呼包盆地第四纪地层与环境演化*

通过对呼包盆地钻孔岩芯进行系统的岩石学、年代学和孢粉带的分析,综合盆地已有的研究成果,并与邻区对比,划分了盆地第四纪地层单位,同时探讨了河套古湖泊在第四纪的演化特征。结果为:1)剖面存在两个假整合接触面(Q₁/Q₂和Q₂/Q₃),可划分为4个沉积旋回,7个孢粉带。2)剖面第四纪地层从下至上依次可划分下更新统(1070±150~844±126kaB.P.)、中更新统沟子板组(520±78~271±40kaB.P.)、上更新统萨拉乌苏组和城川组(108.7±7.8~14.80±1.03kaB.P.),以及全新统。3)河套盆地在第四纪存在4个古湖泊发育时期,即早更新世(80.0~107.0万年或更早),湖盆处于强氧化环境;中更新世早期(43.0~45.1万年),湖盆氧化强度降低;中更新世晚期(19.2~38.5万年),湖盆处于强烈的还原环境;晚更新世(2.4~11.3万年),湖盆处于还原环境。其间隔期为湖泊消亡或大规模萎缩时期,意味着盆地停止下降甚至处于上升隆起的剥蚀状态。

DIVISION OF QUATERNARY BEDS AND ENVIRONMENT EVOLUTION IN HUBAO BASIN IN CHINA

A borehole (40°44.343'N,111°40.171'E; 250.03m deep) has been drilled in the Hubao Basin, a subbasin of the Hetao, Inner Mongolia Autonomous Region of China. Based on systematic analysis on litho-stratigraphy, chrono-stratigraphy, and sporo-pollen of the core, which is integrated with hydrological data and previous research results of the Hetao Basin, as well as compared with the corresponding beds of the neighbouring regions, the article has drawn the following conclusions: (1) There are evidence of two disconformity interfaces in the drilling section. According to the changes of sedimentary grain size and facies, four cycles of sedimentation can be distinguished and seven sporo-pollen zones with different species and assemblages can be recognized during Quaternary in the Hubao Basin. (2) The stratigraphic units of the drilling core section may be setup from bottom as: the late stage of Early Pleistocene (1070±150~844±126kaB.P.), the Middle Pleistocene Gouziban Formation (520±78~271±40kaB.P.), the Late Pleistocene Salawusu Formation and the Chengchuan Formation (108.7±7.8~14.80±1.03kaB.P.), and the Holocene. (3) During the Quaternary, there were four lake-stages in the Hetao Basin: ①During the Early Pleistocene (1070~800kaB.P. or earlier), the basin was in a strong oxidizing environment; ②During the Early Middle Pleistocene (451~430kaB.P.), the oxidizing degree decreased; ③During the Late Middle Pleistocene (385~192kaB.P.), the basin was strong in a reducing environment; and ④During the Late Pleistocene (113~24kaB.P.), the basin was still in a reducing environment. Except for these four lake times, the Hetao paleo-lake basin might have been vanished or shrunk on a large scale. Such an alternative occurrence suggests that the Hetao Basin has had a complicated evolution history of deposition and uplift during Quaternary.