ON KEY PALEONTOLOGICAL HORIZONS AND BOUNDARIES: THE EXAMPLE OF THE STRATIGRAPHY OF THE KUZNETSK BASIN

L. L. KHALFIN

Questions of stratigraphic correlation of deposits, both within individual geological regions and over more extensive territories, up to correlations on a planetary scale, constitute one of the most important tasks of stratigraphy. Without solving this problem, neither paleogeographic reconstructions nor clarification of the geological development of particular territories is possible; consequently, it is also impossible to determine the regularities in the distribution, in space and time, of various mineral resources.

It is known that the principal method of stratigraphic correlation is the paleontological method, and its basis is the proposition stating that identical faunas are geologically contemporaneous (i.e., contemporaneous within the limits of the precision of stratigraphic correlations). Rejection of this initial proposition of biostratigraphic correlation would mean rejection of the use of the paleontological method for solving this range of problems; at present this method is practically the only one and cannot be replaced by any other. Yet in the history of geology, doubts have repeatedly been expressed regarding the validity of this proposition, beginning with T. Huxley’s concept of homotaxis.

As is known, proceeding from the correct proposition of the monophyletic origin of organic forms and from the a priori and (as was later shown) erroneous idea of their very slow and prolonged dispersal, Huxley asserted that identical faunas of regions and countries considerably distant from one another cannot be contemporaneous. These views inevitably had to give rise to another erroneous concept of Huxley’s—the concept of persistence, i.e., the practical invariability of organic forms over very long intervals of geological time. This brought the doctrine of homotaxis into contradiction with Darwinism, since, in accepting homotaxis, it is necessary to admit that organic forms under conditions of distant and prolonged resettlement remain unchanged (only in this case can identical faunas of different countries prove to be substantially different in age).

The successes of paleontology consistently and steadily destroyed the concept of persistence; L. Sh. Davitashvili [13], having analyzed and summarized the relevant materials, demonstrated its complete untenability.

At the same time, numerous biogeographical observations [14; 15, etc.] have shown that organisms are capable of very rapid dispersal over enormous distances; the time required for such dispersal, on the scale of geochronology, is a vanishingly small quantity, lying beyond the limits of precision of stratigraphic correlations. All this allows us to say that the concept of homotaxis is a page from the past in the development of geological knowledge, while the references to homotaxis and to the homotaxial character of deposits that sometimes appear in modern literature belong to the category of anachronisms.

At the same time, the improvement, refinement, and detailing of paleontological and biostratigraphic studies have again posed—although now in an entirely different plane—the question for stratigraphic correlation of the possibility that the same organic forms may appear in different regions and localities at different stratigraphic levels. Because of this, not all and not always biostratigraphic boundaries are boundaries of geological isochroneity. Cases have been established (examples will be given below) in which biostratigraphic boundaries are sliding ones, cutting across levels of geological contemporaneity. In this connection, in those cases where the given question is relevant, a special analysis is necessary for each biostratigraphic boundary in order to prove its isochroneity over the entire area under study.

This question is given special urgency by the circumstance that, as regional stratigraphic schemes are refined, the sliding character—the non-simultaneity—of the boundaries of suites, distinguished by ordinary methods according to their lithological features, becomes increasingly evident. This phenomenon is observed so often, and the scale of the “sliding” of suite boundaries in time proves so significant, that, for example, voices are being raised in American literature [21, p. 64] in favor of excluding suites altogether (formations, in the terminology of American authors) from the number of stratigraphic subdivisions. All this, naturally, draws the attention of geologists to key horizons and boundaries as levels of geological contemporaneity in the sections being correlated.

The questions we have touched upon (as well as a number of other related questions) deserve special consideration. The aims of the present work are more limited: to examine these questions only on the basis of materials on the paleontology and stratigraphy of the Kuzbass, chiefly with the purpose of analyzing the character of the boundaries between suites and subsuites in the new scheme for subdividing the coal-bearing deposits of the basin [34].

It is generally known that the basis of biostratigraphy as a whole is the process of evolution of the Earth’s organic world. All subdivisions of the international stratigraphic scale, which is biostratigraphic by its nature, as well as all local biostratigraphic subdivisions, reflect one or another stage of this general process of evolution of fossil fauna and flora. It is likewise generally known that the evolution of any group of organisms takes place not at some single point, but under conditions of constant migrations caused by changes in physical-geographical conditions.

Both in Russian and in foreign literature there are examples of very detailed studies that have revealed the full complexity of the picture of development of one or another association of organic forms inhabiting a given basin. Among the best and most widely known studies of this type are those, included in textbooks [20, pp. 69, 71], of the fauna of the Tertiary deposits of the Crimean-Caucasian basin and of the Carboniferous deposits of the Moscow syneclise.

As a preface to the presentation of materials relating to the stratigraphy of the Kuzbass, it is especially important to note, along with the broad migrations of entire faunas, the non-simultaneous appearance and disappearance, in different parts of the basin, of the same groups and species. Perhaps the most graphic picture of this phenomenon is provided by the investigations of E. A. Ivanova [17]: because of the uneven manifestation of oscillatory movements within the territory of the Moscow syneclise, there is observed a displacement in space and time of various facies settings and of the biotopes and biocoenoses associated with them. The disappearance and appearance of particular groups of fauna and individual species in specific sections is to a significant extent conditioned by the change of facies settings and by the migration of the biocoenoses associated with them. Thus, the same species of the genus Choristites Fischer have different stratigraphic distributions within the southern and northern limbs of the syneclise.

The significance of such investigations for a correct understanding of the full complexity of the development of fauna or flora, recorded in the deposits of a basin in the form of the corresponding distribution of organic remains through sections and over areas, is obvious: above all, they guard against simplified notions in the field of biostratigraphic correlation of sections.

It should be borne in mind, however, that the most complete and detailed investigations of this kind are the results of numerous works carried out over decades and concern the study of fauna (usually of ideal preservation) enclosed in deposits whose normal stratigraphic succession is clear. In geological practice such conditions are the exception; this applies especially to the eastern regions of the USSR, to which the center of gravity of prospecting, mapping, and geological-exploration work is now shifting. Insufficient knowledge of the geological structure and of the fossil fauna and flora, often combined with complex tectonics and insufficient exposure of the regions, which makes it extremely difficult to trace directly the distribution of organic remains in sections and over areas—these are the usual working conditions of paleontologists dealing with questions of the stratigraphy of Siberia. To this must be added the limited time available to the paleontologist, since the stratigraphic conclusions from his work have a direct bearing on current prospecting and exploration operations.

It does not follow from this, of course, that work under these conditions may be conducted at a lower theoretical level, without taking into account the achievements of modern paleontology and stratigraphy. But it is equally true that it will possess certain distinctive features and requires the development of certain special procedures and methods. A synthesis of this experience may be of definite interest.

In the present article an attempt is made at such a synthesis of work on the paleontology and stratigraphy of the Kuzbass, carried out mainly at the Department of Historical Geology of the S. M. Kirov Tomsk Polytechnic Institute (though, of course, with due regard also to published materials). In doing so, the author has considered it possible, relying on these works and their results, to touch upon certain general questions of stratigraphy as well.

On the Dual Character of Biostratigraphic Boundaries

It is known that one of the criteria permitting the establishment of the presence of breaks between stratigraphically adjacent units is a significant difference in the age of the fauna (or flora) below and above the boundary of contact of these units. By means of age analysis of the fauna (flora), in this case not only the presence of a break is established,

but also its scale (the upper limit of its duration). In a biostratigraphic sense, this break is expressed in the absence from the section of some assemblage of organic remains corresponding to a certain stage in the development of the fauna (flora) inhabiting the given region.

In the case where we have a section of deposits of some region, subdivided into suites by a series of breaks, we shall obviously find in this section a reflection of the history of the fauna that inhabited it, with a certain number of gaps corresponding to the number of breaks. But what is essential is that even in the presence of a continuous series of sediments formed over a sufficiently long time, in many cases we shall not observe in a particular section a successive and continuous development of the fauna.

Practically always the distribution of organic remains through a section reflects two interrelated processes, yet ones that find quite distinct independent expression: the process of evolution, the development of certain associations of organic forms, and the process of migration of these associations. Being interrelated and superimposed upon one another, these processes create a rather complex picture of the distribution of organic remains through the section and form a number of cases and combinations, the analysis of which makes it possible to use paleontological material for stratigraphy most fully. Let us begin by taking two extreme cases and considering them in concrete examples.

Beginning with the Kemerovo district in the north of Kuzbass, along the entire Cis-Salair belt and southward to the district of the city of Stalinsk, no traces whatever of a break in sedimentation are present at the boundary between the Upper Balakhon and Kuznetsk suites. Nevertheless, the boundary between these suites is determined quite clearly paleontologically: it passes 110 m above seam VI Internal in the Prokopyevsk district, 65 m above seam VII Internal in the Kiselevsk district, and at a level 70–75 m above the Kemerovo seam in the Kemerovo district.

