Acritarch Assemblage in Yanchang Formation in Eastern Gansu Province and Its Environmental Implications
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摘要: 为了明确中晚三叠世延长组形成时期鄂尔多斯盆地陆相沉积的属性, 用孢粉分析方法详细研究了陇东西峰油田钻井岩心的微古植物组合, 在主要源岩层——延长组长7段发现了极为丰富的疑源类, 化石丰度高, 但属种单调; 以光面球藻(Leiosphaeridia) 绝对占优势, 经常出现具刺的微刺藻(Micrhystridium).同时代疑源类, 特别是具刺型疑源类生态分布和组合特征对比分析认为, 陇东地区中晚三叠世主要为淡水湖泊环境.出现的具刺疑源类不仅类型单调, 而且棘刺明显弱化, 推测是个别海生具刺类型在淡水环境中经历长期演化的结果.本地区主要烃源岩长7段沉积时期恰好处于湖水淡化过程, 也是最大湖进期, 大规模的湖水补给来源于陆地, 并非海侵.表明中晚三叠世鄂尔多斯盆地已经结束早中三叠世近海湖泊的性质.Abstract: In this paper, the microfossil assemblage from drilling core samples of Xifeng oilfield is discussed with palynologic analystic method, to clarify terrestrial sediment environment of Yanchang Formation in Ordos basin during the Middle and Late Triassic. The research shows abundant acritarchs in the main oil-source rocks, namely in Chang 7 Section of Yanchang Formation. All these acritarchs, highly abundant in fossils, but monotone in species, are dominated absolutely by Leiophaerids acritarchs Leiosphaeridia, often associated with acanthomorphic acritarchs Micrhystridium. The contrast in the ecological distribution and assemblage between contemporaneous acritarchs, especially acanthomorphic acritarchs, illustrates that the sedimentary surroundings were fresh lacustrine. These acritarchs, not only monotone in species but also conspicuous in echinulate process, suggest that some acanthomorphic marine acritarchs survived long-time evolution in fresh water. The sedimentary environment of Chang 7 Section, the main source rock in the area, turned into the climax of lake transgression, indicating the supply of the large-scale lake water body from rivers instead of from the rise in sea level. These evidences prove that the Early and Middle Triassic paralic and lacustrine features had already been inactive in the Middle and Late Triassic Ordos basin.
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Key words:
- Ordos basin /
- Triassic /
- Yanchang Formation /
- acritarch /
- paleoenvironment.
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图 1 陇东地区延长组疑源类钻井地层分布
Fig. 1. Distribution of acritarch from well core of Yanchang Formation in eastern Gansu Province
图 2 荧光显示的光球型疑源类(Leiosphaeridia spp.)
a.分散个体, 20-X44, ×320;b.块状聚合体, 12-X44, ×350
Fig. 2. Fluorescence image of Leiosphaeridia spp.
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