New Pulsation Formations with Archaeocyaths in the Northwestern Part of the Eastern Sayan

K. V. RADUGIN

The geology of the northwestern part of the Eastern Sayan is fairly well known (Bazhenov, 1937; Obruchev, 1932; Vologdin, 1959; Zhuravleva, 1960; Predtechensky, 1937). Nevertheless, quite unexpectedly, at the end of the summer field season of 1962, we made an important discovery in the field of regional geology, tectonics, stratigraphy, and paleontology of the northwestern part of the Eastern Sayan: over a small area of several square kilometers, new pulsation carbonate formations with archaeocyaths of ordinary appearance were found.

Missed by previous investigators, they were found in the folded zone of the Salairides, in the very poorly exposed headwaters of the Murtuk—the left tributary of the Kolba, which flows from the left into the Mana (a tributary of the Yenisei). There these formations with archaeocyaths lie stratigraphically below the thick (more than 3 km) Kolba carbonate geological formation (with Lower Cambrian trilobites). This sequence was previously known as the Kolba Formation of V. M. Chairkin (with its stratotype near the settlement of Vyezzhego Loga). Its trilobites have been described monographically and are well known to investigators of the northwestern part of the Eastern Sayan (Koptev, 1962; Repina, 1960). We emphasize that the new formations with archaeocyaths discovered by us lie below the Kolba sequence, beneath a buried peneplain. For a quite understandable reason, this fact made the new find of archaeocyaths very important.

We therefore could not fail to take measures for a rapid and effective study of the newly discovered archaeocyaths.

Assistance in collecting archaeocyath specimens was provided to us by V. F. Derbenev, M. L. Isaenko, and other students of the S. M. Kirov Tomsk Polytechnic Institute. We express to them our deep gratitude for this.

Since the new pulsation formations of the Murtuk district of the Eastern Sayan were not previously known, it is possible to dispense with a review of the results of earlier investigations of the northwestern part of the Eastern Sayan, especially since detailed surveys of them have repeatedly been given in major works by V. A. Obruchev (in his multivolume History of the Exploration of Siberia and in the three-volume Geology of Siberia), as well as in the works of I. K. Bazhenov (with M. P. Nagorsky) (1937); A. A. Predtechensky (1937–1960); V. V. Khomentovsky (with M. A. Semikhatov) (1957), and others.

It is true that the discovery of several new pulsation formations with archaeocyaths in the northwestern part of the Eastern Sayan not only makes it possible to illuminate the history of research in this region in a new way, but also raises fundamental and highly important questions concerning the boundary of the Cambrian. For the sake of necessary brevity, we shall not touch upon these questions and shall consider them

in another article. Let us note only that at present the need has become ripe for a fundamental revision of the stratigraphic significance and position of the archaeocyaths, the only type of animals that became extinct at the beginning of the Cambrian.

Let us proceed to a brief description of the new strata of the Eastern Sayan with archaeocyaths. But first let us say a few words about their geological position.

The Krasnoyarsk Range (or the northwestern projection of the Eastern Sayan) is composed mainly of Upper Proterozoic rocks, namely of several of its systems. These systems (Slavy, Sayan, Krasnoyarsk, Mana, Chud systems) have been distinguished by us on an objective paleontological basis. A characteristic feature of the tectonic dislocations of the northwestern projection of the Eastern Sayan (or the Mana projection) is the northeast strike of the folds of the Upper Proterozoic, whereas the Salairides there are folded in folds of northwestern strike. This is clearly visible from the main structure of the Kolbina geological formation of the Lower Cambrian.

However, the new strata of the Krasnoyarsk Range described below, united under the name of the Lower Kolbina complex (or series), have not a northwestern strike of folds, characteristic of the Lower Cambrian, but a northeastern one. This raises the question whether it is correct, according to the generally accepted opinion, to assign them to the Cambrian.

Let us proceed to a brief description of the Lower Kolbina complex, to the stratigraphic sequence of its parts.

