新近系保德阶建阶研究新进展

NEW ADVANCES IN THE ESTABLISHMENT OF THE NEOGENE BAODE STAGE

根据与古地磁极性年表的对比 ,山西保德冀家沟剖面含保德动物群的保德组上界年龄为 5 .30 Ma,底界年龄不超过 10 Ma,其红黏土的底界为 8Ma,显示该剖面不存在定义的保德阶 11.2 Ma的下界 ,也没有保存保德阶下部的沉积。《国际地层指南》提倡用选择下界的界线层型来确定年代地层单位 ,其上界应该由后续单位的下界来确定。冀家沟剖面显然不具备保德阶的下界 ,但甘肃临夏盆地郭泥沟剖面含三趾马动物群的红黏土之下还有发育的中中新世沉积出露 ,因此 ,后一个地点应存在保德阶的下界 ,并有保德阶最底部的化石和适合于作古地磁分析的沉积物 。

The upper boundary of the Baode Formation with the Baode Fauna is 5.30Ma, the lower boun dary is later than 10Ma, and the bottom of the red clay is 8Ma at the Jijiagou section in Baode, Shanxi. The present study indicates that the Jijiagou section has no the defined lower boundary of the Baode Stage at 11.2 Ma, and the most representative section of the Baode Formation at Jijiagou does not contain the lower part of the Baode Stage. According to the principles of the modern stratigraphy, the upper and lower boundaries of the well basseted unit stratotype of a complete stage are the boundary stratotypes of this stage. On the other hand, the International Stratigraphic Guide advocates to mainly choose the lower boundary stratotype in order to determine a chronostratigraphic unit, and the upper boundary of this unit should be determined by the lower boundary of its subsequent unit. Although the Jijiagou section has no the lower boundary of the Baode Stage obviously, the Guonigou section in the Linxia Basin of Gansu has well developed Middle Miocene deposits under the red clay with the Hipparion fauna. Therefore, the lower boundary of the Baode Stage should be present at Guonigou. The fossils of the lowermost part of the Baode Stage and the fine deposits for the paleomagnetic dating appear at the Guonigou section. Consequently, the Linxia Basin is a favorable place where the lower boundary stratotype of the Baode Stage may be established.