湖北南漳地区晚震旦世高家山期微化石的新发现——兼论南江上升运动的地质意义

DISCOVERY OF MICROFOSSILS OF THE LATE SINIAN GAOJIASHANIAN AGE IN NANZHANG, HUBEI PROVINCE, CHINA WITH REMARKS ON THE GEOLOGICAL SIGNIFICANCE OF THE NANJIANG UPLIFT MOVEMENT

湖北南漳邓家崖一带 ,晚震旦世时位于扬子台地北缘 ,受到南江上升运动的影响 ,灯影组上、下段之间为一古喀斯特侵蚀面 ,灯影组下段上部地层剥蚀甚多。灯影组上段底部有厚 2 m左右的含磷地层 (相当于陕南的高家山段的一部分 ) ,其中除了隐藻类磷块岩之外 ,还发现产有大量球状绿藻类化石 (Eosphaera gigantea sp. nov.,Megasphaera inornata Chen et L iu和 Protosphaeridium sp. )及似红藻石 ,并产有可能属于软体动物门的小壳化石(N anzhanguncus delicatus gen. et sp. nov.)。这些发现丰富了晚震旦世高家山期生物群的内容。 南江上升运动在整个扬子区都有明显的影响 ,之后的海侵开始时 ,不仅有碎屑岩的沉积 ,并有磷质的输入和磷块岩的形成 ,它促使高家山生物群的繁盛 ,是灯影期沉积地层划分和对比的良好标志。根据这一标志及沉积区的横向追索 ,峡区的灯影组石板滩段是南江上升运动前的沉积地层 ,与蛤蟆井段之间的界线不是一个等时面。所以 ,西陵峡生物群和高家山生物群产出层位不是同期沉积。就目前材料看 ,震旦系建阶条件尚不成熟。

Dengjiaya in Nanzhang County, Hubei Province, was located in the northern margin of the Yangtze Platform in the Late Sinian (Neoproterozoic Ⅲ) (Fig.1). Here the Dengying Formation is divided into the lower and upper members by a paleokarstic erosional surface (Fig.2). At the base of the upper member there is a suite of phosphoritic strata in which a large number of spherical Chlorophyte fossils (Eosphaera gigantea sp. nov., Megasphaera inornata Chen et Liu and Protosphaeridium sp.), rhodolith-like grains and a small shell fossil (Nanzhanguncus delicatus gen. et sp. nov.) probably belonging to Mollusca, as well as phosphatized cryptalgal deposits, are discovered. The Nanjiang Uplift Movement in the Late Sinian occurred in the Yangtze Region. It produced an obvious hiatus in the middle part of the Dengying Formation, and the terrigenous deposits and phosphorites occurred at the beginning of the following transgression that made the Gaojiashan Biota to thrive. The disconformity is a good criterion for the division and correlation of the Dengying Formation. The correlation shows that the Shibantan Member, which yields the Xilingxia Biota, of the Dengying Formation in the East Yangtze Gorges area was probably deposited before the Nanjiang Uplift Movement, therefore, the occurrence of the Xilingxia Biota was not contemporaneous with the Gaojiashan Biota (Table 1). In addition, the boundary between the Hamajing and Shibantan Members of the Dengying Formation in the East Yangtze Gorges area is not synchronous.