PROGRESS IN NATURAL SCIENCE Vol .13 , No .12 , December 2003

Sinian-Cambrian stratigraphic framework for shallow- to deep-water environments of the Yangtze Platform:an integrated approach＊

ZHU Maoy an1 ＊＊ , ZHANG Junming1 , Michael STEIN ER2 , YANG Aihua1 , LI Guoxiang1 and Bernd D .ERDTMANN 2 (1 .LPS , Nanjing Institute of Geology & Palaeontology , Chinese Academy of S ciences, Nanjing 210008 , ;2 .TU Berlin , Sekr . ACK 14 , Ackerstrasse 71-76 , D-13355 Berlin , Germany)

Received June 8 , 2003 ;revised July 7 , 2003

Abstract The Sinian (Terminal Proterozoic)and Early Cambrian shallow- to deep-water sequences of the Yangtze Platform w ere investigated .Based on integrated lithostratigraphic, biostratigraphic, and other approaches, the shallow-w ater sequence from the base of the Sinian (base of the Doushan tuo Formation)to the top of the Qiongzhusian (top of the Yu' anshan Formation)w as subdivided into 12 stratigraphic intervals.These 12 intervals w ere applied in turn to the subdivision and correlation of the sequences present in various facies of the Yangtze Platform .The high-resolution stratigraphic framework here developed can serve as a time frame for ongoing multidisci- plinary analyses of the “ Cambrian explosion” .

Keywords: Neoproterozoic, Sinian, Cambrian, stratigraphy , Cambrian Explosion, Yangtze Platform.

Recent paleontolog ical investigations of the terozoic of Australia[ 5] , SSF zonations fo r the Early Cheng jiang fauna , sm all shelly fossils (SSF' s), and Cambrian of South China[ 6] , and Early Cambrian the Weng' an biota have together demonstrated that biozonations of Siberia[ 7] , interbasinal and global cor- the Sinian (Terminal Proterozoic)-Cambrian transi- relations of these biozones are few in number and un- tion interval of the Yang tze Platform (South China) certain .Due mainly to low species diversity , tax o- is extremely impo rtant for understanding the “ Cam- nomic uncertainty , and sporadic and endemic paleo- brian explosion ,” a major event in the evolutio n of geog raphic distributions , the usefulness of biostrati- life .Sinian-Cambrian sequences ranging in o rigin graphic approaches (standard for the Phanerozoic)in from shallow to deep marine are w ell developed and developing a g lobal time scale fo r the Terminal Pro- exposed throug hout this region .Fo r this reason , an terozoic and Early Cambrian is limited .Therefore , increasing number of g lobal research programs em- other stratigraphic tools must be applied .Based on ploying multidisciplinary analyses of this critical geo- bio-, litho-, chemo-, sequence , and event strati- logical interval have focused on the Yangtze Platform . graphic principles, the present investigation attempts How ever , as on other continents, Sinian-Cambrian to establish a high-resolution stratig raphic framewo rk fossil material from the Yangtze Platfo rm occurs in fo r this interval on the Yangtze Platform . sequences developed in a v ariety of paleoenviron- ments .Because correlation of these facies has not pre- 1 Sinian stratigraphy viously been co nducted at fine scales[ 1, 2] , the tempo- ral and spatial relationships of these sequences gener- Since the designation of the Yang tze Go rges as [ 8] ally are uncertain . Therefore , a high-resolution the ty pe locality for the Sinian System , the Sinian stratigraphy for the Terminal Pro terozoic-Cambrian sections of that area have been investigated intensive- [ 9 ～ 11] transitional interval is needed urgently .Despite global ly .T raditionally , the Sinian has been interpret- stratigraphic studies co nducted during the past several ed as consisting of the interval between the top of the decades, subdivision and correlation of the Terminal Huangling G ranite and the base of the Cambrian Sy s- Pro terozoic and Early Cambrian remain unclear[ 3 , 4] . tem .In addition , the type section fo r the Sinian has Although various biostratigraphic zonations have been consisted of ex posures along the Yangtze River from proposed (e .g .Acritarch zo natio ns for the Neopro- Liantuo to Wang jiaping , in the eastern part of the

