Stromatolite biostratigraphy

Precambrian Research, 15 (1981) 353--371 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

353

STROMATOLITE BIOSTRATIGRAPHY

J. BERTRAND-SARFATI Centre G$ologique et G$ophysique, Universit@ des Sciences et Techniques du Languedoc, Montpellier (France) M.R. WALTER Precambrian Paleobiology Research Group, University o f California, Los Angeles, CA

(U.S.A.) (Accepted for publication February 5, 1981)

ABSTRACT Bertrand-Sarfati, J. and Walter, M.R., 1981. Stromatolite biostratigraphy. Precambrian Res., 15: 353--371. Stromatolites are now used more and more frequently in attempts to solve Precambrian stratigraphic problems. Abundant new data that have become available during the last two years support the view that the methods are valid and useful, provided that rigorous taxonomic procedures are followed. Contrary to earlier indications that Early and Late Proterozoic stromatolites are identical, none of the 53 forms ('form-species') now known from the Early Proterozoic are also reported from younger rocks. Nonetheless, it has become increasingly apparent that correlations based only on group ('form-genus')-level identifications can be misleading. The problems of establishing identity at the form-level indicate the need to substantially improve descriptive methods, and in particular to make more use of numerical techniques. Stromatolite assemblage zones distinguished in Middle to Late Proterozoic sequences are 100--350 Ma in duration, with boundaries at 1650 -+50, 1400 -+50, 1050 -+50, 650--680 -+20 and 570 -+20 Ma.

INTRODUCTION

S t r o m a t o l i t e s are being used in P r e c a m b r i a n s t r a t i g r a p h y n o t o n l y in the t r a d i t i o n a l areas o f t h e U.S.S.R., Africa, Australia a n d India, b u t , also, m o r e r e c e n t l y , in Canada, t h e U.S.A., M e x i c o a n d especially China. Their use in intrabasinal c o r r e l a t i o n {e.g. Bertrand-Sarfati a n d T r o m p e t t e , 1 9 7 6 ) is n o w well d o c u m e n t e d a n d can be c o n s i d e r e d r o u t i n e and u n c o n t r o v e r s i a l and is n o t , t h e r e f o r e , discussed here, a l t h o u g h t h e t a x o n o m i c p r o c e d u r e s r e c o m m e n d e d b y t h e a u t h o r s a p p l y as m u c h t o this field as t o b i o s t r a t i g r a p h y . O n t h e o t h e r h a n d , t h e use o f s t r o m a t o l i t e s in interbasinal and i n t e r c o n t i n e n t a l c o r r e l a t i o n (Preiss, 1 9 7 7 ) , is still insufficiently d o c u m e n t e d t o be c o n s i d e r e d d e m o n s t r a b l y valid, t h o u g h r e c e n t w o r k c o n t i n u e s t o s t r e n g t h e n t h e view

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that the method not only works, but is very useful. The recently published Chinese studies are especially significant in determining the validity of stromatolite biostratigraphy, but we are unable to critically analyze them here because translations of the descriptions have not yet been published. Stratigraphic intervals defined by changes in stromatolite assemblages are 100--350 Ma long, for the period 1650--570 Ma ago (Table I), although TABLE I Subdivision of the Middle and Late Proterozoic based primarily on stromatolite assemblages in the U.S.S.R. (Aksenov et al., 1978) Cambrian 570 -+20 Vendian 650--680 -+20

Riphean

Kudashian (R 4) Karatavian (Late Riphean, Rs) - - 1 0 5 0 -+50 Yurmatinian (Middle Riphean, R2) - - 1 4 0 0 -+50 Burzyanian (Early Riphean, R~ ) - - 1 6 5 0 -+50

there are indications of a possible finer subdivision (Semikhatov, 1976, p. 350). No attempt has yet been made to use stromatolites to zone rocks older (Early Proterozoic and Archean) or younger (Phanerozoic) than this, although, as is discussed below, these rocks do contain distinctive stromatolites. In this review, we have not confined ourselves to a consideration of only Late Proterozoic correlation, because the validity of the methods is best tested by considering the whole of the Proterozoic record. The most serious criticisms of stromatolite biostratigraphy have resulted from studies of Early Proterozoic stromatolites, as it has been asserted that many of these are identical to those from the Late Proterozoic. We pay particular attention to that problem in this review. Steady progress has been made in the understanding of stromatolite morphogenesis (Hofmann, 1977; Horodyski, 1977; Monty, 1977; Waiter, 1977; Semikhatov et al., 1979), an essential ingredient in any interpretative use of stromatolites, whether for biostratigraphy or palaeoecology. Support for stromatolite biostratigraphy has come from the observation (Schopf, 1977} that significant changes in microfossil assemblages occur approximately at the boundaries between the Lower and Middle Riphean and the Upper

