STUDY ON LOWER BOUNDARY OF QUATERNARY IN NORTH CHINA—An Important Climatic-Geological Event in Early Matuyama Chron
An Zhisheng (Xi'an Department of Loess and Quoternary, Academia Sinica)
A study on biostratigraphy, magnetostratigraphy, paleoclimatology and sedimentology of continuous stratigraphic sequences with Plio-Pleistocene age suggests that in the early Matuyama chron of about 2.3-2.4 m. y. B. P., a bigger change in terms of the biological evolution, sedimentary facies and paleoclimate than that in the end of Olduvai subchron recorded an important event throughout the geological period. This event may be considered as the Plio-Pleistocene boundary in the North China. 1. The evidence of mammalian fossils. The Nihewan fauna, including advanced Pleistocene species and a few remains of the Pliocene species appeared about in Olduvai subchron. The first appearance level of Eqqns was placed about 2.4 m. y. B. P. Dongyaozitou fauna bearing many Pliocene elements developed about 2.5 m. y. ago. An. evolutional interval from Prosiphneus stage to Myospalax stage occured about 2.4 m. y. BP. Consequently, a distinct alternation betweeu the Pliocene fauna and the Pleistocene fauna took place in a transitional interval of geomagnetic polarity from late Gauss chron to early Matuyama chron. 2. The evidence of microfossil and other fossils. An appearance datum level of foraminiferal assemblage including Globigerina bulloides, G. pachyderma, Hyalinea baltica etc., and calcareous nannofossil assemblage characterized by Coecolithus pelagicus, Emiliania huxleyi are different from other observed datum level characterized by the brackish water foraminiferal species in warm-temperate shallow water during late Cenozoic in the North China. It represents an open sea with normal salinity and a cool-cold climate. Besides, before or after Matuyama/Gauss boundary variations of ostracoda and vegetation happened. 3. The evidence of sedimentary facies. At 2.4 m. y.- B.P., the dust-loess accumulation started to appear in large scale instead of Pliocene red clay. At the same time, in Nihewan intermontane basin the lacustrine deposits with fine grain forming in fresh water were succeeded by the calcareous deposits with coarse grain forming in shallow water. The fine grain sediments were also succeeded by coarse grain sediments in Yushe basin. CaCO3 content indicating a rather dry-cold climate also obviously increased in fluviolacurine sediments of Beijing Plain at 2.4 m. y. B.P. In a word, the rapid deterioration of climate with an age of 2.4 m. y. in the North China indicates the character of dry-cold glacial climate subjected to the middle latitude area. Meanwhile, an event of biota variation occured at the Matuyama/Gauss boundary. It is supposed that these climatic and biological event shows an important geological level placed about 2.4 m. y. B.P. The level was recorded by the basal beds of Wucheng loess, calcareous sediments with coarse grain in upper part of lower Nihewan formation, upper Yushe formation and Xiadian formation, which may be regarded as the base strata of Pleistocene in the North China temporaryly. The geological event taken place in early Matuyama chron would be closely connected with global climate change, and to some extent with the intense uplift of Tibet Plateau resulted from an intense uplift episode of the Himalayan movement at the Matuyama/Gauss boundary. In the North China, the cool climate started in 3.4 m. y. B.P. An event of colder climate occured between 3.0 and 2.5 m. y. The major event of rapid climatic deterioration characterized by dry and cold took place at 2.4 m. y. B.P. Another event of warm or temperate and humid climate in Olduvai subchron recorded an episode of long term climate change. Since 0.7 m. y. a tendency of dry climate has been getting more and more. All of this may constitute the basic pattern of long term climate change since late Cenozoic in the North China.