On the Upper Precambrian stromatolite standard of North Eurasia

Earth-Science Reviews, 16 (1980): 235--247

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Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

On the Upper Precambrian Stromatolite Standard of North Eurasia M.A. SEMIKHATOV Geological Institute of the U.S.S.R. Academy of Sciences, 109017 Moscow (U.S.S.R.)

ABSTRACT Semikhatov, M.A., 1980. On the Upper Precambrian stromatolite standard of North Eurasia. Earth-Sci. Rev., 16: 235--247. Review of the distribution of the U.S.S.R. Upper Precambrian stromatolites in time and space demonstrates an unrepeated and unidirectional change of their assemblages along w i t h distinct lateral differentiation of the systematic composition of coeval assemblages. Such a differentiation was most pronounced during Middle and Late Riphean time.

INTRODUCTION

The stratigraphic utility of any group of organic remains depends on a standard section which illustrated a certain succession of stratigraphically important taxa and their assemblages. The lateral validity of such a standard is determined by a complex combination of facial, paleoclimatic and paleobiogeographical factors. Using stromatolites in Upper Precambrian (Riphean) stratigraphy, the Uralian stratotype of the Riphean, located in the Variscian folded belt, was considered to be the only. standard in the first stages of the investigations. Successive stromatolite associations distinguished in other regions of the U.S.S.R. were compared with those singled o u t in the stratotype, mostly at the group (informal genera) level (e.g. Keller et al., 1960; Komar, 1966). The general parallelism of their change established in some remote regions was the main empirical evidence for the stratigraphic potentialities of Riphean stromatolites. However, against the background of this general parallelism, the Siberian succession of stromatolites proved to include certain assemblages n o t known in the Urals. As a result, a combined model of the vertical change of stromatolites of North Eurasia was compiled in the late 1960's. It was based on the then accepted correlations of Siberian and Uralian sections. It was the very model with which certain sets of stromatolite taxa established in North America, Australia and Africa were compared, predominantly at the group level (see Preiss, 1976, for bibliography and review). The development of investigations showed a broader time-range for many 0012-8252/80/0000--0000/$ 03.25 © 1980 Elsevier Scientific Publishing Company.

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groups of Upper Precambrian stromatolites than was previously thought (for review see Semikhatov, 1976). It became necessary to put special emphasis in the stratigraphic use of stromatolites on identifying assemblages of forms (informal species) distinguished according to peculiarities of microstructure. Direct dependence of the latter upon the systematic composition of stromatolite-forming algae and the dual (biotic and abiotic) nature of the morphological features used for definition of the groups (see Serebryakov, 1976; Semikhatov et al., 1979a, for review) lead to the same emphasis. The attention concentrated on form assemblages focussed at once on the considerable lateral variations of most of them. This resulted in difficulties in using the stromatolites for interbasinal correlations (e.g. Khomentovsky et al., 1972; Shenfil, 1978, 1979). D I S T R I B U T I O N O F T H E U.S.S.R. R I P H E A N S T R O M A T O L I T E S IN T I M E A N D SPACE

Leaving apart the Riphean of Middle Asia, which requires a significant revision of its paleontological characteristics, four types of stromatolitebearing sections of the U.S.S.R. Upper Precambrian can be distinguished: Uralian, North Siberian, Middle Siberian and South Siberian, each of them having its own combinations and succession of forms, and, to a certain extent, groups of stromatolites. The distribution of these types is shown in Fig. 1. The data on composition, isotopic age and correlation of the composing sections were recently summarized by Semikhatov et al. (1979b), and Chumakov and Semikhatov (1980). The information on the distribution of stromatolites mentioned below is based on several reviews (Komar, 1966, 1978; Semikhatov et al., 1970; Golovanov, 1970; Dolnik and Vorontzova, 1974; Krylov, 1975; Semikhatov, 1976; Shapovalova and Krylov, 1978; Shpunt et al., 1979) and on material from the All-Union Colloquia on stromatolites and the author's personal data. The cited isotopic datings were recalculated in accordance with the value of the decay constants recommended b y the International Geochronological Subcommision. Among Upper Precambrian stromatolite assemblages of North Eurasia the Lower Riphean incorporated in beds with isotopic ages between 1650 + 50 and 1350 + 50 Ma there is one that is laterally less differentiated. Its key elements, Kussiella kussiensis (Maslov), and Gongylina differenciataKomar, are present in all five regions of the paleontologically defined Lower Riphean known in the U.S.S.R.: South Urals, Anabar Massif, Olenek and Udzha uplifts and the Uchura--Maya region (see Fig. 1 for locations). In addition, the Lower Riphean of the Anabar Massif and Olenek uplift are related by the presence of Collonella discreta Komar and Kussiella taeniata Golovanov. The deposits of the Anabar Massif and the Uchur--Maya region contain, in addition to K. kussiensis and Gongylina differenciata, three other forms: Kussiella aequssa Golovanov, Nucleella figurata Komar, and Omachtenia ornachtensis Nuzhnov. Besides the above-mentioned interregional forms,

