藏北羌塘地区侏罗纪颗石藻化石的发现及其意义

藏北羌塘地区出现了两套代表深水相沉积的黑色岩系,成为该区研究中的又一热点。最近通过扫描电镜,首次发现黑色岩系中含有数量丰富但属种比较单一的钙质超微化石——颗石藻(Cocco-lithophore)。经鉴定,主要是Ellipsagelosphaera sp.(椭圆晶球颗石藻未定种),结合菊石化石,认为其时代归属于侏罗纪。这一重大发现标志着在西藏特提斯最东部侏罗纪地层中颗石藻化石的存在。文中初步识别了Ellipsagelosphaera sp.及其分布、时代。地球化学分析表明:含颗石藻的烃源岩有机质丰富,认为这两套深水相黑色岩系的出现特别是以颗石藻输入为主的烃源层可能是因为颗石藻的勃发。颗石藻的发现,对研究区烃源岩的空间分布可能提供了一种新的研究思路和研究方法。

THE DISCOVERY OF JURASSIC COCCOLITHOPHORE IN QIANGTANG AREA OF NORTHERN TIBETAN PLATEAU AND ITS SIGNIFICANCE

Two series of black shales, representing the deep water deposit, are widely distributed in Qiangtang area , northern Tibet, which becomes a hot topic of Qiangtang basin study. Recently, by means of scanning electron microscope, abundant Coccolithophore with minor variation of species was found in the black shale for the first time. The Coccolithophore is one of the calcareous nannofossils. According to the fossil identification the Coccolithophore discovered is mainly the Ellipsagelosphaera species. Considering the coexisted fossils of ammonites we put forward that the time of Coccolithophore is Jurassic. The significant finding shows the existence of Coccolithophore in the Jurassic stratigraphy of the East Tethys, Tibet. In the paper, nannofabric of Coccolithophore is described in detail. We have also discussed the distribution and the time of Ellipsagelosphaera species. Geochemical analysis of cocco-liths-bearing oil source rocks demonstrated that these rocks have abundant organic matters, which may be attributed to the algal blooms, above all, the Coccolithophore blooms. The discovery of Coccolithophore may provide a new thought and a new method of research on special distribution of source rocks in the studied area.