On the Facies Variability of Devonian Deposits of the Rybinsk Depression

V. L. Kokunov

(Presented by the scientific seminar of the Department of Fossil Fuels)

The Rybinsk Depression, situated south of the railway between the cities of Kansk and Krasnoyarsk, is the northeasternmost intermontane depression of the Sayan–Altai folded region. In view of the still unresolved problem of its possible oil and gas potential, associated with Devonian deposits as possible reservoirs of oil and gas, it should remain an object of attention for petroleum geologists, owing to its convenient location relative to transportation routes.

Recently, a number of works have appeared in print devoted to various questions of the geological structure of the Rybinsk Depression; among them is a work by a group of geologists edited by A. A. Borisov [3], where B. A. Fuks gives the stratigraphic subdivision of the Devonian of the depression according to the scheme adopted as unified by the All-Union Conference on the Stratigraphy of Siberia.

Unfortunately, many features of the geological structure of the depression have not been reflected in the unified scheme and may lead to its incorrect interpretation. Thus, in the work of B. A. Fuks [3], the Okler facies of the Assafyevka Formation and the Chargin facies of the Kungus Formation, although with the reservation of facies variability, nevertheless figure as independent formations.

The scheme of subdivision of the deposits filling the Rybinsk Depression is presented in Table 1. For its proper understanding, it is necessary to dwell on the lithologic-genetic features of the Devonian-age deposits composing the Rybinsk Depression.

At the beginning of Early Devonian time, in connection with the powerful uplift of the Eastern Sayan, the southern part of the Rybinsk Depression, directly adjoining it, began to be intensively filled with a thickness of sedimentary rocks of Early Devonian age, which are chiefly the product of erosion (washing away) of the weathering crust of the variegated complex of rocks of the Eastern Sayan.

When comparing the rocks of the foothills of the Eastern Sayan in the area of the village of Assafyevka with the rocks found in the pebbles of the conglomerates, their complete analogy is evident. If in the bordering zone there are granites, gneisses, diorites, gabbro, quartz porphyries, porphyrites, etc., then exactly the same list of rocks is also observed among the pebbles of the basal conglomerate of the Devonian deposits. In the composition of the pebbles there are noted granites, gneisses, diorites, quartz porphyries, porphyrites, felsites, crystalline schists, quartzites,

Table 1

dolomites, light-gray dolomitized limestones, black “stinking” limestones, sandstones and gravelites of gray and brown colors, siltstones, and so-called “conglomerates within conglomerate” (rounded pebbles which themselves consist of conglomerate). Among the pebbles of this conglomerate there are strongly micaceous calcareous siltstones, cryptocrystalline ferruginous limestones, dolomitized limestones, and quartz pebbles.

The formation of the Assafievka conglomerates through the destruction of rocks of the Eastern Sayan is also indicated by the direct dependence of the composition of the conglomerate pebbles on the composition of the basement rocks. Moving northwest from the village of Assafievka, Cambrian formations, predominantly carbonate in composition, replace the Precambrian basement rocks. This is immediately reflected in the composition of the conglomerate pebbles, which here are 90–95% represented by rounded fragments of Cambrian and Precambrian limestones; while the composition of the pebbles of the basal conglomerate near the village of Kan-Okler fully corresponds to the composition of the rocks of the nearest basement. On the attached lithologic-facies map of the Assafievskaya suite (Fig. 1), areas with different pebble compositions are distinguished separately.

Analysis of the spatial distribution of facies and thicknesses of the Lower Devonian deposits indicates that during Assafievskaya time the relief of the source area was rather dissected, and that the Rybinsk Depression began with a narrow initial trough elongated along the Eastern Sayan, with a steeper northwestern limb and a more gently sloping southeastern limb. This is convincingly shown by the great coarseness of the clastic material of the Assafievskaya suite in the western part of the depression and by the gradual transition of the Assafievka conglomerates into sandstones of the Okler facies.