＊ Supported by the National Natural Science Foundation of China (Grant Nos.40232020 , 40172002), the Chinese Ministry of Science and Tech- nology (Grant No .G2000077700), and the Deustche Forchungemeinschaft (Grant No.Er 96132-1) ＊＊ To w hom correspondence should be addressed .E-mail:myzhu @nigpas.ac.cn 952 Progress in Natu ral Science Vol.13 No .12 2003

Yang tze Gorges .In 2001 , the National Stratigraphic the Doushantuo Formation (black shales containing Commission of China presented a new definition of the Miaohe Biota[ 19 ～ 21] ;(5)The Hamajing Member the Sinian Sy stem , restricting this unit to the interval of the Dengy ing Fo rmation (thick bedded dolomite between the top of the Nantuo Tillite and the base of w ith oolites and o ncolites);(6)The Shibantan M em- the Meishucunian S tage (the base of the Tianzhushan ber of the Dengying Formation (dark , laminated M ember of the Dengying Fo rmation)[ 12] .Because of argillaceous limestone rich in Vendotaenia and simple the construction of the great dam of the Yangtze ho rizontal trace fossils (the Xilingxia Biota , see Ref . Gorges, many new sections have been exposed and 22);(7) The Baimatuo Member of the Dengying studied[ 13～ 18] .These studies indicate that the ty pe Formation (thick-bedded dolomite). section from Liantuo to Wang jiaping has two serious shortcoming s :the Doushantuo Formation is not well The foregoing subdivisions of the Sinian are used exposed, and the upper co ntact with the Cambrian here as a stratig raphic standard for subdividing and System is an obvious unconformity . correlating Sinian sequences throughout the Yang tze Platfo rm .Together these sequences represent a v ari- The results of our stratigraphic investigations ety of facies ranging from shallow to deep m arine . show that the sections in the southwestern part of the Mo re than 20 Sinian sections were investig ated along Huang ling Anticline are better developed and ex posed two transects from the inner-platform (shallow-water than the ty pe section .Based on examination of the environment)to the platform slope (deep-w ater envi- sections at Heng duny an (along the new road from ronment)(Fig .1).The Sinian sections from the M aoping to Zigui), Wuhe -Yangjiahe , and Hezi' ao Yangtze Gorges area throug h no rthw est to (Changyan), we have adopted Wang et al.' s[ 2] sug- west and central Hunan (Fig .2), and the sections gestion that the Sinian rocks of the Yang tze Gorges extending from w est of the platfo rm (eastern Yunnan can be subdivided into 7 intervals .Listed in ascending and southern Shaanxi)through southwest to o rder , these are :(1)Member 1 of the Doushantuo southeast Guizhou (Fig .3), were subdivided and Formation (the cap dolomite above the Nantuo roughly correlated .Because of the limitation of the Tillite);(2)Member 2 of the Doushantuo Formation space , our descriptions of these sections w ill be given (dark argillaceous shales with intercalations of argilla- in separate publications .Here w e present two prelim- ceous dolomite);(3)Member 3 of the Doushantuo inary correlatio n charts (Fig s .2 and 3).In these tw o Formation (laminated dolomites);(4)Member 4 of charts , the following rem arks should be noted :

Fig.1 . Simplified palaeogeographic map of the Yangtze Platform du ring Sinian-Camb rian boundary interval, showing tw o transections A and B of investigated sections in Figs.3 and 4 .I =platform interior ;II =transitional zone;III =slope and deep basin . Progress in Natural Science Vol.13 No .12 2003 953

