly and ubiquitously overlie Proterozoic glacial deposits almost on every continent, and are commonly referred to as cap carbonates. Their unusual litho\| fabrics, stratigraphically abrupt basal and upper\|level contacts and strongly negative carbonate isotopic signatures (\{δ\{\}\+\{13\}C\-\{carb.\}\} values range from -7.0‰-0‰) suggest a chemical oceanographic origin, the details of which remain unknown. It is proposed that these enigmatic deposits are related to the destabilization of gaseous hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and internal basins.\; The authors carried out studies on the geochemistry, sedimentology and palaeontology of the Sinian cap carbonates in Guizhou and Hunan provinces, including the occurrence of cap carbonates of unusual fabrics, strongly negative carbon isotopic signatures, and a lot of bitumen nodules. From the results it is suggested that the cap carbonates were formed from solid methane seepage, and it is in agreement with Kennedy’s viewpoint (2001) that the cap carbonates resulted from the rapid precipitation of calcium carbonate in response to solid methane seepage.

Medusoid Fossils from the Sinian Xingmincun Formation of Southern Liaoning Province

Abundant medusoid soft-bodied metazoan fossils have been found in the Sinian Xingmincun Formation(with an isotopic age of 650 Ma) at Qipanmo, Qidingshan and Yangtun of the Dalian-Jinzhou area, southern Liaoning Province; in addition, a number of macroscopic algal fossils have been discovered for the first time in the Getun Formation overlying the Xingmincun Formation. Medusoid fossils include 3 genera and 6 species(all of which are new). Judging from their biological characters, these medusoid fossils are considered to have an intimate affinity with the Ediacara fauna in the terminal Precambrian of Australia, as their characters are close to each other. The authors suggest that these medusoids and the Edicara fauna might be the products of the same period. The discovery of the medusoid fauna in the Xingmincun Formation marks an important event in the evolution of the terminal Precambrian organisms in China, and it is of major practical significance in the study of the Sinian System of China and the correlation of the Sinian strata of northern and southern China.

Lithofacies paleogeography and exploration significance of Sinian Doushantuo depositional stage in the middle-upper Yangtze region, Sichuan Basin, SW China

In recent years, natural gas exploration in the Sinian Dengying Formation and shale gas exploration in Doushantuo Formation have made major breakthroughs in the Sichuan Basin and its adjacent areas. However, the sedimentary background of the Doushantuo Formation hasn't been studied systematically. The lithofacies paleogeographic pattern, sedimentary environment, sedimentary evolution and distribution of source rocks during the depositional stage of Doushantuo Formation were systematically analyzed by using a large amount of outcrop data, and a small amount of drilling and seismic data.(1) The sedimentary sequence and stratigraphic distribution of the Sinian Doushantuo Formation in the middle-upper Yangtze region were controlled by paleouplifts and marginal sags. The Doushantuo Formation in the paleouplift region was overlayed with thin thickness, including shore facies, mixed continental shelf facies and atypical carbonate platform facies. The marginal sag had complete strata and large thickness, and developed deep water shelf facies and restricted basin facies.(2) The Doushantuo Formation is divided into four members from bottom to top, and the sedimentary sequence is a complete sedimentary cycle of transgression–high position–regression. The first member is atypical carbonate gentle slope deposit in the early stage of the transgression, the second member is shore-mixed shelf deposit in the extensive transgression period, and the third member is atypical restricted–open sea platform deposit of the high position of the transgression.(3) The second member has organic-rich black shale developed with stable distribution and large thickness, which is an important source rock interval and major shale gas interval. The third member is characterized by microbial carbonate rock and has good storage conditions which is conducive to the accumulation of natural gas, phosphate and other mineral resources, so it is a new area worthy of attention. The Qinling trough and western Hubei trough are favorable areas for exploration of natural gas(including shale gas) and mineral resources such as phosphate and manganese ore.