关键词: 泥河湾盆地;马圈沟遗址;哺乳动物化石;生物地层和磁性地层年代

Abstract: The Majuangou section (= MJ G, 40°13′3112″E, 114°39′5019″N) lies near Cenjiawan village and on the south of the Sanggan river in the Nihewan Basin. In 1990, Wei Qi first reported a new paleoliths2bearing layer from the middle part of the Majuangou section with the name Banshan Site (Wei Qi, 1994). Two years later, the Hebei Province Institute of Cultural Relic (= HPICR) found another artefact layer named as“Maguangou Site”from the lower part of the same section and did excavation in the next year (Li and Xie, 1998). In 2001, the first two authors gathered numbers of mammal remains and several paleoliths by washing the matrix collected from the“New artefact layer”, 8 meters below the bottom of“Maguangou Site”(Cai and Li, 2003). A test excavation was done and Goudi Site was named by Wei Qi (Wei, 2002). During the HPICR′s excavation seasons from 2001 to 2005, the fourth artefact layer was found and called MJ G Ⅱ. The four artifact layers found in the Majuangou section are, from the top to bottom, Banshan Site, MJ GⅠ( =“Majuangou Site”in Li et al , 1998), MJ G Ⅱand MJ G Ⅲ ( =“New artefact layer”in Cai and Li, 2003 and“Goudi Site”in Wei, 2002) (see Fig11). Zhu et.al (2004) reported magnetostratigraphic results of the four artefact layers mentioned above.
The magnetic age of the MJ G Ⅲ given by Zhu et al ( 2004) is about 1166Ma, while the biostratigraphic age estimated by Cai et al (2003) is suggested to be older than 1180Ma. There is a small disparity between the two results. In 2005, invited by HPICR, the authors undertook the screen-washing work in the Majuangou sites and collected additional fossil mammalian specimens from MJ G I and Banshan Site. The purpose of this paper is to briefly describe the fossil mammals collected in 2001 and 2005 and to ascertain the age of the fossil2bearing beds. Further more, the potential factors interfering with the determination of magnetic chronology for these layers are discussed.
By the way of washing about 315 tons of matrix, 553 small and 6 large mammal specimens were collected from MJ G Ⅲin 2001. They are Sorex sp., Erinaceus sp., Ochotona youngi, Ochotonoides complicidens, Allactaga sp., Spermophilus sp., Cricetulus sp. , Yangia tingi , ? Episiphneus sp., Micromys sp., Chardinomys nihowanicus, Cromeromys gansunicus, Borsodia chinensis, Allophaiomys deucalion, Canidae gen. et sp. indet., Canis sp., Mammuthus trogontherii, Hipparion sp., Rhinocerotidae gen.et sp. indet. and Cervidae gen. et sp.indet. (Fig12 —4).
Only 7 poorly preserved specimens were found after washing about 850 kgs of matrix from MJ GⅠ in 2005. Four species , including Ochotona sp., Cromeromys sp., Borsodia sp. and Allophaiomys sp. can be recognized (Fig15).
Sixty three specimens were collected from about 3 tons of matrix from Banshan artefact layer. These specimens belong to 7 species, Ochotona sp., Ochotonoides sp., Leporidae gen. et sp. indet., Allocricetus ehiki, ? Cricetulus sp., Borsodia chinensis and Allophaiomys pliocaenicus(Fig.6).
The co-occurrence of A. deucalion, C. gansunicus, B . chinensis, ? Episiphneus sp., Y. tingi and C. nihowanicus shows that the MJ G Ⅲmammal fossils assemblage can be correlated with those from Haiyan Formation of the Yushe Basin, Shanxi ( Flynn et al, 1997 ) and Loc. 93001 of Wenwanggou, Lingtai, Gansu (Zheng and Zhang, 2001) . The age of MJ G Ⅲseems to be earlier than that of the end of the Olduvai normal polarity event, and possibly falls within the range of 2110—1180Ma. In addition, the coexistence of rooted Episiphneus and rootless Y. tingi has only been recorded in Chinese early Early Pleistocene (Zheng, 1994) .
In Europe, the earliest appearance of Allophaiomys was once considered as the beginning of Biharian (Repenning, 1992) . The age of the type locality of A. deucalion, Villany25 in Hungary, however, is at present considered as from late Villanyian to early Biharian ( Kowalski, 2001). In addition, the magnetic age of the layer yielding Allophaiomys in Russian Tizdar is 2125—1196Ma (Pevzner et al, 1998) . In the loess section of Xunyi, Shaanxi, A. pliocaenicus appears in the paleosol layer S25 (Xue et al, 2001), which is 1183—118Ma (Ding et al , 2002). It seems to be un2 doubted that A. deucalion occurs before Biharian in Eurasia.
In the Donggou section, about 2 km south of the Majuangou Site, A. deucalion was found in both layers 11 and 16 with C. gansunicus, while A. pliocaenicus in layer 19 with Y. tingi. The first two layers (11 and 16) were all regarded as to be late Villanyian, while the third layer (19) to be early Biharian (Zheng et al, 2006). It is rather closed to the magnetic result (Zhu et al, 2001) that the 81112 m depth of the 121 m Donggou section is with the age of the upper limit of the Olduvai event, which should be equivalent to the position between layers 18 and 19 (Zheng et al, 2006).
The mammal assemblage from MJ G Ⅲshould be contemporaneous with those from layer 11 and 16 of the Donggou section because they all have rootless A. deucalion, rooted C. gansunicus and B. chinensis in common. Their ages should be earlier than 1180Ma, rather than the magnetic result of 1166Ma (Zhu et al, 2004) .
In fact, there exist a few deposit intervals in the MJ G section, such as the exposure of elephantsπ footprints (Xie et al, 2006) and distinct unconformity beneath the MJ G Ⅲartefact layer (Fig17), which may make the Olduvai normal polarity event absent. The question here comes that the normal polarity event measured from 10 m depth under MJ G Ⅲ (Zhu et al, 2004) is or not the real Olduvai event.
Without considering the absence of strata, it is easy to make an oversight on estimating the age by calculating the average deposit velocity between the two normal polarity events. As a matter of fact, the deposit velocity of the total 63m thick section of Majuangou Site should not be homogeneous, especially in its lower part with some deposit intervals. The magnetic dating, therefore, seems to be too young.

Key words: Majuangou Site; Nihewan Basin; Fossil mammals; Biostratigraphic and magnetic age