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On Aphebian stromatolites and Riphean stromatolite stratigraphy
Precambrian Research, 5 (1977) 175--205 175 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ON A P H E B I A N S T R O M A T O L I T E S AND R I P H E A N S T R O M A T O L I T E STRATIGRAPHY
H.J. HOFMANN
Department of Geology, University of Montreal, Montreal (Canada) (Received February 8, 1977)
ABSTRACT Hofmann, H.J., 1977. On Aphebian stromatolites and Riphean stromatolite stratigraphy. °'~ Precambrian Res., 5: 175--205. New occurrences of typical Riphean stromatolite and microphytolite groups are reported and illustrated from typical Aphebian sequences in Canada. These include Minjaria, ColonneUa, and Kussiella from the Mistassini Group of central Quebec, Gymnosolen, Jaeutophyton garganicum, Minjaria, CoilenieUa, Kussiella, Tungussia, Lenia, Osagia, and Vesicularites from the Belcher Supergroup in Hudson Bay, and Asterosphaeroides (Nelcanella) and Radiosus from the Manitounuk Supergroup of the southeastern coast of Hudson Bay. These occurrences are at variance with prevailing views of Riphean "biostratigraphy". They call into question the validity of age assignments based only on stromatolite groups, as well as the premise that prokaryotic evolution is mainly responsible for observed trends in gross morphology. The systematics of stromatolites is re-examined in the second part of the paper, and a more objective, morphometric (stereometric) approach is discussed. Examples illustrate how differences and similarities whitin and between stromatolite groups can be expressed quantitatively. The morphologic attributes chosen comprise the shapes of lamina profiles, cross sections (plan views), and longitudinal sections (silhouettes). Microstructures are illustrated photographically; they were not analysed morphometrically and await further study.
INTRODUCTION A t least five o r i e n t a t i o n s in t h e subdiscipline o f s t r o m a t o l i t o l o g y can be identified: biostratigraphic, s e d i m e n t o l o g i c , biologic, g e o p h y s i c a l , a n d m o r p h o metric. These are n o t m u t u a l l y exclusive, b u t t h e y d i f f e r in t h e emphasis p l a c e d o n t h e objectives p u r s u e d , a n d b y t h e c o n c e p t s a n d i n d e p e n d e n t nom e n c l a t u r e s used. O n e well r e p r e s e n t e d g r o u p includes t h e " b i o s t r a t i g r a p h e r s " , t h o s e w h o see as their m a i n t a s k t h e subdivision a n d interregional c o r r e l a t i o n o f seq u e n c e s based o n detailed investigations o f assemblages o f fossil s t r o m a t o lites, c o u p l e d with r a d i o m e t r i c a g e - d e t e r m i n a t i o n s in t h e i r geologic c o n t e x t . F o l l o w e r s o f this s c h o o l generally e m p l o y a f o r m a l b i n a r y , latinized n o m e n -
176 clatorial system for handling the paleontologic data and for conveying taxonomic information (e.g., "an assemblage of Kussiella kussiensis and Omachtenia omachtensis"). Another group of individuals, also studying fossil sequences, follows a "sedimentologic" approach. The chief interest in this orientation is in sedimentary, paleocological, and perhaps economic aspect of stromatolites, without necessarily attributing chronostratigraphic significance to them. Adherents to this practice tend to employ an informal, descriptive terminology, rather than a formal binary nomenclature (e.g., "SH-V type", or "unlinked, closely spaced intertidal columns with a preferred NW orientation"). A third, "biologic" approach is pursued by those who are studying the problem of stromatolite genesis by observing the biologic and sedimentary entities, processes, and conditions and geochemistry in different modern environments. The nomenclature of stromatolites involves mainly the truly biological taxonomy of the predominant biota, and descriptive terms for mat (or fabric) types, and environmental setting {e.g., "intertidal pustular mat of En tophysalis major"). A fourth approach is applied by a small group who can be said to have a "geophysical" orientation. By quantitative analysis of the laminar rhythmicities the potential astronomic significance of stromatolites is assessed, including such aspects as their utility as geologic calendars, indices of lunar orbital evolution, and seasonality. In these rhythmometric studies the quality of the lamination is of paramount importance (uniformity of laminae, presence of periodicities); taxonomic matters are relatively unimportant. A fifth and not yet well developed line of pursuit is concerned with the purely geometric-mathematical aspects of stromatolite characteristics, and with improving the methods of data collecting and handling, using computers. This "morphometric" approach involves analytic studies in which the objective is to make quantitative statements about the geometric attributes, permitting statistical treatment of the data. This allows for more precision in the specification of attributes and identification of stromatolite "taxa" and environments, and facilitates comparisons and communication as well. It also entails synthetic studies whose aim is to improve our understanding of the common geometric properties and relationships of the basic elements of stromatolites of different origin, and also of similar abiogenic structures. This is achieved by using computers which generate artificial images that simulate natural growth forms. By varying the values assigned to the different fundamental stromatolite parameters, the geometric variations in the graphic displays can be appreciated quantitatively, and direct comparisons can be made with natural forms. The principal aim of this paper is to report and illustrate further new occurences of typical Riphean stromatolites in the Aphebian, to discuss certain major problems confronting the "biostratigraphers", and to suggest possible ways of resolving them. The dilemma is posed both by empirical data inconsistent with some prevailing views of Riphean biostratigraphy, and by some-
177
what haphazard taxonomic practices. The latter can be improved by applying the "morphometric" approach. To avoid ambiguity, the terms used in this paper for stromatolite elements are given in the left column of Fig. 1; following previous usage (Logan et al., 1964; Hofmann, 1969), oncolites (oncoids) are considered to be special, mobile types of stromatolite structures. Allowance is made for the segregation and separate designation of different morphologic elements within bioherms and for the eventual naming of higher rank taxonomic categories (such as the "bioherm series" of Komar et al., 1975). This is comparable to the situation regarding conodonts, where multielement assemblages bear names different from those of the individual elements. STROMALOLITE STRATIGRAPHY
The underlying premise of Precambrian biostratigraphy is that the stromatolite characteristics exhibit secular changes through geologic time. These characteristics are essentially geometric, lithologic (textural), and dimensional; the methods of serial sectioning and graphic reconstruction, and
STROMATOLITE COMPONENTS CONCEPT
OTHERDESIGNATION
THE BED
bioherm, biostrome
THE STRUCTURE
stromatolite column, head, mound, coenoplase
THE LAMINA
stromatoid layer
THE MICROSTRUCTURE
microfabric, spongioatromid taxon
THE TEXTURE
grain-to-grain relationship
TH E G RAIN
o
THE MICROFOSSIL TIME
particle, crystal, mineral taxon, organism, biota
T
Fig.1. Make-up of stromatolite beds. The definition and usage of the terms stromatolith and stromatoid of Kalkowski (1908) are somewhat ambiguous: for discussions see Reis (1908) and Monty (1977).
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microscopic study of thin and polished sections have served to outline such changes. Practitioners in the "biostratigraphic" orientation generally have a paleontological background, and they apply the standard paleontological treatment accorded real organisms, such as erecting formal taxa, designating types, and using a binary latinized nomenclature. This has, for some nonspecialists who use the information, obscured the fact that stromatolites are in part sedimentary structures. These structures with latinized names cannot be regarded as biological taxa, even though the names are Linn~an in form. The Riphean " z o n e s " or " p h y t e m s " represent assemblages of biosedimentary structures and lithologies, and are different from Phanerozoic biozones which are based on assemblages of fossilized organisms; they are, however, comparable to trace fossil assemblages which also consist of biosedimentary structures (see also Hofmann 1969, pp. 40--42). The biostratigraphic scheme was first established for Eurasia, where geological conditions favoured the extensive development of stromatolitic sequences during the Riphean and Vendian (1.65--0.57 Ga). In the same territory, however, stromatolitic Aphebian sequences (2.5--1.7 Ga) are underrepresented. The Precambrian biostratigraphy with its four major subdivisions is thus primarily for the approximately 1 Ga long Riphean-Vendian. In North America, Aphebian stromatolitic sequences are well represented, and over the last few years many of the stromatolite groups supposed to be restricted to the mid- and late Riphean have n o w been identified from these earlier sequences (Hofmann, 1969, 1973; Bell and Hofmann 1974; Donaldson 1975, 1976). Additional examples are given below. Such discrepancies are n o w also being noted on other continents where Aphebian sequences abound (Walter, 1972, pp. 84, 166; Preiss and Walter, 1975; Preiss, 1976). Evidently, then, d o u b t must be cast on age assignments based only on groups, some of which are inadequately defined or overlap with others. Moreover, the precision of the identification at the group levels needs to be re-examined. Until now, morphologic variations between groups have not been expressed quantitatively. Selected data on new Aphebian stromatolite occurrences are summarized in Table 1, and the structures are illustrated photographically and by reconstructions in Figs. 2--13. Photographs were introduced to Precambrian stromatolitology in 1890, and are still the most widely used tool for disseminating morphologic information on gross form and microstructure. Assuming that the photos are unretouched, this m e t h o d is generally objective insofar as it involves relatively little interpretation (other than selecting the sample for illustration and in deciding on print quality--particularly contrast). TAXONOMY AND S T E R E O M E T R Y
The need for a more objective, quantitative approach to stromatolite classification is becoming urgent. The increasingly complex descriptive literature makes it difficult to relate and compare newly found structures with those
179
Fig.2. M i n j a r i a - C o l o n n e l l a - Kussiella biostrome in dark grey, anthraxolitic dolostone of the upper part of the Upper Albanel Fro. (Mistassini Group); W side of Temiscamie Bay, Lake Albanel, central Quebec (51°01.5'N 73°06.5'W). Compare with "columnar stromatolites" in Donaldson (1963, pl. 2). A: Outcrop view of dendroid variant in biostrome. B: Polished longitudinal section; G.S.C. type 48527. C: Large thin section in transmitted, nonpolarized light. Rectangle outlines area of part D. D: Microstructure of central part of column. Rectangle outlines ared in E. E: Detail of microstructure at high magnification. Note grain size differences.
already described. Probable synonyms appear more frequently, and the situation is complicated by the existence of different schools of thought on what are taxonomically significant attributes, by language barriers, by unavailability of certain publications, by difficulties in obtaining type material and
2
3--7; 15, 17, 18--19
8
Min]aria (+ Colonnella, Kussiella)
Gymnosolen (+ Tungussia)
Gymnosolen
Colleniella (+ Kussiella, Stratifera)
10 A--C
Jacutophyton garga- 9 nicum
Illustrations (Figs.)
Groups (and associated groups)
48532
48533
Sanikiluaq section 56°32.7'N 79°14.6'W
48531
Katuk section 56°31.9'N 79°10.0'W
Belcher Islands, Hudson Bay
Sanikiluaq section 56°32.3'N 79°14.6'W
Belcher Islands, Hudson Bay
Sanikiluaq section 56°32.8'N 79°14.4'W
48530
48529
Sanikiluaq section* 56°32.7'N 79°14.6'W Belcher Islands, Hudson Bay
48528
48527
G.S.C. types**
Katuk section* 56°31.9'N 79°09.0'W
Belcher Islands, Hudson Bay
Lake Albanel, central Quebec 51°01.5'N 73°06.5'W
Localities collected
Summary of new occurrences of Aphebian stromatolites and microphytolites
TABLE I
Belcher Supergroup Mayor Fro. (lower part)
Belcher Supergroup McLeary Fm. (top part)
Belcher Supergroup Tukarak Fm. (lower part)
Belcher Supergroup Mayor Fm.
