The Precambrian—Cambrian boundary: A symposium

Earth-Sczence Revzews, 11 (1975) 209--251 © Elsewer Scmntffm Pubhshmg Company, Amsterdam -- Printed m The Netherlands

The Precambrian-Cambrian Boundary: A Symposium J W Cow,e and M F Glaessner (w,th contrJbut,ons by A Boudda, G Choubert, A Faure-Muret, W B Harland, A Yu Rozanov and V E Sawtsky )

ABSTRACT Cowm, J W and Glaessner, M F , 1975 The Precambrlan--Cambrmn boundary A symposmm Earth-Scl R e v , 11 209--251 Contributions to a symposium of the IUGS Subcomm~sslon on Cambrian Stratigraphy at the International Geologmal Congress m Montreal m 1972 are the basis of this paper whmh is intended to gwe guidance to further work of what is now a Working Group on the Precambrlan--Cambrian boundary of the International Geologmal Correlation Programme (UNESCO/IUGS) The proposed principles for the defimtlon of the boundary are based on those accepted by the Comm~sslon on Stratigraphy, particularly on stratlgraphlc classlfmatlon and on defining the Sllurmn--Devoman boundary These matters are discussed by the authors m general terms and by W B Harland m terms of procedure They are illustrated by examples of strat~graphm sequences across the boundary in Siberia given by A Yu Rozanov and discussed crltmally by V E Savltsky New relevant discoveries of Early Cambrmn faunas and their stratlgraphm setting are revmwed by A Boudda el al P E Cloud's contribution, pubhshed elsewhere, and work m Austraha are rewewed by the authors The conclusmns, based not only on the symposmm materml but also on current opinion, are that no smgle sequence whmh could serve as a reference standard for the Precambrmn--Cambrmn boundary has yet been ldentifmd but that many slgnlfmant ones have been described whmh will be further examined Whde the general hmlts for the choice of a boundary d e h m t m n should he between the horizons with fossils of Edmcaran type and those with dwersffied trilobite assemblages, a number of "Working Reference Points" exist m that interval whmh are correlatable horizons of faunal change These cannot be defined simply as horizons of hrst appearance

INTRODUCTION The Subcommlsslon on Cambrian Stratigraphy (Commission on Stratigraphy, International Umon of Geological Sciences) held a symposmm on " T h e P r e c a m b r l a n - - C a m b r y a n b o u n d a r y p r o b l e m a n d t h e q u e s t i o n o f su i t able regional stratotypes" at the 24th International Geologmal Congress m M o n t r e a l , o n A u g u s t 2 4 , 1 9 7 2 Sir J a m e s S t u b b l e f i e l d w a s C h a i r m a n a n d o n e of the Authors (J.W.C) was Secretary of the Subcommlsslon Soon afterw a r d s i t w a s d e c i d e d , w i t h t h e c o n s e n t o f t h e P r e s i d e n t a n d his s u c c e s s o r

210 (A R. Palmer) to pubhsh an a c c o u n t of the proceedings in the form of a review o f the pr obl em u n d e r discussion The reason was the desirability of interesting more geologists in the fundamental problem and presenting the f o u n d a t m n on whmh f u r t h e r work towards its solution could proceed Progress was made subsequently by the formal estabhshment of a Working Group of the Commission on Stratigraphy whmh c a m e d o u t an mvestigatmn o f relevant areas In Siberia in 1973 (Cowie and Rozanov, 1973, 1974) and forwarded a project proposal to the International Geologmal Correlation Programme (UNESCO/IUGS) which was accepted in April 1974 The following review, based on the S y m p o s m m in Montreal, is intended to show to what e x t e n t a consensus of opinion was reached among the particlpants at the outset and what problems were foreseen and defined It is clearly recognised t hat several years of intensive studies are reqmred before possible solutions of the problem of the Precambrian--Cambrian b o u n d a r y can be presented to an m t e r n a t m n a l f o r u m of geologists with the expectation o f acceptance of one or a n o t h e r by consensus Statements of principles,

J.W. Cowm obtained his BSc and PhD from the University of Bristol in 1950 and 1956 respectively His field research has been particularly c o n c e n t r a t e d In Greenland, the Arctic Islands of Canada, and Scotland He is Chairman of the Working Group on the Precambrlan--Cambrlan b o u n d a r y and Secretary of the Subcommlssion on Cambrian Stratigraphy (IUGS and International Geologic Correlation Programme) and he is also a m e m b e r of the Stratigraphy Committee of the Geological Society of L o n d o n

M.F. Glaessner obtained degrees from the Universities of Vienna and Melbourne He worked as an associate of the Vienna Museum of Natural History from 1924 to 1932, mainly on fossil decapod Crustacea, at the Petroleum Research Institute in Moscow from 1932 to 1937 on m m r opa l aeont ol ogy and stratigraphlc correlation, for the Anglo-Iranian Oil Com pany (now BP) and its associated compames, mainly in PapuaNew Guinea and Melbourne (Victoria), and from 1950 at the Geology D e p a r t m e n t of the University of Adelaide (South Australia) where he is now Emeritus Professor and H o n o r a r y Research Associate of the Centre for Precambrlan Research, working on Precambrian and Early Cambrian Metazoa

211

rewews o f interim results, and d e f i m t m n of future research tasks will help to bring a b o u t the required consensus THE PROBLEM OF THE BASE OF THE CAMBRIAN

It is accepted by stratlgraphers that divisions of the stratlgraphlc scale should be considered neither as purely mental concepts, divorced from the concrete sequence of strata, nor as pure assemblages o f fossils whmh wherever e n c o u n t e r e d would define the period concerned Historically, as well as m m o d e m consensus (as far as it exists) the c o n c e p t of the Cambrian Period and System is based on a concrete sequence of strata deposited during an interval o f geological time and containing certain sequences of fossils One problem whmh is c o m m o n to all dlvlsmns of the Phanerozolc strat~graphlcal time scale is th at at the time o f the first estabhshment of its dlvlslons m the first half o f the 19th c e n t u r y neither the changes of rock sequences from one part of the globe to another nor the changes of faunas from place to place were k n o wn Geologmal mapping was then essentially c o n f m e d to Europe and Eastern North America and the blofacms concept, though introduced m 1838, was n o t properly u n d e r s t o o d until the end of the last century Th~s problem is being overcome by the i n t r o d u c t i o n of designated stratotypes The second problem which is n o t dealt with here is that the Cambrian System (and Period) is the first (oldest) of the Phanerozom Systems (and Periods) The dlwsions of the Phanerozom t~me-strat~graphlc scale are operatlonally recogmsed and correlated by assemblages of fossils contained in the rocks, including those o f the s t r a t o t y p e sequences The rocks preceding those which are fosslhferous and c o m m o n l y accepted as Cambrian are generally unfossfllferous, and of t en know n to be much older by various conslderatmns The earher u m t is c o m m o n l y separated from the later one by an u n c o n f o r m i t y Occasmnally such u n c o n f o r m l t m s are followed by strata which are probably Cambrmn but are unfosslhferous In the type region of the Cambrian, m Wales, all these deficmncms have been observed (Cowie et a l , 1972, p 5) It is now know n that Phanerozom time comprises only a b o u t 12% of the time since the origin of the earth or 15% of the time since the f o r m a t i o n of the oldest k n o w n rocks Little-altered stratlgraphm sequences of P r e c a m b n a n age exist which have to be correlated and classffmd by stratlgraphic m e t h o d s A slgmfmant change m these m et hods must result from an apparent progressive failure of the d o m i n a n t Phanerozom stratlgraph~c approach (which relies on fossils) as the stratxgrapher moves downward across the base of the Cambrian into a sequence of Precambrian stlata What defines this boundary9 It cannot be the first appearance of fossils (of any kind) because some Precambrmn fossils are k n o w n and certainly life did n o t start at the beginning o f Cambrmn time The questmn of w het her a guideline could be taken f r om the first appearance of a t a x o n of fossils or of shelly fossils or of ammal trace fossils or from evolutionary factors (bmloglcal or physmo-chemmal) is a m o o t poi nt requiring resolution It cannot be an

212

u n c o n f o r m i t y because a world-wide synchronous u n c o n f o r m i t y would imply a world-wide synchronous catastrophe for which there is, as yet, no evldence It cannot be on geochronologlcally ascertainable date because there are far fewer places where rocks can be dated lsotoplcally than places where the base of the Cambrian has to be defined and mapped Th~s leaves only one method, the d e f i m t m n by means of a stratotype section and its subsequent correlation Where can such a section be found9 This was the problem facing the participants of the 1972 Symposmm at the International Geological Congress m Montreal.

Requzrements for stratotypes "typtcal" faunas Probably the majority of stratlgraphers concerned with the problem here discussed agree that the best possible definition of what is meant by "base of the Cambrian" is through a boundary-stratotype The International Subcommission on Stratlgraphlc Classification (Hedberg, 1972, p . 1 7 ) h a s stated " A boundary-stratotype is a specific point m a specific sequence of rock strata which serves as the standard for deflmtlon and recognition of a stratlgraphlc b o u n d a r y " and "The boundary-stratotypes of a chronostratlgraphlc unit should be chosen m sequences of essentially continuous deposition at or near markers which are as favourable as possible for long-distance chrono-correlatlon An u n c o n f o r m i t y is highly undesirable either as a boundary-stratotype or as the trace of the boundary of a chrono-stratlgraphlc unit because it does not represent an lsochronous horizon " 0 b i d , p 29) "The value of a stratotype is that it provides clear, u m f o r m , and unchanging definition which should mean the same thing to everyone and to which everyone can return for reference" (1bid., p 17) The "reference p o i n t " (Lafltte et al., 1972) or "marker p o i n t " (Harland et a l , 1972) proposals do n o t differ essentially from the boundary-stratotype concept It should be added that the stratotype and reference sections should contain verifiable evidence of actually observable (as against hypothetically constructed) crucial sequences of faunas The Montreal S y m p o s m m of the Cambrian Subcommlsslon provided a review of the possibilities for the estabhshment of a stratotype for the base of the Cambrian Before discussing these posslblhtles, two problems have to be considered. One is the concept of " t y p i c a l " Cambrian fossils, and of a possible r e s t n c t m n of the scope of bmstratlgraph~c defimtmn to certain groups of fossils, disregarding all others. Fossils can only be described as " t y p i c a l l y " Cambrian ~f they are empmcally known to be confined to "typecally" (1 e , stratotyplcally, or rehably correlated) Cambrian rocks They cannot be so described on the basis of concepts of evolutionary bmlogy or any other theoretical considerations. The use of fossils m stratigraphy rests on an empirical basis but n o t on an arbitrary selection of certain kinds of fossils as the only ones capable of characterizing geological periods. It would be an unwise and retrograde step, for example, to consider only trilobites or only archaeocyathlds as w o r t h y of consideration as Cambrian " i n d e x " fossils

213

when practical expermnce pomts to m a n y other posslblhtles. The second problem is that of a proper understanding of the chronostratlgraphm umts proposed as lmmedmtely preceding the Cambrian Period The problem of the Precambman--Cambrmn b o u n d a r y requires conslderatmn of the stratlgraphm subdlvlsmns of the Late Precambrmn and subsequent c o o r d m a t m n of the " w e w from below" with the "vmw from above". Some background knowledge of what precedes the Cambrmn Permd is here brmfly presented, w i t h o u t proposing names or defimtlons for this mterval of time

