垣曲盆地最高阶地风尘堆积形成时代及其构造意义

AGE OF AEOLIAN DEPOSIT ON THE HIGHEST TERRACE IN YUANQU BASIN AND ITS TECTONIC SIGNIFICANCE

对垣曲盆地原土坪阶地剖面进行了详细的沉积学和磁性地层学研究,结果表明,原土坪剖面地层自下而上由水下河湖相-漫滩相-风尘堆积构成,整个剖面记录了一个水位下降、由水下沉积环境转变为气下环境的连续堆积过程。剖面地层记录的1.84～1.62MaB.P.期间由河湖相沉积环境转变为典型风尘堆积的沉积转型及风尘堆积反映出的1.21MaB.P.前后古气候变化事件是对青藏高原青藏运动C幕和昆黄运动的区域沉积响应。开始于1.62MaB.P.的典型风尘堆积底界年龄代表了该级阶地的形成时代,表明该段黄河至少在此时已经形成。

The Yuanqu Basin is a small faulted basin in the east of the Chinese Loess Plateau and formed in the bedrock mountain region in Sanmenxia. Yuantuping terrace is the highest one in Yuanqu Basin which is located at the south of the basin and the north of the Huanghe River. The terrace is l15m thick and includes 6m fluvio sand-gravel deposits,109m fluvio-lacustrine sediments, and loess deposits. This paper mainly studies the 109m thick finegrained sediment on the top of sand-gravel section,which can be divided into g segments with different lithological and structural features and with different colors. The four segments from bottom upward are at 109.0- 94.9m, 94.9 ~84.0m, 84.0 ~ 65.5m, and 65.5- 0m. We systematically collected and tested samples of magnetic susceptibility,granularity, and palaeomagnetism on these sections. Paleomagnetic measurements demonstrate that the Yuantuping terrace records three normal polarity zones（ N1 ~ N3） ,three reversed polarity zones（R1 ~ R3） ,and two N＇（44.8 - 46.5m ） and N （64.8 ~ 65.8m ） unsure normal polarity zones including C1 n, C1 r. 1 n, and C2n corresponded to Cande 95 Time Scale. Paleomagnetic age of the bottom of the 109m terrace which is also the bottom of the R3 reversed polarity zone is 2. 11Ma B. P. depending on the sedimentation rate of the N3 （91.3 ~ 100.8m） normal polarity zone. The judgment of sedimentation rates of the other lithological segments depends on the rates of the Cir. ln,Clr. 2n and C2n. The granularity distributing properties show that all material of the four segments is derived from aeolian deposits. The results of the granule parameters and magnetic susceptibility show different regularity which demonstrates that the four segments were formed in different sedimentation environment. Based on the distribution of lithological segments and the analysis of magnetic susceptibility regularity, dispel dot figures of granule parameters and C-M plots, the sedimention phases can be divided into fluvio-lacstrine-floodplain deposits （Phase Ⅰ ）and aeolian deposits（Phase Ⅱ ）. Phase Ⅰ（109.0 ~ 84.0m,2. 11 ~ 1.62MaB. P. ）includes two sub-phases with their boundary at 94.9m： The lower one is fluvio deposit of sub-phase Ⅰ1 （ 109.0 ~ 94.9m,2. 11 ~ 1.84MaB. P. ） and the upper is floodplain deposit of sub-phase I 2（94.9 ~ 84.0m, 1.84 ~ 1.62MaB. P. ）. The aeolian deposit characters of phase Ⅱ （84.0 ~ 0m, since 1.62MaB. P. ） at 65.5m of the section （ 1.21MaB. P. ） show that the paleoclimate after 1.21MaB. P., especially the glacial-interglacial cycles,changed more markedly than before. In conclusion,the Yuantuping terrace recorded the whole successive depositional progress from subaquatic to subaerial environment. There are three sedimentary environment changes. The segment at 94. 9m of the strata recorded the sedimentary environmental change from subaquatic to subaerial at about 1.84MaB. P. ; the sedimentary record at 84. 0m obviously indicates that the environment changed from fluvial-lacustrine progradation condition to the loess deposition at about 1.62MaB. P. ; the aeolian deposit at 65.5m,obviously bearing a characteristic of aeolian depositional change, suggests that the palaeoclimatie change event occured at 1.21MaB. P. The three sedimentary environment changes are the regional sedimentary responses to the phase C of Qinghai-Tibet Movement and the uplift of Kunhuang（Kunlun-Huanghe）Movement. The occurrence of typical aeolian deposit indicates the completion of the evolution of Yuantuping terrace. The sedimentary change in 1.62MaB. P. indicates that the terrace had been already formed,that there had been extensively exorheistic water system, and that the Huanghe River of this section had already been formed at that time. The analysis of sedimentary progress in Yuantuping section indicates that the physiognomic and environmental evolution progresses reflect the uplift of Tibet Plateau.