Mistassini Group Albanel Fro. (upper part)
Stratigraphic unit
Dolostone
Dolostone
Limestone
Dolostone
Dolostone
Lithology
Low intertidal to subtidal; currents azimuth 060 °
Subtidal
Subtidal
Subtidal
Subtidal
Inferred environment
(3O O
11;16
12
13
20
Lenia (+ Stratifera )
Minjarm
Osagia (+ Vesicularites)
Asterosphaeroides (+ Radiosus) 56.2°_+N 76°30'W
38369
48539
Churchill Sound 56°29.5'N 79°30.3'W Richmond Gulf, SE coast of Hudson Bay
48538
Sanikiluaq section 56°32.5'N 79°14.1'W
Belcher Islands, Hudson Bay
Churchill Sound 56°29.5'N 79°30.2'W
Belcher Islands, Hudson Bay
48537
48536
56°32.6'N 79°14.1'W Churchill Sound 56°29.5'N 79°30.3'W
48535
48534
Sanikiluaq section 56°32.6'N 79°14.0'W
Belcher Islands, Hudson Bay
Sanikiluaq section 56°32.6'N 79°14.2'W
Belcher Islands,Hudson Bay
* For detailed maps and sections refer to Hofmann (1976b) ** Numbers refer to type collection of Geological Survey of Canada, Ottawa
10 D
Tungussia
Manitounuk Supergroup Nastapoka Group
Kasegalik Fm. (upper part)
Belcher Supergroup McLeary Fm. (lower and middle parts)
Belcher Supergroup Kasegalik Fm. (upper part)
Kasegalik Fm. (upper part)
Belcher Supergroup McLeary Fm. (lower and middle parts)
Belcher Supergroup McLeary Fm. (middle part)
Dolostone
Dolostone
Dolostone
Dolostone
Dolostone
Dolostone
Dolostone
Shallow subtidal
Shallow subtidal and intertidal
Subtidal
Supratidal and intertidal; subtidal?
Subtidal
Fig. 3. Outcrop views of Gymnosolen-Tungussia biostrome in grey and buff dolostone of Mavor Fm. at Sanikiluaq, Belcher Islands (56 ° 32.7'N 79 ° 14.6'W). A and B: Two views of central portions of biostrome in Unit G of Fig. 4. Note subhorizontal stylolite at middle of B, truncating columns. Scale graduated in din. C: View of strongly decumbent structures in Unit F of Fig. 4; strong stylolitization causing appression of some columns. D: Plan view, showing variations in size and shape of Gymnosolen f.
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183
examination of field occurences. Some of the problems could be eliminated by a standardization of the criteria adduced in classification, and b y more effective methods of presenting morphologic data used in establishing the taxa. If this standardization were to emphasize presentation in pictorial, graphic, or numerical form, rather than in verbalization, the information would be in a m o d e allowing for more precise definition, more decisive comparisons, better inter-language communication, as well as for more efficient data handling, storage, and retrieval (Hofmann, 1976a). The present lack of consensus also results in the description of new fossil groups and forms based on different concepts, making direct comparisons and evolutionary studies between incongruous taxa difficult. Among the many different methods available (see Hofmann 1969 and Krylov 1975, 1976 for summaries), t w o main ones have emerged for the treatment of
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SKETCH MAP OF BASAL PART OF MAVOR FM. SANIKILUAQ
Fig.4. Sketch map of basal part of Mavor Fro., Sanikiluaq. Area of map is 400 m from N end o f long outcrop of Mavor and Tukarak Fins. at left center of fig. 2 in Hofmann (1976h) (56°32.7'N 79°14.6'W). Position of large glacial erratic given for reference in field. North is to right. Unit A: Grey, massive dolostone with molar-tooth structure and minor edgewise breccia. UnitB: Laminated grey dolostone; some edgewise breccia. Unit C: Grey, massive dolostone. Unit D: Large mounds of laminated dolostone (Colleniella and Stratifera) with intraforrnational intermound breccias. See Fig. 10B and C. Interference ripples in lower part. Unit E: CoUeniella mounds of laminated grey dolostone ; brown weathering dolomitic quartz arenite and quartzose dolostone in channelized, intermound areas. Preferred orientation of mounds 060 °. See Fig. 10A. Unit F: Massive dolostone ; basal portion of biostrome with oblique columns and plates (see Fig. 3C). Unit G: Massive Gymnosolen dolostone. See Figs. 3A, B, D, 5, 6.
184
fossil structures by "biostratigraphers" of the Precambrian. Both of these use a binomial, latinized system of nomenclature -- the traditional vehicle of communication in biology and paleontology for more than 200 years. These two main trends may be identified as that based on the spongiostrome concept of Gfirich (1906) in which the microstructural features serve as the basis for major categories, and that on the stromatolite concept of Kalkowsky, in which the gross form and lamina shape is the predominent group characteristic. (To say this is somewhat of an oversimplifiaction, because individual authors have n o t always adhered to one concept consistently, even within a single publication). There is thus the curious situation where the same speci-
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Fig.5. Graphic reconstructions of Gymnosolen f. in Mavor Fro.; 4 km ESE of Sanikiluaq (Katuk section, 56°31.9'N 79°09.9'W). Block from bed equivalent to Unit G in Fig. 4. G.S.C. type 48528.
185
Fig.6. Microstructures of Gymnosolen f., Mayor Fm., Sanikiluaq and Katuk sections. A: Outcrop view of Unit G (Fig. 4), showing columns with two types of microstructure: well laminated recrystallized dolostone at bottom and recrystallized dolostone with radiofibrate texture in upper parts. B: Thin section under transmitted, nonpolarized light, showing the two types of microstructure. Specimen is from column B in Fig. 5. Rectangles outline fields in parts C and D. G.S.C. type 48529a. C: Enlargement of radiofibrate texture in upper left of B. D: Microstructure of top of column at lower right in B. Rectangle outlines area in E. E: Microstrucutre at high magnification, showing grain size differences between light and dark lamellae. m e n s can have d i f f e r e n t n a m e s , d e p e n d i n g o n w h a t m e t h o d is applied. E x a m ples are Conophyton Maslov, Lerrnontovaephycus V o l o g d i n , Granifer Vologdin, a n d Tschichatschevia V o l o g d i n . This is n o t a m a t t e r o f o m i s s i o n o r o f i g n o r a n c e o f t h e o t h e r classification, b u t o n e o f i n t e n t , w h e r e b y f u n d a m e n -
186
tally different concepts are applied to stromatolite systematics, both classifications being characterised b y Linn~an-like designations. In such a confusing situation it is conceivable that a particular form could be listed twice or more under different names in fossil lists compiled by uninitiated individuals. Gross morphology, lamina shape, and microstructure are the parameters n o w generally considered most important, and this is reflected in the prevailing trend to use the first two in the group concept and microstructure in the form taxa {e.g., Conophyton garganicum). As indicated elsewhere (Hofmann 1975, p. 99), the binomial stromatolite nomenclature now in use is n o t hierarchical (LinnSan) b u t parallel. An equal weight can be ascribed to the morphology and to the microstructure, and perhaps to the profile shape as well; neither is necessarily subordinate to the other as is the species to the genus in the Linn~an system; and their positions, in a natural hierarchy, if
Fig.7. Microstructure of Tungussia portions in Unit F of Fig. 4. G.S.C. type 48529b.
187
any, are undetermined. It would be equally valid to write Garganicum conophyton, as long as Conophyton refers to growth form and garganicum to a microstmcture. If this same microstmcture is found with Jacutophyton, it should be possible to refer to it as J. garganicum. Seen in this light, a system
Fig.8. Gymnosolen f. in bluish grey limestone in lower half of Tukarak Fro. at Sanikiluaq (56°32.8'N 79°14.4'W). A: Outcrop view of erect columns and orange-weathering replacement dolomite patches and dikes. T o p o f biostrome is capped by orange-weathering, laminated grey dolostone showing synsedimentary microfaulting. Scale in dln. B: Cross sections of columns. Scale in dm and cm. C: Thin section under transmitted, nonpolarized light, showing lamination and radiofibrate crystallization fabric. Rectangle in column at lower left marks field magnified in D. G.S.C. type 48530. D: Microstmcture showing laminae and radiofibrate texture.
188
Fig.9. Jacutophyton (Conophyton) garganicum in cherty grey dolostone of upper fifth of McLeary Fm. at Sanikiluaq, Belcher Islands (56°32.3"N 79°14.6'W). A: Outcrop view. Scale in dm and cm. B: Close-up of another branching column with conical laminae. C: Thin section under transmitted nonpolarized light, showing microstructural variations, branching, and contact with matrix at upper left. Rectangle at right margin outlines field in part D. G.S.C. type 48531. D: Microstructure, showing alteration. Thickness of dark layers generally 25--100 ~m, light layers 50--150 urn. Rectangle near b o t t o m center magnified in E. E: Microstructure under high magnification.
Fig. 10. Outcrop views of large structures in Mavor and McLeary Fins. A: Coileniella m o u n d with flanking arenite beds in Unit E of Fig.4. Scale in dm and cm. B: Colleniella-Stratifera m o u n d with flanking flat-pebble breccia in Unit D of Fig.4. Bioherm is 5 m N of S end of outcrop. Compare with fig. 19 of Hofmann (1969). G.S.C. type 48533. C: Bioherm of Kussiella exhibiting furcate branching. Bed equivalent to upper part of Unit D of Fig.4 at Katuk section (56°31.9'N 79°10.0'W). G.S.C. type 48532. D: Tungussia from distinct, 1 m thick marker horizon in lower part of middle member of McLeary Fro., Sanikiluaq (56°32.6'N 79°14.2~). G.S.C. type 48534.
Fig. 11. Lenia f. in lower half of McLeary Fm., Sanikiluaq, Belcher Islands. Compare with "digitate stromatolites" of Donaldson (1963, pls. 4--5). A: Outcrop view of bed with columns in lower member of McLeary Fro. (56°32.6'N 79°14.1'W). Scale in dm. B: Plan view of columns in bed 2 m above Tungussia marker bed (Fig.10 D) at Sanikiluaq (56°32.5'N 79°14.2'W). C: Polished surface with inclined columns from same bed as in A. G.S.C. type 48535. D: Lenia-Stratifera in bed 29 m stratigraphically below bed in A. Thin section, transmitted, nonpolarized light. Rectangle outlines area of part E. G.S.C. type 48536. E: Microstructure, showing relict radiofibrate texture. F: Enlarged view of microstructure from just beneath lower left of E. Note the flared nature of the dark lamellae suggesting possible physiochemical origin of lamination.
o
Pig.12. Minjaria f. in grey dolostone of Kasegalik Fro., N end of Churchill Sound, Belcher Islands (56°29.5'N 79°30.2~V). Compare this also with the pre-Riphean Pitbaria described by Walter (1972, p. 167 ). A: Outcrop showing slightly oblique, longiudinal section of digitate columns. B: Close-up of hand specimen. G.S.C. type 48537a. C: Thin section in transmitted, non)olarized light. Rectangle marks field o f part D. G.S.C. t y p e 48537b. D: Microstructure in central portion o f column in C. ~: Enlarged view of left central portion of D. Note grain size differences between light and dark lamellea.
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192
Fig.13. Osagia f. in grey dolostone, McLeary and Kasegalik Fms., Belcher Islands. Compare with "pisolitic stromatolites" of Donaldson (1963, pl.6). A: Outcrop view of lens with oncoids and catagraphs within Lenia biostrome. 1.2 m above Tungussia marker bed (Fig. 10 D), Katuk section (56°32.0'N 79°10.3'W). B: Thin section of pinkish grey dolostone in transmitted, nonpolarized light. Bed stratigraphically 66 m below Tungussia marker bed, Sanikiluaq (56°32.5'N 79°14.1'W). G.S.C. type 48538. C: Microstructure; enlarged view of upper left part of oncoid in center of B. D: Microstructure more highly magnified; field of view 3 mm below left central part of C. Note slightly flared outlines of dark lamellae, comparable to flaring in Lenia (Fig. 11 F). E: Osagia f. and Vesicularites f. in grey dolostone, Kasegalik Fro., N end of Churchill Sound; adjoining Minjaria locality (56°29.5'N 79°30.3'W). G.S.C. type 48539.