Edzacaran and Vendzan Precam br~an

b~o- and chronostratzgraphzc concepts o f the Late

The Edlacaran was proposed by Termmr and Termmr (1960) as an essentially blostratlgraphlc concept. Since that time the regional stratigraphy of the beds contammg the Edlacara fauna has been clarified {Wade, 1970, Forbes, 1971, Dally 1972) and the global distribution of elements of this fauna has been discussed (Glaessner, 1971) It has become clear that strata contammg archaeocyathlds, hyohthlds and trilobites usually considered w i t h o u t question to be Lower Cambrian m age are preceded by strata contaming elements of the Edlacara fauna In South Austraha up to 420 m of Rawnsley Quartzite (a Member of the Pound Quartzite) and a regional unconformity intervene between this fauna and the putative lowest Cambrian strata which here contain distinctive trace fossils One or two elements of this fauna may extend into Lower Cambrian elsewhere, as is indeed to be expected unless catastrophlst views are accepted, but these instances are hkely to be n o t specific or generm identities but slmllarltms on substantially higher t a x o n o m m levels They do n o t affect the important observation and proposition t h a t an essentmlly non-skeletal fauna, 1 e , one n o t able to form mmerahzed skeletons, existed m the Late Precambrmn. It seems that trace fossils accompanymg it are slgmflcantly simpler and less dlverslfmd than those occurring m the Lower Cambrmn (Glaessner, 1969, Banks, 1970, Daffy, 1972) The only fossils at present considered potentmlly useful m correlating rocks older than the Edlacara fauna are stromatohtes (Walter, 1972, Hofmann, 1973). It is possible that further studies and dlscoverms will make it possible to subdivide what is now seen as the Edlacaran fauna into a succession of faunas but this ~s not y e t possible The most important geological event predating the Edmcaran fauna m varmus regmns is the Late Precambrian me age (Harland, 1964; Chumakov, 1971). Its manffestatmns m boreal regmns are known as the Varanglan (Varegmn or Laplandyan) glaciation The stratlgraphm unit m whmh its sediments occur is often referred to as the Vendmn. It is not necessary for the purpose of the present dlscussmn to rewew the history of this and other terms and concepts in Late Precambrmn stratigraphy It is, however, approprmte to summarlse here the present vmw of B.S. Sokolov who coined the term Vendlan m 1950--1952 (Sokolov, 1971, 1972, 1973) It referred originally to the youngest complex of Late

214 P r e c a m b n a n strata of the Russian Platform which precedes immediately the oldest Cambrian. F u r t h e r knowledge of the Vendlan as an i m p o r t a n t and distractive stage m the history of this regnon came from the stratlgraphlc drllhng campaign of the post-war years The Vendlan consists of three "Serms", the Volhynlan ( L ow er Vendlan) and the R e d k m o and Valdal (Upper Vendlan) It grades upwards m t o the Baltm "Serm s" of the Lower Cambrian, b u t only m some areas o f the Baltic--Moscow synechse and m the V o l h y n o - - P o d o h a n depression while elsewhere the record is incomplete (see Rozanov, p 233, who considers t hat for this reason the Russian Platform is n o t a suitable t y p e region for the base of the Cambnan) Its stratigraphy is complex and variable but it contains distinctive and widespread hthologacal umts such as volcamcs, tflhtes and p h o s p h o n t e s The Vendmn was accepted as a unit of the standard stratlgraphm scale for varmus parts of the world by a n u m b e r o f authors, mostly Russmn, but Sokolov (1971) admits t hat the precise defimtlon o f its lower b o u n d a r y remains a most difficult problem It is clear f r o m Chumakov's (1971) chart that he conslderes the " t f l h t e s " of the Volhynmn "Serms" and their equivalents as Middle Vendmn ( " L a p l a n d l a n " ) and th at he adds as Lower Vendmn the Pachelma "Serms" and its eqmvalents which Sokolov prefers to exclude f r o m it In Slberm the "classmal horizon of the Vendmn glaciation" is "less clear" (Sokolov 1971) but Keller (1972, p 29) refers to the Vendmn ( Y udom m n or E o c a m b n a n ) the following examples f r o m Slberm (1) Vorogov tflhtes of the Yemsel Range, (2) K o l y m a Uplift, (3) Ulutau and Karatau in Kazakhstan, and (4) the correlated Chatkal-Naryn zone m Klrglzm The Y u d o m a " S t a t e " is considered by Keller (1963) and Sokolov as representing the Vendmn in the east (The divergent wews o f Sawtsky and their a t t e m p t e d r e f u t a t m n by Rozanov are discussed below ) This d o m i nant l y carbonate sequence contains m m r o p h y t o h t h s and s t r o mato h tes whmh are considered as distractive Semlkhatov et al (1970) and Keller (1971) consider the Y u d o m m n and its s t r a t o t y p e as preferable to the Vendlan as a Late Precambrmn u m t of the standard stratlgraphm scale but this is opposed, also on priority grounds, by Sokolov who sees the Y u d o m a State (or Complex or F o r m a t m n ) merely as one of the regional facies of the Vendmn The answer to this questmn will also depend on the ultimate assessment of the merits of the stratotypes of the Vendmn on the Russmn Platform and the Y u d o m m n in Slberm This need n o t concern us at this stage Summing up this discussion we fred that the lowest Cambrmn is preceded by the Vendlan, or alternatively the Yudomlan While many would have no d o u b t a b o u t the Precambrmn age of the whole of the Vendmn, its lower b o u n d a r y is in d o u b t It is placed either at the base of the clrcumArctm and European tflhtes or alternatively considerably lower Again, this problem need n o t concern us here but it will have to be solved before the Vendlan or one of its alternatives, whmh are m this respect n o t much clearer, can b e c o m e a recognized part of the standard stratlgraphlc scale The distinctive Edmcaran fauna of the Vendmn has been illustrated by Sokolov (1972, 1973) The questmns t ha t r e m a m to be discussed are where the b o u n d a r y

215 b e tween the Cambrian and Vendmn (or its equivalents) should be placed, how it should be defined, and where it can be seen Before these questmns are considered it will be useful to review the geochronological framework of the problem

Geochronology Arguments a b o u t the dating o f the base of the Cambrian were revmwed by Glaessner m 1963 and only later work and recent discussions will be consldered here Results obtained m the U.S S R on this problem are still based on K-Ar dating, as far as we know This raises the double problem of the posslblhty o f all datmgs being on the one hand t o o y o u n g because of argon losses and on the o t h e r hand being t o o old because of the use of the decay c o n s tan t he = 0 557 10-10 instead of 0 585 • 10 -I 0 accepted by m any British, Amerman and Austrahan workers The large n u m b e r of dates obtained m the U S.S R. f r om Late Precambrlan to Lower Cambrian glaucorotes which are consistent with the stratlgraphm position of the samples (only a minority of samples have been r e p o r t e d as giving ages out of stratlgraphm order) mdmates t ha t argon losses, if t hey occur, are hkely to be u n if o r m for the time interval here considered, 1 e , 550--680 m.y The correction for the decay cons t a nt for this interval is m the order of 4 5% It may be leglhmate to assume t hat the two correctmns, being of the same order of magnitude and o f opposite sense, substantially cancel o u t Confirmation of the acceptability of these numbers can come only from Rb-Sr datmgs m North Amerma, at the present stage of our knowledge (L am bert m Harland and Francis, 1971, p 10) Then we come up agamst the problem of the uncertain half-hfe of 87 Rb whmh necessitates consideration of at least two posslblhtms for each date The relevant, stratlgraphmally best controlled ages are those of the granites of southeastern Massachusetts (reviewed m Harland and Francis 1971, pp 62--64) and the H o l y r o o d Gramte of southeast Newfoundland (last reviewed by Anderson 1972) The age of the H o l y r o o d granltels574-+ 11my (hSTRb=147 10-~1),591my (half-hfeof ~Rb4.85 1010, m t e r m e d m t e be t w een 4.7 and 4 0 10 ~0 years) or 607 m y (h87 Rb = 1 39 • 101 ~) This gramte post-dates the " E d l a c a r a n " Mistaken Point fauna and is older than fossflfferous Lower Cambrian The age of this "non-tril o b i t e " fauna ~s, however, questmnable (Anderson 1972, p 1713) and may be as y o u n g as "Callav~a-zone", L e , Atdabanlan m terms of the Slbermn stratlgraphlc scale The H o p p m Hill gramte samples gave ages which are interpreted as being t o o low and its true age is considered as that of the Northbrldge gramtm gneiss m the same area, 1 e , a b o u t 570 m y (h ~ 7 Rb = 1 39 10 t~) (Lambert, m Harland and Francis 1971, pp 60--62) The H o p p m Hill granite is overlain by Lower Cambrmn with a fauna with Coleololdes and ot her tubular fossils on whmh T h e o k n t o f f (1968) and Palmer (1971, p 211) have c o m m e n t e d recently It seems to be uncertain whether these fossils are of pre-Callawa or of Callawa-zone age On this basis

216 TABLE I K-Ar ages of selected rocks from near the Precambrmn--Cambrlan boundary m the U S S R ( A f t e r S o k o l o v 1971 and sources given by him )

mllhons of years belore the present Basal Cambrian, Siberia *Basal Cambrian, Sunnagln Substage, Tommotlan Stage, R Aldan ( " D v o r t s y " ) , Siberia, g l a u c o m t e

Upper Vendlan, Valdm " s e r i e s " , Moscow synchse, g l a u c o m t e s Upper Yudommn, Nemaklt - - D a l d y n H o r i z o n , glauconlte Upper Yudomlan, glaucomtes Middle Vendian, R e d k m o " S e r i e s " , g l a u c o m t e s Lower Yudommn, glaucomtes Lower Vendmn (Volhyman), Yaryshev "Suite", glaucomte Lower Vendlan (Volhyman), Prlpyat basalts Vendlan, Uka " S u i t e " , Ural Mrs P r e - Y u d o m l a n , Ingdl mtruslves, biotite from carbonatlte Pre-Yudomxan, Inffll mtruslves, nephehne syenite * * P r e - V e n d l a n , Upper Pachelma "Series", glauconltes

050 - - 575 578 620 580 590 595 635 590 640 665 660 690 610

- - 570 ± 15 - - 607 - - 650

- - 690

* F r o m K h o m e n t o v s k y (1972, p 41) ** 6 7 0 - - 6 8 0 according to S o k o l o v (1973, p 205)

most recent authors see n o grounds for disagreement with the tentative but widely accepted dating of the putative "base" of the Cambrian at apprommately 570 m.y. This would vahdate, equally tentatively, the acceptance of the glauconlte ages of rock samples from near the base of the Cambrian as reported m the U.S S R., which is m accordance with the assumptions discussed above. Sokolov ( 1 9 7 2 , p 117) states that for the Vendlan m the U S S.R a time span from 680 ± 20 to 570 ± 10 m y. is accepted This refers to the use of the term sensu strmto, 1.e, from the base o f the northern tllhtes to the base of the Cambrian. The evidence (accepted by Sokolov with reservations) is given m Table I (K-Ar dates after Sokolov ( 1 9 7 1 ) and sources given by him) Sokolov concludes that the slgmflcant coincidence between the dating of rocks near the boundary of the Vendlan and the Cambrian on the Russian Platform (top of Valdal Series and base o f Baltm Stage) and o n the Siberian Platform (top of Upper Y u d o m a and base of T o m m o t Stage) means that "there is n o w n o serious reason for admitting a slgnlfmant difference m the radmgemc ages of the Vendmn m the European part of the U S S R. and m Siberia"

217 SYMPOSIUM CONTRIBUTIONS

1 On standardtzmg the Precambrtan--Cambrzan boundary by W B. Harland (Department of Geology, Umverslty of Cambridge, Great Britain) The base of the Cambrian (whmh can be taken as equivalent to the threshold of Phanerozom time) has long been regarded as of special importance In the history of life But the point m time when the Cambrian Period is t h o u g h t to have begun has been conceived in widely differing ways This has led to great confusion and it is now generally recogmzed that high priority must be given to defining this p o m t m an internationally acceptable way It is also c o m m o n l y agreed that this hmlt should be defined m rock and so become part of the developing Global Standard Stratigraphic Scale Such a procedure was attempted In 1948 with the Phocene--Plelstocene boundary and by 1972 it had become well established by the Sflurmn-Devonian boundary decision Indeed it may be anticipated that within a decade or two most divisions In the stratlgraphic scale will be so standard~zed The history of the decision to define the Sllurmn--Devoman boundary at a point in the rock sequence at Klonk and the final r e c o m m e n d a t m n is set out by the Chairman of the Workmg Group, McLaren (1972) and also summarlzed with a c o m m e n t a r y (McLaren, 1973) The procedure has been expressed m general terms (e g George et a l , 1969, Hedberg, 1972, L a f t t e et a l , 1972, and Harland et al 1972). This paper sets out some practmal reqmrements and theoretical lmplicatmns of this procedure for the Precambrian--Cambrian limit The definition of the boundary at a reference point in a boundary stratotype section precludes all alternative methods of definitmn, for example (a) the age m years of the d e f n e d boundary can only be estimated with inevitable uncertainty so it cannot at the same time be defined geochronometncally, (b) historical events such as m biological evolution or chmatm change should be used m choosing a reference point, and when chosen blostratlgraphy will then provide the principal means of correlatmn, but the Cambrian Period will then cease to be defined by its faunas, on the contrary the established Cambrian Period will then define its faunas Indeed the reason for treating the deflmtlon of the stratlgraphlc scale independently of the stratigraphlc interpretation of natural phenomena is to prowde an objective scale for the study of the phenomena.

