西藏南羌塘盆地中三叠世座倾错组放射虫动物群及其地层学意义

Radiolarian Fauna from Middle Triassic of Zuoqingcuo Formation,Southern Qiangtang Basin,Xizang,China,and Its Stratigraphic Significance

前人研究表明西藏南羌塘盆地存在中三叠世地层,然而一直以来缺乏确凿的古生物化石证据.在西藏改则县古姆乡座倾错地区原被划为晚三叠世日干配错组的地层中测制了两条剖面,在硅质岩样品中分析和鉴定出18属33种(含未定种)放射虫化石,主要分子包括Cryptostephanidium longispinosum (Sashida)、Eptingium manfredi Dumitrica、Falcispongus falciformis Dumitrica、Hindeosphaera spinulosa (Nakaseko et Nishimura)、Muelleritortis firmum (Gorican)、Oertlispongus inaequispinosus Dumitrica,Kozur et Mostler、Paroertlispongus chinensis (Feng)、Paroertlispongus hermi(Lahm)、Paroertlispongus weddigei Lahm、Paurinella aequispinosa Kozur et Mostler、Pseudostylosphaera coccostyla (Rüst)、Pseudostylosphaera compacta (Nakaseko et Nishimura)、Pseudostylosphaera gracilis Kozur et Mostler、Pseudostylosphaera japonica (Nakaseko et Nishimura)、Pseudostylosphaera nazarovi (Kozur et Mostler)、Pseudostylosphaera tenue (Nakaseko et Nishimura)、Triassocampe imitata Bragin、Triassospongosphaera triassica (Kozur et Mostler).座倾错放射虫化石组合指示这段地层时代为中三叠世安尼期晚期-拉丁期早期.结合之前的工作积累,将南羌塘盆地三叠系划分为表示稳定型浅海相碳酸盐岩沉积的下三叠统孜狮桑组、表示半深海-深海相碎屑岩-硅质岩沉积的中三叠统座倾错组、上三叠统日干配错组以及上三叠统-下侏罗统萨门熊组.该发现为了解中三叠世南羌塘地区的真实面貌提供了地层学和古生物学证据,也进一步证明中三叠世南羌塘盆地不是陆地剥蚀区或隆起区,而是一种半深海-深海相沉积区.

Previous studies indicate that there are Middle Triassic strata in the southern Qiangtang basin, which has not been confirmed due to the lack of solid evidence of paleontological fossils. Here two new sections in Zuoqingcuo, Jimu Town, Gaize County, Tibet, were investigated. It is found that the sections belong to the Upper Triassic Riganpeicuo Formation according to the 1 ： 250 000 Regional Geological Survey Report of Dinggu Area. 33 species in 18 genera of fossil radiolarians from the specimens of siliceous rocks were totally obtained and identified. The main components of the radiolarians are Cryptostephanidium longispinosurn （Sashida）, Eptingiurn manfredi Dumitrica, Falcispongus falciforrnis Dumitrica, Hindeosphaera spinulosa （Nakaseko et Nishimura）, Muelleritortis firmum （Gorican）, Oertlispongus inaequispinosus Dumitriea, Kozur et Mostler, Paroertlispongus chinensis （Feng）, Paroertlispongus hermi （Lahm）, Paroertlispongus weddigei Lahm, Paurinella aequispinosa Kozur et Mostler, Pseudostylosphaera coccostyla （Riist）, Pseudostylosphaera compacta （Nakaseko et Nishimura）, Pseudostylosphaera gracilis Kozur et Mostler, Pseudostylosphaera japonica （Nakaseko et Nishimura）, Pseudostylosphaera nazarovi （Kozur et Mostler）, Pseudostylosphaera tenue Nakaseko et Nishimura）, Triassocampe imitata Bragin, Triassospongosphaera triassica （Kozur et Mostler）. The radiolarian assemblage indicates an age of the late Anisian-early Ladinian of Middle Triassic for the two sections. Based on the previous research and the present data, the Triassic strata in the southern Qiangtang basin are divided into （1） the Lower Triassic Zishisang Formation, indicating carbonate deposition in a stable shallow marine environment, （2） the Middle Triassic Zuoqingcuo Formation, a new formation, indicating clasolite to silicolite deposition in a deep-water （bathyal-abyssal） environment, （3） the Upper Triassic Riganpeicuo Formation and the Upper TriassicLower Jurassic Samenxiong Formation. Our results provide stratigraphic and paleontological evidences to understand the features and contents of the Middle Triassic in the southern Qiangtang area, and also further confirm that the Middle Triassic of the southern Qiangtang basin was not an erosional or uplift area, but a bathyal-abyssal depositional area.