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Chapter 10.8 Distribution of Stromatolites in Riphean Deposits of the Uchur-Maya Region of Siberia
10. STROMATOLITES IN BASIN ANALYSIS
Chapter 10.8
DISTRIBUTION OF STROMATOLITES IN RIPHEAN DEPOSITS OF THE UCHUR-MAYA REGION OF SIBERIA S.N. Serebryakov
INTRODUCTION
The use of Riphean stromatolites in basin analysis is rather limited due to inadequate knowledge of their paleoecology. The widely employed extrapolation of the ecological features of the best known (intertidal) Recent stromatolites to Precambrian stromatolites meets well-grounded objections from many scientists (e.g., Maslov, 1959, 1960; Korolyuk and Sidorov, 1965; Trompette, 1969; Walter, 1970a, 1972a; Serebryakov, 1971, 1975; Bertrand-Sarfati, 1972b, c; Hofmann, 1973; Serebryakov and Semikhatov, 1973, 1974; Monty, 197313, c; and others). The most popular method of studying the paleoecology of the Riphean stromatolites, by analyzing microfacies of individual bioherms (e.g. Bertrand-Sarfati, 1970, 1972b), is not faultless either. In particular, the effect of biotic (associated with the systematic composition of algae) factors on the morphogenesis of stromatolites can hardly be detected in local material. This makes, in turn, the determination of the relative role of the environment more difficult. Furthermore, the interpretation of microfacies itself is based mainly on evidence from Recent stromatolites. Some of these faults can be avoided by considering broadly the location of stromatolites in the Riphean deposits of an extensive region. This article is an attempt to determine a relative effect of biotic and environmental factors on the lateral and vertical distribution of stromatolites in the Riphean of the Uchur-Maya region. REGIONAL GEOLOGY
The Uchur-Maya region of southeastern Siberia covers an extensive area (650 x 750 km) in the basins of the right tributaries of the Aldan river and includes three large structural elements (Figs. 1,2): the eastern slope of the Aldan shield, the Uchur-Maya platform of the Siberian craton and the
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Fig. 1. Schematic lithologic-facies profiles of the pre-Yudomian deposits of the UchurMaya region. Legend: 1 = sandstones (predominantly quartz) and conglomerates, grey-coloured; 2 = the same, red-coloured; 3 = siltstones; 4 = argillites; 5 = sandyklayey red-coloured rocks; 6 = variegated argillites of the Neruen suite; 7 = limestones;8 = dolomitic limestone and limey dolomites; 9 = dolomites; 10 = variegated clayey limestones, mark and calcareous argillites of the Totta suite; 11 = rhythmic interstratification of sandstones, sandy oncolitic and stromatolitic dolomites; 12 = the same and chemogenic dolomites; 13 = rhythmic interstratification of sandstones, siltstones, argillites and dolomites; 1 4 = variegated and dark-grey pelitomorphic platy limestones of the Malgina suite; 15 = light-grey and grey, massive and platy dolomites; 16 = grey dolomites with reddish-brown crust of weathering; 17 = dark-grey bituminous limestones of the Neruen suite; 18 = grey-coloured clastic and microphytolitic limestones and dolomites; 19 = red-coloured chemogenic, clastic and stromatolitic limestones and dolomites of the Ignikan suite; 20 = grey and dark-grey chemogenic, clayey-bituminous, clastic and microphytolitic limestones and dolomites of the Ignikan suite; 21 = pre-Riphean formations; 22 = surfaces of regional unconformity; 23 = surfaces of regional weathering; 24 = suite boundary lines; 25 = surface of preYudomian erosion; 26 = K-Ar age of glauconite in m.y. (according to Kazakov and Knorre, 1970); 27-37 = forms of stromatolites (in brackets - morphologies of stromatolites found in the studied material analogous to the given form in microstructure but with a different morphology) :27 = Omachtenia omachtensis Nuzhnov (Stratifera,nodular, rare Kussiella); 28 = Nucleella figurata Komar, Gongylina differenciata Komar, Kussiella kussiensis Krylov (Stratifera, Omachtenia); 29 = Baicalia baicalica (Maslov) (Stratifera), Colonnella kylachii Shapovalova, Suetliella tottuica Komar and Semikhatov (nodular, Stratifera), S. suetlica Shapovalova (Stratifera), Conophyton garganicum Korolyuk; 30 = Malginella malgica Komar and Semikhatov (Stratifera);31 = Malginella zipandica Komar (Stratifera, Irregularia); 32 = Parmites aimicus (Nuzhnov) (Irregularia); 33 = Colonnella ulakia Komar (Baicalia, Tungussia);34 = Minjaria sakharica Komar (Baicalia, Jacutophyt o n ) , Conophyton metula Kirichenko (Jacutophyton), Conophyton lituum Maslov (Jacutophyton,Baicalia, Colonnella, nodular), Baicalia lacera Semikhatov (Jacutophyton, Conophyton), Baicalia ingilensis Nuzhnov (Jacutophyton);35 = Baicalia maica Nuzhnov (Jacutophyton); 36 = Inzeria tjomusi Krylov (Jacutophyton); 3 7 = Inzeria confmgosa (Semikhatov) (Stratifera, Omachtenia?); 38 = area of deep burial of Riphean deposits; 39 = boundary of largest positive structures (Al. Sh. = Aldan shield, Om. Up. = Omnia uplift); 40 = zone of Nelkan fault; 41 = depressions in the Uchur-Maya platform (Uch. D. = Uchur, M.D. = Maya); 42 = Yudoma-Maya trough (Yud. M.T.); 43 = geographic position of lithologic-facies profiles (a-b and c-d, Fig. 1 ; e-f-g-h-i, Fig. 2 ; j - k , Fig. 6); 44 = position of sections shown in Fig. 5.
h
i
h
I
N. SLOPE OF
1
Fig. 2. Schematic lithologic-facies profiles of lower and upper subsuites of the Yudoma suite (Terminal Riphean). For position of profile line see Fig. 1 ;e , f , g , h , i - points of inflection of profile line. Legend: 1 = quartz and feldspar-quartz sandstones and gritstone with subordinate limestones and oncolitic dolomites; 2 = quartz and feldspar-quartz sandstones with subordinate gritstones, siltstones and argillites; 3 = complex, occasionally rhythmic alternation of predominating microphytolitic dolomites with stromatolitic, granular and sandy dolomites and sandstones; 4 = variegated cherty and clayey dolomites, argillites and cherty argillites with subordinate phytogenic and sandy dolomites; 5 = argillites, clayey and indistinctly granular dolomites with rare lenses ofsandstones, stromatolitic and microphytolitic dolomites; 6 = indistinctly granular dolomites with intercalation of argillites, sandy and microphytolitic dolomites; 7 = bituminous and clayey dolomites with rare stromatolites, intercalations of argillites, siltstones and sandstones; 8 = bituminous limestones; 9 = distinctly granular dolomites with particular stromatolites and microphytolites; 10 = indistinctly granular limestones containing glauconite; I 1-1 8 = stromatolites with microstructures described from the following stromatolite forms: 11 = Boxonia grumulosa Komar; 12 = B. ingilica Komar and Semikhatov; 13 = Pankcollenia emergens Komar; 14 = Colleniella singularis Komar; 15 = Jurusania judomica Komar and Semikhatov; 16 = Gongylina nodulosa Komar and Semikhatov; I7 = J. sibirica (Jakovlev); 18 = Linella simica Krylov; 19 = undefined stromatolites.
