临夏盆地的新生代地层及其哺乳动物化石证据

临夏盆地的新生代地层相当发育 ,保存了从渐新世至全新世的连续沉积序列。更为重要的是 ,这些沉积物中含有丰富的哺乳动物化石 ,为划分和对比临夏盆地的新生代地层提供了可靠的证据。然而 ,此前关于这个盆地地层层序和时代的认识有许多矛盾之处 ,地层命名繁复 ,化石证据混乱。近年来我们对临夏盆地的野外考察已理清了沉积序列 ,并在充分的哺乳动物化石证据的基础上重新厘定了各个岩石地层单位所对应的地质时代。临夏盆地的新生代哺乳动物化石以晚渐新世的巨犀动物群、中中新世的铲齿象动物群、晚中新世的三趾马动物群和早更新世的真马动物群最为丰富。

CENOZOIC STRATIGRAPHIC SEQUENCE OF THE LINXIA BASIN IN GANSU,CHINA AND ITS EVIDENCE FROM MAMMAL FOSSILS

The Linxia Basin is situated in the southeastern part of Gansu Province, China, and it is a flexural basin bounded by the Leijishan fault, North Qinling fault, and the Maxian mountain (Fang et al., 2003). The Cenozoic strata are very thick and well exposed in the Linxia Basin, and abundant mammal fossils are discovered from the strata (Fig. 1). Before the 1950s, there were few works done about the Tertiary strata in the Linxia Basin, and the sediments were mostly referred to the informal name “Gansu Group”. Subsequently, the Tertiary red beds in the Linxia Basin were given the formal name Linxia Formation, the age being determined to be Pliocene, and was divided into four lithologic members (GRGIT, 1965). Qiu et al. (1990, 1991) first described the Early and Late Miocene fossils in the Linxia Basin. After comparing the Jiaozigou section with the stratotype of the Linxia Fm., Qiu et al. (1990) and Xie (1991) suggested that the Linxia Fm. limited to the fourth member of the original Linxia Fm., with an age of Late Miocene. Li et al. (1995) further divided the Linxia strata, and renamed different formations from the Late Oligocene to the Holocene. We adopt some of their divisions and revised others, and we recognized the following sequences (from lower to upper): ? Early Oligocene Tala Formation, Late Oligocene Jiaozigou Formation, Middle Miocene Dongxiang and Laogou Formations, Late Miocene Liushu Formation, Early Pliocene Hewangjia Formation, Late Pliocene Jishi Formation, and Early Pleistocene Wucheng Loess (Fig. 2; Table 1).