江汉平原周老镇钻孔砾石统计及其与长江三峡贯通的关系

Statistics of Gravels from a Bore in Zhoulao Town,Jianghan Plain and Its Relationship with Cut-Through of the Yangtze Three Gorges,China

对江汉平原周老镇钻孔砾石层砾石的粒度和砾态进行统计和分析的结果表明：砾石的风化程度普遍较高，砾石的扁度、球度、磨圆度、砾态参数（F／ψ）和各轴分选系数均符合河流相沉积物的特征。通过分析砾石粒度和砾态的变化曲线，可将其划分为3个阶段。阶段Ⅰ和Ⅱ的平均砾径、中值砾径、扁度、球度和砾态参数（F／ψ）相差不大，而阶段Ⅲ与阶段Ⅰ、Ⅱ相差很大，阶段Ⅲ的水动力条件、沉积物比阶段Ⅰ、Ⅱ显著增强并混杂了远源物质。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Ⅲ109．5-117．0m（0．97～1．12Ma．B．P．）处钻孔的沉积环境发生了重大改变，江汉平原水系发生了重要调整，可能是长江贯通三峡。

The statistics and analysis of gravel particle size and shape features from the bore in Zhoulao town,Jianghan Plain indicate that the gravels are generally highly weathered. Their shapes （ rectangular, spheric and round）, F/ψ and all axes sorting coefficient are all in conformance to the features of fluvial facies. The variation curve is divided into three stages according to the analysis of the gravel particle size and shape features. The contrast of the mean value of the average grain size, median grain size, rectangular, sphericity, F/ψ, a-axis sorting coefficient and roundness of the bore gravels in three stages shows that stage Ⅰ is a little different from stage Ⅱ , but stage Ⅲ is greatly different from the former two. This suggests that the water motive of stage Ⅲ enhanced evidently, while the sediment of stage Ⅲ jumbled farther ones, compared with stage Ⅰ and Ⅱ. Based on the analysis of the lithology and content of the bores in Zhoulao Town, and the map of the Quaternary petrographical section in the Jianghan Plain, it is determined that the sedimentary environments changed significantly at the 109.5-117.0 m of the bore in Zhoulao Town and the water system modulated remarkably in Jianghan Plain when the Three Gorges were cut by the Yangtze River.