大同-阳原盆地古泥河湾湖的岸线变化

THE STUDY OF THE CHANGE OF ANCIENT LAKE SHORE IN THE DATONG-YANGYAN BASIN

本文通过大同-阳原盆地古湖岸线遗迹的分析,阐述第四纪古泥河湾湖的出现、发展和消亡,指出湖泊变迁是构造运动和气候变化等因素共同作用的结果,构造运动是主要因素。

The change of the ancient 1 ke shore is an important event of the paleogeo-graphical condition in the Datong-Yangyan basin during pleistocene.This paper, based on some evidences of geomorphology, sedimentology and palaeontology associated with the evolution of the lake, suggests that there are four ancient lake shore lines around the margin of the Datong-Yangyan basin. They are found at heights of 1150-1170m, 1100-1130m,1000-1070m and 900m above the sea level,and shows that the ancient Nihewan lake shore had two periods of advance and three periods of retreat during pleistocene. When the first advance and retreat occurred,the whole basin was almost occupied by water.The shore line had reached up to the piedmond zone, and formed two lacustrine terraces (1150-1170m and 1100-1130m) .This event took place in the early pleis-tocene(Q , ).From the late,early pleistocene(Q31 )to the early, middle pleistocene (Q12),the lake shore decreased about 3-4km and the level of the late water went down to a height of 900m above the sea lavel. It formed a series of lake shore deposits in the Quaternary system.The second advance occurred in the middle pleistocene (Q2), and formed a lake abrasion-accumulation platform (1000-1070m). After this the level of the lake water declined quickly again and the lake disappeared entirely.It occurred from the late, middle-pleistocene to early, late-pleistocene (Q13).To explain the evolution of the lake we must consider two factors comprehensively; tectonic movement and climatic change. The wet-cold climate provided plenty of water in the early pleistocene (Q1)and middle pleistocene(Q2) causing the lake level to increase. The climate, however changed gradually during Q31-Q12 and Q32-Q13,and did not favour the development of the lake.Although the climate changes were an important reason,but the neotectonic movement was a more important cause of the evolution of the Nihewan lake. The tectonic movement had made a confined intermountain basin which provided a place for the development of the lake,and had controlled the elevation and subsidence of the level of lake water by changing the height of the lake outlet. The tectonic movement which started in the late, middle-pleistocene (Q32) had made the adjacent .Huailai basin-a local base level of erosion-go down.W ith down-cut erosion,the lake water flowed from the Datong-Yangyan basin into the Hualai basin, finally,causing the ancient Nihewaa lake to disappear.