Table of Content

20 September 2009, Volume 15 Issue 3

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Article

The Winneshiek Lagerstatte, Iowa, USA and Its Depositional Environments

LIU Huai-bao1, McKAY R M1, WITZKE B J1, BRIGGS D E G2

2009, 15(3):  285-295. 

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Fossil Lagerst¨atten are deposits containing abundant and/or exceptionally preserved fossils, often including soft-bodied tissue. Of these, Konservat-Lagerst¨atten are especially important because they provide not only a much more complete record of the diversity and paleoecology of ancient communities, but also more detailed information on the taxonomy and anatomy of the biota compared to the normal shelly fossil record. However, Konservat-Lagerst¨atten are rare because they require exceptional physical and chemical depositional conditions. Environmental conditions during the Cambrian may have been more favorable for the preservation of soft-bodied organisms than later in the Paleozoic, and such Lagerst¨atten provide evidence of biodiversity changes during the Cambrian explosion. Although the Ordovician Radiation or Great Ordovician Biodiversification Event (GOBE) has been well documented, this is based mainly on the shelly fossil record because soft-bodied organisms are poorly represented during the 45 million years of the Ordovician. Only three Lagerst¨atten, Beecher's Trilobite Bed in upper New York State, the Soom Shale in South Africa, and the recently recognized biota in Manitoba, Canada, approach the extent of soft-tissue preservation in Cambrian deposits. All the three Lagerst¨atten are of Late Ordovicianage. 　The Winneshiek fauna was discovered recently near Decorah, northeast Iowa, USA (Fig. 1). It is preserved in a new stratigraphic unit which consists of greenish-brown to dark-gray sandy laminated shale with abundant organic carbon and pyrite (Fig.2). The new fauna contains both invertebrate and vertebrate fossils; current recovery includes eurypterids and other chelicerates,phyllocarid crustaceans, ostracods, linguloid brachiopods, mollusks, isolated conodont elements and bedding plane assemblages,skeletal elements of jawless fish, various indeterminate fossil forms, and bromalites and other trace fossils (Fig. 3). Most of these fossils are extraordinarily preserved, some with soft tissues or body impressions, establishing the status of this deposit as a Konservat-Lagerst¨atte, the only significant example of Middle Ordovician (Whiterockian) age. .........

The Bradoriida of the Niutitang Formation from Traditional Lower Cambrian in Jinsha County，Guizhou Province

YANG Xing-lian1,2, ZHU Ming-jin1, ZHU Lu-yan1, CUI Tao1

2009, 15(3):  296-303. 

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Some Bradoriida fossils were preserved in the manner of carbonaceous membrane in black shale of the lower part of the Niutitang Formation of the traditional Lower Cambrian (Terreneuvian and Cambrian Series 2 ) in Zhangyangou Village, Yankong Town, Jinsha County, Guizhou Province. They include two genera, one species, one conformis species and one uncertain species as listed below : Kunmingella douville, Kunmingella sp. and Tsunyiella cf. luna. These bradoriid fossils were found in association with the oldest trilobite Tsunyidiscus（ Mianxiandiscus）, showing that their horizon is equivalent to the Abadiella Zone in Yunnan Province. Moreover, the lower horizon of Kunmingella and Tsunyiella may be correlated with the lower part of Qiongzhusian Stage. Kunmingella as an element in western palaeobiogeographical area and Tsunyiella as an element in middle palaeobiogeographical area appear together in Jinsha County belonging to the middle palaeobiogeographical area of early Cambrian bradoriids. Consequently, the range of mictium between the western and middle palaeobiogeographical area can be extend from Changning-Zhengxiong subdistrict to Jinsha and Zhijin Counties. The discovery of numerous bradoriid at the base of the Cambrian enriched the components of the Niutitang biota, and is of importance in studying the evolution of early organisms in Cambrian.

