6
REPORTS Chinese Science Bulletin 2003 Vol. 48 No.2 188 193 Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau SONG Chunhui 1,2 , FANG Xiaomin 1,2 , GAO Junping 1 , NIE Junsheng 1 , YAN Maodu 1,3 , XU Xianhai 1 & SUN Dong 1 1. MOE National Laboratory of Western China’s Environmental Systems & College of Resource and Environment, Lanzhou University, Lan- zhou 730000, China; 2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China; 3. Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA Abstract A number of fossil mammals have been found in the very thick Cenozoic stratigraphy of the Guide Basin in the northeastern margin of the Tibetan Plateau. Some of these are of great significance in mammal evolution and stratigraphic correlation on and around the Tibetan Plateau and North China. However, the chronology of these mam- mals is poorly constrained. Dating of the mammals will not only place precise age constraints on the mammals, but also provide much information on the related stratigraphy that records the uplift process of the Tibetan Plateau. Detailed paleomagnetism of the upper part of the Cenozoic stratigra- phy at He’erjia and Lajigai north of Guide County has re- vealed magnetic chrons that can be correlated to Gauss and 3An chrons, determining the section spanning about 3.1 6.5 Ma and the first, second and third layers of fossil mammals at about 5.25, 5.1 and 4.4 Ma, respectively. Ages of the sig- nificant genus Gazella kueitensis and the Chinese elephant Anancus sinensis are firstly constrained at about 5.25 MaBP and 4.4 Ma, respectively. The mammalian evolution and the associated increase in coarse sediments and sedimentation rate may suggest that the northeastern Tibetan Plateau was uplifted rapidly at that time, and the eastern Tibetan Plateau with its neighboring regions was not high enough to stop mammal exchange between the northern and southern sides of the Tibetan Plateau. Keywords: Tibetan Plateau, Guide Basin, Late Cenozoic stratigra- phy, fossil mammals, paleomagnetism. Great progress has been made with respect to the dating of the Cenozoic eolian sediments and red beds of north and northwest China as part of an overall study of global change, the Asian monsoon and uplift of the Ti- betan Plateau [1 4] . However, absolute ages of many Ce- nozoic fossil mammals and their related stratigraphy are poorly constrained, in contrast to great achievements from paleontological studies of Chinese Cenozoic fossil mam- mals since the 1980s [5 11] . This hinders further develop- ment of Chinese Cenozoic paleontology. Consequently, dating of Cenozoic stratigraphy containing typical Chi- nese fossil mammals in North and West China, especially dating of the appearance and extinction of each genus of chronological significance, and building a complete evo- lutional sequence of Cenozoic fossil mammals of North China from absolute ages will promote further studies of Chinese Cenozoic paleontological stratigraphy, biological evolution and environmental change. We first carried out a paleomagnetic study of the Late Cenozoic stratigraphy in the Guide Basin, Qinghai Province, since these strata contain many fossil mammals, including the Chinese old elephants Anancus sinensis and Anancus cuneatus [6] . These elephants are very important components of Cenozoic fossil mammals and were widely distributed over the whole Eurasian continent and the northern and southern Tibetan Plateau. They appeared within a relatively narrow time range of evolution, hence they are chronologically significant and valuable for stratigraphic correlation. But their absolute ages have not been exactly determined in China. Zheng and his co- workers suggested a Pliocene age for these fossil ele- phants based on the facts that these kinds of elephants out of the Guide Basin are found to occur only in the Baodean Red Clay and Quaternary loess and that the Guide Anancus sinensis can be correlated in characteristics to the similar species from the layer “Yushe ” in the Yushe Basin [6] . It is now believed that Pentalophodon simplex and P. pingliangensis that fall into the same genus as Anancus sinensis and Anancus cuneatus, occurring at the end of the Pliocene or at the beginning of the Pleisto- cene [7—9] . Thus, dating of these two elephants not only places age constraints on the stratigraphy in the Guide Basin and other similar basins in the northeastern Tibetan Plateau, but also provides valuable information on the evolution of mammals between the southern and northern Tibetan Plateau, which have a close relationship with the plateau uplift. 1 Stratigraphy and paleontology fossil layers The Guide Basin is located between the western Songba Gorge and the eastern Longyang Gorge in the upper reaches of the Huanghe River (Yellow River) in northeastern Qinghai Province. It is bounded by the Qinghai Nan Shan (Mts.) and the western Laji Shan (Mts.) to the north, the Baji Shan (Mts.) to the south, the Wali- gong Shan (Mts.) to the west, and the Zamazari Shan (Mts.) to the east. The basin surface has an average alti- tude of about 3100 m and cut down by the Huanghe River by 900 m, forming a set of terraces (at least T1 T7). Guide City sits on terrace T1 at an altitude of 2230 m (Fig. 1). The stratigraphy of the Guide Basin belongs to the Zama Stratigraphic Sub-Province, Songpan Strati- graphic Province, Qilian-Helan Stratigraphic Great Province [10] . Proterozoic, Triassic, Tertiary and Quaternary 188 Chinese Science Bulletin Vol. 48 No. 2 January 2003

Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

Embed Size (px)

Citation preview

Page 1: Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

REPORTS

Chinese Science Bulletin 2003 Vol. 48 No.2 188 193 Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

SONG Chunhui1,2, FANG Xiaomin1,2, GAO Junping1, NIE Junsheng1, YAN Maodu1,3, XU Xianhai1 & SUN Dong1

1. MOE National Laboratory of Western China’s Environmental Systems & College of Resource and Environment, Lanzhou University, Lan-zhou 730000, China;

2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China;

3. Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA

Abstract A number of fossil mammals have been found in the very thick Cenozoic stratigraphy of the Guide Basin in the northeastern margin of the Tibetan Plateau. Some of these are of great significance in mammal evolution and stratigraphic correlation on and around the Tibetan Plateau and North China. However, the chronology of these mam-mals is poorly constrained. Dating of the mammals will not only place precise age constraints on the mammals, but also provide much information on the related stratigraphy that records the uplift process of the Tibetan Plateau. Detailed paleomagnetism of the upper part of the Cenozoic stratigra-phy at He’erjia and Lajigai north of Guide County has re-vealed magnetic chrons that can be correlated to Gauss and 3An chrons, determining the section spanning about 3.1 6.5 Ma and the first, second and third layers of fossil mammals at about 5.25, 5.1 and 4.4 Ma, respectively. Ages of the sig-nificant genus Gazella kueitensis and the Chinese elephant Anancus sinensis are firstly constrained at about 5.25 MaBP and 4.4 Ma, respectively. The mammalian evolution and the associated increase in coarse sediments and sedimentation rate may suggest that the northeastern Tibetan Plateau was uplifted rapidly at that time, and the eastern Tibetan Plateau with its neighboring regions was not high enough to stop mammal exchange between the northern and southern sides of the Tibetan Plateau.

Keywords: Tibetan Plateau, Guide Basin, Late Cenozoic stratigra-phy, fossil mammals, paleomagnetism.

Great progress has been made with respect to the dating of the Cenozoic eolian sediments and red beds of north and northwest China as part of an overall study of global change, the Asian monsoon and uplift of the Ti-betan Plateau[1 4]. However, absolute ages of many Ce-nozoic fossil mammals and their related stratigraphy are poorly constrained, in contrast to great achievements from paleontological studies of Chinese Cenozoic fossil mam-mals since the 1980s[5 11]. This hinders further develop-

ment of Chinese Cenozoic paleontology. Consequently, dating of Cenozoic stratigraphy containing typical Chi-nese fossil mammals in North and West China, especially dating of the appearance and extinction of each genus of chronological significance, and building a complete evo-lutional sequence of Cenozoic fossil mammals of North China from absolute ages will promote further studies of Chinese Cenozoic paleontological stratigraphy, biological evolution and environmental change. We first carried out a paleomagnetic study of the Late Cenozoic stratigraphy in the Guide Basin, Qinghai Province, since these strata contain many fossil mammals, including the Chinese old elephants Anancus sinensis and Anancus cuneatus[6]. These elephants are very important components of Cenozoic fossil mammals and were widely distributed over the whole Eurasian continent and the northern and southern Tibetan Plateau. They appeared within a relatively narrow time range of evolution, hence they are chronologically significant and valuable for stratigraphic correlation. But their absolute ages have not been exactly determined in China. Zheng and his co- workers suggested a Pliocene age for these fossil ele-phants based on the facts that these kinds of elephants out of the Guide Basin are found to occur only in the Baodean Red Clay and Quaternary loess and that the Guide Anancus sinensis can be correlated in characteristics to the similar species from the layer “Yushe ” in the Yushe Basin[6]. It is now believed that Pentalophodon simplex and P. pingliangensis that fall into the same genus as Anancus sinensis and Anancus cuneatus, occurring at the end of the Pliocene or at the beginning of the Pleisto-cene[7—9]. Thus, dating of these two elephants not only places age constraints on the stratigraphy in the Guide Basin and other similar basins in the northeastern Tibetan Plateau, but also provides valuable information on the evolution of mammals between the southern and northern Tibetan Plateau, which have a close relationship with the plateau uplift.