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238 some other taxa are known in the U.S.S.R. Lower Riphean. These are Conophyton cylindricum Maslov, C. garganicum Korolyuk, Jacutophyton sp., Gaia irkuskanica Krylov, and some local endemics present in the type Lower Riphean (the Burzyan group of the South Urals), endemic forms of Anabar,

Conophyton garganicum Korolyuk, Jacutophyton divulgatus Shapovalova of the Olenek uplift, and some others. All the above forms of Conophyton and Jacutophyton are present up to the Middle and lower Upper Riphean. Such a relative abundance of stromatolite forms connecting the Lower Riphean of two or more separate regions is the more noteworthy as the Early Riphean was a time of general high position of the cratons, when relatively small separated basins existed along their margins only (Semikhatov, 1974). The Middle Riphean transgression, covering a considerable part of North Eurasia, not only failed to bring further unification in the composition of coeval stromatolite assemblages, but, on the contrary, was accompanied by considerable differentiation. It was during the Middle (1350-+ 50 to 1000-+ 50 Ma) and Late (1000 + 50 to 680 -+ 20 Ma) Riphean that the above-mentioned four types of sections were the most sharply defined.

Uralian sections In the type Middle Riphean, the Yurmatin Group of the South Urals, stromatolites are known only in its upper (Avzyan) formation (K--Ar age of glauconite: 1220 Ma). These are: Colleniella evoluta Shapovalova, Baicalia aborigena Shapovalova, Svetliella sp. from the lower part, and Conophyton rnetula Kirichenko, C. cylindricum Maslov, Jacutophyton sp. and Baicalia sp. (= B baicalica of Krylov, 1975) from the upper part of the formation. Younger stromatolites of the Urals are incorporated in the Karatau Group, the type Upper Riphean. They are separated from the Avzyan ones by a preKaratau unconformity and by a thick clastic basal formation of the Karatau Group. Six successive assemblages are singled out in the overlying part of the group and three of them (the first, fourth and fifth) are known at the corresponding levels in the Polyudov Ridge (Raaben, 1976). The first Karatau assemblage is composed of Inzeria tjomusi Krylov, Jurusania cylindrica Krylov, and Conophyton lituum Maslov, incorporated in cryptalgalaminates of the lower part of the Katav Formation, sometimes described as Malginella malgica. The second consists of the upper Katav Malginella zipandica Komar as well as Parmites aimicus (Nuzhnov), Conophyton garganicum Korolyuk, C. sp. A, and Kussiella enigmatica Raaben from the lower part of the overlying Sim (Kislyar) Formation. The third, fourth and fifth assemblages are united by various common taxa frequently regarded as characteristic for the uppermost Riphean far beyond the Urals (Krylov, 1975; Raaben, 1976; Shapovalova and Krylov, 1978). The third (upper Sim) assemblage includes Katavia karatavica Krylov, Gymnosolen ramsayi Steinmann, Tungussia nodosa Semikhatov, and Minjaria aff. sakharica Komar. The fourth (lower Min'jar) assemblage contains, in addition to Katavia karatavica and Gymno-