Fig .2 . Sketch chart show ing correlations of the Sinian sections from the Yangtze Gorges to Central Hunan (transection A in Fig .1).1 , Liantuo-W angjiaping section , Yichang , Hubei;2 , Wuhe section , Sandouping , Yichang , Hubei;3 , Zhongling section (Doushantuo Fm)and Yangjiaping section (Dengying Fm), S himen County , NW Hunan ;4 , Tianping section , , Hunan ;5 , Sidouping section , Zhangjiajie , Hunan ;6 , Yanw utan section , , Hunan ;7 , Xixi section , Louxi County , Hunan ;8 , Bajiaotan section , S huangfeng County , Hu- nan .The legends are same as in Fig .3 .

(1)During the Sinian , the Sancha-Tainping- differ from each o ther considerably .In general, the Sidouping area (near Zhang jiajie in Hunan)and the Sinian sequences in the slope area are m uch more con- Yuqing-Taijiang area (Guizhou)w ere located in the densed than that in the basins on the platform , and transition zone betw een the epeiric platfo rm and slope the sequence boundaries in the Yang tze Gorges area environments.The sequences in environments of the are difficult to recognize in the slope area .In particu- inner platform , transitional zone , and the slope area lar , during the interval equivalent to the Dengying 954 Progress in Natu ral Science Vol.13 No .12 2003

Formation , sediments composed of cherts and black stratig raphic markers in the slope area , co rrelation is shales were deposited in the slope area , while carbon- problematic .Other stratigraphic methods are required ate sediments were deposited in the shallow-w ater en- in the future . vironments.Because of the absence of preferred

Fig .3 . S ketch chart showing correlations of the Sinian sections from the w est of the Yangtze Platform to the east of Guizhou (transection B in Fig .1).1 , Piantoushan section , Wangjiaw an , Jinning County , Yunnan ;2 , Hujiaba section , Ninqiang County , S .S haanxi;3 , Songlin section , Zuiyi, Guizhou ;4 , Baidoushan section , Weng' an County , Guizhou ;5 , Yuqing , Guizhou ;6 , W uhe section , , Guizhou ; 7 , Huanglian section , Songtao County , Guizhou ;8 , Xikou section (Doushantuo Fm), , and Bahuang section (Liuchapo Fm) City , Guizhou .Legends:1 , limestone;2 , dolomite ;3 , oolitic dolomite;4 , muddy dolomite;5 , chert ;6 , tillite;7 , shale;8 , siltstone; 9 , phosphatic conglomerate;10, phosphorite ;11 , hiatus;12 , phosphatic nodule ;13 , trilobite. Progress in Natural Science Vol.13 No .12 2003 955

