SOME LITHOLOGICAL AND FACIES FEATURES OF EPIPHYTON LIMESTONES OF THE ANASTASYINO SERIES IN THE NORTHWESTERN PART OF THE EASTERN SAYAN

S. S. GUDYMOVICH

(Presented by Prof. Dr. K. V. Radugin)

Information on the structure, lithology, and facies conditions of the formation of epiphyton limestones is available in the works of V. P. Maslov [4, 5], K. B. Korde [3], Arkhangelskaya et al. [1], and I. T. Zhuravleva [2]. These questions have recently been examined in detail by A. F. Senakoslis [8, 9], using as an example the Lower Cambrian limestones of several districts of the Sayano-Altai folded region.

The author studied the epiphyton limestones of the Anastasyino and Ungut formations (Anastasyino Series) of the Late Precambrian–Lower Cambrian* in the middle course of the Mana River (northwestern part of the Eastern Sayan), in the area of the settlements of Bolshoy and Maly Ungut.

The epiphyton limestones of the Anastasyino Series are organogenic structures that may be called biostromes [2]. For the largest bodies the term “biostele” [4] is more appropriate. The dimensions of the latter considerably exceed those indicated by the above-mentioned authors for similar formations. Thus, the massif of epiphyton limestones (Pecherskaya Formation according to K. V. Radugin; the base of the Ungut Formation according to most authors) 1 km below the settlement of Bolshoy Ungut has a thickness of not less than 25 m (the “Baranchik” rock) and a strike length of 2.5 km. The width of the biostele ranges from several tens to several hundreds of meters. There is every reason to suppose—if only delayed sections are not matched by outcrops of other rocks, in particular interbiohermal ones—that the thickness of this biostele will reach several hundreds of meters, with a strike length of up to ten kilometers.

The internal structure of the biostele is fairly uniform: 85–90% of it consists of organogenic limestones, altered to varying degrees (recrystallized, pseudobrecciated, granulated), composed of layers of epiphytons in a vertical or nearly vertical position during life; the spaces between them are filled with material varying in composition and genesis—from pure micro- to fine-grained carbonate, fine-grained carbonate mud; to a significantly lesser degree—

* The question of the age of the formations is not the subject of consideration in this article. In most stratigraphic schemes of the region they are defined as Lower Cambrian, corresponding to the level of the Ust-Kundat horizon. In resolving this question, the author generally follows K. V. Radugin.

clayey–aleurolitic material and peloidal-detrital material, consisting of fragments of epiphytons and, more rarely, proauloporids. The amount of organogenic carbonate filling the branches of the epiphytons, according to the author’s calculations, ranges from a few percent to 40–42 percent and averages 25% of the rock mass.

In the structure of some varieties of limestones, Girvanella take part, forming felt-like accumulations that once covered the bottom, or occurring as disorderly accumulations of varying density. The appearance of Girvanella, as a rule, is accompanied by some increase in the amount of terrigenous admixture and peloidal-detrital material. Only in isolated cases, in individual thin sections, were peloidal-detrital limestones encountered that were devoid of epiphyton thalli buried in life position. A minor role (up to 10%) in the structure of the epiphyton limestones is played by original rocks, usually [1, 8, 9] assigned to the group of syngenetic breccias. Macroscopically these rocks, like the peloidal-detrital ones, are completely indistinguishable from biohermal* limestones. Under the microscope it is evident that they consist of sharply angular, irregularly shaped fragments of these limestones, no more than 1–1.5 cm in size, cemented by the same material that fills the spaces between the thalli in the biohermal limestones.

According to data from detailed sampling** it is evident that these breccias, like the peloidal-detrital limestones, occur as irregularly shaped, rapidly wedging-out lenses no more than 10–20 cm thick. These breccias undoubtedly formed as a result of the washing-out of a just-hardened sediment under the action of environmental agitation, as indicated by A. F. Senakolis [9] with reference to N. M. Strakhov [10]. It is important to note that the transitions from breccias to the over- and underlying biohermal limestones are rapid, but gradual, without any traces of erosion, etc., and also that their appearance is not accompanied by any changes in the character of the biohermal limestones.