239 solen ramsayi, Tungussia cf. colcimi Raaben, Alternella bianca Raaben, Parmites meridionalis Raaben, and Colonnella sp. in the fifth (upper Min'jar) assemblage, there appear Minjaria uralica, Paramites concrescens Raaben, Poludia polymorpha Raaben, and Conophyton rniloradovici var. krylovi Raaben. The K-Ar datings of the second and third assemblages gave 970-850 Ma, and those of the fourth and fifth assemblages 790--740 and 710-650 Ma respectively. Stromatolites of the sixth assemblage, incorporated in the overlying Uk Formation capping the Karatau Group, are different: Linella ukka Krylov, L. simica Krylov, and Tungussia bassa Krylov. They appear directly above the beds with the glauconite K-Ar age of 6 0 0 - 6 6 0 Ma. This change of stromatolites accompanied by an abrupt change of microphytolites is regarded as a paleontological substantiation of the Upper Riphean--Kudash boundary in the stratigraphic scale for the Precambrian of the U.S.S.R. (Keller et al., 1979; Semikhatov, 1979). Middle Siberian sections The Middle Siberian type of sections has a vast distribution (see Fig. 1) and includes in particular the Uchur--Maya hypostratotype of the Riphean. The correlation of the sections composing this type does not give rise to disagreements, unless certain terrigenous beds of the Yenisei ridge which are devoid of stromatolites are involved. Lower Riphean stromatolites of the Middle Siberian type were considered above. The succession of Middle--Upper Riphean taxa in sections of this type is the most complete in the Uchur--Maya region. It starts with the Svetlyi Assemblage (Shapovalova, 1974) and includes: Collenella evoluta Shapovalova, Colonnella kyllachii Shapovalova, Svetliella svetlica Shapovalova, S. venusta Shapovalova, Baicalia aborigena Shapovalova, B. inventa Shapovalova, B. sp., Litia diformis Shapovalova, Kussiella kussiensis (Maslov), K. vitata Komar, and Conophyton ex gr. garganicurn Korolyuk. The latter three forms extend here from the Lower Riphean, but on the whole the Svetlyi Assemblage strongly differs from older ones not only in its systematic composition, but also in the appearance of mlcrostructures of the new type Canalophorida (after Komar, 1979). This assemblage dated between 1230--1210 and 1160--1000 Ma (K--Ar datings of glauconite) is incorporated in. the Aimchan Group that unconformably rests on the Lower Riphean. The overlying Kerpyl Group contains in its basal Totta Formation (K--At age of glauconite 1000--1160 Ma). Appia topicalis Shapovalova and Compactocollenia tcha]ensis Dolnik. In other sections of the Middle Siberian type no stromatolites are known at the Aimchan and Totta levels. Higher up the sections of this type, a horizon of multicoloured cryptalgalaminates is found. It contains stromatolites (two endemics) in two localities of the Uchur--Maya region only. Younger deposits of this region (Tzypanda Formation capping the Kerpyl Group) incorporates two successive stromatolite assemblages: 1 = Parmites aimicus (Nuzhnov), Malginella zipan-

