RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6003, doi:10.2205/2005ES000183, 2005

The Patomskoe Zone

2005ES000183-fig04
Figure 4
[32]  The sediments in this area, as that in the former, represent various near-continental shelf environments. The Chencha Formation is assigned to the upper part of the Zhuya regional horizon that is overlain by basal beds of the Judoma Horizon correlated with the Vendian [Khomentovsky, 1985]. An uninterrupted, 75-m-thick fragment of the upper part of the Chencha Formation, bearing microfossils, is located on the left bank of the Bol'shaya Chuya River (Figure 4). It is composed of clayey-silty sediments characterized by thin rhythmic bedding and bearing thin limestone layers. The rocks of the Chencha Formation are conformably overlain by quartz gravel- and sandstones of the Zherba Formation of the Judoma Horizon. In the northeastward direction the thickness of the Chencha Formation significantly increases and it mainly contains stromatolite and microphytolithic limestones interbedded with siltstones, marls, and fine-grained quartz sandstones. West of the Bol'shaya Chuya River the Chencha Formation slightly increases in thickness and in its upper portion the carbonaceous sediments replacing upwards shallow stromatolite limestones, indicate the stagnation in this part of the basin. It can be inferred that the major part of the formation was deposited on the inner-shelf carbonate platform [Khabarov, 1999]. The upper Chencha beds in the section on the Bol'shaya Chuya River represent shallow-water conditions of significantly siliciclastic, passive sedimentation that resulted likely from a transverse ledge of the shelf that separated areas of stromatolite constructions in the early Chencha time. In the later Chencha period this ledge separated stromatolite carbonates of the shelf platform in the east from carbonaceous clayey limestones that were deposited westward in a local shelf trough. This depression most likely represented an apical fragment of the deep part of the pre-Vendian foreland basin characterized by a stagnant carbon-producing biolithogenesis [Nemerov and Stanevich, 2001]. Thus the upper Chencha beds in the Bol'shaya Chuya River section were deposited in an upper sublittoral environment with a passive sedimentation regime. The background deposition of silt and organogenic microfossil-bearing layers occurred within a likely isolated by barrier reefs part of the shelf, without a significant influence of storms and intense currents.

[33]  New genera and species from the Chencha Formation were described previously [Stanevich, 1986; Jankauskas et al., 1989]. Subsequently that and additionally available material was studied more thoroughly. The revision revealed a group of forms that, according to morphology and mean size of 7  m m to 35  m m, are comparable to certain members of modern green algae. Almost all preparations from 18 samples contain hundreds of forms that can be assigned to several acritarch genera and species.

2005ES000183-pla02
Plate 2
[34]  Among the Chencha acritarchs Dictyotidium minor Stan. was distinguished (Plate 2, figs. 1-6); in [Stanevich, 1986]: Plate 2, figs. 4-7); subsequently, independently of the author, it was published again within the new genus Dictyotidia [Jankauskas et al., 1989]. The occurrence of forms of Dictyotidium Eis. emend Stapl. in the Precambrian sections was soon confirmed by the description of D. fullerene Butt. from organogenic carbonates of Spitsbergen, synchronous with the Chencha Formation [Butterfield et al., 1994]. The investigation of Dictyotidium minor Stan. under an electron microscope revealed the features indicating its similarity to modern coenobial green algae Pediastrum boryanum (Turp.) Menegh. This species, along with P. kuwraiskyi Schmidle, was discovered among fossil remains and is characterized by a coenobial structure of 4 to 128 and over cells. The cells are differentiated into outer and inner ones, commonly closely accreted by all sides [Tsarenko, 1990; Van den Hoek et al., 1995; Vasser et al., 1989]. The outer cells are grooved, with two non-branched, slightly narrowed to the top shoots, fitted with long, narrow appendages in the coenobium plane. The illustrated on the photo ribs or nodes of D. minor (Plate 2, fig. 3) are most likely the accreted appendages formed by outgrowths of the cell envelope.