355

Riphean and Vendian; these boundaries are presently defined by changes in assemblages of stromatolites. The major obstacle remaining to retard progress in stromatolite biostratigraphy is the confused state of the systematics. Here, the authors advocate a methodological and taxonomic approach that we have individually found effective and reasonable, and which if more widely used will facilitate more rapid progress in this field. The authors have used the three-fold subdivision of the Proterozoic advocated by the Subcommission on Precambrian Stratigraphy (Sims, 1979) and the IUGS Working Group on the Precambrian for the U.S.A. and Mexico (Harrison and Peterman, 1980); the boundaries of the Early, Middle and Late Proterozoic are set at 2500, 1600 and 900 Ma, respectively. The discussion of stromatolite biostratigraphy uses the stratigraphic framework established in the U.S.S.R., where the Riphean has been considered to have four subdivisions (Burzyanian, Yurmatinian, Karatavian and Vendian). More recently in the U.S.S.R., it has been advocated that the Vendian be separated from the Riphean as a unit of equal rank, and an additional Riphean subdivision, the Kudashian, be recognised between the Karatavian and the Vendian {Table I); this scheme has not yet been used in the Soviet stromatolite literature (Chumakov and Semikhatov, 1981). BIOSTRATIGRAPHY

Despite apparently disruptive disputes over stromatolite systematics, there is sufficient agreement among investigators for effective communication to occur and to be expressed, for instance, in published tables of ranges of stromatolite taxa from all over the world (Semikhatov, 1976, 1978c; Komar et al., 1977a; Preiss, 1976b). Most workers use some variant of the taxonomic approach first advocated by Krylov (1963), in which the stromatolites are classified using their gross morphology, lamina shape and microstructure (we refer to the last two together as 'fabric'). We review here recent studies that can be considered within this taxonomic framework. Recent Soviet work has concentrated on Riphean stromatolites from the Siberian platform: from the Anabar area (Shapovalova and Krylov, 1976, 1978; Krylov, 1975; Shpunt et al., 1976; Shenfil, 1978a, b), Yenissei area (Kar pinsky and Shenfil, 1978; Shenfil, 1978c), far-eastern Siberia (Shapovalova, 1976; Shapovalova et al., 1978), Uchur Maya area (Komar et al., 1977b; Serebryakov, 1975, 1976b), and the Patom upland (Dolnik and Vorontsova, 1975; Dolnik, 1976). Vlasov {1977) has described new Riphean stromatolites from the Ural Mountains, and there has been a stratigraphic review of the Lower Riphean of Siberia (Semikhatov and Serebryakov, 1978). There has also been renewed interest in two other fields: Phanerozoic stromatolites of Siberia (Krylov, 1975; Kyrvel, 1976, 1977), and pre-Riphean stromatolites (Dolnik, 1978; Makarikhin, 1978; Krylov and Perttunen, 1978); the pre-Riphean stromatolites are discussed below. Many recent Indian studies have been based on the premise that stroma-