193
of polynomial designations such as that proposed by Maslov (1960) is attractive. Stromatolites have important sedimentary and diagenetic components; in this respect they are like ichnofossils, as already mentioned. (Ichnotaxa may reflect anatomical and behavioural attributes, as well as sedimentary and taphonomic properties, and separate descriptive, ethologic, and stratinomic classifications exist in parallel.) Notwithstanding the general trend in group and form concepts, the systematics of stromatolites, continues to be beset with inconsistencies in the choice of features, and combinations of features, in erecting the taxa. Inspection of the diagnoses of many taxa currently in use reveals that the systematics is literally unsystematic (see also Krylov, 1976, pp. 36--40). Though acknowledged, this state of affairs is not found objectionable by everyone {e.g. Walter, 1972, p. 17). It does, however, impede understanding of the factors which are responsible for the supposed evolutionary trends in stromatolites through the Proterozoic. By isolating each individial factor it will be easier to analyse its changes through time, while at the same time it should encourage the quantification of information that would allow stromatolitology to become more objective; it would also permit better evaluation of models of stromatolite genesis and evolution and their causes. Much has been made of the premise that graphic reconstructions are essential for the identification of groups, yet this methodology itself can be criticised (Hofmann, 1969, 1973, 1976a; Donaldson, 1975). To maintain that extensive longitudinal and transverse sections such as seen in outcrops are insufficient for identification is not only to deny a basic principle and tool of stereology (that sections are representative samplings of volumes as a whole), but is also to deny oneself the use of available rapid and powerful statistical techniques in morphometric analysis. Moreover, the reconstruction method also neglects the factor that the gross morphology of individual structures is almost certainly governed by stochastic processes (the probability that two structures have identical form is very much smaller than the probability that two individuals of a true biological species have the same form). What one desires in making the reconstructions is really to obtain an idea of the modal form (a statistical concept) for a taxon. Such a modal form can, however, also be deduced from sections, for it matters little, statistically speaking, whether we analyze a number of serial sections from one block, or one or two large sections containing a multitude of columns. As well as being more representative, the latter alternative has the advantage of applying equally to structures too large or too small to be handled or serially sectioned. Few would deny the great utility of shape and size analysis of sandstone grains in thin sections where the relevant information is obtained without graphically reconstructing individual grains. A computer assisted method of morphometric analysis of stromatolites was outlined by Hofmann (1974, 1976a). The technique is still being refined, and additional parameters are being developed and applied to Aphe-
194
bian structures. The method is based on partially automated measurements and analysis of shape, size, and orientation functions, using electronic image. analysing equipment. The numerical data extracted from images include measures of areas, perimeters, lengths, convexities, and number of features in a given field o f view. Each structure, or array of structures, is analysed for quantities that express the shape, size, and orientation of four main features of use in stromatolite taxonomy: longitudinal section (silhouette), cross section (plan view), lamina shape, and microstructure. Selected Aphebian stromatolite groups serve as examples to illustrate the method (Figs. 14--19). The data should be compared with reconstructions and photographs shown in Figs. 2--6 and 8--11, which supplement the quantitative data and allow comparison with Riphean groups already described in the traditional way. Table II gives the functions and parameters that have been found useful in quantifying stromatolite attributes. o;~
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Fig.14. Laminosity plot: synopsis of lamina profile shapes. Only simple, even laminae w i t h 1 o r d e r o f c u r v a t u r e are d e p i c t e d . F o r e x p l a n a t i o n s e e t e x t .
195
Lamina profiles Of particular interest in categorizing the shape of lamina profiles is a laminosity plot, a preliminary version of which was introduced by Hofmann {1974, 1976a, fig.5). A more complete and modified rendition of this plot is now given in Fig. 14, which illustrates both pictorially and numerically the LAMINA
PROFILES
Gymnosolen f.
.0.7, 0.5
I PROFILE
• 0.6
FLATNESS
__Fh
.,z
Fv .52
0.5 .75
o z :E --I
<
,81
6,2.68
5~6S
2~
9
0.4
Fv S
.~o ~ ! ~ , * ~ b ~ 3
0.3
o in. w
>
.,
46 "54 2~1~/ 7242.. :~3~
4 .41 el
29"~ \5 .8o
0.2
~Fh.
"e34
3~
.15 -19
.~
4-
=47 .44
"
s
0.4 t
0.5 I
HORIZONTAL
0.6 l
Fh S
0.7 l
0.8 i
0.9 f
LAMINOSITY
Fig.15. Lamina profiles and corresponding laminosity plot for axial and near axial sections of Gymnosolen f. from the block illustrated in Figs. 5 and 6 B. Symbols defined in Table II. Star represents arithmetic mean, close to profile 67. Compare with Fig. 14.
196 variations of basic eliiptical and angulate profiles with one order of curvature. The abscissa is designated the horizontal laminosity index, a measure of the relative horizontal development of laminae; the ordinate is the corresponding vertical laminosity, which expresses their relative vertical extent. The derived profile flatness ratio, Fh/Fv, gives the profile configuration in terms of the smallest plumbed rectangle circumscribing each lamina. The addition of second and higher orders of curvature to the basic first order curve (crinkling) has the effect of displacing the position of profiles towards the origin of the graph (S/[Fh + Fv] increases). The superimposed trend-lines A to E delineate selected families of laminae with specific geometric attributes. Line A contains the loci of rectilinear profiles at different inclinations; it also defines the outer limit of the field of geometric possibility of continuous laminae (S/[Fh + Fv]) is at a minimum for any value of Fh/Fv). Line B is the trend of untilted, rectilinear, symmetrically angulate profiles. Line C represents untilted semiellipses, while line D localizes laterally directed semiellipses. Line E contains plenicinct (completely encapsulating) elliptical laminae such as found in oncoids. Oncoids with penecinct (incompletely encapsulating) elliptical laminae would lie above and to the right of line E, whereas oncoids with plenicinct laminae with t w o or more orders of curvature (e.g., Ottonosia) would fall below and to the left of line E. Similarities and differences between the laminar shapes of the newly reported Aphebian occurrences of Gymnosolen and Lenia are shown in Figs. 15--16. Cross sections (plan views) The circularity index A/Pe 2 is an excellent descriptor quantity for shape. .0.4
Fb 0
'03
Fv
.~
4FV -0.2 ~
"~
1~21.31"1S 8. 1.9
S'ZL 141P.]~.1fl
~ _
~
0
g I
cm
"01
2~ 2" 22,13 8"
Fh
2
07
S
o.8
o.9
io
HORIZONTAL LAMINOSITY
LAMINA PROFILES Fig.16. Lamina profiles and corresponding laminosity plot for random longitudinal sections of Lenia f. from block illustrated in Figs. 11 A and C. Symbols s a m e as for Figs. 14--15. Arithmetic m e a n l a m i n o s i t i e s i n d i c a t e d by star near profile 26. Note that t h e c l u s t e r i n g is t o t h e right of the cluster in Fig. 15.
197
O.Oe
-39
•
• 37 0.07
-'"4t
.3~
:~p
3~'33 .29
"'~
"15+14
"3027
w
.,, .21
-16
•2~4
.12
M "1~18 • 22 rnX
0.06
" I1 "9.10
A 2 I
2
3
4
5
• 34
"23 "6
,0.05
6
*7
7
8
9
10
11
,0.04
12
-4
A
OOe
"2
' 0.03 ~ J : : P e
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
•
8
•
~
e
•
31
32
33
34
35
36
T8
39
2 i
i
37
•
;0
10
5
i
i
6 i
EQUIVALENT CIRCLE DIAMETER
0.08
.39
"~
•
"3
.5
cm
•0.07
~ 2+
"~,
+35 "27 +19 3~"33 .29 "2B ,21 • 38 3"?"31 t20 .25
,4o
,16 -12
"24
M 'J~18 • 22 x m
.0,06
A o
me
• 34
*11 "~.10
,23 ,6
-0.05 .7 -17
-0.04
,4 A
~03
,5
,3
~Pe
.2
"~
I
4Aj ,3P,
4
?
ESTIMATED
cm ,s
WIDTH
Fig.17. Cross s e c t i o n s o f Gymnosolen f. s h o w n in o u t c r o p (Fig. 3 D). S t r o m a t o l i t e struct u r e s m a k e u p 55.5% o f t h e bed. A t c e n t e r left e a c h s t r u c t u r e is s h o w n in isolation a n d n u m b e r e d , w i t h s a m e o r i e n t a t i o n as in integral view at u p p e r left. G r a p h s a t r i g h t s h o w r e l a t i o n b e t w e e n " s h a p e " a n d " s i z e " , using t w o a l t e r n a t e f o r m u l a e . T h e circularities in t h e t w o p l o t s are i d e n t i c a l for individuals, b u t t h e sizes are d i f f e r e n t b e c a u s e d i f f e r e n t f u n c t i o n s were u s e d ; t h i s is p a r t i c u l a r l y n o t i c e a b l e for t h e o d d - s h a p e d s e c t i o n s 1--7. T h e star l a b e l l e d M r e p r e s e n t s t h e a r i t h m e t i c m e a n value o b t a i n e d b y averaging t h e 4 0 individual m e a s u r e m e n t s ; t h e cross l a b e l l e d m r e p r e s e n t s t h e value o b t a i n e d b y a single, integral m e a s u r e m e n t o f t h e field o f cross s e c t i o n s at t h e u p p e r left o f this figure. M a n d m are n o t t o o d i f f e r e n t in this case.
The value is at a maximum for a perfect circle (= 0.0796); the greater the departure from a circle, the smaller the value. For size, a number of different estimators can be employed, and the two selected in Table II were used in the analysis for Gymnosolen f. (Fig. 17).
198
A
10 cm
lit! 9
I0
II
12
13
14
15
16
Igl w 1
2
18
19
"zO 21
22
24
25
26
16
27 28
29 30
31
32
33
7
8
9
}0
II
I?
13
II s..411.6v.
,,..,
23
,4
17
18
19
20
21
22
23
24
29 30
31
15
25 26
32 33 34 35 36 37 38 ]9 4041 4,'
D
C
i
?
3
4
5
6
7
8
9
I0
II
12
13
14
15
16
14
||11Vcuw,,,,,,.
34 35
77 28
4P ~2
5
17 6
qht b 8 * * ,
3
18
17
~ 23
m 24
19
a 2s
q, 26
20
~'z
21
" 28
Fig.18. Longitudinal sections of Gymnosolen-Tungussia illustrated in Fig. 3. A: Copy of portion of Fig. 3 A. Stromatolite structures comprise 60.2% of the bed. Morphometrics of isolated columns A1 to A8 given in Fig. 19. B: Copy of portion of Fig. 3 B. Stromatolite structures comprise 60.5% o f the bed. See Fig. 19 for morphometric data. C: Copy o f portion of Fig. 3 C. Structures are appressed due to stylolite development, and constitute 75.7% o f the bed. See Fig. 19 for morphometric data. D: Longitudinal section of platy tungussid structures in equivalent of Unit G of Fig. 4, in Katuk section 4 km ESE of Sanikiluaq.
199
A third, Pe/~, which is applicable only to shapes without peripheral concavities, was n o t used. An approximate, rapid m e t h o d of estimating these quantities for shape and size in a bioherm is to analyse integrally an extensive surface showing a large n u m b e r (N) of stromatolite cross sections by determining the total area and total perimeter of all, and adjusting the values by the total n u m b e r of features analysed (i.e., determine NA/Pe 2 for shape and 2J[(ZA)/uN] 1/~ for size). Average values are m o r e quickly obtained in this way, but these values are less accurate than mean values obtained by averaging individual determinations (i.e., Z (A ~Pc2) IN; Z (2J[A/u]I,~)/N; and Z (4JA ~Pc)IN). The difference between the integrally and the individually determined values increases with decreasing uniformity of size and shape of the cross sections (see differences between M and m in Fig. 17).
Longitudinal sections (silhouettes) Several descriptor functions are given in Table II; others n o t listed include -,4,-- H O R I Z O N T A L 2
PROTUBERANCY
Ih
1.5
Fh .~
*C7
0.9
°~
• 1.0
*MA
.0.9
.A8
• .. M~
.C2
Iv
•J M B .C3
.0.7
°C2 ¢-
Fv Iv
"u
llrM B
.Cll •~
Iv
F._¥
05
Iv
.~
Fh
•~
z O .<
Fv
Fh
-4 c go m :o
2
F!
0.4
Fv • ~'
03
1.5-
-S5
'0"~AI
• AI
O
.C3
1.:
0.6
< m m -4
Nk
.O.8
Fv
.QI
• °CS 02.0
.too
• glS .AS • m *M~
.C.6
*C7
.C8 ,~,,o
"kMA
.86 • AS .e6
Mc
0.7
• A7 .B7
"A2 *A& "B3
"A3
.A4 .810
lk "C'~
0.8
°~ "A2
.~
.Ca C4- • C'i0 CS..C12 .C1
I ,
2 l
Fh
Iv
[ 3 i
ELONGATION
4 l
2.5:. 5 i
.0.4
Fh
3i
25
Ih
Ih 0i6
.~ 0;7
0;8
0i9
liO
INDEX
F i g . 1 9 . M o r p h o m e t r i c d a t a f o r s e c t i o n s in Fig. 18 A - - C . N o t e t h e t r e n d s o f v a l u e s f o r A a n d B, a n d f o r C. MA = arithmetic mean of columns A1--A8. MB = arithmetic mean of columns B1--B10. MC = arithmetic mean of columns C1--C12. MAB = arithmetic mean of columns A1--A8 and B1--B10.