Consequently, the appearance of these breccias was connected not with shoaling of the area and its entry into the zone of constantly acting agitation of the environment, i.e., to a depth equal to the first tens of meters—which would inevitably have immediately affected the character of the epiphytons—but with irregular, deepest disturbances (strong storms, earthquakes). The rapid lithification of the sediment noted above, almost simultaneous with deposition, which is a necessary condition for the formation of the breccias described, apparently was a characteristic feature of the formation of epiphyton strata, since epiphytons, as attached organisms that calcified during life, could settle only on a hard substrate.

At every moment of its existence the algal buildup was a carpet of epiphytons situated at a depth of less than several tens of meters, in a zone of almost complete absence of environmental agitation [2]. The author considers the latter condition obligatory for epiphytons. Otherwise, taking into account their lifetime calcification—

* The term “biohermal” is used by the author by tradition. It would be more correct to say “biostromal” or “biostellate,” since epiphytons did not build bioherms.

** Thin sections were selected on a 30–40 cm × 5–10 cm grid over an area of several square meters, chosen on the surface of rocky outcrops of epiphyton limestones at the site near the mouth of the Zherzhul River and in the “Baranchik” rock.

naturally, the delicate branchlets of the epiphytons, having an average thickness of 0.05–0.06 mm at a length of 0.3–0.8 mm, would inevitably have been broken. Meanwhile, the excellent preservation of the laminations and the almost complete absence of detritus were noted above.

Ordinary syngenetic breccias of disruption also take part in the structure of the biostromes of epiphyton limestones; they are clearly distinguished macroscopically in the form of beds and lenses from several centimeters to several meters thick, consisting of unrounded fragments enclosing limestones up to tens of centimeters in size, cemented by clayey-carbonate material. The appearance of such breccias in the section is preceded by changes in the structure of the biothermal limestones, expressed in a decrease in the number and height of epiphyton laminations with a simultaneous increase in the content of terrigenous admixture and mud. Further, the organogenic limestones are replaced by massive, largely recrystallized limestones devoid of organic remains. It is through the destruction of these limestones that the breccias described are formed. Their appearance is thus undoubtedly connected with considerable shoaling of the area, or even with its temporary desiccation.

Sometimes the sharp changes in sedimentation conditions described, up to interruptions, are recorded by the appearance in the section, instead of breccias, of packets of bedded carbonate sandstones, often red-colored, underlain by characteristic fine-clastic limestones, which most likely also have a clastic origin [4].

An interesting feature of the interruptions described is the development of a network of stylolitic seams, appearing already in massive varieties, where they are oriented chaotically and are located at distances of several tens of centimeters from one another. As one approaches the breccias and especially the sandstones, the stylolite network thickens to average distances between seams of a few centimeters and acquires an orientation close to the bedding of the rocks; within the packets of red-colored sandstones and fine-clastic limestones it passes into a system of sutural surfaces, coinciding with the bedding and finer, down to microscopic. Above the boundary of maximum shoaling or interruption, one observes, as it were, a mirror reflection of the system of suture-stylolitic surfaces, gradually disappearing.

Thus, in contrast to the data of A. F. Senakolis [8, 9], who regards periodically changing relationships between the rate of growth of laminations and the amount of incoming mechanical material—formed as a result of the destruction, under the action of surf and waves, of newly formed sediment and of the epiphyton laminations themselves—as the principal regulator of the growth of algal bioherms, the author considers that the exceptional preservation of the laminations and the insignificant amount of mechanical mud indicate that the destruction and covering of the laminations by mechanical sediment played practically no role in the growth process of the epiphyton structures described, except in a small number of cases when an increased amount of mechanical sediment, mainly terrigenous aleuritic material, immediately leads to the suppression of the algae—the reduction of the size of the laminations, curvature of the branchlets, and, finally, rapid cessation of their growth.