Response of Hirnantia Fauna to the Environmental Changes before the Second Phase of Late Ordovician Mass Extinction: Example from the Kuanyinchiao Formation at Shuanghe, Southern Sichuan, Southwest China

LI Gui-peng1,2， ZHAN Ren-bin1， WU Rong-chang1,2

2009, 15(3):  304-317. 

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The spatial distribution of the latest Ordovician Hirnantia fauna in relation with the water depth and temperature gradients has been studied extensively and throughly. However, there has been no specific investigation on the temporal changes of the Hirnantia fauna. Based on the samples from the Kuanyinchiao Formation at Shuanghe, Changning County, southern Sichuan Province, China, we analyze the palaeoecological changes of the Hirnantia fauna using taxonomic diversity and individual density,and the environmental changes in relation with the water depth, oxygen concentration and bioturbation levels throughout the Kuanyinchiao Formation. We also discuss the relationships between the community changes and environmental fluctuations. It shows that community diversity and individual density of the Hirnantia fauna generally increased through time and decreased sharply near the top of the formation immediately after the taxonomic diversity reached its peak. Gradual changes of many environmental factors with the glaciation-induced sea level drop might account for the general palaeoecological trends of the Hirnantia fauna, and later sudden rising of sea level might be the cause of the rapid disappearance of the fauna, but the fluctuation of oxygen concentration or bioturbation levels might alter the structure of brachiopod community dramatically in some situations. 　　This study is based on one locality which was in the deeper water environment during the latest Ordovician. Previous studiesshow that temporal change of the Hirnantia fauna in the shallower water settings might have different patterns, but no specific studies were done at this particular site. Later investigation should compare the temporal patterns of Hirnantia fauna in different areas and under different environmental settings. All these works would contribute to understand how the Hirnantia fauna survived and developed in the relatively stable period between two episodes of the end Ordovician mass extinction.

High-Resolution Sequence Stratigraphic Study on the Palaeogene Suweiyi Formation in Kuqa Depression of Xinjiang Region and Discussion on Its Major Controlling Factors

LIU Jing-yan1，DING Xiao-zhong2，QIU Yi-gang1, LIU Jun3, JI Yun-Long1

2009, 15(3):  318-327. 

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Kuqa depression lies at the northern margin of Tarim basin, nearby the southern Tianshan Mountain. The Palaeogene Suweiyi Formation was formed during foreland basin developing phase and thick fluvial-delta and lacustrine sediments were deposited. The unique location of Kuqa depression and the large range successive outcrops of Suweiyi Formation provide favorable room to carry out the high-resolution sequence stratigraphic study and the major controlling factors discussion. Based on the relative distinguishable flooding surface, the Suweiyi Formation is classified into ten 4th order sequences and a number of 5th order sequences or parasequences. The 4th order sequence consists of incised valley fill, delta front mouth bar, prodelta, subaqueous distributary channel, evaporite salt lake and lacustrine deposits. The incised valley sandstone, delta front mouth bar sandstone and the subaqueous distributary channel sandstone of the 4th order sequence Pss1, Pss6, Pss3, Pss5 and Pss8 comprise main reservoir beds. The characteristics of 4th order sequences and the stacking patterns of parasequences indicate that under the tectonic subsidence background, the interplay of climate, lake level fluctuation and depositional supply controlled the development of the high order sequences. The climate and the depositional supply were the crucial factors. The aridity and the humidity effected the fluctuation of lake level and controlled the basin filling and sequence development. The difference of depositional supply of near thrust belt and far thrust belt made the depositional filling and stacking patterns of parasequences different.

Late Triassic Sequence Stratigraphy and Sedimentary Evolution of the Northern Mid-Yangtze Foreland Basin

CHEN Liu-qin1, 2, DUAN Kai-bo1, HUO Rong1, HU Shui-qing2

2009, 15(3):  328-338. 