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dica Komar, rare Conophyton sp. A; 2 = Colonnella ulakia Komar, Baicalia rata Semikhatov, B. aft. rata, Minjaria sakharica Komar, Telemsina sp., Confunda sp. The Pb-isochron age of the Tzypanda dolomites is 980 Ma (concordia plot). The first two representatives of the lower Tzypanda assemblage are found (tentatively at the same level) in some sections of the northeast U.S.S.R. and M. zipandica is also known in the coeval beds in the Turukhansk region, together with Baicalia prima Semikhatov and Tungussia nodosa Semikhatov. No representatives of the upper Tzypanda assemblage were recognized beyond the Uchur--Maya region at the level concerned, b u t some of them do occur in younger beds. The next Lakhanda group of the Uchur--Maya h y p o s t r a t o t y p e incorporate two distinctive stromatolite assemblages. The first one (Lakhanda) is enclosed in the lower (Neryuen) formation of the group. It contains: Baicalia lacera Semikhatov, B. ingilensis Nuzhnov, Conophyton metula Kirichenko, C. cylindricum Maslov, C. lituum Maslov, Jacutophyton cognitum Shapovalova, J. multiforme Shapovalova, J. ramosum Shapovalova, Colonnella sp., rare Jurusania cylindrica Krylov, Inzeria tjomusi Krylov, and some other forms. The K--At age of glauconite from enclosing beds is 920--980 Ma. The time-equivalents of the Neryuen Formation in the Turukhansk region (Derevnin and Burovaya formations), and on the Yenisei ridge (Potoskuy Formation) contain the main forms of this assemblage in combination with some others. In the Turukhansk region these are Baicalia lacera, B. rara Semikhatoy, Conophyton metula, C. lituum, Jacutophyton cognitum, Tungussia nodosa Semikhatov, Gymnosolen (?) tungusicus Shenfil, Colonnella sp., and on the Yenisei ridge Baicalia lacera, B. unca Semikhatov, Conophyton cylindricum, Jacutophyton multiforme, Tungussia nodosa, and Colonnella sp. In other sections of Middle Siberian type only Baicalia sp., Conophyton cylindricum and C. lituum are known at the Neryuen level (on the Kolyma uplift). The second stromatolite assemblage of the Lakhanda Group is present in its upper (Ignikan) formation dated at 870 Ma (K--Ar, glauconite) and 840 +40 Ma (Pb-isochron, dolomite, concordia plot). It consists of Baicalia maica Nuzhnov, Inzeria tjomusi Krylov, I. (?) confragosa (Semikhatov), Jurusania cylindrica Krylov, and rare Jacutophyton sp. Though all representatives of the given assemblage, except for I. (?) confragosa, are known in the underlying beds of the Lakhanda Group (Shenfil, 1978, 1979), the assemblage differs significantly from the preceding one in the composition of Baicalia and in the absence of diverse sets of definite forms of Conophyton and Jacutophyton. It is this very change of stromatolite that was for a long time considered as substantiation of the Middle--Upper Riphean boundary in its Siberian paleontological model (see Semikhatov, 1976; Semikhatov et al., 1979b; Shenfil, 1978, 1979 for discussion). Inzeria t]omusi, usually accompanied by Baicalia maica, is widely distributed in correlated deposits of the northeast of the U.S.S.R. (Kolyma,

241 Okhotsk, Omolon and Taigonos massifs). The key forms of the Ignikan assemblage, accompanied by some others, can be traced in time analogues of the Ignikan Formation in western Middle Siberia. In the Turukhansk region these are Inzeria (?) confragosa, Minjaria uralica Krylov, Gymnosolen sp. from the Shorikha Formation and Inzeria tjomusi, I. nimbifera (Semikhatov) and /. aft. djejimi Raaben from the Turukhansk Formation; on the Yenisei ridge: I. (?) confragosa, I. nimbifera, Minjaria uralica, Baicalia unca Semikhatov, Tungussia nodosa Semikhatov, and T. confusa Semikhatov, from the Kirgitey (Seryi Kluch and Dadykta) Formation. The data presented show that the above part of the Middle--Late Riphean history of Middle Siberia was characterized by lateral expansion of stromatolites and a tendency towards unification of the systematic composition of their contemporaneous assemblages from different regions. These phenomena were taking place against a background of extensive transgression (see Semikhatov, 1974, figs. 32, 33). The latter was interrupted by uplift as well as, on the Yenisei ridge, by folding and intrusion of granitoids with the U--Pb isochron age (on zircon) of 830 + 50 Ma. The latest Upper Riphean sediments (younger then 830--850 Ma) in Middle Siberian sections are either absent or are represented by terrigenous deposits (Ui Group of the Uchur--Maya region and its analogues). The marine transgression of Yudoma time, starting about 660--680 Ma ago, covered vast territories of Siberia (see Semikhatov, 1974, fig. 34), resulting in a profound redistribution of types of sections. During Yudoma time, that terminates the Late Proterozoic and apparently corresponds to the Kudash and Vend of the U.S.S.R. stratigraphic scale, carbonate sediments predominated (Yudoma Group and its equivalents) throughout the Siberian platform and its surroundings, except the southwestern, regions. The composition of the stromatolites enclosed in them is rather uniform. From the north of Siberia to the Uchur--Maya region and Lake Baykal there are Paniscollenia emergens Komar, Colleniella singularis Komar, rarer Boxonia grumulosa Komar that in two or more regions are accompanied by Jurusania (?) judomica Komar et Semikhatov (Olenek uplift, Kharaulakh, Uchur-Maya region, western shore of Lake Baykal), Stratifera irregularis Komar (Anabar, Olenek uplift), Boxonia allahjunica Komar et Semikhatov, Dgerbia grumulosa Dolnik (both in the Baykal area, Patom Highland, Uchur--Maya region), Jurusania sibirica (Yakovlev) and Linella sibirica Krylov (both in the Uchur--Maya region and western shore of Lake Baykal). Thus, the successive stromatolite assemblages of the Middle Siberian Riphean differ from that of the Urals in composition, at the form and to some degree at group levels. Some forms c o m m o n to both regions are in each of them members of different assemblages or even appear in a reverse order. These facts, combined with a lack of radiochronological information explain the differences of opinion about the correlation of Siberian sections with the Uralian one (e.g. Khomentovsky et al., 1972; Keller, 1973; Komar, 1973, 1978; Semikhatov, 1974; Shenfil, 1978; Semikhatov et al., 1979b). At