This sequence was established by means of detailed geological mapping (using eluvium, deluvium, clearings and pits) on a reliable base of the geological map. The stratigraphic composition of the Lower Kolbina carbonate complex is as follows: it is divided into several pulsation formations (from below)—the lower stratum of dolomitized carbonate rocks (Murtuk), then the Belorechensk (village of Pionersky), the overlying Berestyan, and still higher—the Bultusuk.

The total thickness of the Lower Kolbina complex is approximately estimated as considerable (apparently not less than 2–3 km).

Let us begin a brief characterization of the complex of strata with its lower beds.

a) This is the Murtuk carbonate stratum with epiphytons (and with rare archaeocyaths?).

b) Then follow the Pioneer beds, named after the rocky hillock “Pioneer”; they are divided into two parts. The lower part, less than 100 m thick, lies unconformably on gray and light-gray massive dolomitized fine-crystalline carbonate rocks containing equally oriented bushes of epiphytons and rare fragments of archaeocyaths. It consists of dark-gray thin-platy aleuritic-clayey shales (with lighter dolomitic interbeds). In this member, with a total thickness of the first tens of meters, angular and rounded by dissolution fragments of the adjacent underlying rocks were found. The supposition that these fragments come from a tectonic contact breccia or from a boudinaged interbed has not yet been confirmed. Relict tectonic cracks and veinlets were found in them. We regard them as the result of the destruction of coastal cliffs and the deposition of rubble and blocks in the lower beds of the described member or subformation.

Noteworthy are the gray tones of its coloration, whereas the overlying carbonate beds are white and variegated. Its assignment to the Pioneer beds is still provisional.

The upper part of the Pioneer beds (about 100 m thick) consists mainly of massive and platy gray, white, partly pink, mottled dense fine-crystalline marbles (or limestones).

There are interbeds of red calcareous sandstones. There are also interbeds with fragments of white, nearly synchronous or more ancient limestones, which sporadically contain Epiphyton and archaeocyaths. From the main mass of the carbonate Pioneer beds, hyoliths, regular and irregular archaeocyaths, and algae have been collected; these are being treated monographically.

c) The Pioneer beds, possibly constituting an independent pulsation formation, are overlain by the Belorechenskaya sequence of massive, unstratified, fine-crystalline marbles¹ (from below):

1) a lower, poorly exposed part (marbles and ? sandstones);

2) light-colored archaeocyath-bearing fine-crystalline marbles (with small turf-covered intervals, exposure 62 M1);

3) light-colored, partly archaeocyath-bearing fine-crystalline marbles exposed in eluvium (in the middle part with thin beds of oncolitic marbles);

4) light-colored archaeocyath-bearing fine-crystalline marbles exposed in the deluvium of the slope;

5) partly gray, algal, light-colored marbles concealed beneath alluvium.

The total thickness is estimated approximately at no less than 1–1.5 km. The marbles are massive, in places of biohermal facies, and usually unstratified. Although there are few measurements of bedding, the average dip angle has evidently been determined rather accurately, since the dip angle of the underlying and overlying beds is also not small, or is of medium magnitude (55–90°).

Among the varieties of rocks of the sequence described, we note: a) archaeocyath-bearing rocks of biohermal facies, filled with cups of archaeocyaths; b) Epiphyton-bearing rocks and those with abundant Razumovskia; c) oolitic or oncolitic rocks; d) rocks with an incrustational, partly algal structure; e) massive rocks, but without archaeocyaths; f) dolomitized rocks, etc. Sedimentary breccias also attract attention. One of them has a carbonate-quartz cement, with the quartz represented partly by well-rounded large sand grains. Others have carbonate cement and are indistinguishable from the enclosing fine-crystalline marbles. In one of such breccias, a block of archaeocyath-bearing marble was observed, strongly altered (in particular, dolomitized), evidently before the formation of the block. Thus, if relict metamorphism is confirmed, this breccia will prove the existence of archaeocyath-bearing marbles more ancient than itself.

