bY, M arlin B rasier, John C ow ie and M ichael Tavlor D ecisio n o n th e P reca m b rian - C a m b ria n boundary stratotype The G lobal Stratotype Se(,tion and Pointjbr the P ro- dom inate elsew here. It w its for this reas（川that B D aily (unpub- terozoiC- C anibrian botindarv has been agreed upon. lished) em phasised the potential of' [race fossil stratigraphy as a subsidiary gVuide. The botindarY is defi ned in a coastal section near the T his w a s l 'o llow cd in 1974 by a m eeting in Paris. at w hich the loviln Of' Fortune in sotitheaste，二 N etyfl)undland, follow ing Points W ere 1.111aniniously agreed (C ow ie, 1992). C a n ad a . T he prim ary task of the W orking G ro up w as the choice o f a stra- Lotype boundary point-, a secondary task w as the consideration ot associated stratigraphic divisions above and below the bound- a ry . In tro d u c tio n A ny s uccession selected for the boundary point m ust be as con- fi nUOUS as possible, m arine. and as m ono facial as possib le} the m ain m ethod of guidance in selection should be biostratig- T he problem of the Precambrian-Cambrian boundary is part of one raphy although all possible m ethods of correlation should be ol'ihe Lreatcst enigm as of the fossil record; i.e. the relatively abrupt e nliste d . appearance of skeletal fossils and com plex, deep burrow s in sedi- T he 'Edia c a r a' type fauna should be considered as Precam brian- inerriary Successions around the w orld. A s w ill be recounted below , T he 'oleiiellid/t'allotaspid' trilobite faunas should be considered its definition has involved the rolling back of a m ajor frontier in as C am b rian - geology o ver the last three decades. B etw ee n the 'E d iacara' and the trilob ite faun as, tho se fossi川七r－ U ntil the la te 194 0 s. it w a s ass u m e d th at the P recam b rian w as ous successions that Could not be allocated w ith certainty to largely w ithout fossils and that the base of the C am brian w as m arked eith er the Prec am brian o r the C am bria n, sh ou ld hav e the W ork - by the low est appearance of trilobites, e.g. the 'O lenellus Z one' of' ing G roup’S Close attention. W alcott （1890) and W heeler 门947). M ore traditionally, it w as Increased SUpport for w or k o n the boundary w as m ade possi- draw n a[ a reuional Unconform ity below them , c-g. in R ayner（1967). ble in 1974, w hen 'T he Precam brian-C arnbrian B oundary' w as T he first steps tow ards a m ore precise definition of the base w ere accepted its Project 29 by the IG C 13 B oard. A m eeting in C am - taken in 1960. at the N orden IG C in C openhagen. w hen M F G laess- brid ae. U K , in 1978. review ed discoveries Of SUb-trilobitic sm all net- proposed the establishm ent of a Subcom m ission on C am brian ske le ta l fo ssi卜 an d [race fossils fro m aro u nd the w orld , a s w e ll as StratioCraphy, and in 1968 w hen C hairm an C I Stubblefield and Sec- the potential ：）「unacCynetostratigraphy. T he C am bridge m eeting retary J W C ow ie chose the problem of' the 'B ase of the C am brian reco m m ended to the W orking G roup that "T he Precam brian- System ' its one of its first tasks. B y this tim e, the existence o l'a latest C am brian boundary should be placed as close as is practicable to Precam brian fauna w as w idely acknow ledged, e.g- in G laessner and the base Of the oldest stratigraphic unit to yield T ornino tian W ade （1966), and w ork by R ussian gL,eologists w as beginning to (sensu lato) fossil assem blages" (C ow ie, 1978). A lthough there dem onstrate a pre-trilobitic succession of skeletal faunas, w hich w as little support at this tim e for it boundary defined by trace fos- w ere referred to the C am brian System . e.v. in Rozanov （1967). siJs. their potential for the correlation ot strata below the first M eetings on the boundary w ere organized for the IG C in C zechoslo- trilobites w its now being explored (see A lpert, 1977; B rasier, vakia in 1968, and at M ontreal in 1972. M uch discussion ensued at 1979, figure I）． the latter niceling and it 'W orkin.