cL
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Yudoma-Maya trough. The latter is distinguished from the typical platform area by greater and more variable thicknesses, and, in some cases, by the facies of the Riphean deposits. It has been treated lately as an aulacogen. A number of lesser structures can be distinguished within the Uchur-Maya platform; the largest among them are the Uchur and Maya depressions separated by the Omnia uplift. The composition of the deposits of the four subdivisions (phythems) of the Riphean is shown in Figs. 1 and 2. There are terrigenous, terrigenouscarbonate and carbonate units forming extensive sedimentary cycles (e.g., Nuzhnov and Yarmolyuk, 1959, 1963; Komar et al., 1970). Five cycles of this kind (which are regarded as groups) have been distinguished in the pre-Yudomian deposits (Fig. 1).At the bases of all the groups (except the Uii) there are unconformities; these are most distinct on the Uchur-Maya platform. The Yudoma suite (Fig. 2) corresponds to the Terminal Riphean (Yudomian) and overlies the older rocks with regional unconformity. It is intimately associated with the deposits of the Early Cambrian epoch, so that they form a single sedimentary cycle. In the Uchur-Maya region the completeness of the sections and the thickness of the Riphean deposits gradually increase from west to east; the terrigenous suites are subject to the most significant lateral changes (Fig. 1). The variations of the carbonate suites are marked by changes in the proportions of limestones and sedimentary-diagenetic dolomites, and the amount and character of allochthonous material. Unfortunately, the marginal (nearshore) facies of the deposits are preserved only in the Omakhta suite and in the lower subsuites of the Yudoma suite. The variations in the composition of the carbonate rocks can be partially accounted for by fluctuations in the salinity of the basinal waters (e.g. Akulshina et al., 1969). Supply of continental waters containing sodium and magnesium carbonate was evidently also of great importance; this facilitated accumulation of dolomite and limestone-dolomite facies (Serebryakov, 1968, Grigorev et al., 1969). Further from the coast (or as the transgressions developed), these facies were replaced by limestones, much like the phenomenon observed in the Neruen, Ignikan and Yudoma suites. In the carbonate suites of all the zones of the Uchur-Maya region, one can commonly observe traces of local submarine erosion, cross-bedded and wavy-laminated structures, and ripple marks; clastic carbonate rocks (mainly endoclastites) are also widespread in occurrence. Desiccation cracks, typical of terrigencus suites, are found occasionally. This suggests deposition under generally shallow water and relatively high hydrodynamic energy for all of the Riphean sediments of the Uchur-Maya region. This conclusion is supported by the practically ubiquitous occurrence of stromatolites and microphytolites. Stromatolites have been found in all the suites of the region, except the Ustkirba suite. The only stromatolites considered below are those of the carbonate and terrigenous-carbonate
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--
IDESICCATIO)(I
DEEPER WATER
/
E5ii m 2 1703
7n4 ... ... .... ..
Fig. 3. Sedimentary rhythms of the lower part of the Omakhta suite, Uchur River. Legend: 1 = stromatolites; 2 = oncolitic dolomites; 3 = sandy dolomites and dolomitic sandstones; 4 = finegrained silty sandstones; 5 = silty argillites; 6 = desiccation cracks; 7 = pseudomorphs after halite; 8 = ripple marks.
suites. (The stromatolites were studied jointly by the author, Komar and Semikhatov. Komar and Semikhatov made all the taxonomic determinations.) The study of the Uchur-Maya stromatolites indicated that they formed subtidally. This conclusion is based on the uniformity of the stromatolitebearing members of the suites over vast areas, persistence of microstructures within the same bioherm irrespective of its growth relief, and absence of any evidence that the bioherms were subjected to intense erosion. Neither is there any evidence of subaerial exposure of the stromatolites; in particular,
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no desiccation cracks have been found. According to Gebelein (1969), these are the only reliable indication of intertidal or supratidal formation of stro matolites . Among the stromatolite-containing suites of the Uchur-Maya region, the evidence of shallowness is most pronounced in the Omakhta suite of the Uchur depression. There the suite is represented by rhythmically alternating terrigenous and carbonate rocks (Fig. 1). In the direction to the Aldan shield, the rhythms become gradually thinner, the chemogenic dolomites disappear, the rocks get more and more red-coloured, and numerous desiccation cracks and halite pseudomorphs are observed in the clastic members of the rhythms. Rhythmicity in the Omakhta suite in the western, near-shore part of the Uchur depression is manifested by the repeated occurrence of the following rock types (Fig. 3): fine-grained silty sandstones with quartz and, then, dolomitic cement, then sandy dolomites, then oncolitic and stromatolitic dolomites with variable amounts of sand. Some of these rocks recur in the reverse order forming closed asymmetric rhythms (Serebryakov, 1971). In the sandstone beds which begin and conclude each rhythm, thin clay layers are common. Desiccation cracks, ripple marks and salt crystal moulds are confined to these layers, providing evidence of short periods of exposure. At the same levels there are some erosional surfaces. The carbonate members of the rhythms show no evidence of subaerial exposure. It is apparent that the rhythms are of a transgressive-regressive character, stromatolites being confined to the periods of greatest submergence. It is significant that in the eastern zone of the Uchur depression, where there is less evidence of shallow deposition, the transgressive part of the rhythms is terminated by chemogenic dolomites overlying stromatolites. Thus, there is every reason to suppose that even the Omakhta stromatolites, which formed in the shallowest water, formed below the ebb tide level. Thus, the observed distribution of stromatolites in the Uchur-Maya region should reflect their original distribution in the various subtidal environments. These environments differed at least in their position relative to the shore and in some cases in the conditions of sedimentation. Therefore, the first question to be answered is whether these differences affected the lateral distribution of the stromatolites. The second question is whether there exists any correlation between the vertical changes in the stromatolites and the variations in the composition and character of the enclosing rocks. These questions are considered below independently for the various morphological groups of stromatolites and for the stromatolites of a definite microstructure irrespective of their morphology. The stratigraphic basis for analysing the distribution of the stromatolites is provided by the suites (formations) and subsuites which are litho-stratigraphic subdivisions; we are obliged to regard their boundaries as isochronous.
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DISTRIBUTION OF ASSEMBLAGES OF MORPHOLOGICAL GROUPS OF STROMATOLITES
Study of stromatolites of the Uchur-Maya region (Nuzhnov, 1960,1967; Nuzhnov and Shapovalova, 1965, 1968; Semikhatov and Komar, 1965; Voronov et al., 1966; Komar et al., 1970, 1973;Krylov and Shapovalova, 1970b;Komar, 1973)indicated that their groups, defined on a morphological basis, form laterally persistent stable assemblages confined to particular stratigraphic intervals. The spatial distribution of the assemblages was found to have three general features: (1) Stromatolite assemblages occur over practically the whole area of outcrop of the carbonate and terrigenous-carbonate suites; they are not subject to any significant lateral changes. The variations in thickness and facies of the sediments are accompanied by changes in the relative abundance of some groups, the total abundance of the stromatolite and the thickness of their bioherms. The Neruen suite provides a good example. Its thickness (Fig. 1)in the Yudoma-Maya trough is twice that in Uchur-Maya platform. A t the same time there appear in the trough, among the carbonate rocks, beds of chemogenic, bituminous limestones and dolomites which result in a lesser abundance of the phytogenic and phytoclastic rocks. Extensive carbonate bodies appear in the trough in the lower and upper part of the suite represented on the platform mainly by argillites. In spite of these changes in the suite, Baicalia, Conophyton and Jacutophyton, normally forming “Jacutophyton cycles”, dominate everywhere (Shapovalova, 1965, 1968; Serebryakov et al., 1972).In the Yudoma-Maya trough, in comparison with the Maya depression, the individual stromatolite horizons get thicker and bioherms composed of any one group of stromatolites (predominantly Baicalia) are of greater size and more numerous. As a result, the relative abundance of the above groups is somewhat different. The horizons containing Baicalia account, in four sections of the Maya depression, for 58-60% of the total thickness of the stromatolite horizons and, in two sections of the Yudoma-Maya trough, for 67-7076. (2)The composition of the assemblages may remain the same, irrespective of vertical changes in the composition and character of the enclosing deposits. Most spectacular in this respect are stromatolites of the Neruen suite. Conophyton, Jacutophyton, Baicalia and Colonnella are enclosed in its lower part in clastic and chemogenic dolomite varying in their fabrics and colour, and in its upper part, in less variable limestones and dolomitic limestones. These stromatolite constructions frequently cross the boundary of the lithologically different carbonate rocks without changing their morphology. Moreover, all these groups (except Jacutophyton) have been found enclosed in argillites (Fig. 4). Absence of any direct interdependence between the morphology of
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8 not exposed 740m
I
Fig. 4. Stromatolite horizon of the upper subsuite of the Neruen suite, Lyaki River. Legend: 1 = limestones; 2 = argillites;3-5 = stromatolite morphology;3 = Jacutophyton; 4 = Buicaliu; 5 = Colonnella; 6-9 = stromatolite microstructures typical for the following forms: 6 = Baicalia lacera Semikhatov; 7 = B. ingilensis Nuzhnov; 8 = Conophyton lituum Maslov; 9 = C . cylindricum Maslov (for jacutophytons the denominator indicates the microstructure of central column, the numerator that of the branches); 10-14 = colour of rocks: 10 = red; 11 = mottled (red and green); 1 2 = variegated; 13 = grey of various shades; 14 = dark grey to black.
