泥河湾盆地上新世小哺乳动物生物地层学及相关问题讨论

PLIOCENE MICROMAMMALIAN BIOSTRATIGRAPHY OF NIHEWAN BASIN,WITH COMMENTS ON THE STRATIGRAPHIC DIVISION

在泥河湾盆地稻地组与壶流河组、蔚县组命名剖面上分别筛洗小哺乳动物化石。其中花豹沟剖面选取 3个层位 ,发现小哺乳动物化石 1 0种 ;稻地剖面 6个层位 ,发现化石 2 5种。结合小哺乳动物化石组合 ,以及岩石地层追索对比 ,认为壶流河组与蔚县组从岩性上无法分开 ,应予合并 ,保留蔚县组 ;稻地组与蔚县组为同时期沉积 ,二者之间存在一定的相变关系 ,建议废止稻地组 ;仅保留蔚县组作为泥河湾组之下风成红黏土之上的岩石地层单位 ,其时代为中上新世晚期至晚上新世。

The Nihewan (Nihowan) Basin, known from 1926 for the finding of fossil mammals as the type locality of the Early Pleistocene strata of North China—Nihewan bed, has long been the highlighted area for a few generations of paleontologists, archeologists, and sedimentologists.New survey and screen washing for small mammals from two sections along the Huliuhe River were carried out in a field season of 2002. This paper gives the fossil list of the Daodi and Huabaogou sections, and discusses briefly the division and correlation of the formations named for the Pliocene strata.The Huliuhe Formation was previous correlated to the Bahe Formation (Late Miocene). There newly discovered 8 species of small mammals from this level (Fig. 1). The coexistence of the Chardinomys yusheensis, Huaxiamys downsi, Pseudomeriones cf. P. abbreviatus, Pliosiphneus ly- ratus , and Pliosiphneus n. sp. etc., and the durations of these species in the sequences of North China, e. g. Lingtai and Yushe, suggest the age of middle Pliocene (MN 15 equivalent). Though there are only three species from the overlying Yuxian Formation, the derived Chardinomys nihewanicus shows strong evidence for the late Pliocene in age.The Daodi section, about 5 km north to Huabaogou gulley, is much more fossiliferous. 25 species from 8 levels are discovered (Fig. 2). There found only one species of Trischizolagus from the lowest level that is most probably eolian sediments. From Da-1～ Da-2, the assemblage is composed mainly of the middle Pliocene elements such as the primitive Huaxiamys primitivus etc. The upper levels (Da-3～Da-5) produced the most derived Pseudomeriones species, P. complicidens , which was recorded only from the late Pliocene, and other taxa, e.g. Mimmomys orientalis, Chardinomys nihewanicus etc.The Daodi Formation was named previously for the lower part of Nihewan Bed, which was thought to be overlying on the red clay of Yuxian Formation, and of Late Pliocene in age. However, the sediments of Yuxian and Huliuhe formations are not eolian red clay, but fluvial sediments. Moreover, the fossil contents of the Daodi Formation are basically the same with those of the Yuxian Formation. There exists evident facies changes for the fluvial sediments underlying the cross bed of Nihewan Formation. So, the name of Daodi Formation should not be valid for preoccupiedness of the Yuxian Formation.The Huliuhe Formation, only 5m in thickness, and not separable with the overlying strata, should only represent the lower part of the Yuxian Formation.Hence, along the west bank of the Huliuhe River, underlying the Nihewan cross bed, there exists only one formation—Yuxian Formation, overlying on the eolian red clay that is not named till now. The geological age of the Yuxian Formation should be middle to late Pliocene.