Nomenclature and standardtzatmn International agreement concerning this or any boundary requires two kinds of decision, namely on nomenclature and on standardization Nomenclature concerns both the proper names and their relationship m a

Proterozom

Phanerozom

Eon

1i

i

etc

Rlphean Adelaldean

Palaeozoic

etc

Era

etc

Vendlan

Cambrian

ete

Period

]

[

/

/

///J

Comley

J

Varanglan

Edlacarlan

J

etc

Epoch

etc

I Sunnaglman

1

etc

Age

Kenyad]an

Hmrarchm table illustrating one possible position of the P r e e a m b n a n - - C a m b n a n b o u n d a r y

TABLE II

A

etc

~n~agl~lCUS

D regularzs--L t o r t u o s a

etc

Chron

b~

219 scheme o f clasmfmatlon Place names are best used so as n o t to imply that the dlvlmon is defined by any partmular character Span and point schemes o f n o m e n c l a t u r e are bot h feamble Traditionally names are used for t he span bet w e e n boundaries. However, as each b o u n d a r y becomes standardized it may also be useful to name the b o u n d a r y s t r a t o t y p e point (e.g Klonk for Sllurlan--Devoman). With this m mind c o n v e m e n t names for points should be sought Span and p o m t nbmenclatures can coexist and each has distract advantages. Thus a poi nt scheme enables any span to be speclfmd by the t~vo hm l t m g points and ~t allows flex~blhty m introducing new points (e.g. m c o m p u t e r files) A span scheme enables a hmrarchy to be used A hmrarchy has an advantage m the use of higher ranks for general knowledge and for expressing degrees of imprecision The trad~ttonal hwrarchy (eon, era, period, epoch, age, chron) is most easily managed ff no minor dlwmon overlaps an adjacent major dlvlsmn Thus it would be conmstent with tradltmnal usage ff the Precambnan--Cambrlan b o u n d a r y were also to serve as the mltml b o u n d a r y of the Phanerozom Era and so on (see Table II). A developing hzerarchy for latest Precambrmn t~me, along the same hnes as for Phanerozom time will be needed and continuing dlscussmn m N h t clarify some posmbflltms. In any case the b o u n d a r y m question will be part of a more comprehenmve scheme and Table II illustrates one suggestion for the kind of hmrarchmal scheme t hat could develop ff some Russmn--Slbenan rock names were used It will probably take as long to decide a scheme of names as to standardize t h e m so it is well to work towards such agreement Standardlzatmn, however, is the mare objective of the present exercise The operattonal sequence may follow the Klonk precedent m three steps (a} Seek agreement on the age hmlts m bmstratlgraphmal terms within whmh the b o u n d a r y should be decided so as to do least wolence to existing and earher usage. (b) Consider the most promising sectmns spanning this age (e g Siberia, Morocco, Austraha, and Cahfornm) and m each o f the best sectmns define a p o in t or points m each as a focus for the mvestlgatmn of posmble m et hods of tlme-correlatmn t h r o u g h o u t the world These points may subsequently be considered for the u m q u e standard b o u n d a r y reference point. For this reason they should be selected (1) m a well exposed sectmn of relatively u n if o r m marine facms, (2) m a sequence with what seem to be the best characters for correlatmn 0deally with a varmty of fossil taxa plus the possibility of r a d m m e t n c age d e t e r m m a t m n and palaeo-magnetm o n e n t a t m n ) , (3) there should be a good posslblhty for correlatmn of the rocks both above and below the b o u n d a r y point, (4) the sectmn should be bot h mternatmnally accessible and with the posmbfilty of conservatmn, (5) the sectmn should be very fully investigated with pubhs he d d e s c n p t m n (c) Select a single poi nt t hat is best for correlatmn and relate this to the agreed n o m e n c l a t u r e m a r e c o m m e n d a t m n to the Commlssmn of Stratigraphy (after co ns ul t at m n with the approprmte Subcommlssmn) for declsmn by the IUGS

220

Preczsmn ~n d e f m l t m n is a c c om phs he d by estabhshmg this p o i n t m a statable sectmn The ~mprovement comes f r o m e h m m a t m g the unnecessary u n c e r t a i n t y m d e f m l t m n . The inevitable unpreclsmn m correlatmn is n o t affected However, the s t r a t o t y p e with the best posslbflltms of correlatmn b o t h above and below the reference poi nt should be selected, since uncert a m t y m correlatmn is different f or different points and so is affected by the choice o f standard An ob]ectwe standard is t h e r e b y achmved because the physmal poi nt m rock can be ldentffmd by any scmntlst who can pursue his own mvestlgatmn o f any aspect o f the rocks above and below the reference p o m t Global and regmnal stratotypes Any time b o u n d a r y must be standardized at one p o i n t and no more. Thus once the decision is made the sections which have n o t been selected c a n n o t serve as auxlhary s t r at ot ypes to define the b o u n d a r y , nor can additional b o u n d a r y standards be set up. Regmnal s tr a t ot ypes fulfil a di f f er e nt functmn. T h e y are fully described and mvestlgated sequences o f named local umts t hat are chosen for their correlatmn potentml. T h e y are thus b o d y - s t r a t o t y p e s rather than boundarystratotypes. T h e y b e c o m e the basis f or a major n e t w o r k for correlatmn from regmn to region and so p r o w d e standards for correlatmn o f ot her sectmns within the regmn. Reference points to focus research m correlation may be estabhshed by the same p r o ced ur e as for the global stratlgraphm scale but must be named differently so as to avoid confusmn. The p r o ced u r e to set up global b o u n d a r y stratotypes stimulates much work none of whmh is wasted ff the refining of global and regmnal time correlatmn is planned at the same time. Indeed a useful procedure may well be for regmnal groups to set up regmnal chronostratlgraphm scales accordmg to global standards so t hat as c o n v e m e n t the approprmte IUGS a u t h o r i t y may select some regmnal boundaries and estabhsh t h e m for the standard stratlgraphm scale

Operatmns with tzrne and rock The reference p o i n t m the b o u n d a r y s t r a t o t y p e is fLxed m rock Its purpose is to provide an objective fLxed poi n t m time. This is done by concelvmg an eqmvalent instant event m the genesis o f the rock at that p o m t Two b o u n d a r y instants def m e a time span between them. This is the basis of the standard stratlgraphm scale whmh is a scale of points and consequent spans The stratlgraphm age o f rock will be expressed by its relationship to whatever standard scale becomes estabhshed Away from the reference points the b o u n d a r y c a n n o t be dehneated, although it can be conceived as a

221 synchronous time surface separating rocks formed before and after it. Indeed the f u n c t m n of the scale is to prowde agreed terms for expression of estimated tlme-correlatmn of rocks Expressmn of stratlgraphm age using only the span name is adequate for both time and rock (e.g Cambrian time, Cambrian rock) Cambrmn rock was formed m Cambrian time Two parallel hmrarchms (e g permd/system, age/stage) correspond to these alternative expressmns The terms mltml and terminal bougdary can be apphed generally for beginning/base and end/top It is convenient to agree that, insofar as the point may be fixed within a time gap m the rock sequence the hmtus shall belong to the earher time span While some gap m some degree be lnewtable it is highly desirable to avoid any evident break or marked hthologmal change m the succession when selecting the positron of the reference point.

Conventtonal and natural dwls~ons In the interpretation of history and m the search for natural laws man needs artlfmlal conventions The matters referred to above relate entirely to such conventions. A division, such as the Cambnan Period can therefore be decided more or less wisely for sclentlfm convemence A conventional division must n o t be confused with a natural stage m the evolution of the earth or of hfe which may well have been contmuous. The study of evolution needs an artlficml scale with whmh to explore and express the pattern of events While the mvestlgatmn of Nature results m an indefinite change m hypotheses which must be allowed for, it is useful to have a stable artffmlal scale and this stability is prowded by the machinery and the authority of IUGS

2 The problem of the lower boundary o f the Cambrtan by A Yu Moscow)

Rozanov (Geologmal Institute, U S S R

Academy of Science,

Study of the lower boundary of the Cambnan System raises two essential problems (1) the m e t h o d and principle of drawing the boundary, and (2) the selection of the most suitable boundary stratotype. The questions of the lower boundary or boundaries of the Palaeozoic and the Phanerozom are dehberately n o t discussed here because they may be considered to be quite different questions. It seems more useful for the progress of international agreement to follow the pomts of discussion suggested by Stubblefleld and Cowle in Circulars of the Subcommlsslon on Cambrian Stratigraphy and the results of their postal e n q u m e s to members. These and numerous pubhcations of recent years show t h a t specialists studying the Cambrian appear to

222 //~

ARCTIC OCEAN

u~,.,? ~

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Fig 1 Late P r e c a m b r l a n a n d E a r l y C a m b r m n o u t c r o p areas m Siberia (Dmgram, modified f r o m K h o m e n t o v s k y et al , 1 9 7 2 )

be nearly u n a m m o u s m favouring the estabhshment of the base of the Cambrian on a blostratlgraphlcal basis It seems obvious to me t hat a b u n d a n t faunas with skeletons appeared on the globe simultaneously ( in the geological sense of the word) and by these faunas are mean t groups such as Archaeocyathans, hyohthlds, brachlopods, t o m m o t l d s and gastropods. O t he r groups acquired the ability to skeletahse later trilobites, ostracods and others. This earhest skeletal fauna will serve as the best base for the establishment o f the Cambrlan--Precambrlan b o u n d a r y I shall n o t c o n c e n t r a t e on this aspect, since m y concepts have been rather t h o r o u g h l y analysed In a n u m b e r o f papers (Rozanov, 1966, 1967, Rozanov et a l , 1969) and I pass directly to the b o u n d a r y s t r a t o t y p e

223

Although the Symposium on the base of the Cambrian held in Paris in 1958 did not pass any recommendations concerning a possible stratotyplcal section, we can n o w look upon this problem more optimistically. I believe that the Siberian platform (Fig 1) should be considered as one of the most promising regions In the sections of Cambrian and Precambrlan boundary deposits of the Slbermn platform we can speak in general terms of the followmg stratlgraphic subdivisions, which are well traced almost throughout the territory (from below Ul~wards, Fig. 2).