2005ES000183-fig05
Figure 5
[35]  The Chencha "microbiota", along with forms of D. minor, includes rounded and tetrahedral Tchuja and Centrum Stan., the forms morphologically transitional between them, their clusters, and other acritarchs (Figure 5; Plate 2). According to their characters, these forms and especially Centrum Stan. (Plate 2, figs. 14-17, 22) are similar to the modern widespread cosmopolitan species Tetraëdron minimum (A. Br.) Hansg. and Chlorotetraëdron fitridens (Berk - Mannag.) Kom. et Kovac. (Chlorococcales) that are characterized by solitary cells with tetrahedral and polyhedral (quadrangular or hexagonal) outline and straight or concave sides. The envelopes can be three-layered. The asexual reproduction is by way of autospores. Considering the fact that cells of modern T. minimum are flattened, Centrum Stan. is most similar to C. fitridens. Cells of the modern Chlorococcales that are related to acritarchs, possess a wide specific polymorphism [Gorlenko, 1981; Van den Hoek et al., 1995; Vasser et al., 1989]. The Chencha assemblage yields morphotypes with characters of both Tchuja and Centrum Stan. (Plate 2, figs. 19-21). In their specimens the different types of zonal layers and morphologically transitional forms are clearly observed. A thick-walled envelope that often occurs in these forms and that is a generic character for acritarchs Retiforma Mikh. [Mikhailova and Podkovyrov, 1987], is characteristic of aplanospores or akinetes, for instance, of modern Tetrasporales [Gorlenko, 1981; Vasser et al., 1989]. The inner coccoid morphology of both Tchuja zonalis Stan. (Plate 2, fig. 11) and Retiforma tolparica Mikh. (Figure 5) can be explained by the presence of primordial autospores, which is characteristic of autosporangiums of green algae. They are similar in morphology (Plate 2, figs. 8-11) to autosporangiums of modern Chlorella Beijer. According to certain records [Kalina and Puncochárová, 1987], Chlorella includes species with a single-layered envelope missing sporopollenin, whereas the members of Coelastrella Näg. of the same family possess sporopollenin in their envelopes. The forms of the latter genus can produce up to 16 autospores inside a maternal envelope, grow up, and initiate next generation. Considering a high probability of convergent characters in fossil remains, there is another interpretation; namely, the coccoid forms inside acritarch envelopes can represent bacteria cells that posthumously (or symbiotically) replace the alga protoplast in syngenesis [Stanevich, 2003].

[36]  The records on chemical composition of modern Chlorococcales are discrepant and incomplete. It is known that apart from cellulose they include other polysaccharides, pectin, and sporopollenin [Andrejeva, 1998]. The latter represents a resistant polymeric carotenoid incorporated into the envelopes of spores, pollen grains, and acritarchs [Bruck and Show, 1973; Martin, 1993]. With the presence of cellulose, a multi-layered envelope can be formed [Andrejeva, 1998], which is an additional evidence for comparison of Tchuja and Centrum Stan. with the Chlorococcales members. Their zonal lamellar structure can be also explained by the occurrence of maternal envelopes around filial cells. In certain modern species the envelopes can become mucous, diffused, and filial cells are found to be enclosed in common mucilage. Similar structures are conceivably represented in some acritarch morphotypes (Plate 2, figs. 24 and 30). One can commonly observe their elongated and two-layered envelopes surrounded by mucilage of various density (Plate 2, figs. 13, 18, 19, 23-27). They are considered as forms in the cytokinesis stage or can represent the analogues of uniform or local bulges of cell envelopes, occasionally forming bladder-like outgrowths, characteristic of aged algae in some Chlorococcales members.

