兰州黄河Ⅱ和Ⅲ级阶地的地层结构、年龄及环境意义

通过对兰州段黄河Ⅱ和Ⅲ级阶地剖面的地层岩性、粒度分析和光释光测年等,认为剖面中被普遍界定为河漫滩沉积的一套厚几米至几十米具水平层理的细粒砂土层是以冲、洪积与黄土互层的交互式沉积,其开始沉积的时期与气候进入冰期或冰阶相对应。据此可将兰州段黄河阶地剖面地层划分为三部分,即:河流砂砾石层、加积层与风成黄土层,分析了这一地层结构的发育过程,指出沉积相的识别和划分对于阶地研究具有重要意义。

STRATIGRAPHIC STRUCTURES AND AGES OF THE SECOND AND THIRD FLUVIAL TERRACES ALONG THE BANK OF HUANGHE RIVER IN LANZHOU BASIN,WESTERN CHINA,AND THEIR ENVIRONMENTAL IMPLICATIONS

The stratigraphic lithology of fluvial profiles in the second and third terraces(T₂ and T₃)along the bank of Huanghe River in Lanzhou Basin,Western China,was investigated.A section with fine particle size was sampled at 2.5cm intervals above the bottom gravel layer.Granulometric analysis,as well as analysis on some fine quartz particles(4～11μm) selected systemically from optically stimulated luminescence(OSL)samples were used to build up the relative stratigraphic chronology by simplified multi-plate regenerative dosimetry.This set of fine sandy layer of several to tens meters thickness has horizontal bedding texture and is commonly regarded as flood plain deposit.Based on the depositional characterization and chronology of the terrace section(including riverbed deposition,alluvial flat deposition,and overlying aeolian loessial deposition),it was considered as the result of interactive deposition that interlaid by alluvium,proluvium,and loess,due to the vertical aggradation of fluvial accumulation.The OSL ages of the bottoms of the aggradational layer in T₂ and T₃ were 20～24ka and 70～74ka,respectively.They respond to the beginning of glacial period or paraglacial period,indicating that cold and arid climate was advantageous of the development of aggradational layer in terrace surface.The thickness of aggradational layer is related to some factors such as the climatic conditions,hight of terrace surface,etc,but not related directly to water-level hight of main river channel during the flooding period.The difference of ages between the sub-point-bar sandy layers beside the riverbed and the overlying loess-like soil layers in T₂ and T₃ was not big enough,indicating that once the riverbed was exposed out of water and gone out of use,on which the dusty loess could be accumulated and preserved immediately and finally formed into loess-like soil,even under the weak hydrodynamic conditions.This also means that there were no evidently depositional interruption between the formation of terrace and the accumulation of loess-like soil,and the beginning of the accumulation of loess-like soil in every class of the terrace represented approximately the forming time of the corresponding terrace class.But the stable and continuous eolian loess could not be formed until the terrace surface grow upwards to curtain hight and break completely away from the aggradational conditions of mountainous torrent,and its bottom age could be lagged several myriayears after the beginning of the loess-like soil.The discrimination and classification of sedimentary facies of the terrace is very important for geologists and geomorphologists to determine accurately its formation time and to further discuss deeply its generation mechanism.