C O N F E R E N C E R E P O R T S T race Fossils, Sm all Shelly Fossils an d th e Precam brian-Cam brian Boundary St. John's, New foundland, C anada, 8 一18 A ugust 1987 The Precam brian-Cam brian boundary m arks a fundam ental stratigraphic ranges of ichnotaxa are needed for m ore change in Ea rth history, the first developm ent of abundant sections, particularly in A ustralia and the R ussian Platform , skeletal and bioturbating orga nism s. A lthough there is to fu rthe r te st the co rrelation s. A c rita rch s ha ve no t be e n general agreem ent w ith the principle of placing the bound- as w idely studied, but presentations by G . Vidal (Sw eden), M . ary "as close as practical to the first appearance of abun- M oczydow ski (Poland) and X ing Y usheng em phasized their dant shelly fossils," m arked provincialism o f the earliest potential biostratigraphic utility in the boundary interval. sk eletal fossils an d the ir virtua l restric tio n to ca rbo na te facies have ham pered g lobal correlation in the boundary In the past, paleontologic studies in the Precam brian- C am brian boundary interval have focused upon the evolution interval (Cowie, 1985, Episo旦es v. 8, p. 93-98). of the biota. A m ajor them e of the conference was the need In A ugust of 1987, fifty geologists from ten countries m et to to reconsider the effects of environm ental and p reserva- consider a possible stratotype site in eastern N ew foundland. tional factors on the stratigraphic distribution of these The overall them e of the conference w as correlation, and predom inantly benthic fossils. E . Landing and J. M ount dialogue betw een specialists in different disciplines w as (U .S.A .) show ed how first appearances of sm all shelly fossils encouraged by poster sessions and them atic discussions. taxa in any section a re very strongly influenced by these Sm all shelly fossils are critical in zoning the Precam brian- factors, and J.P.A . M agw ood (C anada) and T.P. Crim es C am brian bounda ry interval, particularly in the carbonate m ade sim ilar observations concerning trace fossils. A ll facies of central A sia. In his keynote address, J. Cow ie organism s are environm entally controlled to som e degree, (U .K .) pointed out the need for greater know ledge o f the and only by carrying out detailed sedim entologic analyses in taxonom y and correlation of these fossils. S. B engtson conjunction w ith the paleontologic studies can w e assess the (Sw eden) and S. Conw ay M orris (U .K .) discussed the taxo- relative im portance of evolution, ecology and preservation nom ic problem s presented by the m orphologic variability in controlling the stratigraphic distribution of the fossils. and m ulti-elem ent nature of m any sm all shelly fossils and show ed how careful analysis of large num bers of specim ens can significantly reduce the num ber of synonym ous form s. M .D. Brasier (U .K .) discussed the correlation of the Tethyan carbonates of A sia (including M eischucun), show ing several potential horizons for correlation based on first appearance of various sm all shelly fossil taxa. Inter-continental cor- relation of sm all shelly fossils w as m ore controversial, w ith presentations of several distinct correlations of the three candidate stratotypes. D etailed descriptions of these key sections in Siberia (N . Esako va, U .S.S.R .), M eischucun, C hina (Xing Yusheng, C hina), and Fortune H ead, New found- land (G .M . N arbonne, Canada a nd E . Landing, U.S.A .) also provided lively discussions and a great deal of new infor- m a tio n . O ne m ajor new them e of the m eeting w as the prospect for global correlation using trace fossils, w hich are especially com m on in siliciclastic facies w here shelly fossils typically are rare and poo rly preserved . Correlation in siliciclastic facies is critical, as these deposits com prise nearly 70% of exposed rocks in the boundary interval. In his keynote address, T.P . C rim es (U .K .) outlined three globally corre- la ta ble tra ce fossil zo ne s tha t oc c ur b e lo w the lo w e st trilo bites. In ta lks o n th e W e rn e ck e M o unta in s o f no rth - w estern C anada