The presence of fine horizontal bedding [5], so characteristic of the sandstones of the Okler facies, and, together with it, signs of water currents, indicates, according to D. V. Nalivkin [7], a shallow, flat sandy coast with depths most often no greater than 1.0–1.5 meters; moreover, the formation of thin horizontal layers most likely occurred as a result of the shifting of ripple ridges [4]. The orientation of the ripple marks indicates that the direction of the main currents was from southwest to northeast.

The mineralogical composition of the sandy and aleuritic rocks of the Assaf’evo Formation (Table 2) is approximately the same at its different points, but the ratios of individual components vary noticeably. Whereas the western part of the depression is characterized by a considerable content of rounded rock fragments (up to 45% of the mass of the clastic material) and hydromicas, the eastern part of the depression is characterized by a considerable content of quartz (up to 70% of the mass of the clastic material) and garnet, as well as by better sorting of the material and more perfect rounding of the grains.

In the cement of the variegated conglomerates and sandstones of the Assaf’evo Formation there is invariably present one or another, usually considerable, amount of colloidal material represented by low-water iron oxides. These iron hydroxides cover each grain of clastic material with thin films and, in a dispersed state, enter into the composition of the cement; sometimes accumulations of hydroxides have the form of irregular clots. On average, iron hydroxides constitute 10–15% of the HCl-soluble part of the rocks (conglomerate pebbles were not included in the calculation).

The Assaf’evo deposits formed at the foot of a newly formed land area (the Eastern Sayan in Silurian time represented dry land), dissected by numerous mountain-type watercourses, as is indicated by the size of some pebbles in the conglomerates ($0.75 \cdot 0.5 \cdot 0.3\ \text{m}^3$) and their good rounding. The red color of the deposits is due to a considerable amount of iron compounds released during the destruction of ancient igneous rocks, and also during the weathering of lavas and volcanic tuffs. The presence of thin hydroxide coatings covering the grains of clastic material, as well as of iron colloids in the cement of the rocks, compels one to assume that enrichment of the rocks of the Assaf’evo Formation by them occurred while still on land and in the process of transportation, and then also in the area of deposition, which most probably represented a series of deltas, partly subaerial, as indicated by finds of psilophyte flora. As is known, the first psilophytes had an amphibious mode of life and grew most readily along shores and in delta areas [1]. In this connection it should be noted that it is not accidental that the richest occurrences of psilophyte flora are situated along the margins (eastern and western) of the depression, while the coarsest-clastic facies of the rocks are most widely developed in the area of the Khaidakskii massif and the village of Kubenki. It was from there that the clastic material arrived and then was distributed in the narrow field of the intermontane depression.

The intense downwarping, compensated by accumulation of the sandy-conglomeratic sequence of the Assaf’evo Formation, was accompanied in places by outpourings of basic effusives.

In the Karymovo time following the Assaf’evo, a thick conglomerate–sandy–aleurolitic sequence of arkosic composition accumulated in the southern part of the Rybinsk depression (Fig. 2).

The mineralogical composition of the sandy and aleuritic rocks of the Karymovo Formation is remarkably persistent over the entire enormous area of its distribution, from the basin of the Kana River, where the lower part of the formation is exposed, to the Batogoshka River in the basin of the Berezovka River (Table 2).

As a rule, the rocks of the formation contain up to 45% quartz, up to 25% feldspars, with both acid plagioclases (mainly of the oligoclase–andesine series) and potassium feldspars present. In addition, rounded rock fragments usually occupy up to 30% of the volume of the clastic material. As a rule, they are represented by fragments of siliceous rocks (quartzites), clayey rocks (various clay shales), fragments of quartz–sericite metamorphic rocks, and often acid effusive rocks.

Table 2

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Schematic lithofacies maps of the Devonian suites. Compiled by the author.
(Fig. 1 — Asaph suite; Fig. 2 — Karymovo suite; Fig. 3 — Ivangino suite; Fig. 4 — Kutchug suite; Fig. 5 — Amonash suite.)