356 tolites are useful for biostratigraphy. The classifications used are much the same as those employed elsewhere, but there has been an unfortunate tendency to use outdated nomenclature. Recently published works are those on southern India (Govinda Rajulu and Gowda, 1975; Reddy, 1975; Prasad et al., 1976), Madhya Pradesh (Chandra and Battacharya, 1973; Sen, 1976; Schnitzer, 1977; Srivastava, 1977), Uttar Pradesh and Rajasthan (Raha, 1974; Barman, 1975; Chauhan, 1975; Verma, 1975; Kumar, 1976, Tewari and Kumar, 1977; Anonymous, 1977a,b; Barman et al., 1978; Misra, 1979); some pay special attention to the relationships of stromatolites and phosphorites in the Aravalli Supergroup (Munshi et al., 1974; Banerjee, 1975:; Verma and Barman, 1975; Kumar, 1977). Stromatolites have proven useful in interpreting the structurally-complex Himalayan region (Acharya, 1974; Raha and Sastry, 1976; Prashra, 1977; Sinha, 1977; Mathur, 1981; Raha, 1981). All of these workers use stromatolite assemblage zones comparable with those of Semikhatov (1976). In a large project aimed at a better understanding of this zonation, Indian geologists organized a "Workshop on Stromatolites" in Udaipur in 1978, with a complete review of recent Indian studies (Verma, 1978). Studies of Australian Precambrian and Early Paleozoic stromatolites were designed to test the validity of stromatolite biostratigraphy, and to solve local stratigraphic problems (Cloud and Semikhatov, 1969; Glaessner et al., 1969; Poole, 1976; Preiss, 1976a; Grey, 1978; Preiss and Kinsman, 1978; Walter, 1978; Walter et al., 1979). In general, the results were consistent with those of the Soviet stromatolite biostratigraphers, but significant anomalies were discovered when Early Proterozoic stromatolites were studied (see below). Chinese stratigraphers use assemblages of stromatolites to subdivide and correlate the Middle and Late Proterozoic Sinian Suberathem (Hsing and Liu, 1973; Cao, 1974; Cao and Liang, 1974; Cao and Zhao, 1978a,b; Lion and Cao, 1976; Zhu et al., 1978) {Note that Cao is the official Piny in transliteration of the name earlier rendered as Tsao.) Up to six assemblages are recognized, with the oldest possibly being Early Proterozoic. A great many similarities and c o m m o n taxa at the group level are reported not only with the Soviet assemblages, but also with those described from Australia. Personal observations support the reported close similarities of the Chinese and Australian assemblages. We can note, however, that the 'red algae' (Manicosiphonaceae) reported from the Sinian of China and used in biostratigraphy are inorganic crystal pseudomorphs, probably after gypsum (Glaessner, 1980). Recent studies of North African stromatolites have emphasized their utility in biostratigraphy; these studies are based in the Sahara (Bozhko et al., 1974; Bertrand-Sarfati, 1976a; Bertrand-Sarfati and Trompette, 1976), Mali (Bertrand-Sarfati and Moussine-Pouchkine, 1978}, southern Morocco (Schmitt and Monninger, 1977; Schmitt, 1979) and on the Congolian craton (Cahen, 1972; Bertrand-Sarfati, 1976b). In a brief study of Zambian stromatolites, Gunatilaka {1977} disputed their biostratigraphic usefulness. The

357

Early Proterozoic stromatolites described by Bertrand-Sarfati and Eriksson (1977) are discussed below. Many studies of Canadian stromatolites have been concerned with their paleoenvironmental significance and use in intrabasinal correlation (e.g. Campbell and Cecile, 1975; Cecile and Campbell, 1978; Donaldson, 1976b; Henderson, 1975), but few have explored their use in biostratigraphy (Donaldson, 1976a; Hofmann, 1974, 1977, 1978; Semikhatov, 1978b). None of the publications on Riphean stromatolites, with the exception of those of Hofmann (1974) and Young and Jefferson (1975), use binomial nomenclature and therefore are not considered further in this review (see also Hofmann, 1976; Aitken, 1977; Jefferson and Young, 1977; Aitken et al., 1978a,b; Campbell, 1978, 1979}. Changes in stromatolite morphologies during the Paleozoic have been analyzed using the Visean stromatolites of Nova Scotia (Geldsetzer, 1977). In the U.S.A. the same reserve is observed regarding the biostratigraphic significance of stromatolites, despite the encouraging analysis published by Cloud and Semikhatov (1969). Among various works describing stromatolites from Middle and Late Proterozoic sequences (Cloud et al., 1974; Horodyski, 1976a, b, 1977; Semikhatov et al., 1979; White, 1977; Wright et al., 1978), only a few use binomial nomenclature and then only at the group level (Ford and Breed, 1974; McConnell, 1974, 1975). In Europe stromatolites are n o t widespread. Vendian forms have been described from Greenland (Bertrand-Sarfati and Caby, 1976) and northern Norway (Binns, 1975; Tucker, 1977; Siedlecka, 1978; Bertrand-Sarfati and Siedlecka, 1980); a study of acritarchs by Vidal (1976) suggests that the Greenland stromatolites may be Late Riphean rather than Vendian. Stromatolites have been reported from the Riphean of Czechoslovakia (Pacltova and Pouba, 1978). Early Proterozoic stromatolites from Finland have also been treated taxonomically (Krylov and Perttunen, 1978). There has been no general study of South American stromatolites, but they are being used increasingly for intracontinental correlation in Brazil (Marchese, 1974; Cassedanne and Cassedanne, 1976; Dardenne and Campos Nero, 1976; Fairchild, 1977; Dardenne, 1978, 1979; SchSll, 1978; Campos Nero, 1979). Late Proterozoic stromatolites from Caborca in Mexico have been described and named (Weber et al., 1978a,b). EA R L Y PROTEROZOIC STROMATOLITES