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Detailed sedimentary facies analysis and sequence stratigarphic division of the four measured sections and one well log section are studied on the basis of principles and methods of sedimentolgy and sequence stratigraphy. The patterns of sedimentary facies transformation, types of sequence boundaries, sequence characteristics and their dominating factors are described as well.It shows that the upper Triassic of the northern mid-Yangtze basin can be divided into four third-order sequences (TS1, TS2, TS3 and TS4), and further grouped into one second-order tectonic sequence. The thickness of the upper Triassic is about 1 200 m in Jingmen-Dangyang basin where the four sequences are relatively intact. In contrast, it thins westward to 100~300 m in Zigui and Lichuan counties and eastward to less than 40 m in Hanchuan and E’zhou counties where only one sequence could be confirmed.The unconformity, river rejuvenation surface and lithology and lithofacies transition surface are the main types of sequence boundaries. The sequence stratigraphic framewoks are constructed on the basis of sequence subdivision of four outcrop sections and one well log section, which indicate that the sedimentary fabric of TS3-TS4 is of higher energy than that of TS1-TS2. Under the background of tectonic, paleogeography and paleoclimate comprehensive controlling over the stacking patterns of sequences, the studied area experienced a conversion from the lacustrine-swamp system to the fluvial system, which displays the early sedimentary characteristics of the northern mid-Yangtze foreland basin.

Cenozoic Tectono-Sedimentary Evolution in the Middle Part of Northern Continental Shelf-Slope Region, South China Sea

DING Wei-wei1, LI Ming-bi1, HE Ming2, TANG Yong1, FANG Yin-xia1

2009, 15(3):  339-350. 

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The middle part of the northern continental shelf-slope region lies in the south of the Pearl River Basin. As an important structure unit of the northern continental margin of the South China Sea, it has experienced multiphase tectonic events and bears a thinned crust, upon which Cenozoic rifted-depressions with great hydrocarbon potential are developed. Several seismic profiles crossing the study area are introduced in this article. Based on the interpretations and balanced cross-section restorations of these profiles, combined with structural geology, a Cenozoic tectono-sedimentary model is established. The results show that three stages can be proposed for this region: the rift stage from Paleocene to Eocene, the rift-depression stage from Oligocene to the e Middle Miocene. The sedimentation environment transformed from the fluvial and lake facies to the shallow sea facies, and finally to the deep sea facies. The existence of NW-SE direction mantle flow under the continental crust in the north continental margin of South China Sea made the extension activities migrate from north to south. Results also show that the depressions (such as the Baiyun Sag) on the continental slope are featured by ductile deformation.It appears to be caused by the ductile thinning and necking of the crust in response to hot lithopheric stretching, together with mantle upwelling.

Research Status of Extensional Fault-Propagation Folding

QI Peng1, ZHANG Jun-xia2

2009, 15(3):  351-357. 

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Extensional fault-propagation fold, an important kind of folds in extensional environment, is formed by propagation of the basement blind normal fault into sedimentary cover which was bent and thinned. The deformation is accommodated by trianglelike upward widening distributional shearing, and the up tip of blind fault controls the position of the apical angle of the shear zone. The tri-shear modeling matched well with the material movement in deforming zone. The apical angle of tri-shear zone and the fault propagation to slip ratio (p/s ratio) are both important parameters controlling the geometry and deformation of the extensional faultpropagation fold. The process of extensional fault-propagation folding can be divided into two stages by whether the blind fault propagates through cover strata. During deposition, extensional fault-propagation folding controls the geometry and superimposed pattern of grow strata strongly. The blind fault nucleation point and propagation amount can be determined exactly by analysis of the geometry of reserved growth strata and the development of extensional fault-propagation fold can be finally restored.

Abnormal Changes of Crustal Deformation before Wenchuan 8.0 Earthquake Observed by BSQ Model Digital Tilt Meter

JING Yan1, ZHANG Hong-xu1, SUN Yi2, LI Hong1, FAN Liang-long1, XIONG Yu-zhen1

2009, 15(3):  358-364. 

