Facies (2006) 52: 611–625 DOI 10.1007/s10347-006-0083-z
ORIGINAL ARTICLE
Anna I. Antoshkina
Palaeoenvironmental implications of Palaeomicrocodium in Upper Devonian microbial mounds of the Chernyshev Swell, Timan-northern Ural Region Received: 10 November 2004 / Accepted: 24 April 2006 / Published online: 10 October 2006 C Springer-Verlag 2006
Abstract Nine major microfacies and eight transgressiveregressive cycles have been recognized in the middle Frasnian-Famennian microbial mound of the Shar’yu River section in the Chernyshev Swell, Timan-northern Ural region. Rocks including Palaeomicrocodium at the top of each cycle show signs of brecciation, freshwater leaching, and vadose cementation. Palaeomicrocodium, an enigmatic structure, being close in nature to Microcodium, is useful as a palaeoenvironmental indicator and can also be used in regional correlations. Seven levels of subaerial expose surfaces with Palaeomicrocodium correspond to seven breaks in growth of the microbial mound in the Upper Devonian reef-like formation. The high-amplitude sea-level changes recognized are in good correlation with those in the Moscow Syneclise and are also fixed on the global eustatic curve. Keywords Palaeomicrocodium . Microbial mound . Microfacies . Cycle . Correlation . Timan . Northern Ural region . Upper Devonian Introduction In the Timan-northern Ural region (the so-called Pechora Plate) Lower Palaeozoic reefs have been recognized only within the Ural area. During the Late Devonian, various organic build-ups, anoxic and hypoxic facies became widespread in the inner-shelf areas. Anoxic sediments, rich in organic material, are known as productive hydrocarbon source rocks. The Upper Devonian facies were formed as a result of the basal Frasnian transgression, which drowned large areas on the north-eastern margin of the East EuA. I. Antoshkina () Institute of Geology, Komi Science Center, Ural Branch, Russian Academy of Sciences, 54 Pervomayskaya St., 167982 Syktyvkar, Komi Republic, Russia e-mail:
[email protected] Tel.: + 8212-425353 Fax: + 8212-425346
ropean craton. In the Early Frasnian, active rifting was accompanied by magmatism. During the Middle FrasnianTournaisian, the Pechora Plate was characterised by a thermal subsidence (Malyshev 2000). Extensive shallow-water areas with barrier and isolated reefs developed on a margin of the remaining shelf in the western parts of the region studied. In the central part of the region, bank-like carbonate massifs and starved basins became widespread. The starved basins were characterised by accumulation of the bituminous deep-water sediments (condensed cephalopod limestones, shales, bedded cherts) of the Domanik facies (Menner et al. 1991; Menner and Shuvalova 2000). The term “Domanik” was introduced by A. A. Keyserling in 1844 to indicate the Upper Devonian source rocks (oil shales) enriched in organic carbon (kerogen) which were determined in the South Timan at the Domanik Creek. In addition, the Late Devonian facies set includes also terrigenous carbonate deposits (so-called depression basin fills). The Upper Devonian rocks, now exposed on the Chernyshev Swell, were formed in a middle-shelf environment. The Upper Devonian units with microbial mound in the Shar’yu River section are Middle Frasnian to Upper Famennian in age. The Lower Frasnian strata are recognised in the Locality 68 only (Fig. 1) and are dated by the occurrence of stromatoporoids Trupestomata cf. bassleri (Lec.) (samples Sha68-177-94, Sha68-178-94). The massive-bedded skeletal-peloidal limestones with calcispheres and sparse nodules of the cyanobacteria Girvanella are common in this locality. The lowermost unit of the microbial mound in the Locality 86 (Sha86-171-94), represented by bioclastic limestones with abundant small calcispheres and foraminifers, is lithologically similar to one of the uppermost Lower Frasnian limestone beds from the Locality 68 (Sha68-172-94). This fact suggests that the bioclastic limestones interval separates the Lower and Middle Frasnian units. The lithologically similar bioclastic limestones in the base of the Domanik Stage in the Sher-Nyadejta River reefogenous succession yield conodonts of the Polygnathus timanicus Zone (Menner et al. 1991; Gobanov et al. 1992).