The discovery of morphologically diverse Early Proterozoic (Aphebian) stromatolites in Canada (Hofmann, 1969b; Hoffman, 1974, 1976; Donaldson, 1976a) focused attention on apparently close similarities with Late Riphean forms from the Soviet Union. The resulting renewed interest in stromatolites of this age has led to a number of recent publications (Walter, 1972; Preiss, 1976a; Bertrand-Sarfati and Eriksson, 1977; Dolnik, 1978; Hofmann, 1978; Krylov and Perttunen, 1978; Makarikhin, 1978; Semikhatov, 1978b,c).

358 TABLE II Forms (form-species) of stromatolites reported from pre-Riphean rock units Taxon

Reference

Occurrence

Group time-range

Riphean

LMUT Alcheringa narrina Asperia aspera * * Butinella boreale B. digitus B. ambigua Calevia olenica * Columnacollenia rantamaa

Walter, Semikhatov, Makarikhin, Makarikhin, Makaxikhin, Makarikhin, Krylov and Perttunen,

Columnaefacta asymetrica**Krylov and Perttunen, Conophyton biformatum Semikhatov, C. infernum Semikhatov, C. minisculum Semikhatov, Confunda confuta Semikhatov, Discorsia discorsa Semikhatov, Externia externa * * Semikhatov, Gruneria biwabikia Cloud and Semikhatov, G. biwabikia Lian and Cao, Jacutophyton inferum Semikhatov, Katernia africana * * Cloud and Semikhatov, K. perlina Bertrand-Sarfati and Eriksson, Kussiella cf. tuanshanziensis Lian and Cao, KussoideUa limata Semikhatov, Preiss, Minjaria granulosa Hofmann, Mistassinia wabassinon Nordia laplandica Krylov and Perttunen, Butin, Olenia rasus Omach tenia kvartsimaa * * K r y lo v a n d Perttunen, Paniscoilenia clivosa* Makarikhin, Pilbaria boetsapia Bertrand-Sarfati and Eriksson, Bertrand-Sarfati and P. inzeriaformis Eriksson, P. perplexa Walter, P. perplexa Semikhatov, Bertrand-Sarfati and Radiatina iso tropa * * Eriksson Sapinia fucoides Bertrand-Sarfati and Eriksson Segosia (?) minija Dolnik,

Fo

1972 1978b 1978 1978 1978 1978 1978

1978 1978b S 1978b S 1978b E 1978b E 1978b E 1978b S 1969 V B Fo 1976 1978b S 1969 T 1977 T 1976 1978b S 1976a N 1978 M 1978 1966 1978 1978 1977

K K K K Fi

x x x x x x x

X

Fi

x

X X X X

X

X X X X

X

X X X X

X

X X X X

E

X X X X

X

X X X X X

1977 1978

T

X

X

Fi K Fi K

x x x x x x x x x

1977 T 1972 D 1978b S T

X

X

T

1977

X

X

X

P

x

X

359 TABLE II (continued)

Taxon

Straticonophyton icon Stratifera biwabikensis* S. biwabikensis S. elongata* S. hearnica* S. kahocheUa* S. laxa* S. tenica* Sundia muris Tarioufetia yilgarnia Tibia cristata * *