This boundary is established by the replacement of the Late Balakhon pelecypod fauna by the Early Kuznetsk fauna; descriptions of these faunas are available in published works [31; 33]. Here it is sufficient to note the following circumstances: the Late Balakhon fauna is freshwater, the Early Kuznetsk fauna is brackish-water; the latter is not a derivative, a product of the development of the former; it is an immigrant fauna that entered the Kuznetsk depression together with the influx of brackish waters into it and almost everywhere displaced the aborigines that had previously lived here in freshwater basins. Thus, in the present case we are dealing with a clear biostratigraphic boundary reflecting a certain episode in the migrations of two different faunas.

As an example illustrating the other extreme case, let us take the distribution of plant remains in the Erunakov suite. The stratotype section of this suite along the left bank of the Tom River above the village of Erunakovo has repeatedly been subjected to detailed study, in particular phytostratigraphic study. Figure 1 gives the distribution of various plant species through this section, according to the observations of S. G. Gorelova [12]. In this figure we see a very gradual change of the flora, the disappearance of some forms and the appearance of other forms at different stratigraphic levels. This is a picture of the evolution, the development, of a certain plant community, and although the flora as a whole changes substantially throughout the section, we cannot draw here biostratigraphic boundaries as distinct as the boundary between the Upper Balakhon and Kuznetsk suites.

Nevertheless, M. F. Neuburg [25], who studied the flora from this section, divided it into five phytostratigraphic horizons, named

Fig. 1. Distribution of various plant species in the section of the Yerunakovskaya Formation along the Tom River above the village of Yerunakovo.
Drawing by S. G. Gorelova.

Visible taxon labels in the figure, from left to right:

Sphenobaiera sibirica Neub.; Noeggerathiopsis minuta Radcz.; Angaropteris arta Radcz.; Annularia anthriscus (Chachl.); Zamiopteris striata Radcz.; Annularia shorensis Yolcz.; Nephropteris acutiqua Neub.; Noeggerathiopsis chinskensis Radcz.; Noeggerathiopsis subaurita Radcz. et Gorel.; Pityostrobus (?) kinnersleyi Gorel.; Comia (?) pumilio Neub.; Pecopteris venikiensis (Chachl.); Cladophlebis acuta Zal.; Noeggerathiopsis andreyi Radcz.; Sphenostoma steeredta Neub.; Nephropteris taigana Yolcz.; Noeggerathiopsis beflani Neub.; Cladophlebis ostrovskensis (Chachl.); Noeggerathiopsis microvensis Gorel.; Sphenophyllum tomense Gorel.; Zeilleropteris longifolia Martyn.; Lepidophloios acuminatum Neub.; Noeggerathiopsis aequalis Schmalh.; Noeggerathiopsis khadkinia Gorel.; Koretia tomiensis Radcz.; Nephropteris elongata Neub.; Lepidophyllum astemelepides (Feist.); Phoenicopsis Nirovana Radcz.; Nephropteris schmalhauseni Radcz.; Tjujaseea typica Radcz.; Annularia lanceolata Neub.; Nephropteris micina Radcz.; Nephropteris cordata Radcz.; Cladophlebis neuroglina Radcz.; Jacutostrobus munigatica Radcz.; Cladophlebis tomiensis Radcz.; Cladophlebis begejifoecia Radcz.; Pityostrobus panita Zal.; Chibopteris baremboensis Radcz.; Syncladiopteris cuneata Radcz.; Sphenopteris obliqua Zal.; Annularia neurovensis Neub.; Sphenophyllostachys petnikvensis Gorel.

(from bottom to top) the Suryekovo, Turnovo, Korovikhino, Borisovo, and Erunakovo horizons. Each of these horizons has its own paleobotanical characteristic; moreover, in this respect floristic complexes (and, accordingly, horizons) of a twofold kind are distinguished. In the terminology of M. F. Neuburg, on the one hand there are horizons characterized by principal floras; these are the Suryekovo, Korovikhino, and Erunakovo horizons. These horizons are separated, are delimited, by horizons (the Turnovo and Borisovo) with mixed floras, consisting of components of stratigraphically adjacent principal floras. In essence, these horizons are extended boundaries between horizons containing the principal floras.

This is a general phenomenon for all cases in which, in a continuous section, the development (evolution) of some fauna or flora is observed and it is precisely this process of evolution that is taken as the basis for stratigraphic subdivision: in this case the biostratigraphic horizons are separated not by any levels or surfaces, but by certain stratigraphic intervals containing a mixed fauna (flora). Thus we ascertain the existence in nature of a twofold character of biostratigraphic boundaries:

a) definite and sharp boundaries, boundary-levels—when the development of the fauna (flora) in a given region (or at a given locality) was complicated or interrupted by a fairly abrupt change in conditions and the associated resettlement of organic forms;

b) indefinite and diffuse boundaries, stratigraphic boundary-intervals with a mixed fauna (flora)—when, in a section, we see a reflection of the pattern of successive development of the fauna (flora) inhabiting the given region, gradually changing its composition over the course of time.

A fine graphic expression of the process of development of a series of faunal associations, interrupted by repeated incursions of new associations, is the well-known scheme, repeatedly reproduced in the literature, of the history of the mollusk fauna of the Upper Tertiary basins in the Crimean-Caucasian region according to V. P. Kolesnikov.

Let us repeat that the two examples given above are extreme cases, considered in isolation. In actual reality, when studying sections and deposits of entire regions (basins), we often observe a combination, a superposition of phenomena of resettlement of organisms upon the general process of evolution, as we shall show later by the example of the paleontology of Kuzbass.

Speaking of the second type of biostratigraphic boundaries, it should be noted that their indefiniteness is, in turn, due to two causes. First, it is the appearance—already noted above—of new forms at different stratigraphic levels; and second, it is connected with the presence of numerous transitional forms between species. In sufficiently complete collections, between an ancestral species and the species derived from it, there are usually found transitional links connecting them, which are regarded now as varieties of one of these species (usually the original species), now as independent species. As an illustration one may cite (Fig. 2) the genetic relationships between species of the pelecypod group, very widespread in the deposits of the Erunakovo Formation—the group Anthraconauta pseudophillipsii Fedotov [32, p. 114].

Relationships of this kind between genetically related species can be observed only in those cases where the continuous development of one fauna (flora) or another is recorded in the section. Conversely, in the first of the cases we have considered, since the Early Kuznetsk and Late Balakhonka faunas are not linked by succession, there are no—

there are also forms that might be regarded as transitional between these faunas.

Fig. 2. Diagram of genetic relationships among species of the group Anthraconauta pseudophillipsii Fedotov.

Considering Fig. 1, which gives us a picture of the gradual development of the flora of the Erunakovskaya Suite, we become convinced that this development is not entirely smooth and uniform. In fact, this figure, on the one hand, clearly shows the absence in this flora of index assemblages of forms in the usual sense of the term, i.e., assemblages whose distribution would be limited to the bounds of a definite horizon. But at the same time certain boundaries are indicated, at which—or, more precisely, near which—not single forms but at least several forms appear and disappear, for example at the level of coal seams 38 and 49.

The phenomenon noted in the preceding paragraph makes possible a biostratigraphic subdivision of sections in which we observe the continuous development of a flora (fauna) already on the basis of a quantitative accounting of the forms appearing and disappearing at different levels, although this subdivision cannot be as sharp as in the first case. One of the methods that may be recommended in this case consists in distinguishing horizons within which a definite composition of species is preserved, although each of them also extends beyond the limits of the given horizon: some species appear near its lower boundary, while others disappear near the upper one (a third group of species, which appear below the given horizon and pass above its upper boundary, is immaterial). For example, for “horizon 6,” distinguished by S. G. Gorelova [12, p. 6] between coal seams 38 and 49 of the stratotypic section of the Erunakovskaya Suite, the following are characteristic:

a) forms appearing near its lower boundary (seam 38) and passing into higher horizons: Phyllotheca turnaensis Gorel., Koretrophyllites minutus Radcz., Annularia lanceolata Radcz., Noeggerathiopsis Khalfinii Gorel., N. insignis Radcz., Nephropsis cordata Radcz., Niazonia stellata Radcz.;

b) forms that appeared below “horizon 6” but end their existence near its upper boundary (seam 49): Annularia uskovensis Gorel., A. batchatensis (Chachl.) Radcz., Noeggerathiopsis minuta Radcz., N. acutangula Neub., Nephropsis tomiensis Zal., Crassinervia arta Radcz.

For “horizon 6” S. G. Gorelova cites not a single index form: it is characterized by a combination of the two groups of species listed above, although all of them have a considerably wider and, moreover, different vertical distribution¹. In other words, in the present

in this case, certain species with a broad vertical range, when occurring together, acquire the significance of index forms for a narrow stratigraphic interval (horizon).

The main difficulty in using this method lies in the fact that new collections of flora (fauna) usually alter ideas about the vertical distribution of species in the direction of expanding it. Let us give several examples (see the table).