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Fig. 1 Location of the studied area in the Timan-northern Ural region A and locality map of studied outcrops along the Shar’yu River B, Chernyshev Swell: S1 l, Lower Silurian, Llandovery; S1 w, Wenlock; S2, Upper Silurian; D3 fr, Upper Devonian, Frasnian; D3 fm, Famennian; C1 v + s, Lower Carboniferous, Visean + Serpukhovian; C2, Middle Carboniferous; C3 , Upper Carboniferous. The digits above thick black lines show a location of Upper Devonian sections. The digits in a circle show the location of the Upper Devonian sections
Brachiopods, collected from the same section, do not allow yet detailed biostratigraphic subdivision of the Shar’yu microbial mound. Occurrence of the ichthyolith Srunius rolandi (Gross) in sample Sha86-171-94 indicates that these strata are of Middle Frasnian age. According to Ivanov (1999: Table 3), the chondrichthyan Protacrodus cf. vestus found in sample Sha86-145-94 is characteristic of Upper Frasnian deposits in the Russian Arctic. The Frasnian-Famennian boundary in the section studied is located in the middle part of the carbonate massif, in the interval between the uppermost occurrence of Palmatolepis gigas (in sample Sha86-145-94) and the lowermost of P. triangularis (in sample Sha 86-126-
94; Antoshkina 2000). The same conodont taxa define the Frasnian-Famennian boundary in the Sher-Nyadejta River section (Ovnatanova et al. 1991). The conodont Apatognathus sp. is typical for the Upper Famennian in the SherNyadejta River reefogenous deposits as well (Ovnatanova et al. 1991). Occurrence of the foraminifer Quasiendothyra sp. (in sample Sha86-79-94) dates the uppermost strata of the Shar’yu River microbial mound as Upper Famennian. The foraminifer Quasiendothyra communis is typical for the uppermost Famennian limestones in the Chernyshev Swell, including the reefogenous successions (Pershina 1962; Eliseev 1963). The upper contact of the microbial mound is not exposed. In the Shar’yu River succession,
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stratigraphical unconformity occurs between the Upper Devonian and Lower Carboniferous (Middle-Upper Visean) limestones (Eliseev 1963; Tsyganko et al. 1985). A study of the carbonate massif revealed the presence of Palaeomicrocodium in the microbial boundstones (Antoshkina 2000, 2002). In addition, spheres resembling problematic Palaeomicrocodium aggregates were also found in packstones near the Frasnian-Famennian boundary in the Subpolar Urals, at the Syv’yu River section (Yudina et al. 2002). This taxon was first recognised and described as microcodiaceans by Mamet and Roux (1983: 95) in the Upper Devonian reefal breccias from the Bonaparte Gulf Basin of Australia. It was originally described as a cluster of spherical and some other microscopical structures of the genus Microcodium. According to Mamet and Roux (1983), Palaeomicrocodium is the most ancient taxon of the genus Microcodium but the environmental conditions in which they existed were similar. Microcodium, characterised by specific calcite grains of organic origin, was first recognised and described by Gluck (1912). He attributed it to the green (siphonaceous) algae. Maslov (1967, 1973) recorded the taxon in the lagoonal and freshwater deposits of the Permian and Tertiary age. He considered it to be blue-green algae of an uncertain systematic position characterised by atypical forms of calcification. Klappa (1981), who studied Microcodium from the Tertiary and Pleistocene caliche and recent soils, proved it to be the result of calcification of symbiotic associations between soil fungi and cortical cells of higher plant roots. According to Kosir (2004), the widespread occurrence of Microcodium in almost unaltered shallow marine limestones indicates that its formation took place during early stages of palaeosol development, probably reflecting specific types of vascular plants of a pioneer community that were able to colonise carbonate substrates during the early phases of subaerial exposure. The origin of Microcodium is of great palaeoenvironmental importance: its occurrence could be regarded as a criterion for the recognition of palaeosols and subaerial reworking of the deposits. In this paper a detailed sedimentological analysis of the Shar’yu microbial mound is presented the first time. The purpose of this study is to demonstrate that Palaeomicrocodium, a distinctive structure, closely related to Microcodium, can be used as a palaeoenvironmental indicator but also in the regional correlations.
Geological setting The Chernyshev Swell is a narrow, slightly raised plateau located in the eastern part of the Timan-northern Urals region (Fig. 1). It is more than 400 km long and up to 40 km wide in the central part, and is composed of the strongly folded Upper Ordovician to Jurassic rocks that are subdivided into three structural stages obviously separated by stratigraphical, sometimes angular unconformities. The structural stages were formed during three periods: the Upper Ordovician-Carboniferous preorogenic phase, the Permian-Triassic orogenic phase, and the MesozoicCenozoic postorogenic phase (Timonin 1998). The Shar’yu River is a tributary of the Usa River that cuts the Chernyshev Swell in its central part. The Upper Devonian deposits in the Chernyshev Swell are represented by shallowwater platform and reef-like carbonates (carbonate massifs), by bituminous sediments (with siliceous, terrigenous and carbonate components) of the Domanik facies formed in starved basin conditions and by terrigenous carbonate deposits interpreted as depression basin fills (Pershina 1962; Tsyganko et al. 1985). The succession of the rock complex is also exposed in the Shar’yu River. The carbonate massif studied is exposed on the right bank of the Shar’yu River, close to the mouth of the Durnoj Creek (Fig. 1B, Locality 86). The section is more than 2 km long. About 300 m downstream from the mouth of the Durnoj Creek (Locality 72 in Fig. 1B), about 8 km to the south along the Shar’yu River (Locality 64) and about 3–4 km to the west (Locality 89), the same stratigraphical interval is represented by basinal facies. The Frasnian (62 m thick in the Locality 64, Fig. 1B) and lower Famennian (90 m thickness in Locality 62) are represented by limestones, marls, argillites, siliceous and dark bituminous shales, bedded cherts, limestones with lenses of argillite. They pass to the Middle-Upper Famennian (up to 530 m in thickness in Locality 62) clinoform carbonate units: limestones, clayey limestones, marls and argillite with nodules of cherts (Tsyganko et al. 1985). The Shar’yu organic structure seems to have been formed on an isolated shallow-water bank surrounded by bituminous carbonate-siliceous-terrigenous sediments of basinal origin. Microbial mound composition
Materials and methods Samples for this study were collected from the Upper Devonian carbonate massif at the Shar’yu River located in the central part of the Chernyshev Swell. Samples (124) were collected for the sedimentological and palaeontological analysis from the 400-m-thick Shar’yu reef-like succession. The interval between samples varied from 1.2 to 28.5 m.