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Continuous ground tilt observation is important in crustal deformation studies. The new BSQ model digital vertical pendulum tilt meter installed at Jinhe observation station in Deyang city, Sichuan Province caught abnormal ground deformation obviously before the Wenchuan 8.0 earthquake. According to the observation results, beginning from half a month before the earthquake, the ground tilt rates at the Jinhe station became tens of times stronger compared to that of before, and the tilt direction was about NW 25°. Besides, the BSQ model tilt meter here also got relatively complete information about stress accumulation and release before and during the earthquake process. It provided important information for studying strong earthquake process. The article also introduced the observation results from other kinds of tilt meters before the Wenchuan earthquake installed near the Longmenshan fault. This research proved that the BSQ model digital tilt meter can get more useful information about the deformation of ground surface, and worthy to be popularized.

Morphological and Hydrological Effects of WenchuanEarthquake on Modern Rivers

XU Ling1, DAI Fu-chu1, TU Xin-bin1, WANG Ming-ming2

2009, 15(3):  365-370. 

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The Ms 8.0 Wenchuan earthquake strongly shocked at 14:28 on May 12, 2008. The seismogenic structure belongs to the Longmenshan fault zone. Its surface rupture belts include those formed along the NE striking Yingxiu-Beichuan and Pengxian-Guanxian faults and the NW striking Xiaoyudong fault. The surface ruptures are characterized by thrust faulting with strike-slip movement. The surface ruptures, landslides, fall and dammed lakes were surveyed with great efforts after the Wenchuan earthquake. However, the morphological effects of Wenchuan earthquake on rivers were not analyzed or discussed. Based on our field survey on earthquake geomorphology, this paper summarizes the morphological effects of Wenchuan Earthquake on rivers including waterfalls by consistent direction fault scarps, river slipping, earthquake fault lakes/poodles and riverway changing. Their developing trends are discussed. The hydrological effects of Wenchuan Earthquake on rivers are also analyzed.The Wenchuan earthquake increased the river silt and sediment contents and changed the river water quality. Furthermore, it heightened the connections between rivers and the groundwater systems.

Application of Occurrences of Gypsum Minerals in Saline Lacustrine Facies Analysis and Source Rocks Organic Geochemistry Evaluation:A Case Study of Wendong Subdepression

LIU Qing1, 2， ZHANG Lin-ye1， SONG Guo-qi3， SONG Chang-yu1， WANG Ru1

2009, 15(3):  371-379. 

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Based on the occurrences and characteristics of the gypsum and other markers, the sedimentary facies of the Paleogene saline lacustrine deposits in Wendong subdepression was studied, and the source rocks of different facies were evaluated according to the organic geochemistry analysis. It shows that the association of gypsum minerals reflects the changes in sediment environment, lake water depth, lake water salinity and bottom oxygen content etc., and possesses close relationship with sedimentary facies and source rocks development. Accordingly we established the saline lacustrine facies and source rocks development model. The saline mudflat and dry mudflat facies is dominated by disseminated, cemented and nodular gypsum minerals, the source rocks belong to no or poor source rocks with low organic abundance. The playa facies is dominated by enterolithic and lenticular gypsum minerals, has relatively high organic abundance and can form good to excellent source rocks. The saline deep lake facies is dominated by laminated gypsum minerals and can develop excellent source rocks with high organic abundance. This is of great significance in Paleogene saline source rocks evaluation of China.

Geochemical Characteristics and Oil-Source Analysis of Crude Oils in the Yanchang Formation of Upper Triassic from Ordos Basin

WANG Chuan-yuan1,2, DUAN Yi2, CHE Gui-mei3, ZHAI Rui-hua4,ZHENG Chao-yang2, WU Bao-xiang2, KANG Yan5

2009, 15(3):  380-386. 

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The biomarkers of crude oils of Yanchang Formation in Ordos Basin were studied in this paper, and the oil-source correlation and their genesis were discussed. The data indicate that the studied oils are of the same genetic type and the oil-forming matrixes are algae and higher plants. The crude oils were formed under the weak reducing and fresh water environments. The crude oils are mature. The crude oils in the Yanchang Formation of Upper Triassic were mainly originated from the Chang-7 oi1 bed group.

