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西藏中部的南木林县乌郁盆地属于新生代山间盆地,新近纪沉积发育完整,富含植物和孢粉化石,在研究青藏高原隆升及植被—气候演化中具有重要意义。我们对Spicer等(2003)研究层位进行了孢粉分析,发现这里的孢粉组合总体是以桦科花粉占统治地位、同时又含有大量的松科花粉和丰富的蕨类孢子,明显不同于宋之琛、刘金陵(1982)所报道的以山毛榉科高含量为特征的孢粉植物群。这一发现与李浩敏、郭双兴(1976)指出的芒乡组内存在两个不同的植物化石组合的意见一致。不仅如此,综合已知的植物和孢粉资料,芒乡组甚至可能含有三个植物化石组合(层位),即下部以桦科含量丰富的组合、中部以高山栎—高山绣线菊高含量为特征的组合、以及上部以单子叶草本植物和松科为主的组合。根据植物和孢粉化石资料以及放射性同位素年代数据,芒乡组时代应属于早中新世到中中新世早期。另一方面,长期以来乌郁盆地的地层命名系统相当繁乱,在实际应用中存在许多同义不同名或同名不同义现象,盆地名称也莫衷一是,给后人的研究带来了种种不便甚至误解。参照地层规范并结合近年来化石和放射性同位素年龄数据等新的研究进展对乌郁盆地新生界地层系统进行了梳理,明确了各地层单元的名称、涵义和时代,自下而上的序列为:日贡拉组(渐新世,上部可能进入中新世)、芒乡组(早中新世晚期至中中新世早期,约18~15 Ma)、来庆组(中中新世晚期至晚中新世早期,主体年龄为15~8 Ma)、乌郁群(晚中新世至上新世,8~2.5 Ma,不排除顶部进入更新世的可能)。
The Wuyu Basin in South Mulin County in central Tibet belongs to the Cenozoic mountain basin. The sediments of the Neogene are rich in vegetation and sporo-pollen fossils, which are of great significance in studying the uplift of the Tibetan Plateau and vegetation-climate evolution. We analyzed the sporopollen in the study horizon of Spicer et al. (2003) and found that the sporopollen assemblage in this area is dominated by birch pollen and contains a large amount of Pinaceae pollen and abundant fern spores, which is obviously different from Song Zhichen, Liu Jinling (1982) reported to be high content of beech family of pollen flora. This finding is consistent with the observation that there are two different fossil assemblages in the Mangxiang Formation, as pointed out by Li Haomin and Guo Shuangxing (1976). In addition, according to the known plant and sporopollen information, the Mangxiang Formation may even contain three plant fossil assemblages (horizons), ie, the lower part is rich in birch family, the middle part is highland oak - As a combination of features, and the upper part of the monocotyledonous herbs and Pinaceae dominated combination. According to the data of plant and sporo-pollen fossils and the age of radioisotopes, the age of the Mangxiang Formation should belong to the early Miocene to early Middle Miocene. On the other hand, stratigraphic naming system in Wuyu Basin has long been quite chaotic. In practice, there are many synonymous synonyms with different names or synonymous meanings, and the names of basins are not consistent. All these inconveniences and even misunderstandings are caused to future generations. With reference to the stratum norms and combined with the recent research progress of fossil and radioisotope age data, the stratigraphy system of Cenozoic strata in the Wuyu Basin was sorted out. The names, meanings and times of each stratum were clarified. The bottom-up sequences are as follows: Rigung Formation (Oligocene, upper part may enter the Miocene), Mangxiang Formation (late Early Miocene to Mid-Miocene, about 18 ~ 15 Ma), Laiqing Formation (late Miocene to Late Miocene, the main age is 15 ~ 8 Ma), Wuyu group (late Miocene to Pliocene, 8 ~ 2.5 Ma, does not rule out the possibility of the top into the Pleistocene).