The carbonate massif is mainly exposed in the forest and in whole, its strata dip at an angle 30–40◦ to the north-east. We can only approximately estimate the dimensions of the massif. It can be traced more than 2 km along the Shar’yu River valley and approximately about two kilometres also along the right bank of the Durnoj Creek (Fig. 1B, Locality 86). Poor exposure of the massif made it difficult to tell whether it was a circular mound or an elongated swell.
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General framework In general, this reef-like carbonate massif shows variations in lithofacies expressed by vertical and lateral alternations of massive microbial and mat-like stromatolite boundstones, massively bedded fenestral limestones, microbial and skeletal packstones/grainstones. They include poorly sorted bioclastic material, pelsparite and clotted, dense micrites. In places, the reefal rocks are strongly recrystallised. The vertical zonation recognised in the Shar’yu microbial mound apparently reflects successive steps in its development, leading to progressive changes in lithology and fossil associations (Fig. 2). No lithological evidence of a sharp environmental shift during the growth of the Shar’yu organic structure has been found. However, microfacies study proved that changes in the composition of the massif recognised were controlled by sea-level fluctuations, of both eustatic and tectonic origin. Similar eustatic and tectonic control was shown by House et al. (2000) on an example of the Frasnian reefs in the Southern Timan and by Belyaeva et al. (2002) on the Middle Frasnian-Tournaisian reefal complex of the Timan-Pechora Province. The poorly sorted fine-grained mud matrix and the dominance of not rounded particles suggest the prevalence of quiet water conditions during formation of these strata, probably below the fair-weather wave base. This conclusion is supported by the lack of a proper talus around the organic structure in the adjacent areas and by in situ filamentous species such as Epiphyton and Shariuphyton. Nine major microfacies recording a shallowing upward trend of the sea during the growth of the massif were recognised. They are briefly characterized below. A. Bioclastic-peloidal packstone/grainstone This type of rock occurs together with peloidal-micritic grainstones and microbial boundstones (Fig. 3F). Abraded skeletal fragments dominate, with sparry and recrystallised micrites as calcite cement. The rock also contains unsorted (0.2–6 mm in size) fragments of brachiopods, ostracodes, crinoids, charophytes, Girvanella nodules, microbial lumps/peloids, locally abundant calcispheres, and some intraclasts of microbial boundstone, wackestone and micritised fossils. B. Brachiopod coquina with micritic layers Within the microbial boundstones lenses and accumulations of brachiopod debris locally occur. The large lenses (from 10–15 cm up to 1.5–2 m in length and from 2–3 cm up to 10–25 cm in thickness) contain brachiopods (Plectorhynchonella, Junnaenellina, Cyrtospirifer, Chonetipustula, and Plicatifera) and echinoderm remains in places. These lenses occur in thin micritic limestone layers in
the uppermost part of the mound. Micritic limestones contain scattered brachiopod shells as well. Brachiopod fragments usually show a subparallel orientation. Rare sponge spicules, styliolinids, ostracodes, and charophyte fragments occur. Also some crinoid stem fragments, up to 4–8 cm in length, have been found together with the brachiopod shells. C. Microbial boundstone A characteristic feature of the microbial boundstone is a massive or cryptobedding preservation style with thicknesses of beds ranging from 0.6 m up to several metres (Figs. 3A, 4B, C). The limestones consist mainly of calcareous microbes or cyanobacteria such as Renalcis, Girvanella, Izhella, Shuguria, Epiphyton, Sphaerocodium, Chabakovia, rare Ortonella, Nuia, Shariuphyton, of solenoporids, and also contain rare calcispheres/radiolarians, foraminifers, and charophytes. Fragments of ostracodes, gastropods, brachiopods, unidentified bioclastic material and calcareous ooids are mainly scattered but sometimes form small “lenses” within the boundstone. The Renalcis-Izhella-Shuguria-Epiphyton association is abundant throughout the mound, but in the Famennian part of the build-up it forms biostrome-like beds up to 0.6– 0.9 m in thickness. In general shape they are similar to the Famennian microbial biostromes described by Menner et al. (1991, Fig. 3B) on the Sher-Nyadejta River. D. Stromatolite boundstone The stromatolite boundstone fabric is characterised by curvilinear structures (Figs. 3B, 5F). These structures show distinct margins and sometimes are in direct contact with the massive microbial boundstones and other microfacies. The stromatolite fabric consists of individual microbial laminae that are crinkle or wavy. Cement comprises of multiple individual layers (generations) of calcite that are accentuated by thin dark films and formed various overgrowths on the sediments. Calcified microbes and cyanobacteria (Rothpletzella, Renalcis, and Girvanella) interspersed with microbial laminae, are associated with rare skeletal remains (ostracodes, foraminifers and calcispheres) and form stromatolitic encrustations on peloidal debris. As the laminae do not form closed circles around the peloidal debris, the microbial encrustation did not result from the coalescence of individual columns or “heads”, but rather grew as a rough undulate surface characterised by mounds or swellings and depressions or embayments. They have a basic geometric form comparable to hemispheres measuring 1–15 cm in height and 60–80 cm in width. Rare bioherm-like bodies of stromatolite boundstone, up to 80 cm in height and 1–1.5 m in width, could form mats extending about 10 m.
