河套盆地托克托台地湖相层研究

RESEARCH ON THE LACUSTRINE STRATA OF THE TUOKETUO MESA, HETAO BASIN, CHINA

野外观察表明托克托台地的湖相层与上覆的黄土之间存在风化剥蚀面。文章选择了一个剖面点系统地采集了孢粉样品,同时在湖相层底部、顶部和黄土层底部采集了3个光释光样品;另外在一个观察点的不整合于上新统之上的河流相中采取了一个OSL样品。通过对这些数据结果的分析,并综合已有的地层年代、孢粉数据和水文工程地质钻孔资料,试图取得对托克托台地湖相层形成年代、环境及台地演变过程的合理认识。结果表明:1)湖相层相当于邻区的萨拉乌苏组,形成于12～10万年以前的晚更新世早期;黄土相当于邻区的城川组,形成于8万年之后的晚更新世晚期;形成于3万年左右的湖相层是晚更新世晚期同期沉积黄土相变的结果。2)古气候变化明显,晚更新世早期(12～10万年)气候温和湿润,水量充沛,推测最高湖水位介于海拔1050～1080m之间,在河套盆地形成了一个大湖泊———“河套古湖”,与“萨拉乌苏湖”是分开的;晚更新世晚期(10万年以后)气候干冷,湖水后退,同时意味着黄河开始外流,3万年时期的湖水位为海拔1020m左右。3)上新统地层遭受风化剥蚀使地表处于准平原化,在这一时期和林格尔断裂和鄂尔多斯北缘断裂发生明显位错;约12万年以后,和林格尔断裂停止位错,沉积了晚更新世早期的湖相层,约10万之后湖水向盆地中心退却,并在8万年开始沉积黄土,湖水再也未扩展到台地之上,在这期间鄂尔多斯北缘断裂继续位错直至晚更新晚期大约3万年左右停止,台地最终形成。

A weathering-denuding surface separates the lacustrine strata and a thick layer of the loess in the Tuoketuo mesa. In order to ensure the environment and age of the lacustrine strata, as well as the origin and development the mesa, we chose to sample sporo-pollen by the numbers, at the same time to gather 3 samples of optically stimulated luminescence （OSL） dating, from the bottom to top of the lacustrine strata as well as at the bottom of the loess. In addition, we obtained one OSL sample in the fluvial facies, and this fluvial facies is unconformable on the Pliocene Series. Based on these new data and known data on the age of strata, sporo-pollen and the bore of hydrographic geology, we conclude： （1） the lacustrine strata formed during the early stage of the Late Pleistocene （ca.120-100kaB. P. ago）, and is equal to the Salawusu Formation of its neighboring position. The loess formed during the late stage of the Late Pleistocene （after about ca.80kaB. P.）. The lacustrine sediments withthe age of ca. 30kaB. P. were the results of the top loess facies change in the same term. （2） The palaeoclimate change is evident during the Late Pleistocene. The climate during the early stage of Late Pleistocene （ca.120-100kaB. P.） was mild and humid, with the highest level of the lake water at 1050-1080m above the sea level. This ancient lake was called as ＂Hetao ancient lake＂ and different from ＂Salawusu ancient lake＂. However, the climate during the late stage of Late Pleistocene （since ca.100kaB. P.） was dry and cold, and the level of lake water dropped; at the same time it means that the Huanghe river began to become an exterior river. The level of the lake water was 1020m±above sea level at ca. 30kaB. P. （3） The earth＇s surface was weathered and denuded until the forming of peneplane after the Pliocene Series had deposited and diagenesis had finished. Meanwhile, the Helingeer fault and the northern margin fault of Ordos was active and had displaced. The lacustrine sediments had deposited during the early stage of the Late Pleistocene while the displacement of the Helingeer fault stopped. The level of the lake water fell back towards the center of the Hetao basin after ca. 100kaB. P., and the loess had begun to deposit since ca. 80kaB. P. up to the Holocene Epoch. Until ca. 30kaB. P., the displacement of the northern margin fault of Ordos had ceased, and the Tuoketuo mesa finally shaped.