藏南江孜地区早白垩世甲不拉组钙质超微化石的特征及其地层意义

The characteristics of Early Cretaceous calcareous nannofossils from Jiabula Formation in Gyangze area,southern Tibet,and their stratigraphic significance

藏南江孜一带的侏罗纪—白垩纪海相地层被划分为维美组和甲不拉组。通过偏光显微镜和扫描电镜观察和分析,发现甲不拉组黑色页岩中含有保存较为完好的钙质超微化石。研究区的生物丰度和分异度偏低,经鉴定以椭圆球石科(Ellip-sagelosphaeraceae)生物群为主,数量相对丰富,但属种比较单调。超微化石指示甲不拉组为早白垩世沉积的产物。结合菊石和双壳类化石,认为甲不拉组地层时代属于早白垩世贝利阿斯期至凡兰吟期(Berriasian-Valanginian)。这一发现标志着西藏特提斯东部早白垩世地层中钙质超微化石的存在。甲不拉组早白垩世钙质超微化石的发现,为该地区海相地层的时代划分、对比提供了新的资料,也为寻找海相侏罗纪—白垩纪地层界线指示了目标。

The marine Jurassic-Cretaceous boundary strata are outcropped in southern Tibet.The strata in Gyangze area are divided into Weimei and Jiabula Formations.The samples were observed and analyzed by means of polarizing light microscope（PLM） and scanning electron microscope（SEM）,and the well-preserved calcareous nannofossils were found for the first time in the black shale of Jiabula Formation along the Jiabula-Gokou section.Biological abundance and diversity are relatively low in the study area.According to the fossil identification the biota discovered is mainly the Ellipsagelosphaeraceae fauna which is abundant with minor variation at the genus and species level.The nannofossils indicate that the Jiabula Formation is a product of Early Cretaceous deposition.Considering the coexistent fossils of ammonites and bivalves,the authors hold that the age of Jiabula Formation is the Berriasian-Valanginian of Early Cretaceous.The significant finding shows that the Early Cretaceous nannofossils occur in the eastern Tethys.The discovery of nannofossils provides new evidence for the age of marine strata and their stratigrphic correlation,and also indicates the possibility of the marine Jurassic-Cretaceous boundary study.