bY, Marlin Brasier, John Cowie and Michael Tavlor D eci si on on the P r ecambr i an- Cambr i an boundar y str atotype The Global Stratotype Se(,tion and Point j br the Pro- dominate elsewhere. It wits for this reas（川that B Dai ly (unpub- terozoiC- Canibrian botindar v has been agreed upon. li shed) emphasised the potential of' [race fossi l strati graphy as a subsidi ary gVuide. The botindarY is defi ned in a coastal section near the T his wa s l 'oll ow cd i n 1974 by a meeting in Pari s. at which the loviln Of' Fortune in soti theaste，二 Netyfl )undland, fol lowi ng Points Were 1.111anini ousl y agreed (Cowie, 1992). Canada. T he pri mary task of the Working Group was the choice of a stra - Lotype boundary poi nt-, a secondary task was the consideration ot associ ated str atigraphic di visions above and below the bound- a ry . I nt r oducti on Any su ccession selected for the boundary point must be as con- fi nUOUS as possible, mari ne. and as monofacial as possible} the main method of guidance i n selecti on should be biostrati g- T he problem of the Precambrian-Cambrian boundary is part of one raphy al though all possible methods of correl ation shoul d be ol' i he Lreatcst eni gmas of the f ossi l record; i.e. the relatively abrupt enl is ted. appear ance of skeletal fossi ls and complex, deep burrows i n sedi- T he ' Edia ca ra ' ty pe fauna should be considered as Precambri an- inerriary Successions around the world. As wi ll be recounted below, T he 'oleii el lid/t'al lotaspid' tr ilobite faunas shoul d be consi dered its defi nition has i nvol ved the roll i ng back of a maj or frontier in as Cambrian- geology over the last three decades. B etween the ' E d i ac ara' and the tril obite faunas, those f os si 川七r－ U nti l the late 194 0 s. i t was ass um e d th a t the Precambrian was ous successions that Could not be al located with certai nty to largely wi thout fossil s and that the base of the Cambri an was mar ked ei ther the Precambri an or the Cambr ian, should hav e the W ork - by the l owest appearance of tril obi tes, e.g. the ' Olenell us Z one' of' i ng Group’S Cl ose attention. Walcott （1890) and Wheeler 门947). More traditional ly, it was I ncreased SUppor t for wor k o n th e boundary was made possi - drawn a[ a reuional Unconformi ty below them, c-g. i n Rayner （1967). ble i n 1974, when 'T he Precambrian- Carnbr ian Boundary' was T he fi rst steps towards a more precise defi ni ti on of the base were accepted i ts Project 29 by the IGC13 Board. A meeti ng i n Cam- taken i n 1960. at the Norden IGC i n Copenhagen. when M F Glaess- bridae. UK , in 1978. reviewed discoveri es Of SUb-tr i lobitic smal l net- proposed the establ ishment of a Subcommission on Cambrian s keletal fos s i 卜 and [race fos s i ls f rom around the w or ld, as well as StratioCraphy, and i n 1968 when Chair man C I Stubblefi eld and Sec- the potential ：）「unacCynetostrati graphy. T he Cambr idge meeti ng retary J W Cowie chose the problem of' the ' Base of the Cambrian recommended to the Wor ki ng Group that " T he Precambr ian- System' its one of its fi rst tasks. By thi s time, the existence ol' a latest Cambr ian boundar y should be placed as close as is practi cabl e to Precambrian fauna was widel y acknowledged, e.g- i n Glaessner and the base Of the oldest stratigraphic uni t to yield T orninoti an Wade （1966), and work by Russi an gL,eologi sts was begi nni ng to (sensu lato) fossi l assemblages" (Cowie, 1978). Although there demonstrate a pre-tr ilobitic succession of skeletal faunas, which was l ittle suppor t at thi s ti me for it boundary defi ned by trace fos- were referred to the Cambrian System. e.v. i n Rozanov （1967). siJs. their potenti al for the correlati on ot strata bel ow the fi rst Meetings on the boundary were organized for the IGC in Cz echoslo- tr i lobites wits now being explored (see Al per t, 1977; Brasier , vaki a in 1968, and at Montreal i n 1972. Much di scussion ensued at 1979, fi gure I ）． the latter ni celi ng and it ' Worki n.