The fossils of the Belorechenskaya sequence are of great importance.

d) The Berestyanskaya pulsation formation, apparently lying unconformably on the Belorechenskaya, is exposed scarcely worse than all the others. Nevertheless, its basal beds form two favorably situated cliffs, where their occurrence is clearly visible, and they are easily traced in the form of insoluble fragments of variegated, red-green shales and sandstones beneath carbonate rocks. There are also other bedrock exposures with clear bedding of the layers. On the basis of all data, the occurrence of the Berestyanskaya sequence in a syncline of NE strike has been established; with an axis plunging to the SW, it is enveloped from the N, NW, and SE by archaeocyath-bearing marbles of the Belorechenskaya sequence.

The stratigraphic succession of the Berestyanskaya formation is as follows (from below):

1) a basal subformation of greenish sandstones and red calcareous-clayey shales with lenses and interbeds of “limestones”;

2) a substantially carbonate subformation with an archaeocyath fauna, with rare brachiopods, hyoliths, and Epiphyton;

3) the upper subformation of gray, dark-gray (to black), and other calcareous quartz sandstones, passing into sandy limestones or marbles, likewise dark-colored.

The upper subformation is so distinctive that in the future it may be separated as an independent pulsation formation, distinguished by the development within it of quartz sandstones. The thickness of the Berestyan Formation is estimated at no less than several hundred meters.

d) The Bultusuk Formation, apparently at least half a kilometer thick and, according to preliminary data, lying unconformably on the Berestyan pulsation formation and its basement, is exposed in the very overgrown interfluve of the Murtuk and Bultusuk rivers. For this reason it has been studied far from sufficiently.

Nevertheless, a first idea of its lithological composition and fossils has been obtained. Its division into three parts is emerging. We note the characteristic enrichment of the beds in the lower part of the middle formation (or subformation) with quartz sandstones, up to the appearance of interbeds of quartz sandstone with calcareous cement, or even quartzite. In addition, in the basal part of the Bultusuk Formation, even breccia-like beds or grits with small angular fragments of white archaeocyath limestones have been found.

The Bultusuk Formation as a whole is carbonate. It lacks dolomites. Among the features of its composition we note the presence in some beds of thin interlayers, small lenses, and more complex bodies of brownish microquartzite (or chalcedonite). Other carbonate beds consist of a sedimentary organogenic-clastic rock that should be called calcareous iglolite (as distinct from sandstones), since they consist of needle-shaped fragments of threads. As far as is known, this is a new type of sedimentary rock. Among them in the formation described there are two varieties: one with a chaotic arrangement of fragments of threads or needles, and the other, formed in a mobile environment, with their parallel orientation (under the influence of currents).

Also very interesting is a variety of fine-crystalline marbles with inclusions and veinlets of black solid bitumen.

There are other characteristic features of the composition of the Bultusuk Formation (for example, the presence of the thinnest hyolite interlayers). But in the main mass it consists of ordinary platy, bedded, or massive fine- or microcrystalline marbles, usually of light, more often almost white tones.

The upper part of the Bultusuk Formation consists of white-pink massive, unbedded archaeocyath buildups.

Thus, the Bultusuk Formation is fairly clearly distinguished from the neighboring underlying formations and still more from the overlying formations by its lithology, especially by the features of its appearance noted above.

This distinctiveness is also supported paleontologically. As in the underlying formations of the Lower Kolba complex described, archaeocyaths, epiphytons, and Razumovskii occur in it. But the hyolites in it are rather peculiar, more often very small. Traces of trilobites have been found in the Bultusuk Formation.