242 present it appears to me, as before, that the most reliable correlation places the Lakhnada and Ui groups of the Uchur--Maya h y p o s t r a t o t y p e and their Siberian analogues equivalent to the type Upper Riphean -- a pre-Uk part of the Karatau Group of the Urals (for discussion see Semikhatov et al., 1979b). Thus, the Uralian model of the Middle--Upper Riphean boundary (based on the unconformity beneath the Karatau Group), adopted in the stratigraphic scale of the U.S.S.R. Precambrian, seems to separate in the Siberian sequences upper Tzipanda and Lakhanda stromatolite assemblages.

North Siberian Sections The sections composing the North Siberian type differ from one another in composition of Middle and Upper Riphean deposits. Their correlation is based on K--At datings of glauconite, on local succession of stromatolites, and partly on lithostratigraphic criteria. Beds containing stromatolite assemblages of limited variety rest conformably on the above-mentioned Lower Riphean. These are: the Arymas Formation of the Olenek uplift with Baicalia minuta Komar, Conophyton lituum Maslov, Jacutophyton multiforme Shapovalova and the lower part of the Lower Yusmastakh Formation of the Anabar Massif, incorporating Baicalia minuta, Conophyton metula Kirichenko, C. garganicum Korolyuk, and endemic Sacculia tegimentum Golovanov. The K--At ages of the above beds are 1220--1160 and 1230 Ma, respectively. In younger deposits of the north, in the Debengda Formation of the Olenik uplift, as well as in the middle and upper parts of the Lower Yusmastakh Formation of the Anabar Massif, the sets of stromatolites are more diverse and contain some common forms. These are: Anabaria radialis Komar, A. divergens Komar, Stratifera nekulachica Golovanov, confined to the above-mentioned subdivisions, as well as taxa of wide vertical and lateral distribution: Conophyton garganicum Korolyuk, C. lituum Maslov, C. metula Kirichenko, and Jacutophyton cognitum Shapovalova. Besides the just-mentioned forms in the Debengda Formation, there are J. multiforme Shapovalova, Conophyton lituum Maslov, Baicalia sp., and in the upper part Malginella zipandica Komar, whereas in Lower Yusmastakh deposits, Svetliella cf. venusta Shapovalova, Omachtenia omachtensis Nuzhnov, Gongylina differenciata Komar and some local endemics occur. Near the base of Upper Yusmastakh deposits, above K--Ar datings of glauconite of 1140--1120 Ma, the Uchur--Maya Appia topicalis Shapovalova was identified along with Colonnella cormosa Komar (Shapovalova and Krylov, 1978). Owing to the presence of some c o m m o n forms and even some similarities in their succession, the just-described stromatolite assemblages of the Olenek uplift and Anabar Massif seem to correlate, though this is contradicted by the K--Ar data on Upper Yusmastakh rocks. Glauconite K--Ar ages of the Arymas (1220--1160 Ma) and of the Debengda formations (lower part: 1140--1080 Ma; upper part: 1040 Ma),

