从陕西旬邑上新生界剖面看黄土高原新近系/第四系界线

新近系 /第四系界线在黄土高原大都以磁性 M/ Ga界线为界进行划分 ,它与岩性分界往往并不一致 ,后者都低或高于前者一小段距离。根据旬邑剖面的岩性、孢粉、古动物、磁化率、粒度及地化资料 ,并参考了其他一些剖面的岩性 ,认为以二者之间的过渡层底为界进行划分比较合理

DISCUSSION ON THE NEOGENE-QUATERNARY BOUNDARY--BASED ON THE XIAQIANG UPPER CENOZOIC PROFILE OF XUNYI COUNTY, SHAANXI PROVINCE

An Upper Cenozoic profile developed at Xiaqiang of the Xunyi area in Shaanxi Province is a set of earthy deposits. Its upper part, yellow in color, belongs to the loess paleosol sequence of the Pleistocene and Holocene of Quaternary, 141.2m thick and the lower part, red and about 90m thick, is the deposits of the Upper Miocene and Pliocene of Neogene. The paleomagnetic boundary of the Matuyama (Quaternary) and Gauss (Neogene) in this profile is situated at L 43 . Below this M/G boundary line, there are two beds, the lower part of L 43 , S 43 , and the lowest layer of loess, L 44 , 3.6m thick in total. This lowest loess layer has been called “transitional bed”. In the lower part of the transitional bed, there are many small brecciated gravels, abundant in the bottom, descreasing in number upwards, and disappearing near the top of this bed. The gravels are red clayey silt rocks coming from the underlying beds of the Neogene, 1~4mm is size, half angle edged and half round in shape, having a scattered distribution. This bed looks like a sort of “basal conglomerate”. Which suggests, in a certain degree, the beginning of a suite of new sediments in a new period. A lower jaw of Youngia omegodon has been discovered from this bed. To obtain more evidences, several kinds of analyses, such as those of sporo pollen, susceptibility (Fig.1), grain size (Fig.2), and geochemistry (Fig.3) have also done. The results, represented by curves of these analyses indicate an obvious change when passing through the bottom line of the lowest layer of loess. Therefore, if consider comprehensively, this transitional bed should be included in the deposits of the Quaternary, and the bottom line of this bed would be the boundary between the Quaternary and Neogene, i.e. Q/N. It may be interpreted as that during the time from the latest Pliocene to the earliest Pleistocene, the climate suddenly became colder and colder which may be caused by the violent uplift of the Qing Zang (Tibet) High Plateau and the rapid spread of the arctic ice cap. Winter monsoon was getting stonger and the aeolian erosion was increased. The strong wind (along with the effect of some flowing water perhaps) eroded the bared surface of the Neogene deposits, carried the gravels of red silt rock away for some distance and then deposited them together with loess. Certainly, a possible influence of an earth crust movement (neotectonic movement) could not be ruled out. This lowest layer of loess can be seen in the sections of Jiaxian, Shaanxi Province and Renjiapou of Lintai, Gansu Province.