Because of its complex history,the Yellow River is a good case for fluvial geomorphology studies.The Jinshaan Gorge is one of the key parts to understand the evolution of the Yellow River.Since 1922,researchers have made great progress in the study of the evolution of the Yellow River in the Jinshaan Gorge area.But there are some disputes still remained about the Tangxian Surface,an erosion surface formed in the Miocene and Pliocene,and the alluvium on it.The main divarication is whether the Tangxian Surface is a terrace of the Yellow River or the alluvium on the Tangxian Surface is formed by the ancient Yellow River.Baode-Fugu area,located in the north part of the Jinshaan Gorge,is famous for the Neogene red clay sediment in which Hipprion fauna is unearthed.The recent studies of red clay offer a chronological data to discuss the age of both the red clay,other contemporaneous sediment and the underneath erosion surface of Tangxian Stage.The present paper is based on the survey of eighteen sediment outcrops on the Tangxian Surface in the BaodeFugu area.Twenty one samples were collected from gravel layers for sediment analysis.A topographic profile across the Yellow River shows that there is a wide bedrock valley stretching along the modern Yellow River,which is about 170m above the river level.According to the previous magnetostratigraphic studies of the red clay and fluvial sediment,the Tangxian Surface was formed at least 8.OMa ago.Gravel statistic result shows that the gravel sitting on the Tangxian Surface is ＞ 1.5 in the complexity index of components and is characterized by an average roundness of ca.2 or ＞2,i.e.sub-rounded and rounded,with a variance of ＞0.5.The rock type of the gravels indicates a mixture of distant and local source.Combining the evidences above,it is reasonable to conclude that there used to be a river flowing along a similar course direction of modern Yellow River in this region during the Middle Miocene.Though it is much smaller than the Yellow River today,it is a part of the ancient river when the Yellow River connected the Weihe and Hetao basins and it can be named as the Ancient Yellow River.With the incision of the Jinshaan Gorge,the Tangxian Surface became the highest terrace of the Yellow River in the study area.On both side of the Tangxian Surface,gravels interbeded with the red clay are mainly deposits of the tributaries.Their relatively simple components (ca.1 in complexity index),poor roundness (＜ 1.5 in average,sub-angular)and a small variance of less than 0.32 suggest a local source of the sediment.%晋陕峡谷是研究黄河演化的关键河段之一,许多学者对晋陕峡谷以及黄河的演化进行了研究,但还没有形成统一认识,特别是晋陕峡谷区唐县面上是否存在古黄河沉积物以及唐县面是否属于黄河阶地,学术界尚存在不同见解.本文以晋陕峡谷北段保德-府谷地区唐县期基岩侵蚀面上18个沉积剖面的野外观察为基础,在各露头剖面砾石层及现代黄河中采集21个样品进行砾石特征统计分析,结果表明:保德-府谷地区存在与现代黄河峡谷延伸方向一致的唐县期基岩宽谷,其形成时代至少在8.0Ma之前.唐县期宽谷底部各剖面砾石成分复杂性指数大多高于1.5,平均磨圆度接近2或在2以上,以次圆和圆为主,但磨圆度方差在0.5以上,砾石中远源组分比例大,具有长距离搬运和来源丰富的特征,由此推断该地区在中新世中期已经存在一条规模较大的河流,是现代黄河贯穿之前的一部分,即古黄河.晋陕峡谷的下切使唐县期宽谷高出现代河面约170m,成为该地段黄河的最高一级阶地.该地区红土层中下部的砾石透镜体,砾石成分简单,复杂性指数在1左右,磨圆普遍较差,除个别剖面,平均磨圆度都在1.5以下,以次棱角状为主,且不同砾石磨圆程度相差不大,磨圆度方差在0.32以下,具有近源堆积的特点,应该是来自河谷两侧支流的洪积物.
展开▼
