MAGNETOSTATIGRAPHY OF AN EARLY-MIDDLE MIOCENE LOESS-SOIL SEQUENCE IN THE WESTERN LOESS PLATEAU OF CHINA

Earlier studies on two parallel sections (QA-I and QA-II) in the Qinan region from the western Loess Plateau demonstrated that loess deposition in northern China started by 22 Ma ago, indicating the onset of Asian desertification and winter monsoon circulation by the early Miocene time. A third section, 218.2 m thick, about 30 km east of the previously studied sections, is geomagnetically dated in this study. The upper portion (0-166.5 m) of this section consists of alternating typical loess and soil units while the lower part (166.5 m-218.2 m) is water-reworked, but the material was derived from loess deposits.

Thermal demagnetization was performed on 853 samples at an average interval of 25 cm. Samples were demagnetized in a MMTD-600 Thermal Demagnetiser and measured using a 2G three-axis cryogenic magnetometer, both installed in field-free space. Most of the samples yield a stable characteristic remanent magnetization (ChRM) above 350 degree and 91% of the samples gave reliable characteristic remanence directions.

The obtained magnetic polarity zonation is well correlative with the portion from the Chron C6Ar to Chron C5r.3r of the GPTS without any significant hiatus. Extrapolation based on sedimentation rate indicates a time span of the sequence from 21.4 Ma to 11.6 Ma BP. The boundary between the typical loess-soil sequence and water-reworked lower portion is dated for 19.6 Ma.

Lithostratigraphy and magnetic susceptibility of the sequence are consistent with the geomagnetic results. They show high similarity and correlativity with those of the QA-I and QA-II sections. These results firstly confirm our earlier studies that loess deposition started in northern China by the early Miocene, and secondly demonstrates that the stratigraphy of the Miocene loess-soil sequences is spatially correlative, having therefore great potential for the study of regional/global climate changes during the Miocene time. The Miocene loess-soil sequences, combined with the well-known Quaternary loess-soil sequence and the Late Miocene-Pliocene Hipparion Red-Earth formation, provides a unique continuous eolian record of paleoclimates since the early Neogene.