ISSN 00244902, Lithology and Mineral Resources, 2014, Vol. 49, No. 6, pp. 461–472. © Pleiades Publishing, Inc., 2014. Original Russian Text © A.I. Antoshkina, E.S. Ponomarenko, N.A. Kaneva, 2014, published in Litologiya i Poleznye Iskopaemye, 2014, No. 6, pp. 493–505.
Fenestral Limestones: Specific Features of Late Devonian Seas in the Timan–Northern Ural Region A. I. Antoshkina, E. S. Ponomarenko, and N. A. Kaneva Institute of Geology, Komi Scientific Center, Ural Branch, Russian Academy of Sciences, ul. Pervomaiskaya 54, Syktyvkar, 167982 Russia email:
[email protected] Received May 21, 2013
Abstract—The article is dedicated to the lithological–paleoecological analysis of Upper Devonian fenestral limestones from different areas of the Timan–northern Ural region, which reflect paleogeographic settings from the coastal zone to the marginal part of the shelf. It is established that their main structural elements are represented by fenestrae, peloids, calcispheres, and occasional microzoo and phytobenthos remains. The presence of a dark micritic envelope on calcispheres is their specific textural feature. The envelope of calci spheres represents mineralized vegetable mucus, which reflects their planktonic type of dwelling. These struc tures include representatives of radiolarian skeletons, Charophyceae and Chlorophyceae algae, and foramin iferal tests. The trophic system consists of five levels. Carbonate sediments accumulated in relatively shallow water and lagoonal settings occupied mostly by tidal and shallowwater subtidal microbial mats. In these lagoons, intermittently limited circulation stimulated the formation of anoxic conditions in bottom waters. Development of anoxic conditions in the bottom water layer, sulfate reduction, and freshwater influence determined the formation of a specific paleocoenosis of fenestral limestones, which could not form biogenic frameworks. DOI: 10.1134/S0024490214060029
Fenestral limestones are present in reeflike sequences of different world regions and various ages: Late DevonianTurnaisian, Late Permian, Mesozoic (Flügel, 2004). The term “fenestral fabrics” was first used to characterize Upper Triassic supratidal lime stones (Tebbutt et al., 1965) with abundant voids (fenestrae), which formed different types of lamina tion. They were defined in limestones with “open space structures” (Wolf, 1965), which resulted from the partial or complete filling of fenestrae with sedi ments and/or cement. In the Paleozoic section of the Timan–northern Ural region, fenestral limestones occur among Upper Devonian reeflike strata. Inas much as these rocks contain abundant remains of cal cisphere organisms, the taxonomic affinity of which is still debatable, such limestones were first termed as spheroidal (Maksimova, 1977) and spheromicrite patterned (Shuiskii, 1981). The Famennian carbon ates of such type include showings of oil in the Khor eiver Depression of the Timan–Pechora Province. In this connection, their investigation represents both scientific and practical interest as potential reservoir for hydrocarbons. In is believed that calcispheres can not be used for interpretation of paleoecological envi ronments in paleobasins. The fenestral limestones of the region under study contain abundant clacispheres of variable morphology and size. The analysis of all the
available data on these organisms, the lithological composition of these limestones, and their paleo coenotic structures was aimed at defining criteria for paleocological reconstructions of the Late Devonian marine basins. It is conceivable that these criteria may be used for reconstructing paleoenvironments in other basins, which include fenestral limestones. MATERIALS AND METHODS In total, over 200 samples and thin sections were investigated. The samples were obtained from differ ent areas: South Timan (Sed’yu River), Khoreiver Depression (boreholes of the Oshkotyn (OSH), Vos tochnaya Kolva (VK), and Central Khoreiver (CKH) areas), Chernyshev Swell (Shar’yu River), and North ern Urals (Podcherem and Un’ya rivers). They reflect paleofacies profile across the coastal zone, middle shelf, and its marginal part. Such a profile provides opportunity to investigate the taxonomic composition of clacispheres in different parts of the Late Devonian basin and reconstruct structures of their paleo coenoses. For reconstructing depositional environ ments, we have analyzed the composition of carbon and oxygen isotopes and organic matter in carbonate rocks. Analytical investigations were conducted at the Institute of Geology (Komi Science Center, Ural