Reference

Hofmann, Hofmann, Lian and Cao, KrylovandPerttunen, Semikhatov, 1978b Semikhatov, Semikhatov, Semikhatov, Makarikhin, Preiss, Bertrand-Sarfati and Eriksson, T. plumata Bertrand-Sarfati and Eriksson, Topinamboura insulata Bertrand-Sarfati and Eriksson, Tungussiaheterostroma Preiss, Vertexa termina Semikhatov, Archaeozoon septentrionale Hofmann, Collenia (?) ferrata Hofmann, CoUeniakona Twenhofal, Cryptozoon walcotti Hofmann, Hadrophycus immanis F e n t o n and Fenton, Klimetia marginata Makarikhin, K. torosa Makarikhin,

Occurrence

Group "t~e-range

~ ~ ~

h~ Riphean

1978 1969b 1976 1978

~ o~

M B

x x x x x x C Fi

S S S S

1978b 1978b 1978b 1978 1976a

K N

x x x x x x x

1977

T

x

1977

T

x

1977 T 1976a N 1978b S 1971 Sn 1969b B 1919 Ko 1971 Sr 1939 Na 1978 1978

K K

x x x x x

x x x x x

x x x x x

x x x x x

x

x x x x x x x x x x x x x x x x x x x x x x x

*Stratiform; **pseudocolumnar; (B) Biwabik Formation and Gunflint Formation (ca. 2.0 Ga); (C) Chuanglingkou Formation (1.7--1.9 Ga); (D) Duck Creek Dolomite (ca. 2.0 Ga); (E) Epworth Group (1.8--2.5 Ga); (Fi) Finland (rocks ca. 2.0 Ga); (Fo) Fortescue Group (2.8 Ga); (K) Karelia (rocks 1.6--1.9 Ga); (Ko) Kona Dolomite (ca. 2.3 Ga); (M) Mistassini Group (>1.7 Ga); (N) Nabberu Basin (1.7 Ga?); (Na) Nash Formation (1.7--1.9 Ga); (P) Prebaical (1.6--2.5 Ga); (S) Great Slave Supergroup (1.8--2.2 Ga); (Sn) Snare Group (1.7--2.5 Ga); (Sr) Steeprock Group (ca. 2.6 Ga?); (T) Transvaal Dolomite (ca. 2.2 Ga); (V) Ventersdorp 'System' (2.6 Ga). The indicated subdivisions o f the Riphean are Lower, Middle, Upper and Terminal (or Vendian). The new subdivision indicated in Table I, incorporating the Kudashian, is not yet in general use. The references given are to recent publications, where they are available, not necessarily to the original authors of the taxa.

360 These studies show that Early Proterozoic stromatolites are diverse and distinctive. Thirty-eight groups and 53 forms have been described (Table II). Of these, 24 groups and all 53 forms are unknown from younger rocks. Only 14 of these groups are also recorded from the Riphean, and among these 14, 8 are long-ranging, occurring through all or most of the Riphean. Some anomalies have been found (the authors have excluded all reported 'anomalies' where identifications are taken to group level only, it is considered that such reports are based either on inadequate material or insufficient study). Preiss (1976a) has reported forms of Minjaria, Tarioufetia and Tungussia (Table II) from the Nabberu basin (1.7 Ga?); these three groups are known elsewhere only in much younger rocks, although Tarioufetia is an only recently-diagnosed group, as y e t known from few localities, so its apparently disjunct distribution may well be a result of insufficient sampling. It is clear from the work of Hofmann (1977), among others, that more anomalies are likely to be discovered as work progresses; it is essential, though, that these reports be based on standard taxonomic procedures in order that the work can be assessed fully and critic~dly. It is inevitable, in the early stages of the use of any fossil group for biostratigraphy, that numerous 'anomalous' occurrences will be found, as knowledge of the distribution of each taxon improves. With time, these 'anomalies' will become part of the accepted distribution of the taxa. There is n o w a considerable b o d y of evidence that Early Proterozoic stromatolites are different from younger forms. At the taxonomic level of group, however, this difference is minimal, and many groups once thought to be characteristic of the Late Proterozoic have now been reported from Early Proterozoic rocks. Early and Late Proterozoic stromatolite assemblages cannot be reliably distinguished using only group level identifications. SYSTEMATICS