1
200
TABLE II Selected parameters for stromatolite groups Function measured
Quantity calculated
Lamina profiles F v = height F h = width S = length (= perimeter/2)
Fv/S = vertical laminosity index Fh/S = horizontal laminosity index F h / F v = profile flatness
Cross sections (plan views) A = area of structure A, = area of matrix Pe = perimeter of A J = scale conversion factor N = number of individuals
Shape
Shape A/Pe 2
= circularity
Size = equivalent circle diameter = mean width
2Jx/A/~ 4JA/P e
Abundance* IOOA/(A+A,)
Longitudinal sections (silhouettes) F v = height F h = width I v = vertical intercept I h = horizontal intercept A = area of structure A~ = area of matrix Pe = perimeter of A J = scale conversion factor N = number of individuals
= % stromatolite structure in bed
Shape and orientation = circularity = elongation index = vertical protuberancy index = horizontal protuberancy index = intercept ratio
A/Pe ~ Fv/F h Iv/Fv Ih/F h Iv/Ih
Size JA/Iv
= mean width of erect column
Abundance* IOOA/(A+A,)
= % stromatolite structure
in bed *Determined integrally for arrays of structures in bed. s u c h q u a n t i t i e s as t h e r a t i o o f n u m b e r o f b r a n c h e s t o t h e n u m b e r o f i n d i v i dual structures, and the shape of cumulative frequency curves of horizontal c h o r d s ( H o f m a n n , 1 9 7 6 a ) . T h e p a r a m e t e r s c o m p i l e d h e r e are t h e v e r t i c a l a n d h o r i z o n t a l p r o t u b e r a n c y i n d e x e s , a n d t h e e l o n g a t i o n i n d e x . T h e r a t i o I v / F v is a m e a s u r e o f t h e d e g r e e o f v e r t i c a l b r a n c h i n g o f a s t r u c t u r e , a n d I h ] F h gives the a b u n d a n c e of lateral p r o j e c t i o n s and niches. In practice, it seems preferable to plot the reciprocals of these two protuberancy indexes on linear scales, b e c a u s e t h e y a l l o w f o r b e t t e r s e p a r a t i o n o f p o i n t s f o r s t r u c t u r e s w i t h f e e b l y d e v e l o p e d b r a n c h e s o r p r o j e c t i o n s , a n d also b e c a u s e t h e c o o r d i n a t e s a r e f i n i t e ( t e r m i n a t i n g a t a v a l u e o f 1). Fig. 1 9 i l l u s t r a t e s p l o t s o f t h e s e f u n c t i o n s f o r G y m n o s o l e n f. T h e y p r o v i d e a striking, q u a n t i t a t i v e e x a m p l e of the great variability in shape b e t w e e n ind i v i d u a l s t r u c t u r e s w i t h i n a v e r y s m a l l v o l u m e o f t h e s a m e b e d . T h i s is s o m e -
201 thing which can be perceived qualitatively by inspection of the graphic reconstructions, and just as well by the silhouettes {Figs. 5 and 18) and photographs. Insofar as size is concerned, the ratio J A / I gives a very good estimate of the average width of erect and suberect columns, particularly when applied integrally to longitudinal sections (JZNA/ZiVIv)"The value thus obtained is probably generally more representative-of the bed as a whole than either of the values obtained from a horizontal section, which represent only a particular level within the bioherm. Micros tructure The microstructures of the Aphebian stromatolites have not been analysed quantitatively,although estimates can be obtained with parameters such as those suggested by Hofrnann (1976a). The same qualifying statements relating to errors in integralmeasurements, made earlieron in the lastparagraph under the heading "cross sections (plan views)", also apply to integral measurements on rnicrostructure. To complete the illustrationof the Aphebian stromatolites and to permit visual comparison with microstructures of Riphean forms by the traditional qualitative and subjective methods, the Aphebian microstructures are also shown in Figs. 2, 6--9, and 11--13. As with other attributes,considerable variabilityis encountered here too. The microstructures await further study and re-evaluation of the taxonomy at the form level.(The publication of a proposed atlas of microstructures (Monty, 1976), would be an enormously useful endeavour, particularlyif the photographs were to be reproduced at some standardized scale(s).This could serve as a startfor a concerted effort to quantify the microstructural attributes.) The microstructure is a feature of primary importance, for it relatesdirectly to the lithologicaspect at the mesoscopic scale that is the basis for mapping and correlatingin the field.Moreover, the biologicalinfluence or control of primary microstructure, (and hence also its possible evolutionary and chronostmtigraphic importance), appears to be less difficultto demonstrate than the influence on, or control over, gross morphology. An extreme interpretation and application of this is the classificationof "algae" of Vologdin (1962), in which diagenetic effectsare not sufficientlyappreciated. MICROPHYTOLITES Microphytolites (catagraphs) have also been used chronostratigraphically in the Riphean-Vendian. As for stromatolites,a biologic origin and evolutionary tendencies have been assumed. It is advisable,however, to consider most of these entitiesat least as dubiofossils;some are clearly pseudofossils (e.g.,intraclasts,ooids and recrystallizedooids). Their chronologic utility thus is doubtful; on the other hand, they appear to be most useful for de-
0
203
termining past environmental settings. The reported microphytolite "evolution" appears to reflect a succession of different lithofacies. Examples of Aphebian occurrences of Vesicularites, Radiosus, and Asterosphaeroides (Nelcanella), microphytolite groups generally reported from the Riphean-Vendian, are illustrated in Figs. 13 and 20. CONCLUSIONS
At least some stromatolite and microphytolite groups used in making age assignments in studies of Riphean-Vendian sequences occur also in Aphebian rocks. This, together with other empirical data on evolution and ecology of prokaryotes and stromatolite development, indicates that the validity of the "biostratigraphy" based on supposedly evolving stromatolite and microphytolite groups is in need of reexamination. Moreover, the current m e t h o d of defining stromatolite groups can be improved. Equipment now available allows for a variety of quantitative statements to be made about stromatolite morphology. The " m o r p h o m e t r i c " approach, concentrating on studies of individual attributes and their changes through time, rather than studies using "taxa" incorporating divergent concepts, should simplify the determination of long term trends (if any) in these biosedimentary structures. The parallel (nonhierarchical) nature of the group-form classification must be acknowledged. The task of developing a well-founded theory of stromatolites is still before us. A viable theory will integrate data from all the orientations of stromatolitology mentioned in the introduction, and perhaps others yet to be developed. Interpretations of data from one field must be consistent with data from other fields. The information should be intelligible to all practitioners; communication between those working on different objectives is essential. However, this is at present impeded by a multitude of divergent nomenclatures which reflect different fundamental concepts, and different professional backgrounds and degrees of specialization of those working with the structures. A standardized basic terminology for the various attributes and concepts {particularly in the "biostratigraphic" orientation) would seem to be a desirable ingredient for such a unified theory; relationships should be expressed simply and with the least a m o u n t of ambiguity. Fig.20. Aphebian catagraphs in thin sections under transmitted, nonpolarized light. A: Vesicularites f. in grey dolostone, Kasegalik Fro. Belcher Islands. Same thin section as Fig. 13 E. (56°29.5'N 79°30.3'W). G.S.C. type 48539. B: Radiosus f. and Asterosphaeroides (NelcaneUa) f. (small) in light grey dolostone from Nastapoka Group (Manitounuk Supergroup), Richmond Gulf area, SE coast o f Hudson Bay. Specimen collected by F.R. Joubin. G.S.C. t y p e 38369. C : Asterosphaeroides (Nelcanella) f and ooids (obliterated Radiosus). Same thin section as B. D: Enlargement of Asterosphaeroides (Nelcanella) bodies, showing finer crystallinity in dark portions. Rectilinear, palimpsest radiofibrate structure and blunt, angulate ends, suggest genesis by radial crystallization. Same thin section as B.
204 ACKNOWLEDGEMENTS
I gratefully acknowledge support received in the field from R.T. Bell and D. Krause in the Belcher Islands, and M. Rocheleau in the Lake Mistassini area. F. Souli~, who prepared most of the large thin sections used in this study, developed a more efficient method of preparing them. F.R. Joubin provided the sample from the Richmond Gulf area, illustrated in Fig. 20 B--D. The work was financially supported by the National Research Council of Canada under grants no. A 7 4 8 4 and E3952. REFERENCES
Bell, R.T. and Hofmann, H.J., 1974. Investigations of the Belcher Group (Aphebian), Belcher Islands, N.W.T. Geol. Assoc. Can. Mineral. Assoc. Can., Program Abstr., 8. Donaldson, J.A., 1963. Stromatolites in the Denault Formation, Marion Lake, Coast of Labrador, Newfoundland. Geol. Surv. Can. Bull., 1 0 2 : 3 3 pp. Donaldson, J.A., 1975. Proterozoic sedimentary rocks of Canada and the stromatolite controversy. Geol. Soc. Aust., First Australian Geol. Congr., Adelaide, May 1975. Abs. : 23. Donaldson, J.A., 1976. Aphebian stromatolites in Canada: implications for stromatolite zonation. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam. Chap. 7.3, pp. 371--380. G~rich, G., 1906. Les Spongiostromid~s du Vis~en de la Province de Namur. M~m. Mus. R. Hist. Nat. Belg., 3: 1--55. Hofmann, H.J., 1969. Attributes of stromatolites. Geol. Surv. Can., Pap. 6 9 - - 3 9 : 7 7 pp. Hofmann, H.J., 1973. Stromatolites: characteristics and utility. Earth-Sci. Rev., 9: 339-373. Hofmann, H.J., 1974. Stromatolites: stromatoid morphometrics. Geol. Soc. Amer., Abs. Progr. : 798. Hofmann, H.J., 1975. Australian stromatolites--essay review. Geol. Mag. 112: 97--100. Hofmann, H.J., 1976a. Stromatoid morphometrics. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam: Chap. 2.5, pp. 45--54. Hofmann, H.J., 1976b. Precambrian microflora, Belcher Islands, Canada: significance and systematics. J. Paleontol., 50: 1040--1073. Kalkowsky, E., 1908. Oolith und Stromatolith im norddeutschen Buntsandstein. Dtsch. Geol. Ges., 60: 68--125. Komar, Vl. A., Krylov, I.N., Raaben, M.E., Semikhatov, M.A., Serebryakov, S.N., and Zhuravleva, Z.A., 1975. Phytolites in the Precambrian stratigraphy. Sympos. Correlation of the Precambrian, Moscow, September 1975, Abstr.: 71--73. Krylov, I.N., 1975. Stromatolity rifeya i fanerozoya SSSR. Tr. Geol. Inst. Akad. Nauk SSSR, 274 : 243 pp. (in Russian, with English table of contents). Krylov, I.N., 1976. Approaches to the classification of stromatolites. In: M.R. Walter (Editor), Stromatolites. Developments in Sedimentology, 20. Elsevier, Amsterdam, pp. 31--43. Logan, B.W., Rezak, R. and Ginsburg, R.N., 1964, Classification and environmental significance of algal stromatolites. J. Geol., 72: 68--83. Maslov, V.P., 1960. Stromatolity. Tr. Geol. Inst. Akad. Nauk SSSR, 41 : 188 pp. (in Russian). Monty, C., 1976. Note, Stromatolite Newsletter, 4: 23. Monty, C., 1977. Evolving concepts on the nature and the ecological significance of stromatolites: a review. In: E. Fl~gel (Editor), Fossil Algae. Springer-Verlag, Berlin Heidelberg, pp. 15--35.
205
Preiss, W.V., 1976. Proterozoic stromatolites from the Nabberu and Officer Basins, Western Australia, and their biostratigraphic significance. Geol. Surv. South Aust., Rept. Inv. 47 : 51 pp. Preiss, W.V. and Walter, M.R., 1975. Stromatolites of the Australian Precambrian: use in intra- and intercontinental correlations. Sympos. Correlation of the Precambrian, Moscow, September 1975, Abstr.: 74--75. Reis, O.M., 1908. Kalkowsky: Uber Oolith und Stromatolith im norddeutschen Buntsandstein. Neues Jahrb. Min., Geol. Pal., 2: 114--134. Vologdin, A.G., 1962. Drevneishie vodorosli SSSR. Akad. Nauk SSSR, Moscow, 656 pp. (in Russian). Walter, M.R., 1972. Stromatolitesand the biostratigraphyof the Australian Precambrian and Cambrian. Palaeontol. Assoc. London, Spec. Pap. 11:190 pp.