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Fig 2 Correlation of sections of the boundary deposits of the Cambrian and Precambnan of the Siberian platform (Rozanov et al , 1969 ) 1, hght-grey limestones, 2, dolomites, 3, sandstones, 4, conglomerates, 5, thin-layered and thin platy carbonate rocks, 6, wavylayered limestones and dolomites, 7, argillaceous red limestones, 8, dark grey, brecclated, algal hmestones, 9, shale, m places red (slanting shading), 10, the suggested Cambrlan-Precambrlan boundary, 11, the boundary between Tommotlan and Atdabaman stages, 12--14, boundaries between zones, 15, conditional boundaries, 16, archaeocyathaids, 17, hyohthlds, 18, trilobites, 19, M o b e r g e l l a , 20, gastropods, 21, groups of vague systematic allocation, 22, algae, 23, P a l e o h n a and A n a b a r l t e s , 24, Yudomian (Yu2 -- Nemaklt-Daldyn horizon) deposits, 25, Rhlphean deposits

224 (1) Yudomlan (Yu 1 + 2 and Yu 2), characterized by stromatohtes, oncohtes (Zhuravleva, 1964, Rozanov et al., 1969, Semlkhatov et a l , 1970) and sporadm elements of the Edlacarlan fauna (Sokolov 1972), its upper part called the Nemaklt--Daldyn (=Manykal) horizon contains the tubes Anabarites and Paleohna, c o n o d o n t o m o r p h o u s fossils and the first Gtrvanella and Renalc~s (2) Tommotlan stage subdivided into the zones Aldanocyathus sunnagmlcus (oldest), Dokzdocyathus regular~s (with subzones Lapworthella tortuosa and L bella), and D lenawus (youngest) These deposits are characterized by a very a b u n d a n t complex of skeletal fossils archaeocyathan, sponge, hyollthld, h y o h t h e l m m t h l d , gastropod, t o m m o t l d , brachlopod, e t c , as well as by numerous algae. In the first zone of the T o m m o t l a n stage CA sunnagln~cus) were f o u n d over 60 specms of skeletal fossils In the T o m m o t l a n stage no trilobites were recorded Worth attention is a wide distribution of Mobergella rad~olata Bengtson (formerly mentioned m our papers as M brastadz) in the upper zone of the Tommotlan stage. This level can be well fixed m the sectmns of the EastEuropean platform where I dlstmgulsh these layers as the Glebovo horizon (3) Atdabaman stage t h a t is also subdivided mto some zones according to archaeocyathans (Zhuravleva et a l , 1969) and contains in the lowermost parts the well-known Fallotapsts and Profallotaspts and m the rest numerous Olenelhdae (Khomentovsky and Repma, 1965). In general the Atdabanmn stage corresponds to the deposits of the Holrma zone (or the " H o l m m n " horizon) of the European sectmns. In almost all sectmns of the Slbermn platform the Yudomlan, Tommotlan, and Atdabanmn stages are represented by carbonate deposits Th~s fact makes these sectmns most important However, it would be wrong to think that all the problems dealing with the Cambrmn--Precambrlan boundary on the Siberian platform are solved uneqmvocally. There are three mmn questmns to be discussed here (a) Whether the sectmns of the middle course of the Aldan river are representative enough~ Some researchers beheve that the Cambnan--Precambrmn boundary coincides here with the llthologmal boundary (replacement of dolomites by hmestones) (b) Whether the Nemaklt--Daldyn horizon should be attributed to the Cambrian 9 (c) Does the faumstm complex lying above the Nemaklt--Daldyn horizon m the sectmns of the North Siberian platform belong to the A sunnagmzcus zone, or is it an older complex9 As to the first pomt, I shall say t h a t on the Aldan river the A sunnagmlcus zone complex has already been observed m the uppermost parts of the Yudommn Here, for instance, according to the observatmns of V.V. Mlssarzhevsky and those of mine, the following successmn of rocks ~s exposed m the sectmn situated at 7 km above the Ulakhan Sulugur nver m o u t h (Fig 3)

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(1) Near the w a t e r - h n e are e x p o s e d b l a c k o n c o h t m d o l o m i t e s w i t h concret i o n s o f f h n t Vlszble t h m k n e s s - - 0.3 m (2) T h i n - l a m i n a t e d , locally b r e c c l a t e d h g h t - g r e y d o l o m i t e s - 0 85 m (3) Massive, o n c o h t l c d o l o m i t e s , r u t h f l m t y c o n c r e b o n s - - 1 1 m (4) Free-grained, massive, passing a l o n g the strike into the l a m i n a t e d d o l o m i t e s - - 0 8 m. (5) DolomltlC, a u t h l g e m c b r e c c i a - - 0 3 m (6) Massive, m places w a v y , t h i n - l a m i n a t e d , h g h t - g r e y d o l o m i t e s - - 0 65 m (7) An i n t e r l a y e r o f a u t h l g e m c b r e c c i a and w a v y - l a m i n a t e d d o l o m i t e s - 03m (8) On t h e s h g h t l y w a v y surface o f t h e u n d e r l y m g r o c k s t h e r e is a b e d o f g l a u e o m t m , c a l c a r e o u s s a n d s t o n e t h a t ~s n o t p e r s i s t e n t m t h i c k n e s s A m o n g d e t n t a l m a t e r i a l o n e can m e e t shells o f b r a c h l o p o d s , as well as Aldanocyathus cf vlrgatus, Sunnagmia imbncata, Cambrotubulus decurvatu% Ttksztheca hcts, Torelella curvae, ete T h e s e fossils e n a b l e us to a t t r i b u t e this l a y e r with c o n f i d e n c e to t h e z o n e o f A sunnagmzcus -- 0 1 m (9) T h i n - l a m i n a t e d , in places w a v y - l a m i n a t e d d o l o m i t e s - - 0 7 m (10) B r e c c m t e d , vesmular, h g h t - g r e y d o l o m i t e s w i t h m l c r o p h y t o h t e s Nubeculantes abustus a n d u n d e t e r m i n e d f r a g m e n t s of shell fossils - - 0 3 m (11 ) O n c o h b c d o l o m i t e s o f grey or s h g h t l y p m k c o l o u r - - 0 2 m T h e a b o v e r o c k s f r o m m e m b e r 1 to 11 m e l u m v e b e l o n g t o the Y u d o m m n A b o v e are d e p o s i t s o f t h e P e s t r o t s v e t f o r m a t m n t h a t rest on an u n e v e n surface o f Y u d o m m n d o l o m i t e s ( w i t h p o c k e t s u p to o n e m e t r e d e e p ) T h e c o n t a c t b e t w e e n t h e f o r m a t m n s as well as l a y e r 8 are e x c e l l e n t l y e x p o s e d m t h e given o u t c r o p , a n d can be c o n t i n u o u s l y t r a c e d o v e r 1 5--2 k m (12) G r e y - g r e e n , p i n k , g r e y g l a u e o m t m h m e s t o n e s p a c k e d with o r g a m c r e m a i n s A t t h e base o f t h e m e m b e r t h e r e is o b s e l ~ e d a c o n s i d e r a b l e a d m t x -

226 ////

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ture o f d e t n t a l material and a higher c o n t e n t of glaucomte Small bloherms of archaeocyathans were f o u n d m these layers t o o -- 3.2--5 m Layer 12 has an a b u n d a n t c o m p l e x of the A sunnagmtcus zone, and higher, m highly red clayey hmestones was found even so low down, a c o m p lex of the zone D regulans ( R ozanov et a l , 1969) S o m e w h a t higher up m the Aldan river course can be observed similar relations at the b o u n d a r y of the formations and those of the Cambrmn and P r e c a m b n a n d es cnbed by V V M~ssarzhevsky m the o u t c r o p located 7 km westwards o f the village of Ugmo (Fig 4) However, here the fauna was observed directly m onc oht m dolomites of the u p p e r m o s t Yudom]an (an analogue of layers 9--11 of the previous section) and includes Chancellorla and Clrcothecldae Worth a t t e n t m n is the fact t hat a wash-out characterized by the presence of pockets is f o u n d within the zone A sunnagmlcus In addition, on the Sukharlkha river (the Igarka regmn) the com pl ex of the zone A sunnagm~cus can be observed bot h m the last t w o metres of the S u k h a n k h a f o r m a t m n , grey hmestones that are only m some places secondarfly d o l o m l b z e d , and m the 0 9--1 0-m think layers of the overlying formaUon of red clayey hmestones ( R oz a nov et a l , 1969) (Fig 5). In ot her words, despite the fact t h a t the appearance of the complex of the A smmagmzcus zone takes place m pure hmestones of the Sukhar~kha f o r m a t m n , the result o b t a m e d ss the same as on the Aldan river It should be said as well t ha t study o f the character of porosity of the Y u d o mmn dolomites reveals their metasomatm features As to wh eth e r the N e m a h t - D a l d y n horizon belongs to the Cambrmn, I would hke to say the following (a) The co mp l e t e hst of fossils f ound m the Nemaklt-Daldyn horizon certainly can by no means be com pa r ed to the greater varmty know n m the T o m m o t m n stage

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~ ~ c l a r l s &hut - ~ryptaporocyathus C ~ u n ~ c a n e n s l s Zhur C~psu~osyathus -----Fr~ Suasaec~at~ms Do~idocyathus D r e g ~ l a r l s ahur D l e n a l c u s Moz maltatoc~athus • --~idanoc~athus ~i s u n n s g i n i c u s I Z h u r ~ AI v i r g ~ t h u s ( Z h u r ) ~I. d r a ~ u n o v ~ (ioz) ~i ~katscf~enkol ~Vol A1 a n a b ~ r e n s ~ s ( V o l ) ~l tu r b ~ d u s (aoz) AI t e z m u ~ m u r u s Vol. Al.l~ar%aensls ioz 1 ~l.c~ut~s(~oz.) doDustocyathus M. b e l v e d e r l ioz S . r o b u s t u s kVol i ~ ~ious ~oz d dot a t u S ~OZ M.moor x QVol ) dgrDic~athus ~o.mo~oIICUS VOI. ~Iblrec~athus --Tenner~c~at~us otuy~ lumulocyathus I p r x m u s ~oz. Nochorolc~at~ue i~ v u l ~ a r l s Zhur ~rabxl~s Zuur ~ , ÷ i d a n ~ c u s ~nur I~ o c c u l a t u s Zhur D . s u o e r v a c u u s ~uz s_r]d]a~,lus ~oz. m ~ i t l f o r m l s ~oz -

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+

DorsollnevltuS D o c t l c i s t a t u s Sjss D. s e d e c g s t a t a s b y e s aorilithes K b l l a b l a ~ u s ~iss.

Fig 7 D l s t n b u U o n of h y o h t h l d s ( A c c o r d i n g t o R o z a n o v et al , 1 9 6 9 )

(b) We can guarantee s o m e tens o f s p e c m s o f t h e really s k e l e t a h s e d f a u n a a m o n g fossils o f t h e z o n e A sunnagmmus, there are n o such rehable fossils in t h e N e m a k l t - - D a l d y n h o r i z o n (Figs 6 - - 9 ) Indeed, it is m o s t likely that Anabarttes and Paleohna are representatives

230 cb fommotlan

Yudomlan

stage

w Nemaklt-Daldyn norizon

+ only Atdabarlan

i. I

~tage

zone A 1 zone D reguzone D suD.nagl larls lenalnlCUS s u b z o n e SMDzone c a s ~o~tuu Della ------

~elclonell= nemella

B. s e o t a t a n O

I

~acutzca

(~is~, k [hlsS~

parula Miss m~lycanlca ulnella

"

~

-

--

--

b.aSdab~ulca (~is~.) + G.S&VIZzk - llse + Tsmnuell~ ~ ÷ I elata LISS Igerella I ungulata LISS I monstrosa ~iss Anabarella A p l a n a Vost ~.indecor~ L~ss Latouchella L k o r o b k o v l ~Vost I L.memora~lllS ~iss ------Aldanella

~ ~

A.rozoa~OVl ~ I S S --

;~m, --"~ .

.

.

.

