The Sedimentary Facies And Paleoenvironment Records Of Late Tertiary In Lanzhou Area

Lanzhou Area is located on the northeastern margin of the Loess Plateau and the Tibet Plateau. Its stratum of Tertiary is very important for the research that may indicate the course of the Tibet Plateau uplift and the aeolian sediment. The Gaolanshan Section is 210 meters high and the stratum belongs to the age of late Tertiary. After collecting samples at intervals of 0.25 meter from the whole section,we got 901 samples as a result.Grainsize,magnetic susceptibility and chroma experiments were accomplished in laboratory. Using the result of experiment of grainsize, we can divide the deposit into several components according to differently formative cause by means of distribution function. Meanwhile, distribution function as well as the ratio of the different components can also be obtained. Combined with the principle of magnetic susceptibility and chroma,it shows that the deposit of the Section is an eolian deposition and there are also with 15 layers of sand rock which were accumulated by river. The analysis of paleoenvironment implies:1.The sedimentary stratum of the section had changed from being fluvial deposit to eolian deposit at 7Ma B.P..This transformation means the beginning of internal Asia continent drying and the conclusion is consistent with the Loess Plateau.2.Lanzhou Area was an open plain and its lithosphere was very stable in 6～5.2Ma.The climate was wet and warm during the contemporaneity. Discontinuous fluvial sand rocks were came into being because of the frequent tectonic activities,Climate turned cold and drought during the stage of 5.2～3.5Ma.3.The lithosphere of Lanzhou Area began to become relatively depressed at 3.5Ma.The WuQuan conglomerate is an evidence that the phase A of QingZang movement had taken place.