Magnetostratigraphy of the Neogene Strata in Xunhua Basin, Qinghai Province

Xunhua basin is one of the inter-mountainous basins distributed in northeastern margin of the Tibetan plateau with over 1 000 m thick lacustrine and fluvial sediments of the Cenozoic. These sediments record both tectonics and climate changes and provide great potential for deciphering how tectonic uplift and climate change interact with each other. However, the absence of high resolution chronology of these sediments has prevented us from understanding their relationship. Accordingly, we performed a detailed magnetostratigraphic study of the Neogene lacustrine and fluvial sediments at the Xigou Section in the Xunhua basin. 560 samples were collected from 280 sites throughout ~560 m thick lacustrine and fluvial sedimentary sequences. A total of 267 samples yielded well defined characteristic remanent magnetization components (ChRMs) by stepwise thermal demagnetization and were used to establish the magnetostratigraphy of the Xigou Section. Based on the vertebrate fossils found at the upper part of the section and characters of the magnetozones, the magnetostratigraphy of the Xigou Section can correlate well with the geomagnetic polarity timescale (CK95), which suggests a depositional age of ~14.6 Ma to ~5.0 Ma. On the basis of the analysis of the sedimentary faceis at the Xigou Section, we are convinced that the Xunhua basin shrinked gradually during ~14.6 to 5.0 Ma. One huge thick conglomerate layer (~20 m thickness) appeared firstly at ~7.3 Ma and was considered to be caused by rapid uplift of the surrounded mountains, which was almost coinstantaneous with rapid uplift of the Tibetan plateau at ~8.0 Ma, implying the synchronous uplift of the Tibetan plateau.