Q~iss

-- ,11~l~m----?----

A,a~tleborensls Q S h a l e r et F o e r s t e ) A.crasea ~lse. A.utchurlca ~iss A . o p e r o s a ~,iss Pela&~ella P lorenzl (tubayasnl) +

Fig 8 Dlshlbutlon of gastropods (According to Rozanov et al , 1969 ) o f w o r m s , but m such a case their t u b e s (or d w e l h n g b u r r o w s ) c a n n o t be c o m p a r e d m s l g m f m a n c e w i t h the s k e l e t o n s o f m o l l u s c s , a r c h a e o c y a t h a n s , b r a c h l o p o d s , etc (c) T h e p r e s e n c e o f faunal and floral e l e m e n t s o f t h e N e m a h t - - D a l d y n h o r i z o n distributed e l s e w h e r e in t h e u n d o u b t e d Cambrian enables us to say that w e deal w i t h c o n t i n u o u s s e c t i o n s and are observing the P r e c a m b r l a n - Cambrian natural s u c c e s m o n m the e v o l u t i o n o f the organic k i n g d o m s Lastly, a q u e s t i o n has b e e n r e c e n t l y raised by V E. Savltsky H o w e v e r , I t h i n k that it is a m l s u n d e r s t a n d m g V E Savltsky draws a t t e n t i o n to the fact that m the n o r t h o f t h e p l a t f o r m in the l o w e r c o m p l e x there are m a n y gastropods, w h e r e a s t h e s e are a l m o s t absent m the A sunnagmzcus z o n e o n the Aldan river I t h i n k that this c o n t r a d i c t i o n is n o t slgmflcant, b e c a u s e the a b s e n c e o f representatives o f a n y group d o e s n o t m e a n a n y t h i n g For mstance, there are n o a r c h a e o c y a t h a n s m Enghsh s e c t i o n s Yet, w e c a n n o t say that Cambrian 1 o f England is older than Cambrian 2 o f Siberia ol vine versa In a d d i t i o n V V M l s s a r z h e v s k y m 1 9 6 9 f o u n d o n t h e Y u d o m a rive1 a t y p i c a l l y n o r t h e r n c o m p l e x o f g a s t r o p o d s t o g e t h e r w i t h the fossils o f the A s u n n a g m t c u s z o n e ( S e m l k h a t o v et a l , 1 9 7 0 ) In c o n c l u m o n , I s h o u l d h k e to say that m t h e Siberian p l a t f o r m there are a

231

~J

¢o

lommotlan

Yudomlan I I

e

sta6e

Nemakit-Da/dym horizon + only Atdabanian

zone zone D reouzone D A I sunlarls lenaln~olnl-b totL Della cus cue ~uoea

_n~olitnellus ~ tenuls ~iss v]a~m~rov~e

stage

~iss

H islt~ous MIss n.~randis ~Iss Torellella sUrvae .lee i len~ifor~is ($$ss) I blConvexal~iss k~chtozla Anabarltes trls~lcatue llss trlBtlCh~S ~IS8 A trlp~r~itus ~isa. n trlo~rl~at~8 ~lS8 ternarlus .iss. n Co~posltus miss ~nbrotubulus C d e c u r v a t u s ~llee. ~oleolella

II

C . n l l l l n ~ s l LaVSS.] ~oleolus Q , $ r i ~ o n l s ~,/ss ¢oleololdeS C.trl..emlnatus ~±ss Saoellldlt~s I0~ o~ia i ~ ozlows~ ll(i,~iSS . n l a n a ~ ..iss. i ~ o n a t ~ ~ lea; C~Enella b~rbows~=e IiS3 el~nella ,

tor%uoe~ ul a Leil= leg CI C h O d & C ~ k~u31

o

j+

c e u t ~ Z ~ files + i o tic' e l l a I izifund~ LOIll Jrih~ liC .

@

-- ----

w

z%~e

s~!s

_h~.lie~11

I l'Ou

~rDa

Zig

_ _

.,. s~

Fig 9 Distribution of t o m m o t l d e s , H y o h t h e l m l t h e s , etc ( A c c o r d i n g to R o z a n o v et al , 1969 )

number of sections suitable for a stratotype of the Cambrlan--Precambrlan boundary Many of them, however, are in areas which are hardly accesslb]e Therefore I would like to draw attention to the sections of the middle course of the Aldan river (Fig 10) This region is the most suitable, since in the Lena--Aldan region are found the type sections of the Yudomlan and Tommotlan stages

)

)

0

UDOMIAN

lldonocyothus

lurnefocf

sunnog/n/cu$

lorluo$o

T/l~s/lheco I/c/s

Lopwoflhello

Lopworlhe//o hello

Pnolcu$- M o / a / h e c o

lok~docyolhus regulor/s

)ok/docyolhus

lumbers at heod of columns or)d [ongslde them cross-refer to the ommotlan Stoge publication by ozanov et a~ 1969

Dvortsy 1405 A - 2 2 4

3o

M407

yucteleekh

~io

-226A

1408/74

~r8

I-~----114

[-~iz

- M408/4 M408/2

5

-M408/10 M408/6-A225/13

M408 A-225

IIakhan - S u l u g u r

5 L

5

5

r

~3 .o ~n

233

Finally, a few words about the sections of the East-European platform Some scientists (M.F. Glaessner, B S Sokolov) have supposed that it was possible to choose a Cambrmn--Precambrian boundary stratotype on this territory Without going Into details, I present here Fig 11 (p 234) which clearly shows the vulnerability of this opinion

3 The Precambrzan--Cambrlan boundary problem and the questlon of suitable regional stratotypes by V E Sawtsky (Siberian Research Institute of Geology, Geophysics and Mineral Deposits) (1) A methodical approach to the deflmtlon of the Cambrian lower boundary cannot principally differ from that to the resolution of slmllm questions when the boundanes between other great Phanerozoic subdivisions are estabhshed That is why "specml" principles for definition of Cambrian lower boundary as well as for boundaries of its zonal and stage subdivisions must n o t exist According to conventional ideas and recommendations of the International Subcommlssion for Stratigraphic Classification of the lUGS, the reliable age meaning of boundarms may be defined on the basis of palaeontology but only in continuous sectmns of marine deposits whmh belong to a single palaeogeographic region (palaeobiogeographm province) The presence of transition layers having fauna of " m i x e d " ("transition") type between large subdivisions ~s the most typmal feature of a continuous section Isolation of these layers mto independent stratigraphic subdivisions is the most important way for defining the section's intervals The boundary to be found is determined from the foot or top of these layers according to arrangement and priority (2) The above mentioned being taken into consideration, the problem of the Cambrian lower boundary m Siberia appears to be that (a) None of the known sectmns satisfies the necessary reqmrements in full measure (b) The region of the middle course of the Aldan river, which is consldered by some researchers to be the best one In which to define the Fig 10 Sections of the boundary deposits of the Cambrian and Precambrlan of the middle course of the river Aldan 1, dolomites, 2, red argillaceous hmestones with thin mterlayers of grey hmestones, 3, grey hmestones, 4, oncohtm dolomites, 5, argillaceous limestones, 6, stromatohtes, 7, argllhtes, 8, argillaceous hmestones, brown-grey or hlac grey, 9, grey argillaceous wavy-laminated hmestones, 10, a batch of red strongly argillaceous hmestones, enrmhed with phosphatm fossils, 11, crystalline rocks, 12, (a) conglomerates, (b) sandstones, 13, marls, 14, bloherms of archaeocyathlds, 15, argillaceous dolomites, 16, wavy-laminated argillaceous dolomites, 17, brown-grey dolomites, 18, stromatohtes, 19, possible position for the basal Lower Cambrian boundary, 20, boundarms between zones

234

W Latv,a

Eston,a

Moscow

Mjosa

basra

(Norway)

"Izhora beds"

MIDDLE CAMBRIAN

Leman Stage

I b~ formation

z

Vergale


horizon

® ® ® ® ®

z"'

beds Kokumogm beds

nn <~ '~ }o ~ t--

Lukah

(=Talsy) hor,zon

Ventova ~

~

formation Lukah beds

Glebovo horizon z .~ --

Lontowa LU

horizon

o

Lontowa or "Blue C l a y " formahon

i

Key

-

o

o

conglomerate

@@®

Fe-oohtlc

® Mobergella

o f--

Rowno horizon

PRECAMBRIAN VENDIAN

Kothn formahon

Fig 11 Correlation of the Lower Cambrian section of the Baltm and the section of the Moscow s:ynechse

s t r a t o t y p e o f the Cambrian lower b o u n d a r y , does n o t satisfy the most imp o r t a n t condition for the d e t e r m i n a t i o n of chronostratlgraphm boundaries the sections must be monofaclal Here the b o u n d a r y is suggested to be m the c o n t a c t zone of sharply different hthofacms Y u d o m a (hght dolomite) and Pestrotsvet (varmgated clayey hmestone) There is no corresponding palaeontologmal charactenstm for transition-beds f rom Precambrmn to Cambrmn in this region the proposed b o u n d a r y at the base of the T o m m o t l a n subdwlslon coincides with the sharp hthologmal dl sconform l t y The abrupt appearance of more than 30 specms (23 genera) of archaeocyathans, hyohthel-

235

mmths, gastropods and other groups and the so-called "first zonal complex with skeletal fosmls" m the stratotype of the Tommotlan stage (the Sunnagm horizon) can only be explained by faunal migration (c) The most favourable sections for defimtlon of the Cambrian lower boundary are those of Zapadnoe Pnanabarye (between the K o t u y and the K o t u y k a n rivers), where there are contmuous sections of most richly fossfllferous transition-beds -- from Precambnan to Cambrian -- the Nemaklt-Daldyn horizon (= Anabarltes trlsulcatus zone, Savltsky, 1971) Here are widely spread remnants of worms (7) Anabar~tes tr~sulcatus, pogonophores Sabelhdzte~ sp (= Paleohna Sokolov), algae Ep~phyton mopmatum, Gtrvanella problemat~ca, Renalczs polyrnorphus, Renalczs s p , Mzc~ocodlum laxum and mmrophytohtes of the so-called Yudoma complex Nubecularztes antzs, N orblcularts, N abustus, N varms, Volvatella zonahs, N breuts The layers with abundant gastropods, h y o h t h e l m m t h s and hyohthlds Oelandwlla lzorobl~ow--Anabarella plana zone (Yegorova and Sawtsky, 1969) follow above Thus, the problem of the lower boundary of the Cambrmn of Slberm is trod up with the problem of the age of the Nemaklt--Daldyn horizon and I consider these tranmtmn layers to belong to the Cambrian system (3) At present there exists the o p m m n that the regmnal ehronostratlgraphm scale of tranmtmn beds from Precambrmn to Cambrmn of the Siberian platform consists of three subdlvlsmns of stage rank Yudommn, Tommotmn and Atdabaman I should hke to draw a t t e n t m n to the fact that th~s scale is of a compomte character Stratotypes of these subd~v~smns are rudely separated m different regmns (the Yudoma river, the Aldan river, the Lena river), they are m different hthofacms, they are characterized by different bmfacms, belong to different facms regmns and palaeo-bmgeographm provmces. The Yudommn complex belongs to the Olekma--Kotuy regmn (dolomites of the central part of the basra), the T o m m o t l a n stage belongs to the A n a b a r - S y n s k region (reef hmestones of the marginal part of the shelf) and the Atdabanmn stage belongs to the Yudoma--Olenek regmn (temgenous and carbonate depomts of the mner (7) shelf zone) On these grounds the sequence mentioned above does not meet the reqmrements of true chronostrat~graphm scales Still undecxded are the questmns of the relatmns between the Nemak~t-Daldyn and Yudommn (m the narrow sense) horizons as well as the Sunnagmlcus-hczs and Korobkom-plana zones (m the broad sense) whmh are connected with the problem of border eorrelatmn of separated stratotypes of different facms subdlwsmns According to Rozanov et al (1969), Yegorova and Sawtsky (1969) the Korobkovt-plana zone (Yudoma--Olenek regmn) is characterized by wide d l s t n b u t m n of polychaetes (7) Anabantes (5 specms), of gastropods Anabarella (4 specms), Latouchella, Igorella, of certam Camemda (Fomzchella mfundtbuhforma and hyohthlds Kugdatheca uoluta, Allatheca comca, Ctrcotheca conmca, C kutemlkov~, being completely absent m the Sunnagmlcus-hcts zone of the Aldan river (Anabar--Synsk regmn)

236

4 The terminal Precambrtan and Lower (Morocco)

Cambrian

of

the Antz-Atlas

by A Boudda (Servme de la Carte G~ologlque du Maroc, Rabat), G Choubert and A Faure-Muret (Bureau de Cartographm Ggologlque Internatlonale, Pans)