Such cases are very numerous. It is impossible to explain them only by different authors’ differing understanding of the scope of species; the whole point is that S. G. Gorelova had at her disposal considerably more extensive and more systematically conducted collections of flora.

The reason for the difficulties in establishing the actual vertical distribution of forms, in the case when we trace step by step the evolution of some flora (fauna), is understandable: each of the forms initially appears in a small number of specimens, and before its disappearance the number of its representatives usually likewise gradually decreases. Under these conditions it is, of course, difficult to catch the moment of appearance and the moment of disappearance of a form, and new collections can always introduce new refinements. It is quite another matter when, as a result of a sufficiently abrupt change in conditions, one fauna (flora) gives way to another, as happens at the boundary between the Upper Balakhonka and Kuznetsk formations.

The mismatch between the horizons distinguished by S. G. Gorelova [12, table 4] in the section of the Erunakovskaya Formation and the horizons previously distinguished by M. F. Neuburg [25] in the same section is explained, in particular, by refinement of the boundaries of the vertical distribution of species. For the same reason, the principal floras distinguished by M. F. Neuburg require a revision of their composition.

In the case under consideration, the establishment of biostratigraphic boundaries is also aided by qualitative analysis of newly appearing forms—namely, by identifying among them new progressive elements. Thus, in the same section of the Erunakovskaya Formation, at the level of seam 60, S. G. Gorelova draws an important stratigraphic boundary according to the first appearance, within this Upper Permian flora, of forms of Mesozoic aspect (representatives of the genera Tychopteris Zal., Iniopteris Zal., Rhipidopsis Zal., Chiropteris Zal., and some others); quantitatively these forms occupy a subordinate position, but, qualitatively constituting a new association, they acquire important stratigraphic significance.

It should, however, be noted specifically with respect to these forms that their appearance in the Kuzbass was probably caused not by their origination in place, but by immigration from somewhere outside: within the local flora, despite—

because, despite its being well studied, one cannot indicate forms that could be regarded as ancestral in relation to these Mesozoic elements. It is precisely because of this sharp distinction from the rest of the flora that their appearance in the section provides a sufficiently definite biostratigraphic boundary. In other words, in the present case there is a certain analogy with the appearance of an immigrant Early Kuznetsk fauna, marking the boundary between the Upper Balakhonka and Kuznetsk formations. True, this case has its own special features, consisting in the fact that the appearance of immigrant forms is not accompanied by the extinction or emigration of those that had inhabited the area earlier. We shall return to this case later, when considering the question of key stratigraphic horizons.

Additional possibilities for stratigraphic purposes are provided by the study of the individual variability of species in combination with tracing them through the section. In this case, those gradual changes in forms that lead to the appearance of new species are recorded. Studies of this kind, begun in application to the flora of Kuzbass by M. D. Parfenova, all concern the same flora of the Yerunakovo Formation, from the same section; they have been partly published [1] and are partly being published in the present collection. Therefore here we may confine ourselves to giving only a few examples. Thus, when the leaves of Noeggerathiopsis angustifolia Neuburg are considered from the standpoint of the value of the ratio of the length of their apex to the width of the leaf (which varies within the limits from 0.5 to 1.09), it becomes clear that near the level of seam 38 (the lower boundary of the “horizon b” of S. G. Gorelova considered above), leaves for which this ratio exceeds 0.8 disappear. In the leaves of Lepeophyllum acteonelloides (Gein.) Zal. the same ratio varies within the limits of 0.7–1.8, and upward through the section the number of leaves with a long apex decreases regularly, while the number of short-apex leaves likewise increases regularly. There is also a regular appearance upward through the section of seeds of Angarocarpus tychtnensis (Zal.) Radcz. with an ever greater value of the ratio of their width to length, etc.

The study of the anatomical structure of the epidermis of leaves, stems, and seeds—also first begun in application to the fossil flora of Kuzbass by M. D. Parfenova [1]—may have still more substantial significance for stratigraphy. This method makes it possible to distinguish species that are not distinguishable morphologically. Thus, among specimens belonging to the morphological species Angarocarpus tychtnensis, there proved to be species possessing substantially different cellular structures of the cuticle and having different vertical distributions.

Using the flora of the Yerunakovo Formation as an example, we have considered the possibilities and methods of biostratigraphic subdivision that are based on the study of the evolution of a certain plant community. We have become convinced that these methods, approaches, and possibilities are diverse, far from exhausted, and become more varied as this flora is studied in greater depth. But at the same time, the very nature of the process underlying biostratigraphic constructions—namely, the successive evolution of the flora—predetermines the character of biostratigraphic boundaries for this case: they will always yield, in sharpness and definiteness, to boundaries drawn on the basis of migrations of faunas and floras, and in the general case will have the character not of levels (surfaces), but of certain stratigraphic intervals (horizons).

On migrations of biocenoses and faunas (floras)

Let us return to the consideration of the biostratigraphy of Kuzbass, specifically to the distribution of floral remains in the section of the Yerunakovo Formation. The materials of M. F. Neuburg and S. G. Gorelova cited above show

picture of the continuous development of the flora of this suite. However, these data represent a certain generalization of the distribution of floristic remains actually observed in the section. In reality, a fully continuous distribution of plant remains through the section is not observed: in any interval of the section, beds containing plant remains alternate with beds lacking them; most often the sandstone beds lack identifiable plant remains, containing usually only plant detritus, carbonized fragments of branches, and sometimes petrified wood. This kind of discontinuity in the distribution of plant remains through the section is apparently explained mainly by conditions of fossilization of plant remains—favorable or unfavorable to varying degrees (chiefly leaves and seeds, by which the species listed by M. F. Neuburg and S. G. Gorelova are established). But the discontinuity in the distribution of plant remains through the section also has another form of manifestation, reflecting not the conditions of their burial, but certain synvital phenomena—namely, the migration of biocenoses under the influence of local changes in their conditions of existence. This applies equally to plant and animal cenoses. This phenomenon and its significance for stratigraphy can be demonstrated quite clearly by the example of the Kolchugino fauna of pelecypods.

The Kolchugino fauna is widespread in the deposits of the Ilyinka and Erunakovka suites, although it had already appeared in the Kuznetsk suite [3, p. 90]. Its most typical representatives are the genera Microdontella Lebedev and Microdonta Khalfin, together with which the genera Anthraconauta Pruvost and Palaeanodonta Amalitzky are abundantly present; somewhat more rarely—Abiella Ragozin and Anthraconaja Tr. et Weir.

This fauna has been described repeatedly from various districts of the Kuzbass, and it seemed that everywhere it was represented abundantly in quantitative terms and diversely in species composition. Only in 1945, in the Prisalaïr belt of the Kuzbass, in deposits transitional between the Ilyinka and Erunakovka suites, was this fauna found in an extremely impoverished state [3, p. 110]: its impoverishment is expressed in an extremely poor composition (almost exclusively the genera Microdonta and Microdontella), in dwarf shell sizes, and in its extreme quantitative scarcity; highly characteristic is the complete or almost complete absence of anthraconautids, which are very abundant in the normal Kolchugino fauna. It is also very characteristic that the flora accompanying this fauna likewise proved to be depressed—consisting almost exclusively of small-leaved cordaites.

Later, as a result of the investigations of O. A. Betekhtina [5], a depressed fauna of the same type was found in a number of districts of the Kuzbass in deposits stratigraphically higher. It turned out that in some districts such a fauna appears repeatedly in the section, alternating with the normal Kolchugino fauna. It also turned out that in these higher beds the depressed pelecypod fauna is accompanied by an abundant and diverse flora that shows no signs of depression.

Figure 3 presents the distribution of pelecypods in the lower part of the stratotypical section of the Erunakovka suite. This figure clearly shows the uneven distribution through the section of the fauna as a whole, and of anthraconautids in particular: in the intervals from coal seam 49 to coal seam 33, from coal seam 13 to coal seam 6, and downward from a level 105 m below coal seam 1, the fauna is extremely scarce, and anthraconautids occur only singly, as very small specimens. Conversely, in the intervals from only

from the above-mentioned level to bed 6, from bed 13 to bed 33, and above bed 49 the fauna is abundant, while the anthraconauts are diverse, have normal dimensions, and are present in large numbers.

It is obvious that such a replacement of two types of a single fauna reflects not the general process of its evolution, but migrations of these faunal types under the influence of changing conditions. We shall discuss the possible causes that induced the fauna to make these migrations in the next section; here it is important to note two circumstances:

1) these migrations of depressed and normally developed biocoenoses were short-lived: each time the fauna returned to the given area in the same species composition in which it had previously left it;

2) in some cases these migrations were, moreover, not distant, but occurred over a limited area within the basin: this is proved by the fact that, according to the observations of O. A. Betekhtina, in sections of the Erunakovskaya Formation of certain deposits the position of particular horizons with different types of fauna does not coincide, i.e., they are situated at different levels. Obviously, in this case we are dealing with the reflection in the section of one area or another (deposit) of local changes in conditions, which is why the corresponding horizons will have only local significance.