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Fig. 2 Succession of facies, and semi-quantitative distribution of fossils in section 86 (see Fig. 1B for location) on the Shar’yu River (FWWB fair-weather wave base)
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E. Fenestral-clotted and peloidal limestone
G. Carbonate pseudobreccia
The matrix of these limestones mainly consists of clotted micrite (microbial) with locally preserved cell moulds with abundant calcispheres (50 µm in diameter), filled with sparry calcite cement (Fig. 3D, E). The cell moulds comprise of poorly-preserved wispy or micritic tubules, which may represent calcimicrobial taxa (such as Rothpletzella), but bioclasts are small in size and have a scattered distribution. Fenestrae with curved or flat bases and with irregular, ragged tops are common. In the Famennian part of the build-up, one of the most distinct sedimentary structures is the fenestral fabric with calcispheres. Study of similar limestones from the Upper Devonian mound of south-western Siberia (from the Nyurol’ Depression) show that most of the calcispheres and several of the structures earlier identified as foraminifers were re-interpreted as radiolarians (Sedaeva and Vishnevskaya 2000, 2001; Vishnevskaya and Sedaeva 2002). Many of the radiolarians were subjected to bioerosion as a result of calcimicrobial activity on them. The process often resulted in a complete micritisation of the radiolarians so that their remains are preserved as “calcispheres”. Our studies also have shown that many calcispheres in the microfacies studied have to be regarded as radiolarians (Fig. 3E).
Limestone or dolomite replacements of limestone grains occur as autoclasts/intraclasts that are generally poorly sorted with angular to subangular shape and surrounded by sparry cement (Figs. 4A, E, G, 5E). Sometimes similarity of the close located fragments outlines is distinctly visible. Angular intraclasts were produced by the desiccation or dehydratation and mechanical erosion of lithified microbial rocks within the intertidal and supratidal zones.
F. Peloidal and microbial wackestone/packstone/grainstone Non-skeletal or peloidal material is an important component in most of the microbial mound rocks, although they are rarely identifiable in the field (Fig. 3C). Micritic lumps have a wide range in diameter (from 0.3 up to 15 mm) and they are the most abundant components in the limestones. Some of the peloids have a calcimicrobial matrix. The peloids are characterized by poor “sorting,” irregular shapes and diverse sizes. Coats of microbial micrite and structureless micrite on grains or peloids are locally common. During diagenesis, the original outline of recrystallized skeletons got lost except for the micrite coat. Calcispheres and/or radiolarians, foraminifers, charophytes (Umbella), and sometimes oolites are common components in the fine-grained peloidal and micritic sediments.
Fig. 3 Photomicrographs of middle Frasnian-Famennian lime-
stones. A Microbial boundstone constructed by Izhella-RenalcisEpiphyton association (Microfacies C), sample Sha86-103-94. B Stromatolite boundstone formed by species Rothpletzella and Girvanella (Microfacies D), sample Sha86-129-94. C Peloidal grainstone with large fragment of a recrystallized calcisponge (Microfacies F), sample Sha86-115-94. D Fenestral clotted micrite containing calcispheres-radiolarians (Microfacies E), sample Sha86-127-94. E Radiolarians micritized by calcimicrobes (Microfacies E), sample Sha86-94-94. F Bioclastic-peloidal micritic grainstone composed of crinoid ossicles, thin ostracode shells, and small calcispheres or forams (Microfacies A), sample Sha86-115-95. Scale bar for A–D and F is 1 mm
H. Dolomite after microfacies F, E, G, and unidentified sediments Mottled dolosparites and dolomicrites are characterized by traces of primary textures, high porosity, and in some parts by strong weathering of rock features. I. Brecciated limestone with in situ Palaeomicrocodium The most characteristic feature of these various limestones is destratification and brecciation, high porosity of rocks, and the presence of leaching cavities (Figs. 4B–D, 5A– D, F). Breccias with fine- and medium-sized dark microbial boundstone intraclasts encrusted by a white fibrous calcite form lens-like beds and pockets (Fig. 5D). Microscopically, the limestones with Palaeomicrocodium have an extremely heterogeneous structure where the following components are recognized: microbial boundstones; breccias formed by fine- and medium-sized boundstone fragments (Fig. 5E); variably oriented fractures and fenestral cavities filled with sparry calcite. We can see a fragment of this texture in Fig. 4C where the primary microbial boundstone structure was converted to the complex structural aggregate due to appearance and disappearance of fractures. Sometimes enlarged fenestral porosity, which was probably caused by vadose(?) freshwater dissolution (Fig. 3D, F), and vadose-like silt (Fig. 4F) can be recognized. Palaeomicrocodium aggregates in brecciated rocks appear as broad rosettes (0.1–0.3 mm in diameter) and “flowerlike” branches (from 2 up to 5.2 mm in length) that show clear traces of an internal non-calcified central cavity. But relatively more elongated cavities may sometimes be filled by micrite (Figs. 4C, 5A–D). The cross section morphology of the spherical bodies, and their arrangement in clusters, form and outline of segments and large central cavities are compared to Palaeomicrocodium from the Frasnian reef in the Bonaparte Gulf Basin, Australia (Mamet and Roux 1983: pls. 8/18–19, 9/1–10) and from the Lower Cambrian microbial-archaeocyathan build-ups of Italy (Cherchi et al. 1997: pl. 3/4–7). Environmental implications Changes of the depositional environments represented by the above-mentioned microfacies are shown in Fig. 2.