1 Stratigraphy and paleontology fossil layers

The Guide Basin is located between the western Songba Gorge and the eastern Longyang Gorge in the upper reaches of the Huanghe River (Yellow River) in northeastern Qinghai Province. It is bounded by the Qinghai Nan Shan (Mts.) and the western Laji Shan (Mts.) to the north, the Baji Shan (Mts.) to the south, the Wali-gong Shan (Mts.) to the west, and the Zamazari Shan (Mts.) to the east. The basin surface has an average alti-tude of about 3100 m and cut down by the Huanghe River by 900 m, forming a set of terraces (at least T1 T7). Guide City sits on terrace T1 at an altitude of 2230 m (Fig. 1). The stratigraphy of the Guide Basin belongs to the Zama Stratigraphic Sub-Province, Songpan Strati-graphic Province, Qilian-Helan Stratigraphic Great Province[10]. Proterozoic, Triassic, Tertiary and Quaternary

188 Chinese Science Bulletin Vol. 48 No. 2 January 2003

Page 2: Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

Fig. 1. Location map of the Erdaogou-Lajigai section (upper) and the measured Erdaogou cross section (lower).

stratigraphy is exposed in the basin. The Tertiary strati-graphy is very thick and is widely distributed at an angu-lar unconformity with the underlying Proterozoic and Tri-assic strata or at pseudoconformity or conformity with the overlying Quaternary strata. In 1885, L. V. Loczy named a set of Cenozoic piedmont-fluviolacustrine red beds the Guide Series or Guide Formation and estimated their ages in the Pliocene according to Myospalax arvicolinus found in the red beds. Recently, some researchers suggested dif-ferent divisions of the red beds. The fluviolacustrine sediments containing a typical Hipparion fauna at He’erjia were regarded as the Xiadongshan Formation, and the above coarse fluviolacustrine sediments as the Shangtan Formation[12 14]. A huge number of fossil mam-mals were found in the Xiadongshan Formation in the Guide and Hualong Basins. The mammals fall into ten genus, but present a simple pattern similar to the typical Baodean (late Miocene) Hipparion fauna in North Chi-na[13], such as, Sinocastor sp., Gomphotherium sp., Hip-parion platyodus, Chilotherium sp., Axis sp., Palaeotra-gus cf. decipiens and Gazella gaudryi. The fossil mam-mals found in the Shangtan Formation at He’erjia fall into eight genus, mainly Anancus sinensis, A. cuneatus, Hap-parion cf. tchicoicum, Axis shansiulis, Chilotherium sp., Gazella kueitensis, Mymomys sp., Felis sp., Ochotona sp. and Leporidae indet.[6,13]. Anancus sinensisv and A. cuneatus are never found in the Baodean Hipparion Red Earth[6], whereas Hipparion cf. tchicoicum is regarded as a Hipparion with changed primitive features which are only seen in the Ruscinian or Gaozhuang strata[13], suggesting an age of the Pliocene for the Shangtan Formation[6,13]. All of these studies provide general chronolgical constraints on detailed magnetostratigraphy. Our observations[14] show that the Cenozoic strati-