There are two main problems in the systematics of stromatolite biostratigraphy: firstly, can the principles of paleontological t a x o n o m y be applied to organosedimentary structures built by communities of microorganisms, and secondly, which features and descriptive methods will provide the most useful and reproducible classification? One result of the lack of general agreement a b o u t these problems is that Krylov (1976) was able to identify 12 separate classifications of stromatolites. Biological taxa are distinguishable because animals and plants do not uniformly intergrade. Some character clusters (species) are more successful than intermediate clusters, and are therefore more abundant. Two sets of observations indicate that this is also true of stromatolites. Firstly, where outcrop is good and stromatolites have been studied over considerable distances within single beds or formations, it has been observed that character clusters (forms) are stable, enabling taxa to be widely recognised. Examples include Acaciella australica and Inzeria intia from the Amadeus basin (Walter, 1972), Conophyton ressoti and Tungussia globulosa from the Taoudeni basin

361 (Bertrand-Sarfati, 1972; Bertrand-Sarfati and Trompette, 1976), and the "Jacutophyton cycles" of the U.S.S.R. (Serebryakov, 1976a,b). Secondly, extant microbial communities do not uniformly intergrade, and frequently replace each other abruptly, presumably reflecting adaptation to subtly different environments. This is well shown in the intertidal zone of Hamelin Pool, Shark Bay, Australia, where, for instance, juxtaposed areas of smooth mat, pustular mat and tufted mat have sharp, distinct borders and form discrete patches and zones (Logan et al., 1974; Golubic, 1976). So there are good reasons for considering that the refined methods evolved by taxonomists to permit the storage and communication of a vast amount of information can equally well be applied to stromatolites. Reduced to a minimum, the principles of biological classification applicable to stromatolites are these (Mayr, 1969, pp. 82--83,142--143): (1) "it is the taxon which gives us the characters, not the reverse"; (2) "the weight given to a . . . character depends largely on its constancy in the given group"; (3) "characters that unequivocally designate categorical rank do not exist"; (4) "a member of a taxon [does] not need to possess all the characters of the taxon and that such a deviant component of the taxon (e.g. species in a genus) [does] not need to be excluded and placed in a separate taxon". Taxa are recognised because of the clustering of characters (Hutchinson, 1968). Once taxa have been identified, this information can conveniently be communicated by the use of names (as has been clearly expressed by Cloud and Semikhatov, 1969). The nomenclature must be regulated and, at least for the present, it seems best to follow the rules of the International Code of Botanical Nomenclature. The decisions as to which features to use in taxonomy and what relative significance to give to each feature will remain with individual taxonomists. Inevitably, these decisions will vary among individuals and with time. It is futile to attempt to make the taxonomy inflexible by specifying, for instance, that groups will be defined by lamina shape and forms by microstructure. Within systematics, only nomenclature can and should be regulated by adherence to a code; classification is a matter for individual judgement. It is possible, nonetheless, to offer some guidelines for the selection and weighting of characters. Studies of Holocene stromatolites and careful interpretative analyses of ancient stromatolites are beginning to make possible the interpretation of fabric (microstructure and lamina shape), indicating that many features of the fabric are biologically generated (see the review by Semikhatov et al., 1979). On the other hand, some microstructural features are diagenetic and therefore of dubious taxonomic value {they may still have some value (Komar, 1976), since patterns of diagenetic alteration can reflect the primary inhomogeneities of a biogenic microstructure). Similarly, some features of gross morphology are clearly environmentally produced (Logan et al., 1974). A taxonomy that is intended for biostratigraphic and palaeobiologic use will therefore give little weight to these abiogenic features.