The terminal Precambrtan o f the Antt-Atlas P r e c a m b n a n III the last major orogenic cham of the South Moroccan Precambrlan (the Eastern Anti-Atlas, Fig 12), whose presumed age is of the order of +1,000 m y., is covered with major d l s c o n f o r m l t y by a very large volcamc co mp lex know n as P r e c a m b n a n III or Ouarzazate series (Choubert, 1948) It consists mainly of rhyolitic (lavas, lgnlmbnte, tuff, breccia etc ) and andesltm complexes Its m a x i m u m thickness is several thousand meters (Oued Dra gorges) Some red d e t n t a l deposits are associated with the volcanlcs Th ey b e com e considerable in the u p p e r subdivision of this system (TlOUlne serms) Some lenses of lacustrine limestone containing Collema are also f o u n d

Fig 12 Dlstrlbutxon of fossil localltms of the Lower Cambrian of the Anti-Atlas

237 This epoch of " s u b s e q u e n t volcanism" (of H. Stllle) and d e t n t a l sedimentation of continental ongm can be m t e r p r e t e d as a molasse stage of the last Precambrlan orogemc chain A d o u d o u m a n : marine sedimentation starts after ~ shght dl sconform l t y at the top of Precambrlan III. The strata, given the name of A d o u d o u m a n by C h o u b er t (1957) are dolomites and m o n o t o n o u s dolomltm hmestones reachm g a thickness of 2,000 m (m the valley of the Oued Adoudou, south of Tlznlt) This "S4rle des Calcalres m f 4 n e u r s " , however, starts m the western half of the Anti-Atlas with a "basal series", generally consisting of a basal conglomerate of no great thickness, with very constant dolomltlc beds and a thickness o f greenish schist w~th a horizon of coarse sandstone at about the middle The average thmkness is 100 m, with a m a x i m u m of 400 m m the Anzl subsidence p o c k e t The dolomites of the L ow e r A d o u d o u n l a n contain amino-acids of algal o n g m (studmd by M Praschnovsky, Wurzburg) similar to those of the Nama and Damara series m Southwest Africa and the shghtly oide~ series of Bamb m and Minas m Brazil (Boudda 1972) The reduced coastal facies also contain Collenza Volcanic activity was very reduced and strictly locahsed during the A d o u d o u n m n Examples are the trachytlc and andesltic volcanoes of Alougoum and E1 Gloah R a d l o m e t n c study (Rb-Sr) of these rocks has revealed a very widespread Hercynlan overprinting Rehct ages, however, mdmate 900 my " S 4 n e he-de vln" ("Wine-lees series") (Upper A d o u d o u m a n ) The Calcalres mf6rmurs are followed by an epoch o f regression, durmg which the sea retreated m the Western part o f the Anti-Atlas (Tlzmt regmn) The rest of the chain has been covered by reddish-wolet form at m ns of continental o n gm This is the "Serle he-de-wn" ("Wine-lees serms") which is referred to the Upper A d o u d o u m a n and consists of conglomerates in the East, sandstones ( " T l k l r t sandstones" of L Genti1, 1924) m the Ouarzazate regmn, and then becomes enriched with violet pehtes with salt crystal pseudomorphs West o f Tazenakht, levels of dol om i t e appear m the midst of these pehtes Their n u m b e r increases towards the west m the region of oscillation of seashores, with a pr e dom i nance of continental influences m the east and marine facms m the west The m a x i m u m thickness is of the order of 600 m In these marine levels Collenta and " b u s h y " S t r o m a t o h t e s are found, these are at present under study The " S 4 n e he-de-vln" may be taken to be substantially c o n t e m p o r a r y with the tlllltes of West Africa ( T a o u d e n m basra), whose radlometrlc age is at present believed to be 650- - 620 m y

The problem of the lower hmzt of the Lower Cambrian At the end of the regressive epoch of the "Sgrle he-de-vln" a new marine transgression coming f r o m the west invaded the greater part of South

238 Morocco. This is represented now by the hmestones and dolomltlC hmestones, generally rich m Collema, whmh have been called the "Calcalres Sup~neurs" (400--450 m think). This series was at first assigned entirely to the Lower Cambrian (Choubert, 1952--1959) Later, m vmw of the absence of palaeontological proof it was placed at the summit of the Upper A d o u d o u m a n The present tendency is to lower the base of the Lower Cambrian again to the upper boundary of the "Serm he-de-vln" and to begin this stage with the big marine transgression represented by the Calcalres Supeneurs These latter carbonates seem to be merely a cover facms of the platform characterlsmg the stable zones of the northern border of the Afrman craton In the north, on the other hand, m the direction of the Upper Atlas subsldence trench, this hmestone facms is gradually replaced by shale--hmestone formations with trilobites Thus, within the Western Anti-Atlas, m the Issafene synchne, the upper limestone facies goes up as far as Hup6's Zone IV (see below) while on the edge of the Sous plato, at Amouslek, the shale--hmestone facies begms as early as Zone I and archaeocyathlds are found in the last 30--50 m of the Calcalres Sup~rleurs Finally, even further north, in the Caledoman--Hercynlan geosynchne of the Upper Atlas, in the Ounem, the shale--hmestone facms begins at the top of the " S e n e he-de-wn" which is here partly marine It is in this shale--hmestone serms of the Atlas that trilobite faunas have recently been discovered by A Boudda, accompanied first by J P Schaer m 1964 and later by K Sdzuy m 1971 The Calcmres Supdneurs of the southern border of the Sous Plato (north flank of the Anti-Atlas) are followed by the following formatmns (1) First, the hmestone layers (30--50 m) just above the Calcmres Supgneurs contain archaeocyathlds Lenses of shales with trilobites (at present under study) are also found, but only very locally (2) Shale--hmestone serms (or Amouslek stage) This includes Hup~'s (1952) hrst three trilobite zones and has an average thickness of 200--250 m, it can be divided into Zone I (oldest) Fallotasp~stazemmourtens~s and Bzgotmops, Zone II Choubertella, Zone III (youngest) Dagumasp~s, Resserops, Fallotaspzs and others together with a great abundance of lnartmulate brachmpods The hmestone beds of these schists contain archaeocyath~ds and may yield bmherms (especially on the south flank of the Anti-Atlas). (3) The shale Serms (Issafene stage) also includes three zones of Trilobites and has a thickness of 100--150 m Zone IV (oldest) Antatlas~a, Zone V Bondonella, Neltnena and, as before, Antatlasza, Zone VI (youngest) Saukzanda and Gtgantopygus On the north flank of the Anti-Atlas this zone is invaded by "Calcmres

239 scormc~s" (scoriaceous hmestones) and therefore has no trilobites Its type is taken from Issafene, where Zone VI is partmularly rmh m trilobites of many different kmds, mostly named by Hup~ (including Callav~a, Longlanda, Alanzs~a, Strenuella, Aglagaha, Rochaeva, Termterella, Paget~ellus and others (4) The Sandstone Series, or Ashnr stage (near Gouhmme) is accompamed on the borders of Sous by thick volcamc tufts 150--200 m thick) There is only one zone of trilobites Zone VII Term~erella and Gent~laspzs, charactensed by the appearance of numerous Protolemdae, worthy of m e n t m n also are Serrod~scus, Klerulf~a, Nevadla and there are others The sandstone series marks the end of the sedimentary cycle of the Lower Cambrmn The sandstones and volcamcs correspond to a regressive phase whmh terminates this cycle, hence the name "Gr'es t e r m m a u x " (terminal sandstone), m the Upper Atlas their lateral equivalents are conglomerates, representing emergence (5) With the O u n k e n n'Ourmast serms (near Agdz) whmh is largely transgressive, the sedimentary cycle of the Middle Cambrian begins, it is very constant, but thin (60 m) It corresponds to Zone VIII whmh ymlds Myopsolenus magnus, Protolemdae and Klngaspzs (6) This last and youngest serms continues upwards with thick and monotonous serms of shales of the Middle Cambrmn including Paradoxtdes near the base

Lower Cambrian of the Ounem (south flank of the Upper Atlas) The shale-hmestone serms of the Ouenln has a thickness of nearly 400-450 m. It has no fewer than 11 levels of trilobites (ldentffmd by A Boudda and K Sdzuy m 1971), the first of whmh is at the top of the "Sgrm he-dev m " They are small trilobites, close to Blgotlnopsldae and at present under study by K Sdzuy at Wurzburg (Boudda and Choubert, 1972) As well as the trilobites, oncohtes (Gzrvanella) and stromatohtes m "bushes" or bloherms of varying size are also found m sandstone beds They belong to a shallow facies with rlpplemarks and mud-cracks at certain levels, as well as slumpmg. The next beds are characterlsed by a substantial development of volcanic pyroclasts, unfortunately devoid of fossils The pyroclasts have already appeared lower down, m the upper part of the shale-hmestone series Towards the top of this pyroclastlc complex the first lenses of hmestone with archaeocyathlds of the Ouneln are found The first trilobites m common with the Anti-Atlas are Antatlasza (Hup~'s Zones IV or V) This horizon is found above the pyroclastm complex, in the nelghbourhood of the first archaeocyathld horizons Another trilobite horizon provides Rochaeva, a genus identified by Hupd m 1959, but the descrlptmn of whmh has n o t been pubhshed (Hupg, 1969)

240 This horizon, which corresponds to Zone VI of the Anti-Atlas, follows the second d e v e l o p m e n t of h m e s t o n e lenses with archaeocyathlds Higher still, conglomerates are f ound m assocmtmn with pyroclastm deposits T h e y indicate emergence m nelghbourmg regmns and should correspond substantmlly with the "Gr'es t e r m m a u x " of the Anti-Atlas (Zone VII) These conglomerates contain pebbles from precedmg layers m whmh have been f o u n d specimens of the trilobite Rochaeva The f o u r t h and last archaeocyathld level of O une m is intercalated within the conglomerates The vertmal sectmn of the L ow er Cambrmn of Ounem ends with layers contalmng Protolenldae (Zone VIII of the Ant~-Atlas) The ht hol ogy can be either shale and sandstone or h m e s t o n e of the "Mzcmacca breccsa" t y p e These latter hmestones rich m Protolemdae, a c c o m p a m e d m the lower layers by Kmgaspzs and m the uppe~ layers by Paradoxzdes This hmestone therefore corresponds exactly to the c o m m o n l y accepted limit between the Lower and Middle Cambrmn It is, moreover, followed by the typmal thick shale series of the Middle Cambrian The total thickness of the lower Cambrmn m the Ouneln is of the order of 800 m

Correlatzon The exact correlation of the successive levels of the two vertmal sections is n o t y e t estabhshed with precision, smce Hup~'s f~rst three trilobite zones have n o t y e t been identified m the O u n e m Just as m the case of the upper hmestones of the Issafene, the Antatlasla Zone IV is the first dated level of the Ounem In this last vertmal sectmn, however, the Antatlas~a level is preceded by a non-fosslhferous pyroclastm serms which may perhaps correspond partly to the Amouslek stage On the o t h e r hand, it has been possible to relate the O unem shale--hmestone series very closely to the Anti-Atlas developments by wrtue of the recent discovery by A Boudda at Alt Bouzmane (on the South border of the Eastern Sous) of fauna similar to the primitive trilobites This deposit is found lmmedmtely above the "Serm he de w n " just hke the f~rst fosslhferous level of the Ouneln sectmn.