An analogous, but at times still more frequent and finer alternation of biocoenoses (only no longer of depressed and normally developed ones, but of brackish-water and freshwater ones) is observed in places in sections of the Promezhutochnaya Subformation [4] and in the lower parts of the Kuznetskaya Formation [31]. The same picture is also observed with respect to the flora: in the article by O. A. Betekhtina [7], published in the present volume, it is shown how, in the section of an individual deposit, local horizons can be distinguished that reflect migrations of phytocoenoses (or assemblages of phytocoenoses), differing partly in their composition and partly in their general character.

At one time [31, p. 14] we called such migrations intra-areal migrations of biocoenoses, and the repeated appearance of the latter in the section—intraformational recurrence of biocoenoses. These migrations, from the standpoint of the interests and tasks of stratigraphy, should be distinguished from broader migrations, during which the fauna (flora) leaves the limits of a given region (basin) as a whole, usually for a longer, but sometimes also for a very short, time. Migrations of this kind we shall consider somewhat later.

Let us return to Fig. 1 and Fig. 3 and compare them from a theoretical point of view, with the aim of elucidating the processes reflected by these graphs. Let us note, first of all, that Fig. 3 can easily and simply be reduced to the type of Fig. 1. To do this it is sufficient, in Fig. 3, to remove the quantitative characterization of species distribution and simply to connect by a straight stratigraphic line the extreme points of occurrence of each species. As a result of such an operation we would obtain a graph completely analogous to that shown in Fig. 1: the various species likewise will appear and disappear at different stratigraphic levels; this will be a picture of the evolution of the fauna of pelecypods of the Erunakovskaya Formation, entirely analogous to the picture of the evolution of the flora reflected in Fig. 1.

As has already been noted, the evolution of the population of any basin proceeds under conditions of constant migrations of biocoenoses in connection with the inevitable shifts of facial environments (stations and biotopes) inhabited by these biocoenoses. This pattern of migrations is additionally reflected by graphs of the type shown in Fig. 3; i.e., these graphs more fully reflect the history of the given fauna (flora): in them we see how, upon the general process of evolution of the given fauna (flora), the migrations of the biocoenoses composing it are superimposed.

Fig. 3. Distribution of various species of pelecypods in the section of the Erunakovo Formation along the Tom River near the village of Surnekova. Drawing by O. A. Betekhtina.

Visible labels in the figure include: Genus Microdontella, genus Anthraconaia, genus Anthraconauta, genus Anthracomya, Anthraconauta phillipsi, Ostracoda, “remains of small pelecypods,” “remains of small phyllopods,” and Horizons I–IV.

In this sense we indicated above (p. 48) that these processes are closely interconnected and find a distinct, separate expression in the section.

We shall now briefly dwell on the practical side of the question—on clarifying the significance of migrations of biocoenoses for purposes of stratigraphy. This significance consists in the following: in any section it is possible to distinguish certain stratigraphic intervals (horizons), as a rule very fine ones, recognizable already by the general character of the fauna and flora contained in them. This is important in the following respects.

First, the species entering into the composition of these complexes often have a very wide vertical distribution and are not index forms; they may be found both considerably above and considerably below the given horizon—in deposits of identical facies; but immediately above and below the given horizon they are absent, being replaced by representatives of other biocoenoses characteristic of somewhat different facies. In other words, within a limited area over which local changes in conditions extended and caused a replacement of biocoenoses, even indifferent forms and groups may be used for detailed stratigraphic subdivision and correlation.

Second, such horizons, characterized by entire biocoenoses, are usually recognized not by the species composition but by the generic, and even more general, composition of the organic remains and by the overall character of the fauna and flora. This makes it possible to distinguish and trace such horizons already in the course of the primary documentation of boreholes—during the description of core by workers of geological-exploration parties, i.e. directly in the course of geological-exploration work. The practical application of this method is the subject of the article by O. A. Betekhtina and L. L. Khalfin printed in the present collection.

It should be emphasized in every way that a stratigraphic subdivision based on this method is applicable only to a limited area, since the replacement of biocoenoses is conditioned, as already noted, by local changes in conditions, and, consequently, the succession of biocoenoses in the section will not coincide for different districts. But, as experience has shown, within the limited area of an individual deposit the biostratigraphic intervals distinguished by this method are often the best marker horizons, making it possible reliably to correlate sections in boreholes, along exploration lines, and in exploratory areas.

Not only the examples cited above, but the entire experience of biostratigraphic investigations shows that processes of migration of organic forms and communities have a direct bearing on the solution of concrete stratigraphic problems. Therefore we shall allow ourselves to dwell on this circle of questions, of course not in the broad scope that this important question deserves, but only to the extent to which its consideration is necessary for illuminating the materials on the stratigraphy of Kuzbass. Nevertheless, we must begin with certain general premises.

From the point of view of the interests of stratigraphy, two cases of the dispersal of organisms should be distinguished: dispersal in the absence or rapid disappearance of barriers, and dispersal in the presence of barriers that, with the passage of time, slowly move in space (the barriers being facies settings that cannot be inhabited by the group of organisms under consideration).

The first case may occur, for example, when one fauna (flora) or another gains access to the limits of a territory (water area) not previously inhabited by it, but suitable for it or having become suitable for its

existence (for example, when previously separated basins are joined, or during the ingression of a basin that has rapidly filled some depression), or when, in one or another part of a sufficiently extensive basin (territory), a new form or group of forms arises. Numerous observations on the dispersal of various groups of modern organisms show that such dispersal takes place extremely rapidly over enormous distances; and for this case the fundamental principle of biostratigraphy fully retains its force—the geological contemporaneity of identical faunas throughout the area of their distribution (from the moment of their free dispersal until the onset of extinction, which cannot be simultaneous over the whole territory occupied by the faunal range). These will be the so-called cosmopolitan forms, which, as the composition of both modern and fossil faunas shows, are by no means an exceptional phenomenon.

The second case is a slow displacement in space and time of facies settings together with the organisms inhabiting them. In this case the rate of dispersal of organisms is limited by the rate of migration of the facies. Let us recall the example already cited of the diachronous appearance of the same brachiopod species within the northern and southern wings of the Moscow syneclise—in parallel with the movements of the corresponding facies.

On a broader scale, a regular displacement (chronologically—upward) of the boundaries of the stages of the Upper Devonian in the states of New York and Pennsylvania, during movement from east to west, has been established [24, pp. 40–42], with a corresponding time displacement of the faunas characterizing these stages. However, a priori one may suppose that in all such cases the difference in the stratigraphic position of the same forms cannot be significant: if the facies shift too slowly, i.e., if the process of faunal migration is stretched over a long time, this fauna, by virtue of the general laws of evolution of the organic world, cannot fail to change noticeably and, consequently, at some distance from the initial area of its habitat it will already appear in a different species composition.

A very instructive section of the Cambrian deposits of the Grand Canyon (Colorado), compiled by McKee [21, p. 65], may serve as a good illustration of what has been said. Within these deposits there is the Bright Angel suite (formation in the understanding of American authors), consisting of green argillaceous shales with interbeds of more coarse-grained rocks. Both the upper and lower boundaries of this suite, when traced in a latitudinal direction, shift very significantly: in the west the base of the suite is marked by the fauna of the lower Lower Cambrian (Olenellus), while 350 km to the east, at the same distance from the footwall of the suite, there is a horizon with Middle Cambrian trilobites (Glossopleura and others).

The chronological limits that bound differences in the stratigraphic position of one and the same fauna in different districts or regions are determined by the still uninvestigated relationship between the rate of facies migration and the rates of evolution of organic forms. In any case, this difference, for each species, lies within the limits of the time of its existence; for rapidly evolving groups (archistratigraphic groups), these limits are always narrow, and therefore deposits characterized by identical representatives of such groups may be regarded, within the accuracy of stratigraphic constructions, as geologically contemporaneous.

In the Kuznetsk Basin, throughout the Upper Paleozoic, both of these cases are observed (Fig. 4).

The first of these is the settlement within the basin of the early Kuznetsk fauna (see above, p. 48). As already noted, at the end of the time of formation of the Upper Balakhonka suite (the Usyaty sub-suite), a Late Balakhonka fauna was widespread in the Kuznetsk Basin; it apparently inhabited fairly extensive and interconnected fresh-water bodies. The beginning of the time of formation of the Kuznetsk suite was marked by the penetration (ingression) into the basin of brackish waters, together with which the Early Kuznetsk fauna rapidly spread over almost the entire area of the basin.

This fauna, sharply different from the Late Balakhonka fauna, has the closest resemblance to the so-called Alykaevo fauna [31, p. 38], which characterizes the Lower Balakhonka suite—chiefly the Alykaevo sub-suite. This resemblance is expressed in the presence in both faunas of very close representatives of the genus Augea Khalf in and of the group Mrassiella magniforma Rag., which are absent from the Late Balakhonka fauna. The resemblance is so considerable that at first these faunas were called [31] the first Alykaevo and the second Alykaevo, in order to emphasize the recurrent character of the latter.

