甘肃龙担早第四纪黄土堆积古地磁年代研究

临夏盆地位于青藏高原东北边缘,是青藏高原与黄土高原衔接带上的一个晚新生代断陷盆地。龙担剖面地处临夏盆地的中央部分,产有大量哺乳动物化石,从化石证据中得出与早更新世午城黄土同时期的黄土沉积,但是岩性看起来又与中国东部黄土高原上典型的午城黄土有差别。本文基于龙担剖面中黄土-古土壤序列的古地磁研究、粒度以及磁化率分析,主要对正极性N3的极性归属问题进行阐述,N3对应于Matuyama极性时中下部的Olduvai极性亚时期,建立了该剖面的年代地层序列。龙担动物群的上化石层位于Olduvai极性亚时内部,时代为1.77～1.95Ma,从而得出龙担动物群的上化石层年龄为1.9Ma,下化石层位于Reunion极性亚时之下,时代为2.25Ma。

PALEOMAGNETIC CHRONOLOGY OF QUATERNARY STRATIGRAPHY OF THE LONGDAN SECTION IN GANSU PROVINCE OF CHINA

Linxia Basin,situated in the northeast belt of the Tibetan Plateau,is a Late Cenozoic depression basin bounded by the Tibetan Plateau and the Chinese Loess Plateau.The Longdan section(35°31'31.6″N,103°29'0.6″E)lies on the east slope of Longdan,a small village of township Nalesi in the south of the Dongxiang Autonomous County,Linxia Hui Nationality Autonomous Prefecture.The Longdan mammal fauna was discovered at the base of the Early Pleistocene loess deposits at Dongxiang,where the lithology is different from the typical Wucheng Loess on the Chinese Loess Plateau.The rich fossils contain many new species and the major two layers of fossils are in the loess beds.Geologically the fossiliferous area is located in the central part of the Linxia Cenozoic sedimentary basin.Tectonically it is an intermountain fault basin,bordered by the Leijishan major fault in the south and the north Qinling and Qilianshan major faults in the north.The section is 51.6m thick above the gravel layer,including the 1.6m Late Pleistocene Malan Loess on the top and the other loess-paleo～sol sequences in the middle of the section.The base of the section is the Jishi Formation,consisting of gravel layer of 13～17m thick.In this study,972 bulk samples were collected with an interval of 5cm and other 401 oriented samples were taken with a magnetic compass.In the laboratory,the paleo～magnetism,medium grain size and the susceptibility of the loess-paleo～sol in the Longdan section were analyzed.From the stratigraphic analysis,the Longdan section from the top 0.3m to the bottom 51.6m,containing 5 normal polarities(N1～N5)and 5 reversal polarities(R1～R5).The paleo～magnetic results show N3 is the Olduvai subchron in the middle of the Matuyama chron,and then the chronology of the Longdan mammal fauna is constructed along the section.The Olduvai subchron with the age of 1.77～1.95Ma is found just in the upper fossiliferous level of Longdan mammal fauna.The geological age of the upper fossiliferous level of Longdan mammal fauna is estimated to be about 1.9Ma.The lower fossiliferous level is below the Reunion subchron and its age is estimated to be 2.25Ma.