青海柴达木盆地第四纪下限

THE LOWER QUATERNARY BOUNDARY IN THE CHAIDAMU BASIN OF QINGHAI PROVINCE

根据青海柴达木盆地9个钻孔岩心的岩石地层学,磁性地层学、气候地层学、微体古生物学、地球化学、岩相学、新构造运动等学科的综合研究,本文认为,青海柴达木盆地大约在2.48Ma B.P.前后,曾发生一系列的环境变化,可以用2.48Ma B.P.为第四纪下限。这一见解与中国其它地区工作结果是一致的。这一环境剧变的原因,应当归于青藏高原的隆升。以往文献中所谓3.00Ma B.P.的第四纪下限,当是更早的构造运动的表现。

The Chaidamu Basin of Qinghai Province is the second largest basin in China, where the sequence of Late Cenozoic with great thickness can be found, but systematic stratigraphic work have not been made yet. Based on the systematic stratigraphic researchs (including lithostratigraphy, palaeomagnet-ostratigraphy, climatostratigraphy, micropalaeontology, geochemistry, lithofa-cies and neotectonics) in a total of nine standard boreholes in the Chaidamu Basin, this paper tries to give the Lower Quaternary boundary in the basin. On basis of neotectonic movement, the studied area can be subdivided into three subregions as follows: subregion I (northwestern part)-relatively rising one, subregion Ⅱ (southeastern part)-intensive subsidence one and subregion Ⅲ (middle part)-transitional one between the first two subregions. Sporopollen analysis suggests that there exists a boundary for climatostratigraphy. Under it arbor pollen is rich and occupies over 20% of the total sporopollen, and pollen of subtropical and warm coniferous and broadleaf trees is also abundant. Above the boundary the amount of arbor pollen reduced and subtropical elements were rarely seen,but herb pollen increased obviously. It indicates that climate went into a cold-dry stage. This event lasted during 2.564-2.508 Ma B.P.. At about 2.88 Ma B.P. the margin area of the lake basin in the subregion I rose due to the effects of neotectonic movement. At borehole No. 05 of Chahanslatu, gravel-bearing medium-grained sand of 17.74 m in thickness occurs near the boundary of Gauss/Matuyama polarity chron and this means that there was an intensive tectonic movement at about 2.48 Ma B.P. and it continued to about 2,00 Ma B.P,.In Mahai area, gypsum appears, which has been considered as the mark for the lower boundary of Quaternary System. In Dalangtan area (boreholes No. 02 and No. 36), salt formation formed very early, at about 4.00 Ma B.P., but in sulphate only gypsum and calcareous mirabilite are found, only since 2.48 Ma B.P., have sulfate-mirabilite and natronkalisimonyite occurred. Obviously the environmental change at about 2.48 Ma B.P. in the studied area was related to the uplifting of Qinghai-Tibet Plateau and also to the changes of magnetic field and geoid. So we consider that 2.48 Ma B.P. should be the age of lower Quaternary boundary.