ON A P H E B I A N S T R O M A T O L I T E S AND R I P H E A N S T R O M A T O L I T E STRATIGRAPHY
H.J. HOFMANN
Department of Geology, University of Montreal, Montreal (Canada) (Received February 8, 1977)
ABSTRACT Hofmann, H.J., 1977. On Aphebian stromatolites and Riphean stromatolite stratigraphy. °'~ Precambrian Res., 5: 175--205. New occurrences of typical Riphean stromatolite and microphytolite groups are reported and illustrated from typical Aphebian sequences in Canada. These include Minjaria, ColonneUa, and Kussiella from the Mistassini Group of central Quebec, Gymnosolen, Jaeutophyton garganicum, Minjaria, CoilenieUa, Kussiella, Tungussia, Lenia, Osagia, and Vesicularites from the Belcher Supergroup in Hudson Bay, and Asterosphaeroides (Nelcanella) and Radiosus from the Manitounuk Supergroup of the southeastern coast of Hudson Bay. These occurrences are at variance with prevailing views of Riphean "biostratigraphy". They call into question the validity of age assignments based only on stromatolite groups, as well as the premise that prokaryotic evolution is mainly responsible for observed trends in gross morphology. The systematics of stromatolites is re-examined in the second part of the paper, and a more objective, morphometric (stereometric) approach is discussed. Examples illustrate how differences and similarities whitin and between stromatolite groups can be expressed quantitatively. The morphologic attributes chosen comprise the shapes of lamina profiles, cross sections (plan views), and longitudinal sections (silhouettes). Microstructures are illustrated photographically; they were not analysed morphometrically and await further study.
INTRODUCTION A t least five o r i e n t a t i o n s in t h e subdiscipline o f s t r o m a t o l i t o l o g y can be identified: biostratigraphic, s e d i m e n t o l o g i c , biologic, g e o p h y s i c a l , a n d m o r p h o metric. These are n o t m u t u a l l y exclusive, b u t t h e y d i f f e r in t h e emphasis p l a c e d o n t h e objectives p u r s u e d , a n d b y t h e c o n c e p t s a n d i n d e p e n d e n t nom e n c l a t u r e s used. O n e well r e p r e s e n t e d g r o u p includes t h e " b i o s t r a t i g r a p h e r s " , t h o s e w h o see as their m a i n t a s k t h e subdivision a n d interregional c o r r e l a t i o n o f seq u e n c e s based o n detailed investigations o f assemblages o f fossil s t r o m a t o lites, c o u p l e d with r a d i o m e t r i c a g e - d e t e r m i n a t i o n s in t h e i r geologic c o n t e x t . F o l l o w e r s o f this s c h o o l generally e m p l o y a f o r m a l b i n a r y , latinized n o m e n -
176 clatorial system for handling the paleontologic data and for conveying taxonomic information (e.g., "an assemblage of Kussiella kussiensis and Omachtenia omachtensis"). Another group of individuals, also studying fossil sequences, follows a "sedimentologic" approach. The chief interest in this orientation is in sedimentary, paleocological, and perhaps economic aspect of stromatolites, without necessarily attributing chronostratigraphic significance to them. Adherents to this practice tend to employ an informal, descriptive terminology, rather than a formal binary nomenclature (e.g., "SH-V type", or "unlinked, closely spaced intertidal columns with a preferred NW orientation"). A third, "biologic" approach is pursued by those who are studying the problem of stromatolite genesis by observing the biologic and sedimentary entities, processes, and conditions and geochemistry in different modern environments. The nomenclature of stromatolites involves mainly the truly biological taxonomy of the predominant biota, and descriptive terms for mat (or fabric) types, and environmental setting {e.g., "intertidal pustular mat of En tophysalis major"). A fourth approach is applied by a small group who can be said to have a "geophysical" orientation. By quantitative analysis of the laminar rhythmicities the potential astronomic significance of stromatolites is assessed, including such aspects as their utility as geologic calendars, indices of lunar orbital evolution, and seasonality. In these rhythmometric studies the quality of the lamination is of paramount importance (uniformity of laminae, presence of periodicities); taxonomic matters are relatively unimportant. A fifth and not yet well developed line of pursuit is concerned with the purely geometric-mathematical aspects of stromatolite characteristics, and with improving the methods of data collecting and handling, using computers. This "morphometric" approach involves analytic studies in which the objective is to make quantitative statements about the geometric attributes, permitting statistical treatment of the data. This allows for more precision in the specification of attributes and identification of stromatolite "taxa" and environments, and facilitates comparisons and communication as well. It also entails synthetic studies whose aim is to improve our understanding of the common geometric properties and relationships of the basic elements of stromatolites of different origin, and also of similar abiogenic structures. This is achieved by using computers which generate artificial images that simulate natural growth forms. By varying the values assigned to the different fundamental stromatolite parameters, the geometric variations in the graphic displays can be appreciated quantitatively, and direct comparisons can be made with natural forms. The principal aim of this paper is to report and illustrate further new occurences of typical Riphean stromatolites in the Aphebian, to discuss certain major problems confronting the "biostratigraphers", and to suggest possible ways of resolving them. The dilemma is posed both by empirical data inconsistent with some prevailing views of Riphean biostratigraphy, and by some-
177
what haphazard taxonomic practices. The latter can be improved by applying the "morphometric" approach. To avoid ambiguity, the terms used in this paper for stromatolite elements are given in the left column of Fig. 1; following previous usage (Logan et al., 1964; Hofmann, 1969), oncolites (oncoids) are considered to be special, mobile types of stromatolite structures. Allowance is made for the segregation and separate designation of different morphologic elements within bioherms and for the eventual naming of higher rank taxonomic categories (such as the "bioherm series" of Komar et al., 1975). This is comparable to the situation regarding conodonts, where multielement assemblages bear names different from those of the individual elements. STROMALOLITE STRATIGRAPHY
The underlying premise of Precambrian biostratigraphy is that the stromatolite characteristics exhibit secular changes through geologic time. These characteristics are essentially geometric, lithologic (textural), and dimensional; the methods of serial sectioning and graphic reconstruction, and
STROMATOLITE COMPONENTS CONCEPT
OTHERDESIGNATION
THE BED
bioherm, biostrome
THE STRUCTURE
stromatolite column, head, mound, coenoplase
THE LAMINA
stromatoid layer
THE MICROSTRUCTURE
microfabric, spongioatromid taxon
THE TEXTURE
grain-to-grain relationship
TH E G RAIN
o
THE MICROFOSSIL TIME
particle, crystal, mineral taxon, organism, biota
T
Fig.1. Make-up of stromatolite beds. The definition and usage of the terms stromatolith and stromatoid of Kalkowski (1908) are somewhat ambiguous: for discussions see Reis (1908) and Monty (1977).
178
microscopic study of thin and polished sections have served to outline such changes. Practitioners in the "biostratigraphic" orientation generally have a paleontological background, and they apply the standard paleontological treatment accorded real organisms, such as erecting formal taxa, designating types, and using a binary latinized nomenclature. This has, for some nonspecialists who use the information, obscured the fact that stromatolites are in part sedimentary structures. These structures with latinized names cannot be regarded as biological taxa, even though the names are Linn~an in form. The Riphean " z o n e s " or " p h y t e m s " represent assemblages of biosedimentary structures and lithologies, and are different from Phanerozoic biozones which are based on assemblages of fossilized organisms; they are, however, comparable to trace fossil assemblages which also consist of biosedimentary structures (see also Hofmann 1969, pp. 40--42). The biostratigraphic scheme was first established for Eurasia, where geological conditions favoured the extensive development of stromatolitic sequences during the Riphean and Vendian (1.65--0.57 Ga). In the same territory, however, stromatolitic Aphebian sequences (2.5--1.7 Ga) are underrepresented. The Precambrian biostratigraphy with its four major subdivisions is thus primarily for the approximately 1 Ga long Riphean-Vendian. In North America, Aphebian stromatolitic sequences are well represented, and over the last few years many of the stromatolite groups supposed to be restricted to the mid- and late Riphean have n o w been identified from these earlier sequences (Hofmann, 1969, 1973; Bell and Hofmann 1974; Donaldson 1975, 1976). Additional examples are given below. Such discrepancies are n o w also being noted on other continents where Aphebian sequences abound (Walter, 1972, pp. 84, 166; Preiss and Walter, 1975; Preiss, 1976). Evidently, then, d o u b t must be cast on age assignments based only on groups, some of which are inadequately defined or overlap with others. Moreover, the precision of the identification at the group levels needs to be re-examined. Until now, morphologic variations between groups have not been expressed quantitatively. Selected data on new Aphebian stromatolite occurrences are summarized in Table 1, and the structures are illustrated photographically and by reconstructions in Figs. 2--13. Photographs were introduced to Precambrian stromatolitology in 1890, and are still the most widely used tool for disseminating morphologic information on gross form and microstructure. Assuming that the photos are unretouched, this m e t h o d is generally objective insofar as it involves relatively little interpretation (other than selecting the sample for illustration and in deciding on print quality--particularly contrast). TAXONOMY AND S T E R E O M E T R Y
The need for a more objective, quantitative approach to stromatolite classification is becoming urgent. The increasingly complex descriptive literature makes it difficult to relate and compare newly found structures with those
179
Fig.2. M i n j a r i a - C o l o n n e l l a - Kussiella biostrome in dark grey, anthraxolitic dolostone of the upper part of the Upper Albanel Fro. (Mistassini Group); W side of Temiscamie Bay, Lake Albanel, central Quebec (51°01.5'N 73°06.5'W). Compare with "columnar stromatolites" in Donaldson (1963, pl. 2). A: Outcrop view of dendroid variant in biostrome. B: Polished longitudinal section; G.S.C. type 48527. C: Large thin section in transmitted, nonpolarized light. Rectangle outlines area of part D. D: Microstructure of central part of column. Rectangle outlines ared in E. E: Detail of microstructure at high magnification. Note grain size differences.
already described. Probable synonyms appear more frequently, and the situation is complicated by the existence of different schools of thought on what are taxonomically significant attributes, by language barriers, by unavailability of certain publications, by difficulties in obtaining type material and
2
3--7; 15, 17, 18--19
8
Min]aria (+ Colonnella, Kussiella)
Gymnosolen (+ Tungussia)
Gymnosolen
Colleniella (+ Kussiella, Stratifera)
10 A--C
Jacutophyton garga- 9 nicum
Illustrations (Figs.)
Groups (and associated groups)
48532
48533
Sanikiluaq section 56°32.7'N 79°14.6'W
48531
Katuk section 56°31.9'N 79°10.0'W
Belcher Islands, Hudson Bay
Sanikiluaq section 56°32.3'N 79°14.6'W
Belcher Islands, Hudson Bay
Sanikiluaq section 56°32.8'N 79°14.4'W
48530
48529
Sanikiluaq section* 56°32.7'N 79°14.6'W Belcher Islands, Hudson Bay
48528
48527
G.S.C. types**
Katuk section* 56°31.9'N 79°09.0'W
Belcher Islands, Hudson Bay
Lake Albanel, central Quebec 51°01.5'N 73°06.5'W
Localities collected
Summary of new occurrences of Aphebian stromatolites and microphytolites
TABLE I
Belcher Supergroup Mayor Fro. (lower part)
Belcher Supergroup McLeary Fm. (top part)
Belcher Supergroup Tukarak Fm. (lower part)
Belcher Supergroup Mayor Fm.
Mistassini Group Albanel Fro. (upper part)
Stratigraphic unit
Dolostone
Dolostone
Limestone
Dolostone
Dolostone
Lithology
Low intertidal to subtidal; currents azimuth 060 °
Subtidal
Subtidal
Subtidal
Subtidal
Inferred environment
(3O O
11;16
12
13
20
Lenia (+ Stratifera )
Minjarm
Osagia (+ Vesicularites)
Asterosphaeroides (+ Radiosus) 56.2°_+N 76°30'W
38369
48539
Churchill Sound 56°29.5'N 79°30.3'W Richmond Gulf, SE coast of Hudson Bay
48538
Sanikiluaq section 56°32.5'N 79°14.1'W
Belcher Islands, Hudson Bay
Churchill Sound 56°29.5'N 79°30.2'W
Belcher Islands, Hudson Bay
48537
48536
56°32.6'N 79°14.1'W Churchill Sound 56°29.5'N 79°30.3'W
48535
48534
Sanikiluaq section 56°32.6'N 79°14.0'W
Belcher Islands, Hudson Bay
Sanikiluaq section 56°32.6'N 79°14.2'W
Belcher Islands,Hudson Bay
* For detailed maps and sections refer to Hofmann (1976b) ** Numbers refer to type collection of Geological Survey of Canada, Ottawa
10 D
Tungussia
Manitounuk Supergroup Nastapoka Group
Kasegalik Fm. (upper part)
Belcher Supergroup McLeary Fm. (lower and middle parts)
Belcher Supergroup Kasegalik Fm. (upper part)
Kasegalik Fm. (upper part)
Belcher Supergroup McLeary Fm. (lower and middle parts)
Belcher Supergroup McLeary Fm. (middle part)
Dolostone
Dolostone
Dolostone
Dolostone
Dolostone
Dolostone
Dolostone
Shallow subtidal
Shallow subtidal and intertidal
Subtidal
Supratidal and intertidal; subtidal?