(2)The so-called cap-dolomite at the base of the argillaceous siltstone (the interio r of the Yang tze Sinian is widely recog nized at the base of the Platfo rm ).Oolitic textures and oncolites in the Doushantuo Fo rmation , except in a few basins in the dolomite of the Hamajing Member indicate a high-en- w est (eastern Yunnan and Ning qiang , S .Shaanxi) ergy facies w hich overlies above the black shale at the and in the deeper slope area .The reason for the ab- top of the Doushantuo Fo rmation .The sea-level drop sence of the cap-carbonate in the basin of the Yangtze from the Doushantuo to Dengying Fo rmations can be Platform may be the shallow-water environment with recog nized in all facies on the Yang tze Platfo rm . inputs of clastic sediments from the nearby landmass . Based on this fact , the phospharite interval at the top The absence of the cap-carbonate in the deep slope en- of the Doushantuo Formation in the Yangjiaping area vironment may be due to deposition below the carbon- may be equiv alent to the Hamajing Member in ate compensation boundary .However , further inves- Yangtze Gorges .Similar situation m ay also exist in tigation is required to recognize the equivalent unit of the Weng' an phospharite mine area , w here the upper the cap-carbonate in these sequences . part of the upper phosphorate could be correlated w ith the Ham ajing M ember .How ever , the detailed co rre- (3)A black shale horizon is well developed at lations of the transitional interval of the Doushantuo the top of the Doushantuo Fo rmation in almost all pa- and Dengy ing Formations require more investigations leoenvironments of the Yangtze Platform , as show n in file scales by comprehensive methods in more sec- in Fig .2 .This horizon marks the base of a new se- tions . quence and marine transg ression , and therefo re can be used as a stratigraphic marker .How ever , corre- (6) The base of the middle member of the sponding horizon in the sections from the western Dengying Fo rmation , w hich generally is uncon- Yang tze Platform and central Guizhou requires fur- formable , represents the beginning of a new se- ther detailed w ork on these sections (Fig .3). quence .It is probable that a tectonic event o r sea-lev- (4)During the interval of the Doushantuo For- el change occurred during the interval of the middle mation , phosphorite horizons w ere deposited in cer- Dengying Formation . Therefore , the Shibantan tain limited areas .The best-know n occurrences are (Yangtze Gorges), Gaojiashan (South Shaanxi)and located in the area of the Kaiyang and Wen' an phos- Jiucheng (eastern Yunnan)members of the Dengying phorite mines in central Guizhou .In northwest Hu- Formation can be considered as representing the same nan , the Dongshanfeng phosphorite mine occurs at interval.However , the dark limestone of Shibanan the top of the Doushantuo Form ation in the Yangjiap- Member deposited in the offsho re basin , w hereas the ing area .In central Hunan , phosphorite horizons are siliciclastic rocks of the Gaojiashan and Jiucheng distributed over a large area in the middle of the Members deposited in the coastal basins w ith inputs Doushantuo Formation (fo r instance in the Xixi phos- of clastic sediments from nearby landmass (Figs .2 and 3).Co rrelation suggests that the Xilingxia biota phorite mine).Other occurrences include sections at [ 27] the Caoyang Phosphorite Mine (Shang rao , Jiang xi), is equivalent to the Gaojiashan bio ta . the Baichu Phosphorite M ine (Baokang , Hubei), and (7)Biostratig raphically , Sinian acritarch zona- Chadian (M ianxian , South Shaanxi)[ 23] .The phos- tions have not been recog nized at fine scales. phorite facies is a very significant indicator of paleo- Megafossil zonation is even more difficult .Up to ceanog raphic and paleoenvironmental changes ;how- now , only problematical tube fossils such as Cloudi- ever , detailed correlations of these phosphorite ho ri- na , w hich occurs at the top of the Deng ying Forma- zons are questionable .Because of problematic correla- tion and below the first occurrence of SSF' s , may be tion, the exact age of the w ell preserved algae and considered as a stratig raphic marker .A stage-level u- embryo fossils in the Weng' an biota[ 24～ 26] and other nit of the Sinian w ould be defined by the fist occur- phophorites of the Doushantuo Formation remain un- certain .Therefore , w hether the age of the Weng' an rence of Cloudina , which can be used for global cor- bio ta is equivalent to that of the Miaohe biota is un- relation of the Neoproterozoic III .However , the dis- clear . tribution of Cloudina on the Yang tze Platform is still limited to the south Shaanxi area[ 27, 28] .It would be a (5)The Dengying dolomite is divided into three future focus to look for new occurrence of Cloudina parts by a middle unit that consists of thin-bedded , in other areas of the Yangtze Platfo rm , and to under- argillaceous , laminated carbonates or variegated stand w hat controls distribution of Cloudina . 956 Progress in Natu ral Science Vol.13 No .12 2003

