临夏盆地上中新统的年代地层划分与对比

CHRONOSTRATIGRAPHIC SUBDIVISION AND CORRELATION OF THE UPPER MIOCENE OF THE LINXIA BASIN IN GANSU,CHINA

中国上中新统的陆相地层非常发育,其中在华北地区广泛分布的红黏土堆积富含三趾马动物群化石。甘肃临夏盆地的上中新统即是一套红黏土地层,其岩石地层单位名称为柳树组。已发现柳树组的大量化石地点,按其在剖面中出现的层位可以自下而上分为4个动物群,已根据代表性地点依次命名为郭泥沟、大深沟、杨家山和青卜山动物群。古地磁测年数据显示,柳树组涵盖了整个晚中新世的沉积,即包括上中新统下部的灞河阶和上中新统上部的保德阶。郭泥沟、大深沟和杨家山动物群属于灞河阶,青卜山动物群属于保德阶。灞河阶底界如与海相Tortonian阶一致,则这条界线(古地磁C5r.2n下界)存在于临夏盆地沉积序列中,年龄为11.6Ma,三趾马的首现位置略高于此界线,年龄为11.1Ma。保德阶底界如与海相Messinian阶一致,可以在柳树组和西藏吉隆的沃马组中准确划定这条界线的位置。由于山西保德是保德阶的命名地点,因此保德阶的底界层型应选择在保德县腰庄乡冀家沟剖面保德组靠近红黏土底部的古地磁C3Br.1r之内,年龄为7.25Ma,三趾马Hipparion forstenae的首现可作为其生物标志。

Terrestrial strata of the upper Miocene are well developed in China, among which the widely distributed red clays in northern China contain very rich mammalian fossils of the Hipparion fauna. The upper Miocene of the Linxia Basin in Gansu Province consists of a sequence of red clays of the Liushu For- mation, in which many fossil localities were found. The fossil assemblages were subdivided into four mammalian faunas from the bottom to the top according to their horizons in the sections, and respectively named as the Guonigou, Dashengou, Yangjiashan, and Qingbushan faunas based on the representative localities. Paleomagnetic dating shows that the Liushu Formation covers the entire upper Miocene, including the lower upper Miocene Bahean Stage and the upper upper Miocene Baodean Stage. The Guonigou, Dash- engou and Yangjiashan faunas belong to the Bahean, while the Qingbushan fauna belongs to the Baodean. If the lower boundary of the Bahean Stage correlated with that of the marine Tortonian Stage （lower boundary of Chron CSr. 2n）, it occurs in the sedimentary sequence in the Linxia Basin, with an age of 11.6 Ma,and the first appearance datum of Hipparion is slightly higher than this boundary, with an age of 11.1 Ma. If the lower boundary of the Baodean Stage is correlated with that of the marine Messinian Stage, its position can be accurately determined in the Liushu Formation and also in the Woma Formation of Gyirong, Tibet. Because the Baodean was derived from Baode County, Shanxi Province, the lower boundary stratotype of this stage should be based on the Jijiagou section of Yaozhuang Township in this county, within Chron C3Br. lr near the base of the red clays in the Baode Formation and with an age of 7. 25 Ma, and the first appearance datum of Hipparion forstenae can be regarded as a biostratigraphic marker.