243 along with the stratigraphical position of the formations, strongly suggest its correlation with the Aimchan and the Kerpyl groups of the Uchur--Maya region, respectively. Noteworthy is that generally dissimilar coeval stromatolite sets of the Debengda and Kerpyl deposits contain one c o m m o n form -Malginella zipandica -- that seems to be located in both regions at approximately the same level. A second form, with a relatively narrow time-range, connecting North and Middle Siberian sections is Appia topicalis. Among the diverse set of Debengda stromatolites, only the above mentioned forms of Conophyton and Jacutophyton (except J. divulgatum) extend up into the overlying lower part of the Khaipakh Formation (K--Ar age of glauconite, 980--1000 Ma). The correlatable parts of the Upper Yusmastahk Formation incorporate only some endemic Stratifera, whereas the coeval beds of the Udzha and Kharaulakh uplifts are devoid of stromatolites. In the uppermost horizons of the North Siberian Upper Riphean deposits, confined at the top to the pre-Yudoma unconformity, the stromatolite assemblages are completely different from older ones. On the Olenek uplift in middle Khaipakh deposits (K--Ar age of glauconite, 890--910 Ma), there are Inzeria tjomusi Krylov, Jurusania cylindrica Krylov, as well as Gymnosolen furcatus Komar that extends up into upper Khaipakh beds and is accompanied there by Boxonia vera Shapovalova and Kotuicania torulosa Komar. The upper part of the Upper Yusmastakh deposits of Anabar contains Gymnosolen furcatus, Kotuicania torulosa, Boxonia lissa Komar and some purely local forms. In the lower part of the Udzha Formation of the Udzha uplift Inzeria t]omusi and Jurusania cf. cylindrica were reported, and higher up the section Gymnosolen ramsayi Steinmann (Shapovalova and Krylov, 1978; Shpunt et al., 1979). This formation is cut by dikes with K--At ages of 820--890 Ma. Considering the isotopic data, all the just-listed stromatolites should be attributed to the lower part of the Upper Riphean.

South Siberian sections The Middle and Upper Riphean sections of South Siberian type contain systematically poor stromatolite assemblages, represented mostly by endemic forms. This circumstance, combined with practically complete absence of isotopic datings and differences in composition of deposits in the major regions of development of the South Siberian Riphean, create problems of correlation (e.g. Khomentovsky et al., 1972; Dolnik and Vorontzova, 1974). We adopt the correlation worked out by Irkutsk geologists (bibliography and discussion, see Semikhatov et al., 1979b). Two stromatolite assemblages were singled out in the Middle--Upper Riphean sequences of the region. The first is present in Upper Ballaganakh and Kadalikan deposits of the Baykal--Patom Highland, and in their analogues in the north Baykal Highland and along the western shore of Lake Baykal; in eastern Sayan the assemblages become extinct. This assemblage is characterized by endemic forms of Baicalia and Stratifera (B. hirta Dolnik, B. mari-

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inica Dolnik, S. sarmensis Dolnik, etc.) which in some places are accompanied b y local forms of Conophyton (C. cadilnicum Korolyuk) and sporadically appearing taxa distributed b e y o n d South Siberia. These are Compactocollenia tchajensis Dolnik from the lower part of the Goloustnaya Formation of the north Baykal Highland and reported by T.A. Dolnik in the Sen' Formation of the eastern slope of the Baykal--Patom Highland, Baicalia aff. lacera Semikhatov, B. cf. rara Semikhatov, Conophyton metula Kirichenko, C. garganicum Korolyuk, Parmites aimicus (Nuzhnov), and Svetliella cf. venusta Shapovalova. It is worth noting that representatives of three various assemblages of the Uchur--Maya regions are mixed in the Sen' Formation. The upper stromatolite assemblage of South Siberia, known in the Zhuya Group of the Baykal--Patom and north Baykal Highlands, and in the Tagul Formation of the northern slope of eastern Sayan incorporates Inzeria gigantea Dolnik, I. tchencha Dolnik, Tinnia patomica Dolnik, Patornia ossica Krylov, and Lenia ]acutica Dolnik, accompanied in each of the above areas by some local endemic forms of the citated groups. The group composition of this assemblage suggests its Late Riphean age, and by their stratigraphic position directly under Yudomia deposits the enclosing beds can be compared with the Ui Group of the Uchur--Maya region. Noteworthy is that in the latter, composed mostly of terrigenous rocks, the Patomian Tinnia patomica appears. The lower stromatolite assemblage of South Siberia is usually related to the Svetlyi--Lakhanda assemblages of the Uchur--Maya region, when judged by its group composition, although some researchers (Khomentovsky et al., 1972; Shenfil, 1978} consider its age to be younger. CONCLUSION The above brief review of Riphean stromatolites of the U.S.S.R. has confirmed, first of all, the unrepeated and unidirectional character of their successions. At the same time we have seen that the composition of these successions changes considerably from place to place. During the entire Late Proterozoic a distinct lateral differentiation of stromatolite assemblages existed over the territory of North Eurasia. It was most pronounced during Middle and Late Riphean time, when the stromatolite-bearing sections of the U.S.S.R. were grouped into four types, each characterized by its peculiar sets and successions of stromatolites, at the form and partly at the group levels. In different sections of any one type, the composition of coeval stromatolite assemblages does not remain strictly the same either. However, the key members of such assemblages stretch over all the area of development of a given t y p e of sections, in each particular area being, so to speak, 'overgrown' with some local forms, and sometimes accompanied by taxa of a wide lateral distribution common to sections of another type. The role of paleogeographical and paleofacial factors in such a differentiation of composition of coeval stromatolite assemblages is obvious. Yet something else

