Late Devonian Turbidite System and Tectonics of Zhen'an Basin at Northern Margin of South Qinling
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摘要: 南秦岭北部山阳断裂和板岩镇断裂之间的镇安盆地在晚泥盆世发育—套浊流沉积。通过对这套浊积岩系内部岩相、相组合、古流向等详细研究，结果证明其内部由斜坡沉积、水道—天然堤沉积、舌状体沉积以及盆地平原等单元组成，然而它们的空间组合及内部特征并不代表一个典型的水下扇沉积体系，而是反映一个由线状物源供给的缓坡型浊积岩体系。晚泥盆世浊积岩体系是随中记盆世碳酸盐台地发生沉陷而随之发生的，反映镇安盆地当时为一个相对狭窄的断陷式盆地。区域构造分析证明，盆地的形成和发展与南、北秦岭发生初始碰撞而引起的俯冲板块前缘挠曲隆升和上部地壳伸展作用有关。Abstract: Zhen'an basin, situated at the northern margin of South Qinling, is bounded to the north by the Shanyang fault and to the south by the Banyanzhen fault, respectively. It was a rifted basin and filled mainly with siliciclastic turbidites during Late Devonian.The turbidite system is made up of the Jiuliping formation (Upper Devonian), which can be divided spacially into two segments, the northern Jiuliping formation and southern Jiuliping formaion, according to facies characteristics. The lower part of the northern Jiuliping formation is composed of slope fine-grained sediments, inclusive of hemipelagites and fine-grained turbidites, but commonly cut by gullies which are filled with coarse-grained gravity-flow deposits. In contrast, the lower part of the southern Jiuliping formation consists of channel-levee and lobe deposits, but typical thinning-and-fining-upward, or thickening-and-coarseningupward sequences are not well developed. The channel-levee and lobe units are blanketed with basin plain deposits, they are then changed into thin-layered deep-water carbonate rocks.Clearly, the whole Upper Devonian sequences show a deepening-upward tendency. In addition, it is demonstrated on the basis of analysis of paleocurrents and source areas that the siliciclastic turbidite system was fed from a linear source area to the north, but turbidity current was deflected eastwards along the basin axis. The turbidite system studied is not a classical submarine fan but one that has much in common with the ramp turbidite system model.The turbidite system and basin development are apparently influenced by tectonism, sealevel fluctuation and sediment supply. The rifting leads to the drowning of Middle Devonian carbonate platform and the initiation of siliciclastic turbidite system, and the uppermost deepwater carbonate rocks or carbonate turbidites are believed to be related to sea-level rising. The Late Devonian initiation and development of Zhen'an Basin resulted from the peripheral upwell and induced extension in the upper crust at the northern margin of the subducting South Qinling plate in consideration of regional tectonism.
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Key words:
- qinling /
- zhen'an basin /
- turbidite system /
- devonian
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