Calcareous nannofossils from the Jurassic black shales in the Qiangtang Basin, Northern Tibet (China): New records of stratigraphic ages and palaeoceanography
Calcareous nannofossils from the Jurassic black shales in the Qiangtang Basin, Northern Tibet (China): New records of stratigraphic ages and palaeoceanography
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藏北羌塘盆地侏罗纪黑色页岩中的钙质超微化石：地层年龄和古海洋学的新记录
DOI：
10.1127/nos/2018/0464
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发表时间：
2019-01
影响因子：
2.2
通讯作者：
Yi Haisheng
中科院分区：
文献类型：
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作者：
CHEN Lan;Yi Haisheng
藏北羌塘盆地侏罗系黑色页岩(中国)被认为是最重要的油气勘探源岩之一。到目前为止，对这些沉积物的准确测年一直受到菊石记录非常差的阻碍。在这里，我们首次详细研究了毕罗湖和安多114站两个剖面的侏罗纪富有机碳海相沉积中的钙质超微化石。尽管超微化石保存较差到中等，物种丰富度也相当低，但海尔维蒂卡Ansulasphaera的记录允许在这两个剖面中识别出NJ12超微化石带，并对应于巴松晚期至卡洛维亚。在这两个部分都有可能延伸到基本的牛津池。尽管球虫的保存情况普遍较差，但在扫描电子显微镜下，在两地的样本中都观察到了华氏球虫的整个球体。同样，在两个区段，在NJ12区观察到的小型Watznaueria显著增加，紧随其后的是碧罗Co区段的直立线直立鼠的显著高峰。这种超微化石组合表明，在所研究的区间内，地表水的营养水平从中营养条件逐步增加到富营养条件。在西北特提斯边缘的卡洛维安-牛津期地层中也报告了类似的趋势。西北部和东部(西藏)特提斯之间地表水生产力增强的共同记录支持了一种可能是气候驱动的超区域事件。这种高产条件可能是西藏和其他地方中侏罗世富含有机质矿床形成的最重要因素。
The Jurassic black shales of the Qiangtang Basin, northern Tibet (China) are acknowledged to be amongst the most important source-rocks for petroleum exploration. To date, a precise dating of these sediments has been hampered by a very poor ammonite record. Here, we present for the first time a detailed study of calcareous nannofossils from the organic carbon-rich Jurassic marine sediments of two sections, Biluo Co and Amdo 114 station. Although nannofossils are poorly to moderately preserved, and species richness is quite low, the record of Ansulasphaera helvetica allows the identification of the NJ12 nannofossil Zone in both sections and corresponds to an age of late Bathonian to Callovian. An extension into the basal Oxfordian is possible in both sections. In spite of the generally poor preservation of coccoliths, entire coccospheres of Watznaueria were observed under the SEM in samples from both localities. Similarly, in both sections, a significant increase in small-sized Watznaueria was observed in the NJ12 Zone, being followed by a prominent peak of Zeugrhabdotus erectus in the Biluo Co section. Such a nannofossil assemblage indicates a progressive increase in the trophic level of surface waters from meso-to eutrophic conditions in the studied interval. A similar trend is reported from Callovian–Oxfordian strata in the NW Tethyan margin. The common record of enhanced surface-water productivity between the NW and East (Tibet) Tethys supports a supraregional, probably climatically driven event. Such high-productivity conditions may have been the most important factor behind the formation of mid-Jurassic organic-rich deposits in Tibet and elsewhere.