周老镇钻孔砾石层重矿物特征及地质意义

Heavy mineral characteristics of gravel layers in the Zhoulao Town borehole

利用江汉平原周老镇钻孔的岩心样品,分析研究了钻孔岩心沉积物10层砾石层中填充物的重矿物组分及分布特征。结果显示周老镇钻孔10个砾石层自下而上可以分为3个矿物段。对比3个阶段稳定重矿物、非稳定重矿物的变化特征:阶段Ⅰ、阶段Ⅱ,沉积物主要以近源物质为主,含有少量的远源物质;而阶段Ⅲ,沉积物主要以远源物质为主,含有少量的近源物质。结合周老镇钻孔岩性及钻孔砾石含量和江汉平原第四系岩相剖面特征,判断在阶段Ⅲ,深度112.30～109.05m(1.17～1.12MaB.P.)处,钻孔沉积环境发生了重大改变,江汉平原水系发生重要调整,长江三峡贯通形成。

A borehole was drilled at Zhoulao Town in Jianghan plain, and then ten gravel layers in core sediments were analyzed in order to study heavy mineral components and distribution characteristics of the fillings. The results show that the ten gravel layers can be divided into three mineral stages from bottom to top. A comparative study of the stable and unstable heavy minerals of the three stages has revealed that the variation characteristics are as follows： sediments of stage 1 and 2 are composed mainly of distal materials with a small amount of proximal materials, whereas the major sediments of stage 3 are distal materials mixed with a certain amount of proximal materials. Based on core lithology and gravel contents as well as Quaternary hthofacies features of Jianghan plain, the authors hold that at the depth of 112.30-109.05 m in the borehole （1.17-1.12 Ma.B.P., namely stage 3）, the sedimentary environment changed remarkably, and the water system of the Jianghan plain underwent considerable variation, which finally led to the formation of the Three Gorges on the Yangtze River.