Parasequence-Scale Organic Facies Variations and Its Significance in Nonmarine Basin
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摘要: 柴达木盆地北缘侏罗系由非海相冲积—三角洲及湖泊沉积体系构成。托阿尔阶的沉积序列为下部的冲积体系向上过渡为滨浅湖、中深湖沉积体系,后者发育一系列在垂向上向上变粗—变浅的旋回层并构成准层序。在准层序级别,有机相参数发生规律性变化。岩石的有机质含量向上增加,显示与粉砂质含量呈正相关。其原因可能是冲积物源的植物碎屑和粉砂颗粒具有相似的水动力均衡条件。有机母质具有高等陆生植物和低等水生生物输入的双重旋回特征。成熟度居于临界状态 (镜质体反射率平均 0.57% )。自生的黄铁矿 /菱铁矿比与沉积物堆积速率之间存在一定的联系。湖泛面位于准层序的下部且直接上覆高有机质的暗色泥岩。Abstract: The Jurassic of the northern Qaidam basin is widely recognized as the nonmar ine sediments, and interpreted as alluvial-delta and lacustrine depositional system. The depositional sequence of the Tianshuigou Fm. within the lower Jura ss ic mainly composed of the alluvial system at the base and lower and the deep lac ustrine system in the upper. Geochemical and optical analyses were undertaken on the minor cyclothems, a progradationally coarsening-and shallowing-upward paraseque nce (cycle) within the major lacustrine deposits in which Type III land-plant derived kerogen is overwhelming. The total organic carbon cont ent of the fine-grainned lithology increases upwards through the parasequence and is positively correlated with silt content, presumably due to the hydrodynam ic equivalence and common alluvial source of phytoclasts and silt particles. The section is marginally mature ( about 0.57% vitrinite reflectance). The distri bution of pyrite and siderite is apparently related to sediment accumulation rate,with frequent appearance of siderite in the lower part of the para sequence. The lacustri ne flooding surface at the base of the parasequence is overlain by black shale, and thought to have been terminated by an abrupt and major rise in relative lac ustrine level,which may be perhaps the result of tectonically driven changes in subsidence.
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Key words:
- organic facies /
- parasequence /
- nonmarine /
- Jurassic /
- Qaidam basin
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