Subtidal
Fig. 3. Outcrop views of Gymnosolen-Tungussia biostrome in grey and buff dolostone of Mavor Fm. at Sanikiluaq, Belcher Islands (56 ° 32.7'N 79 ° 14.6'W). A and B: Two views of central portions of biostrome in Unit G of Fig. 4. Note subhorizontal stylolite at middle of B, truncating columns. Scale graduated in din. C: View of strongly decumbent structures in Unit F of Fig. 4; strong stylolitization causing appression of some columns. D: Plan view, showing variations in size and shape of Gymnosolen f.
Ov t~
183
examination of field occurences. Some of the problems could be eliminated by a standardization of the criteria adduced in classification, and b y more effective methods of presenting morphologic data used in establishing the taxa. If this standardization were to emphasize presentation in pictorial, graphic, or numerical form, rather than in verbalization, the information would be in a m o d e allowing for more precise definition, more decisive comparisons, better inter-language communication, as well as for more efficient data handling, storage, and retrieval (Hofmann, 1976a). The present lack of consensus also results in the description of new fossil groups and forms based on different concepts, making direct comparisons and evolutionary studies between incongruous taxa difficult. Among the many different methods available (see Hofmann 1969 and Krylov 1975, 1976 for summaries), t w o main ones have emerged for the treatment of
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SKETCH MAP OF BASAL PART OF MAVOR FM. SANIKILUAQ
Fig.4. Sketch map of basal part of Mavor Fro., Sanikiluaq. Area of map is 400 m from N end o f long outcrop of Mavor and Tukarak Fins. at left center of fig. 2 in Hofmann (1976h) (56°32.7'N 79°14.6'W). Position of large glacial erratic given for reference in field. North is to right. Unit A: Grey, massive dolostone with molar-tooth structure and minor edgewise breccia. UnitB: Laminated grey dolostone; some edgewise breccia. Unit C: Grey, massive dolostone. Unit D: Large mounds of laminated dolostone (Colleniella and Stratifera) with intraforrnational intermound breccias. See Fig. 10B and C. Interference ripples in lower part. Unit E: CoUeniella mounds of laminated grey dolostone ; brown weathering dolomitic quartz arenite and quartzose dolostone in channelized, intermound areas. Preferred orientation of mounds 060 °. See Fig. 10A. Unit F: Massive dolostone ; basal portion of biostrome with oblique columns and plates (see Fig. 3C). Unit G: Massive Gymnosolen dolostone. See Figs. 3A, B, D, 5, 6.
184
fossil structures by "biostratigraphers" of the Precambrian. Both of these use a binomial, latinized system of nomenclature -- the traditional vehicle of communication in biology and paleontology for more than 200 years. These two main trends may be identified as that based on the spongiostrome concept of Gfirich (1906) in which the microstructural features serve as the basis for major categories, and that on the stromatolite concept of Kalkowsky, in which the gross form and lamina shape is the predominent group characteristic. (To say this is somewhat of an oversimplifiaction, because individual authors have n o t always adhered to one concept consistently, even within a single publication). There is thus the curious situation where the same speci-
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Fig.5. Graphic reconstructions of Gymnosolen f. in Mavor Fro.; 4 km ESE of Sanikiluaq (Katuk section, 56°31.9'N 79°09.9'W). Block from bed equivalent to Unit G in Fig. 4. G.S.C. type 48528.
185
Fig.6. Microstructures of Gymnosolen f., Mayor Fm., Sanikiluaq and Katuk sections. A: Outcrop view of Unit G (Fig. 4), showing columns with two types of microstructure: well laminated recrystallized dolostone at bottom and recrystallized dolostone with radiofibrate texture in upper parts. B: Thin section under transmitted, nonpolarized light, showing the two types of microstructure. Specimen is from column B in Fig. 5. Rectangles outline fields in parts C and D. G.S.C. type 48529a. C: Enlargement of radiofibrate texture in upper left of B. D: Microstructure of top of column at lower right in B. Rectangle outlines area in E. E: Microstrucutre at high magnification, showing grain size differences between light and dark lamellae. m e n s can have d i f f e r e n t n a m e s , d e p e n d i n g o n w h a t m e t h o d is applied. E x a m ples are Conophyton Maslov, Lerrnontovaephycus V o l o g d i n , Granifer Vologdin, a n d Tschichatschevia V o l o g d i n . This is n o t a m a t t e r o f o m i s s i o n o r o f i g n o r a n c e o f t h e o t h e r classification, b u t o n e o f i n t e n t , w h e r e b y f u n d a m e n -
186
tally different concepts are applied to stromatolite systematics, both classifications being characterised b y Linn~an-like designations. In such a confusing situation it is conceivable that a particular form could be listed twice or more under different names in fossil lists compiled by uninitiated individuals. Gross morphology, lamina shape, and microstructure are the parameters n o w generally considered most important, and this is reflected in the prevailing trend to use the first two in the group concept and microstructure in the form taxa {e.g., Conophyton garganicum). As indicated elsewhere (Hofmann 1975, p. 99), the binomial stromatolite nomenclature now in use is n o t hierarchical (LinnSan) b u t parallel. An equal weight can be ascribed to the morphology and to the microstructure, and perhaps to the profile shape as well; neither is necessarily subordinate to the other as is the species to the genus in the Linn~an system; and their positions, in a natural hierarchy, if
Fig.7. Microstructure of Tungussia portions in Unit F of Fig. 4. G.S.C. type 48529b.
187
any, are undetermined. It would be equally valid to write Garganicum conophyton, as long as Conophyton refers to growth form and garganicum to a microstmcture. If this same microstmcture is found with Jacutophyton, it should be possible to refer to it as J. garganicum. Seen in this light, a system
Fig.8. Gymnosolen f. in bluish grey limestone in lower half of Tukarak Fro. at Sanikiluaq (56°32.8'N 79°14.4'W). A: Outcrop view of erect columns and orange-weathering replacement dolomite patches and dikes. T o p o f biostrome is capped by orange-weathering, laminated grey dolostone showing synsedimentary microfaulting. Scale in dln. B: Cross sections of columns. Scale in dm and cm. C: Thin section under transmitted, nonpolarized light, showing lamination and radiofibrate crystallization fabric. Rectangle in column at lower left marks field magnified in D. G.S.C. type 48530. D: Microstmcture showing laminae and radiofibrate texture.
188
Fig.9. Jacutophyton (Conophyton) garganicum in cherty grey dolostone of upper fifth of McLeary Fm. at Sanikiluaq, Belcher Islands (56°32.3"N 79°14.6'W). A: Outcrop view. Scale in dm and cm. B: Close-up of another branching column with conical laminae. C: Thin section under transmitted nonpolarized light, showing microstructural variations, branching, and contact with matrix at upper left. Rectangle at right margin outlines field in part D. G.S.C. type 48531. D: Microstructure, showing alteration. Thickness of dark layers generally 25--100 ~m, light layers 50--150 urn. Rectangle near b o t t o m center magnified in E. E: Microstructure under high magnification.
Fig. 10. Outcrop views of large structures in Mavor and McLeary Fins. A: Coileniella m o u n d with flanking arenite beds in Unit E of Fig.4. Scale in dm and cm. B: Colleniella-Stratifera m o u n d with flanking flat-pebble breccia in Unit D of Fig.4. Bioherm is 5 m N of S end of outcrop. Compare with fig. 19 of Hofmann (1969). G.S.C. type 48533. C: Bioherm of Kussiella exhibiting furcate branching. Bed equivalent to upper part of Unit D of Fig.4 at Katuk section (56°31.9'N 79°10.0'W). G.S.C. type 48532. D: Tungussia from distinct, 1 m thick marker horizon in lower part of middle member of McLeary Fro., Sanikiluaq (56°32.6'N 79°14.2~). G.S.C. type 48534.
Fig. 11. Lenia f. in lower half of McLeary Fm., Sanikiluaq, Belcher Islands. Compare with "digitate stromatolites" of Donaldson (1963, pls. 4--5). A: Outcrop view of bed with columns in lower member of McLeary Fro. (56°32.6'N 79°14.1'W). Scale in dm. B: Plan view of columns in bed 2 m above Tungussia marker bed (Fig.10 D) at Sanikiluaq (56°32.5'N 79°14.2'W). C: Polished surface with inclined columns from same bed as in A. G.S.C. type 48535. D: Lenia-Stratifera in bed 29 m stratigraphically below bed in A. Thin section, transmitted, nonpolarized light. Rectangle outlines area of part E. G.S.C. type 48536. E: Microstructure, showing relict radiofibrate texture. F: Enlarged view of microstructure from just beneath lower left of E. Note the flared nature of the dark lamellae suggesting possible physiochemical origin of lamination.
o
Pig.12. Minjaria f. in grey dolostone of Kasegalik Fro., N end of Churchill Sound, Belcher Islands (56°29.5'N 79°30.2~V). Compare this also with the pre-Riphean Pitbaria described by Walter (1972, p. 167 ). A: Outcrop showing slightly oblique, longiudinal section of digitate columns. B: Close-up of hand specimen. G.S.C. type 48537a. C: Thin section in transmitted, non)olarized light. Rectangle marks field o f part D. G.S.C. t y p e 48537b. D: Microstructure in central portion o f column in C. ~: Enlarged view of left central portion of D. Note grain size differences between light and dark lamellea.
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192
Fig.13. Osagia f. in grey dolostone, McLeary and Kasegalik Fms., Belcher Islands. Compare with "pisolitic stromatolites" of Donaldson (1963, pl.6). A: Outcrop view of lens with oncoids and catagraphs within Lenia biostrome. 1.2 m above Tungussia marker bed (Fig. 10 D), Katuk section (56°32.0'N 79°10.3'W). B: Thin section of pinkish grey dolostone in transmitted, nonpolarized light. Bed stratigraphically 66 m below Tungussia marker bed, Sanikiluaq (56°32.5'N 79°14.1'W). G.S.C. type 48538. C: Microstructure; enlarged view of upper left part of oncoid in center of B. D: Microstructure more highly magnified; field of view 3 mm below left central part of C. Note slightly flared outlines of dark lamellae, comparable to flaring in Lenia (Fig. 11 F). E: Osagia f. and Vesicularites f. in grey dolostone, Kasegalik Fro., N end of Churchill Sound; adjoining Minjaria locality (56°29.5'N 79°30.3'W). G.S.C. type 48539.