stromatolites and the character of the enclosing rocks in the Neruen suite has been already considered in the description of the “Jacutophyton cycles” (Serebryakov et al., 1972). Yet the superposition in bioherms of different stromatolite morphologies can evidently be attributed to ecological factors which may have operated over extensive areas (see Ch. 6.4,fig. 7). This contradiction can obviously be accounted for by the fact that cyclic variations in the morphology of stromatolites and local variations in the character of rocks were controlled by different causes. For instance, the rate of sedimentation and intensity of subsidence are poorly manifested in the character of carbonate sediments; the role of these factors in the morphogenesis of stromatolites seems to be significant (Bertrand-Sarfati, 1972c; Serebryakov et al., 1972). The possibility that the environment may affect the shape of stromatolites but not the character of the sediment has been suggested by Maslov (1959) and Preiss (1973a).
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(3) Different assemblages of stromatolites are found in lithologically similar beds of different ages; these assemblages may vary through a homogeneous section of carbonate rocks. Such a change in assemblages occurs in the uniform dolomites of the Tsipanda suite (see Fig. 1).In its lower part there are stratiform stromatolites (Mulginellu, Strutiferu, Irregulariu); in the middle part, stratiform and stratiform-columnar types (Irreguluria, Purrnites) are encountered, and in the upper part, these are replaced by columnar structures (Colonnellu, Buiculiu, Tungussiu, occasional Minjuriu, and forms resembling Tilernsinu). There is no lithological evidence that these changes in stromatolite assemblages are dependent on any changes in ecological conditions. Though such a dependence cannot be absolutely denied, the fact that the changes in morphological assemblages coincide with variations in the microstructures of the stromatolites indicates that these changes are probably due to changes in the communities of stromatolite-forming algae. Dolomites similar in composition, colour and fabric to the Tsipanda ones occur in the Yudoma-Maya trough a t the base of the overlying Neruen suite. They are separated from the rocks of the Tsipanda suite by a regional weathering surface and a thin (5-20m) bed of argillites. Minjuriu and Tilemsinu (?) have not been found in these dolomites but Jucutophyton and Conophyton are abundant. This change in the morphological assemblage is accompanied by a partial change in the microstructures; but some of the stromatolites of the boundary horizons of the Tsipanda and Neruen suites have similar microstructures. The Neruen stromatolites assemblage differs from the Tsipanda one not only in composition but also in the appearance of the cyclic structure of the bioherms (Jucutophyton cycles). Consequently, the change in the associations at the boundary of the suites under consideration was caused not only by the change in algal communities but also on the changes in the environment. It is significant that these changes are not manifested in the macroscopic and petrographic appearance of the rocks. This is not an exceptional example of the repetition of uniform carbonate beds a t various stratigraphic levels. The dolomites of the Svetly and the Tsipanda suites are similar in their composition and position in the sedimentary cycles. Dolomites of the Svetly type have been encountered in the Neruen and Ignikan suites. Yet the stromatolite assemblages in these suites are different. The lithologically similar deposits are unlikely to have formed, in all cases, under different conditions. It is much more probable that we have to deal here with the age variability of stromatolite morphology; this view is reinforced by the fact that there are corresponding variations in their microstructures (see below). The specific features of the distribution of the stromatolite assemblages allow two conclusions. Firstly, the vertical change in assemblages often has no direct relationship to changes in the enclosing deposits. Though the effect of environment on the stromatolites in the sections can be established in
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some cases, the general sequence of assemblages can hardly be explained without admitting that it was dependent on the evolution of the stromatoliteforming algae. Secondly, there is very little relationship between the distribution of assemblages, the conditions of sedimentation and the distance from the coast. This does not exclude the effect of environmental factors on stromatolite morphology within the bioherms or even in certain areas of basins (see Ch. 6.4). The effect of these factors is not manifested in the characteristics of the rock, which makes their interpretation more difficult. Those ecological factors which affected the morphology of the stromatolites hardly influenced their microstructure. In fact, the intrabioherm variability of stromatolites is normally unaccompanied by microstructural changes (Komar and Semikhatov, 1965, 1968b; Komar, 1966; Krylov, 1967a, 1972; Serebryakov, 1971; Walter, 1972a). The abundant evidence of the Recent and ancient stromatolites (see Ch. 7.1) suggests that particular microstructures owe their formation to particular communities or species of algae. If this is correct, the distribution of stromatolites with particular microstructures must to some degree reflect the original distribution of the stromatolite-forming communities. DISTRIBUTION OF STROMATOLITES WITH PARTICULAR MICROSTRUCTURES
The specific features of microstructure usually provide the basis for distinguishing forms (“species”) of stromatolites; the concept of “group” is sometimes identified with the concept of “stromatolites of a certain microstructure”. Krylov (1967a, 1972) has repeatedly discredited such identifications, stating that whether a form does or does not belong to some group, according to the current classification, must be determined by the morphology of the structures. Meanwhile, the same microstructure frequently occurs in stromatolites with diverse morphologies characteristic of different groups. Because the classification of microstructures remains an unsolved problem, I have referred in the following analysis to the microstructures described at the time of the establishment of each particular form. The distribution of stromatolites with particular microstructures in Riphean deposits of the Uchur-Maya region has recently been considered by Komar et al. (1973). Four types of distribution have been recognized: (1) The same microstructure is characteristic of all the stromatolites of a suite or some portion of a suite over the whole area of its distribution. For example, a considerable part of the Malgina suite is composed of Malginella malgica Komar et Semikhatov and Stratifera of a similar microstructure. In the lower part of the overlying Tsipanda suite, Malginella, Stratifera and Irregularia have a common but different microstructure (that described from Malginella zipandica Komar). The boundary between the two microstructures coincides (not always exactly) with the sharp change in the composition of
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the rocks at the boundary of the suites. The next level of microstructural change is within the uniform Tsipanda dolomites: in their middle part, there occurs Pamites aimicus (Nuzhnov) and Irreguluriu with a similar microstructure. Y A 2
. CHELASIN 3
PLLYW-VU
4
L +@
1)
Fig. 5. Vertical range of stromatolites with particular microstructures, in some sections of the Neruen and Ignikan suites (ranges in proportion to thicknesses). For position of sections see Fig. 1. Letters given in circles are stromatolites with microstructures described from the following forms: c = Conophyton cylindricum; It = C. lituum; mt = C. metula; s = Minjaria sakharica; u = Colonnella ulakia; i = Baicalia ingilensis; 1 = B. lacem; m = B. maica; t = Inzeria tjomusi; cn = I . confragosa.
