RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6003, doi:10.2205/2005ES000183, 2005
[19] Until recently microfossils were almost unknown in thoroughly studied Neoproterozoic rocks of the Sayany zone [Dol'nik, 2000; Dubin et al., 1969; Khomentovsky et al., 1972] (Figures 1 and 2). Our research showed that the sediments of the region yield acritarchs that were previously found in a lot of Late Precambrian "microbiotas".
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Figure 3 |
[21] The analysis of sedimentation textures and composition of siltstones and sandstones revealed that three studied sections represent fragments of a lateral succession of the Ipsit shelf environments. In this series two groups of environments are recognized. Sections 1 and 2 (Figure 3) are characteristic of the first group. Section 2 demonstrates the formation of an infra- and supratidal carbonate platform and the following transgression with deposition of laminite siltstones in the proximal inner shelf part. The deeper shelf environment is represented by low-angle cross-bedded sandstones and siltstones of Section 1 that were deposited in the zone between wave bases of calm weather and storms. The immature composition of clastic material, from feldspathic-quartz to arkose, and slightly rounded fragments indicate a prevailing erosion of granitoid massifs lacking a developed waste mantle.
[22] Silty sandstones of Section 3 (Figure 3) represent the second environmental group. In the Mount Krasivaya scar one can observe numerous tempestite layers composed of fine-grained sandstones, mainly 10 cm to 100 cm thick, which form composite beds up to 250 cm thick and sets with the thickness up to 10-15 m. Tempestites are characterized by a typical for storm beds succession of textures and by lumpy cross-lamination [Dott and Bourgeois, 1982; Walker and Plint, 1992]. As a whole the sedimentologic analysis of the lower subformation of the Ipsit Formation permits the inference about deposition of mainly proximal storm deposits on a gentle slope and rare penetration of inner shelf environments in the zone. The clastic component of silt- and sandstones is represented by feldspar and, scarcely, quartz, which indicates a prevailing erosion of syenite massifs.
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Plate 1 |
[24] Microfossil depositions associated with the carboniferous-ferruginous layers of Sections 1 and 2, were studied in slides derived from siltstone and argillite beds (Plate 1, figs. 1-3). The most numerous small rounded or rhomboidal forms number first hundreds of specimens. Tens of Octoedryxium div. sp. specimens of size 14 mm to 24 mm are observed on a slide; in rare cases certain species can be identified among them (Plate 1, figs. 3-7). Some forms are characterized by a reticulate inner structure and radial morphology. Rounded, thick-walled specimens of Margominuscula rugosa Naum. em. Jank or Retiforma sp. are also present (Plate 1, fig. 25). Judging from red color of many forms or their inner elements, the polymerous envelopes contain ferric hydroxides. It should be noted that the inner structure of microfossils is more difficult to recognize in thin sections than in macerated preparations, owing to a background superimposition of mineral particles.
[25] The organogenic material of the second group from Section 3 is represented in both preparations and slides by a pellicular detritus and similar acritarch composition (Plate 1, Sample 130/6, 8). Though Leiosphaeridia Eis. em. Downie et Sar. forms of size 60 m m to 450 m m (Plate 1, figs. 28-30, 33-37, 40) number in the first hundreds of specimens, they constitute no more than 5% of coalificated detritus that possesses a similar with acritarchs degree of textural degradation. The same ratio is observed in slides, where similarly preserved thallus fragments occur as well (Plate 1, fig. 32). The plant detritus bearing microfossils is associated with dark siltstone and argillite layers of 3 mm to 15 mm thick reflecting the episodes of poststorm calming. This sediment represents a concentrated deposition of plant remains stirred up from more shallow parts of the shelf. The conservation of organogenic material resulted from subsequent poststorm deposition of a fine silt and pelite, almost barren of organic matter.
[26] Under an electron microscope the piercing of the degraded specimen of Leiosphaeridia div. sp. by pyrite crystals of likely syngenetic origin is observed (Plate 1, fig. 29). In the slides one can see the transfixion of specimens by rock fragments, which explains the presence of holes in some organic-walled forms. For instance, in the form referred to Leiosphaeridia aff. kulgunica Jank. (Plate 1, fig. 34) the hole produced by pressing in of a 60- mm quartz grain can be taken on examination in preparations for a biologically important character [Jankauskas et al., 1989].
