ON THE ANGULAR UNCONFORMITY BETWEEN THE ANASTASYA AND UNGUT SEQUENCES IN THE NORTHWESTERN PART OF THE EASTERN SAYAN

S. S. GUDYMOVICH

(Presented by Professor K. V. Radugin)

The stratigraphy and tectonics of the thick, predominantly carbonate complex of rocks developed in the basin of the middle and upper reaches of the Mana River and the headwaters of the Bazaykha River, and lying between the base of the Kolba Formation (Series) of the Lower Cambrian and the certainly Proterozoic formations of the Kuvai and other formations, have not yet been sufficiently clarified. In particular, this concerns the question of the tectono-stratigraphic relationships of the Anastasya and Ungut sequences¹, which are included in this complex.

Without touching in detail on questions of the stratigraphy and age of these sequences, we shall note only the following: the majority of investigators of the area [2, 3, 6], while interpreting in different ways the details of the geological structure of the pre-Kolba deposits, regarded the relationships of all the sequences (including the Anastasya and Ungut) with one another, and also with the overlying Kolba Formation, as generally conformable, sometimes connected by gradual transitions, which, in addition to paleontological data, served for them as a basis for assigning these deposits to the lower beds of the Lower Cambrian. K. V. Radugin [4, 5], as a result of many years of investigations, came to the conclusion that the pre-Kolba deposits are of Late Precambrian age; he subdivided them into a number of systems, divisions, and stages. In contrast to other geologists, K. V. Radugin notes a large number of stratigraphic and tectono-denudational breaks in the structure of these deposits.

The difference in the viewpoints of the above-mentioned investigators concerning the relationships of the Anastasya and Ungut sequences is well illustrated by material from the area of the middle course of the Mana River, in the sector below and above the settlement of Maly Ungut.

V. V. Khomentovskii et al. [6] saw here a gradual transition from the limestone-sandstone-shale Leibin Subformation to the massive dolomites of the Zherzhul Formation proper. A. A. Predtechenskii [2] wrote of the absence of a clearly visible angular unconformity between the sandstone-shale-limestone (Anastasya?) sequence and the Ungut Formation. I. I. Koptev [1] believes that the Ungut massive limestones and dolomites are separated from the underlying upper parts of the Anastasya Formation by an insignificant break, evidence of which he considers to be breccias, occurring in some outcrops and confined to the boundary of the formations.

¹ We deliberately do not use the term “formation” with respect to these sequences, in view of the major discrepancies among different authors in their volume, age, and stratigraphic position.

K. V. Radugin [4] assigned the limestone–sandy–shale sequence to the Maloungut Formation, and the overlying massive epiphyton limestones, underlain by the Ungut massive dolomites, to the Pechersk Formation. He considers the contact between the formations to be unconformable, with a tectono-denudational break manifested in it, separating the Krasnoyarsk and Mana systems distinguished by him.

In the summer of 1963 the author studied the relationships between the Anastasiya and Ungut sequences in this area. The results of the work constitute the content of the present article.

Two areas were studied in detail: 1) the vicinity of the settlement of Maly Ungut, 2) the area of Semyon Rock, located 12 km from the first area upstream along the Mana. In the first area both sequences, well exposed on the right side of the valley of the Mana River, form a large anticlinal fold with an axis plunging to the north-northeast and a broad arch-like hinge area composed of rocks of the Anastasiya sequence. The overlying Ungut sequence composes the limbs and the periclinal closure of the fold (Fig. 1).

Fig. 1. Schematic geological map of the area of the settlement of Maly Ungut.
1 — graywackes of the Kolba Formation (?); 2 — massive light dolomites of the Ungut sequence; 3 — massive light epiphyton limestones of the lower part of the Ungut sequence; 4 — greenish-gray bedded dolomites of the Anastasiya sequence; 5 — flysch-like interbedding of dark-gray clayey limestones, shales, and carbonate sandstones of the Anastasiya sequence; 6 — carbonate breccias in the upper part of the Anastasiya and at the base of the Ungut sequence; 7 — collection points of epiphyton flora; 8 — boundary of unconformable (transgressive) bedding; 9 — boundary of conformable bedding; 10 — fold axis in the Anastasiya sequence; 11 — fold axis in the Ungut sequence; 12 — bedding elements.