graphy in the Guide Basin is composed of three distinct lithologic parts. The first (lower) part is the distinct pur-ple-reddish Eogene sand conglomerate, sandstone and silty mudstone exposed in the margin of the basin, such as, at Ashigong. The second (middle) part is a motley (dark red, grayish, greenish, and brown) mudstone and siltstone intercalated with some sandstone and grayish-bluish mar-lite exposed in the central basin. The third (upper) part is huge thick bed of grayish boulder conglomerate exposed over the basin. The upper and middle parts of the strati-graphy are well exposed along the Nongchun River (Fig. 1). At the mouth of the river are motley mudstones and siltstones, exposed in the Erdaogou section at He’erjia and the Lajigai section, where many fossil mammals were excavated previously. These strata belong in order to the top of the second part of the stratigraphy and have a large change in facies to the north (to the head of the river or to the margin of the basin), from motley mudstone to light brownish siltstone, grayish, dull yellowish sandstone and conglomerate. Above this strata is thick boulder conglom-erate, well exposed in the area from Duoleichang to Gan-jia, representing rapid uplift of the Tibetan Plateau (Fig. 1). Hence detailed dating of the mammal-containing Er-daogou and Lajigai sections can provide valuable infor-mation regarding the stratigraphic division of the basin and its adjacent region, Tibetan Plateau uplift, and pale-ontologic evolution. The measured Erdaogou section is 227 m thick, con-taining three layers of fossil mammals. The stratigraphy in the section can be divided into 14 lithofacies associations (LAS11 LAS14, which are further subdivided into 55 successions) according to lithology, sedimentary structure and fossil mammals (Figs. 1 and 3). The characteristics of the stratigraphy and mammals are briefly described below,

Chinese Science Bulletin Vol. 48 No. 2 January 2003 189

Page 3: Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

REPORTS

with a comparison to the former division by Zheng et al.[6]. LAS14. Brownish red mudstone intercalated with gray-ish-greenish mudstone, 12 m thick; 2 Magnetostratigraphy and ages of fossil mammals LAS12 13. Grayish-green fine sandstone and brownish red silty mudstone intercalated with fine gravelly sandstone (the whole sequence can be subdivided into three layers correspond-ing to successions 31 33 of Zheng et al.[6]), 16 m; LAS11. Grayish-green mudstone intercalated with gray-ish-greenish sandstone (subdivided into three layers, corre-sponding to succession 30 of Zheng et al.[6]), 8.5 m; LAS10. Interbedded brownish yellow and grayish-green sandstones, containing the third layer of fossil mammals: Anancus sinensis, Axis shansiulis and Gazella kueitensis (corre-sponding to succession 29 of Zheng et al.[6]), 14.5 m; LAS9 8. Grayish-blue, brownish red mudstones interca-lated with brownish yellow, grayish-green fine sandstone, con-taining snails, shells, mussels and fragmented fossil mammals (subdivided into ten layers, corresponding to successions 23 28 of Zheng et al.[6], 30.5 m; LAS7. Brownish red and grayish-green mudstones inter-calated with thick grayish or brownish yellow fine gravelly sandstones and sandstones (subdivided into 5 layers, corre-sponding to succession 18 22 of Zheng et al.[6]), 35 m; LAS6. Brownish yellow, grayish fine gravelly sandstones with some muddy siltstone at the top, containing the second layer of fossil mammals: Mymomys sp., Ochotona sp. and Ga-zella kueitensis (corresponding to succession 17 of Zheng et al.[6]), 5.5 m; LAS5. Grayish-green, brownish red mudstone and silty mudstone intercalated with grayish sandstone (subdivided into six layers, corresponding to successions 11 16 of Zheng et al.[6]), 15 m; LAS4. Grayish mudstone, containing the first layer of fossil mammals: Hipparion sp., Chilotherium sp., Gazella kueitensis and Axis shansius (corresponding to succession 10 of Zheng et al.[6]), 3.5 m; LAS2 3. Interbedded grayish-green and brownish red mudstones intercalated with grayish fine gravelly sandstone and sandstone, containing fossil snails and mussels (divided into 16 layers, corresponding to successions 1—9 of Zheng et al.[6]), 56 m; LAS1. Interbedded grayish-green, brownish red mudstones and grayish fine gravelly sandstone, containing fossil snails and mussels (subdivided into 9 layers), 20 m. The measured Lajigai section is 180 m thick and is divided upwards into 9 lithofacies associations (LAS11LAS19) (Fig. 3) as follows: LAS11 14. Same as the LAS11 LAS14 in the Erdaogou section above, 45 m thick; LAS15. Dull yellowish, light reddish mudstone, siltstone and sandstones intercalated with some thin sandy fine conglom-erate, containing fossil mammals: Hipparion cf. tchicoicum, Hipparion sp., 55 m; LAS16. Interbedded dull yellowish, light reddish siltstone and sandy fine conglomerate, 16 m; LAS17. Thick grayish sandy fine conglomerate interca-lated with dull yellowish sandstone, 14 m; LAS18. Interbedded dull yellowish, grayish siltstones,

sandstones and fine conglomerate, 25 m; LAS19. Thick grayish conglomerate intercalated with dull yellowish sandstone, 25 m.

