The Study Of Magnetostratigraphy And Paleoenvironment Evolution Of Neogene Sedimentary Sequences In Lanzhou Area

The absolute depth of Gaolan Hill section in Lanzhou is 240m totally, 422 paleomagnetic samples were collected with 0.5m intervals in the section. Thermal demagnetizer and cryogenic magnetometer were used to systematically measure the magnetic remanence from room temperature(25℃) up to 500℃with 50℃step in order to dertimime magnetostratigraphy of the Gaolan mountain section. The date span of Gaolan Hill section is from 6 to 3.5MaB.P..Insert some date points into the known date points with Linear interpolation in order to obtain the age scale of Gaolan Hill section. And make up the susceptibility change curve along with the stratum age, in the result, talking about the change of geological structure and climate in Neogene.First, the trait of Magnetic polarity of Gaolan mountain section is that 2 long negative Polarity Subchron are in both sides and 4 short positive Polarity Subchron are inside. It is identical with Gilbert Chron. The date span of Gaolan Hill section is from 6 to 3.5MaB.P..Second, the age of Wuquan conglomerate bottom is about 3.5MaB.P..It is little later than the age of Jishi conglomerate which is about 3.58Ma. They are impacted by the A Screen movement of uplift of the Qinghai-Tibet Plateau.Third, Eolian sequence accumulation is occurred from 7 MaB.P. at least. Ancient monsoon climate and western aridification had been formed. The age is identical nearly with the bottom age of eolian sequence accumulation in Loess Plateau.Fourth, Eolian sequence accumulation rate had increased significantly in the period of 3.5～1.48MaB.P.. But, because of being of Wuquan conglomerate and Huanghe river, the stratum of Lanzhou is discontinuous. So the accurate age of change of eolian sequence accumulation rate haven't been found out.