The Anastasiya sequence (the Maloungut Formation according to K. V. Radugin) is generally represented by flysch-like interbedding of dark-gray clayey limestones, shales, and calcareous sandstones, with the sections of the sequence differing noticeably in different limbs of the fold. In the eastern limb, limestones play a significant role; the thickness of their beds reaches several meters. In the upper parts of the section, closer to the contact with the overlying Ungut sequence, thin lenticular interbeds of sedimentary breccias begin to occur, consisting of fragments of the underlying limestones, shales, and sandstones. Here, sandy limestones with a large number of small fragments of light epiphyton limestones are also widely developed. Occasionally, among the bedded rocks of the sequence, isolated large blocks of light massive limestones, in places epiphyton limestones, up to several meters in diameter, are encountered. It is interesting that the bedding of the enclosing rocks abuts at a right angle against the lateral surfaces of such blocks. Finally, in the very uppermost parts of the section of the Anastasiya sequence on

on the eastern limb of the fold there appear interbeds of massive light-colored Epiphyton limestones up to 2–3 m thick.

Tracing the upper part of the sequence directly in the outcrops of the right side of the Mana River valley from the eastern limb of the fold across its crest to the western limb, one can see a gradual decrease in the thickness of the limestone beds and the disappearance of detrital limestones, with an increase in the role of shales and sandstones; as a result, in the section of the Anastasya sequence on the western limb of the structure, the greater part of it is represented by the flysch-like interbedding described above. Only the very top of the sequence on the western limb is composed of interbedded gray and dark-gray dolomites, calcareous dolomites, and, more rarely, limestones; moreover, when these rocks are traced along strike directly in the outcrops, facies transitions from one to another are visible, the limestones sometimes becoming lighter in color and massive.

The deposits of the Ungut sequence on the eastern limb of the anticline begin with Epiphyton limestones up to 200 m thick (the Pechera suite according to K. V. Radugin). At the base of these limestones, in some outcrops, there occur sedimentary breccias consisting of large blocks, up to several meters in size, of light-colored massive Epiphyton limestones, completely unrounded. The spaces between these blocks, especially closer to the base, are filled with fine-clastic material of the same limestones and of the layered rocks of the underlying Anastasya sequence. In other outcrops, similar breccias or other obvious traces of a break are absent, and what is seen is, as it were, a rather rapid, but non-abrupt, transition from the Anastasya sequence to the Ungut sequence, expressed in the gradual disappearance of sandstones and shales, the lightening of the limestones with an increase in the thickness of their packets, and the disappearance of the bedded texture up to the appearance of massive light-colored limestones already assigned to the Ungut sequence. Above the limestones lie light-colored massive dolomites, locally graphitic. The relationships between the limestones and dolomites have not been finally clarified, since their contact is strongly obscured by subsequent dolomitization.

On the western limb of the fold, in the most exposed locality—the right side of the Mana River valley, 2 km below the settlement of Malyi Ungut—a sharp contact between the Anastasya and Ungut sequences is visible: thin-bedded gray dolomites with interbeds of clay shales are abruptly replaced by massive light-colored dolomitized marbles with a characteristic network of thin black graphite veinlets.

Both sequences, as already noted, form a large anticlinal fold. The dip angles of the rocks of the Anastasya sequence on the limbs of the fold gradually increase from 10–15° in the immediate vicinity of the crest to 60–70° on the periphery, which is especially clearly visible on the western limb. In addition, the rocks of the Anastasya sequence on the western limb of the anticline, in the middle part of the section exposed in the basement of the terrace on which the settlement of Malyi Ungut stands, are complicated by small additional folds, asymmetric eastward to the point of overturning. The amplitude of the folds reaches several meters. In this same area there are small, steeply dipping tectonic dislocations of the reverse-fault type, with amplitudes of up to several meters, most often confined to the axial surfaces of the overturned folds. The strike of the axial planes of the folds and of most of the small disjunctive structures—northeast 15–20°—coincides with the general strike of the axes of the main structures. I. I. Koptev believes that this minor folding accompanies a major fault, which he traces along the axis of the main anticline in the Ungut sequence on its western limb. We do not have

there are as yet no data for tracing this disjunctive fault. The bedding elements of the rocks of the Ungut sequence on the eastern limb of the anticline differ hardly at all from the bedding elements of the Anastasya rocks; on the western limb, measurements in the Ungut rocks are absent.