We collected samples at 2 m intervals in the Er-daogou section at He’erjia temple and at 2 3 m intervals in the Lajigai section, yielding a total of 180 oriented block samples. Each block sample was cut into three ori-ented cubic samples at 2 cm × 2 cm × 2 cm. The first and second sets of the cubic samples from the Erdaogou sec-tion and the first set of cubic samples from the Lajigai section were subject to thermal demagnetization using the American ASC TD-48 thermal demagnetizer in the MOE National Laboratory of Western China’s Environmental Systems, Lanzhou University. Remanent magnetization was measured on an AGICO JR-5A spinner magnetometer. Both the thermal demagnetizer and magnetometer are placed in a large Helmholtz magnetic shielded cage with geomagnetic field less than 20 milligausses in the central area, which is 20 cm in diameter. Representative samples from different lithologies in the section were chosen for 18 steps of progressive thermal demagnetization from 25 to 685 to understand remanence-carrying mag-netic minerals. Most samples show an obvious change of magnetic intensity and direction at about 200 , indicat-ing the removal of a viscous remanent magnetization; after 350 400 , some after 500 , remanence shows a stable direction to the original, with two intensity drops at 580 and 685 , suggesting that a characteristic rema-nence is isolated after those temperatures and that mag-netite and hematite are probably the major remanence carrying minerals (Fig. 2). Thus, the rest of the samples were subjected to 2 3 steps of progressive thermal de-magnetizations at temperatures that would can isolate the characteristic remanent magnetization. The third set of the cubic samples from the Erdaogou section and the second set of the cubic samples from the Lajigai section, total at more than 200, were subjected to 4 steps (550, 580, 600 and 650 ) of thermal demagnetization in the Laboratory of Paleomagnetism in the University of Michigan, then were put on a 2G cryogenic magnetometer in a magneti-cally shielded room for remanent magnetization meas-urement. Final characteristic paleomagnetic directions were obtained after Fisher average of directions from the three sets of sub-cubic samples from each level and then were used to calculate the virtual geomagnetic polarity (VGP) that are plotted as a function of thickness in Fig. 3. Six clear normal polarity zones (N1 N6) and six reversed ones (R1 R6), 3 clear normal zones (N1 N3) and 2 reversed ones (R1 R2) are detected in the Er-daogou and Lajigai sections, respectively (Fig. 3). Taking into account the fossil mammals, this suggests a Pliocene

190 Chinese Science Bulletin Vol. 48 No. 2 January 2003

Page 4: Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

Fig. 2. Progressive thermal demagnetization of the representative samples in the Erdaogou section, He’erjia.

age for the stratigraphy. The observed magnetozones in the Erdaogou section are found to have good match with the chrons between the Gilbert and 3An of the geomag-netic polarity timescale (GPTS) of Cande and Kent[15]. N1

N4 correlate with the four subchrons in the Gilbert re-versed polarity chron, i.e. the Nunivak, Cotchiti, Sidufall and Thvera, respectively, and the R1 R5 correlate with the reversed zones in the Gilbert, and the rest N5, N6 and R6 correlate with chrons 3An.1n, 3An.2n and 3Ar, respec-tively (Fig. 3). The normal polarity zone N3 at the bottom of the Lajigai section should also record the subchron Nunivak, because it occurs in the same marker bed of sandstone as N1 in the Erdaogou section. Thus, the R2 in the Lajigai section should be correlated with the top part of the Gilbert reversed polarity chron (Fig. 3). The upper and middle part of the Lajigai section is characterized largely by long normal zones. We regard them as records of the Gauss normal chron, with the R1 correlating to the Mammoth subchron, rather than the combination of the Kaena and Mammoth subchrons. The relatively long span of R1 in the Lajigai section may be caused by the fast increase of sedimentation rate as indicated by increasing size and content of gravels from the bottom of the N2 (Figs. 3 and 4) (note over 300 m of boulder conglomerate is observed on the measured Lajigai section). This huge thick bed of conglomerate can be correlated with the Jishi conglomerate in the nearby Linxia Basin which paleo-magnetically spans the whole Gauss chron[4]. Thus, the N1 is the analogue of the normal polarity subchron between Kaena and Mammoth (Fig. 3). The thickness vs. age plot of the interpreted chrons shows a linear relationship with sedimentation rate, which increases significantly from since about 6.2 Ma, 5.2 Ma and 3.6 Ma (Fig. 4). This co-incides with the fact that coarse sandstone and conglom-erate are deposited at those times. All these indicate that the above magnetic interpretation is plausible. Thus, the whole measured stratigraphic sequence is estimated to span from about 6.5 3.1 Ma and the first, second and third fossil mammal layers are determined at about 5.25,

