晚更新世晚期呼包盆地环境演化与地貌响应

对大青山山前台地沉积地层剖面的沉积学、年代学研究结果表明,晚更新世晚期呼包盆地环境发生过两次重大转变。约47-28kaB.P.的呼包盆地湖泊水位一直维持着较高的水平,湖相沉积沿河伸入大青山内;约28kaB.P.以后,湖泊逐渐退缩,山区及山前地带沉积特征转为以河流加积为主,并一直维持到约22kaB.P.;此后山区河流由加积转变为下切,最大下切深度达100m。断裂两侧的地貌分析表明,在约22kaB.P.之前大青山山前断裂的构造活动不明显;约28kaB.P.的湖泊-河流转换事件是与区域性的气候变化有关。约22kaB.P.的环境突变事件主要由大青山山前断裂的构造活动引起,其垂向活动速率约为4-6mm/a;晚更新世晚期以来大青山山前断裂的稳定时期对应呼包盆地的湖相环境时期,而断裂的强烈活动时期对应气候的冷干时期。

GEOMORPHOLOGICAL RESPONSE TO ENVIRONMENTAL CHANGES DURING THE LATE STAGE OF LATE PLEISTOCENE IN HUBAO BASIN

The landforms on the north margin of Hubao basin are mainly controlled by the Daqingshan piedmont fault, forming a belt of conspicuous geomorphological variation. On the south slope of the Mt. Daqingshan, there ubiquitously exits a piedmont terrace north to the Daqingshan piedmont fault, the surface of which is 120m above the fan surface to the south of the fault. The terrace is composed of a set of lacustrine-stream deposits, which can be divided into a lower and an upper part. The lower part consists of grayish yellow, yellowish green silt and fine sand, containing thin layers of sand and gravel with fine horizontal bedding. This part was deposited under lacustrine environment with an age of 47-28kaB.P. The upper part consists of gravel of varying size and low psephicity with inclined bedding. This part was deposited under stream environment with an age of 28 - 22kaB.P. The synchronous deposits in the river valleys nearby form a river terrace 100m above the present stream level, the structure of which is the same as that of the piedmont terrace. The stratigraphic sections record two environmental change events, one is lacustrine-stream transition event, occurring before 28kaB.P. , when the lake of Hubao basin retrogressed; the other is aggradation-down-cutting transition event, occurring after 28kaB.P. , when the streams in the area transited to downcutting from aggradation.Stratigraphic records demonstrate that the piedmont belt undergoes different depositional environment in the late stage of Late Pleistocene. The lacustrine environment started no later than 47kaB.P. , and sustained to 28kaB.P. During this period, the lake almost covered the whole Hubao basin, and the broad valleys in the area were under lacustrine environment, too. The depositional features in both the basin and the mountain valleys show that the Daqingshan piedmont fault was inactive during 47-22kaB.P. Comparing with the environmental changes in other lakes in North China, we can infer that the lacustrine-stream transition in Hubao basin was the response to regional climate change associated with the global climatic change.From 22kaB.P. to present, the greatest downcutting depth of the rivers in the area reaches 100m. The difference of the elevation of the lacustrine deposits cross the fault is 58-136m. These features prove that the Daqingshan piedmont fault has been intensely active since 22kaB.P. , its slip rate is about 4-6mm/a.Since the late stage of Late Pleistocene in Hubao basin, the inactive stage of the Daqingshan piedmont fault was synchronous with the period of the lacustrine environment, while the active stage of the fault synchronous with the cool and dry period.Our study in the Daqingshan piedmont indicates that the integral environmental explanation resulted from both geomorphological and depositional records is more comprehensive and reliable than that from a single record for the site where both geomorphological and depositional records are well preserved.