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Sequence Stratigraphy of the Desert System: A Case Study of the Lower Cretaceous in the Kuqa Basin in Xinjiang, Northwestern China

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Acta Geologica Sinica(English Edition) 2004年第3期78卷 744-755页

作者： MEIMingxiang yubingsong JINWeiguang SchoolofEarthSciencesandNaturalResources ChinaUniversityofGeosciencesBeijing100083

The Lower Cretaceous strata in the Kuqa Basin in Xinjiang are marked by a set of arid red beds. Several types of sedimentary fades can be identified in this set of arid red beds: mudstones of the plaza and intracontinental sebkha, aeolian sandstones, sandy conglomerates of the intermittent river, conglomerates of the pluvial fan, etc. These types of sedimentary facies constitute a typical desert system. Therefore, the Cretaceous strata in the Kuqa Basin provide a favorable condition for studies of sequence stratigraphic divisions of the desert system. With the rise and fall of the base level of the sedimentary basin, cyclicity is clearly revealed in stratigraphic records, which helps the identification of the third-order sequences. Based on the cyclicity in stratigraphic records, 5 third-order sequences can be found in the strata of the Early Cretaceous in the Kuqa Basin. These sequences comprise a second-order tectonic sequence. The primary feature of these third-order sequences is of an upward-fining sedimentary succession formed by a succession of 'coarse sediments of the alluvial system-fine sediments of the lake system'. The result of this study shows that aeolian sandstones are the best reservoirs of natural gas in the Cretaceous strata in the Kuqa Basin, and that the Kela-2 gas field is the first large gas field dominated by aeolian sandstone reservoirs in China.

关键词： sequence stratigraphy desert system Cretaceous Kuqa Basin Xinjiang northwestern China

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Characteristics of Rare Earth and Trace Element Patterns in Bedded Cherts from the Bottom of the Lower Cambrian in the Northern Tarim Basin, Northwest China and Their Genetic Significance

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Acta Geologica Sinica(English Edition) 2005年第2期79卷 215-224页

作者： yubingsong DONGHailiang CHENJianqiang LIXingwu LINChangsong StateKeyLaboratoryofGeologicalProcessesandMineralResources ChinaUniversityofGeosciencesBeijing100083China DepartmentofGeology MiamiUniversityOxfordOH45056USA

The sedimentary sequence of the Lower Cambrian is a key interval to reveal the early evolution history of the Earth and there occur widespread cherts worldwide. These cherts possibly carry important information to decipher the breakup process of the Rodinia Continent. Black rock series at the bottom of the Lower Cambrian in the Northern Tarim Basin, China, is composed of black shales interbedded with thin-bedded cherts. Ten chert samples were systematically collected from two outcrops at Xiaoerbulak and Sogatbulak, 8.8 and 7.5 m thick respectively. The cherts were crushed, and were analyzed for trace element and rare earth concentrations. Trace elements such as V, Cu, Zn, U, Pb, Ba, Cd, Ag, Mo, As and Sb are highly enriched, and others such as Rb, Zr, Cs, Hf, Ta, W, Tl, Bi and Th are highly depleted in the cherts. These trace element patterns suggest that the cherts may be of deep crustal origin. The low ratios of Th/U and Rb/Sr further suggest that the cherts are of earth interior sources or received hydrothermal input during their deposition. Chondrite- normalized Eu/Eu* value markedly decreases upward in the section from 5.54 at the lowermost to 0.73 at the top, and NASC-normalized Eu/Eu* value decreases from 8.05 to 1.03. The relatively high Eu/Eu* ratio for the cherts from the northern Tarim Basin is most likely due to a hydrothermal input (e.g., Eu/Eu* ~10). The systematic decrease of Eu/Eu* ratio from the bottom to the top of the section reflects that the hydrothermal input is the largest in the lowermost portion of the section and gradually decreases upward. The chondrite-normalized Ce/Ce* ratio ranges from 0.42 to 0.83, with an average of 0.60. North American Shale Composite (NASC)-normalized Ce/Ce* ratio ranges from 0.42 to 0.79, with an average of 0.57. Negative Ce anomalies are distinct. ΣREEs in the cherts generally increase from 10.50 ppm at the bottom to 35.97 ppm at the top of the sampled section. NASC-normalized (La/Lu) N ratio decreases from 2.72 a

关键词： black rock series and chert rare earth and trace elements depositional environment northern Tarim Basin Northwest China

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