5.1 and 4.4 Ma, respectively. 3 Discussion and conclusion

Bohlin studied fossil antelope in the Guide Basin in 1938 and named it Gazella kueitensis[16]. But because its exact locality in the section was unknown, later research-ers only could roughly estimate its age in the Villa-franchean of the European terrestrial mammal stages. Zheng and his colleagues excavated Gazella kueitensis in the first, second and third layers of fossil mammals in the Erdaogou section at He’erjia, which allowed us to deter-mine its exact stratigraphic level and its approximate age[6]. Later this mammal was found by other researchers in the Pliocene stratigraphy in other places. Our paleo-magnetic measurements determine its living age at about 5.3 4.4 Ma, namely at the beginning of the Pliocene, enabling these mammal remains to be useful for strati-graphic correlation. The Anancus sinensis in the third layer of fossil mammals in the Erdaogou section is a widely-distributed mammal in North China, and thus it is very significant for stratigraphic correlation. But its precise age has not been known. Our paleomagnetism dates its age at about 4.4 Ma, namely during the late Early Pliocene. Since similar Anancus was found in the Siwalik stratigraphy in South Asia in the south margin of the Himalayan-Tibetan Pla-teau, we estimate that the eastern Tibetan Plateau and its neighboring mountains were not high enough to prevent this mammal exchange between the southern and northern Tibetan Plateau at the latest, about 4.4 Ma. This inference coincides with the plateau uplift history suggested by the changes of sedimentation rate and lithology. The occur-rence of a hugely thick set of boulder conglomerates in the upper Cenozoic stratigraphy of the Guide Basin (Fig. 3) suggests rapid uplift of the Tibetan Plateau. The Er-daogou section is under this set of conglomerates, and changes in facies to interbedded sandstone and conglom-erate toward the mountains. Under the Erdaogou section are grayish-greenish lacustrine fine sediments that are widely distributed over the basin including the marginal

Chinese Science Bulletin Vol. 48 No. 2 January 2003 191

Page 5: Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

REPORTS

Fig.

3.

Cor

rela

tion

of m

agne

tozo

nes

of t

he E

rdao

gou

and

Laj

igai

sec

tions

with

the

geo

mag

netic

pol

arity

tim

esca

le (

GPT

S)[1

5].

(a),

(c)

, (e

) T

he

stra

tigra

phy,

mag

netic

dir

ectio

ns a

nd z

one

of th

e E

rdao

gou

sect

ion;

(b)

, (d)

, (f)

the

stra

tigra

phy,

mag

netic

dir

ectio

ns a

nd z

one

of th

e L

ajig

ai s

ectio

n.

192 Chinese Science Bulletin Vol. 48 No. 2 January 2003

Page 6: Magnetostratigraphy of Late Cenozoic fossil mammals in the northeastern margin of the Tibetan Plateau

Fig. 4. The thickness-age relationship of the interpreted chrons from the Erdaogou and Lajigai sections and their determined sedimentation rate change.

areas of the mountains, indicating a lowland sedimentary environment. Therefore, the stratigraphy in the Erdaogou section just represents a transitional stage for the north-eastern Tibetan Plateau, rising slowly from a lowland en-vironment, then rapidly for a high plateau. Change of the sedimentation rate in the Erdaogou and Lajigai sections lends further support to the above inference, which dem-onstrates clearly that the sedimentation rate began to in-crease from about 5.2 Ma, and to rapidly increase by over twofold since 3.6 Ma (Fig. 4). Thus, both mammalian evolution and changes of sedimentary environment and rate imply that the northeastern Tibetan Plateau was not high enough to prevent the migration of animals between the southern and northern plateau in the early Pliocene, in good agreement with our previous conclusion that the northeastern Tibetan Plateau began to uplift rapidly from about 3.6 Ma as suggested by geomorphologic and strati-graphic evolution of the Linxia Basin[17].