245 must be involved, for the two most persistent assemblages -- those of the Yudomian and Lower Riphean -- were formed under contrasting paleogeographical conditions; one under conditions of extensive marine transgression, the other in isolated basins existing along the periphery of uplifted cratons. The analysis of the behaviour of certain taxa of stromatolites in space and time multiplies the already-known examples of heterochronous appearance of some forms and groups of stromatolites in various sections of the Upper Proterozoic of the U.S.S.R. Such heterochroneity is proved by Minjaria uralica and Gyrnnosolen ramsayi that are constantly associated in the Uralian sections to the upper Upper Riphean, whereas in various regions of Siberia they extend down into its lower part, i.e. into beds older then 830--850 Ma. At the same time it turns out that the entire assemblages with persistent composition in rather remote sections can be geologically diachronous. The clear-cut example of this phenomenon is the reverse succession of the assemblage Malginella zipandica--Parmites aimicus--Conophyton sp. A and the combination of Inzeria tjomusi--Jurusania cylindrica observed in the Urals and the Uchur--Maya region. The available geological data consider the first assemblage as diachronous. The examples of diachronous appearance of some taxa or assemblages of stromatolites are usually observed in passing from one type of section to another; such examples within one type are rare, and the range of the time transgression observed is smaller. Along with these anomalies, a number of other stromatolite taxa or their associations appear, in the light of the most up-to-date data, still to maintain their stratigraphic ranges over vast areas. They are present in several sections at approximately the same level, penetrating from one type of section to another. Examples are Baicalia aborigena and Colleniella evoluta of the Uchur--Maya Aimchan Group and Uralian lower Avzyan deposits; Appia topicalis of the Uchur--Maya, Udzha and Anabar sections; associations of Inzeria tjomusi--Jurusania cylindrica, and in Siberia also I. (?) confragosa (outside the older assemblage predominated by certain forms of Baicalia, Conophyton, and Jacutophyton) traced in the beds aged at about 850--1000 Ma in the sections of the Uralian, North Siberian and Middle Siberian types; Yudomian assemblage of Paniscollenia emergens, Boxonia grumulosa, Colleniella singularis and other forms, etc. Thus, the present-day data on the vertical and lateral distribution of stromatolites does not disprove at all one of the most reliable pieces of evidence of stratigraphic importance of s t r o m a t o l i t e s - parallelism in changes in their assemblages in remote sections. Yet, these data do show that ideas put forward in the late 1960's about their uniform changes throughout North Eurasia were oversimplified. On being studied more carefully, the pattern of stromatolite distribution breaks into a complicated mosaic of particular successions characterizing rather vast though clearly confined regions. The system of such mosaics is united by c o m m o n trends of change in the systematic composition of stromatolites in time and by appearance of some

