柴达木盆地第四系介形类化石带与磁性柱

QUATERNARY OSTRACODE ZONES AND MAGNETO STRATIGRAPHIC PROFILE IN THE QAIDAM BASIN

柴达木盆地东部晚上新世－全新世湖相沉积巨厚，无明显间断，微体化石丰富。区内已建厚度超过２０００ｍ的第四系磁性柱中，记录了布容、松山和高斯极性时带。由于有剖面上部的同位素测年数据和剖面中、下部的介形类化石带序列的对比成果配合验证，布容、松山极性时下限的确定和布拉克、莫纳、琵琶湖、奥尔杜威、马默思等亚极性时的鉴别依据充足可信。据此极性年表标定的晚上新世－第四纪介形类化石带序列中１２个标准化石的时限得以识别。古生态研究与壳体元素分析的资料表明，始见于距今３０５万年的Ｍｉｃｒｏｌｉｍｎｏｃｙｔｈｅｒｅｓｉｎｅｎｓｉｓ指示低温水体，该化石带的出现标志着中新世以来柴达木盆地古气候的首次明显变冷。

The lacustrine deposits of Late Pliocene-Holocene in the eastern Qaidam Basin are extremely thick, without any marked gaps, and abundant in microfossils. In the Quaternary polarity columnar section established in the area, with a thickness over 2 000m, Brunhes, Matsuyama and Gauss polarity chrons (zones) are recorded. The palaeomagnetic determination has been tested and verified with the isotopic dating data (in the upper part of the section) and the data from correlation of the fossil ostracode zone sequence providing evidence for the Gauss/Matsuyama and Matsuyama/Brunhes transition zones, while the identifications of the Blake, Mono, Biwa, Olduvai and Mammoth subchrons are also reliable. Based on the polarity time scale mentioned above, the geological ranges or palaeomagnetic datings of the index fossils in the ostracode zone sequence are recorded in (ascending order) as: Ilyocypris qingxuensis , ranging from the early period of Late Pliocene to 3.37Ma BP; Microlimnocythere reticulata, ranging from the middle period of Late Pliocene to 3.05-3.01Ma BP; M.sinensis , FAD3.05-3.01Ma BP,LAD 2.77-2.70Ma BP; Heterocypris salina salina and Eucypris inflata, from about 2.73Ma BP to Recent; Qinghaicypris crassa, FAD 2.515Ma BP, LAD 2.39-2.34Ma BP; Prionocypris gansenensis , from 2.18Ma BP to Recent; Ilyocypris inermis, from 1.83Ma BP to Recent; Candona neglecta, from 1.21Ma BP to Recent; Limnocythere inopinata, from 0.97Ma BP to Recent, Cytherissa lacustris ,from 0.75Ma BP to Recent; Candona condida, from 0.21 Ma BP to Recent. Achievements from paleoecological research and final analysis on individual valves of M.sinensis show the species as an indicator of cold water, and so this species appears in the first period of a low temperature since the Late Miocene epoch.