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The subenvironments deduced from the microfacies can be grouped into two macroenvironments: submarine and subaerial. The first is characterized by the prevalence of subtidal low or middle energy conditions, and the second, essentially by the development of supratidal environments. The palaeoecological analysis of the submarine microfacies reveals anomalous marine conditions during the deposition. Only restricted faunal associations of cyanobacteria, small calcispheres and foraminifers were able to exist. Yudina et al. (2002) suggest that the prolonged environmental stress in the NE-Laurussian shelf habitats is mostly ascribed to the increased oxygen deficiency and/or unbalanced nutrient dynamics in the disturbed greenhouse climatic and active synsedimentary tectonic setting. Active tectonics in the region during Late Devonian is indicated by montmorillonite of volcanic origin found in the phtanites of the Domanik facies (Merts et al. 1990). The Shar’yu organic build-up communities were dominated by various cyanobacteria associated with radiolarians, foraminifers, ostracodes, gastropodes and brachiopods (Antoshkina 2000). According to Sapelnikov et al. (1999), the Renalcis-Epiphyton Community (microfacies C) belongs to the Benthic Assemblage 3 of Boucot (1975). The environment was quiet, evidently below and also protected from the fair-weather wave base level. Microfacies B, D, E but also F-I formed in similar environments. The microfacies A indicate low-energy conditions on the flank of the organic structure. Microfacies I, with corncob-like structure of Palaeomicrocodium, could possibly represent periods of non-deposition because they show signs of brecciation, microkarstification (Fig. 4D), vadose(?) freshwater leaching (Fig. 5G), and cementation (Fig. 5D). According to Martin-Chivelet and Gimenez (1992), the presence of charophyte remains in microfacies suggests admixture of freshwater. Fig. 4 Photomicrographs and polished slabs of middle Frasnian-
Famennian limestones, and details of microfacies G–I. A Polished slab of carbonate pseudobreccia. Angular microbial and peloidal wackestone intraclasts surrounded by calcite that result from diagenetic process. There are Palaeomicrocodium aggregates here (Microfacies G, I), sample Sha86-107-94. B Polished slab of the brecciated microbial boundstone with Palaeomicrocodium (Microfacies I). Degree of reworking increase upward in the layer: from primary fabric constructed by Renalcis-Shuguria-Epiphyton-Izhella association (light colonies) to a very heterogenic structure with secondary porosity. Probably three main processes took place: cracking, produced by successive wetting and drying of the subaerally exposed microbial boundstone; pedogenic microkarst, and microdissolution, which produced enlarged pores and can be influenced by texture (cracks, matrix-grain contacts, primary porosity see in C), sample Sha86166-94. C Photomicrograph of detail from B. Palaeomicrocodium (P) occur as various “flower-like” branches. D Hand sample of the microbial boundstone with Palaeomicrocodium showing carbonate breccias in the top, interpreted as microkarst breccia (Microfacies I), sample Sha86-166a-94. E Detail of A showing formation of pseudobreccia by newly cementation followed by cracking of primary deposit (Microfacies G). F Vadose-like silt in carbonate pseudobreccia (Microfacies I), sample Sha86-117-94. G Pseudobreccia containing clotted and peloidal intraclasts, whose outlines though separated, would fit into each other like the pieces of a jigsaw puzzle (Microfacies G), sample Sha86-94-94. Scale bar for A–B and D is 1 cm, for C and E–G is 1 mm
Non-skeletal or peloidal material is an important component of the above-mentioned microfacies. One of the most distinct sedimentary structures is the fenestral fabric with calcispheres and foraminifers. But as it appeared most of the calcispheres have to be considered as remains of radiolarians (Vishnevskaya and Sedaeva 2002). Many of the radiolarians were affected by biocorrosion as a result of calcimicrobe activity on them that result in complete micritization of the radiolarians in that their remains are present as spheres only. Walls of the initially siliceous skeletons became incrusted by micrite calcite which transformed into poly-drusy calcite crystals, monocrystals or aggregates towards the centres of cavities. A gradual upward increase in the degree of micritization is a general characteristic of the Shar’yu Famennian reef-like limestones. A matching biosiliceous acme near the FrasnianFamennian boundary is seen in the sections in the TimanPechora Basin (Afanasieva 2000). On the basis of the study of the Palliser Formation (Famennian) of western Canada, Peterh¨ansel and Pratt (2001) demonstrated that the evident increase of bioeroders during the Late Devonian is directly connected with a rise in the nutrient availability on the carbonate platform. Black shale (Domanik facies) episodes probably reflect eutrophical events in the Late Devonian epicratonic sea. According to Racki (1999), there is evidence for wide development of siliceous biota within benthos communities during the Frasnian-Famennian extinction. Siliceous communities, adapted to more eutrophic conditions, have thrived in the stressed niches. Such biosiliceous signal during the major biotic crises is mostly explained as result of a large-scale increase in volcano-hydrothermal activity during plate-boundary re-arrangements. A biosiliceous acme is a worldwide phenomenon in the Frasnian-Famennian interval, known primarily from Laurussian shelves (Racki 1999). Afanasieva (2000) concluded that in Late Devonian diversity of zooplankton reached its highest peak in Palaeozoic. There is no evidence of submarine incrustation cement and reefal talus which might suggest to existence of a waveresistant rigid reef framework with a large primary porosity. The communities of small, attached, vagile and infaunal benthic organisms with the sessile Receptaculites (e.g., in the Sher-Nyadejta River reefogenous rocks according to Gobanov et al. 1992) suggest that the organic frame formed in a quiet, fully oxygenated environment within the photic zone. These communities occupied the carbonate bank slopes with increased nutrient supply where they were shielded from the destructive wave action and skeletal debris flows. It is possible that the top of the mound reached the intertidal/supratidal environments at times. Some rocks of microfacies G and I show indications of microkarst dissolution (Fig. 4D), brecciation (Figs. 4A–E, 5E), freshwater dissolution (Fig. 5G), occurrence of incipient vadose-like pisoids, vadose-like silt (Fig. 4E) and prismatic calcite cement (Figs. 4A, 5D).
