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2008 Vol. 19, No. 3

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Tectonics in the Northwestern West Philippine Basin
Xianglong NI, Shiguo WU, Ryuichi Shinjo
2008, 19(3): 191-199.

The West Philippine basin (WPB) is a currently inactive marginal basin belonging to Philippine Sea plate, which has a complex formation history and various crust structures. Based on gravity, magnetic and seismic data, the tectonics in West Philippine basin is characterized by amagma spreading stage and strike slip fractures. NNE trending Okinawa-Luzon fracture zone is a large fracture zone with apparent geomorphology and shows a right-handed movement. The results of joint gravity-magnetic-seismic inversion suggest that the Okinawa-Luzon fracture zone has intensive deformation and is a transform fault. Western existence of the NW trending fractures under Ryukyu Islands Arc is the main cause of the differences between south and north Okinawa Trough. The Urdaneta plateau is not a remained arc, but remnant of mantle plume although its lava chemistry is similar to oceanic island basalt (OIB).

Megascopic Symmetrical Metazoans from the Ediacaran Doushantuo Formation in the Northeastern Guizhou, South China
Yue WANG, Xunlian WANG, Yuming HUANG
2008, 19(3): 200-206.

In South China, various megascopic symmetrical metazoan fossils were found in the upper Doushantuo (陡山沱) Formation of the Neoproterozoic Ediacaran. The worm-like fossil is characterized by modern taxological annelid, for many metameres, parapodia, one possible tentale, an alimentary canal, and a dorsal vessel. The triradiate discoidal fossils belong to Trilobozoa, and the octaradiate discoidal fossil might be Ctenophora. All these fossils indicate that the megascopic metazoans have appeared in the Doushantuoian of Eidacaran and imply that the symmetrical metazoans must have originated at least 550 Ma ago.

An Update of Conodonts in the Induan-Olenekian Boundary Strata at West Pingdingshan Section, Chaohu, Anhui Province
Laishi ZHAO, Jinnan TONG, Suxin ZHANG, Zhimin SUN
2008, 19(3): 207-216.

The Lower Triassic in Chaohu (巢湖) area, Anhui (安徽) Province, China, is well developed and its sequence is typical in South China. After a brief introduction of the Induan-Olenekian boundary of Chaohu, this article presents some new data on conodonts. More than ten times of conodont samplings and investigations have recovered thousands of conodont specimens, which are especially rich in the Induan-Olenekian boundary strata at the West Pingdingshan Section in Chaohu City, Anhui Province. The most distinctive forms are the conodonts of the Neospathodus dieneri group and N. waageni group. The first occurrence of N. waageni eowaageni, which is regarded as the indicator of the Induan-Olenekian boundary, is situated at 40.49 m above the base of Yinkeng (殷坑) Formation. Some key conodonts and seven new specimens are introduced.

Preservation of Radiolarian Diversity and Abundance in Surface Sediments of the South China Sea and Its Environmental Implication
Muhong CHEN, Lanlan ZHANG, Lili ZHANG, Rong XIANG, Jun LU
2008, 19(3): 217-229.

Quantitative analysis of radiolarian preservation in surface sediments from different regional environments of the South China Sea (SCS) is discussed in this article for illustrating their relationship. The results show that species diversity and abundance of radiolarian in the surface sediment increase with water depth, indicating no obvious dissolution of radiolarian shell in the deep SCS. Upwelling activity and volcanic eruption are both in favor of radiolarian propagation, and cause the settlement enrichment of radiolarian shell. Whereas, an abnormal reduced radiolarian fauna in deeper slope sediment may result from the remain of turbid flow that brings sediments from shallow area, such as shelf and upper slope. Radiolarian species are mainly composed of tropic-subtropic types, and include some cold or polar species, which indicates that a mistake in paleoceanographic analysis should arise from the monospecific index. However, synthesis radiolarian indexes in sediments may well respond to the ecologic and depositional environments in the SCS.

Calculating Pollution Indices by Heavy Metals in Ecological Geochemistry Assessment and a Case Study in Parks of Beijing
Qingjie GONG, Jun DENG, Yunchuan XIANG, Qingfei WANG, Liqiang YANG
2008, 19(3): 230-241.