- Group on the Precarribrian- Cam- Candidates fo r t he P recani br ian- Cani brian GS S P w ere dis - brian Boundary' (PC-CBWG) was for med, wi th J W Cowie as its cussed in some detai l at a mectingC. in Br istol, England in 1983. and Chairman. Some of the history of the Working Group has been sum- three were selected for (urther consideration, as fol low s . mari zed elsewhere (Cowie, 1992). Ul akfi an-SLII.UgLir on the Aldan River in castern Si b eri a, ot the In ef fect, thi s Worki ngC Group w a s s etting i tself the chal leng- f or mer USSR (now in Russia)-, here the boundary level (at the i na task of defi ni ng the ' bottom li ne' of the biostr ati g r aphic scal e; base of bed 8) lay in carbonate faci es, withi n a succession of i .e. ［（）di scover, naine and i nterpret f ossi ls where few had been smal l skeletal fossils and al gae that l ay below the earl iest found before. Many of the fossi ls unearthed over the fol lowing archaeocyathans, brachiopods and other markets ot bTommoti an decades were not r eferable to previousl y known groups and their type' : thi s section was well known and well studied. potenti al for strati gr aphic correlation was, therefore, completely T he section at Mei shucun near Kunmi ng, in Yunna n Provi nce untes ted. of souther n Chi na. Here, the boundary level (Marker B) lay T he W or kin gV Gr ou p held i ts fi rst fi eld meeti ngV i n S iberi a in wi thi n a phosphori te facies and was marked by the abrupt 1973, sponsored by the Acaderny of Sciences of the USS R, to con- appearances of phosphati sed inicromol kiscs and pr oblematica. sider possi ble str atotypes For the Precambri an- Cambr ian bound- T his s ection was w ell -studied but l ittle known outsi de of ary al ong the middle reaches of the Al dan and L ena ri vers in China. Y akuLia, eastern Si beria. Here, 29 Forei gMn gCeologi sts were i ntro- S ev eral s ec ti ons on the B uri n Peninsula of southeastern New - duced to the 'T ommoi ian I'auna' and the secti ons of R ozanov and foundland, Canada; here both small skeletal fossils and trace fos- others （1969）一Members di scussed, not for the l ast ti me, the ori gin s i ls w ere know n to occur i n it rnix ed carbonale-s i licicl astic s uc- of' the lowest smal l shel ly fossi l (S SF ) asscrnblagl e i n bed 8 at cessi on. Only outline studies were avai lable from thi s region and Ulakhan Sul ugur, that was refer red to the base Ot the T ommoti an no preci se section was pi npoi nted. Stage (Cowie and Roz anov, 1974). Discussi ons also focused on At this meeti ng, i t was agai n dec i de d tha t the boundary strato- the relationship between li thofaci es and biofacies, notably the type should be pl aced " as close as practi cabl e to the l owest known problem of correlatinCg carbonate shelf facies with archaeocy- appearance of diverse shelly fossils with a good potential for corre- athans (as in Siberia) with clastic shelf to basinal facies that pre- lation" (Cowie, 1985). Such an emphasis upon SS17s provi ded a great ， 广 ＿1 1 1 气， 污 卜 ， I ， 了 Sti1 111.11L IS ［。、their study. but there was growi ng concern about thei r however- that [lie whole section was remagnetised i n Ordovi cian util ity for con-cl ation. A preli minary mandate For ill e MeishUCUII SCC- times. A Wor king Group visi t to the Burin Peninsul a in 1979 was ti on was del'erred at the Moscow IGC in 1984, when i t was recog- l'ollowed up by i mportant ,round work on SSFs. trace lossi ls and nised that greater international aIgreement oil SSF taxonomy was Ii [ hostr at igraphy (Bengi son and Fletcher, 1981．1983). Further necessary- 'I’h i s led to -,in "SSF Workshop" i n Uppsala in 1986, orga- researches Culmi nated in a series ol' papers which detai led the strati - nized bv S Benatson. It call now be seen that thi s mecting resulted i n gIOraPphic dli、S1h1-ibLUlti（o川n （o）f trace l O SS jl S, SS[7s and l ithostrati -raphy several new thrusts.