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Cycles of sedimentation The transition between microfacies demonstrated in Fig. 2 reflects repeated shallowing to a subaerial environment during microbial mound growth. The regressive phases are identified by the appearance of microfacies G to I. Periodic exposure of the microbial mound top has determined cyclic reoccurrence of these microfacies. Eight transgressive-regressive cycles separated by brecciated horizons (Fig. 6) are recognized. Cycle I consists of microfacies A, C, E, C and I where A (lowermost unit) is interpreted as flank microfacies, C (dominating) – as microbial bioherms or biostromes, E – as lagoonal sediments and I – as the offshore bar’s exposed surface (Fig. 6). Palaeomicrocodium aggregates, the common constituent of microfacies I, are thought to represent specific calcification, or probable cyanobacterial filaments reminiscent of the living cyanophyte Rivularia (Monty 1995) or, like Microcodium, that aggregate around terrestrial plant rootlets in association with fungi (Klappa 1981) or a precipitation of calcium carbonate within the root cortical cells (Kosir 2004). Terrestrial plants might grow in a supratidal or probably an intertidal environment. The sinuous longitudinal sections of central cavity of Palaeomicrocodium aggregates are clearly visible in Fig. 5A–D. The transgressive-regressive cycle II contains units assigned to intercalating microfacies C (dominating), E, D, and G, F (in the upper part). This cycle is characterized by widespread lagoonal deposits and is terminated by an interval of brecciated stromatolite boundstone with Palaeomicrocodium (Fig. 5F). The transgressive-regressive cycle III differs from others by consisting only microfacies H and I. The dolostones of microfacies H of the cycle may probably correspond to the Frasnian-Famennian boundary level (grey zone in Fig. 6) characterized by a major regional regression, and by the development of evaporites below it in the Timan-Pechora Basin (House et al. 2000; Menner and Shuvalova 2002). Microfacies I correspond to the uppermost dolostone unit of this cycle and is characterized by a widespread and wellexpressed large porosity. The next transgressive-regressive cycle (IV) represents mostly microfacies E, F with rare C, G. In general, it formed in shallower (mainly lagoonal) environments compared to the underlying cycles. Fig. 5 Photomicrographs of microfacies I: A–C Palaeomicrocodium
devonicum Mamet and Roux (Microfacies I). A Palaeomicrocodium (P) and Renalcis (R), sample Sha86-90-94; B sample Sha86-16694; C sample Sha86-122-94. D Detail of F showing laminated cement fringe around of a void. These laminations were produced by alternation of radial sparite and thinner micritic layers. Palaeomicrocodium (P) is located close to the void wall. E Circumgranule porosity originated by dessication and fenestrae (left) – by dissolution (Microfacies I), sample Sha86-122-94. F Stromatolite boundstone constructed by Rothpletzella-Girvanella-Renalcis association with Palaeomicrocodium (P) and larger fenestral porosity that could form in subaerial environments by vadose(?) freshwater leaching (Microfacies I); sample Sha86-141-94. G Detail of A. Leaching of shells (Microfacies I). Scale bar is 1 mm for all images
The upper transgressive-regressive cycles V, VI, VII contain units assigned to the microfacies accordingly: cycle V – D, E, F, C, F, G, cycle VI – H, D, F, C, F, D, C, E, and cycle VII – C, E, D, F, E, C that clearly indicate frequent changes in the microbial mound environments. The increasing importance of microfacies E and F in upper cycles results from the development of low-energy lagoonal environments. As noted above, the fine-grained microbial peloids and biocorrosion of bioclasts (radiolarians, crinoids, ostracodes, and brachiopods) imply that microbial organisms could be active bioeroders. The bioerosion process contributed to the formation of peloids by degrading carbonate debris in the mound. The uppermost depositional cycle VIII evidently lacks topmost units. It consists of alternating microfacies E, F, B and A. The accumulation of various brachiopods in lenses containing lime mud indicates that the skeletal material was concentrated episodically as winnowed lags. Cycle VIII deposits share many similarities in composition and texture with other cycles of the mound, particularly in the abundance of peloids. However, the presence of more diverse brachiopods, fragments of the problematic hydroid, Fistulella, and thin layers of micrite suggest a slightly different environmental setting. Lithological and palaeontological attributes of the uppermost beds of the cycle VIII indicate that the topmost sediments of the mound accumulated in flank environments. They reflect the final stage of the microbial mound growth. The microbial mound studied represents a series of largescale transgressive-regressive cycles (T-R) reflecting the lagoonward migration of both biohermal and flanking sediments. A notable feature in the development of the organic structure or reef ecosystem is its repeated establishment in subaerial environments, marking transgressive pulses between the cycles. The subaerial exposure beds, marked by brecciated horizons (Fig. 6), are briefly characterized below. Subaerial exposure indications Seven levels distinguished as the tops of the T-R cycles in subaerial exposure environments through the Shar’yu carbonate succession are recognized on the basis of macroand microlithological features. The interpretation of these horizons is based on the following indicators: 1. field shape as porous rocks (up to 60 cm thickness) with a variation in the weathering colour, reflecting a change in lithology (Fig. 4B). Sometimes these coloured rocks are traced laterally in outcrop approximately up to 10 m, 2. extremely heterogeneous structure with creaking, pedogenic-like microkarst and unsorted breccias (Fig. 4B–E), 3. association of carbonate pseudobreccias and microkarst structures as lens-like and pocket-like cave filling (Fig. 4A, D), 4. brecciated horizons have probably undergone a vadose meteoric leaching and diagenesis; the presence of cav-