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c o m m o n forms or specific groups of rather narrow time-range, and their unrepeated assemblages at some approximately coeval marker levels. The main current goal of the stratigraphic use of Riphean stromatolites is the elaboration of detailed models of lateral and vertical distribution of their taxa within particular large regions, the mutual correlation of these models on the basis of geological, radiometric and paleontological data, and to be able to establish, in this way, an integral pattern of distribution of stromatolite taxa in space and time. ACKNOWLEDGEMENTS

Among many persons with whom I discussed the problems elucidated in the paper, I would like especially to mention V.A. Komar and V.Yu. Shenfil. The manuscript had been critically read by P.E. Cloud and B.M. Keller who made valuable comments.

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247 Krylov, I.N., 1975. Riphean and Phanerozoic stromatolites of the U.S.S.R., Tr. Geol. Inst. Akad. Nauk S.S.S.R., 2 7 4 : 2 4 3 pp. (in Russian). Preiss, W.V., 1976. Intercontinental correlations. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam, 359--370. Raaben, M.E., 1976. Perspectives of subdivisions of the Upper Precambrian biostratigraphy. In: Stratigraphy, Sedimentology, Precambrian Geology, Nauka, Moscow, pp. 198--208 (in Russian). Semikhatov, M.A., 1974. The stratigraphy and geochronology of the Proterozoic. Tr. Geol. Inst. Akad. Nauk S.S.S.R., 2 5 6 : 3 0 2 pp. (in Russian). Semikhatov, M.A., 1976. Experience in stromatolite studies in the U.S.S.R. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam, pp. 337--357. Semikhatov, M.A., 1979. The new stratigraphic scale for the Precambrian of the U.S.S.R. : an analysis and lessons. Izv. Akad. Nauk S.S.S.R., Ser. Geol., 1979 (II): 5--22. Semikhatov, M.A., Komar, V.A. and Serebryakov, S.N., 1970. The Yudomian Complex of stratotypical area. Tr. Geol. Inst. Akad. Nauk S.S.S.R., 2 1 0 : 2 0 7 pp. (in Russian). Semikhatov, M.A., Gebelein, C.D., Cloud, P.E., Awramik, S.M. and Benmore, W.C., 1979a. Stromatolite morphogenesis - - p r o g r e s s and problems. Can. J. Earth Sci., 16: 992--1015. Semikhatov, M.A., Aksenov, E.M., Becker, Yu.R., Velicanov, V.A., Dolnik, T.A., Kabankov, V.Ya. and Solontsov, L.F., 1979b. Subdivision and correlation of the U.S.S.R. Riphean. In: Upper Proterozoic Stratigraphy of the U.S.S.R. (Riphean and Vendian). Nauka, Leningrad, pp. 6--42 (in Russian). Serebryakov, S.N., 1976. Biotic and abiotic factors controlling the morphology of Riphean stromatolites. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam, pp. 321--336. Shapovalova, I.G., 1974. Riphean stratigraphy and stromatolites of the northern part of Yudoma-Maya depression. Nauka, Novosibirsk, 139 pp (in Russian). Shapovalova, I.G. and Krylov, I.N., 1978. Stromatolite correlation of the Riphean preYudomian deposits of the East and North of Siberian Platform. In : Upper Precambrian and Lower Cambrian Stratigraphy of the Eastern Part of the Siberian Platform. Yakutsk, pp. 3--37 (in Russian). Shenfil, V.Y., 1978. Problems of correlations by mean of Riphean stromatolites in Siberia. In: New Data on Late Precambrian Stratigraphy and Paleontology of the Siberian Platform. Nauka, Novosibirsk, pp. 22--37 (in Russian). Shenfil, V.Y., 1979. Stromatolite definition of the Upper Riphean lower boundary and correlation of Siberian parastratotype with Uralian stratotyoe of the Riphean. In: Upper Proterozoic Stratigraphy of the U.S.S.R. (Riphean and Vendian). Nauka, Leningrad, pp. 61--66 (in Russian). Shpunt, B.R., Shapovalova, I.G., Shamshina, E.A., et al., 1979. Proterozoic of the NorthEast of Siberian platform. Nauka, Novosibirsk, 215 pp. (in Russian). [Received and accepted August 19, 1980]