Legend for the maps

A — continental facies.
1 — interbedding of sandstones and conglomerates (conglomerates predominate; the pebbles contain various igneous rocks).
2 — interbedding of sandstones and conglomerates (conglomerates predominate; the pebbles contain carbonate rocks).
3 — sandstones.
4 — interbedding of aleurolites and sandstones (aleurolites are rare).
5 — interbedding of sandstones and aleurolites (sandstones are rare).
B — lagoonal-continental facies.
6 — interbedding of red-colored sandstones and aleurolites with beds of limestone.
7 — interbedding of sandstones with rare beds of aleurolites and argillites.
8 — sugary quartz sandstones (Chargin facies — beach deposits).
9 — interbedding of red-colored aleurolites with beds of oolitic limestone.
10 — boundaries visible in all studied suites.
11 — principal directions of transport of clastic material.
12 — area of denudation with dissected relief.
13 — area of denudation with gentle relief forms.
14 — conditional facies boundaries.

For the rocks of the Karymovo Formation, contact cement, contact cement of contact, and more rarely film cement are characteristic. Usually the rocks contain 90–95% clastic material, and cement accounts for only 5–10% of the total rock mass. The cement is represented, as a rule, by carbonate material, rather intensely recrystallized.

The accumulation of the thick sandy–silty sequence of the Karymovo Formation took place in the same original narrow downwarp where the Assaf’evo Formation was deposited (Fig. 2). But the conditions under which this sequence formed were somewhat different. Evidently, the source area had already been strongly leveled in comparison with Assaf’evo time. As the erosional process waned, increasingly fine sediments were deposited, and in the upper parts of the Karymovo Formation we already have a thick (up to 700 m) unit of dark-brown siltstones with very characteristic types of bedding. Some of them should be discussed separately. For the siltstones of the Karymovo Formation, which have a conchoidal fracture and the ability to break into angular fragments, forming “cracklers,” diagonal cross-bedding of the stream type is characteristic, produced as a result of the migration of various types of sand bars [2], expressed as shoals. The ability to form “cracklers,” together with finely aggregated extinction, are features of deposits of colloidal origin [7]. This marks the end of the first stage in the formation of the Rybinsk depression.

In Middle Devonian time, as a result of the expansion of the downwarping zone to the northeast, part of the southwestern framing of the Siberian Platform was drawn into subsidence; some expansion of the zone of sediment accumulation is also observed in the south of the depression (Fig. 3). This is recorded by the transgressive overlap of rocks of the Ivashkhino (Pavlovsk) Formation onto Paleozoic deposits of different ages in the south of the depression, and by the presence of deposits of the formation on the Uyar swell¹), where they rest directly on the basement.

The accumulation of deposits of this formation took place already in a calmer setting than in the preceding epochs, and this, of course, left its imprint on its lithologic composition. One of the characteristic features of the formation is the presence of limestone beds in its section. Another feature is the predominance of fine-clastic silty rocks over coarse-clastic ones and the subordinate role of conglomerates and gravelites.

Comparison of the mineralogical composition of the sandy and silty rocks of the formation from its various localities shows (Table 2) that the latter have a rather consistent mineralogical composition; moreover, a very characteristic feature is the significant percentage of cementing material in the total mass of the rocks (from 10 to 20%). The content of rock fragments in the sandstones of the formation ranges from 10 to 25%, with a characteristic predominance among them of rounded fragments of quartzites and limestones.

The limestone interbeds have a very interesting feature. The limestones are distinguished by considerable facies variability and often, along the strike, pass into calcareous siltstones. At the same time, in the section of the formation there is an increase in the content of coarse-grained sandstones and gravelites. The limestones themselves contain a significant admixture of terrigenous material in the form of grains of quartz and feldspars. Chemical analysis of the limestones showed that the amount of material insoluble in HCl

¹) Here and elsewhere we use the names of the structural elements of the depression after M. A. Zharkov and B. A. Fuks [3].

the residue usually ranges within 6–10%, but not infrequently reaches as much as 40% of the total mass. At the same time, the dolomites included in the limestones usually account for 2–4% of the HCl-soluble part of the rocks.