Sedimentary Facies and Facies Model of the 3rd Member of Shahejie Formation in the Western Sag, Liaohe Fault Basin

ZHANG Zhen1,2，BAO Zhi-dong1,2，TONG Heng-mao1,2，WANG Yong3

2009, 15(3):  387-397. 

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The sandbodies in the 3rd Member of Shahejie Formation in Western Sag with a huge sedimentary thickness and a wide distribution are the most important reservoirs for petroleum accumulation in the deeply buried sequences in Liaohe basin. Based on core, drilling, logging and seismic data, seven types of sedimentary facies are identified in this area, i.e., alluvial fan, fan delta, beachy turbidite fan, off-shore turbidite fan, fault trough gravity flow channel, slump turbidite fan and lacustrine facies, in which the multistory sandbodies in the fan delta and turbidite fan comprise the best reservoirs in the area. Five sets of depositional system are mainly developed during the sedimentation of the 3rd Member of Shahejie Formation, with the alluvial fan-fan deltasublacustrine fan depositional system dominant; At the early-middle period of the 3rd Member of Shahejie Formation, the lake basin showed transgression depositional sequence. The sandbodies of fan delta and off-shore turbidite fan in western slope area gradually moved eastward and the distribution range expanded little by little, while in eastern steep slope area, the facies belts of each subfacies of fan delta narrowed down gradually and the sandbodies of beachy turbidite fan were distributed in belts; At the late period of the 3rd Member of Shahejie Formation, the lake basin showed dereliction depositional sequence. The distribution range and thickness of sandbodies in the western slope area succeeded the characteristics of the earlier period on the whole,however, the facies belts of each subfacies of fan delta in eastern steep slope area widened gradually and went forward to the center of depressed area of the lake basin; The features of distribution and migration of sedimentary facies belts indicate that the sedimentary framework was controlled by several factors, including palaeotopography, boundary growth fault, relative change of lake surface and provenance efficiency.

Characteristics and Controlling Factors of Mesozoic Clastic Reservoirs in Zhuanghai Area, Shandong Pvorince

SHUAI Ping

2009, 15(3):  398-406. 

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The diagenesis, reservoir space and controlling factors of Mesozoic clastic reservoirs in Zhuanghai area have been researched in this paper through core observation, thin section determination, scanning electron microscope and X-diffraction analysis. The results show that, the clastic reservoirs experienced strong compaction, cementation and metasomatism as well as polyphase dissolution and crack formation. The diagenetic processes can be divided into four stages including early diagenesis B,middle diagenesis A1 and A2 and middle diagenesis B. There exist two chief types of reservoir space: the one is fractures including structural fractures, compaction fractures and cleavage cracks; the other is secondary pores including feldspar dissolution pores and carbonate cement pores which are related to atmospheric water dissolution and organic acid dissolution during the course of organic matter evolution in Shahejie Formation and Dongying Formation. The Mesozoic clastic reservoirs in Zhuanghai area are chiefly affected by sedimentation, diagenesis and tectonism. Sedimentation controlled the textures of clastic rocks such as matrix,sorting and rounding, and further affected the texture of primary pores, which served as basement for later dissolution. As the migration pathways of fluids, the faults and unconformity surfaces supplied acidic media for dissolution. Therefore, the scale of faults and unconformity surfaces controlled the properties of clastic reservoirs obviously. The closer the distance from faults and unconformities is, the better the reservoir properties are. In the areas of favourable sedimentation and geotectonic conditions with good primary pore structure, weak compaction and cementation, and well developed fractures, the strong dissolution, good primary pores and reservoir properties were formed, leading to development of several secondary pore zones around the unconformity surfaces, and the most favorable secondary pores occurred on the top of Mesozoic during the middle diagenetic stage A1.