Conclustons The foregoing brief description shows t hat trilobites existed and could be preserved in stratlgraphm levels much older than previously considered. In the case o f the fauna of the Amouslek stage, Hupe (1957) had already stressed that the degree of evolution o f their trilobite fauna was more primitive than that of the specms of the "Olenellus z o n e " m Europe In the Ounem shale--hmestone series (which could be provisionally called the " O u n e m stage"), the fauna is even more primitive, preceding the Bzgotmops of Zone I In these circumstances, must we lower the base of the L ow er Cambrian, or must we create a new stage (or several new stages) which would

241

precede the "Olenellus Z o n e " and, sn the last analysis, take its place below the Cambrmn9 Stages of this kind have already been proposed, such as the Edmcarmn (H and G Termmr 1960) for the fauna of the terminal Precambrlan m Austraha, or the T o m m o t m n (A Yu R oz a nov et a l , 1969) for the fosslhferous layers w i t h o u t tnlobltes of the earhest Lower Cambrian m the U.S S.R (Siberia and elsewhere). The specml feature of the Ounem fauna is t hat ~t revolves trilobites It ~s hoped t hat their study will make it possible to o u t h n e certam parallels betweetl these different faunas

5 Possible stratotype sequences for the basal Palaeozow m North Amertca after P E Cloud ( D e p a r t m e n t of Geologmal Scmnces, University of Cahfornm, Santa Barbara, U S A ) (Review) Cloud (1973) bases his approach to a defl m t l on of the base of the Camb n a n on the separation of this c o n c e p t f rom that of the beginning of the Phanerozolc or o f the Palaeozom This is a clear alternative to the hnkxng of the three terms whmh is a d o p t e d m Harland's diagram (Table II, p 218) Cloud states " I n the absence of t y p e or reference sectmns for Phanerozom and Paleozolc, we have only their etymologmal lmphcatlons and usage on whmh to base our concepts and defimtlons If Phanerozom means anything it means "visible h f e " , with the strong l m p h c a t m n t hat it refers to ammal hfe that is wslble to the naked eye Thus rocks t hat contain obvious and clearly visible Metazoa such as those m the Edmcanan, must be Phanerozom, whatever else th ey are Likewise Palaeozom means ancmnt hfe, again w~th the lmphcatlon of ammal hfe Cambrian has no inherent etymologmal lmphcations and may mean whatever we want ~t to mean, as long as it ~s correlatable m some way to some of the rocks originally included m the t ype Cambrmn o f North Wales. I think that the base of the shelly fauna ~s not a bad place for t he base of the Cambrian, especmlly if, as seems hkely, the f~rst appearance of authentm Scohthus and Dtplocratermn can also be used to mark this level. But, m my wew, this is n o t a good place for the base of the Palaeozom and the Phanerozom " (Cloud, 1973, p 194 ) On this basis, Cloud exammes four sequences m N ort h Amerma whmh may contain the base of t he Palaeozom For each sequence he ~dentlfms, with more or less certainty, an mterval labelled as " C a m b r m n or Sub-Camb r m n " whmh is younge r than the Pr ot er o zom and older than the unquestloned Cambrmn with shelly faunas or at least with trace fossils beheved to represent t h e m (Rusophycus) The " candi dat e sequences" t hat may contain the transltmn from P r o t e r o z o m to Phanerozom (equated with t hat from pre-Palaeozom t o Palaeozom) and also rocks of Edmcarmn age and the Edmcarmn-Cambrmn trans~tmn occurs m f o ur areas the Appalachmn regmn, the Canadmn Cordillera, east-central Alaska and the southwestern Great Basra,

242 especially eastern California In southeastern Newfoundland Cloud includes m his Cambrmn or Sub-Cambrian (to whmh we may refer as the cntmal interval) the C o n c e p t m n G r o u p or part of it, the Cabot G roup and the R a n d o m F o r m a t m n (see now also Anderson, 1972) In the Canadian Cordillera the cntmal interval ~s the upper part of the Mmtte Group, w~th trace fossils, and the mam part of the McNaughton F o r m a t m n up to the equivalents of a Fallotaspzs-honzon occurring f ur t her to the west A think sequence m east-central Alaska and Y ukon T e r r i t o r y "offers some promise of perhaps containing the P r o t e r o z o m - - P h a n e r o z o m and Edmcarlan--Cambrmn translt m n s " In the Whlte--Inyo Mountains the cntmal interval is designated as the Reed Dolomite, below the occurrence of Wyattta but Cloud states t hat the u n c o n f o r m a b l y underlying Wyman F o r m a t i o n is of uncert am age and that the sequence is n o t a promising prospect as a complete s t r a t o t y p e succession for the base o f the Cambrian and related transltmns In the southern Death Valley Regmn, also m Cahfornm, the Stlrlmg Quartzite and part of the J o h n m e F o r m a t m n may fall m the cntmal interval Cloud expects the base of the Cambrian near the base of the Wood Canyon (with Rusophycus) and the t o p of the Proterozom within or near the top of the J o h n m e F o r m a t m n While apparently favouring California for a regmn whmh may yield at least a composite s t r a t o t y p e sequence for the transitions of interest, Cloud comes to the conclusmn that we are n o t y e t ready to identify the " b e s t " North Amerman s t r a t o t y p e either for the P r o t e r o z o m - - P h a n e r o z o m transltmn or for the base of the Cambrian and t ha t more work is needed This conclusion appears to be still vahd at the time of writing of the present review

6 Austraha In a paper entitled " T h e base o f the Cambrian and the first Cambrian faunas" B Dally gave m 1972 a progress report which included new data f r o m the South Australian sections (Fig 13) He concluded t hat "a defimtlon o f the base of a Cambrian System with reference to Precam bnan tllhtes is unrealistic and t hat the classical and t l m e - h o n o u r e d blostratlgraphm approach should be r e t a i n e d " (Daily, 1972, p 13) He claimed that trace fossils can be used as "stratlgraphm indicators" m sediments w i t h o u t shelly faunas when t h e y are c o n t e m p o r a n e o u s with shelly faunas elsewhere It seems to be his opinion that shelly faunas always mdmate a Lower Cambrian or y o u n g e r age He f o u n d the trace fossil Rusophycus m " l o w e r m o s t Cambrian" sediments, the age being suggested on the basis of u n c o n f o r m a b l e relationships with lower beds containing the Edlacara fauna and on the occurrences of D~plocraterton and Plaglogmus m the n e x t higher Form at i on He considers Rusophycus as p r o o f of a trilobite fauna existing at this time even though their shells do n o t normally appear until later, 1 e m the succeeding hmestones (Fig 14) Daffy clmms (p 13, abstr ), " I t is d e m o n s t r a t e d that b o t h the first appearance of shelly faunas and the first appearance of trilobite shelly fossils were dlachronous within the Early Cambrian" A final conclu-

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slon is th at the earhest Cambrian faunas can be looked u p o n as consisting of two c o n t e m p o r a n e o u s suites (1) " a shelly suite u p o n whose presence our existing c o n c e p t of the Cambrian Pemod hmges", and (2) "a trace fossil suite which by correlation with the shelly faunas can be used to recogmze cont e m p o r a n e o u s Cambrian depomts". Dally later c o m m e n t e d (pers c o m m , Jan 1974) t hat the placement of the Precambnan--Cambrlan b o u n d a r y in Australia is based on a direct comparison of the Austrahan trace-fossil assemblages with those within the t y p e provmce (the British, Scandmavlan and Baltm regions), not abl y Sweden and Norway It should be noted, however, t h a t unconform i t i es with discordance occur m all these areas. The oldest Austrahan shelly faunas f o u n d to date are placed m the early part of the T o m m o t l a n Stage of Siberia (see Fig. 2) but these are everywhere y o u n g e r than the oldest trace-fossil assemblages until n o w assigned to the Cambrian within the Austrahan c o n t m e n t Daffy claims world-wide slgnlfmance for the Australian sectmns because there is such a well defined sequence o f faunas on either side of the Precambrlan--Cambrlan b o u n d a r y , as he conceives its positron Central Austraha In the Amadeus Basin m the centre of Austraha, the Arumbera Sandstone is conformable, at least locally, with the underlying Pertatataka F o r m a t i o n

245

m whmh the Olymptc Member contains the u p p e r tflhtes. The lower tfihtes o f thzs basra occur m the n e x t lower Areyonga F o r m a t m n . The Arumbera Sandstone was fzrst described (wrongly as a greywacke) from the Ellery Creek sectmn by Pnt char d and Qumlan (1962) It reaches m a x i m u m thicknesses o f over 1,300 m m the east and 800 m m the west In some sectmns zt can be dzwded into f our umts, red sfltstones being followed by sandstones twine, wzth a conglomerate locally developed at the t op of the ftrst sandstone umt. In ot he r sectmns zt zs more clearly dtvzded into three members, the u p p er szltstones forming a p r o m m e n t stratzgraphm marker whmh ~s here informally referred to as the M~ddle Arumbera Daffy (papers m preparatmn) now wzshes to restrmt the t er m Arumbera F o r m a t m n to the " r e d coloured clastms" of the lower of three umts and gzve F o r m a t m n status and names to the mzddle and uppe r um t s (Fzg. 15). In one sectmn a p e n n a t u h d - h k e fossil szmflar to a spectes described from the Edmcara fauna, "Rangea" cf longa Glaessner and Wade, was f o u n d at Deep Well, at the base of the Arumbera Sandstone There seems to be a structural break above the lower Arumbera sandstones at th~s locahty The mzddle and u p p e r umts appear to be mzssmg. The lower sandstones are here overlain by the evapontm, c o n t o r t e d Chandler Lzmestone Above zt zs the " O r d m n " Giles Creek Dolomite Accordmg to Op~k (1968), ~ts fauna is of W~tt,ams Bore o:

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246 Lower Middle Cambrian age Others consider it as u p p e r m o s t Lower Cambrian ("Protolenus--zone") If this is h y p o t h e t m a l l y equated to the B o t o m a Stage of Siberia, then the underlying Chandler Limestone, the T o d d River Dolomite which overlies the Arumbera c o n f o r m a b l y in the Ross River section, and the upper two members of the Arumbera would correspond to the Atdaban and T o m m o t Stages in the sense of Rozanov These correlations c a n n o t be blostratlgraphmally proved at present In the sections at Ross River. Williams Bore and Juha Creek, the lower half o f the Arumbera contains only few, uncharacteristic trace fossils The u p p er half contains such fossils abundantly, including characteristic Lower Cambrian forms such as Plaglogmus, Psarnmwhnltes, Phycodes, and arthropod tracks The T o d d River Dolomite into which this f o r m a t i o n grades upward contains unldentifmd archaeocyathlds and the brachlopod (9) Mtcromztra m Its lower part, 160 m above the base of the middle m e m b e r of the Arumbera This is Daily's Faunal Assemblage 2, L ow er Cambrmn (Daily, 1956) The T o d d River D ol om i t e is overlain by the Giles Creek Dolomite The geologists of the Bureau of Mineral Resources who have carrmd out regmnal mapping (Wells et al., 1967) and have measured many sections over a distance o f 250 km, east and west of Ahce Springs, consider the Arumbera s e d lmen tatmn as c ont i nuous from Late Precambrmn to Cambrian There are no obvious indications o f any basw environmental change within the Arumbeta F o r m a t i o n whmh would a c c ount for the sudden appearance of abundant Cambrmn trace fossils m the middle m e m b e r The biostratlgraphm sequence in Central Australia is remarkably close to t hat described by Banks (1970) f r o m n o r t h e r n N or w a y but the stratigraphm distance from the top of the Up p er mo s t tflhte to the Breivlk F o r m a t i o n with its Cambrmn trace fossils ~s only ab o ut 500 m while m the Amadeus Basm Its equivalent is a b o u t 1,500 m The scarcity of shelly fossils pr obabl y makes the record of the Precambrlan--Cambrmn transltmn m the Amadeus Basin less suitable as a standard for mter-regmnal correlation than the Siberian or Moroccan sequences It is, however, ~mportant for regmnal correlations over vast areas of the N ort hern Territory, South Australia and Western New South Wales, as shown by Daily (1972, and papers m press). CONCLUSIONS As we have stated m the i nt r oduct i on, this revmw is intended to be more than a hlstormal record of a symposium held in 1972 The conclusions arising from the papers presented and discussed at the s y m p o s m m are therefore intended to outline n o t merely the past but the present consensus of opinion as far as we can discern It, and problems for future work as t h e y present themselves now (1) The world-wide search for a standard sequence of strata deposited during Late Precambrlan and Early Cambrian time, in u n i f o r m facms and