[37]  A combination of diversity and at the same time morphological similarity of forms of the Chencha taphocoenosis indicates their affiliation to a single assemblage. Judging from their morphology and by analogy with modern specimens, they likely conducted a planktonic mode of life. Deposition of microfossils in the layers resulted from the absence of intense hydrodynamics during the sediment deposition. Planktonic forms were likely associated with a macrophytic stage of algae life (Plate 2, fig. 28), that does not exclude a facultative benthic existence [Van den Hoek et al., 1995; Vasser et al., 1989]. Modern members of Chlorococcales include unicellular, colonial, and coenobial forms with alternating sexual and asexual reproduction [Gorlenko, 1981; Van den Hoek et al., 1995; Vasser et al., 1989]. The morphological varieties of acritarchs represent a morphotype series reflecting different stages of life cycle of certain modern green algae. Figure 5 shows the alternation of generations. The reproductive process is isogamic. The generated gametes merge together forming a zygote, which after a rest stage produces zoospores that turn into multangular polyhedral cells. Within the polyhedrons, subsequent to their germination, the new coenobiums are formed. Preservation of zoospores in the sediments, especially in the Precambrian, is unlikely owing to the chemical composition of their envelopes that are of glycoprotein nature. Zoospores of certain Chlorococcales species are characterized by the absence of cell envelopes [Andrejeva, 1998]. In modern algae, on deterioration of habitat conditions, the more resistant aplanospores are generated instead of zoospores. They were most likely retained in fossil remains along with the coenobial and other reproductive cells.

[38]  It should be noted that in modern members of Chlorococcales the sexual reproduction is comparatively rare. However, we presume that in the Chencha microbiota this type of reproduction, namely, its heterogamic variety, was much more common. This inference is confirmed by numerous findings of elongated and isometric forms consisting of two and over cells. The cytokinesis stage presumably represented by them is more preferential for the formation of mucous pellicle that includes numerous cells and makes up the plankton matrix of green algae. The forms or depositions of acritarchs of different outline enclosed in a less dense, veil-like matter, correspond to that stage. The matter most likely represented a mucilage composed of resistant polymers.

[39]  Thus the correlation of major acritarch morphotypes from the Chencha Formation with reproductive stages of modern green algae is quite acceptable owing to a distinct morphological similarity and peculiarities of the envelopes' chemical composition. Their biological interpretation conforms with the reconstruction of hydrodynamically passive and photically favorable environment of the "microbiota" locality. The discussed material yields trichome fragments of oscillatorias (Plate 2, fig. 29) that likely represent remains of stromatolite-forming cyanobacteria assemblages inhabiting shallower parts of the shelf carbonate platform.

2005ES000183-fig06
Figure 6

The Baikal-Muya Zone. Muya Region

[40]  In the Muya region (Figure 6) the sedimentary volcanogenic rocks distinguished as the Kelyana Subseries (sequence), were referred to either Lower Proterozoic [Salop, 1964] or Lower Riphean [Bulgatov, 1983; Mitrofanov, 1978]. The first found microfossil assemblage that is partially discussed in the paper, included the forms known in the region only from the Zhuya beds underlying the sediments of the Vendian Judoma Horizon [Stanevich and Zheleznyakov, 1990]. Subsequently the Neoproterozoic age of the Kelyana sequence was confirmed by a number of trustworthy radiologic datings [Rytsk et al., 1999, 2000, 2001]. The establishment of relative synchrony of geologic bodies from different zones of the outer and inner belts of SBFS permitted the reconstruction of Neoproterozoic geodynamic environments in the area [Nemerov and Stanevich, 2001; Stanevich and Perelyaev, 1997]. The sedimentary volcanogenic rocks in the Muya region (Figure 6) contain rich microphytologic remains, only partially reported [Stanevich and Faizulina, 1992]. The acritarch assemblages are mainly represented by forms examined solely in the sediments of SBFS. In the paper we discuss most thoroughly the morphology, habitat conditions, and inferred nature of Floris Stan., which forms retain their volume and original flowery morphology in fossil remains.