In the example considered, the settlement of the early Kuznetsk fauna over the territory of the basin is connected with a substantial change in the character of the water bodies and, as a consequence of this, with the disappearance of the Late Balakhonka fauna that had lived here earlier. Thus, the distinctness of the biostratigraphic boundary between the Upper Balakhonka and Kuznetsk suites is due to the complete replacement of one fauna by another.

Another example of penetration into the basin and settlement over its area by immigrant forms is the appearance of the so-called “forms of Mesozoic aspect” in the flora of the Erunakovo suite (see p. 51). Apparently, these species, having arisen and formed outside Kuzbass, penetrated at a certain moment into the Kuznetsk Basin; moreover, this was not connected with any appreciable changes in the physicogeographical setting that could have had a substantial effect on the flora living here. The new settlers simply became admixed with the quantitatively predominant plant aborigines; therefore the boundary marked by their appearance is not as clearly expressed as in the first of the examples considered by us.

Another case of migrations is illustrated by the examples considered above of intra-areal migrations of biocoenoses, or of certain assemblages of biocoenoses, under the influence of local fluctuations in living conditions, i.e., in connection with migrations of facies within the basin.

It is quite obvious that, in the general case, this movement of facies takes place gradually and rather slowly, but it can be observed only by tracing the replacement of facies over an area. Yet even in individual sections (Fig. 3) the boundaries, for example, between horizons with normal and with impoverished faunas are not entirely sharp: different forms appear and disappear not at one level, although close to it. This marks the initial and final moments of migrations that began slowly and likewise ended slowly; moreover, the different components of the biocoenoses did not take part in these migrations simultaneously.

The specific causes of such migrations of biocoenoses, at the present level of our knowledge, cannot always be precisely indicated, but they are sufficiently varied and changed over the course of time. Thus, in some cases the leading cause was a change in the degree of salinity of the water bodies located in the given region. This is very clearly expressed in the alternation of biocoenoses in the lower part of the Kuznetsk suite.

in the area of the town of Stalinsk and in the intermediate subformation in the Kemerovo district (see below, p. 66).

It is considerably more difficult to establish the causes of the migration of the various biocenoses constituting the freshwater Kolchugino fauna; this question comes down to determining the causes that exerted a suppressive effect on this fauna. Since pelecypods are stenionic organisms (for modern pelecypods the limiting value is a pH equal to 6), it may be assumed that one of the suppressive factors could have been an increase in the concentration of hydrogen ions in the water of the lakes they inhabited, quite possible with the wide development of bogs and processes of peat formation throughout the entire time of formation of the deposits of the Il’inskaya and Erunakovskaya suites. At present one thing can be stated with certainty: these suppressive factors differed at different times. This conclusion follows from the fact that the character of the flora accompanying the suppressed biocenoses of pelecypods differs in different horizons.

As already noted, in one case this accompanying flora also has a suppressed character; in other cases, horizons with a suppressed fauna contain an abundant and diverse flora. Obviously, in the first case the action of the suppressive factors extended also to the terrestrial flora and to the population of water bodies, i.e., these factors had a sufficiently universal character; in the second case they affected only the aquatic fauna.

It is important to note that in all regions where several horizons with a suppressed fauna have been established, one of them, confined to the transitional beds between the Kazankovo-Markinskaya and Uskatskaya subformations, is at the same time characterized by a suppressed flora, while stratigraphically higher there are horizons with a suppressed fauna and a flourishing flora. In due course, analyzing the composition of the fauna and flora of the lower horizon, their generally suppressed character, the nature of their burial, and the composition of the enclosing rocks, we came to the conclusion that “this fauna inhabited small and shallow water bodies of stagnant and semi-flowing type amid a uniform cordaite forest” and that “the principal cause of the suppression of our fauna was the severe climatic conditions that prevailed during the formation of the deposits with this fauna” [31, pp. 110, 111]; hence the conclusion was drawn that deposits with a suppressed fauna and flora may serve for purposes of stratigraphic correlation within the basin as a whole, since “it is improbable that climatic changes could have had a narrowly local character” [31, p. 116]. This is precisely the universal factor that had an equally unfavorable effect on both the flora and the fauna.

If this supposition is correct, then it must be assumed that the deterioration of the climate (probably cooling) extended over the entire basin, and the more stenothermal (thermophilic) groups of fauna and flora left its limits and returned to it only after the restoration of the former climatic conditions. In other words, here there is a certain analogy with the behavior of the brackish-water Alykaevskaya fauna, which at the end of the time of formation of the Nizhnebalakhonskaya suite left the Kuznetsk depression and returned to Kuzbass only at the beginning of the formation of the deposits of the Kuznetskaya suite, together with the ingress of brackish waters (see p. 57). The difference consists only in the fact that between the emigration and return of the brackish-water fauna much time elapsed, and this fauna returned in an already noticeably changed composition, whereas the time during which severe climatic conditions existed in the depression was not long, and after this time had passed the fauna and flora returned in their former species composition.

Fig. 4.
Main stages in the history of the pelecypod fauna of the Kuzbass in the Upper Paleozoic.
I and III, brackish-water faunas: 1 — Alykaevskaya fauna; 1a — superstitious elements of the Alykaevskaya fauna; III — Early Kuznetsk fauna.
II and IV — freshwater faunas: II — Late Balakhonskaya fauna; IIa — relicts of the Late Balakhonskaya fauna; IV — Kolchuginskaya fauna; IVa — genus Neamigenia Khalfin; IVб — group Palaeanodonta pseudolongissima–Palaeanodonta glossitiformis; IVв–IVг — oppressed Kolchuginskaya fauna.
A — European immigrants: A₁ — genera Anthraconauta Pruvost, Anthraconia Tr. et Weir, Naiadites Dawson; A₂ — genus Palaeanodonta Amalitzky.
Б — Gorlovka immigrants (genus Yavorsk’iella Khalfin).
1a — emigration of the Alykaevskaya fauna beyond the limits of the Kuzbass; 1б — its subsequent return; 2a and 2₁a — emigration beyond the limits of the Kuzbass; 2б and 2₁б — return to the Kuzbass of stenobiontic components of the Kolchuginskaya fauna (genera Anthraconauta and Palaeanodonta).

It is considerably more difficult to decipher the conditions under which an impoverished fauna and a flourishing flora coexisted, especially since here we encounter, in turn, two cases. We have already spoken of one of them: these are the horizons that have a strictly local character and that, apparently, are connected with a change in the type of fauna under the influence of local fluctuations in pH. But in addition to this there exists at least one horizon with precisely such a paleontological characteristic (impoverished fauna—flourishing flora), which may be regarded as widespread throughout the entire basin, just as the horizon with impoverished fauna and impoverished flora at the boundary between the Kazankovo-Markino and Uskat subformations.

Between these two horizons lies the Uskat subformation, characterized (everywhere except the Prisalair belt) by an unimpoverished Kolchugino fauna and a flourishing flora. At the upper boundary of the Uskat subformation, i.e., at the boundary between the Ilyinka and Yerunakovo formations, the hydrological conditions of the bodies of water situated in the Kuznetsk depression changed strongly, although in a manner unknown to us, which had an extremely unfavorable effect on the pelecypod fauna inhabiting them. If this boundary were marked only by the replacement of a normal fauna by an impoverished fauna, we could not say how simultaneous this replacement was over the whole area of Kuzbass, since we do not know the nature of the changes that brought about the impoverishment of the fauna.

But, apparently, the same changes that so substantially affected the aquatic fauna also affected the flora, but not in the form of a generally depressing factor (as happened at the end of the time of formation of the Kazankovo-Markino subformation). This was expressed in a noticeable, though not very abrupt, change in the composition of the flora—the disappearance near this boundary of some forms and the appearance of others. Let us note that this is the same boundary (coal seams 38–39) at which we have already recorded, according to S. G. Gorelova, intensified form-generation in the composition of local plant groups between the II and III Chertino floristic complexes—a boundary that is well traceable in different regions of Kuzbass [6, p. 195]. These changes in the composition of the flora and the replacement of the normal fauna by an impoverished one made it possible precisely at this level to draw the boundary between the Ilyinka and Yerunakovo formations, as was done (although not entirely consistently for all districts) in the stratigraphic scheme of 1954.

If we consider the processes that interest us at the present moment in the light of migrations of associations of organic forms, then from the materials cited above we must draw the following conclusions: at the boundary between the Ilyinka and Yerunakovo formations, the most stenobiontic groups of pelecypods (above all, almost all anthraconautids) were forced once again temporarily to leave the limits of the Kuznetsk depression, while newly arising plant forms spread over its territory.

The materials cited above, concerning various cases of migrations and dispersal of the Upper Paleozoic fauna and flora in Kuzbass, are needed by us chiefly for considering the question of the so-called key stratigraphic horizons.

