Magnetostratigraphy and palaeoclimatic significance of Late Neogene aeolian sequence of Lanzhou area

Abstract

Lanzhou area is located on the northwestern brim of the Loess Plateau and the northeastern margin of the Tibetan Plateau, where the Tertiary strata have special significance to the study of aeolian deposition and uplift of the Tibetan Plateau. Gaolan Hill section is in the southern Lanzhou City with a total thickness of 240 m. 422 paleomagnetic block samples were obtained from the field with an approximate spacing interval of 0. 5 m (local refinement). All samples were demagnetized in a thermal demagnetiser systematically, and then the Nature Remanent Magnetization (NRM) was subsequently measured using a superconducting magnetometer from room temperature to 500 degrees C in 50 degrees C steps. The results show that the measured polarity column of the top strata of the Gaolan Hill section contains two large segments of negative polarity with four small segments of normal polarity events in between, which is consistent with the typical characteristic of the Gilbert Chron, and the normal polarity of the two ends corresponds to Chron2A. 3n and Chron3An. in respectively. The palaeomagnetic age of the Gaolan Hill section was determined ultimately as 6 similar to 3. 5 Ma. Based on this chronological frame, the initial accumulation of Wuquan conglomerate was approximately at 3. 5 Ma, indicating a strong uplift of the,Tibetan Plateau, namely the Phase A of the Tibetan movement. The lithologic characters changed from fluviolacustrine sandstone to aeolian Red Clay with thin interval of greyish-white sand layers at the depth of 602 m. It can be inferred from the rock strata and age that the development age of aeolian sequence in Lanzhou area was 7 Ma at least, which is in accordance with the bottom age (7 similar to 8 Ma) of the aeolian sequence of the eastern and central Loess Plateau, suggesting that their accumulation processes are unitary.