在对汉江上游河谷进行野外考察的基础上,就郧县盆地汉江I级阶地及其风成黄土-古土壤覆盖层沉积学和理化性质进行了研究,并且采用OSL方法进行了测年断代。获得了26个OSL年龄数据,证实厚层风成黄土L_1底部的年龄在25 ka BP,而剖面底部风成黄土 -冲积砂层交互层(T_1-al_2)年龄范围在55-25 ka BP之间。地层年龄说明I级阶地上黄土的堆积过程基本连续,汉江I级阶地的发展经历了早期新构造抬升与河流下切阶段(55-25 kaBP)和晚期阶地面稳定接受沉积(25-0 ka BP)两个阶段。距今55 ka BP前后,新构造运动抬升和汉江下切作用加剧,河漫滩相沉积层开始脱离水面并接受风尘堆积物。这个过程持续到距今约25 ka BP,期间河水不时地淹没阶地面,从而形成了风成黄土 -冲积砂交互的沉积学特征。距今约25 ka BP 以来,河水不再淹没阶地,汉江1级阶地最终形成,阶地面开始接受连续的风尘堆积。在汉江下切作用加剧的同时,全球性末次冰期的发展也逐步进入冰盛期,风尘活动强烈,在阶地表面堆积形成了厚层黄土。汉江I级阶地形成以来,气候的变化使黄土覆盖层受到不同程度的风化成壤改造,形成了黄土 -古土壤地层,其地层序列从下向上依次为:河流相砂卵石层(T_1-al_1)黄土-冲积砂互层(T_1-al_2)马兰黄土(L_1)过渡黄土(L_t)古土壤(S_0)近代黄土(L_0)表土(MS)。这个层序记录汉江上游流域自从25 ka BP以来的气候变化经历末次冰期后东南季风逐渐加强、中全新世季风强盛、晚全新世季风衰退和气候变干的演变模式。
英文摘要:
The first river terrace within the Yunxian Basin along the upper reaches of Hanjiang River was investigated in the field work. The sediment profiles were sampled systematically. The sedimentological characteristics, and physical and chemical properties were analyzed in laboratory. The samples were dated using OSL method. The results show that, the bottom age of the thick loess L_1 is 25 ka BP, and the age range of the eolian loess-alluvial sand inter-beds (T_1- al_2) is between 55-25 ka BP. The age distribution suggests that deposition process of the loess sequence in Hanjiang River valley is essentially continuous. The first river terrace of the Hanjiang River has experienced two development stages, including the tectonic uplift and river incision (55-25 ka BP), and then the stabilized eolian loess deposition (25-0 ka BP). Around 55 ka BP, the river incision started because of the intensified tectonic uplifting. The eolian loess accumulation on the uplifted flood plain was frequently interrupted due to the overbank flooding. This process continued around 25 ka BP and the inter- beds of eolian loess and alluvial sand (T_1-al_2) were therefore formed. To about 25 ka BP, the first river terrace had formed and eolian loess accumulated continuously on the terrace land. At the same time, with the river incision, the last glaciation entered the maximum stage and the thickeolian loess covered the terrace because of the increased dust storms and dustfalls. Since the formation of the terrace about 25 ka BP, the eolian loess experienced varying degrees of weathering and pedogenesis in different periods because of monsoonal climate change. The stratigraphic paleosol-loess sequence on the first terrace from the bottom to the top is listed as fluvial gravel (T_1-al_2) eolian loess and alluvial sand inter-beds (T_1-al_2) Malan loess (L_1) transitional loess (L_t) paleosol (S_0) recent loess (L_0) modern soil (M_s). This stratigraphic sequence has recorded a monsoonal climate change model since 25 ka BP, including the gradually intensified southeast monsoon during the late glacial and early Holocene periods, the strongest monsoon in the mid- Holocene, and the monsoon recession and the climate drying during the late Holocene.