On key horizons and key boundaries

The concept of a marker horizon is well known to geologists, who widely use marker horizons in their practical work. The concept of a key horizon requires clarification, if only because marker and key horizons are sometimes regarded as synonyms. Thus, in the Geological Dictionary of VSEGEI [10, p. 95] the following definition of a key horizon is given:

“A bed or stratum within a mass of rocks, distinguished by lithological features: color, composition, the presence of any inclusions, interbeds, etc., or by a complex of organic remains, and retaining its features over a considerable area, which makes it possible to use it for tracing and correlating sections and in geological mapping (synonym—marker horizon).”

This is the definition of a marker horizon; for a key horizon it is insufficient. The essence of the matter is that, with a slow and prolonged displacement of facies, lithological horizons (including marker horizons) traceable over an area are chronologically displaced, crossing stratigraphic boundaries at one angle or another. Thus, during a slowly developing transgression, a basal conglomerate that is a good marker horizon will, at different points within the area of its development, have such a different geological age that this difference must already be taken into account in stratigraphic correlations. There is no need here to dwell in greater detail on this phenomenon, first investigated by N. A. Golovkinskii [11], whose ideas have recently again attracted the attention of Soviet geologists [29; 16; 22, etc.].

In most cases the difficulties caused by the sliding character of the boundaries of lithological horizons and suites are overcome by means of the paleontological method—as in the case of the lower boundary of the Bright Angel Formation (see above, p. 56). In the Kuzbass, as a result of the decisions adopted at the 1954 conference, the conventional boundaries between suites, drawn along the first or last coal seam of workable thickness in the section, were replaced by biostratigraphic ones, which was a major and important step forward in improving the stratigraphic scheme of the basin [34].

But, as we have already established in the preceding section of the article, biostratigraphic horizons and paleontological boundaries may in some cases also be sliding, although within narrower limits than lithological boundaries. In light of this circumstance, the question of key horizons—understood as horizons that are strictly coeval, or that come as close as possible to coevality, over the entire area of their development—acquires special interest. Considerable attention is paid to this question in American geological literature, but it still awaits systematic elaboration.

Some remarks on this question, based on materials on the stratigraphy of the Kuzbass, will be made after a brief note of a general character.

Thus, key horizons are horizons that may be considered geologically isochronous over the entire area of their development.

In many cases, though (as already indicated above) not always, marker horizons play the role of key horizons. At the same time it should be emphasized once again that the isochrony referred to in the definition of a key horizon given above can only rarely be complete and certain. But even in those cases when the processes responsible for the emergence of the typical features of a marker horizon spread over a considerable area with great speed, the marker horizon acquires the character of a key horizon. By great speed we mean a speed comparable with the rate of dispersal of organisms in the absence of barriers and, in any case, exceeding the rate of displacement of the basin’s facies; evidence of the latter is the transition of the marker horizon from one facies into another (see below). In practice, with respect to almost any marker horizon, a specific analysis is necessary in order to determine to what extent that horizon may be regarded as a key horizon.

It is possible, however, to have reference horizons that do not have the character of marker horizons, since the latter are associated with the notion of their sufficiently obvious features by which these horizons can be recognized and used in the geologist’s field work. Reference horizons, by contrast, may also be such that their characteristics can be established only in the course of office (laboratory) work (microfauna and microflora, a slight admixture of volcanic ash, etc.). For a marker horizon the most important feature is its evident difference from the enclosing rocks; for a reference horizon, its isochronism.

Reference boundaries are the boundaries of regional stratigraphic subdivisions, isochronous over the entire area of development of the latter. These are, on the one hand, the boundaries of reference horizons, and in a number of cases (though far from always) also the boundaries of larger subdivisions (subformations, formations, series).

By their nature reference horizons may be very diverse; they may be both lithological and paleontological. Their classification has not been worked out; moreover, not all types of reference horizons have been identified. Let us give several examples of lithological and paleontological horizons.

The ideal case of a reference horizon is a layer of volcanic ash or bentonite, because of its strictly identical age and its independence from facies settings [24]. Falling from the air after a volcanic eruption, the ash layer covers sediments of different facies; subsequently, sediments of those same facies will be deposited on top of the ash layer, which thus, in a section, will pass from one facies into another, crossing the boundaries of lithological horizons.

Confidence in the strictly identical age of bentonite beds is determined by the conditions of their formation. In other cases, not only is there no such confidence, but in general reference horizons will not be absolutely of the same age. As already indicated, what is decisive in this case is the very fact that the horizon passes from one facies into another: the crossing by a marker horizon of facies boundaries gives it the significance of a reference horizon.

A good example of the identification of similar reference horizons in the Permian deposits of the Donbass is found in the work of L. P. Nesterenko [27]: by means of these horizons, passing from facies to facies, the named author refined the stratigraphy of the Permian deposits of the Donbass, which previously had been based on considerably less persistent marker horizons in the form of beds of salt, gypsum, dolomite, and limestone. Similar lithological reference horizons, passing from the Muab limestone formation into the Bright Angel shale formation, are described by McKee in the work already cited above [21].

In the practical work of geologists in Kuzbass, dealing with its facially variable coal-bearing deposits, certain—usually thick—beds of sandstones, gravelites, or carbonaceous argillites are used as marker horizons, and above all coal seams. For individual districts of Kuzbass this method is quite admissible, but one must emphatically warn against the still-continuing attempts to extend it to the entire territory of the basin, despite the firmly established fact of great facies variability across the area of the basin, even of whole formations and subformations.

Thus, it has long since been established that the Krasnoyarka Formation, which figured in the stratigraphic schemes of Kuzbass, in reality proved to be a deltaic facies of the Ilyinka Formation. It has likewise been firmly established, on a reliable paleontological basis, that the Usyat Subformation, containing...

in the southern half of the basin, the thick coal seams in the north (in the Kemerovo district) are replaced by a thin (70–75 m) member of sandy-clayey rocks that contains no workable coal seams. Likewise, the Uskat subformation in different districts of Kuzbass is either industrially coal-bearing or practically barren of coal.

Against the background of such considerable facies variability of formations and subformations—showing, in particular, that peat-forming processes developed within the depression unevenly and not simultaneously—attempts at long-distance correlation of individual seams or members must be decisively rejected. Any coal seam, changing its thickness over an area, passes from workable into nonworkable and pinches out altogether. Only over the area where a coal seam has been traced directly by exploration workings can it serve as a marker horizon. It is known that even in neighboring deposits (especially those subordinate to the Kolchugino series) the coal seams do not correlate.

It is not impossible that in the future, with a more comprehensive study of the paleogeography and the history of formation of the basin, key lithological horizons will be discovered that are distributed over its entire territory or over a considerable part of it, but as yet such horizons are not known. Consequently, at present, for the parallelization of sections from different districts of Kuzbass we must rely on paleontological key horizons.

In considering the question of paleontological key horizons theoretically, we must first of all take into account the more or less close connection between the remains of benthic organisms and lithofacies, i.e., the dependence during life of these organisms on bottom facies environments. Therefore, if different facies environments exist within the basin (as usually is the case), they will be inhabited by different biocoenoses, which will migrate together with the displacement of the corresponding facies. Correlation of facially different deposits with the aid of such organisms obviously encounters considerable difficulties.

Great assistance in this case may be provided by organisms inhabiting the pelagial: their remains will be buried in deposits of various facies, facilitating the stratigraphic correlation of the latter. An analogous role is played by plant pollen, the great importance of which for correlating heterofacial deposits was emphasized by V. V. Menner [23].

As is known, pollen can be transported by wind over enormous distances and can fall out over a vast area, becoming admixed with deposits of various facies. In this process there is a certain analogy with the fall of volcanic ash, but from the point of view of distinguishing key horizons there is also a definite difference. The fall of volcanic ash on the scale of geological time is an almost instantaneous event, owing to which distinct ash layers arise in the section that are convenient for recognition. By contrast, the settling of pollen from the air is a practically permanent process; theoretically, even under these conditions one can imagine the appearance of a key horizon (for example, by the first appearance in a given section of pollen of new plant forms that arose on a neighboring continent), but concrete examples of such horizons are not known to us, not to mention that such horizons cannot be identified under conditions of field work.

The question of paleontological key horizons in general has not been worked out. McKee [21, p. 69] gives one case: the emergence of such horizons “as a result of accidental and temporary breaches of groups of organisms through barriers between individual faunal provinces.” This is an important and, perhaps, sufficiently widespread, but still

a particular case, alongside which various other cases of the emergence of key paleontological horizons may be established; we shall try to show this using the stratigraphy of Kuzbass as an example. We shall first formulate the general premise (as it appears to us) for the emergence of paleontological key horizons: paleontological key horizons and boundaries arise in all cases when changes in conditions causing the extinction and resettlement of organic forms rapidly spread over a considerable territory, or when throughout the entire territory there are no barriers to the dispersal of newly arisen or immigrated forms. From the standpoint of this general proposition, let us consider the paleontological key horizons and boundaries in the section of the coal-bearing deposits of Kuzbass.

