黄河豫西段形成演化研究

The Yellow River between Sanmenxia and Xiaolangdi is a bridge connecting the upper and lower reaches of the Yellow River, therefore plays a critical role in the research on the formation and evolution history of the whole reach. Despite some advance in understanding the formation and evolution of this key reach in recent years, the detailed explanation for this challenge issue is still imperfect resulting in an unfavorable setting for the study on the formation and evolution of the Yellow River. Fluvial terrace is landform that was at one time constructed and maintained as the active floor of a river but is now abandoned, while planation surface is generally regarded as an ancient topographic surface. According to fluvial geomorphology, these geomorphic surfaces can record the formation and evolution history of the related river, being considered as a direct archive of long-timescale fluvial history. Recent studies reveal that two conspicuous planation surfaces and a series of well-preserved fluvial terraces were formed along the reach of the Yellow River between Sanmenxia and Xiaolangdi. In this propose, a combined approach of field investigation, difference GPS measurement, remote sensing, and DEM analysis is adopted to establish a complete sequence for these planation surfaces and fluvial terraces by outlining their distribution and characters. The formation ages of these geomorphic surfaces will be determined by the multiple approaches of magnetostratigraphy, pedostratigraphy, optically stimulated luminescence, and C-14. Furthermore, the composition characteristics of the fluvial deposits on these geomorphic surfaces will also be analyzed by petromineralogy, elemental tracer technique, and fabric measurement to reconstruct river directions during the formation processes of different geomorphic surfaces. These obtained archives in fluvial sediments and geochronology will be assembled to show the scenario of the Yellow River formation between Sanmenxia and Xiaolangdi. This work can not only cooperate with the achievement on the Yellow River development along the Jinshaan Gorge to provide new insight into the formation of the middle reach of the Yellow River, but also is useful to evaluating the neotectonic movement and geologic hazard in this reach.

黄河豫西段（三门峡-小浪底）是黄河中下游的过渡河段，在黄河形成演化研究中处在关键的位置。尽管近年来关于该段黄河形成演化研究有一定进展，但仍没有比较完善的认识，这对研究黄河形成演化十分不利。河流阶地是废弃的古河漫滩，夷平面是古地貌面，它们记录了地质时期河流形成演化的历史，是研究河流形成演化的重要载体。已有研究表明，黄河豫西段分布典型的夷平面和阶地。本项目拟通过野外考察、差分GPS测量、遥感和DEM分析研究该段黄河沿岸夷平面和阶地的分布与特征，建立层状地貌面序列；通过磁性地层学、黄土-古土壤地层学、OSL和14C测年方法确定它们的形成年代；通过岩石/矿物、示踪元素、沉积组构等方法研究地貌面物质成分特征，恢复各地貌面发育时期河流的流向，进而探讨黄河豫西段形成过程。该研究不仅能与相邻晋陕峡谷段黄河研究成果联系，建立关于整个黄河中游形成过程的较系统认识，也有助于本区新构造运动定量研究和地质灾害评价。

黄河起源于青藏高原东北部，向东流经一系列盆地、山脉、构造断裂系统，最终汇入渤海。它的形成和演化过程是揭示东亚地貌格局、构造活动和内陆侵蚀演化历史的基础，具有重要研究价值。其中，豫西段黄河西起三门峡东至小浪底是中下游的过渡河段。它的形成被认为是指示黄河水系东流格局出现的重要标志，在整个黄河形成演化研究中处在关键的位置。本项目以重建黄河豫西段的形成演化过程为主要研究目标，设计了四个方面的研究内容：①在黄河豫西段层状地貌面分布研究中，不但确定了一级夷平面，而且新发现了更高一级河流阶地，建立了夷平面至5级河流阶地完整的地貌面序列。通过横断面对比，绘制了地貌面位相图，发现这些地貌面发生强烈上凸变形，指示萧山相对隆升；②在层状地貌面物质成分特征研究中，确认了堆积在夷平面上的砾石层分布于萧山东麓。通过分析和对比重矿物组合特征以及砾石层的组构和岩性特征，认为夷平面上这套砾石层有别于河流阶地砾石层、现代黄河河道沉积物和上三门组沉积物，揭示夷平面发育过程中，萧山东麓虽然已有河流形成，但未与三门古湖取得水系联系，说明豫西段黄河尚未形成。③在层状地貌面形成年代研究中，根据夷平面和最高级河流阶地上覆风成沉积物的磁性地层低界年龄，限定夷平面形成于3.63 Ma，最高级河流阶地形成于1.2 Ma。其中，最高级河流阶地的年代与三门峡谷下游的最高级阶地年代一致，限定了豫西段黄河形成于3.63-1.2 Ma间。④在黄河豫西段形成演化研究中，结合上述三方面研究成果，认为青藏高原向东挤出，导致汾渭地堑拉张、萧山相对隆升。差异性抬升不但瓦解夷平面，导致地堑收窄、堆积上三门组沉积物，还驱动萧山东麓河流系统向东扩展，最终在3.63-1.2 Ma间切开萧山，袭夺了黄河中上游水系，至此豫西段黄河出现，黄河东流入海格局形成。大量内陆沉积物倾泻入海，导致沉积速率在1.0 Ma左右剧增，为洋盆陆相沉积物的堆积速率变化找到了除构造活动和气候变化的第三个控制因素。