[41]  Long-term studies of stratigraphic units in the Baikal-Muya zone mainly revealed their geological [Bulgatov, 1983; Salop, 1964] and geodynamic [Bozhko et al., 1999; Gusev et al., 1992; Konnikov et al., 1994; Levitskii and Odintsova, 1986; Stanevich and Perelyaev, 1997] peculiarities. A debated character of such questions as the units' range, their chronological succession, and correlation within the region, results significantly from the folded-faulted nature of sequences and the occurrence of thrust-nappe structures [Kovalenko et al., 1995; Stanevich and Perelyaev, 1997]. Typical structures are the fragments of isocline flanks complicated by upthrow strike-slip faults. A consideration of repeated occurrence of section fragments in these structures and tracing of certain lithocomplexes in the studied area allowed the elucidation of the most probable range of the units. Unfortunately, the results of the known [Bulgatov, 1983; Salop, 1964] and later [Stanevich and Faizulina, 1992; Stanevich and Perelyaev, 1997] stratigraphic research were mainly not considered in the development of the new legend for geological maps of the Muya region [Rytsk et al., 2001]. For instance, its stratigraphic succession lacks a number of described in the literature units (Chayangro, Dzhalagun, Uryakh, etc.) and it is not improbable that the corresponding rocks are represented under other names. This conclusively complicated the understanding of as it is many-varied framework of the Precambrian stratigraphy in the Muya region. Therefore, we use the stratigraphic units based on the sections with established structure and reconstructed succession of beds and range (Figure 6), which to a greater extent meets certain standard requirements (Stratigraphic Code. 2nd, supplemented edition, 1992). The exception is the Kelyana sequence (subseries); its sedimentary volcanogenic rocks up to now lack a type description.

[42]  The most complete sections representing microfossil habitats are recovered on the Bolshoi Yakor River (Figure 6, Section 1) and in the low reaches of the Kelyana River (Section 2). The sedimentary volcanogenic deposits of the Yakor and Ust'-Kelyana sequences are an element of island-arc structural sedimentation complex and were deposited in a marine marginal paleobasin. The Yakor section is characterized by an alternation of thick rhythm-members and beds bearing pyroclastic material of various size. Thin-bedded and massive vitroclastic tuffites, sandstones, and siltstones occur at the base of the 70-m-thick sequence. The heterogranular sandstones and siltstones grade into tuffites. Their clastic material is represented by extrusive fragments, tuffs, crystals of feldspar, pyroxene, quartz, and by volcanic glass splinters. The silty pelitic, pelitic, or ash-clayey, mainly porous, cement includes sericite, secondary quartz secretions, and carbonaceous matter. Carbonate interbeds are represented by chlorite-bearing limestones and calcareous argillites (calcilutites) with a slightly flaser structure owing to a small admixture of carbonaceous matter. Pure dark limestones associated with silty argillites with a carbonaceous content reaching up to several percent and bearing the greatest number of microfossils, are less common.

[43]  The middle, 600-m-thick part of the Yakor sequence is composed of tuffites, tuff gravelstones, and tuffs with a slight admixture of epiclastic material. The tuffites are characterized by a combined composition, presence of angular extrusive fragments and of their phenocrysts. The tuffs are composed of angular, poorly sorted fragments of andesites, andesite-dacites, and dacites with various textures, glass phenocrysts, and matrix; they are homogeneous and lack baking traces. The sediments are characterized by different sedimentary structures, namely, various types of cross- and parallel bedding, normal and reversed graded bedding, and slump folds, that indicate a deposition from turbidity currents and influence of submarine flows. The upper, 200-m-thick part of the discussed section is composed of pelitic, rarely silty psammitic, coarsely platy, black, acidic ashstones, bearing thin terrigenous interbeds. The sequence includes thin metabasalt bodies and rhyodacite sills.