(8)The strong neg ative excursion of C iso topes Meishucunian phosphorite ;(2) The Zhongyicun is detected at the interval betw een the extinction of Member consists of phosphorite and phosphotized Cloudina and first occurrence of SSF' s, w hich has dolomite w ith abundant SSF' s .Two SSF assem- been also globally recog nized .Therefore , we suggest- blages (the Anabarites trisulcatus-Protoherzina an- ed that the negative excursion event could be the best abarica Zone and the Siphogonuchites triangularis- marker for defining an interval at the top of the Sini- Paragloborilus subglobosus Zone)have been recog- an , as w ell as at the top of the Neoproterozoic III in nized ;(3)The Dahai Member consists of dolomite o ther continents . and limestone and contains the Watsonella (Her- aultipegma)yunnanensis Zone , w hich is the third (9)It is uncertain w hether the middle and upper SSF assemblage ;(4)The late Meishucunian interval parts of the Dengying Formation are no t developed in betw een the base of the Shiyantou Fo rmation and the the north and central Guizhou as suggested by Cao et ho rizon containing the first occurrence of trilobites. [ 29] al . , e .g .Songling sectio n in Zuny i, Baidoushan The base of the Shiyantou Formatio n is marked by a section in Weng ' an County and section in Yuqing sharp contact between carbonate facies and black sili- (sections 3 ～ 5 in Fig .3).Correlation of the Dengy- ciclastic sequences .The fourth SSF assemblage (the ing Formation in this region requires further study . Sinosachites flabelliformis-Tannuolina zhangwen- 2 Sinian-Cambrian boundary stratigraphy tangi Zone)is recognized in this interval ;(5)The Qio ngzhusian interval contains the two earliest trilo- The Sinian-Cambrian boundary interval of the bite zones (the Parabadiella Zone and the Eo- Yang tze Platform was investig ated intensively during redlichia-Wutingaspis Zone ) as well as the the 1980s (see Ref .[ 6] ).However , previous studies Chengjiang biota . focused on the biostratigraphy of SSF' s and trace fos- In order to understand the biological and geologi- sils, w hich occur primarily in areas containing expo- cal evolution across the Sinian-Cambrian transition in- sures of shallow-w ater facies .Among these areas , terval , we investigated more than 50 sections in shal- eastern Yunnan is of special im portance because about low- to deep-w ater environments of the Yang tze Plat- 200 m of the pre-trilobite Cambrian sequence is well form .Our results demonstrate that this critical inter- exposed here and contains abundant SSF' s and trace val shows dramatic differences between the sections, fossils .The Sinian-Cambrian boundary interval in and that unconformities occur in nearly all sections eastern Yunnan has traditionally been regarded as the except those in central Hunan and southeast Guizhou , best candidate for the Early Cambrian stratoty pe not w hich w ere located in the deep-w ater slope environ- only of China , but also of the w orld .In addition , the ment .Fig .4 show s preliminary correlations of some M eishucun section is the ty pe section .However , representative sections.In this correlation chart , the there are tw o major unconformities in this section , follow ing important rem arks should be noted : one at the base of the Zhongyicun M ember and the o ther at the top of the Dahai Mem ber .For this rea- (1)Based on trace fossils and SSF' s , the base of son , it has been proposed that sections in the Da- the Cambrian in eastern Yunnan can be placed at the hai[ 30] and Yong shan[ 31] areas in northeastern Yunnan base of the Zhongyicun Member .The base of the be chosen as a new strato type section of the Sinian- Cambrian in other shallow-w ater sections on the plat- Cambrian boundary interval . form generally is marked by the base of a phosphatic ho rizon with SSF' s .In these sections , the base of The Sinian-Cambrian boundary stratigraphy in the Cambrian is an unconformity .The exact place- eastern Yunnan w as review ed in detail by Zhu et [ 32] ment of the Sinian-Cambrian boundary in the transi- al . .Five intervals, w hich can be used as stan- tional sequences of the slope area is uncertain because dards for subdivision and co rrelation of the Sinian- of the lack of SSF' s and o ther stratig raphic marker . Cambrian boundary interval in other areas on the Yang tze Platform , are described in ascending o rder as (2)The best Sinian-Cambrian boundary inter- follow s:(1)The Daibu Mem ber is composed of lami- val, o ne w ithout any obvious unconformities , is locat- nated silicate w ith intercalations of laminated dolomite ed either in a few basins in the western part of the and black shale .The unit represents the interval be- Yangtze Platform or in the slope area of the south- tween the thick-bedded dolomite of the Deng ying eastern part of this platfo rm .The representative sec- Formation and the first occurrence of SSF' s in the tions are those in far no rtheastern Yunnan (the Dahai Progress in Natural Science Vol.13 No .12 2003 957