The distribution of microstructures in the Ignikan suite is particularly interesting (Fig. 1).This suite is composed of clastic, microphytolitic, and chemogenic carbonate rocks of diverse composition in which dolomites typical for the Uchur-Maya platform are gradually replaced by limestones in the Yudoma-Maya trough. Yet the vertical sequence of stromatolites persists over the whole area of occurrence of the suite (Fig. 5). This allows three regional biostratigraphic subdivisions to be distinguished in the suite. The facies variations in the deposits consist only of some irregularities in the distribution of stromatolite bioherms and thickness variations in the beds containing stromatolites of a particular microstructure. (2) Mass concentrations of stromatolites of some definite microstructure are confined to definite associations of deposits. The Yudoma suite (Fig. 2) can be used to illustrate this. The distribution of its stromatolites exhibits two opposite tendencies (Semikhatov et al., 1970). On the one hand,
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Fig. 6 . Schematic lithologic-and-facies profile of the lower Yudoma subsuite. For position of profile line see Fig. 1. Legend: I = sandstones; 2 = argillites; 3 = siltstones; 4 = dolomites; 5 = limestones; 6 = darkcoloured bituminous and clayey-bituminous limestones; 7 = sandy dolomites; 8 = variegated cherty-dolomite and clayey-herty-dolomite rocks; 9 = catagraphs; 10-1 1 = stromatolites with microstructures typical for the following forms: 10 = Boxonia grumulosa; I 1 = Jurusania judomica; 12 = K-Ar age of glauconites in m.y. (according to Kazakov and Knorre, 1970).
stromatolites with the same microstructure are present in rocks of diverse composition and genesis, for example in cross-bedded quartz sandstones, horizontally laminated clayey dolomites and massive pelitomorphic limestones. On the other hand, mass concentrations of stromatolites with a definite microstructure are confined to definite associations of rock types, which causes the stromatolites to be irregularly distributed. Thus, most
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common among sandstones and microphytolitic dolomites are stromatolites which have the microstructures described for Boxonia grumulosa Komar, B. ingilica Komar and Semikhatov, Paniscolleniu emergens Komar and Colleniella singularis Komar; stromatolites with the microstructures described for Jurusaniu judomica Komar and Semikhatov occur in cherty and clayey dolomites which probably formed in the deeper sea; those with the characteristic Gongylina nodulosa Komar and Semikhatov microstructure are found in various types of pure carbonates. The boundaries of the rock associations with their contained stromatolite assemblages may be diachronous (Fig. 6). Some of the rock associations occur repeatedly in the section due to the cyclic structure of the Yudoma suite. However, among its stromatolites there are forms which are confined to only one of the subsuites. Thus, Boxonia grumu Zosa and some other morphologically different stromatolites of a similar microstructure do not pass into the deposits of the upper subsuites despite the passage upwards of its characteristic rock association. (3) Stromatolites of various microstructures are distributed in a disorderly fashion within a suite. Though some sequence in the change or alternation of microstructure can be observed in some sections, it is not maintained throughout the depositional area. The Svetly and Neruen suites can provide an example of this. Among the stromatolites of the Neruen suite, seven varieties of microstructures were distinguished (Fig. 5 ) . Five of them are encountered practically in the whole time-range of the suite, overlapping and replacing one another in vertical and lateral directions without any visible regularity. Moreover, in some cases one stromatolite structure may have different microstructures in its different parts (e.g. some Jacutophyton, see Serebryakov et al., 1972). (4) Stromatolites occurring in different sections of the suites have different microstructures. This type of distribution has been found in the Omakhta suite in two isolated sections: in the Uchur depression and in the northern part of the Yudoma-Maya trough (see Fig. 1). These considerations do not contradict the previously made suggestion that the spatial distribution of stromatolites of a certain microstructure reflects the distribution of communities of the stromatolite-forming algae. If this is so, it is unlikely that there was any direct dependence of the change of communities in time on environmental variations. This conclusion is supported also by vertical distribution of assemblages of morphological groups of stromatolites (see above). The condition in the Riphean basins of the Uchur-Maya region allowed, in some cases, the development of one or several coenoses over the whole of its ar6a; in other cases they were predominantly confined to limited areas. In some comparatively rare cases, changes in the algal communities might be caused by the same environmental factors which affected the morphology of the stromatolites. This view is supported by the occasional coincidence in
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a change of stromatolite morphology and change in their microstructure (some of the “Jucutophyton cycles”). In general, the following hypothesis may be proposed t o explain the distribution of stromatolites in the Neruen suite (type 3): in the Neruen basin there existed at the same time five similar and closely related communities (species?) of algae which gave rise to stromatolites with five different microstructures. Each of the communities could develop within the entire range of the conditions present. For this reason the spatial distribution of the communities was irregular and their appearance in the given point of the basin was fortuitous. The difference in the microstructures of the Omakhta stromatolites (type 4 distribution), can be, in all probability, accounted for by the existence of two isolated basins populated by different communities (species?) of algae. The same type of distribution of stromatolites with definite microstructures is observed in suites differing in composition (for instance, in the Svetly and Neruen) while different types are encountered in the lithologically similar suites: in the Svetly and Tsipanda, Neruen and Ignikan. This suggests that the realization of one type or another was determined not by the conditions of sedimentation but by the particular character of the algal communities. CONCLUSION
(1) In the Uchur-Maya region, there were, during the Riphean, extensive shallow-water basins where the conditions of sedimentation were rather stable in space and time. As already pointed out in the literature (e.g., Keller et al., 1968; Trompette, 1969; Bertrand-Sarfati, 1972c) such basins were, evidently, characteristic of the Riphean. (2) Stromatolites of the Uchur-Maya region formed subtidally as did, probably, the majority of other Riphean stromatolites (see Monty, 1973b, c; Serebryakov and Semikhatov, 1973,1974). They formed practically over the entire area of the basin. Therefore, u priori use of Riphean stromatolites as indicators of an intertidal or a nearshore environment is unfounded. (3) Variations in the distribution of different communities (species?) of algae seem to have been the result of their different residence against environmental changes. The range of conditions allowing formation of stromatolites by one kind of alga or another was rather wide, which reinforces the interpretation of the subtidal formation of the stromatolites. (4) The dependence of the distribution of the Uchur-Maya subtidal stromatolites on the conditions of sedimentation is poorly manifested. A moderate supply of terrigenous material did not limit the development of the stromatolites. It was not infrequent that short periods of terrigenous sedimentation failed to change the stromatolite assemblages (for example, in the Svetly and Neruen suites). Such changes, however, did occur within
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633
homogeneous carbonates. Stromatolites were, normally, unaffected by changes in the carbonate composition of the rocks. Stromatolites of similar morphology and microstructure occur in limestone, sedimentary-diagenetic dolomites and mixed limestone-dolomite rocks. Thus, there is no reason to regard the Riphean stromatolites as indicators of definite salinity conditions in the basin. (5) The morphology of the stromatolites, and sometimes also the distribution of the algal communities was greatly affected by environmental factors which are poorly, or not at all, manifested in the lithological characteristics of the rocks. The effect of these factors is more apparent in the analysis of the intrabioherm variability of stromatolites (see Ch. 6.4). (6) Vertical variations in the algal communities and, as a result, in the morphological and microstructural assemblages of stromatolites, seem to have been associated mainly with the evolution of the stromatolite-forming algae. It is not mere chance that a similar sequence of stromatolites has been observed in other Riphean sections within the U.S.S.R. (see Ch. 7.1). (7) The conditions under which the Riphean stromatolites formed were, probably, essentially different from the conditions of growth of Recent algal structures.