The presence of considerable amounts of clastic material, dolomitization, and also the psammitic, conglomerate-like, and cryptocrystalline structures of the limestones—all this indicates that in Ivashikhino time a lagoonal-continental environment prevailed in the territory of the Rybinsk Depression.

Volcanic activity is also associated with the epoch of formation of the suite, expressed as the intrusion of diabase dikes and sheets of basic effusives in the eastern, as well as in the extreme western, part of the Rybinsk Depression.

The time of formation of the suite marks a turning point in the history of the development of the Rybinsk Depression, opening the next stage of development—a stage more similar to the platform type of development.

After a certain break in sediment accumulation, and also after folding, expressed in an angular unconformity between the Ivashikhino and the overlying Kungus suites, subsidence of the Rybinsk Depression continued. At the same time, the area of sediment accumulation in Kungus time expanded considerably farther to the northeast (Fig. 4). The underground connection between the Eastern Sayan and the Yenisei Ridge, and even the southern part of the Siberian Platform, was drawn into subsidence (deposits of the Kungus suite have been recorded in the neighboring Kansk-Taseevo Depression, which belongs to the Siberian Platform). The most intense sagging of the basement is confined to the area of the Balai and Borodino troughs.

In Kungus time there occurred the accumulation of variegated sediments, represented by calcareous siltstones with subordinate interbeds of sandstones and calcareous pseudoconglomerates (concretions). The large amount of fine material in the section of the suite testifies to the relative remoteness of the source areas and to considerable leveling of the denudation area.

The mineralogical composition of the sandy and silty rocks of the Kungus suite (Table 2) is considerably poorer than in the preceding suites. Of considerable interest is the distribution of feldspars in the rocks of the suite. Near the southern boundary of the distribution of the suite deposits, the amount of feldspars is about 25% of the total mass of clastic material. With increasing distance to the northeast and toward the central part of the depression, the feldspar content gradually falls to 5–10%, while the percentage content of quartz noticeably increases (from 40 to 70%), as does that of clay minerals. The content of rounded rock fragments in the rocks of the Kungus suite decreases considerably. It should be noted that this is the only suite in the Devonian section for which a content of syngenetic glauconite is characteristic. The clayey material of the rocks of the Kungus suite is impregnated with fine disseminations of iron hydroxides.

The indistinct bedding of the rocks of the suite indicates that their deposition proceeded uniformly over a long interval of time.

Of great interest is the presence in the section of the Kungus suite of very peculiar formations, for which the name “pseudoconglomerates” has become established. Study of these formations under the microscope showed that they are composed of rounded fragments of limestone, usually of cryptocrystalline structure, cemented by the same calcareous material with an admixture of terrigenous material. The latter is usually represented by fragments of quartz, and, rarely, feldspars. The formation of such bodies took place in the zone of wav-

...deposits under mixing of terrigenous and limestone material, with the deposited material being reworked again.

The section of the Kunguska suite ends, in the southern part of the depression, with a member of light-gray quartz sandstones of the Chargino facies, which gradually pinch out toward the center of the depression. In their composition (the sandstones consist almost 95% of quartz grains), the roundness of the grains, and the character of the bedding, the sandstones of the Chargino facies are very reminiscent of formations typical of beaches.

Thus, if this fact is taken into account, we obtain evidence for a reduction in the area of the Kunguska basin toward the end of Kunguska time. At the same time, as is seen on the lithologic-facies map (Fig. 4), the reduction in the area of sedimentation took place only in the southern part of the depression.

