The Formation And Evolution Of Aeolian Environment In The Gonghe Basin

The Gonghe Basin is located on the desert-loess transition zone across northwestern and north China,which is influenced climatically by both the Asian monsoons and the high-level Westerlies.The unique pattern of geomorphological evolution of the basin makes it an ideal region to investigate formation and evolution of aeolian environment.To date,there are no systematic researches on such an aspect in the Gonghe Basin.The landform evolutions of Gonghe Basin actually play an crucial role in creating aeolian environments by affecting sediment supply patterns and by producing depositional places for aeolian deposition.Investigations on the geomorpholocal processes in the basin is not only important in constraining major forcing factors for occurrences of aeolian activity,but also provide a valuable study case to understand general patterns of formation and evolution of the deserts and sandy lands in China and even other areas in the world.Here we present the results of seven new outcrop sections and eight outcrop sections previously published in the Gonghe Basin.Based on the optically stimulated luminescence(OSL)and the K-feldspar post-IR Infra-Red Stimulated Luminescence(IRSL)dating,the chronological frames of these outcrop sections were established.In combination with climatic proxies,including grain size,loss on ignition(LOI)and elemental compositions,we attempted to investigate the connection between the spatio-temporal differentiation of aeolian deposits and geomorphological processes in the Gonghe Basin.In addition,major controlling factors for the formation and evolution of aeolian environment in this basin were discussed.The mian conclusions of this study are as follows:(1)Stratigraphic and chronological data sets of the outcrop sections show that,fossil dune sands mainly occurred in three periods,i.e.at ~203±7 ka,from 176.2±15.1 ka to 143±27.8 ka,and from 108±7 ka to 86±5 ka.Elevations of the aeolain sand deposits vary from 2875 to 3194 m a.s.l.At ~203±7 ka when the Gonghe basin was in the course of infilling,aeolian sand deposited on the northern edge of the basin.From 176.2±15.1 ka to143±27.8 ka,aeolian sand occurred at the lower elevations of the modern basin.According to geomorphological evolution of the Gonghe Basin,the negative topography(relative to the plateau surface of the Gonghe Basin,3200 m a.s.l.)of areas surrounding Gengga Lake,Yingde Lake and Shazhuyu River may have been developed from a planation surface,which should be corresponding to the surface of the Second Tara terrace.Thus aeolian sand and/or fossil dune sands might deposit on riverbanks and/or surrounding areas of lakes,when the Gonghe Basin underwent infilling by fluvio-lacustrine sediments.From 108±7 ka to 86±5 ka,the Gonghe paleo-lake was disintegrated,due to intensive incisions of the Yellow River,the exposure of terraces abandoned by the Yellow River provides depositional places for aeolian deposits,leading to a broad aeolian environment in the basin.(2)Aeolian environment can be regarded as a function of climatic changes,wind strength,and geomorphological evolution.However,dominant factors are variable from time to time.At ~203±7 ka and from 176.2±15.1 ka to143±27.8 ka,aeolian sand which deposited on riverbanks and/or surrounding areas of lakes was controlled mainly by wind strength and sand supply.From 108±7 ka to 86±5 ka,the disintegration of the Gonghe paleo-lake and exposure of the fluvial terraces of the Yellow River provided depositional places and abundant sand material for aeolian sand accumalation.During this period,the aeolian sand deposition would be mainly ascribed to changes in vegetation conditions,which is in turn affected by effective moisture at those sites.