Chapter 10.8
DISTRIBUTION OF STROMATOLITES IN RIPHEAN DEPOSITS OF THE UCHUR-MAYA REGION OF SIBERIA S.N. Serebryakov
INTRODUCTION
The use of Riphean stromatolites in basin analysis is rather limited due to inadequate knowledge of their paleoecology. The widely employed extrapolation of the ecological features of the best known (intertidal) Recent stromatolites to Precambrian stromatolites meets well-grounded objections from many scientists (e.g., Maslov, 1959, 1960; Korolyuk and Sidorov, 1965; Trompette, 1969; Walter, 1970a, 1972a; Serebryakov, 1971, 1975; Bertrand-Sarfati, 1972b, c; Hofmann, 1973; Serebryakov and Semikhatov, 1973, 1974; Monty, 197313, c; and others). The most popular method of studying the paleoecology of the Riphean stromatolites, by analyzing microfacies of individual bioherms (e.g. Bertrand-Sarfati, 1970, 1972b), is not faultless either. In particular, the effect of biotic (associated with the systematic composition of algae) factors on the morphogenesis of stromatolites can hardly be detected in local material. This makes, in turn, the determination of the relative role of the environment more difficult. Furthermore, the interpretation of microfacies itself is based mainly on evidence from Recent stromatolites. Some of these faults can be avoided by considering broadly the location of stromatolites in the Riphean deposits of an extensive region. This article is an attempt to determine a relative effect of biotic and environmental factors on the lateral and vertical distribution of stromatolites in the Riphean of the Uchur-Maya region. REGIONAL GEOLOGY
The Uchur-Maya region of southeastern Siberia covers an extensive area (650 x 750 km) in the basins of the right tributaries of the Aldan river and includes three large structural elements (Figs. 1,2): the eastern slope of the Aldan shield, the Uchur-Maya platform of the Siberian craton and the
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Fig. 1. Schematic lithologic-facies profiles of the pre-Yudomian deposits of the UchurMaya region. Legend: 1 = sandstones (predominantly quartz) and conglomerates, grey-coloured; 2 = the same, red-coloured; 3 = siltstones; 4 = argillites; 5 = sandyklayey red-coloured rocks; 6 = variegated argillites of the Neruen suite; 7 = limestones;8 = dolomitic limestone and limey dolomites; 9 = dolomites; 10 = variegated clayey limestones, mark and calcareous argillites of the Totta suite; 11 = rhythmic interstratification of sandstones, sandy oncolitic and stromatolitic dolomites; 12 = the same and chemogenic dolomites; 13 = rhythmic interstratification of sandstones, siltstones, argillites and dolomites; 1 4 = variegated and dark-grey pelitomorphic platy limestones of the Malgina suite; 15 = light-grey and grey, massive and platy dolomites; 16 = grey dolomites with reddish-brown crust of weathering; 17 = dark-grey bituminous limestones of the Neruen suite; 18 = grey-coloured clastic and microphytolitic limestones and dolomites; 19 = red-coloured chemogenic, clastic and stromatolitic limestones and dolomites of the Ignikan suite; 20 = grey and dark-grey chemogenic, clayey-bituminous, clastic and microphytolitic limestones and dolomites of the Ignikan suite; 21 = pre-Riphean formations; 22 = surfaces of regional unconformity; 23 = surfaces of regional weathering; 24 = suite boundary lines; 25 = surface of preYudomian erosion; 26 = K-Ar age of glauconite in m.y. (according to Kazakov and Knorre, 1970); 27-37 = forms of stromatolites (in brackets - morphologies of stromatolites found in the studied material analogous to the given form in microstructure but with a different morphology) :27 = Omachtenia omachtensis Nuzhnov (Stratifera,nodular, rare Kussiella); 28 = Nucleella figurata Komar, Gongylina differenciata Komar, Kussiella kussiensis Krylov (Stratifera, Omachtenia); 29 = Baicalia baicalica (Maslov) (Stratifera), Colonnella kylachii Shapovalova, Suetliella tottuica Komar and Semikhatov (nodular, Stratifera), S. suetlica Shapovalova (Stratifera), Conophyton garganicum Korolyuk; 30 = Malginella malgica Komar and Semikhatov (Stratifera);31 = Malginella zipandica Komar (Stratifera, Irregularia); 32 = Parmites aimicus (Nuzhnov) (Irregularia); 33 = Colonnella ulakia Komar (Baicalia, Tungussia);34 = Minjaria sakharica Komar (Baicalia, Jacutophyt o n ) , Conophyton metula Kirichenko (Jacutophyton), Conophyton lituum Maslov (Jacutophyton,Baicalia, Colonnella, nodular), Baicalia lacera Semikhatov (Jacutophyton, Conophyton), Baicalia ingilensis Nuzhnov (Jacutophyton);35 = Baicalia maica Nuzhnov (Jacutophyton); 36 = Inzeria tjomusi Krylov (Jacutophyton); 3 7 = Inzeria confmgosa (Semikhatov) (Stratifera, Omachtenia?); 38 = area of deep burial of Riphean deposits; 39 = boundary of largest positive structures (Al. Sh. = Aldan shield, Om. Up. = Omnia uplift); 40 = zone of Nelkan fault; 41 = depressions in the Uchur-Maya platform (Uch. D. = Uchur, M.D. = Maya); 42 = Yudoma-Maya trough (Yud. M.T.); 43 = geographic position of lithologic-facies profiles (a-b and c-d, Fig. 1 ; e-f-g-h-i, Fig. 2 ; j - k , Fig. 6); 44 = position of sections shown in Fig. 5.
h
i
h
I
N. SLOPE OF
1
Fig. 2. Schematic lithologic-facies profiles of lower and upper subsuites of the Yudoma suite (Terminal Riphean). For position of profile line see Fig. 1 ;e , f , g , h , i - points of inflection of profile line. Legend: 1 = quartz and feldspar-quartz sandstones and gritstone with subordinate limestones and oncolitic dolomites; 2 = quartz and feldspar-quartz sandstones with subordinate gritstones, siltstones and argillites; 3 = complex, occasionally rhythmic alternation of predominating microphytolitic dolomites with stromatolitic, granular and sandy dolomites and sandstones; 4 = variegated cherty and clayey dolomites, argillites and cherty argillites with subordinate phytogenic and sandy dolomites; 5 = argillites, clayey and indistinctly granular dolomites with rare lenses ofsandstones, stromatolitic and microphytolitic dolomites; 6 = indistinctly granular dolomites with intercalation of argillites, sandy and microphytolitic dolomites; 7 = bituminous and clayey dolomites with rare stromatolites, intercalations of argillites, siltstones and sandstones; 8 = bituminous limestones; 9 = distinctly granular dolomites with particular stromatolites and microphytolites; 10 = indistinctly granular limestones containing glauconite; I 1-1 8 = stromatolites with microstructures described from the following stromatolite forms: 11 = Boxonia grumulosa Komar; 12 = B. ingilica Komar and Semikhatov; 13 = Pankcollenia emergens Komar; 14 = Colleniella singularis Komar; 15 = Jurusania judomica Komar and Semikhatov; 16 = Gongylina nodulosa Komar and Semikhatov; I7 = J. sibirica (Jakovlev); 18 = Linella simica Krylov; 19 = undefined stromatolites.