and the C hapel Island Form ation of eastern N ew foundland, N arbonne and his co-w orkers em phasized the fact that trace fossils are cosm opolitan, and the sam e genera and species can be used to zone silicielastic strata w o r ld w id e . J. Paczesna (Poland), G . K um ar (India) and Jiang Z hiw en F i gur e 1 : S om e m em b e rs o f th e P r ec am br ian -C am br ian (C hina) also pointed out the biostratigraphic potential of B o und a ry W ork in g G ro up ex am in in g th e ba s al p ar t o f trace fossils. H ow ever, careful m orp hologic a nalysis, 亡力 e s e c t i o n a t F o r t u n e H e a d 7h e a rro w p o in ts to rigorous taxonom y and detailed docum entation of the t 力e can d id a te bo un d ar y s tr a to typ e p o in t . E PISO D ES, Vol. 10, N o. 4, D ecem ber 1987 339 门 ｜ ｜ C on ference R ep orts ｜ ｜ I l we R ecently, there has been so m e concern about the possible Phanerozoic- type trace fossils (Phycodes pedum Z one), presence of disconform ities in candidate stratotype see- w hich approxim ately corresponds w ith the first appearance tions. P.H. Signor (U .S.A .) applied J.M . Sadler's statistical of sim ple sm all shelly fossils (Sa b ellid ite s c a m b rien sis analyses of stratigraphic co m pleteness to the three candi- in te rva l) in the sec tio n . date stratotypes and concluded that the N ew foundland see- tion had a m uch higher statistical likelihood of stratigraphic Som e concerns w ere expressed about the relative scarcity of com pleteness in the boundary interval than the other tw o sm all shelly fossils in the im m ediate vicinity of the bound- candidate stratotypes, but even it is not unduly thick for the ary, and the distinct possibility that the A cadian orogeny length of tim e available for deposition. m ay ha ve reset the paleom agnetic and geochronom etric signatures of the section. These considerations w ere o ffset In another keynote add ress, S. Bengtson pointed out that his by the extrem e accessibility of the section, the apparent collections of the three sm all shelly fossils assem blages at absence of disconform ities or m arked facies changes in the M eischucun have few if any species in com m on, and he boundary interval, the presence of the distinctly Preca m - hypothesized that m arked facies shifts or stratigrapbic brian fossil assem blage in the underlying strata (H arla niella breaks m ay account for this. Several participants suggested podolica Zone), and the excellent prospects 而下 global that detailed sedim entologic analyses be carried out on all co rre la tion . candidate stratotypes to help ascertain the degree of environm ental fluctuation and stratigraphic com pleteness in A t the end of the field trip, seven of the ten voting m em - the boundary interval. bers of the Precam brian-C am brian Boundary W orking G roup (o f the IU GS International C om m ission on Stratigraphy) A nother new developm ent on the Precam brian-Cam brian present voted that the section and horizon w ere suitable for boundary concerns the absolute age of this boundary, a boundary stratotype. F urther deliberations w ill be held in traditionally regarded as approxim ately 570-600 M a. In a Bristol in M ay of 1988 before a postal ballot o f the full contribution to the guidebook, S. Conw ay M orris arg ued that thirty-one voting m em bers is held. lithostratigraphic relationships and available dates from the A valon zone strongly suggest a younger age, perhaps around The conference and field trip w ere sponsored by the W orking 530 M a. His argum ents received strong support from A . G roup, and partly suppo rted by the Natural Sciences and Benus (U.S.A .) w ho reported a date of 565 1 3 Nla (based on Engineering R esearch C ouncil of C anada and the C anadian U -P b in z irc on s) fo r an a sh be d he re a t M istak e n Po in t N ational C om m ittee for IG C P (Project 29). Special thanks containing Preca m brian m e tazoans. The stratigraphic are due to D r. C hris Barnes for m aking the spacious cam pus position of this bed is m ore than 7 km belo w the Precam - and resources of M em orial University of N ew foundland brian-C am brian boundary horizon.