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Fig. 6 Lithostratigraphical column, location of samples, inferred relative sea level and stratigraphic interpretations for the Middle FrasnianFamennian microbial mound in the Shar’yu River section. Sharp but nearly conformable sea-level drop surfaces (rocks of microfacies I) separate transgressive-regressive cycles I–VIII. Calibration of relative sea-level curve is based on the microfacies analysis. Note the grey zone and a dotted curve corresponding to the Frasnian-Famennian boundary interval based on conodont data. Key explanation: A Bioclasticpeloidal packstone/grainstone, B Brachiopod coquina, C Microbial boundstone, D Stromatolite boundstone, E Fenestral-clotted and peloidal limestone, F Peloidal and microbial wackestone/packstone/grainstone, G Carbonate pseudobreccia, H Dolomite and unidentified sediments, I Brecciated limestone with Palaeomicrocodium
ities filled with internal sediments and vadose-like silt (Figs. 4F, 5D, G), 5. Palaeomicrocodium aggregates, the important constituent of microfacies I only, are probably specific calcification structures in these environments (Figs. 4A, C, 5A–D, F).
The above-mentioned criteria are characteristic of supratidal and lagoonal facies following the conclusions of Bathurst (1980), Cox et al. (1989) and Cherchi et al. (1997) as probable subaerially emerged environments. As Palaeomicrocodium is a calcified organism, specific calcification of filaments might occur in desert crust or caves where the cyanobacteria are living in a subaerial en-
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vironment (Cox et al. 1989) and in shallow hypersaline environments, which might be exposed to periods of emersion or freshwater influx (Golubic 1983). The development of microfacies I suggests supratidal/intertidal environments. Possibly there is evidence of meteoric cementation in some biocomponents (Figs. 4G, 5E, G). These observations may be best explained by a complex cementation process: initial submarine cementation was followed by subaerial exposure of the partially lithified crust (Figs. 4B–D, 5D, F). According to Bathurst (1980), meteoric leaching and calcite cementation took place as a minor event, possibly associated with a seaward migration of a freshwater lens followed by the retreating sea. This could occur prior to the next transgression and renewed microbial mound growth. As Skaug et al. (1982) have shown in the Lower Permian bioherms, horizons with Microcodium represent local subaerial exposures of the top of a bioclastic barrier bar sequence. Similar microfacies with Palaeomicrocodium (indicated as 4B and 4C) have been described by Casier et al. (2002: 230) from the Upper Devonian section at Psie G´orki. Most of the sediments of their microfacies 4A and 4B have probably undergone a vadose meteoric diagenesis. As the main features of these microfacies they mark erosional discontinuities, keyvug structures and irregular cavities associated with the oblique stratification and internal sediments (vadose silt). Discussion Sedimentological analysis of the Middle FrasnianFamennian microbial mound in the Chernyshev Swell section has been difficult to carry out because of the great homogeneity of the carbonate rocks. Most of the cycles recognized basically consist of poorly stratified limestones with undetermined microfacies changes. Cycle III in reality probably includes two smaller cycles which cannot be separated due to the intensive dolomitization of its lower part. Probably this indicates that the basin was characterized by a lower sea level in the Early Famennian time (Zhemchugova 2000). Clusters of spheres of the Palaeomicrocodium are attached to different substrates: microbial boundstones, stromatolite crusts, carbonate pseudobreccias and peloidalcarbonate grainstones. Probably, palisade-like aggregates of Palaeomicrocodium appeared as thin crusts on slightly lithified sediments during the exposure phases. Similar boundstones with Palaeomicrocodium occur in the Middle Devonian limestones of the Trois-Fontaines Formation, Dinant Basin of Belgium, where they were associated with the top of the “first biostrome” (Mamet and Preat 1992). The uppermost part of these rocks was affected by subaerial diagenesis and vadose cementation (Mamet and Preat 1985). Associations of Palaeomicrocodium with Renalcis and Epiphyton have been found in the Lower Cambrian stromatolitic boundstones of SW Sardinia (Cherchi et al. 1997). The Planu Sartu South section rocks with Palaeomicrocodium are characterized by segmented fab-