cL
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Yudoma-Maya trough. The latter is distinguished from the typical platform area by greater and more variable thicknesses, and, in some cases, by the facies of the Riphean deposits. It has been treated lately as an aulacogen. A number of lesser structures can be distinguished within the Uchur-Maya platform; the largest among them are the Uchur and Maya depressions separated by the Omnia uplift. The composition of the deposits of the four subdivisions (phythems) of the Riphean is shown in Figs. 1 and 2. There are terrigenous, terrigenouscarbonate and carbonate units forming extensive sedimentary cycles (e.g., Nuzhnov and Yarmolyuk, 1959, 1963; Komar et al., 1970). Five cycles of this kind (which are regarded as groups) have been distinguished in the pre-Yudomian deposits (Fig. 1).At the bases of all the groups (except the Uii) there are unconformities; these are most distinct on the Uchur-Maya platform. The Yudoma suite (Fig. 2) corresponds to the Terminal Riphean (Yudomian) and overlies the older rocks with regional unconformity. It is intimately associated with the deposits of the Early Cambrian epoch, so that they form a single sedimentary cycle. In the Uchur-Maya region the completeness of the sections and the thickness of the Riphean deposits gradually increase from west to east; the terrigenous suites are subject to the most significant lateral changes (Fig. 1). The variations of the carbonate suites are marked by changes in the proportions of limestones and sedimentary-diagenetic dolomites, and the amount and character of allochthonous material. Unfortunately, the marginal (nearshore) facies of the deposits are preserved only in the Omakhta suite and in the lower subsuites of the Yudoma suite. The variations in the composition of the carbonate rocks can be partially accounted for by fluctuations in the salinity of the basinal waters (e.g. Akulshina et al., 1969). Supply of continental waters containing sodium and magnesium carbonate was evidently also of great importance; this facilitated accumulation of dolomite and limestone-dolomite facies (Serebryakov, 1968, Grigorev et al., 1969). Further from the coast (or as the transgressions developed), these facies were replaced by limestones, much like the phenomenon observed in the Neruen, Ignikan and Yudoma suites. In the carbonate suites of all the zones of the Uchur-Maya region, one can commonly observe traces of local submarine erosion, cross-bedded and wavy-laminated structures, and ripple marks; clastic carbonate rocks (mainly endoclastites) are also widespread in occurrence. Desiccation cracks, typical of terrigencus suites, are found occasionally. This suggests deposition under generally shallow water and relatively high hydrodynamic energy for all of the Riphean sediments of the Uchur-Maya region. This conclusion is supported by the practically ubiquitous occurrence of stromatolites and microphytolites. Stromatolites have been found in all the suites of the region, except the Ustkirba suite. The only stromatolites considered below are those of the carbonate and terrigenous-carbonate
623
STROMATOLITESIN BASIN ANALYSIS
--
IDESICCATIO)(I
DEEPER WATER
/
E5ii m 2 1703
7n4 ... ... .... ..
Fig. 3. Sedimentary rhythms of the lower part of the Omakhta suite, Uchur River. Legend: 1 = stromatolites; 2 = oncolitic dolomites; 3 = sandy dolomites and dolomitic sandstones; 4 = finegrained silty sandstones; 5 = silty argillites; 6 = desiccation cracks; 7 = pseudomorphs after halite; 8 = ripple marks.
suites. (The stromatolites were studied jointly by the author, Komar and Semikhatov. Komar and Semikhatov made all the taxonomic determinations.) The study of the Uchur-Maya stromatolites indicated that they formed subtidally. This conclusion is based on the uniformity of the stromatolitebearing members of the suites over vast areas, persistence of microstructures within the same bioherm irrespective of its growth relief, and absence of any evidence that the bioherms were subjected to intense erosion. Neither is there any evidence of subaerial exposure of the stromatolites; in particular,
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S.N.SEREBRYAKOV
no desiccation cracks have been found. According to Gebelein (1969), these are the only reliable indication of intertidal or supratidal formation of stro matolites . Among the stromatolite-containing suites of the Uchur-Maya region, the evidence of shallowness is most pronounced in the Omakhta suite of the Uchur depression. There the suite is represented by rhythmically alternating terrigenous and carbonate rocks (Fig. 1). In the direction to the Aldan shield, the rhythms become gradually thinner, the chemogenic dolomites disappear, the rocks get more and more red-coloured, and numerous desiccation cracks and halite pseudomorphs are observed in the clastic members of the rhythms. Rhythmicity in the Omakhta suite in the western, near-shore part of the Uchur depression is manifested by the repeated occurrence of the following rock types (Fig. 3): fine-grained silty sandstones with quartz and, then, dolomitic cement, then sandy dolomites, then oncolitic and stromatolitic dolomites with variable amounts of sand. Some of these rocks recur in the reverse order forming closed asymmetric rhythms (Serebryakov, 1971). In the sandstone beds which begin and conclude each rhythm, thin clay layers are common. Desiccation cracks, ripple marks and salt crystal moulds are confined to these layers, providing evidence of short periods of exposure. At the same levels there are some erosional surfaces. The carbonate members of the rhythms show no evidence of subaerial exposure. It is apparent that the rhythms are of a transgressive-regressive character, stromatolites being confined to the periods of greatest submergence. It is significant that in the eastern zone of the Uchur depression, where there is less evidence of shallow deposition, the transgressive part of the rhythms is terminated by chemogenic dolomites overlying stromatolites. Thus, there is every reason to suppose that even the Omakhta stromatolites, which formed in the shallowest water, formed below the ebb tide level. Thus, the observed distribution of stromatolites in the Uchur-Maya region should reflect their original distribution in the various subtidal environments. These environments differed at least in their position relative to the shore and in some cases in the conditions of sedimentation. Therefore, the first question to be answered is whether these differences affected the lateral distribution of the stromatolites. The second question is whether there exists any correlation between the vertical changes in the stromatolites and the variations in the composition and character of the enclosing rocks. These questions are considered below independently for the various morphological groups of stromatolites and for the stromatolites of a definite microstructure irrespective of their morphology. The stratigraphic basis for analysing the distribution of the stromatolites is provided by the suites (formations) and subsuites which are litho-stratigraphic subdivisions; we are obliged to regard their boundaries as isochronous.
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DISTRIBUTION OF ASSEMBLAGES OF MORPHOLOGICAL GROUPS OF STROMATOLITES
Study of stromatolites of the Uchur-Maya region (Nuzhnov, 1960,1967; Nuzhnov and Shapovalova, 1965, 1968; Semikhatov and Komar, 1965; Voronov et al., 1966; Komar et al., 1970, 1973;Krylov and Shapovalova, 1970b;Komar, 1973)indicated that their groups, defined on a morphological basis, form laterally persistent stable assemblages confined to particular stratigraphic intervals. The spatial distribution of the assemblages was found to have three general features: (1) Stromatolite assemblages occur over practically the whole area of outcrop of the carbonate and terrigenous-carbonate suites; they are not subject to any significant lateral changes. The variations in thickness and facies of the sediments are accompanied by changes in the relative abundance of some groups, the total abundance of the stromatolite and the thickness of their bioherms. The Neruen suite provides a good example. Its thickness (Fig. 1)in the Yudoma-Maya trough is twice that in Uchur-Maya platform. A t the same time there appear in the trough, among the carbonate rocks, beds of chemogenic, bituminous limestones and dolomites which result in a lesser abundance of the phytogenic and phytoclastic rocks. Extensive carbonate bodies appear in the trough in the lower and upper part of the suite represented on the platform mainly by argillites. In spite of these changes in the suite, Baicalia, Conophyton and Jacutophyton, normally forming “Jacutophyton cycles”, dominate everywhere (Shapovalova, 1965, 1968; Serebryakov et al., 1972).In the Yudoma-Maya trough, in comparison with the Maya depression, the individual stromatolite horizons get thicker and bioherms composed of any one group of stromatolites (predominantly Baicalia) are of greater size and more numerous. As a result, the relative abundance of the above groups is somewhat different. The horizons containing Baicalia account, in four sections of the Maya depression, for 58-60% of the total thickness of the stromatolite horizons and, in two sections of the Yudoma-Maya trough, for 67-7076. (2)The composition of the assemblages may remain the same, irrespective of vertical changes in the composition and character of the enclosing deposits. Most spectacular in this respect are stromatolites of the Neruen suite. Conophyton, Jacutophyton, Baicalia and Colonnella are enclosed in its lower part in clastic and chemogenic dolomite varying in their fabrics and colour, and in its upper part, in less variable limestones and dolomitic limestones. These stromatolite constructions frequently cross the boundary of the lithologically different carbonate rocks without changing their morphology. Moreover, all these groups (except Jacutophyton) have been found enclosed in argillites (Fig. 4). Absence of any direct interdependence between the morphology of
S.N. SEREBRYAKOV
626
8 not exposed 740m
I
Fig. 4. Stromatolite horizon of the upper subsuite of the Neruen suite, Lyaki River. Legend: 1 = limestones; 2 = argillites;3-5 = stromatolite morphology;3 = Jacutophyton; 4 = Buicaliu; 5 = Colonnella; 6-9 = stromatolite microstructures typical for the following forms: 6 = Baicalia lacera Semikhatov; 7 = B. ingilensis Nuzhnov; 8 = Conophyton lituum Maslov; 9 = C . cylindricum Maslov (for jacutophytons the denominator indicates the microstructure of central column, the numerator that of the branches); 10-14 = colour of rocks: 10 = red; 11 = mottled (red and green); 1 2 = variegated; 13 = grey of various shades; 14 = dark grey to black.