Characteristics of Mesozoic Basin of the Northern Qaidam:A Case Study on Dachaidan Depression

LOU Qian-qian1,2, XIAO An-cheng1,2*, YANG Hao3, HUANG Hong-sheng4, DING Wei-xing4,SHEN Zhong-yan1,2, WANG Liang1,2, CHEN Yuan-zhong5, SHEN Ya5, WANG Li-qun6, ZHOU Su-ping6

2009, 15(3):  407-416. 

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The Mesozoic strata are the key of hydrocarbon target in the northern Qaidam Basin. Combined the seismic data and geologic field materials, the residual distribution maps of Jurassic and Cretaceous strata and the fault system maps were generated.Meanwhile, the cross sections before and after erosion in Jurassic and Cretaceous times were revealed by cross section restoration.Based on the synthetic analyses of Jurassic and Cretaceous residual strata distribution, faults systems and cross-section evolution,we proposed that the northern Qaidam Basin experienced three phases of basinal evolution during the Mesozoic: A series of small faulted-basins developed in the Early Jurassic northern Qaidam in an extensional environment; Afterwards, these small faultbounded basins were covered by a larger depression basin in the Middle Jurassic to Early Cretaceous; During the Late Cretaceous,the northern Qaidam basin was reversed and wholly uplifted, resulting in formation of palaeo-uplifts.

Relationship of Carbon and Oxygen Isotopic Characteristics of Limestone with Hidden Granite in Shuiximiao Mining District, Limu Area

ZHOU Peng-fei, MA Dong-sheng*, CAI Yang, DING Hai-feng, YAO Chun-yan

2009, 15(3):  417-428. 

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Studies on the carbon and oxygen isotopic compositions of the Upper Devonian Rongxian Formation limestone in Shuiximiao mine district，Limu area show that the carbon and oxygen isotope composition characteristics can provide important information about hidden granite upwarps and the fluids involved. The temperature of the interaction between the fluid driven by granite intrusion and the overlying Rongxian Formation limestone was about 110℃. The carbon and oxygen isotopic compositions of the fluids were δ18OSMOW=-3‰, δ13CPDB ≤-7‰, and the water-rock ratios (w/r) were less than 5. This water-rock interaction between mixed fluids of magmatic water and meteoric water and wall-rock limestone decreased the carbon and oxygen isotopic composition values of surface limestone, forming negative anomalies. The higher the temperatures and water-rock ratios (w/r) of the interaction, the more obvious the negative anomalies of the carbon and oxygen isotopes. Research shows that for the oxygen isotopes, the decrease of limestone δ18O values was controlled by the water-rock ratios (w/r) and temperatures, and the decrease of limestone δ13C values was mainly related to the water-rock ratios (w/r). Therefore, the carbon and oxygen isotopic composition variation of outcropped carbonate rocks in Limu mining area can be used to indicate the position of underlying hidden granite upwarps for geochemical exploration.

Morphological Evolution during the Formation of Carbonate Spherulite in Citrobacter freundii Culture

MA Heng, LI Fu-chun*, SU Ning, LI Xue-lin

2009, 15(3):  429-436. 

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A large number of microbes can mediate the formation of carbonate spherulite. Tailing the process of carbonate spherulite formation is the key to understand the mechanism. Using optic microscope and electron microscope, the whole process of carbonate spherulite formation under the mediation of Citrobacter freundii was traced in this study. The results showed that nanometer crystals formed firstly, and later gradually aggregated which were embedded by organics. Finally, spherulite structure formed with radiating inner structure and prismatic rhombic surface texture. During this process, several intermediate structures occurred, mainly with the morphologies of flowers, dumbbells, cauliflowers, spindles and crosses. The structural evolution offers insight into the mechanism of carbonate spherulite formation. It might be considered as the microbial signature in carbonate sediments and rocks. Because previous studies have been rarely conducted for the carbonate precipitation by Citrobacter freundii,this study will provide more information to microbial-mediated carbonate spherulite formation.