Fig .4 . S ketch chart show ing correlations of the Sinian - Cambrian boundary interval on the Yangtze Platform .1 , Meishucun section , Jinning County , Yunnan ;2 , Dahai section , Huize County , Yunnan ;3 , Maidiping section , Emei, Sichuang ;4 , Kuanchuangpu section , Ninqiang County , S S haanxi;5 , Gezhongw u section , Zhijin Coun ty , Guizhou ;6 , Heishapo section , S onglin County , Guizhou ;7 , Wangjiaping section , Yichang , Hubei ;8 , Yanjiahe sectrion , Yichang, Hubei;9 , Yangjiaping section , , NW Hunan ;10 , Sancha-Daping section , Zhangjiajie, Hu- nan ;11 , Huanglian section , S ongtao County , Guizhou ;12 , Lijiatuo section , Yuanling County , Hunan .The legends are same as in Fig .3 . and Xiao tan sections), the Emei area (the Meidiping carbonate and siliciclastic rocks .This contact is the section), and central Hunnan (the Lijiatuo section , most easily recognized boundary in nearly all environ- near Yuanling). ments of the Yangtze Platform , and represents a ma- jo r tectonic event . (3) The base of the Shiy antou Formation in eastern Yunnan is marked by a sharp contact betw een (4 ) In eastern Yunnan , the base of the 958 Progress in Natu ral Science Vol.13 No .12 2003