Pollution index is a powerful tool for ecological geochemistry assessment. The commonly used pollution indices by heavy metals in soils and sediments were classified as two types of single index and integrated index in an algorithm point of view. Four single indices of contamination factor (or concentration factor), ecological risk factor, enrichment factor, and index of geo-accumulation were illustrated, and the reference values for calculating single indices were distinguished into background levels and threshold pollution values. Eight integrated indices were divided into two groups. One group is suitable for the normal distribution single indices including the sum, average, weighted average, vector modulus, and Nemerow pollution indices, and the other for log-normal distribution including the product, root of product, and weighted power product pollution indices. Using background levels as reference values, five contamination classes were divided, and the terminologies are suggested for the single and integrated indices to unify the assessment results. Software of EGAPI was developed in a single document interface to calculate the four single and eight integrated indices by heavy metals to assess the quality of soil and sediment ecological geochemistry. Pollution indices by heavy metals of Cu, Pb, and Zn in soils in parks of Beijing were calculated using EGAPI software, and these five contamination classes and terminologies suggested in this study were evaluated and used. Results of integrated indices of Cu, Pb, and Zn in soils indicated that the soil qualities are unpolluted as a whole and varied from low polluted to unpolluted status from the center to the outskirts of Beijing City.

Evolution of the Moxizhuang Oil Field, Central Junggar Basin, Northwest China
Huayao ZOU, Yuanchun ZHANG, Jianzhang LIU, Jiannan SHI
2008, 19(3): 242-251.

Current oil saturation in the Moxizhuang (莫西庄) Oil Field in central Junggar (准噶尔) basin was evaluated by logging interpretation and measured on core samples, and the paleo-oil saturation in both the pay zones and water zones was investigated by grain-containing-oil inclusion (GOI) analysis. The pay zones in this field have low oil saturation and display low resistivity and small contrast between pay zones and water zones, and are classified as low-porosity, low oil saturation, and low resistivity reservoirs. Both the current low oil-saturation pay zones and the water zones above 4 365 m have high GOI values (up to 38%), suggesting high paleo-oil saturation. The significant difference between current oil saturation from both logging interpretation and core sample measurement and paleo-oil saturation indicated by GOI analysis suggests that this low oil-saturation field evolved from a high oil-saturation pool. Lateral re-migration and spill of formally trapped oil owing to changes in structural configuration since Neogene was the most plausible mechanism for oil loss in the Moxizhuang Oil Field. The combined effects of differential accumulation in the charge phase and the differential re-migration and spill of accumulated oil in Neogene are responsible for the complicated correlation between residual oil saturation and porosity/permeability of the reservoir sandstones and the distribution of low oil-saturation pay zones and paleo-oil zones (current water zones).

Composition and Provenance of Sandstones and Siltstones in Paleogene, Huimin Depression, Bohai Bay Basin, Eastern China
Jin-liang ZHANG, Xin ZHANG
2008, 19(3): 252-270.

This study was conducted to distinguish the compositions and provenance of sandstones and siltstones in the Ek1–Es3 members of Huimin (惠民) depression. The samples have been analyzed for petrographic, major element and selected trace element compositions (including REE). The results show that the sandstones from the first member of Kongdian (孔店) Formation (Ek1) have higher quartz compositions than those from the fourth member (Es4) and the third member (Es3) of Shahejie (沙河街) Formation. The alkali feldspar/plagioclase ratio in the A-CN-K diagrams decreases in the order Ek1 > Es4 > Es3, which suggests that the Ek1 member had a more alkali feldspar-rich granitoid source and more intense weathering of the source than the Es4 and Es3 members. The mineral distributions in the A-CN-K diagrams also indicate that the sandstones and siltstones in the three members underwent K-metasomatism. Extrapolation of the sandstones and siltstones back to the plagioclase-alkali feldspar line in the A-CN-K diagram suggests a high average plagioclase to alkali feldspar ratio in the provenance (tonalite to granite). In addition, the chemical index of weathering (CIW) and chemical index of alteration (CIA) parameters of the sandstones and siltstones suggest that the weathering of the first cycle material was intense, and the CIW decreases in the order Ek1 > Es4 > Es3. Trace element ratios suggest all the sedimentary rocks were mostly derived from granitoids. Elemental ratio plots (e.g., Th/Sc vs. Eu/Eu*) of sandstones and siltstones suggest a mix of a granodiorite-tonalite source as a source of the sandstones and siltstones. The sandstones and siltstones of Es3 and Es4 members also contain higher Eu/Eu* and lower Th/Sc ratios than the sandstones and siltstones of Ek1 member. As is consistent with the petrography, it suggests that the sandstones and siltstones of Es3 and Es4 members are derived from a source with less granite and more granodiorite-tonalite than the sandstones and siltstones of Ek1 member.

Changes of the Hailuogou Glacier, Mt. Gongga, China, against the Background of Global Warming in the Last Several Decades
Yuan-qing HE, Zong-sheng LI, Xiao-mei YANG, Wen-xiong JIA, Xian-zhong HE, Bo SONG, Ning-ning ZHANG, Qiao LIU
2008, 19(3): 271-281.