One of the most clearly and comprehensively characterized boundaries in this section is the boundary between the Lower and Upper Balakhon formations. Initially this boundary (as the boundary between the Alykaev and Intermediate units, in the new scheme—subformations) was established by V. D. Fomichev [30] on the basis of changes in lithology and coal-bearing character. Subsequently it received paleontological confirmation. At this boundary there appear not merely new plant forms, but among them plant remains of a character that are absent lower in the section. These are the morphological genera Lepeophyllum, Crassinervia, which, in the opinion of M. F. Neuburg [26, p. 297], are tiny (bud) scales of cordaites. Their appearance indicates that the climate, previously uniform, from this moment becomes seasonal, with rather sharp fluctuations during the course of the year. We have already noted above that such climatic changes cannot have a narrowly local significance and must have embraced a territory even broader than Kuzbass as a whole. The appearance of forms similar to lepeophyllums should be regarded as simultaneous for all of Kuzbass.

Additional evidence is provided by the fauna of the Alykaev subformation; this abundant and diverse brackish-water fauna disappears at once at the boundary under consideration. There is reason to believe, as R. N. Benediktova [4] notes, that its disappearance was caused not so much by climatic reasons as by significant freshening of the basins inhabited by it, or by some other, as yet unexplained causes. In any case, we have grounds to regard the boundary between the Lower and Upper Balakhon formations, marked by the disappearance of the Alykaev fauna and the appearance of leaves of the bud-scale type, as simultaneous for the entire Kuznetsk Basin, i.e., as having the character of a key paleontological boundary.

The boundary between the Upper Balakhon and Kuznetsk formations has the same key character, as we can see from consideration of the character of the fauna and flora of the Usyat subformation. This so-called Late Balakhon fauna is characterized above all by the gigantism of its components [31]. Geologists have long taken note of the “horizon with large pelecypods,” occurring (according to the modern stratigraphic subdivision) in the upper part of the Usyat subformation; this horizon was used as a marker long before the detailed study of the Late Balakhon fauna.

Let us recall that this fauna is freshwater, and note that it is accompanied by a luxuriantly developed flora with unusually large-leaved cordaites, Annularia, etc. During the formation of the deposits of the Usyat subformation in Kuzbass, optimal conditions existed for the development of both terrestrial flora and freshwater fauna. And then suddenly these conditions came to an end, replaced by sharply changed conditions at the boundary between the Upper Balakhon and Kuznetsk formations.

Above this boundary, almost everywhere in Kuzbass, the freshwater Late Balakhonian fauna is replaced by the brackish-water Early Kuznetsk fauna, in which recurrent elements of the Alykaev fauna play an important role. We have already considered above (p. 57) the invasion and dispersal of this fauna. This is a case that partly corresponds to what Mac-Kee noted (“a sudden breakthrough of fauna”); at the same time, the flourishing flora of the Usyaty subformation is replaced by the extremely scanty, impoverished flora of the Kuznetsk formation. No gradual transitions, no combinations between such different organic associations as the fauna and flora of the Usyaty subformation and the fauna and flora of the lower Kuznetsk formation are observed anywhere, with the exception of one area (see p. 66): this is a typical key paleontological boundary.

Let us emphasize once again that the invasion of a fauna leads to the emergence of a key boundary only in the case when the fauna that has broken through finds conditions suitable for its existence, or when it (as in our specific example) invades together with new conditions (brackish waters) that spread rapidly over all, or almost all, of the territory of the given region. But if the latter condition is not met, the invasion of a new fauna does not lead to the appearance in the section of a key horizon or key boundary, since this fauna does not spread over the entire area of the given region.

In Kuzbass this case may be illustrated by the fauna of the Ostrog formation. Whereas in the north of the basin this formation contains a long-known marine fauna, in which brachiopods predominate [3], in the south the fauna of the Ostrog formation, collected by O. M. Martynova and studied by R. N. Benediktova, proved to be entirely different: brachiopods are completely absent from it, and brackish-water pelecypods predominate, close to those of the Alykaev subformation [33, p. 74].

In the light of the analysis given above of three stratigraphic boundaries, proceeding from the general proposition formulated above concerning key paleontological horizons and boundaries and based on the materials set out in the preceding section, we may with full justification assign to key stratigraphic boundaries the boundary between the Kazankovo-Markino and Usyaty subformations, drawn along the top of the horizon with depauperate fauna and depauperate flora, and the boundary between the Lenino and Gramoteino subformations, characterized by the dispersal in the Kuznetsk depression of immigrant “plants of Mesozoic aspect.”

The above review of limited material, relating only to the stratigraphy of Kuzbass, shows that key paleontological boundaries and horizons may arise under different conditions, as a result of different processes. Summarizing what has been set forth, we may note the following cases of key paleontological boundaries:

1. Boundaries connected with the invasion of a fauna owing to the removal of barriers standing in the way of its dispersal; the cause of the migration of faunas is, in the final analysis, tectonic movements; here, in turn, two cases are possible:

a) the invading fauna encounters conditions quite suitable for life (the same as those in which it lived in the neighboring area); this may occur, for example, when land separating two marine basins with identical hydrological conditions is flooded. In this case one should expect reciprocal migrations and mixing of both faunas, with the probable displacement of the less adapted forms. This is the case of the “faunal breakthrough” according to Mac-Kee; it has not been observed in Kuzbass;

b) the invasion of the fauna is the consequence of a rapid change in the environment, for example, during an ingression of the sea flooding low-lying areas of land; it is accompanied by the death or departure of the previously inhabiting fauna;

the boundary is marked by a sharp change of faunas; in the Kuzbass—the invasion of the early Kuznetsk fauna.

1. Boundaries associated with the dispersal of newly arisen forms; there is no direct connection with tectonic movements; here two cases are also possible:

a) the dispersing group of organisms arose within the given territory (basin) and, having for one reason or another attained mass development, spreads widely over the area of the basin, into the stations and biotopes proper to it. In the Kuzbass, for example, the gigantic anthraconauts and mrasciellids of the late Balakhonka fauna, or new plants of the II Chertinsk complex (at the boundary of the Ilyinskaya and Erunakovskaya formations);

b) the dispersing forms are immigrants, having arisen somewhere beyond the limits of the given territory or water area and having reached it at a certain moment in time; finding here conditions corresponding to their needs, they rapidly occupy this new territory, mixing with the local population; in the Kuzbass—the appearance of plants of Mesozoic aspect at the boundary of the Leninskaya and Gramoteinskaya subformations.

1. Boundaries associated with significant and sufficiently rapid changes of climate, especially clearly reflected in the composition and character of the flora; the appearance of plants reflecting seasonal climatic fluctuations, and of oppressed and flourishing associations of plants and animals; in the Kuzbass—the boundary between the Upper Balakhonka and Kuznetsk formations, the boundary between the Kazankovo-Markinskaya and Uskatskaya subformations.

2. Boundaries characterized negatively: by the disappearance (partly—extinction, partly—emigration) of a fauna. The cause is a sharp and on the whole so unfavorable change in conditions that the place occupied by the given fauna remains practically unoccupied. This case is apparently rare, but it does occur: in the Kuzbass, after the disappearance of the rich Alykaevka fauna, fauna is practically absent everywhere in the intermediate subformation.

3. A special category is formed by palynological boundaries, since in this case the decisive factor is not the resettlement or evolution of organisms, but the mechanical process of pollen transport, chiefly by wind.

The survey presented, of course, does not set itself the task of giving an exhaustive classification of key paleontological boundaries; its purpose is to draw attention to this range of questions and to demonstrate the non-sliding character of the principal biostratigraphic boundaries in the scheme of subdivision of the coal-bearing deposits of the Kuzbass adopted at the 1954 conference.

In conclusion it must be recalled that natural phenomena and conditions, because of their extraordinary diversity and complexity, usually do not fit completely within the rigid frames of our classifications. This also applies to key horizons. Even for the most clearly expressed of them, caused by migrations and by the replacement of entire faunas, particular cases are possible when a fauna driven out by changed conditions finds within the limits of the given basin (region) some possibilities, some refuges, in which it continues to exist (even if in impoverished composition) after its disappearance throughout all the rest of the basin. Returning again to the paleontology of the Kuzbass, we can show this by the example of that brackish-water fauna of pelecypods, which has already been mentioned more than once above; let us summarize these references in the form of a brief review of the history of this fauna (see Fig. 4).

Its first appearance was recorded by R. N. Benediktova in the deposits of the Ostrogskaya Formation at one of the localities (village of Kameshok) in the south of the Kuzbass. Sporadically and locally its representatives are present in the Mazurov—

subformation, the deposits of which are in general very poor in fauna. It reaches its flourishing and mass development during the formation of the sediments composing the Alykaevskaya subformation; this, properly speaking, is the Alykaevskaya fauna.

But at the boundary between the Alykaevskaya and intermediate subformations this abundant fauna suddenly disappears. Until very recently we believed that at this time it left Kuzbass entirely, especially since its representatives had recently been found in NE Kazakhstan [2].

