A PRELIMINARY STUDY ON THE EVOLUTION OF SOUTHEASTERN MARGIN IN TENGGER DESERT

Since the Quaternary the southeastern margin of the Tengger Desert have existed three large geomorphologic unites Yellow River valley, piedmont plateau and eoliansand plain. The Quaternary strata sedimentary facies changed at varying geomorphologic units. During the early Pleistocene, the Yellow River valley deposited by fluvial pebble and silty sand and the piedmont plateau with gravels. During the middle Pleistocene eoliansand occurred in the highest terrace of Yellow River, and the eolian sand is dated to 330 430±58 134 a B.P.by TL, in which calcareous crust and sandy paleosols can be seen; further results of element analysis for this eolian sand profile show that the content of CaCO 3 in calcareous crusts are up to 30 73%~47 32%, and the content of Ca, Mg, Sr/Ba, Co, V, Cr, Ni are ralatively high, while in paleosand layer the content of CaCO 3 is only 1%~2% and with a higher content of K, Na, Al, Fe, Mn, Ti etc.. During the late Pleistocene piedmont plateau was consisted of alternating accumulation of gravel, eoliansand and sandy loess or eolian sand and lancastrine facies, and eolian sand continued accumulating in the eolian plain and the higher terrace of Yellow River; pollen analysis for paleosand profile demonstrated the plant cover during sandy loess accumulation was arid and semiarid steppe dominated by Chenopodiaceae, Artemisia, Gremineae, Ephedre etc.. During the Holocene eolian plain and Yellow River valley is characterized by the alternating of ancient eolian sand, sandy loess and sandy paleosols or calcareous plantroots. It can be seen that, since middle Pleistocene, the southeastern margin of Tengger Desert has undergone a series of normal and reverse alternating from the occurrence of shifting sand to the spread or fixation, semifixation and reduction of the desert to grassland as well as weakly pedogenic processes.Simultaneously, the bioclimatic zones in the area also underwent alternating from dry,cold and windy desert, desert steppe to semiarid steppe, semiarid or semihumid sparsely wooded steppe. The evolution processes is controlled by the coupling between the glacial and interglacial climate fluctuations and the prosperity and decline of eastern Asian monsoon.