All the foregoing, as well as the rich fish and crustacean fauna, indicate that in Kunguska time lagoonal-continental conditions existed within the depression; moreover, the Kunguska body of water had a saline regime, as indicated by the syngenetic nature of glauconite and, to some extent, by the estherian fauna. The Kunguska body of water probably had a connection with the water body of the Nazarovo depression and farther with the Minusinsk basins. This is indicated by the analogous character of the ichthyofauna, as well as by the direct continuation of deposits of the Kunguska suite beyond the Yenisei River.

A considerable decrease in the thickness of the suite in the north of the depression, as well as in the neighboring Kansk-Taseevo depression, indicates that the Eastern Sayan served as the main source area, while the South Yenisei Ridge played only a subordinate role.

The character of the deposits suggests that the Kunguska basin did not possess any considerable depths.

The overlying deposits of the Amonashka suite consist of a unit of limestones alternating with sandstones and siltstones (Fig. 5). It must be noted that the character of the deposits and the almost complete absence of fauna indicate a chemogenic origin of the limestones of the Amonashka suite, while the considerable amount of syngenetic silica and chalcedony indicates significant contamination of the Amonashka basin by products of eruptions (siliceous material). The configuration of the Amonashka basin indicates that by the end of Late Devonian time no connection with the Nazarovo depression existed. The good areal persistence of the limestones, owing to which they served as a reliable marker horizon in various constructions, indicates approximately identical conditions of deposition throughout the entire area of distribution of the suite.

The chemical analyses of the limestones of the Amonashka suite that were carried out showed a significant percentage of insoluble residue (up to 20%), as well as the absence of dolomitization. The limestones of the Amonashka suite generally have a cryptocrystalline structure, but limestones with psammitic and oolitic structures are encountered rather often, especially along the margins of the Amonashka basin.

In conclusion it is necessary to note that in the history of formation of the Devonian deposits of the Rybinsk depression two main stages can be outlined. In the first stage their formation took place in an intermontane trough bounded by the Eastern Sayans and by a subterranean connection of the Eastern Sayans with the South Yenisei Ridge. The deposits formed during this stage are distinguished by a stronger metamorphism of the rocks than in the subsequent stage. The metamorphism of the rocks is expressed in a rather intense recrystallization of the cementing material, which led to mineral neoformations, and in a stronger compaction of the pri-

clastic sediment, which is expressed in the large volumetric proportion of Lower Devonian rocks.

In the second stage, sediment formation took place essentially in a fore-mountain basin, since deposits of Upper Devonian age extend into the neighboring Kansk–Taseevo depression, which belongs to the structures of the Siberian Platform.

Among the facies distinguished, the sandy facies, as the area of distribution of possible reservoirs, and the lagoonal–continental facies are of greatest practical interest.

REFERENCES

1. Ananiev A. R. On the Lower Devonian flora of the southeastern part of Western Siberia. Problems of the Geology of Asia, vol. 1, 1954.

2. Bruins E. P. Analysis of rhythmicity in the structure of sedimentary sequences. In: Methodological Guide to Geological Surveying and Prospecting. Gostoptekhizdat, 1954.

3. Vasiliev V. G., Katenov E. N., Karasev I. P. et al. Geological Structure of the South of the Siberian Platform and the Oil Potential of the Cambrian. Gostoptekhizdat, 1957.

4. Vassoevich N. B. Bedding in the light of the theory of sedimentary differentiation. Proceedings of the Academy of Sciences of the USSR, Geological Series, no. 5, 1950.

5. Kuzmin A. M. Bed and interbed. Transactions of the Mining-Geological Institute, Academy of Sciences of the USSR, West Siberian Branch, issue 11, 1950.

6. Nalivkin D. V. The Doctrine of Facies, vols. I–II. Publishing House of the Academy of Sciences of the USSR, 1956.

7. Rukhin L. B. et al. Reference Manual on the Petrography of Sedimentary Rocks, vols. I–II. Gostoptekhizdat, 1958.