[44]  A similar composition and character of deposits is observed in the section of the Kelyana River low reaches (Figure 6, Section 2). Certain peculiarities of this section referred to the Ust'-Kelyana sequence significantly supplement the characteristics of the island-arc sedimentogenesis. The middle part of the section bears an association of lenslike sandy dolomites and quartz sandstones. The upper part of the section represents a transgressive rhythm, when with a decreasing upward grade of clastic material, the tuffaceous admixture declines and the content of carbonaceous matter grows. The upper member of the Ust'-Kelyana sequence is represented by dark to black carbonaceous silty pelitic shale bearing insignificant content of acidic volcanite fragments of silty psammitic size.

[45]  Thus the above-reported properties of sediments correspond well to backarc basin environments [Reding, 1990; Stanevich and Perelyaev, 1997]. They are characterized by asymmetric and irregular distribution of various facies, contrasting differential depths, different content of volcanogenic material, turbidity currents, and slump processes. The backarc-basin sediments, owing to a reworking in subduction zones, are commonly retained only in mantle fragments [Reding, 1990]. The analogous tectonic situation occurs in the described areas [Konnikov et al., 1994; Stanevich and Perelyaev, 1997].

[46]  The deposits were produced by volcanoes of the paleoisland arc and were deposited as a volcanoclastic apron at its foot. The matter of subaerial and/or subaqueous eruptions arrived in the basin in the form of turbidity currents and by the way of deposition through water mass (ash fall), subsequently undergoing repeated redeposition. Clayey and carbonate facies were deposited either below the distal zones of volcanoclastic aprons or during sedimentation pauses between the explosion and clastic material inputs. The latter is most likely for the Kelyana River section. Whereas the carbonaceous microfossil-bearing silty argillites from the Bolshoi Yakor section were formed synchronously with volcanic activity but were deposited at a significant depth almost without a volcanoclastic input, in the Kelyana River section we recorded several different paleoenvironments. It is firstly a sufficiently shallow association of quartz sands and dolomites. Taking into account the findings of stratiform stromatolites in the Kelyana River upper reaches, we most likely deal there with barrier reef fragments of an offshore stripe of the island arc. The underlying thin-bedded silty argillites contain a rich microfossil assemblage. The upper beds of the Ust'-Kelyana sequence also bearing microfossils, represent another environment. It was a comparatively deep zone, where in stagnant conditions the carbonaceous biogenic siltstones bearing an explosion admixture, were formed.

[47]  Thus it is seen that the initial filling of the marginal paleobasin with sediments has come about from different sources, at the background of slight volcanic activity, and resulted in deposition of pelagic clayey, to some extent carbonaceous and calcareous sediments bearing a volcanic ash admixture. The deposition of carbonaceous matter likely occurred in relatively deep zones, where terminal grades of turbidity currents were deposited, as well as in shelf troughs in stagnant conditions of terrigenous-biogenic sedimentation. Taking into account a variable microfossil composition in different environments, we infer that in the described microfossil localities we deal with a heterogeneous assemblage bearing both autochthonous and allochthonous forms.

[48]  We processed 141 sample from terrigenous sediments of the Muya region, 90 of which contained microfossils referred to over 70 species and intergeneric, mainly acritarch taxa [Stanevich and Faizulina, 1992]. All of them, except for Leiosphaeridia Eis., em. Downie et Sar., possess volumetric envelopes and therefore most likely retained the primary characters. The most interesting morphology is that of Floris Stan. [Stanevich and Zheleznyakov, 1990].