stromatolites and the character of the enclosing rocks in the Neruen suite has been already considered in the description of the “Jacutophyton cycles” (Serebryakov et al., 1972). Yet the superposition in bioherms of different stromatolite morphologies can evidently be attributed to ecological factors which may have operated over extensive areas (see Ch. 6.4,fig. 7). This contradiction can obviously be accounted for by the fact that cyclic variations in the morphology of stromatolites and local variations in the character of rocks were controlled by different causes. For instance, the rate of sedimentation and intensity of subsidence are poorly manifested in the character of carbonate sediments; the role of these factors in the morphogenesis of stromatolites seems to be significant (Bertrand-Sarfati, 1972c; Serebryakov et al., 1972). The possibility that the environment may affect the shape of stromatolites but not the character of the sediment has been suggested by Maslov (1959) and Preiss (1973a).
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(3) Different assemblages of stromatolites are found in lithologically similar beds of different ages; these assemblages may vary through a homogeneous section of carbonate rocks. Such a change in assemblages occurs in the uniform dolomites of the Tsipanda suite (see Fig. 1).In its lower part there are stratiform stromatolites (Mulginellu, Strutiferu, Irregulariu); in the middle part, stratiform and stratiform-columnar types (Irreguluria, Purrnites) are encountered, and in the upper part, these are replaced by columnar structures (Colonnellu, Buiculiu, Tungussiu, occasional Minjuriu, and forms resembling Tilernsinu). There is no lithological evidence that these changes in stromatolite assemblages are dependent on any changes in ecological conditions. Though such a dependence cannot be absolutely denied, the fact that the changes in morphological assemblages coincide with variations in the microstructures of the stromatolites indicates that these changes are probably due to changes in the communities of stromatolite-forming algae. Dolomites similar in composition, colour and fabric to the Tsipanda ones occur in the Yudoma-Maya trough a t the base of the overlying Neruen suite. They are separated from the rocks of the Tsipanda suite by a regional weathering surface and a thin (5-20m) bed of argillites. Minjuriu and Tilemsinu (?) have not been found in these dolomites but Jucutophyton and Conophyton are abundant. This change in the morphological assemblage is accompanied by a partial change in the microstructures; but some of the stromatolites of the boundary horizons of the Tsipanda and Neruen suites have similar microstructures. The Neruen stromatolites assemblage differs from the Tsipanda one not only in composition but also in the appearance of the cyclic structure of the bioherms (Jucutophyton cycles). Consequently, the change in the associations at the boundary of the suites under consideration was caused not only by the change in algal communities but also on the changes in the environment. It is significant that these changes are not manifested in the macroscopic and petrographic appearance of the rocks. This is not an exceptional example of the repetition of uniform carbonate beds a t various stratigraphic levels. The dolomites of the Svetly and the Tsipanda suites are similar in their composition and position in the sedimentary cycles. Dolomites of the Svetly type have been encountered in the Neruen and Ignikan suites. Yet the stromatolite assemblages in these suites are different. The lithologically similar deposits are unlikely to have formed, in all cases, under different conditions. It is much more probable that we have to deal here with the age variability of stromatolite morphology; this view is reinforced by the fact that there are corresponding variations in their microstructures (see below). The specific features of the distribution of the stromatolite assemblages allow two conclusions. Firstly, the vertical change in assemblages often has no direct relationship to changes in the enclosing deposits. Though the effect of environment on the stromatolites in the sections can be established in
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some cases, the general sequence of assemblages can hardly be explained without admitting that it was dependent on the evolution of the stromatoliteforming algae. Secondly, there is very little relationship between the distribution of assemblages, the conditions of sedimentation and the distance from the coast. This does not exclude the effect of environmental factors on stromatolite morphology within the bioherms or even in certain areas of basins (see Ch. 6.4). The effect of these factors is not manifested in the characteristics of the rock, which makes their interpretation more difficult. Those ecological factors which affected the morphology of the stromatolites hardly influenced their microstructure. In fact, the intrabioherm variability of stromatolites is normally unaccompanied by microstructural changes (Komar and Semikhatov, 1965, 1968b; Komar, 1966; Krylov, 1967a, 1972; Serebryakov, 1971; Walter, 1972a). The abundant evidence of the Recent and ancient stromatolites (see Ch. 7.1) suggests that particular microstructures owe their formation to particular communities or species of algae. If this is correct, the distribution of stromatolites with particular microstructures must to some degree reflect the original distribution of the stromatolite-forming communities. DISTRIBUTION OF STROMATOLITES WITH PARTICULAR MICROSTRUCTURES
The specific features of microstructure usually provide the basis for distinguishing forms (“species”) of stromatolites; the concept of “group” is sometimes identified with the concept of “stromatolites of a certain microstructure”. Krylov (1967a, 1972) has repeatedly discredited such identifications, stating that whether a form does or does not belong to some group, according to the current classification, must be determined by the morphology of the structures. Meanwhile, the same microstructure frequently occurs in stromatolites with diverse morphologies characteristic of different groups. Because the classification of microstructures remains an unsolved problem, I have referred in the following analysis to the microstructures described at the time of the establishment of each particular form. The distribution of stromatolites with particular microstructures in Riphean deposits of the Uchur-Maya region has recently been considered by Komar et al. (1973). Four types of distribution have been recognized: (1) The same microstructure is characteristic of all the stromatolites of a suite or some portion of a suite over the whole area of its distribution. For example, a considerable part of the Malgina suite is composed of Malginella malgica Komar et Semikhatov and Stratifera of a similar microstructure. In the lower part of the overlying Tsipanda suite, Malginella, Stratifera and Irregularia have a common but different microstructure (that described from Malginella zipandica Komar). The boundary between the two microstructures coincides (not always exactly) with the sharp change in the composition of
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629
the rocks at the boundary of the suites. The next level of microstructural change is within the uniform Tsipanda dolomites: in their middle part, there occurs Pamites aimicus (Nuzhnov) and Irreguluriu with a similar microstructure. Y A 2
. CHELASIN 3
PLLYW-VU
4
L +@
1)
Fig. 5. Vertical range of stromatolites with particular microstructures, in some sections of the Neruen and Ignikan suites (ranges in proportion to thicknesses). For position of sections see Fig. 1. Letters given in circles are stromatolites with microstructures described from the following forms: c = Conophyton cylindricum; It = C. lituum; mt = C. metula; s = Minjaria sakharica; u = Colonnella ulakia; i = Baicalia ingilensis; 1 = B. lacem; m = B. maica; t = Inzeria tjomusi; cn = I . confragosa.