rics, occurrence of boring traces and vadose pisoids. A similar association of Palaeomicrocodium with the cyanobacteria Renalcis, Izhella, Chabakovia, and Shuguria occurs in the Shar’yu River microbial mound and in the Frasnian mud mound of Belgium (Boulvain and Coen-Aubert 1992; Mamet and Boulvain 1992). In their global review of the Frasnian mud mounds of Belgium, Boulvain and Coen-Aubert (1992) described in detail many sections of the mounds. They recognized several levels with Palaeomicrocodium in the studied mound successions at Beauchateau (up to seven levels), Hautmont (three levels) and Huccorgne (two levels). Although many researchers have studied Belgium mud mounds, special sedimentological analysis of the rocks with Palaeomicrocodium has not been carried out. Now Gouwy and Bultynk (2000), using the graphic correlation method for ten complete Frasnian sections in the Belgian Ardennes, have recognized four major third-order (500,000–5 million years in duration) transgression-regression cycles in this time interval. The base of the first cycle lies in the Givetian. The global Frasnian transgressive-regressive cycles of Johnson et al. (1985) show three major transgressive pulses in the interval from the latest Givetian up to the Late Frasnian which can be correlated with the first, the second and the fourth transgressive pulses recognized in the Ardennes. As these cycles were recognized also in some areas of Euramerican palaeocontinent (Gouwy and Bultynk 2000), they could possibly be considered as eustatic. Study of the Middle Frasnian-Tournaisian oil and gasbearing complex in the Timan-Pechora Province shows 24 breaks in the evolution of shelf margin reefs and shallowwater shelf sedimentation (Belyaeva et al. 2002). They were subdivided into three groups: tectonic (one break of Late Famennian-pre-Visean), eustatic (five breaks in the Middle Frasnian-Famennian) and accumulative (16 beaks in Middle Frasnian-Famennian). The absence of more complete published information about these levels does not allow a correct comparison of these breaks with Shar’yu mound cycles. According to House et al. (2000, Fig. 11), the southern Timan Middle-Late Frasnian sea-level curve, based on the distribution of conodonts and ammonoid faunas, shows two large-scale cycles corresponding to the Domanik-Vetlasyan (Middle Frasnian) and Syrachoy-Livny (Upper Frasnian) stages. They correlate well with the cycles I and II of the Shar’yu mound succession. Comparison of the relative sea-level curve deduced for the Shar’yu microbial mound on the base of the microfacies analysis with the sea-level curve of the Moscow Syneclise (Alekseev et al. 1996) shows a general correlation between major Late Devonian transgression and regression events. As was suggested by Alekseev et al. (1996, Fig. 9), the Devonian sea-level curve by Johnson et al. (1985) is in good accordance with the curve for the Moscow Syneclise. It is possible that some differences of the curve for the Shar’yu microbial mound (Fig. 6) result from the location of this mound at a shallow-water bank within the basin. Thus, occurrence of the studied Palaeomicrocodium from the Shar’yu River Upper Devonian microbial mound de-
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posits in the topmost parts of the cycles ( = subaerial exposure levels) give an opportunity to use it as a possible indicator of sea-level changes. Relative water depth and transgressive-regressive cycles can also be identified based on lithological changes. Microbial mound formed during a relative sea-level rise. The carbonate productivity by the microbial builders was generally adequate to keep pace with the rising sea level. On the other hand, brecciation and microkarstification in the topmost beds of the cycles indicate to subaerial exposure of the microbial mound, taking place during the sea-level fall. Revealing seven levels of brecciated rocks with Palaeomicrocodium in the Middle Frasnian-Famennian microbial mound indicates to, at least, seven sea-level changes during its formation. At the end of each cycle the top of the mound reached the supratidal environment. Lime sediments deposited on a shallow-water bank required only a small decrease in the sea level to be subaerially exposed, to be infiltrated by meteoric waters and to be affected by vadose conditions. As a result of these processes the boundstone structures of the mound were brecciated and the growth of the organic build-ups stopped. Such fluctuations were one of the negative factors that finally terminated the formation of the Late Devonian reef ecosystems at a microbial mound phase. To conclude, seven transgressive-regressive cycles have been recognized in the studied section. Although the Upper Devonian reef-like deposits have been recognized in different tectonic and sedimentation settings, their regional character suggests an allocyclic origin. Thus, they could be related to large-amplitude changes in the relative sea level. This hypothesis is supported by good correlation between the depositional succession recognized in the Shar’yu mound and of the global eustatic curve of Johnson et al. (1985). Therefore, the correlation of various organic structures with Palaeomicrocodium could help in the establishment of important regional tectonic events. It is possible that detailed study of better exposed and more complete organic successions will provide researchers such a chance. Acknowledgments I would like to express my thanks to Prof. Pierre Bultynck, Drs. Philippe Lapointe and Grzegorz Racki for their helpful advice about new information in this field. I am especially grateful to Dr. Peep M¨annik for linguistic help and also Dr. Vadim Shuysky (recently deceased), Drs. Boris Chuvashov, Svetlana Remisova, Alexandra Yudina, Alexander Ivanov, and Tat’yana Beznosova, for assistance in determining the cyanobacteria, foraminifers, conodonts, ichthyoliths and brachiopods. I am grateful to Gennadij Semenov, Petr Yukhtanov and Irina Rudakova for considerable help with photos and computer graphics. Many thanks to Drs. F. Boulvain and S. Kershaw (reviewers) for their constructive critics and useful comments, and Prof. Dr. A. Freiwald for editing of the manuscript. This paper is a contribution to the IGCP 509 Project.
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