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为了解滇西弥渡盆地中晚更新世以来的断陷特征及环境演化,开展了弥渡盆地各级阶地沉积物的热释光测年、沉积相和孢粉的研究工作。通过野外调查发现弥渡盆地西侧至少发育5级阶地,对应于5次间歇断陷阶段,重点调查弥渡盆地西侧的奇家营Ⅰ级阶地、龙华寺Ⅲ级阶地、平安庄Ⅳ级阶地、大树坪Ⅴ级阶地。Ⅰ级、Ⅲ级、Ⅳ级和Ⅴ级阶地热释光测年值分别为200.48±22.05ka,239.06±26.30ka,320.85±35.29ka和332.07±36.59ka。年代学分析结果表明:滇西弥渡盆地的地貌格局形成约33万年来的间歇式断陷运动,而且表现出早期断陷速率快,晚期断陷速率减缓的趋势。从沉积特征分析:早期沉积特征中出现半深湖沉积物,说明湖盆断陷速率相对较快,反映出该段时期程海断裂带南段的活动性相对较强;晚期主要以湖泊近岸、浅湖为主,说明湖盆断陷速率相对较慢,反映出该段时期程海断裂带南段的活动性相对较弱。通过阶地堆积物沉积相和孢粉分析研究显示,更新世中晚期弥渡盆地沉积环境主要为山间盆地河湖环境,气候由凉爽湿润→温暖湿润→凉爽湿润转变的特点。弥渡盆地西缘、北缘发育较多洪积扇,洪积扇以线性排列,东缘洪积扇较少,多为单个扇体,说明盆地西、北侧断陷速率相对较快。盆地西侧的洪积扇,由北向南规模逐渐变小,堆积物由粗变细,扇面坡度由大变小,说明北段断陷运动较强。这一现象说明盆地近代具有向北掀斜的趋势。
In order to understand the faulted-down characteristics and environmental evolution of mid-late Pleistocene in the Midu Basin, western Yunnan, the thermoluminescence dating, sedimentary facies and sporopollen of the sediments of terrace terraces in the Midu Basin have been carried out. Through field investigation, it is found that there are at least 5 terrace stages on the west side of the Midu Basin, corresponding to 5 intermittent faulting stages. The first stage of the Qijiaying Stage, the third stage of the Longhua Temple, , Daping Ping Ⅴ terrace. The pyro-luminescence dating of Ⅰ, Ⅲ, Ⅳ and Ⅴ terraces were 200.48 ± 22.05 ka, 239.06 ± 26.30 ka, 320.85 ± 35.29 ka and 332.07 ± 36.59 ka, respectively. The geochronological analysis shows that the geomorphic pattern of the Midu Basin in western Yunnan Province formed an intermittent rifting movement of about 330,000 years and showed the tendency of early tectonic decay rate and late tectonic decay rate. From the sedimentary characteristics analysis: Semi-deep lake sediments appear in the early sedimentary characteristics, indicating that the lake basin faulting rate is relatively fast, reflecting the relatively strong activity in the southern segment of Chenghai fault zone during this period; lately, , Mainly shallow lakes, indicating that the rate of rifting in the lake basin is relatively slow, reflecting the relatively weak activity in the southern segment of the Chenghai fault zone during this period. The analysis of sedimentary facies and sporopollen in the terrestrial sediments shows that the sedimentary environment of the mid-late Pleistocene is characterized by rivers and lakes in the mountain basin and the climate changes from cool to warm → humid to cool and wet. There are more alluvial fans developed in the western and northern margin of the Midu basin. The alluvial fans are arranged linearly with fewer alluvial fans in the eastern margin, mostly single fans, indicating that the rate of rifting in the west and north of the basin is relatively fast. The alluvial fan on the west side of the basin gradually decreases in size from north to south, and the sediment changes from coarse to thin and the slope of the sector decreases from large to large, which indicates that the north segment faulting movement is strong. This phenomenon shows that the modern basin has a tendency to tilt northward.