193
of polynomial designations such as that proposed by Maslov (1960) is attractive. Stromatolites have important sedimentary and diagenetic components; in this respect they are like ichnofossils, as already mentioned. (Ichnotaxa may reflect anatomical and behavioural attributes, as well as sedimentary and taphonomic properties, and separate descriptive, ethologic, and stratinomic classifications exist in parallel.) Notwithstanding the general trend in group and form concepts, the systematics of stromatolites, continues to be beset with inconsistencies in the choice of features, and combinations of features, in erecting the taxa. Inspection of the diagnoses of many taxa currently in use reveals that the systematics is literally unsystematic (see also Krylov, 1976, pp. 36--40). Though acknowledged, this state of affairs is not found objectionable by everyone {e.g. Walter, 1972, p. 17). It does, however, impede understanding of the factors which are responsible for the supposed evolutionary trends in stromatolites through the Proterozoic. By isolating each individial factor it will be easier to analyse its changes through time, while at the same time it should encourage the quantification of information that would allow stromatolitology to become more objective; it would also permit better evaluation of models of stromatolite genesis and evolution and their causes. Much has been made of the premise that graphic reconstructions are essential for the identification of groups, yet this methodology itself can be criticised (Hofmann, 1969, 1973, 1976a; Donaldson, 1975). To maintain that extensive longitudinal and transverse sections such as seen in outcrops are insufficient for identification is not only to deny a basic principle and tool of stereology (that sections are representative samplings of volumes as a whole), but is also to deny oneself the use of available rapid and powerful statistical techniques in morphometric analysis. Moreover, the reconstruction method also neglects the factor that the gross morphology of individual structures is almost certainly governed by stochastic processes (the probability that two structures have identical form is very much smaller than the probability that two individuals of a true biological species have the same form). What one desires in making the reconstructions is really to obtain an idea of the modal form (a statistical concept) for a taxon. Such a modal form can, however, also be deduced from sections, for it matters little, statistically speaking, whether we analyze a number of serial sections from one block, or one or two large sections containing a multitude of columns. As well as being more representative, the latter alternative has the advantage of applying equally to structures too large or too small to be handled or serially sectioned. Few would deny the great utility of shape and size analysis of sandstone grains in thin sections where the relevant information is obtained without graphically reconstructing individual grains. A computer assisted method of morphometric analysis of stromatolites was outlined by Hofmann (1974, 1976a). The technique is still being refined, and additional parameters are being developed and applied to Aphe-
194
bian structures. The method is based on partially automated measurements and analysis of shape, size, and orientation functions, using electronic image. analysing equipment. The numerical data extracted from images include measures of areas, perimeters, lengths, convexities, and number of features in a given field o f view. Each structure, or array of structures, is analysed for quantities that express the shape, size, and orientation of four main features of use in stromatolite taxonomy: longitudinal section (silhouette), cross section (plan view), lamina shape, and microstructure. Selected Aphebian stromatolite groups serve as examples to illustrate the method (Figs. 14--19). The data should be compared with reconstructions and photographs shown in Figs. 2--6 and 8--11, which supplement the quantitative data and allow comparison with Riphean groups already described in the traditional way. Table II gives the functions and parameters that have been found useful in quantifying stromatolite attributes. o;~
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Fig.14. Laminosity plot: synopsis of lamina profile shapes. Only simple, even laminae w i t h 1 o r d e r o f c u r v a t u r e are d e p i c t e d . F o r e x p l a n a t i o n s e e t e x t .
195
Lamina profiles Of particular interest in categorizing the shape of lamina profiles is a laminosity plot, a preliminary version of which was introduced by Hofmann {1974, 1976a, fig.5). A more complete and modified rendition of this plot is now given in Fig. 14, which illustrates both pictorially and numerically the LAMINA
PROFILES
Gymnosolen f.
.0.7, 0.5
I PROFILE
• 0.6
FLATNESS
__Fh
.,z
Fv .52
0.5 .75
o z :E --I
<
,81
6,2.68
5~6S
2~
9
0.4
Fv S
.~o ~ ! ~ , * ~ b ~ 3
0.3
o in. w
>
.,
46 "54 2~1~/ 7242.. :~3~
4 .41 el
29"~ \5 .8o
0.2
~Fh.
"e34
3~
.15 -19
.~
4-
=47 .44
"
s
0.4 t
0.5 I
HORIZONTAL
0.6 l
Fh S
0.7 l
0.8 i
0.9 f
LAMINOSITY
Fig.15. Lamina profiles and corresponding laminosity plot for axial and near axial sections of Gymnosolen f. from the block illustrated in Figs. 5 and 6 B. Symbols defined in Table II. Star represents arithmetic mean, close to profile 67. Compare with Fig. 14.
196 variations of basic eliiptical and angulate profiles with one order of curvature. The abscissa is designated the horizontal laminosity index, a measure of the relative horizontal development of laminae; the ordinate is the corresponding vertical laminosity, which expresses their relative vertical extent. The derived profile flatness ratio, Fh/Fv, gives the profile configuration in terms of the smallest plumbed rectangle circumscribing each lamina. The addition of second and higher orders of curvature to the basic first order curve (crinkling) has the effect of displacing the position of profiles towards the origin of the graph (S/[Fh + Fv] increases). The superimposed trend-lines A to E delineate selected families of laminae with specific geometric attributes. Line A contains the loci of rectilinear profiles at different inclinations; it also defines the outer limit of the field of geometric possibility of continuous laminae (S/[Fh + Fv]) is at a minimum for any value of Fh/Fv). Line B is the trend of untilted, rectilinear, symmetrically angulate profiles. Line C represents untilted semiellipses, while line D localizes laterally directed semiellipses. Line E contains plenicinct (completely encapsulating) elliptical laminae such as found in oncoids. Oncoids with penecinct (incompletely encapsulating) elliptical laminae would lie above and to the right of line E, whereas oncoids with plenicinct laminae with t w o or more orders of curvature (e.g., Ottonosia) would fall below and to the left of line E. Similarities and differences between the laminar shapes of the newly reported Aphebian occurrences of Gymnosolen and Lenia are shown in Figs. 15--16. Cross sections (plan views) The circularity index A/Pe 2 is an excellent descriptor quantity for shape. .0.4
Fb 0
'03
Fv
.~
4FV -0.2 ~
"~
1~21.31"1S 8. 1.9
S'ZL 141P.]~.1fl
~ _
~
0
g I
cm
"01
2~ 2" 22,13 8"
Fh
2
07
S
o.8
o.9
io
HORIZONTAL LAMINOSITY
LAMINA PROFILES Fig.16. Lamina profiles and corresponding laminosity plot for random longitudinal sections of Lenia f. from block illustrated in Figs. 11 A and C. Symbols s a m e as for Figs. 14--15. Arithmetic m e a n l a m i n o s i t i e s i n d i c a t e d by star near profile 26. Note that t h e c l u s t e r i n g is t o t h e right of the cluster in Fig. 15.
197
O.Oe
-39
•
• 37 0.07
-'"4t
.3~
:~p
3~'33 .29
"'~
"15+14
"3027
w
.,, .21
-16
•2~4
.12
M "1~18 • 22 rnX
0.06
" I1 "9.10
A 2 I
2
3
4
5
• 34
"23 "6
,0.05
6
*7
7
8
9
10
11
,0.04
12
-4
A
OOe
"2
' 0.03 ~ J : : P e
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
•
8
•
~
e
•
31
32
33
34
35
36
T8
39
2 i
i
37
•
;0
10
5
i
i
6 i
EQUIVALENT CIRCLE DIAMETER
0.08
.39
"~
•
"3
.5
cm
•0.07
~ 2+
"~,
+35 "27 +19 3~"33 .29 "2B ,21 • 38 3"?"31 t20 .25
,4o
,16 -12
"24
M 'J~18 • 22 x m
.0,06
A o
me
• 34
*11 "~.10
,23 ,6
-0.05 .7 -17
-0.04
,4 A
~03
,5
,3
~Pe
.2
"~
I
4Aj ,3P,
4
?
ESTIMATED
cm ,s
WIDTH
Fig.17. Cross s e c t i o n s o f Gymnosolen f. s h o w n in o u t c r o p (Fig. 3 D). S t r o m a t o l i t e struct u r e s m a k e u p 55.5% o f t h e bed. A t c e n t e r left e a c h s t r u c t u r e is s h o w n in isolation a n d n u m b e r e d , w i t h s a m e o r i e n t a t i o n as in integral view at u p p e r left. G r a p h s a t r i g h t s h o w r e l a t i o n b e t w e e n " s h a p e " a n d " s i z e " , using t w o a l t e r n a t e f o r m u l a e . T h e circularities in t h e t w o p l o t s are i d e n t i c a l for individuals, b u t t h e sizes are d i f f e r e n t b e c a u s e d i f f e r e n t f u n c t i o n s were u s e d ; t h i s is p a r t i c u l a r l y n o t i c e a b l e for t h e o d d - s h a p e d s e c t i o n s 1--7. T h e star l a b e l l e d M r e p r e s e n t s t h e a r i t h m e t i c m e a n value o b t a i n e d b y averaging t h e 4 0 individual m e a s u r e m e n t s ; t h e cross l a b e l l e d m r e p r e s e n t s t h e value o b t a i n e d b y a single, integral m e a s u r e m e n t o f t h e field o f cross s e c t i o n s at t h e u p p e r left o f this figure. M a n d m are n o t t o o d i f f e r e n t in this case.
The value is at a maximum for a perfect circle (= 0.0796); the greater the departure from a circle, the smaller the value. For size, a number of different estimators can be employed, and the two selected in Table II were used in the analysis for Gymnosolen f. (Fig. 17).
198
A
10 cm
lit! 9
I0
II
12
13
14
15
16
Igl w 1
2
18
19
"zO 21
22
24
25
26
16
27 28
29 30
31
32
33
7
8
9
}0
II
I?
13
II s..411.6v.
,,..,
23
,4
17
18
19
20
21
22
23
24
29 30
31
15
25 26
32 33 34 35 36 37 38 ]9 4041 4,'
D
C
i
?
3
4
5
6
7
8
9
I0
II
12
13
14
15
16
14
||11Vcuw,,,,,,.
34 35
77 28
4P ~2
5
17 6
qht b 8 * * ,
3
18
17
~ 23
m 24
19
a 2s
q, 26
20
~'z
21
" 28
Fig.18. Longitudinal sections of Gymnosolen-Tungussia illustrated in Fig. 3. A: Copy of portion of Fig. 3 A. Stromatolite structures comprise 60.2% of the bed. Morphometrics of isolated columns A1 to A8 given in Fig. 19. B: Copy of portion of Fig. 3 B. Stromatolite structures comprise 60.5% o f the bed. See Fig. 19 for morphometric data. C: Copy o f portion of Fig. 3 C. Structures are appressed due to stylolite development, and constitute 75.7% o f the bed. See Fig. 19 for morphometric data. D: Longitudinal section of platy tungussid structures in equivalent of Unit G of Fig. 4, in Katuk section 4 km ESE of Sanikiluaq.
199
A third, Pe/~, which is applicable only to shapes without peripheral concavities, was n o t used. An approximate, rapid m e t h o d of estimating these quantities for shape and size in a bioherm is to analyse integrally an extensive surface showing a large n u m b e r (N) of stromatolite cross sections by determining the total area and total perimeter of all, and adjusting the values by the total n u m b e r of features analysed (i.e., determine NA/Pe 2 for shape and 2J[(ZA)/uN] 1/~ for size). Average values are m o r e quickly obtained in this way, but these values are less accurate than mean values obtained by averaging individual determinations (i.e., Z (A ~Pc2) IN; Z (2J[A/u]I,~)/N; and Z (4JA ~Pc)IN). The difference between the integrally and the individually determined values increases with decreasing uniformity of size and shape of the cross sections (see differences between M and m in Fig. 17).
Longitudinal sections (silhouettes) Several descriptor functions are given in Table II; others n o t listed include -,4,-- H O R I Z O N T A L 2
PROTUBERANCY
Ih
1.5
Fh .~
*C7
0.9
°~
• 1.0
*MA
.0.9
.A8
• .. M~
.C2
Iv
•J M B .C3
.0.7
°C2 ¢-
Fv Iv
"u
llrM B
.Cll •~
Iv
F._¥
05
Iv
.~
Fh
•~
z O .<
Fv
Fh
-4 c go m :o
2
F!
0.4
Fv • ~'
03
1.5-
-S5
'0"~AI
• AI
O
.C3
1.:
0.6
< m m -4
Nk
.O.8
Fv
.QI
• °CS 02.0
.too
• glS .AS • m *M~
.C.6
*C7
.C8 ,~,,o
"kMA
.86 • AS .e6
Mc
0.7
• A7 .B7
"A2 *A& "B3
"A3
.A4 .810
lk "C'~
0.8
°~ "A2
.~
.Ca C4- • C'i0 CS..C12 .C1
I ,
2 l
Fh
Iv
[ 3 i
ELONGATION
4 l
2.5:. 5 i
.0.4
Fh
3i
25
Ih
Ih 0i6
.~ 0;7
0;8
0i9
liO
INDEX
F i g . 1 9 . M o r p h o m e t r i c d a t a f o r s e c t i o n s in Fig. 18 A - - C . N o t e t h e t r e n d s o f v a l u e s f o r A a n d B, a n d f o r C. MA = arithmetic mean of columns A1--A8. MB = arithmetic mean of columns B1--B10. MC = arithmetic mean of columns C1--C12. MAB = arithmetic mean of columns A1--A8 and B1--B10.