2005ES000183-fig09
Plate 3
[49]  Forms of Floris Stan. bearing large outgrowths (Plate 3, figs. 4-13) resemble spherical crystal druses, leading to a suggestion [Golovenok and Belova, 1995] on their mineral nature (fluorite - CaF2, sellaite - MgF2 ). A questionable character of this inference is evident from the analysis of maceration process. Additionally, these acritarchs were observed in various and obviously noncrystal-like forms ( Floris sp. (ad lib. Retiforma sp., etc.)). For comparison we can offer an example of the long-known acritarchs Octoedrixium Rud. (Plate 3, fig. 16) that possess a distinct crystallographic outline and to a greater extent can be assigned to "abiogenic" forms. It should be noted that formation of crystals in the course of colloid particles' interaction during maceration can take place in certain circumstances. However, such processes need a separate study and description. The discussed Floris forms represent typical organic-walled microfossils, as confirmed by their occurrence in petrographic slides derived from silty argillites in microphytologic samples that bear numerous other forms (Plate 3, figs. 1-3, 6, 7, 10, 14).

[50]  In the Kelyana River section (Figure 6) hundreds of microfossil specimens were studied in Slides 1009/8, 19, 1837/6, 18, 21, derived from carbonaceous siltstone and silty psammitic tuffites with a matrix composed of fine-grained albite-quartz-sericitic aggregate penetrated by carbonaceous matter. The content of the latter reaches 5% to 8% of a slide area. The andesite and andesite-dacite fragments represent an obvious explosion component.

[51]  In the Bolshoi Yakor River section (Figure 6) the microfossil-bearing residue was derived from foliated silty argillites. The study of Slides 182/20-1, 2, 3, and 1067/6 revealed that matrix consists of a fine-grained aggregate of sericite, chlorite, and small (5-40 mm) quartz grains. The relict initial bedding is underlined by lenses and extended spots of carbonaceous matter and by microfossil chains (Plate 3, figs. 1 and 3). Lenses and spots of ferruginous carbonate are common. Single corroded sand-sized fragments of feldspar crystals are recorded. Composition of the shales, considering their lateral grading to indubitable tuffs and tuffites, is determined by explosion (or volcanomictic) admixture of silty-clayey size.

[52]  The carbonaceous admixture is scattered throughout the rock. Most of microfossils are associated with its depositions and clots that underline a discontinuous bedding. Their amount is estimated at one to several thousand forms (0.5-1.5%) for every examined slide. The most numerous are rounded nontransparent corpuscles of size 2-20 mm to 30 mm, conventionally identified as Protosphaeridium div. sp. (the rejected genus, [Jankauskas et al., 1989]). The forms with distinct features of outer and inner morphology, allowing their assignment to certain acritarch genera and species [Jankauskas et al., 1989; Stanevich and Faizulina, 1992] are relatively scarce. Among them are Bavlinella sp., Floris sp., F. cf. radiatus Stan., F. cf. vitimus Stan. et Zhel., Margominuscula rugosa Naum. em. Jank., Pterospermopsimorpha (?) sp., Retiforma sp., Synsphaeridium Eis., and their various depositions and accretions (Plate 3, figs. 2, 14, 20). The ferruginous background of the deposit is underlined by reddish or orange color of some forms that commonly possess a zonal structure.

[53]  In many cases the morphology of Floris Stan. is determined by large outgrowths that commence in the central area of the forms. The biological nature of both these and other remains is a rather questionable issue. The sufficiently peculiar primary characters of Floris and the reconstruction of their burial and habitat environment permit the actuopaleontological comparison and inference about their natural taxonomic affiliation.

[54]  The discussed Neoproterozoic acritarch assemblage of the Muya region, like many others, most likely represents a taphocoenosis composed of remains of different origin. This is witnessed by the occurrence of forms that sharply differ in a set of morphological characters. An additional consideration is the inference about the forms' deposition during sedimentation pauses resulting in the deposition of heterogeneous material and united by a small specific weight. At the same time the biological affinity of acritarchs Floris Stan. is inferred from the morphological unity of specimens and a strong difference between their set of features and that of other forms of the assemblage. The systematic character of the outgrowths' set and outlines in over 100 specimens of Floris indicates a highly probable presedimentation origin of these features. There is one more, in our opinion important factor resulting in a preservation of microfossil primary characters. It is an ability of certain forms to retain their volume and properties during diagenesis and initial stage of greenschist facies of regional metamorphism [Nemerov and Stanevich, 2001; Stanevich, 2003].