Acknowledgements The authors would like to thank Profs. Zheng Shaohua, Qiu Zhanxiang and Gu Zhugang for their helpful comments, and Profs. Li Jijun and An Zhisheng for their reviewing the manuscript. Many thanks are also due to Miao Yunfa, Bai Jinfang, Yang Shengli and others for their field assistance. This work was co-supported by the Na-tional Natural Science Foundation of China (Grant No. 49871010), the “Hundred Talents Project of the Chinese Academy of Sciences (Ren-Jiao-Zi[2000]005) and the National Key Basic Research Project on the Tibetan Plateau (Grant No. G1998040809).

References

1. Liu Tungsheng, Loess and the Environment (in Chinese), BeijingScience Press, 1985, 245 247.

2. Yue, L. P., Zhang, Y. X., Hipparion fauna and magnetostratigra-phy in Hefeng, Jingle, Shanxi Province, Vertebrata Palasiatica (in Chinese with English abstract), 1998, 36(1): 76.

3. Sun, D. H., Liu, T. S., Cheng, M. Y. et al., Magnetostratigraphy and paleoclimate of red-clay sequences from the Chinese Loess Plateau, Sci. China, Ser. D, 1997, 40: 337.

4. Fang, X. M., Li, J. J., Zhu, J. J. et al., Chronology and division of

the Cenozoic stratigraphy in the Linxia Basin of Gansu Province, Chin. Sci. Bull. (in Chinese), 1997, 42 (14): 1457.

5. Qiu, Z. X., Huang, W. L., Guo, Z. H., Fossil Hipparion in China, Chinese Paleontologic Records (175) (in Chinese), Beijing: Geo-logical Publishing House, 1987, 225

6. Zheng, S. H., Wu, W. Y., Li, Y., Late Cenozoic mammalian faunas of Guide and Gonghe Basins, Qinghai Province Vertebrata Palasi-atica (in Chinese with English abstract), 1985, 23(2): 89.

7. Xue, X. X., Notes on two new Brevirostrine mastodonts from Shensi, Vertebrata Palasiatica (in Chinese with English abstract), 1962, 6(2): 173.

8. Xue, X. X., Yue, L. P., Zhang, X. Y., Comparison of boundaries of magnetic, biologic and lithologic stratigraphy, in the Shanyang Basin, Sci. Chin., Ser. B, 1994, 24(4): 413.

9. Zong, G. F., A new species of pentalophodon from Pingliang, Kansu, Vertebrata Palasiatica (in Chinese with English abstract), 1979, 17(1): 81.

10. Li, C. K, Wu, W. Y., Qiu, Z. D., Chinese Neogene: subdivision and correlation, Vertebrata Palasiatica (in Chinese with English abstract), 1984, 22(3): 163.

11. Tedford, R. H., Flynn L. J., Qiu, Z. X., Standards of mammalian biologic and magnetic stratigraphy of Cenozoic basins, Vertebrata Palasiatica, 1991, 11(4): 519.

12. Geological and Resources Bureau of Qinghai Province, Regional Geology of Qinghai Province (in Chinese), Beijing: Geological Publishing House, 1985.

13. Gu, Z. G., Bai, S. H., Zhang, X. T. et al., Division and correlation of the Neogene in the Guide and Hualong Basins of Qinghai Prov-ince, J. Stratigr. (in Chinese), 1992, 16(2): 96.

14. Song, C. H., Fang, X. M., Gao, J. P. et al., Cenozoic tectonic uplift and sedimentary evolution of the Guide Basin in the northeastern margin of the Tibetan Plateau, J. Sediment. (in Chinese with Eng-lish abstract), 2001, 19(4): 496.

15. Cande , S. C., Kent, D. V. , Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic, J. Geo-phys. Res., 1995, 100: 6093.

16. Bohlin, B., Einige Jungtertiäre und Pleistozäne Caviconier aus Nord-China, Nova Acta Reg. Soc. Sc. Upsala, Sr. , 1938, 11(2): 140.

17. Li, J. J., Fang, X. M., Uplift of Qinghai-Tibetan Plateau and En-vironmental Change, Chin. Sci. Bull., 1999, 44(23): 2217.

(Received June 25, 2002)

Chinese Science Bulletin Vol. 48 No. 2 January 2003 193