[27] Small microfossil specimens occur in the redeposited detritus (Plate 1, fig. 38). On examination under electron microscope the interesting remains were recorded that differ from known microfossils (Plate 1, fig. 39). They represent an outgrowth composed of tubes with well-pronounced stomas. The outgrowth is connected by another end to desintegrated detrital matrix that includes acritarchs Leiosphaeridia div. sp.
[28] As is seen, the two studied groups of environments are associated with different acritarch assemblages. The records mentioned above and previous investigations [Nemerov and Stanevich, 2001; Stanevich and Faizulina, 1992; Stanevich et al., 1999] permit the inference on the acritarch nature. Among the first group the forms of Centrum Stan., Floris Stan. and Octoedryxium Rud. can be referred to allochthonous taxa, which is confirmed by their random arrangement in relation to other forms and to inner textures of the layers. Orthorhombic outline of Octoedryxium Rud. can result from the crystallization of captured sulphur and its subsequent diffusion from cells of aerobic sulfur bacteria in the diagenesis [Stanevich et al., 1999]. This suggestion agrees well with their occurrence in the subaerial carbonate platform zone, where they likely conducted a benthic mode of life. Morphological features of star-shaped Floris Stan. and zonal Centrum Stan. indicate their more likely planktonic mode of existence. These layers also yield small volumetric forms of Bavlinella Shep. and others, which in contrast to the above-mentioned microfossils are clearly associated with lenses and spots of carbonaceous matter. Their attribution to the saprophytic benthic bacteria in situ is confirmed by numerous examinations of such forms in slides of carbonaceous silty pelites barren of other taxa from the Valyukhta, Goloustnaya, Dzhalagun, Kachergat, and other SBFS formations [Nemerov and Stanevich, 2001; Stanevich and Faizulina, 1992].
[29] The allochthonous character of acritarchs of the second group is beyond question. Their specimens are completely flattened, corroded and, like plant detritus, are at final stages of degradation. There is no alternative yet to the evident enough conclusion on correspondence of Leiosphaeridia Eis. forms to reproductive organs of brown algae [Kir'yanov, 1986]. It is believed that their destructed forms and algae detritus were transported from shallow areas to the deeper zone of tempestite deposition. Further destruction manifested not only in small coccoid forms related to bacteria, but in evolution of more advanced organisms, occurred there. The tubular forms "growing" from the organogenic mass of allochthonous origin likely represent remains of benthic red algae. Compared to other algae, they are mostly characterized by a parasitic mode of life, rapid adaptation to changing environmental conditions, and by distribution at greater depths [Vasser et al., 1989]. There is likely a fragment of overtopped thallome growing on the transferred algae remains and rapidly buried under silty deposit.
[30] The recognized two ecobiological acritarch groups, despite their somewhat debatable biological interpretation, include relics of different superior taxa of natural classifications. However, the morphological and paleoecologic differences of the forms only indirectly manifest their natural taxonomic position. The first group includes microfossils of various morphology, which are unified by such features as volumetric envelopes and the presence of inner characters. Their dimensions range from 5 mm to 20 mm, at times reaching 44 mm. The second group of microfossils is almost monogeneric and contains forms of mean size from 60 m m to 450 m m. In the Ipsit Formation they are mainly characterized by crumpled and flattened specimens and a high degree of destruction of them and associated plant matrix. The latter also contains volumetric forms of bottom bacteria destructors, similar to small microfossils of the first group.
[31] Thus a distinct difference between acritarchs of the two groups, which in this context can be explained only by their biological nature, is manifested in the dissimilar tolerance to the common factors of syngenesis and initial metamorphism. This difference indirectly confirms the suggested attribution of acritarchs to various taxa of algae and bacteria.
Citation: 2005), Precambrian microfossil-characterized biotopes from the southern margin of the Siberian craton, Russ. J. Earth Sci., 7, ES6003, doi:10.2205/2005ES000183.
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