1
200
TABLE II Selected parameters for stromatolite groups Function measured
Quantity calculated
Lamina profiles F v = height F h = width S = length (= perimeter/2)
Fv/S = vertical laminosity index Fh/S = horizontal laminosity index F h / F v = profile flatness
Cross sections (plan views) A = area of structure A, = area of matrix Pe = perimeter of A J = scale conversion factor N = number of individuals
Shape
Shape A/Pe 2
= circularity
Size = equivalent circle diameter = mean width
2Jx/A/~ 4JA/P e
Abundance* IOOA/(A+A,)
Longitudinal sections (silhouettes) F v = height F h = width I v = vertical intercept I h = horizontal intercept A = area of structure A~ = area of matrix Pe = perimeter of A J = scale conversion factor N = number of individuals
= % stromatolite structure in bed
Shape and orientation = circularity = elongation index = vertical protuberancy index = horizontal protuberancy index = intercept ratio
A/Pe ~ Fv/F h Iv/Fv Ih/F h Iv/Ih
Size JA/Iv
= mean width of erect column
Abundance* IOOA/(A+A,)
= % stromatolite structure
in bed *Determined integrally for arrays of structures in bed. s u c h q u a n t i t i e s as t h e r a t i o o f n u m b e r o f b r a n c h e s t o t h e n u m b e r o f i n d i v i dual structures, and the shape of cumulative frequency curves of horizontal c h o r d s ( H o f m a n n , 1 9 7 6 a ) . T h e p a r a m e t e r s c o m p i l e d h e r e are t h e v e r t i c a l a n d h o r i z o n t a l p r o t u b e r a n c y i n d e x e s , a n d t h e e l o n g a t i o n i n d e x . T h e r a t i o I v / F v is a m e a s u r e o f t h e d e g r e e o f v e r t i c a l b r a n c h i n g o f a s t r u c t u r e , a n d I h ] F h gives the a b u n d a n c e of lateral p r o j e c t i o n s and niches. In practice, it seems preferable to plot the reciprocals of these two protuberancy indexes on linear scales, b e c a u s e t h e y a l l o w f o r b e t t e r s e p a r a t i o n o f p o i n t s f o r s t r u c t u r e s w i t h f e e b l y d e v e l o p e d b r a n c h e s o r p r o j e c t i o n s , a n d also b e c a u s e t h e c o o r d i n a t e s a r e f i n i t e ( t e r m i n a t i n g a t a v a l u e o f 1). Fig. 1 9 i l l u s t r a t e s p l o t s o f t h e s e f u n c t i o n s f o r G y m n o s o l e n f. T h e y p r o v i d e a striking, q u a n t i t a t i v e e x a m p l e of the great variability in shape b e t w e e n ind i v i d u a l s t r u c t u r e s w i t h i n a v e r y s m a l l v o l u m e o f t h e s a m e b e d . T h i s is s o m e -
201 thing which can be perceived qualitatively by inspection of the graphic reconstructions, and just as well by the silhouettes {Figs. 5 and 18) and photographs. Insofar as size is concerned, the ratio J A / I gives a very good estimate of the average width of erect and suberect columns, particularly when applied integrally to longitudinal sections (JZNA/ZiVIv)"The value thus obtained is probably generally more representative-of the bed as a whole than either of the values obtained from a horizontal section, which represent only a particular level within the bioherm. Micros tructure The microstructures of the Aphebian stromatolites have not been analysed quantitatively,although estimates can be obtained with parameters such as those suggested by Hofrnann (1976a). The same qualifying statements relating to errors in integralmeasurements, made earlieron in the lastparagraph under the heading "cross sections (plan views)", also apply to integral measurements on rnicrostructure. To complete the illustrationof the Aphebian stromatolites and to permit visual comparison with microstructures of Riphean forms by the traditional qualitative and subjective methods, the Aphebian microstructures are also shown in Figs. 2, 6--9, and 11--13. As with other attributes,considerable variabilityis encountered here too. The microstructures await further study and re-evaluation of the taxonomy at the form level.(The publication of a proposed atlas of microstructures (Monty, 1976), would be an enormously useful endeavour, particularlyif the photographs were to be reproduced at some standardized scale(s).This could serve as a startfor a concerted effort to quantify the microstructural attributes.) The microstructure is a feature of primary importance, for it relatesdirectly to the lithologicaspect at the mesoscopic scale that is the basis for mapping and correlatingin the field.Moreover, the biologicalinfluence or control of primary microstructure, (and hence also its possible evolutionary and chronostmtigraphic importance), appears to be less difficultto demonstrate than the influence on, or control over, gross morphology. An extreme interpretation and application of this is the classificationof "algae" of Vologdin (1962), in which diagenetic effectsare not sufficientlyappreciated. MICROPHYTOLITES Microphytolites (catagraphs) have also been used chronostratigraphically in the Riphean-Vendian. As for stromatolites,a biologic origin and evolutionary tendencies have been assumed. It is advisable,however, to consider most of these entitiesat least as dubiofossils;some are clearly pseudofossils (e.g.,intraclasts,ooids and recrystallizedooids). Their chronologic utility thus is doubtful; on the other hand, they appear to be most useful for de-
0
203
termining past environmental settings. The reported microphytolite "evolution" appears to reflect a succession of different lithofacies. Examples of Aphebian occurrences of Vesicularites, Radiosus, and Asterosphaeroides (Nelcanella), microphytolite groups generally reported from the Riphean-Vendian, are illustrated in Figs. 13 and 20. CONCLUSIONS
At least some stromatolite and microphytolite groups used in making age assignments in studies of Riphean-Vendian sequences occur also in Aphebian rocks. This, together with other empirical data on evolution and ecology of prokaryotes and stromatolite development, indicates that the validity of the "biostratigraphy" based on supposedly evolving stromatolite and microphytolite groups is in need of reexamination. Moreover, the current m e t h o d of defining stromatolite groups can be improved. Equipment now available allows for a variety of quantitative statements to be made about stromatolite morphology. The " m o r p h o m e t r i c " approach, concentrating on studies of individual attributes and their changes through time, rather than studies using "taxa" incorporating divergent concepts, should simplify the determination of long term trends (if any) in these biosedimentary structures. The parallel (nonhierarchical) nature of the group-form classification must be acknowledged. The task of developing a well-founded theory of stromatolites is still before us. A viable theory will integrate data from all the orientations of stromatolitology mentioned in the introduction, and perhaps others yet to be developed. Interpretations of data from one field must be consistent with data from other fields. The information should be intelligible to all practitioners; communication between those working on different objectives is essential. However, this is at present impeded by a multitude of divergent nomenclatures which reflect different fundamental concepts, and different professional backgrounds and degrees of specialization of those working with the structures. A standardized basic terminology for the various attributes and concepts {particularly in the "biostratigraphic" orientation) would seem to be a desirable ingredient for such a unified theory; relationships should be expressed simply and with the least a m o u n t of ambiguity. Fig.20. Aphebian catagraphs in thin sections under transmitted, nonpolarized light. A: Vesicularites f. in grey dolostone, Kasegalik Fro. Belcher Islands. Same thin section as Fig. 13 E. (56°29.5'N 79°30.3'W). G.S.C. type 48539. B: Radiosus f. and Asterosphaeroides (NelcaneUa) f. (small) in light grey dolostone from Nastapoka Group (Manitounuk Supergroup), Richmond Gulf area, SE coast o f Hudson Bay. Specimen collected by F.R. Joubin. G.S.C. t y p e 38369. C : Asterosphaeroides (Nelcanella) f and ooids (obliterated Radiosus). Same thin section as B. D: Enlargement of Asterosphaeroides (Nelcanella) bodies, showing finer crystallinity in dark portions. Rectilinear, palimpsest radiofibrate structure and blunt, angulate ends, suggest genesis by radial crystallization. Same thin section as B.
204 ACKNOWLEDGEMENTS
I gratefully acknowledge support received in the field from R.T. Bell and D. Krause in the Belcher Islands, and M. Rocheleau in the Lake Mistassini area. F. Souli~, who prepared most of the large thin sections used in this study, developed a more efficient method of preparing them. F.R. Joubin provided the sample from the Richmond Gulf area, illustrated in Fig. 20 B--D. The work was financially supported by the National Research Council of Canada under grants no. A 7 4 8 4 and E3952. REFERENCES
Bell, R.T. and Hofmann, H.J., 1974. Investigations of the Belcher Group (Aphebian), Belcher Islands, N.W.T. Geol. Assoc. Can. Mineral. Assoc. Can., Program Abstr., 8. Donaldson, J.A., 1963. Stromatolites in the Denault Formation, Marion Lake, Coast of Labrador, Newfoundland. Geol. Surv. Can. Bull., 1 0 2 : 3 3 pp. Donaldson, J.A., 1975. Proterozoic sedimentary rocks of Canada and the stromatolite controversy. Geol. Soc. Aust., First Australian Geol. Congr., Adelaide, May 1975. Abs. : 23. Donaldson, J.A., 1976. Aphebian stromatolites in Canada: implications for stromatolite zonation. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam. Chap. 7.3, pp. 371--380. G~rich, G., 1906. Les Spongiostromid~s du Vis~en de la Province de Namur. M~m. Mus. R. Hist. Nat. Belg., 3: 1--55. Hofmann, H.J., 1969. Attributes of stromatolites. Geol. Surv. Can., Pap. 6 9 - - 3 9 : 7 7 pp. Hofmann, H.J., 1973. Stromatolites: characteristics and utility. Earth-Sci. Rev., 9: 339-373. Hofmann, H.J., 1974. Stromatolites: stromatoid morphometrics. Geol. Soc. Amer., Abs. Progr. : 798. Hofmann, H.J., 1975. Australian stromatolites--essay review. Geol. Mag. 112: 97--100. Hofmann, H.J., 1976a. Stromatoid morphometrics. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam: Chap. 2.5, pp. 45--54. Hofmann, H.J., 1976b. Precambrian microflora, Belcher Islands, Canada: significance and systematics. J. Paleontol., 50: 1040--1073. Kalkowsky, E., 1908. Oolith und Stromatolith im norddeutschen Buntsandstein. Dtsch. Geol. Ges., 60: 68--125. Komar, Vl. A., Krylov, I.N., Raaben, M.E., Semikhatov, M.A., Serebryakov, S.N., and Zhuravleva, Z.A., 1975. Phytolites in the Precambrian stratigraphy. Sympos. Correlation of the Precambrian, Moscow, September 1975, Abstr.: 71--73. Krylov, I.N., 1975. Stromatolity rifeya i fanerozoya SSSR. Tr. Geol. Inst. Akad. Nauk SSSR, 274 : 243 pp. (in Russian, with English table of contents). Krylov, I.N., 1976. Approaches to the classification of stromatolites. In: M.R. Walter (Editor), Stromatolites. Developments in Sedimentology, 20. Elsevier, Amsterdam, pp. 31--43. Logan, B.W., Rezak, R. and Ginsburg, R.N., 1964, Classification and environmental significance of algal stromatolites. J. Geol., 72: 68--83. Maslov, V.P., 1960. Stromatolity. Tr. Geol. Inst. Akad. Nauk SSSR, 41 : 188 pp. (in Russian). Monty, C., 1976. Note, Stromatolite Newsletter, 4: 23. Monty, C., 1977. Evolving concepts on the nature and the ecological significance of stromatolites: a review. In: E. Fl~gel (Editor), Fossil Algae. Springer-Verlag, Berlin Heidelberg, pp. 15--35.
205
Preiss, W.V., 1976. Proterozoic stromatolites from the Nabberu and Officer Basins, Western Australia, and their biostratigraphic significance. Geol. Surv. South Aust., Rept. Inv. 47 : 51 pp. Preiss, W.V. and Walter, M.R., 1975. Stromatolites of the Australian Precambrian: use in intra- and intercontinental correlations. Sympos. Correlation of the Precambrian, Moscow, September 1975, Abstr.: 74--75. Reis, O.M., 1908. Kalkowsky: Uber Oolith und Stromatolith im norddeutschen Buntsandstein. Neues Jahrb. Min., Geol. Pal., 2: 114--134. Vologdin, A.G., 1962. Drevneishie vodorosli SSSR. Akad. Nauk SSSR, Moscow, 656 pp. (in Russian). Walter, M.R., 1972. Stromatolitesand the biostratigraphyof the Australian Precambrian and Cambrian. Palaeontol. Assoc. London, Spec. Pap. 11:190 pp.