In this connection the author believes that a significant amount of carbonate material filling the spaces between the laminations—at any rate, the pure micro-fine-grained carbonate filling the spaces between the branchlets within the laminations and in the immediate

in close proximity to the latter, while in the most “pure” varieties, characterized by mature forms of epiphytons completely filling the interbed spaces, it was formed chemically according to the known scheme of the effect of algal activity on the surrounding environment. The fundamental possibility of such a process is acknowledged by all researchers [4, 10]. The methods of formation and types of chemogenic calcite in modern and ancient reefs have been examined in detail by Newell [6].

The biostrome described is sharply replaced eastward by a sequence of massive light-colored dolomites of the Ungut Formation, which then extend far to the east, upstream along the Mana River. The direct contact of the sequences in the “Baranchik” cliff, which sharply cuts the bedding, is secondary, caused by epigenetic dolomitization. Consequently, the zone of the primary transition from limestones to dolomites lay somewhere farther east. To the west, the epiphyton limestones also rather sharply wedge out, being facies-replaced by a flysch-like sequence of dark-gray clayey limestones, carbonate sandstones, shales, and siltstones assigned to the Anastasyino Formation, which bears indications of formation under shallow-water conditions—interbeds of conglomerate-breccias and slump folds confined to certain limestone beds [11]. Cases observed by the author in which these folds were eroded before deposition of the next layer also confirm their slump origin.

Thus, a definite paleogeographic scheme can be outlined for the arrangement of the facies areas of the described sector during the time of formation of the upper Anastasyino and lower Ungut formations:

a) a western shallow-water area, apparently coastal;

b) an eastern area of open sea or a large lagoon with increased salinity of the water. At the boundary of these areas there was a fairly narrow zone of normal marine conditions suitable for the existence of epiphytons. All this agrees with the views of V. V. Khomentovsky and A. A. Predtechensky [7, 11] that during the period when the Mana trough was being filled with sediments, land existed to the southwest of it in the form of cordilleras, from which material was transported into the trough.

REFERENCES

1. N. A. Arkhangel’skaya, V. N. Grigor’ev, K. K. Zelenov. Facies of Lower Cambrian deposits of the southern and western margins of the Siberian Platform. Transactions of the Geological Institute, Academy of Sciences of the USSR, issue 33, 1960.

2. I. T. Zhuravleva. Early Cambrian organogenic structures on the territory of the Siberian Platform (in the book Organism and Environment in the Geological Past). “Nauka,” 1966.

3. K. B. Korde. Cambrian algae of the southeast of the Siberian Platform. Transactions of PIN, vol. XXXIX, 1961.

4. V. P. Maslov. Geological-lithological studies of reef facies of the Ufa Plateau. Transactions of the Institute of Geological Sciences, issue 118, geological series, No. 42, 1950.

5. V. P. Maslov. Fossil calcareous algae of the USSR. Transactions of Geological Sciences, issue 160, 1956.

6. H. D. Newell. Sedimentation structures in Permian reef limestones of the “Journal of Geology,” vol. 63, No. 4, 1955.

7. A. A. Predtechensky. Ancient uplift of southern Siberia (on the question of the ancient temenid of Asia). Materials on Regional Geology. Gosgeoltekhizdat, 1960.

8. A. F. Senakolis. On the composition of Cambrian limestones in some sections of the Sayan-Altai region. Proceedings of TPI, vol. 127, issue 1, 1964.

9. A. F. Senakolis. Cambrian reefogenic limestones of the Batenëv Ridge. Geology and Geophysics, No. 7, 1966.

10. N. M. Strakhov. Principles of the Theory of Lithogenesis. Vol. 1, 3rd ed. Academy of Sciences of the USSR Press, 1962.

11. V. V. Khomentovsky, M. A. Semikhatov, L. N. Repina. Stratigraphy of Precambrian and Lower Cambrian deposits of the western part of the Eastern Sayan. Academy of Sciences of the USSR Press, 1960.