甘肃秦安糜子湾剖面中新世风尘堆积的磁性地层学研究

文章对西部黄土高原糜子湾黄土-古土壤剖面进行了磁性地层学研究。该剖面厚度为164.4m,记录了15个正磁极性段和15个负磁极性段,可与标准极性表中从C5r.3r到C5En之间的极性带对比。根据极性界限用平均沉积速率外推,得到该剖面顶界年龄为约11.6MaB.P.,底界年龄为约18.5MaB.P.。糜子湾剖面位于已发表的QAⅠ中新世黄土剖面以东约30km,两剖面相同时代的地层在岩性地层结构和磁化率曲线波动上具有高度的一致性,表明中新世黄土-古土壤序列与第四纪黄土-古土壤序列一样,地层在空间上有很好的可对比性。文章的结果为从空间角度研究中新世环境变化行为提供了新剖面的年代地层框架。

MAGNETOSTRATIGRAPHY OF THE MIZIWAN MIOCENE EOLIAN DEPOSITS IN QIN′AN COUNTY (GANSU PROVINCE)

Earlier studies on two sections (QA-Ⅰ and QA-Ⅱ) in the Qin′an County from the western Loess Plateau demonstrated that loess deposits started by 22MaB.P., indicating the onset of Asian desertification and winter monsoon circulation by the Early Miocene time. Another Miocene loess section, Miziwan section with a thickness of 164.4m, 30km in east to the QA-Ⅰ section, is geomagnetically dated in this study. Miziwan section is composed of two profiles, Miziwan and Shewang. Thermal demagnetization was performed on 523 oriented samples collected from Miziwan profile and 392 samples from Shewang section at an interval of 25/20cm. Samples were demagnetized in a MMTD-60 Thermal Demagnetiser and measured using a 2G three-axis cryogenic magnetometer, both installed in field-free space. The progressive demagnetization on 118 representative samples at a temperature range from 100℃ to 680℃ shows that the characteristic remnant magnetization(ChRM) can be isolated between 350℃ and 550℃. To improve efficiency, other samples were demagnetized between 250℃ and 525/550℃. The obtained magnetozones at the Miziwan section can be well correlated with the geomagnetic polarity timescale (GPTS) of chrons C5r.3r to C5En. Extrapolation based on sedimentation rate indicates a time span of the sequence from 11.6MaB.P. to 18.5MaB.P. The basal age of Miziwan section is younger than that of QA-Ⅰ and QA-Ⅱ sections. Therefore, no loess-soil sequences older than 22MaB.P. have been found in the western Loess Plateau so far. 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, which is the typical character of the eolian deposits. These results firstly confirm our earlier studies that loess deposition started in Northern China by the Early Miocene, and secondly demonstrate 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.