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

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 are studied to analyze their origin and climate environment information. Combined with regional geological data, the 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.