Great change, associated with global warming, has occurred at the Hailuogou (海螺沟) glacier, Mt. Gongga (贡嘎), China, since the early 20th century. Various data indicate that the glacier has retreated 1 822 m in the past 106 years, with an annual mean retreat of 17.2 m, and the front elevation has risen by 300 m since 1823. Comparison of glacier variations and temperature fluctuations in China and the Northern Hemisphere, over the last 100 years, indicates that glacier retreat stages occurred during the warm phase, and vice versa. Mass balance records during 1959/60-2003/04 have shown that the glacier has suffered a constant mass loss of snow and ice. The accumulated mass balance, -10.83 m water equivalent, indicates an annual mean value of -0.24 m water equivalent. The correlation between the mass balance and temperature is significant, which also indicates that climatewarming is the crucial cause of glacier loss. Local hydrological and climatic data demonstrate that runoff from the glacier has been increasing both seasonally and annually. The correlation analysis and trend analysis indicate that ice and snow melted water is the main cause of an increase in the runoff. As the climate has become warmer, changes in the glacier surface morphology have obviously occurred. These include a decrease in glacier thickness, enlargement of glacial caves, and reduction of the size of clefts on the glacier surface. The ablation period has lengthened and the ablation area has expanded. A variety offactors thus provide evidence that the Hailuogou glacier has suffered a rapid loss of snow and ice as a result of climatic warming.

Investigation of Curie Point Depth in Sulu Ultrahigh-Pressure Metamorphic Belt, Eastern China
Qing-qing QIAO, Qing-sheng LIU, Ning QIU, Yuan-yuan FU, Su-tao ZHAO, Yao WU, Tao YANG, Zhen-min JIN
2008, 19(3): 282-291.

The Curie point depth of continental crust can reflect the regional tectonic pattern and geothermal structures. Analysis of magnetism is an efficient way to obtain the Curie point depth on a regional scale. This study systematically investigated the Curie point depth of Sulu (苏鲁) ultrahigh pressure (UHP) metamorphic belt (33°40′N to 36°20′N and 118°E to 120°E, ca. 60 000 km2), eastern China using aeromagnetic data. The results show that the Curie point depth of the Sulu region varies from 18.5 to 27 km. The shallowest Curie point depth (ca. 18.5 km) is located in Subei (苏北) subsidence, where the estimated temperature gradient value is about 31.35 ℃/km, which is comparable with the measured value of 30 ℃ /km. In addition, a two-dimensional numerical solution of the heat conduction was used to calculate the temperature field to a depth of 30 km along the profile from Tancheng (郯城) to Lianshui (涟水) with a length of 139 km. The steady state model solved using the finite element method shows that the temperature around the Curie point depth is about 585.36 ℃, which is close to the Curie temperature (580 ℃) of magnetite at atmospheric pressure. These results provide newinsights into the tectonic and continuous thermal structures of the Sulu UHP metamorphic belt.

Creep Damage Characteristics of Soft Rock under Disturbance Loads
Zhi-liang FU, Hua GUO, Yan-fa GAO
2008, 19(3): 292-297.

This article focuses on the process of rock creep damage and micro-damage evolution properties of gray green mudstone under impacting disturbance load conditions for the first time using the real time computerized tomography (CT) testing technique. The results indicate that axial load comes into limit strength neighborhood, rock micro-crack links into larger crack, creep rate increases in a short time, larger plastic deformation happens; this is called disturbance accelerating creep stage. When rock is within limit strength neighborhood, there occurs creep micro-damage under smaller disturbance load. When disturbance load is larger, rock directly enters into disturbance accelerating creep stage, failure occurs instantaneously. On the basis of experimental research, the CT scanning method was used to describe the creep micro-damage of soft rock, also helpful in the prediction of roadways' service life and evaluation of geotechnical engineering stability.

Compacting Deformation Engineering Characteristics of Weathered Soft Rock Mixture in Subgrade
Xiao-qing LI, Qing-ping JIN
2008, 19(3): 298-306.

The engineering characteristics of weathered soft rock are important contents of soft rock mechanics. They also play a significant role in compacting deformation, which has been known to exert a significant amount of influence on the stability of highway filling subgrade engineering. In an effort to investigate this aspect of the problem, compacting tests and unconfined compressive strength tests have been carried out on weathered argillaceous slate and pelitic siltstone rocks, which are broken and graded before the test. The testing results indicate that the relationships of both between stress and strain and between axial strain and tangent modulus are exponential relationships; the size of the grain plays some influence on the deformation modulus, whereas the water content impacts the compressive strength greatly, which shows quadratic function; the unconfined compressive strength is linear with the dry density of loose soft rock mixtures. Therefore, the water content must be controlled in both the design and the construction of subgrade engineering of soft rock filling, and at the same time some effective measures should be taken to reach the requirement of compaction.