362 Criticisms of the taxonomic use of the gross morphology of columnar stromatolites reveal a lack of appreciation of a simple geometric fact: a stromatolite is nothing more than the sum of its laminae. The morphology is an expression of the lamina shape and size and lamina stacking (Hofmann, 1969a). It follows that reconstruction is a valid and essential method that must not be abandoned until effective means of describing lamina shapes and stacking are developed. It also follows that a full description of lamina shape and stacking would obviate the need for the reconstruction of the morphology. At present, reconstruction can rarely be avoided {one exception is in the identification of Conophyton). Several authors have reported taxa from the Early Proterozoic without having used methods that can precisely allow the description of gross morphology. These authors have correctly stated that abundant field observations can often lead to an adequate understanding of morphology, where outcrop is very good; b u t they have been unable to convey this understanding because of inadequate documentation. For this reason their reports are of limited value. This practice, and that of some workers of using small specimens uncontrolled by satisfactory field information, has led to confusion and many spurious reports. The practice of giving group-level identifications without the necessary prior research should be discontinued. It is essential that, at this early stage, stromatolite biostratigraphers use only the most rigorously applied methods. One or two small specimens unaccompanied by informative field photographs and sketches, and an informative description of morphological variability observed in the field, are worse than useless: they can be misleading. Appropriate field methods have been described b y Preiss (1976c). Krylov {1975) has emphasized the importance of field observations, and is developing a new taxonomic approach based on the recognition of 'bioherm series'. " F o r most bioherms it is possible to put all constructions from them into rather distinct series of variations. Such series (call them bioherm series) are all the main morphological variants from one bioherm, or uniform bioherms from one bed, with a uniform microstructure (or complex of microstructures). They are the totality of morphological modifications of one species or one regular association of algal species that built the stromatolites. Under different conditions (sometimes the difference is only in position within the bioherm) the same alga could build morphologically different constructions. And the totality of such modifications, instead of seeming limitless in diversity, is quite distinct and n o t very large" (translated from Krylov, 1975, p. 76). The recognition of bioherm series provides additional characters of use in t a x o n o m y . What we advocate is the application of standard paleontological systematic principles to stromatolites, because they have been proven to be highly effective for other fossil groups and there is no valid reason for thinking that they are inapplicable in this field. This is n o t to say that the present descriptive methods must be continued. On the contrary, it is certainly desirable to

363

follow the numerical geometrical approach advocated b y Hofmann (1969a, 1976, 1977), which should lead to more clearly reproducible results and obviate the need for excessively time~onsuming and frequently inaccurate 'graphical reconstruction'. We should n o w attempt to use these or similar parameters, b u t until their usefulness is established, it will be necessary to continue with reconstruction. CONCLUSIONS

Particular attention in this review has been paid to the two outstanding problems that beset stromatolite biostratigraphers: firstly, the suggestion that Early and Late Proterozoic stromatolites are the same, and therefore that their use in biostratigraphy is negated; and secondly, the apparently confused state of the systematics. Another fundamental problem is the question of what could have caused the reported changes in stromatolites; that has been discussed elsewhere recently (e.g. Preiss, 1977; Walter, 1977), with the conclusion that changes observed through the Proterozoic microfossil record (e.g. Schopf, 1977) could provide the necessary explanation, particularly when it is recognised that subtle changes in cyanobacterial physiology could result in changes of stromatolite morphology. The many reports that Early and Late Proterozoic stromatolites are very similar, or the same, are all based on comparisons at the group ('form-genus'} level. There are no reports of Late Proterozoic forms ('form~species') having been found in Early Proterozoic or Archean sequences, despite the fact that 53 forms have n o w been described from these older sequences. Nonetheless, most intercontinental correlations have been based only on group-level similarities of assemblages. It is n o w clear that such similarities can be misleading and must be interpreted with caution. As far as is possible, correlations should be based on form-level identifications, b u t any a t t e m p t to do this is hampered b y the problems of establishing identity at this level. The criteria that are useful for distinguishing taxa are gross morphology and fabric (lamina shape and microstructure}. Gross morphology is only an expression of lamina shape and lamina stacking and its use in systematics is likely to be superseded by the use of quantitative measures of those features. Such a quantitative descriptive approach will simplify the t a x o n o m y and make the results more reproducible. At present, though, there has been insufficient development of the necessary quantitative techniques to enable this change to be made, hence the continued use of graphical reconstruction is advocated. To facilitate the change to quantitative methods, each description should be accompanied by abundant tracings of lamina profiles. There is no reason to consider that conventional paleontological systematic procedures are n o t applicable to stromatolites. Taxa can be defined from clusters of characteristics, and each such cluster can be given a Linnean binomial name, an approach that has a proven capacity to allow the ordering and