247

with some features whmh can be used as a basis for long-range correlation, must continue No single sequence whmh would satisfy all cmtena for a boundary stratotype has y e t been ldentlhed (2) In addition to the sequences m Siberia, Morocco and Austraha whmh are described here briefly and on whmh further work is m progress, and to strata m the U S.A revmwed elsewhere by Cloud (1973), important sections and correlations have been discussed m two volumes on the Cambrian of the Bmhsh Isles, Scandinavia, the Amerman Continent and part of the Arctic, edited by C.H Holland (1971, 1974) These sections and correlations will also be considered m future work The Acado-Baltm region, including the Russian Platform, can still be worth considering, n o t withstanding Rozanov's vmw (p. 233) (3) There is substantial agreement on the lower and upper hm~ts w~thm whmh an agreed base of the Cambrmn must be defined Strata containing the Edlacara fauna and its equivalents are pre-Cambrmn The stratlgraphm interval m which this distractive fauna occurs has been named Edmcaran, Vendmn, Varanglan or Eocambman It commences m many places with glaclgene sediments. Strata containing certain dlversffmd trilobite faunas ("Holm~a Zone", "Olenellus Zones", etc ) are unquestmned Lower Cambrmn These strata are underlam m m a n y sections by sediments which do not contain trilobites They may contain trace fossils of possible arthropod origin, among others, or tubular fossils and others of problematic omgm, and also archaeocyathlds or brachmpods (4) A possible procedure for selection of a boundary reference point ~s outhned by Harland (p 219) Its core is the suggestion to select sequences containing suitable data for correlation ("ideally a variety of fossil taxa plus the posslblhty of radlometric age determination and palaeo-magnet~c correlatmn"). In practme, "a successmn of events potentmlly useful m correlation" (Harland m Holland 1974, p 35) should be selected at different places, their observable sequence could be considered as synchronous (5) One of the results of the mspectmn of sectmns m S~bema by the Working 13roup (Cowm and Rozanov, 1973, 1974) was the selectmn of a number of "Working Reference Points" which could provide a focus for further work (6) The consequences of the selection of one such " e v e n t " , the bmstratlgraphic change from the Zone of Aldanocyathus sunnag~n~cus to that of Doteldocyathus regular~s--Lapworthella tortuosa and its apphcatmn as a Period b o u n d a r y is shown m Harland's Table II (p 218) It should be noted however, that this is selected as an example, n o t a proposal for a boundary, and that the base of the Cambrian is n o t necessarily and generally acceptable as the base of the Palaeozom Era and of the Phanerozom Eon (see Cloud, p 241) (7) It follows from careful consldelatlon of the recorded observations and their stratlgraphm interpretation that the selection of a reference standard for the Precambrlan--Cambrmn boundary cannot be reduced to recording

248

the first appearance of any mdlwdual t a x o n or kind of fossil The first appearance e.g o f a genus or specms of trilobite or of trilobites of any kmd m the geologmal record at any partmular l ocaht y reflects (a) present knowledge, and (b) chance preservatmn. Such records can be rendered mapphcable as standards by new discoveries, whereas a d e f m l t m n of the t y p e "Assemblage X seen to change at the chosen hor i z on to Assemblage Y " is less easily contradmted, partmularly ff it appears as one of a sequence of brutal transitions affecting varmus kmds of organisms The first appearance of shelly faunas is o f t e n considered as a statable reference event for the begmnmg of the Cambrmn (see Rozanov, p. 222) This is, at the required level of preclstun o f stratlgraphm correlation of this boundary, subject to the same objectmn as the f~rst appearance of taxa, m addition to the dfffmulty of defm m g " s h e l l y " (cf. tubes of Anabar~tes m the Yudommn, orgamc-walled tubes of Sabelhdztes m the Late Precambrmn) Bmlogmal evolutmn progressed from soft-bodmd metazoans to others with m m e r a h z e d skeletons about the beginning o f Cambrmn time, but the use of this observatmn as a reference poi nt m the stratlgraphm scale or a b o u n d a r y marker m speclfm se& m ent ary sequences c a n n o t be proposed w~thm suffmmntly precise strat~graphlc hm~ts It concerns n o t a single event but a whole host of e v o l u t m n a r y steps Recourse to a definition based on an agreed p o i n t m a c o n t m u o u s sequence of strata is the only reasonable approach to the solutmn of a b o u n d a r y problem m stratigraphy and this approach now dominates efforts to solve the partmularly dfffmult p r o b l e m o f d e f m m g the base of the Cambrian

REFERENCES Anderson, M M , 1972 A possible time span for the Late Precambrlan of the Avalon Peninsula, Southeastern Newfoundland in the hght of worldwide correlation of fossils, tllhtes, and rock umts within the succession Can J Earth Scl , 9 1710--1726 Banks, N L , 1970 Trace fossils from the Late Precambrlan and Lower Cambrian of Flnnmark, Norway Geol J , Spec Issue, 3 19--34 Boudda, A 1972 Comptes Rendus d'actlvlt~ pour l'ann6e 1971 du Servme de la Carte G~ologlque du Maroc, pp 57--58 Boudda, A and Choubert, G , 1972 Sur la hmlte mf~rleure de Cambrmn du Maroc C R Acad Scl Paris, 275, S~r D 5--8 Choubert, G 1948 Note sur la g~ologm de l'Antl-Atlas Rep Int Geol Congr, 18th, London, 1948, pt 14 29--44 Choubert, G , 1952 Hlstolre g~ologlque de l'Antl-Atlas, m G~ologm du Maroc Congr G~ol I n t , 19e, Alger, 1952, Monogr Region, S~r 3, Maroc, 6 77--194 Choubert, G , 1952a Introduction stratlgraphlque le Precambrmn III et le G~orgmn de l'Antl-Atlas In P Hup~ (Edlteur), Contribution fi l'Etude de Cambrmn Inf~rmur et du Pr~cambrmn III de l'Anti-Atlas Marocam Notes M~m Serv G~ol ~ a r o c , 103 17--39 Choubert, G , 1954 Vue d'ensemble sur l'Infracambrmn et le Pr~cambrmn de l'Antl-Atlas (Maroc) C R Comm Assoc Serv G~ol Afr R ~ u m o n d e Nalrobl 1954 105--116 Choubert, G , 1957 L'Adoudounlen et le Pr~cambrmn III dans l'Antl-Atlas In Les relations entre Pr~cambrmn et Cambrmn Coll Int Centre Nat Rech Scl Paris, 76 143--162

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250 Hup~, P , 1960 Sur le Cambrmn mf~rmur du Maroc Rep Int Geol Congr 21st, Copenhagen, 8 75--83 Keller, B M , (Editor), 1963 Verkhmy Dokembrly (Upper Precambman) In Strahgratlya SSSR,716pp (InRusslan) Keller, B M , 1971 Vend i Yudomly (VendJan and Yudomlan) Bull hloscow Soc Naturahsts, Geol Ser 46 (3) 19--27 Keller, B M , :1972 Vehkm oledenemya v lstorn zemh (Great glaciations m the history of the Earth) Soy Geol , 9 26--35 (In Russian) Kohmentovsky, V V and Repma, L N , 1965 Nlzhmy Kembrly Stratotlpmheskogo Razreza Slblrl (The Lower Cambrian of the Stratotype Section of Siberia ) Akad NaukSSSR,S~b Otdel,199pp Khomentovsky, V V , Shenhl, V Yu , Yakshm, M S and Butakov, Ye P , 1972 Opornye razrezl otlozhemy verkhnego dokembllya i nlzhnego kembrlya Slblrskoy platformy (Standard sections of Upper Precambrlan and Lower Cambrian sediments of the SLberlan platform) Trans Inst Geol Geophys Acad Scl, Slb Branch, 14:l (In Russmn) Lafltte, R et al , 1972 Essm d'accord international sur les probl~mes essentmls de la stratlgraphm C R Somm Soc G~ol France, pp 36--45 McLaren, D J , 1972 Report from the Committee on the Sflurlan-Devoman Boundar5 and Stratigraphy Geol Newsl, 4 268--288 Oplk, A A , 1968 The Ordmn Stage of the Cambrmn and its Austrahan Metadoxldldae In Palaeontologlcal Papers Aust Bur Mm R e s o u r , Bull, 92 113 Prltchard, C E and Qulnlan, T , 1962 The geology of the southern half of the Hermann.~burg I 250,000 sheet Aust Bur Mln Res R e p , 61 Rozanov, A Y u , ]966 Problems of Cambrmn lower boundary Itogl Naukl, Obshch Geol Stratlgr VINITI, Moscow Rozanov, A Yu , 1967 The Cambrmn lower boundary problem Geol Mag , 104 4] 5-434 Rozanov, A Yu , Mlssarzhevsky, V V , Volkova, N A , Voronova, L G , Krylov, I N , Keller, B M , Korolyuk, I K , Lendzlon, K , Mlkhmak, P , Pykhova, N G , and Sldorov, A D , 1969 T o m m o t s k y yarus i problema mzhney gramtsy kembrm (The To m m o t l an Stage and the problem of the Cambrian lower boundary ) Trans Geol Inst Acad Scl U S S R (206 3 8 0 p p ) ( I n R u s s m n ) Sawtsky, V E , 1970 O prawlakh opredelemya mzhney granltsy kembrl:ya 1 krupnykh khronostratlgraflchesklkh podrazdeleml fanerozoya (Rules for the determination of the lower boundary of the Cambrian and of larger chronostratlgraphm divisions of the Phanerozom) Trudy SNIIGIMS, 110 11--23 (In Russian) Semlkhatov, M A , Komar, V A and Serebryakov, S N , 1970 Yudomsky kompleks stratotlpmheskoy mestnostl (The Yudoma complex of the stratotypmal area) Trans Geol Inst Acad Scl U S S R , 2 1 0 Sokolov, B S , 1971 Vend severa yevrazn (Vendmn of Northern Eurasia ) Akad Nauk SSSR,SIb O t d , G e o l G e o h z 6 13--22 (In Russian) Sokolov, B S , 1972 Vendsky etap v lstorn zemh (The Vendmn Stage m the Earth's history) In 24th Int Geol Congr Dokl Sov G e o l , Palaeontology, pp 114--124 Sokolov, B S , 1973 Vendlan of Northern Eurasia In Proc 2nd Int Syrup Arctic Geology, Am Assoc Pet Geol M e r e , 19 204--218 Te~mler, H and Termmr, G , ]960 L'Edlacarmn, premier ~tage pal~ontologlque Rev Gen S c l , 6 7 79--87 Theokntoff, G , 1968 Cambrmn blogeography m New England In E-An Zen, W S White, J B BradLey and I B Thompson Jr (Editors), Studms of Appalachmn Geology, Northern and Maritime Intersclence, New York, N Y , pp 9--22 Wade, M , :1970 The stratlgraphlc distribution of the Edmcara fauna m Austraha Trans R Soc S A u s t , 9 4 87--104 Walter, M R , 1972 Stromatohtes and the bmstratlgraphy of the Austrahan Precambrmn and Cambrian Palaeontol Assoc , Spec Pub , ]1 1--191

251 Wells, A T , Ranford, L C , Stewart, A J , Cook, P J and Shaw, R D , 1 9 6 7 The geology of the north-eastern part of the Amadeus Basra, Northern Terrltory Aust Bur Mm Resour, R e p , 113 Yegorova, L ! and Savltsky, V E , 1969 Kembrllskaya stratlgraflya 1 blofatslya S]blrskoy platformy (Zapadnoye Prlanabarlye) [Cambrian stratigraphy and blofacms of the Siberian platform (Western Pnanabarlye) ] Trudy SNIIGIMS, 43 Zhuravleva, I T , Korshunov, V I and Rozanov, A Y u , 1969 Atdabansky yarus I ego podtwerzhdeme na osnovaml arkheotslat v stratotlpmhesko'm razreze (The Atdabaman Stage and tts substantiation based on Archaeocyatha m the stratotype section ) In Biostratlgraphy and Paleontology of the Lower Cambrian m Siberia and the Far East, I Nauka, Moscow, pp 5--223 (Received September 25, 1974)