- G roup on the Precarribrian-C am - C and id ate s f o r the P recanibrian- C anib rian G S S P w ere d is- brian B oundary' (PC -C B W G ) w as form ed, w ith J W C ow ie as its cussed in som e detail at a m ectingC. in Bristol, England in 1983. and C hairm an. Som e of the history of the W orking G roup has been sum - th ree w e re se lecte d fo r (u rther co n sid eratio n, a s fo llow s. m arized elsew here (C ow ie, 1992). U lakfian-SLII.UgLir on the A ldan R iver in castern Sib er i a , ot the In effect, this W orkingC G roup w a s setting itself the challeng- form er U SSR (now in R ussia)-, here the boundary level (at the ina task ofdefi ning the 'bottom line' ofthe biostratig raphic scale; base of bed 8 ) lay in carbonate facies, w ithin a succession of i.e. ［（）discover, naine and interpret fossils w here few had been sm all skeletal fossils and algae that lay below the earliest found before. M any of the fossils unearthed o ver the follow ing archaeocyathans, brachiopods and other m arkets ot bTom m otian decades w ere not referable to previously know n groups and their type': this section w as w ell know n and w ell studied. potential for stratigraphic correlation w as, therefore, com pletely T he section at M eishucun near K unm ing, in Y unna n Province u ntested . o f southern C hina. H ere, the boundary level (M arker B ) lay T he W o rkingV G r oup held its first field m eetingV in S iberia in w ithin a phosphorite facies and w as m arked by the abrupt 1973, sponsored by the A caderny of Sciences of the U SS R , to con- appearances of phosphatised in icrom olkiscs and problem atica. sider possible stratotypes For the Precam brian-C am brian bound- T h is sectio n w a s w e ll-stu d ied b ut little kn o w n o utsid e o f ary along the m iddle reaches of the A ldan and L ena rivers in C hina. Y akuLia, eastern Siberia. H ere, 29 ForeigMn gCeologists w ere intro- S ev eral s e ctio ns o n th e B urin P en in su la of so u th eastern N ew - duced to the 'T om m oiian I'auna' and the sections of R ozanov and foundland, C anada; here both sm all skeletal fossils and trace fos- others （1969）一M em bers discussed, not for the last tim e, the origin sils w ere kn ow n to occ ur in it rn ix ed carb on ale-silic iclastic suc- of' the low est sm all shelly fossil (S SF ) asscrnblagle in bed 8 at cession. O nly outline studies w ere available from this region and U lakhan Sulugur, that w as referred to the base Ot the T om m otian no precise section w as pinpointed. Stage (C ow ie and R ozanov, 1974 ). D iscussions also focused on A t this m eeting, it w as again dec id e d that the boundary strato- the relationship betw een lithofacies and biofacies, notably the type should be placed "as close as practicable to the low est know n problem of correlatinCg carbonate shelf facies w ith archaeocy- appearance of diverse shelly fossils with a good potential for corre- athans (as in Siberia) w ith clastic shelf to basinal facies that pre- lation" (C ow ie, 1985). Such an em phasis upon SS17s provided a great ， 广 ＿1 1 1 气， 污 卜 ， I ， 了 Sti1 111.11L IS ［。、their study. but there w as grow ing concern about their how ever- that [lie w hole section w as rem agnetised in O rdovician utility forcon-clation. A prelim inary m andate Forille M eishUCUII SCC- tim es. A W orking G roup visit to the B urin Peninsula in 1979 w as tion w as del'erred at the M oscow IG C in 1984, w hen it w as recog- l'ollow ed up by im portant ,round w ork on SSFs.