北羌塘盆地中南部侏罗纪布曲组古生物特征与沉积环境

Palaeontological characteristics and sedimentary environments of the Jurassic Buqu Formation in the Northern Qiangtang Basin

本文通过对北羌塘盆地北坳陷中南部胜利河、东湖及毛毛山等地区侏罗纪布曲组6条剖面及其中丰富的腕足类、双壳类等古生物资料和岩石组合特征的研究,将布曲组地质时代划为中侏罗世巴通期(Bathonian)至早卡洛夫期(Callovian),还可能跨入早巴柔期(Bajocian)。通过本文研究和区域对比,认为布曲组的沉积时代在北羌塘盆地存在穿时性。依据岩石组合特征,布曲组沉积充填物三分性明显,下部和上部为一套以微晶结构、粒泥结构为主的低能碳酸盐岩,中部为一套以高能的亮晶粒屑灰岩为主的碳酸盐岩。结合古生物生态习性,布曲组沉积环境总体为近岸浅水开阔台地–台地边缘碳酸盐岩沉积体系,构成多个沉积旋回。这一基础资料对下一步分析北羌塘盆地坳陷中南部布曲组岩相古地理提供了支撑。

Based on studies of the six sections of the Jurassic Buqu Formation in Shenglihe, Donghu and Maomaoshan areas in the south central parts of the North Depression of the Northern Qiangtang Basin, their lithological characters, and their rich paleontological data(including abundant brachiopods and bivalves), we propose that the age of the Buqu Formation is constrained to the Middle Jurassic Bathonian to early Callovian(probably extending to early Bajocian). Results from our observations and regional correlations indicate that the Buqu Formation in the Northern Qiangtang Basin is diachronic. Characteristics of rock assemblages of the Buqu Formation indicates that the sediments can obviously divided into three parts. Both the lower and upper parts consist of a series of low-energy carbonate rocks dominated by microcrystalline and grain-muddy structures, and the middle part consists of a series of high-energy carbonate rocks dominated by pelsparite. Combined with the palaeoecological habits of the paleontological, the overall depositional environments of the Buqu Formation are considered as a near-shore shallow water open platform and a platform margin carbonate sedimentary system, which constitute multiple depositional cycles. This basic data provides a foundation for further analysis of the lithofacies paleogeography of the Buqu Formation in the south central parts of the Northern Qiangtang Depression.