Magnetostragraphic Study On The Drill Boreholes In Dalangtan Area Of The Qaidam Basin And Its Paleoenvironmental Significance

The chronological framework of Quarternary strata based on magnetostratigraphy in the Qaidam Basin has been carried out by many researchers during the last century. This framework, however, cannot be suitable for the high resolution study of paleo-environmental change because of lower sampling density and precision of instruments, which need to be revised by the new careful study.In order to discuss the paleo-environmental change of Dalantan area and Qaidam basin, we carried out rock magnetic and magnetostratigraphic investigations on Liang-ZK02and Liang-ZK06borehole in the Dalangtan area of the northwestern part of the Qaidam Basin. Rock magnetic and scanning electron microscope results suggest that greigite, magnetite and hematite are the main magnetic carriers in the sediments of two boreholes. Greigite is very probably authigenic origin that was formed during the deposition, which carried both normal and reversal magnetic polarities, and showed mostly consistent with the polarities defined by magnetite/hematite. Magnetostratigraphic results show that the B/M boundary of Liang-ZK02borehole is located at315m, and the Jaramillo subchron and Olduvai subchron are located in405~430and772-816m, respectively. The excursion event of Norwegian-Greenland Sea in the Brunhes Chron, and Cobb Mountain event, Bjorn event, Gardar event, Gilsa event, Reunion â…  and Reunion â…¡ events in the Matuyama Chron are also discovered. The B/M boundary of Liang-ZK06borehole is located at a depth of326m, and the Jaramillo subchron and Olduvai subchron are located in427-457and904-952m, respectively. The Cobb Mountain and Gardar events are found in the lower part of the borehole.Three large environmental changes occurred around2.5-2.2Ma,1.2-0.7Ma and0.4Ma, are identified based on integrated analysis of evaporate rock, clastic rock and pollen records in the Liang-ZK02and Liang-ZK06boreholes. These three events can be traced through entire Qaidam basin. The former two environmental changes may be related to the uplift of Tibetan Plateau, and the later need to be confirmed weather its relation with the uplift of Tibetan Plateau. Paleo-climate in the Qaidam Basin changed from cool, wet climate to cold, dry climate in1.2-0.7Ma because of the uplift of the Tibetan Plateau. It can also be inferred that at least6extreme cold weather conditions revealed after0.9Ma based on the appearance of the mirabilites in the boreholes.Gradual increasing composition and variety of evaporate rock indicate the tendency of aridification. However, this trend induced through the uplift is not intensified gradually, the climate became wetter in the humid period since the end of Early Pleistocene, which might result from intensified strength of summer monsoon caused through the uplift of Tibetan Plateau, where the glacier and snow covered areas increased.