This disappearance of the Alykaevskaya fauna makes the reference boundary between the subformations mentioned especially distinct. But quite recently the staff of the Kemerovo party of the trust “Kuzbassuglegeologiya” for the first time discovered an abundant pelecypod fauna in the deposits of the intermediate subformation in one of the sectors of the Kemerovo district. R. N. Benediktova [4], who studied and described this fauna, notes that it is undoubtedly most closely connected with the Alykaevskaya fauna, and that the latter found in one of the districts in the north of Kuzbass a certain refuge, where it continued (although in an altered state) to exist longer than in the rest of the territory of Kuzbass. The location of this refuge in the northwest of the basin is natural: it was precisely in this direction that the Alykaevskaya fauna retreated together with the regression of brackish waters from the Kuznetsk depression.

The subsequent fate of this fauna is unknown to us up to its new, likewise sudden, appearance in Kuzbass in the form of the Early Kuznetsk fauna, replacing in the section the Late Balakhonka fauna. This is yet another distinct reference boundary. But in this case as well we know one point where the change of conditions, which entailed a change of faunas, does not take place immediately and definitively: this is the district of the town of Stalinsk (the Tom River below the village of Starokuznetsk). Here, in the section of the lower beds of the Kuznetsk formation, beds with Early Kuznetsk fauna alternate with beds containing Late Balakhonka fauna. Let us note that this is, apparently, the southernmost point reached in its spread from the northwest by the Early Kuznetsk fauna. Somewhere here lay the southern boundary of the brackish-water basin (basins?) of that time, and it is not surprising that it is precisely here that in the section we see an alternation of deposits with brackish-water and freshwater faunas. The latter (the Late Balakhonka fauna) persisted here somewhat longer than in the more northern parts of the basin.

Summarizing what has been said, let us note that a delay in the disappearance of both faunas is characteristic, as an exception, only of individual districts; that both the very fact of this delay and the districts in which it is observed are determined by the paleogeographic conditions of the corresponding moments of time; and that these exceptions do not diminish the significance of the reference paleontological horizons for the stratigraphy of the basin as a whole.

On the regional significance of reference stratigraphic boundaries

The main thesis, to the development of which we shall devote the concluding part of this article, amounts to the assertion that reference horizons and reference boundaries are regional-stratigraphic elements and cannot be applied in the international stratigraphic scale.

In this connection it must be emphasized that there exist two principal categories of stratigraphic subdivisions, forming systems of subordinate units: the international stratigraphic scale, with its two aspects (geochronological and biostratigraphic), and regional stratigraphic schemes. As early as 70 years ago S. N. Nikitin and F. N. Chernyshev [28, p. 140] emphasized the fundamental distinction between these categories and noted that ignoring this distinction is the cause of many errors, failures, and confusion. This and other

the profound ideas developed by S. N. Nikitin and F. N. Chernyshev in the work mentioned were not appreciated by their contemporaries and subsequently, unfortunately, were forgotten. Even in our own time much confusion is introduced into stratigraphy because of a failure to understand, in the words of N. S. Nikitin and F. N. Chernyshev, “the dual character of geological classifications,” i.e., because of a failure to understand the specificity and the profound differences between the subdivisions of the international stratigraphic scale, on the one hand, and regional stratigraphic schemes, on the other.

Here we do not have the opportunity to examine in detail the question of the relationship between the units of regional stratigraphy and the units of the international scale, and must confine ourselves to very concise formulations.

As has already been said, the units of the international scale and the regional-stratigraphic units differ in their essence, in their nature. The former are (or, more precisely, should be) a reflection of stages in the evolution of the organic world of the Earth as a whole; adjacent units of the international scale are not so much separated as connected by intermediate, transitional units with a mixed fauna. Such is the character of the development of the organic world: this development has no hiatuses; qualitatively different stages of it are connected by transitions of the type of gradual changes. Therefore, in the international scale, based on the evolution of the organic world, there are and can be none of those sharp boundaries which many stratigraphers so persistently and so unsuccessfully try to find.

A regional stratigraphic scheme is an expression of the geological history of a given region: the principal regional-stratigraphic subdivisions must correspond to definite stages in the geological development of that region; in this sense, correctly distinguished subdivisions are natural units of regional stratigraphy. They are real geological bodies with a definite material composition, not repeated either in time or beyond the limits of the given natural region. They are often sharply and definitely delimited (tectono-denudational breaks, abrupt change of facies). If these boundaries are also recorded paleontologically, then not on the basis of the development of some fauna (flora), but on the basis of the replacement of one fauna by another as the result of migrations.

From the standpoint of the approach characterized above, known in the literature as historical-geological or tectono-stratigraphic (since tectonic movements constitute the skeleton of geological history), the tasks of regional stratigraphy are the distinction, the establishment of the sequence, and the elucidation of the character of the boundaries of natural subdivisions by means of the complex method, which was characterized by G. P. Leonov [18, p. 34] in the following way:

“Stratigraphic schemes of the regional stage are usually developed by the complex method and are based on the lithological and paleontological features of the deposits, traces of interruptions, analysis of oscillatory movements of the Earth’s crust, and the identification of periodicity in the process of sediment accumulation and other features of the deposits, reflecting to one degree or another the general course of the geological development of the corresponding region.”

It should be emphasized with complete definiteness that the historical-geological or tectono-stratigraphic method is a method of stratigraphic subdivision, but not of stratigraphic parallelization. Failure to understand this circumstance is the cause of various misunderstandings; for example, some authors persistently “prove” that with the aid of the tectono-stratigraphic method it is impossible to carry out stratigraphic correlation. But no one has ever even tried—

to do this. Correlation of the stratigraphic columns of different regions with one another and with the international stratigraphic scale is always carried out by methods of biostratigraphic correlation, by dating regional stratigraphic units with the aid of the paleontological method, i.e., by determining their position on the international scale.

The character of the boundaries of regional-stratigraphic subdivisions may reflect various aspects of the geological history of a region. In a number of cases these boundaries clearly reflect the tectonic life of the latter. This applies above all to tectono-denudational boundaries. They correspond to the case when the leading motif in the tectonic history of a region (over a certain interval of time) is macro-oscillatory movements, an alternation of subsidences and uplifts of considerable amplitude, the uplifts being accompanied by the emergence of the given region from below the level of sedimentation for a more or less prolonged period. Folding movements of varying intensity and varying character may appear as a concomitant factor. In the Kuzbass such is the boundary between the Malyiševskaya and Torbaganskaya series—a boundary corresponding to a break in sediment accumulation with erosion of the underlying deposits, traces of ancient weathering, angular and azimuthal unconformity, and a large chronological gap in the assemblages of organic remains.

In other cases sedimentation over a very long time proceeds continuously; in sections of deposits of such structures we shall not find tectono-denudational breaks¹, but we may encounter other manifestations of tectonic movements. Thus, in the subdivision of rhythmically bedded sequences, natural stratigraphic boundaries may correspond to moments of changes (even if purely quantitative ones) in the regime of micro-oscillatory movements. In the Kuzbass the boundary between the Ilyinskaya and Erunakovskaya formations has this character; they differ, in particular, in the thicknesses of the cycles composing them [6].

Macro-oscillatory movements may find clear paleontological expression even in the absence of tectono-denudational breaks—in the case of an abrupt replacement of one fauna by another, as has been established for the boundary between the Upper Balakhonka and Kuznetsk formations. The boundary between these formations is also interesting in another respect: its character changes over the area of the basin. If, beginning with the northern districts of the Kuzbass, along its Prisalaïr belt and as far as the district of the city of Stalinsk in the south, this boundary is marked by the replacement of the Late Balakhonka fauna by an Early Kuznetsk one, then in the east, in the Prialatau belt, it is characterized by a basal conglomerate at the base of the Kuznetsk Formation [8]. Both the change of faunas and the presence of this conglomerate are different expressions of one and the same process: rapid subsidence of the depression and transgression of the water masses filling it.

In other cases the phenomena reflected in the character of the boundaries of regional stratigraphic subdivisions show no obvious connection with tectonic movements; for example: the appearance of plants of Mesozoic aspect at the boundary between the Leninskaya and Gramoteinskaya subformations; climatic changes at the boundaries between the Lower and Upper Balakhonka formations and between the Kazankovo-Markinskaya and Uskatskaya subformations.

But in both the one and the other case, the processes that caused the emergence of key boundaries are evidently regional, not universal, in character. Applying the terminology of G. P. Leonov [18; 19], we may

to say that the radius of action of key stratigraphic boundaries and horizons is limited to the territory of the given natural geological region.

Specifically, as applied to Kuzbass, we can supplement this conclusion with one more point, namely: the boundaries between suites and subsuites in the scheme adopted in 1954 are not only biostratigraphic, but also key ones, i.e., isochronous for the entire area of the basin. This is the fundamental difference between the 1954 scheme and the previous one, in which the boundaries between suites, drawn along the lower or upper coal seam of working thickness, were entirely artificial, being knowingly located at different levels in different parts of the basin.

REFERENCES CITED

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Tomsk Polytechnic Institute
named after S. M. Kirov.

1. This does not exclude the presence of short-term breaks of the diastem type at the boundaries of cycles. ↩↩