New Findings Of Lacustrine Sediments At South-Eastern Piedmont Of The Langshan Mountain And Its Implication For Environmental Status Of The Jilantai Basin In Middle-late Pleistocene

The Jilantai Basin is located on the western frontier of the present Asian summer monsoon domain,and on the left shoulder section of great bend of Yellow River in Hetao Basin.So more sensitive to the evolution of Asian summer monsoon and the evolution of the Yellow River.Several lake shorelines were preserved in the Jilantai Basin.Dating on the highest shoreline of the Jilantai Basin supports that there was once a huge lake covering most area of the Jilantai Basin and Hetao Plain sometime before 60 ka.Dating results of drilling stratum located inside the basin(K-feldspar OSL dating)support once existed > 60 ka lake in the study area,but lack of complete lake sediment records that greater than 60 ka at the edge of Jilantai basin.Therefore,This study intends to work at Southeastern Piedmont Terraces of the Langshan Mountain located on the frontier of Dengkou Subuplift,Trying to find geological evidence of the development of the megalake and measure the OSL age by the quartz grains,Combined with geological evidence and dating results of the close neighboring Jilantai Subdepression to understanding the environmental status of the Basin in Middle-Late Pleistocene.The primary conclusions of this paper are as follows:(1)The platform of southeastern piedmont of Langshan Mountain,studied profile(named LS)with altitude about 1068 m,preserving complete lacustrine sediments records—a >15 m thick section composed mainly of gray green lacustrine strata that silt is the dominated components.It support that the Dengkou Subuplift of Jilantai Basin was a lake environment continuing a long time.Furthermore,the result of ostracoda identifying suggest that Limnocythere inopinata is the main species,demonstrating sedimentation unit formed in stable water condition.The S32 profile situating in Jilantai Subdepression emerging with altitude about 1068 m,emerging lacustrine silty clay with color of greyish-green and thickness about 5 m.Ostracoda species dominated by Limnocythere inopinata,support strata formed at water environment during a long-term.Similar altitude,Sedimentary characteristics and ostracoda species suggest that the lacustrine strata of the platform of southeastern piedmont of Langshan Mountain(LS profile)and the Jilantai Subdepression(S32 profile)formed at different places of one lake.(2)In this study,we measuremented 6 quartz samples by using the single aliquot regenerative dose(SAR).Although individual aliquots OSL signals close to saturation in which three samples,but most of the aliquots OSL signals provides effective age information.(3)Quartz OSL dating results inditates that the chronological frame of lacustrine strata at the platform of southeastern piedmont of Lang Mountain and the Jilantai Subdepression are >129 ka ? 75 ka and >105 ka ? 71 ka respectively.And quartz OSL dating results support the profile LS and S32 formed in the same period Reliability of Quartz OSL dating results suggest that optical signal near saturation,just providing a youngest burial age of the lake.OSL ages and stratum information jointly support the Jilantai Subdepression and Dengkou Subuplift had existed stable lake in Pleistocene,the lake experienced a short time regression then with fluctuating rise at approximately 129 ka,and the high lake stand lasted till 71 ka.(4)In OSL dating,the conventional SAR method is time-consuming to get its paleo-dose for old samples.Therefore,during data processing,this article attempts to examine the applicability of rapid measurements that reported recently in the study area,i.e.,test the application potential of Standardized growth curve(SGC)and global standardised growth curve(g SGC)in the region.The study results of 6 samples from profile S32 and LS indicates that,SGC applicability in the study area is poor,and g SGC has been successfully applied in three samples,you can get the same dose-response curve in the <250 Gy range.