Summing up the foregoing, we note that the similar bedding elements in the rocks of both sequences, as well as the outwardly gradual transition observed here and there from the Anastasya to the Ungut, do indeed create the impression of conformable occurrence with minor local breaks (the presence of breccia in some outcrops). The anticlinal structure in both sequences was also regarded as unified, as was shown on all small-scale maps. But in compiling a detailed geological map of the area the following became clear (see Fig. 1): the axial planes of the anticlinal folds in the Anastasya and Ungut sequences do not coincide; the axis of the fold in the Anastasya sequence, if prolonged, passes beneath the limestones, dipping monoclinally to the east, of the lower part of the Ungut sequence and, most importantly, the base of the Ungut sequence lies at different horizons of the Anastasya sequence. The latter becomes especially striking if one compares the thickness of the Anastasya sequence on the different limbs of the anticline, counting from the fold axis to the base of the Ungut sequence. On the eastern limb it is 320 m, whereas on the western limb it is no less than 1800 m (it is difficult to give an exact figure for the thickness because of breaks in the bedrock exposures and the above-noted minor dislocation of part of the section of the sequence near the settlement of Malyi Ungut. The latter, however, in no way prevents establishing the correct succession of deposits, and the error in determining the thickness in this sector, taking into account the small number of folds and disjunctive faults and their small amplitude, does not exceed 10–15%).

Such a discrepancy in thicknesses on the limbs of the structure cannot be explained by repetition of the section caused by the presence of a major disjunctive fault proposed by I. I. Koptev, since, first, we did not observe such repetition (although we acknowledge that this is rather difficult to do in so monotonous a flysch-like sequence), and, second, if such a disjunctive fault does exist, then analysis of the geological map compiled by I. I. Koptev himself shows that the downthrown block is the western one; i.e., the true discrepancy in thicknesses would be still greater.

The disappearance of 1.5 km of the thickness of sediments of the Anastasya sequence on the eastern limb of the anticline, as compared with the western limb, when the width of the fold is only 3.5 km, can be explained only by erosion of the dislocated sequence.

Evidently, after its formation, as a result of the action of a phase of tectogenesis, the Anastasya sequence was crumpled into large, rather gentle folds of northeastern strike and only then underwent denudation, which led to the erosion of 1.5 km of sediment thickness. After deposition of the Ungut rocks, both sequences, together with younger formations, were subjected to the action of the next, more significant phase of tectogenesis, which formed the principal structures in the region. Moreover, the strike of these structures remained northeastern.

Thus, the above-described anticlinal fold, composed of rocks of the Anastasya and Ungut sequences, is not a single whole, but consists of two structural stages. The fold in the Ungut sequence, formed in a later phase, is inherited.

Apparently, in this second phase, and not in the first, the minor additional folding in the Anastasya sequence also formed. Confirmation of this is provided by the fact that exactly the same folding is present with the

same strike of axes in the younger sequences, in particular in the Ust-Leibinskaya and Kolbinskaya formations.

The conclusions just drawn are confirmed by the results of study of the second area as well. Here, in the bend of the Mana River, in rocky exposures on the right side of the valley, a large anticlinal fold with gently dipping limbs is visible (Fig. 2); deposits of both sequences take part in its structure. The lower part of the Anastasya sequence is composed of black bedded limestones with an insignificant amount of clayey shales; higher in the section these rocks quite gradually give way to massive, at first dark, and then gray and light-gray epiphyton limestones, which make up the upper part of the section of the Anastasya sequence in this area.

Above lie massive white and pinkish dolomites of the Ungut sequence. The contact between the sequences is fairly sharp, but uneven, complicated by secondary dolomitization, as well as by thin zones of brecciation and karst pockets filled with fragments

Fig. 2. Schematic section—sketch of the right side of the Mana River valley immediately below Semenov Cliff. I — bedded dark-gray limestones of the Anastasya sequence; II — dark and light massive epiphyton limestones of the upper Anastasya sequence; III — light massive dolomites of the Ungut sequence; 1 — carbonate breccias at the base of the Ungut sequence; 2 — points where epiphyton flora was collected; 3 — conformable boundary between bedded and massive limestones of the Anastasya sequence; 4 — boundary of angular unconformity between the Anastasya and Ungut sequences; 5 — tectonic disturbance complicating the crest of the anticline.

of the surrounding dolomites and limestones, which completely obscures the primary relationships of the sequences. The bedded limestones of the Anastasya sequence in the core of the main anticline are crumpled into additional, very tight asymmetric folds with vertical eastern limbs of anticlines, striking NE 20° and dipping to the NW at an angle of 45–50° with western limbs. The amplitude of the folds reaches 10–15 m. This additional folding is disharmonic in character, since it dies out as one passes from the bedded limestones to the massive limestones and dolomites. Undoubtedly, this folding is a phenomenon of the same order as the additional folding of the Anastasya sequence in the first area.