In conclusion, we shall dwell somewhat longer on the paleontological characterization of the Lower Kolba complex. Its paleontology is distinctive. Its originality is expressed, above all, by the presence of a significant group of very small primitive archaeocyath-like forms (protoarchaeocyaths). Among them are forms with round, elliptical, semi-oval, and other sections. There are also other striking forms:

a) monocyaths with an unusual transverse section of the cup (lens-shaped, subtriangular, sickle-shaped, oval-trapezoidal, rect-

angular, biconvex with the same and with different convexity on both sides, oval-angular, etc.);

b) mono-walled forms with a flaring opening and mushroom-shaped forms;

c) mono-walled forms with oval pores elongated in a spiral, rather than vertically or horizontally, as well as those in which this arrangement becomes normal in the upper part;

d) very small mono-walled forms with a diameter of about 1 mm, with sparse pores, or else with a thick wall and inclined-horizontal canals;

e) Archaeolynthus (or a new genus) with a rib on the dorsal valve;

f) small chalice-shaped forms (diameter up to 2 mm);

g) very small, apparently primitive, two-walled archaeocyaths, porous, with a diameter apparently of about 1 mm;

h) docidocyathids (?) with hexagonal pores and a small diameter;

i) an archaeocyath apparently with a thin (2–3 mm) mono-walled initial part of the cup;

k) other peculiar archaeocyaths—some with very large oval pores in the inner wall, others with a small septal coefficient of about 2, others with a flaring opening;

l) loculicyathids with a small diameter (1–2–3 mm);

m) an archaeocyath, possibly from the cyclocyatellids, but with a cup diameter of about 3 mm;

n) small forms of the genus Retecoscynus with a diameter smaller than that of the Lower Kenyadian species Retecoscynus retetabulae Zhur. (about 3 mm instead of 6–8 mm);

o) small irregular archaeocyaths, with a diameter of 0.5–2 mm, with a thick and thin outer wall, some of which are apparently close to Rhizacyathus radix from Australia;

p) apparently a new genus of corals without an inner wall, with long septa extending halfway to the radius, with an interweaving of thin rods in the center;

r) forms resembling cribricyathids, with a small (up to 1/4 mm) diameter, partly with an oval cross-section, etc. Finally, we note the presence of a species scarcely distinguishable from an okulicyathid.

Thus, the features of the archaeocyathan fauna noted above are indeed distinctive. How is this to be explained? At present it is still too early to give definitive explanations.

It is still necessary to await the completion of the monographic treatment of the archaeocyaths of the Lower Kolba complex of strata that we have collected. However, even now it is becoming clear that the youngest of them are no younger than the most ancient Sanashtykol forms. There are grounds for placing some horizons at the level of the Kameshkov beds. However, there are also indications of an age of the Belorechensk beds that is apparently synchronous with the Kenyadian–Sunnaginian age.

LITERATURE

1. I. K. Bazhenov, M. P. Nagorskii. Geology of the area of the city of Krasnoyarsk. Materials on the geology of Krasnoyarsk Krai, issue 1, 1937.

2. A. G. Vologdin. On the onto-phylogeny of archaeocyaths. Transactions of the Institute of Animal Morphology, issue 27, 1959.

3. I. T. Zhuravleva. Archaeocyaths of the Siberian Platform. Moscow. Publishing House of the Academy of Sciences of the USSR, 1960.

4. I. I. Koptev. Abstract of dissertation. 1962.

5. V. A. Obruchev. History of geological exploration of Siberia, Moscow. Publishing House of the Academy of Sciences of the USSR, 1932.

6. V. A. Obruchev. Geology of Siberia, vol. 1. Moscow. Publishing House of the Academy of Sciences of the USSR, 1935.

7. A. A. Predtechenskii. Section of the ancient Paleozoic in the area of the city of Krasnoyarsk. Problems of Soviet Geology, No. 7, 1937.

8. L. N. Repina, V. V. Khomentovskii, and M. A. Semikhatov. Regional Stratigraphy of the USSR, vol. 4. Stratigraphy of Precambrian and Lower Paleozoic deposits of the western part of the Eastern Sayan. Trilobite assemblages of the Cambrian of the western part of the Eastern Sayan. Moscow. Publishing House of the Academy of Sciences of the USSR, 1957.

1. Such rocks are usually called limestones, but, being at the base of a 6–7 km geosynclinal sequence, they could not have avoided at least weak recrystallization under high temperature and pressure. ↩