兰州东盆地最近1.2Ma的黄河阶地序列与形成原因

RIVER TERRACES OF THE YELLOW RIVER AND THEIR GENESIS IN EASTERN LANZHOU BASIN DURING LAST 1.2Ma

毗邻青藏高原的兰州地区的黄河阶地系列是地面抬升和气候变化的信息载体。通过古地磁、光释光测年及黄土一古土壤地层序列对比，初步确定了兰州东盆地1．2Ma以来主要有两个阶地发育时期。第1个时期是1．24—0．86Ma，黄河至少发育了4级阶地，其形成时代分别为：1．24Ma，1．05Ma，0．96Ma和0．86Ma；第2时期为最近0．13Ma，黄河发育3级阶地，时代分别为0．13Ma，0．05Ma和0．01Ma。每级阶地河漫滩顶部都有一层古土壤发育，表明黄河下切形成阶地发生在古土壤开始发育的冰期向间冰期的过渡阶段，但是并非1．2Ma以来的每次冰期一间冰期的气候交替都能引起黄河下切形成阶地。气候变化只是阶地形成的必要条件之一，不是充分条件。对比黄河下切速率和阶地年代序列发现，地面上升仍然是影响黄河下切的重要因素。只有在地面上升速率达到一定程度的时候（例如：1．2～0．8Ma和最近0．13Ma），气候变化才能导致黄河堆积与下切交替形成阶地；而在地面上升缓慢时期（例如：0．80—0．13Ma），即使发生了大幅度的气候变化，黄河也没有阶地记录。

Fluvial terraces of the Yellow River in Lanzhou region,which lies to the northeast margin of the Tibetan Plateau, are good archives for studying tectonic and climatic changes. Based on field investigations, pedostratigraphy, magnetostratigraphy and optically stimulated luminescence （OSL） dating, two periods of terrace formation in the Yellow River were revealed. One is 1.24 - 0. 86Ma in which 4 terraces were developed,their ages are 1.24Ma, 1.05Ma,0. 96Ma and 0. 86Ma respectively. Another is the last 0. 13Ma in which 3 terraces were formed,their ages are 0. 13Ma,0.05Ma and 0. 01Ma respectively. All the terraces have similar lithology that there is a paleosol with a thickness of several meters developed on the top of fluvial silt. It shows that the Yellow River incised during the transition from glacial to interglacial and there may be a link between the formation of terraces and glacial-interglacial climatic cycles. However,it is not all glacial-interglacial transitions in which terraces formed during the last 1.2Ma, that indicates that climate change is not the only factor controlling river incision and terrace formation. The incision of river depends on tectonic uplift. The incision rates of the Yellow River indicate that tectonic uplift is active between 1.2Ma and 0.86Ma as well as the last 0. 13Ma in Lanzhou region. Comparison the river terrace formation with the periods of high incision rates indicates that tectonic uplift perhaps plays an important role on river terrace formation.