临夏盆地黑林顶剖面磁性地层学及其意义

MAGNETOSTRATIGRAPHY AND IMPLICATIONS OF THE HEILINDING SECTION,THE LINXIA BASIN,GANSU PROVINCE,CHINA

位于青藏高原东北缘的临夏盆地沉积了厚层的新生代地层,含有丰富的新生代哺乳动物化石。准确确定其地层年代对研究我国西北地区哺乳动物分布格局、演化过程和生态环境变化以及高原隆升过程具有重要的科学意义。盆地南缘黑林顶剖面含有两个层位的哺乳动物化石,详细的磁性地层学研究表明,沉积物的主要载磁矿物是磁赤铁矿、磁铁矿和赤铁矿,剖面年龄约为12.1～5.2Ma,依据沉积速率推算出剖面上部的三趾马动物群地质年代为6.3Ma,下部铲齿象动物群年代为11.1Ma。

The Linxia Basin, a depression bounded by the Leijishan Fault in the west, Xiqinling Fault in the south and Maxianshan Mountain in the northeast in the northeastern edge of the Tibetan Plateau, is very sensitive to the climatic and environmental changes. Many effective research programs have been carried out in this region. Substantial amounts of mammalian fossils have been discovered in the thick Late Cenozoic sediments of the Basin. Through detailed exhumation in this region there have found 5 most typical mammalian faunas. The Late Oligocene Dzungariotherium fauns, the Middle Miocene Platybelodon fauns, the Late Miocene Hipparion fauns, and the Early Pleistocene Equus fauns. The Heilinding section lies on the south margin of the Linxia basin with a total of 227 m Miocene deposits from the top to the bottom. The fossils Hipparion sp. and Platybelodon sp. were excavated in the red clay layer of the upper Liushu Formation and sandstone of Dongxiang Formation respectively. Analysis of thermal demagnetization of natural remanent magnetization （NRM） indicates that hematite, maghemite, and magnetite are primary magnetic minerals in the sediments. According to the high-resolution magnetostraitigraphic research on this section, 14 normal polarities （N1-N14） and 14 reversal polarities （R1- R14） are interpreted, compared with the GPTS of Cande and Kent, from the top 15 m to the bottom 227 m systematically. Based on this result, sedimentation rates and geological age of the fossils-bearing strata can be defined. The Hipparion fauna is constrained at 6.27 MaBP and the Platybelodon fauna is dated 11.12 Ma.