Qionzhusian is defined by the first occurrences of 3 Carbon isotope stratigraphy trilobites. At this same level , bradoriids , bra- [ 35] chiopods , lipped hyolithes , and other distinct shelly Lambert et al. first published a complete fossils also first appear , and these fossils are readily Sinian C iso tope profile based on 33 samples from the distinguishable from the SSF' s in strata immediately fo rmer type section of the Yangtze Gorges , and co rre- below the Qiong zhusian .Therefore , the base of the lated it w ith sections in Siberia and Morocco .Using [ 36] Qiong zhusian can be recog nized in sections containing 98 samples, Yang et al . conducted a more detailed mega-fossils .However , fossils generally are absent in C isotope investig ation of the same section .Because the basalmost Cambrian black shale of the Yangtze of the problems w ith the ty pe section mentioned Platform .In this unit we use a geochemical ho rizon above (e .g .the absence of cap dolomites at the base containing exceptionally high Ni-Mo concentrations as and the presence of a majo r unconformity at the top), a stratigraphic marker fo r defining the base of the the C iso topic curve needs to be revised .Multiple in- vestig ations on the C isotopes of the Sinian in the Qiong zhusian.The reason for this decision is that a [ 17, 18] geochemically corresponding layer occurs at the base Yangtze Gorges were published recently .Al- of the Qiongzhusian in eastern Yunnan , and has been though their samples were collected from the better- w idely detected in the basalmost Cambrian black shale developed sectio ns than the ty pe section , the resolu- of the Yang tze Platfrom .In general , this ho rizon is tion of their data is still very poor .Sinian C isotopic located at the base of the “ stone coal” interval and studies have also been carried out in other few sec- above a black shale interval containing abundant phos- tions ex cept that in the Yangtze Go rges (e .g . Doushantuo Formation at Weng ' an[ 37] ;Dengying phatic nodules .Our investig ations show that the [ 38] black shale interval w ith phosphatic nodules generally Formation in Chengjiang ), but C iso topic data are at the basal part of the Niutitang and Jiumenchong only from the limited intervals sporadically located in Formations is very condensed and is equivalent to the different areas . interv al betw een the base of the Shiyantou Formation As part of our facies analy ses of the Yang tze and the base of the Qiongzhusian in eastern Yunnan Platfo rm , w e selected more complete Sinian sections (Fig .4). w ith stable carbonate facies for hig h-resolution C iso- (5) Integ rated co rrelation indicates that pre- topic study .The Doushantuo Formation at Tianping trilobite Lower Cambrian strata are not well devel- near Zhangjiajie , and the Dengying Formation at oped in shallow-water facies and are condensed in Yangjiaping (no rthw est Hunan ) and Hujiaba in deep-w ater facies .The best , thickest sections are lo- Ningqiang (South Shaanxi), w ere sampled .Prelimi- cated in eastern and northeastern Yunnan (the Zhujia- nary results (Fig .5)show a similar pattern of the Terminal Proterozoic C isotope variation summarized qing and Shitantou Formations), in Emei in central [ 39] Sichuan (the M aidiping Formation and the low er by Jacobsen & Kaufman .In general, our hig h- member of the Jiulaodong Fo rmations), in Ningqiang resolution curve exhibits 3 negative excursions at the in southern Shaanxi (the Kuanchuanpu Formation base , middle and top of the Sinian , respectively , and and the low er member of the Guojiaba Formation), 2 positive excursions in betw een the negative excur- sions.Surprisingly , the curve is exactly identical to and south of the Yang tze Gorges (the Yanjiahe For- [ 3] [ 40] mation and the lower member of the Shuijintuo For- that proposed by Knoll and Shield , demonstrat- mation).These sections represent sequences in the ing that the new C isotopic data , w hich will be pub- basins of platform interior (Fig .4). lished later , can be applied to other Sinian sequences and be used fo r g lobal correlations . (6)The upper boundary of the Qiongzhusian is defined by (i)occurrences of Cang langpuian trilobites Carbon isotopic research on the Precambrian- (the Y iliangella assemblage), (ii)the ex tinction of Cambrian boundary interval has been conducted on the M eishucun and M aidiping sectio ns by Brasier et Eoredlichia and Wutingaspis in shallow-w ater fa- [ 41] [ 42] [ 43 , 44] cies , and (iii)the occurrence of Hupeidiscus in deep- al. , Chen et al . , and Shen et al . .Be- w ater facies .Correlatio n show s that the Qiongzhusian cause the M eishucun section has tw o major unconfor- interv al consists of muddy and silty rocks w ith abun- mities, one at the base of the Zhangyicun Member dant fossils in shallow-w ater facies , and black shales and the other at the top of the Dahai Member , the C w ith few fossils in deep-w ater facies[ 33 , 34] . isotopic curve fo r the Xiaotan section (Yongshan , no rtheastern Yunnan) represents a more complete Progress in Natural Science Vol.13 No .12 2003 959

show s a general stratig raphic sequence and fossils oc- currences during the Sinian-Early Cambrian interval. Age estimates of some important horizons are given in Fig .5 based on our integ rated global correlations. Although the Sinian-Early Cambrian stratig raphic framew ork presented here is still preliminary , it should serve as a basic time frame for investig ations of biological and environmental evolutio n during the Neoproterozoic-Cambrian transitional interval of the Yangtze Platform .

Acknowledgement Fieldwo rk was assisted by Zhang Shi- shan, Jiang Zhiwen , Luo Huilin, Qi Dunlun , He Ting gui , He Hongw ei, T ai T ongshu, Z hao Yuanlong , Yang Xinglian and Bernd Weber.T he authors thank Xue Yaosong , Qian Yi, He T inggui, Z hang Qirui and C .Heubeck fo r helpful discussions, and Chen Juny uan , Zhang Q irui , and Feng Hongzhen for criti- cal review s, and Heyo V an Iten for English improvement of the manuscript .

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