[55]  An extreme resistance to pressure is characteristic of a lot of bacteria groups [Kuznetsov et al., 1962]. Therefore, in the first variant of interpretation the Floris forms were compared with bacteria [Nemerov and Stanevich, 2001; Stanevich, 2003]. A similar "flowery" morphology is recorded in cells of modern aerobic gemmated prostecobacteria [Hoult et al., 1997; Schlegel, 1987]. Their occurrence in native basins and chemoorganotrophic nutrition correspond to the conditions characteristic of Floris forms. However, the latter are several times larger than modern bacteria. This may be explained by the participation in metabolic process of biophilous elements incoming during volcanic eruptions, which could result in bacteria gigantism.

[56]  Another interpretation emerges from a comparison of acritarchs Floris Stan. with dinoflagellates, long-known among fossil remains. Their findings were recorded in the sediments from the Silurian up to Cenozoic [Tappan and Loeblich, 1973; Tasch, 1973]. The investigation of fossil dinoflagellates revealed that they were likely the dominating producers in the Paleozoic biosphere [Tasch, 1973]. Their diversity rapidly increased during the Jurassic and Cretaceous, whereupon a number of cystogenous species began to decline [South and Wittique, 1990]. Certain dinoflagellates are retained in the sediments in the form of siliceous endoskeleton, thecae, or cysts morphologically similar to members of the genus Floris Stan. The tracing of diverse dinoflagellates from the Paleozoic and their yet incomplete studies [Tappan, 1980] permit the inference that they likely occurred in the Early Paleozoic [Evitt, 1963; Meien, 1987] and probably in still older sediments. Their fossil remains are known as hystrichospheres and are assigned, along with other cystogenous fossils, to acritarchs [Meien, 1987; South and Wittique, 1990]. Modern dinoflagellates represent mobile unicellular, rarer colonial organisms, with dorsoventral morphology. Their cell envelope is armored, composed of few polygonal shields joined by narrow or deep sutures. Dinoflagellates frequently possess hypnospores, cysts or hypnocysts. Cysts are common for dinophytes and are resistant to unfavorable environment. Some cysts are morphologically almost identical to vegetative cells (for instance, those of Peridinium Ehr. reminiscent of certain Floris members), whereas other are sharply different. The modern members inhabit both fresh-water basins and seas. Fossil dinoflagellates are characteristic of marine sediments [South, 1990; Tasch, 1973]. The Floris forms are derived from marine sediments where they, judging from their morphology, conducted a planktonic mode of life.

[57]  There is no evidence that the Early Paleozoic acritarchs compared with dinoflagellates [Evitt, 1963; Meien, 1987] had a siliceous skeleton. Undoubtedly, the acritarchs are composed of sufficiently resistant cell envelopes that include sporopollenin [Bruck and Show, 1973; Martin, 1993] and are extracted from rock through a rigid processing by acids. The analogous resistance is characteristic of the Floris forms. A periplast, pellicle, and armor (theca), also resistant to acids, refer to cell envelopes of modern dinoflagellates, which have various modifications. A dinophyte theca consists of several layers. Some dinoflagellates possess theca microfibril plates that are located at the cell surface and form peculiar, sometimes fanciful covers with outgrowths. Among modern 10-60- m m-sized dinophytes there are forms lacking a siliceous skeleton. Taking into account a poor notion of ancient algae, diverse morphology and composition of modern dinoflagellates, and the morphological and dimensional correspondence, we can refer the Floris forms to a dinoflagellate group. This inference is confirmed by the fact that the group is known as one of the oldest among algae [Evitt, 1963; Meien, 1987].

