A Preliminary Study On The Lacustrine Sediment Sequence Of Late Pleistocene In Hetao Basin, Inner Mongolia, China

The upstream of The Yellow River had developed many Paleolakes. The formation and evolution of these Paleolakes record the evolution process of The Yellow River and relationship with regional change of tectonic and environment. As a typical area of the upstream of The Yellow River, we choose Hetao basin as the study area for this paper, expecting to disclose the evolution sequence of Hetao Paleolake through investigation of lacustrine sediments.Detailed studies of the borehole and lacustrine strata indicate that Hetao basin discontinuously develop Paleolake since Late Pleistocene, but the limitation of the traditional dating techniques about this period constrains the farther study of this Paleolake. We select ¹⁰Be and OSL dating technique to constrains the formation of the Tuoketuo mesa which is in the east margin of the Hetao basin, the result indicate that the Tuoketuo lacustrine strata is formed during Late Pleistocene(before156ka—90ka). Through the sedimentology and chronology of the Tuoketuo mesa, we acquire that the mesa experienced the early lacustrine facies and late fluvial—eolian deposit, then combining with other study results we can farther conclude that Hetao Paleolake can divide into three stages restricted by sedimentary record and the age.(1) Hetao Paleolake began to develop at Late Pleistocene on the basis of middle Pleistocene lake. OSL dating of the lacustrine sediments on Yinshan Mountain piedmont indicate that Hetao Paleolake developed a unified megalake at about 100ka, which extended from the southwest of Jilantai Salt Lake to the east of Tuoketuo mesa. It was bordered towards the south and north by the northern margin of the Ordos Plateau and the piedmont along Bayanwulashan—Langshan—Seertengshan—Wulashan—Daqingshan. At that moment the paleolake's level is very high so that over 40m lacustrine strata deposited in Tuoketuo.(2) A significant change happened to Hetao Paleolake since 80ka. Aeolian sediments such as loess deposited in Tuoketuo, and lacustrine hiatus occurred at the basin margin, which showing Hetao paleolake began to outflow and contract, however, there was lacustrine sediments record in the center of the basin. (3)There are no longer lacustrine strata deposited in Hetao area after 24ka, showing that the Hetao paleolake is entirely extincted and the paleogeography is similar to now.Formation and extinction of the Paleolake have close relationship with evolution of The Yellow River and regional change of tectonic and climate, preliminary study show that the paleolake development in Late Pleistocene have the relationship with strongly depression of Hetao bsian caused by Gonghe movement and warm climate during interglaciation; the constraction of paleolake at about 80ka is duo to the cold weather in the glaciation and outflow of the Yellow River, and the paleogeography is similar to now after finally extinction of the lake at about 24ka.