Thus, in this area as well, the presence of a gradual transition from the bedded limestones of the Anastasya sequence to massive ones, and the replacement of the latter upward in the section by massive Ungut dolomites, creates the impression of conformable bedding of the sequences. But if one traces the massive epiphyton limestones of the upper Anastasya

strata across the axis of the main fold in the direction from the western limb to its arch, one can see how their thickness gradually decreases, and within the arch the dolomites of the Ungut sequence lie directly on the bedded limestones of the Anastasya strata, being separated from them by a layer of strongly recrystallized conglomerate no more than 1 m thick, consisting of fragments of the underlying black limestones and light epiphytonal limestones, analogous, judging by the forms of the epiphytons, to the above-described massive limestones of the upper Anastasya strata. The cement of the conglomerates is dolomitic. Having crossed the arch of the fold to its eastern limb, we again encounter massive epiphytonal limestones, whose thickness increases with distance from the arch.

The absence of massive limestones of the Anastasya strata in the arch of the anticline might be explained by primary facies variability of the Anastasya strata, caused, say, by the existence of an uplift (subsequently coinciding with the arch of the fold) during the deposition of the upper parts of the Anastasya strata. But we did not observe any traces of such variability in the lithological composition of the Anastasya strata along the strike throughout the entire area.

Thus, in our opinion, in this area as well the rocks of the Anastasya strata, after their formation, underwent the action of a phase of folding, expressed in the formation of a broad arch-like anticline with gentle limbs. During the hiatus preceding deposition of the Ungut strata, the arch of this anticline was eroded to a depth of no less than 50–70 m, which corresponds to the visible thickness of the massive epiphytonal Anastasya limestones on the limbs of the fold. After deposition of the Ungut dolomites, both strata were again jointly subjected to folding, with the formation of disharmonic folds in the Anastasya strata and of a large inherited anticline in the Ungut strata.

Thus, in contrast to existing views on the absence of an angular unconformity between the Anastasya and Ungut strata in the area of the middle course of the Mana River [1, 2], and even on the presence of a continuous gradual transition between them [6], on the basis of the results of detailed mapping we have attempted to prove: first, the presence of an angular and denudational unconformity between the Anastasya and Ungut strata in this area; second, the presence of an independent pre-Ungut phase of folding, which gathered the Anastasya strata into calm, broad folds of northeastern strike; third, the inherited character of the folds in the Ungut strata—during the action of the following, more significant phase of folding, which formed the main structures in the area, anticlines in the Ungut strata were laid down in the sites of anticlines already existing in the Anastasya strata, preserving the same strike of the axes.

REFERENCES

1. I. I. Koptev. Stratigraphy and paleontology of the Lower Cambrian of the northwestern part of the Eastern Sayan (abstract of dissertation), Tomsk, 1962.

2. A. A. Predtechensky. The Cambrian system. Eastern Sayan. Geology of the USSR, vol. 15, 1961.

3. A. A. Predtechensky, A. P. Shcheglov. Biostratigraphic scheme of the Cambrian deposits of the Altai-Sayan folded region. Materials on the regional geology of Western Siberia. Proceedings of SNIIGGiMS, issue 24, 1962.

4. K. V. Radugin. General characteristics of the Precambrian and metamorphic strata of Siberia. Bulletin of ZSGU, issue 1, 1957.

5. K. V. Radugin. The problem of the Proterozoic of central and western Siberia in the works of V. A. Obruchev. In: Ideas of Academician V. A. Obruchev on the geological structure of northern and central Asia and their further development, Academy of Sciences of the USSR, 1963.

6. V. V. Khomentovsky, M. A. Semikhatov, L. N. Repina. Stratigraphy of the Precambrian and Lower Paleozoic deposits of the western part of the Eastern Sayan. Regional Geology of the USSR, vol. IV, 1961.