[58]  The allochthonous character of the studied microfossil assemblage and distinct morphological difference between its constituent groups indicate their likely different biological nature. The planktonic mode of life of the Floris forms that we compare with dinoflagellates, is supported by their scarce occurrence in slides. Contrastingly, the rounded, simple forms and their depositions numbering in thousands of specimens in a single slide (Plate 3, figs. 1-3, 7, 14, 15, 20), most likely represent a saprophytic bacterial benthos that produced the major part of a sediment carbonaceous component. The forms identified as Bavlinella Schep. (Plate 3, fig. 19) and correlated with sulfate-reducing bacteria [Nemerov and Stanevich, 1; Stanevich, 2003] are referred to the same type. The scarce forms of Octoedryxium Rud. (Plate 3, fig. 16), reasoning from their correspondence to shallow-water aerobic sulfur bacteria [Nemerov and Stanevich, 1; Stanevich, 2003], are most likely the allochthonous remains. As indicated above, most of the assemblage forms retain the volume and characters in a fossil state. However, we also encountered single corroded and crumpled specimens (Plate 3, fig. 21). These remains can be referred to acritarchs Leiosphaeridia Eis., em. Downie et Sar. Diverse members of this genus are well-known from the Late Precambrian and Paleozoic shelf sediments and, according to morphology and size, can be in general defined as eucaryotic algae [Jankauskas et al., 1989].

[59]  The stratigraphic importance of the discussed microfossil assemblage is determined by the range of vertical and lateral distribution of its members. Among known acritarchs the Adara Fomb. forms from the Cambrian of Spain [Fombella, 1977] are the most similar to Floris Stan. The Floris forms are known from the pre-Vendian sediments in the sections of both outer and inner belts of the SBFS (Figures 1 and 2). The same stratigraphic interval distinguished as the Zhuya regional horizon contains species of the Subassemblage IIIa of the Siberian platform [Stanevich and Faizulina, 1992]. In the Muya region the following forms of that subassemblage, noted for the same first occurrence level, were encountered: Bailikania (Trestsh.), Granomarginata sp., Micrhystridium insuetum Trestsh., Paracrassosphaera (Rud. in Trestsh.) (Plate 3, fig. 17), Sibiriella sp., Centrum Stan. (Plate 3, fig. 18), Dictyotidium sp., and Tchuja Stan. Their findings permitted to infer the Neoproterozoic age of the sedimentary volcanogenic rocks in the Muya region. At the same time, the forms of the discussed assemblage are little known outside the SBFS. This circumstance, despite the occurrence of morphologically complicated microfossils in the uppermost Late Riphean, results in only regional stratigraphic significance of the assemblage at the modern stage of investigation.

[60]  The above-reported comparisons based on a vast but heterogeneous complex of records, do not unambiguously define the biological affiliation of the discussed microfossils. However, there is no question that the microfossil assemblage from the Neoproterozoic sediments of the Muya region consists of different groups of microorganisms. Among them the least doubtful is the assignment of simple forms to saprophytic bottom bacteria making up a carbonaceous component of sediments. The rest microfossil specimens that occur in lesser amounts, are characterized by a more complicated morphology, which likely partially reflects their initial structure. This allows their reference to biological objects of different nature and the inference about their allochthonous character. The acritarchs Floris Stan. owing to their relatively large size, can be referred to dinoflagellate remains. A different interpretation, considering the lack of unambiguous markers for such comparisons, seems less preferable. The coincident results of independent research are perhaps a single criterion of correct paleobiological interpretations.


RJES

Citation: Stanevich, A. M., V. K. Nemerov, Yu. K. Sovetov, E. N. Chatta, A. M. Mazukabzov, V. I.  Perelyaev, and T. A. Kornilova (2005), Precambrian microfossil-characterized biotopes from the southern margin of the Siberian craton, Russ. J. Earth Sci., 7, ES6003, doi:10.2205/2005ES000183.

Copyright 2005 by the Russian Journal of Earth Sciences

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