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Branch, Russian Academy of Sciences). Carbon and oxygen isotope compositions were determined with a Delta V Advantage mass spectrometer by the Contin uous Flow method using a Gas Bench II equipment for the preliminary sample treatment. The δ13С values are given in promille units relative to the PDB standard; the δ18O values, relative to the SMOW standard relative to the international standard NBS 19 (TS limestone). Error in the determination of δ13C and δ18O values did not exceed ±0.1‰ (1σ). Samples for the analysis were extracted by very thin drills from rock areas least sub jected to secondary alterations: recrystallization, dolomitization, and leaching. Spectra of combina tional light scattering for defining the structure of organic matter were recorded with a Horiba Jobin Yvon LabRam HR800 mass spectrometer. Before and after etching, the samples were examined under the analytical scanning microscope ISM6400 combined with Xray spectral microprobe. In the first case, we obtained information on the presence of different mineral phases in limestones. In the second case, tex tural relationships were determined. LITHOLOGICAL–GEOCHEMICAL FEATURE OF FENESTRAL LIMESTONES The Late Devonian stage in development of the Timan–northern Ural sedimentary basin was charac terized by the formation of thick (300–600 m or more) organogenic constructions composed of alternating lenticular thickbedded microbial and stromatolite boundstones, ooid, lumpy, and fenestral limestones and their dolomitic varieties (Antoshkina, 2003). The distinctly horizontal intercalations of Upper Devo nian fenestral limestones 0.6–1.5 m thickness are rep resented by light gray rocks with differentscale bed ded structure due to the presence of light fine to coarsetextured patterned of sparite calcite (Fig. 1). These rocks are vaguely expressed among massive ree fal rocks, although they are more abundant in the bed ded backreef sections. The situation, when fenestral limestones form no distinct intercalations, is exempli fied by the massive middle FrasnianFamennian reefal rocks cropping out along the Shar’yu River (Chernyshev Swell). In this section, the fenestral vari eties are recognizable only under the microscope and characterized by the matrix mostly consisting of lumpy pelitomorphic microbial calcite (Antoshkina, 2006). In the Frasnian–Tournaisian reefal sequence cropping out at the Sher Nyadeita River in the north ern part of the Chernyshev Swell, fenestral limestones form distinct layers confined to its Famennian inter val, where they alternate with ooid, bioclastic, and Amphipora limestones and saccharoidal dolomites (Menner et al., 1991). The wide distribution of such rock associations in the Famennian is thought to be connected with the general cease of reef formation, extension of the carbonate bank, and exposure of its inner lagoon facies. The upper Frasnian reefal thick
observable in large rocky outcrops at the Sed’yu River in the South Timan is largely composed of stroma tolitic and stromatolitelike (Porostromata and Spon giostromata) secondary dolostones with sporadically distributed remains of gastropods, bivalves, brachio pods, rare Rugosa corals, stromatoporoids, and Amphiphora (Ponomarenko and Kaneva, 2012). Fenestral and ooid limestones alternating with bio clastic ones represent the main lithotypes in coeval shallowwater deposits. The biohermshaped bodies up to 2.5 m thickness cropping out above massive rocks along the Sed’yu River are composed of fenestral, intr aclastic–peloid, and coniatoid–intraclastic lime stones without of biohermal textures. The main textural components in fenestral lime stones are represented by peloids, calcispheres, calcar eous algae, cyanobacteria, and sporadic fragments and whole microbenthic fossils (Fig. 2). Peloids (0.05–0.80 mm in size) constituting up to onethird of the rock volume are in fact bio chemogenic or purely biogenic structures. By their origin, they may be polygenic: fecal structures repre senting pelitomorphic lumps and clots, calcimicro bial, and microbial. The presence of peloids, which frequently enclose calcispheres, within fenestrae pro vides grounds for their assignment to intraclasts, prob ably, related to the dissolution of walls during the for mation of fenestrae in the pelitomorphic sediment. Different forms of peloids may represent the micri tized organic remains. The thin sections demonstrate successive stages of the transformation of skeletal remains into peloids owing to the biocorrosion activity of microorganisms (Figs. 2a, 2b, 2d). The presence of bioclastic material within microbial clots or lumps may indicate the decisive role of cyanobacteria and bacteria in producing the pelitomorphic calcareous muddy substrate. The formation of microbial films around any particles of the muddy substrate allows them to retain their primary shapes. The matrix of peloid material in fenestral limestones of the Shar’yu River section consists mostly of lumpy pelitomorphic microbial calcite with locally preserved fragments of celluar structure (Antoshkina, 2006). Calcispheres representing spherical biogenic struc tures from