364 retrieval o f a vast a m o u n t o f i n f o r m a t i o n , a n d t h a t is relatively easy a n d conv e n i e n t t o use. Such an a p p r o a c h has t h e a d d i t i o n a l b e n e f i t o f bringing rigor to a n y field w h e r e it is used well. S t r o m a t o l i t e b i o s t r a t i g r a p h y is u n d o u b t e d l y n e e d e d , as the m a n y a t t e m p t s t o use it d e m o n s t r a t e . It seems t o w o r k , b u t critical analysis, or m o r e imp o r t a n t l y , progress, is i m p e d e d b y the i n a d e q u a c y o f s o m e t a x o n o m i c procedures. R e m e d i e s are available, and w h a t is required n o w is an intensive test o f t h e d i s c r i m i n a t o r y p o w e r o f t h e n e w l y a d v o c a t e d q u a n t i t a t i v e characteristics, so t h a t t h e m o s t useful o f t h e m can be selected and p u t into use as s o o n as possible. ACKNOWLEDGEMENTS Critical c o m m e n t s f r o m H.J. H o f m a n n , W.V. Preiss, G. Y o u n g a n d R. T r o m p e t t e have been especially helpful. REFERENCES Acharya, S.K., 1974. Stratigraphy and sedimentation of the Brexa Group. Himalayan Geol., 4: 102--116. Aitken, J.D., 1977. New data on correlation of the Little Dal Formation and a revision of map unit "H5". Geol. Surv. Can., Pap. 77-1A, pp. 131--135. Aitken, J.D., Long, D.G.F. and Semikhatov, M.A., 1978a. Progress in Helikian stratigraphy, graphy, Mackenzie Mountains. Geol. Surv. Can., Pap. 78-1A, pp. 481--484. Aitken, J.D., Long, D.G.F. and Semikhatov, M.A., 1978b. Correlation of Helikian Strata, Mackenzie Mountains--Brock inlier--Victoria Island. Geol. Surv. Can., Pap. 78-1A, pp. 485--486. Aksenov, E.M., Keller, B.M., Sokolov, B.S., Solontzov, L.F. and Shul'ga, P.L., 1978. General scheme for the stratigraphy of the upper Precambrian of the Russian Platform. Akad. Nauk. S.S.S.R., Izv., Ser. Geol. 12:17--34 (in Russian). Anonymous, 1977a. Stromatolites in Raipur Limestone. Oil---Coal News, 14: 1--5. Anonymous, 1977b. Algal stromatolites in Delhi Supergroup. Geol. Surv. India News, 8: 1--9. Banerjee, D.M., 1978. Chemical rhythmicity in the Precambrian laminated phosphatic stromatolites and its bearing on the origin of algal phosphorite. Indian J. Earth Sci., 5: 102--110. Barman, G., 1975. Algal stromatolites from the so called Trans-Aravalli Vindhyans of western Rajasthan. Proc. 1st Int. Geophytological Congr., Lucknow, 1975. Barman, G., Verma, K.K. and Puri, S.N., 1978. Biostratigraphic zonation of stromatolite bearing horizons of Udaipur Dist. Rajasthan. J. Geol. Soc. India, 19 : 264--267. Bertrand-Sarfati, J., 1972. Les stromatolites du Pr~cambrien sup~rieur du Sahara nordoccidental; inventaire, morphologie et microstructures des laminations. Correlations stratigraphiques. Centre de Recherches sur les Zones Arides, Paris, Publ. C.N.R.S., G~ol. No. 14, 240 pp. Bertrand-Sarfati, J., 1976a. An attempt to classify Late Precambrian stromatolite microstructures. In: M.R. Walter (Editor), Stromatolites, Elsevier, Amsterdam, pp. 251--259. Bertrand-Sarfati, J., 1976b. Pr~cisions sur l'~ge Pr~cambrien sup~rieur des stromatolites du group de la Musindozi, Malagarasien du Burundi. Ann. Soc. G~ol. Belg., 99 : 49--64. Bertrand-Sarfati, J. and Caby, R., 1976. Carbonates et stromatolites du sommet du Groupe d'Eleonore Bay (Pr~cambrien terminal) au Canning Land (Groenland oriental). Grin. Geol. Unders. 119, 1--51.

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