大兴安岭北段东坡扎兰屯地区高位砂砾石层研究及其地质意义

STUDY ON THE HIGHSTAND SAND-GRAVEL LAYERS IN ZALANTUN AREA,NORTHERN DAXINGANLING MOUNTAINS:Geological Implication

通过对大兴安岭北段东坡扎兰屯东南前新生代基岩上高位砂砾石沉积的展布、沉积特征、砾组特征及孢粉分析等研究,分析本套砂砾石层的成因及其气候环境记录,并结合区域资料,探讨其形成时代和新构造意义.分析结果表明,研究区高位砂砾石层可划分为Ⅰ级、Ⅱ级和Ⅲ级高位砂砾石层,沉积时代初步厘定为早全新世、晚更新世和中更新世.3级高位砂砾石层均为河流作用的冲积成因,水动力条件较强,略具冲积扇特征,分别为雅鲁河Ⅰ级、Ⅱ级阶地沉积记录和嫩江Ⅲ级阶地沉积记录.孢粉分析显示,Ⅱ级和Ⅲ级高位砂砾石层沉积时为针叶森林草原环境,气候温凉干旱;Ⅰ级高位砂砾石层沉积时为落叶阔叶森林草原环境,气候温暖偏干,其后变为草原荒漠环境,气候寒冷干旱.雅鲁河主河道晚更新世以来由东向西略有迁移,嫩江主河道中更新世以来由西向东不断迁移,雅鲁河断裂和嫩江断裂所夹的断块中更新世以来存在3期抬升.

A large amount of highstand sand-gravel layers is well developed on the pre-Cenozoic basement in southeast Zalantun area on the east slope of Northern Daxinganling Mountains. The distribution, sedimentary characteristics, gravel fabric and sporopollen records of the sand-gravel layers information. Combined with regional geological data, the are studied to analyze their origin and climate environment forming ages and neo-tectonic implication are discussed. The results show that, the highstand sand-gravel layers in the research area can be divided into Ⅰ-、Ⅱ- and Ⅲ-level, which were formed in Early Holocene, Late Pleistocene and Middle Pleistocene epochs, respectively. The 3 layers are all strong hydrodynamic alluvial deposits, with a few characters of alluvial fan, corresponding to the sedimentary records of the first and second terraces of the Yalu River and the third terrace of the Nenjiang River, respectively. The sporopollen analysis indicates an environment of coniferous forest and grassland with a cool and dry climate when the Ⅱ- and Ⅲ-levels of highstand sand-gravel layers were formed, and an environment of deciduous and latifoliate forest and grassland with a warm and dry climate during the formation of Ⅰ-level, followed by an environment of grassland and desert under a cold and dry climate. The main channel of the Yalu River has slightly changed from east to west since Late Pleistocene, while the main channel of Nenjiang River has been changing constantly from west to east since Middle Pleistocene. The block between Yaluhe fault and Nenjiang fault has undergone 3 times of uplift since Middle Pleistocene.