The distribution of microstructures in the Ignikan suite is particularly interesting (Fig. 1).This suite is composed of clastic, microphytolitic, and chemogenic carbonate rocks of diverse composition in which dolomites typical for the Uchur-Maya platform are gradually replaced by limestones in the Yudoma-Maya trough. Yet the vertical sequence of stromatolites persists over the whole area of occurrence of the suite (Fig. 5). This allows three regional biostratigraphic subdivisions to be distinguished in the suite. The facies variations in the deposits consist only of some irregularities in the distribution of stromatolite bioherms and thickness variations in the beds containing stromatolites of a particular microstructure. (2) Mass concentrations of stromatolites of some definite microstructure are confined to definite associations of deposits. The Yudoma suite (Fig. 2) can be used to illustrate this. The distribution of its stromatolites exhibits two opposite tendencies (Semikhatov et al., 1970). On the one hand,
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Fig. 6 . Schematic lithologic-and-facies profile of the lower Yudoma subsuite. For position of profile line see Fig. 1. Legend: I = sandstones; 2 = argillites; 3 = siltstones; 4 = dolomites; 5 = limestones; 6 = darkcoloured bituminous and clayey-bituminous limestones; 7 = sandy dolomites; 8 = variegated cherty-dolomite and clayey-herty-dolomite rocks; 9 = catagraphs; 10-1 1 = stromatolites with microstructures typical for the following forms: 10 = Boxonia grumulosa; I 1 = Jurusania judomica; 12 = K-Ar age of glauconites in m.y. (according to Kazakov and Knorre, 1970).
stromatolites with the same microstructure are present in rocks of diverse composition and genesis, for example in cross-bedded quartz sandstones, horizontally laminated clayey dolomites and massive pelitomorphic limestones. On the other hand, mass concentrations of stromatolites with a definite microstructure are confined to definite associations of rock types, which causes the stromatolites to be irregularly distributed. Thus, most
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common among sandstones and microphytolitic dolomites are stromatolites which have the microstructures described for Boxonia grumulosa Komar, B. ingilica Komar and Semikhatov, Paniscolleniu emergens Komar and Colleniella singularis Komar; stromatolites with the microstructures described for Jurusaniu judomica Komar and Semikhatov occur in cherty and clayey dolomites which probably formed in the deeper sea; those with the characteristic Gongylina nodulosa Komar and Semikhatov microstructure are found in various types of pure carbonates. The boundaries of the rock associations with their contained stromatolite assemblages may be diachronous (Fig. 6). Some of the rock associations occur repeatedly in the section due to the cyclic structure of the Yudoma suite. However, among its stromatolites there are forms which are confined to only one of the subsuites. Thus, Boxonia grumu Zosa and some other morphologically different stromatolites of a similar microstructure do not pass into the deposits of the upper subsuites despite the passage upwards of its characteristic rock association. (3) Stromatolites of various microstructures are distributed in a disorderly fashion within a suite. Though some sequence in the change or alternation of microstructure can be observed in some sections, it is not maintained throughout the depositional area. The Svetly and Neruen suites can provide an example of this. Among the stromatolites of the Neruen suite, seven varieties of microstructures were distinguished (Fig. 5 ) . Five of them are encountered practically in the whole time-range of the suite, overlapping and replacing one another in vertical and lateral directions without any visible regularity. Moreover, in some cases one stromatolite structure may have different microstructures in its different parts (e.g. some Jacutophyton, see Serebryakov et al., 1972). (4) Stromatolites occurring in different sections of the suites have different microstructures. This type of distribution has been found in the Omakhta suite in two isolated sections: in the Uchur depression and in the northern part of the Yudoma-Maya trough (see Fig. 1). These considerations do not contradict the previously made suggestion that the spatial distribution of stromatolites of a certain microstructure reflects the distribution of communities of the stromatolite-forming algae. If this is so, it is unlikely that there was any direct dependence of the change of communities in time on environmental variations. This conclusion is supported also by vertical distribution of assemblages of morphological groups of stromatolites (see above). The condition in the Riphean basins of the Uchur-Maya region allowed, in some cases, the development of one or several coenoses over the whole of its ar6a; in other cases they were predominantly confined to limited areas. In some comparatively rare cases, changes in the algal communities might be caused by the same environmental factors which affected the morphology of the stromatolites. This view is supported by the occasional coincidence in
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a change of stromatolite morphology and change in their microstructure (some of the “Jucutophyton cycles”). In general, the following hypothesis may be proposed t o explain the distribution of stromatolites in the Neruen suite (type 3): in the Neruen basin there existed at the same time five similar and closely related communities (species?) of algae which gave rise to stromatolites with five different microstructures. Each of the communities could develop within the entire range of the conditions present. For this reason the spatial distribution of the communities was irregular and their appearance in the given point of the basin was fortuitous. The difference in the microstructures of the Omakhta stromatolites (type 4 distribution), can be, in all probability, accounted for by the existence of two isolated basins populated by different communities (species?) of algae. The same type of distribution of stromatolites with definite microstructures is observed in suites differing in composition (for instance, in the Svetly and Neruen) while different types are encountered in the lithologically similar suites: in the Svetly and Tsipanda, Neruen and Ignikan. This suggests that the realization of one type or another was determined not by the conditions of sedimentation but by the particular character of the algal communities. CONCLUSION
(1) In the Uchur-Maya region, there were, during the Riphean, extensive shallow-water basins where the conditions of sedimentation were rather stable in space and time. As already pointed out in the literature (e.g., Keller et al., 1968; Trompette, 1969; Bertrand-Sarfati, 1972c) such basins were, evidently, characteristic of the Riphean. (2) Stromatolites of the Uchur-Maya region formed subtidally as did, probably, the majority of other Riphean stromatolites (see Monty, 1973b, c; Serebryakov and Semikhatov, 1973,1974). They formed practically over the entire area of the basin. Therefore, u priori use of Riphean stromatolites as indicators of an intertidal or a nearshore environment is unfounded. (3) Variations in the distribution of different communities (species?) of algae seem to have been the result of their different residence against environmental changes. The range of conditions allowing formation of stromatolites by one kind of alga or another was rather wide, which reinforces the interpretation of the subtidal formation of the stromatolites. (4) The dependence of the distribution of the Uchur-Maya subtidal stromatolites on the conditions of sedimentation is poorly manifested. A moderate supply of terrigenous material did not limit the development of the stromatolites. It was not infrequent that short periods of terrigenous sedimentation failed to change the stromatolite assemblages (for example, in the Svetly and Neruen suites). Such changes, however, did occur within
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homogeneous carbonates. Stromatolites were, normally, unaffected by changes in the carbonate composition of the rocks. Stromatolites of similar morphology and microstructure occur in limestone, sedimentary-diagenetic dolomites and mixed limestone-dolomite rocks. Thus, there is no reason to regard the Riphean stromatolites as indicators of definite salinity conditions in the basin. (5) The morphology of the stromatolites, and sometimes also the distribution of the algal communities was greatly affected by environmental factors which are poorly, or not at all, manifested in the lithological characteristics of the rocks. The effect of these factors is more apparent in the analysis of the intrabioherm variability of stromatolites (see Ch. 6.4). (6) Vertical variations in the algal communities and, as a result, in the morphological and microstructural assemblages of stromatolites, seem to have been associated mainly with the evolution of the stromatolite-forming algae. It is not mere chance that a similar sequence of stromatolites has been observed in other Riphean sections within the U.S.S.R. (see Ch. 7.1). (7) The conditions under which the Riphean stromatolites formed were, probably, essentially different from the conditions of growth of Recent algal structures.