MAIN FEATURES OF THE GEOLOGICAL STRUCTURE OF THE TALANOVSKY GRABEN

(NORTHWESTERN SLOPE OF THE KUZNETSK ALATAU)

V. Z. Mustafin, V. G. Kryukov, N. S. Lykhina

(Presented by Prof. Dr. A. M. Kuzmin)

The Talanovsky graben is located in the northwestern spurs of the Kuznetsk Alatau, being confined to the northern part of the so-called Pervomaisk Sinyan–Cambrian intermediate volcanic zone [8]. The features of the geological structure of the area were also affected by the proximity of the Kuznetsk–Alatau regional tectonic fault, which determined the form of the structure described. In regional plan, the latter acquired the shape of a wedge 1.5–4 km wide, extending in a meridional direction for 25 km. This wedge is situated between branches of the Tersinsky folding zone, which bound the structure on the west and east. Northwestern and northeastern disturbances gave rise to the block structure of the graben (Fig. 1).

Within the graben, formations of the Telbess series of the Lower–Middle Devonian are predominantly developed. They are underlain and bordered by intensely dislocated sedimentary–metamorphic rocks of the Sinyan and by metamorphosed effusive–sedimentary deposits of the Lower Cambrian. Along with them, intrusive bodies of the Cambro-Ordovician (Martaiginsky) and Devonian (Telbessky) intrusive complexes, varying in composition, are widely distributed in the area described. Since bodies of the latter are most closely connected genetically with the stratified section of Devonian effusives, we depart from the generally accepted scheme of describing stratigraphic and intrusive formations, characterizing them in age sequence.

Sinyan deposits, combined into the Yenisei suite, are developed southeast and northwest of the graben, spreading widely beyond its limits. In the studied area they were also observed along the eastern contact of the graben, from which these formations are separated by a strip of Cambrian rocks. The Yenisei suite is composed mainly of gray and dark-gray bituminous limestones, as well as metamorphic shales and banded silicilites.

The Sinyan age of the Yenisei suite in the Sayan–Altai region has been determined on the basis of the study of organic remains, and the faunal and floral assemblage discovered in the area of the Sarala River [1] makes it possible to regard the age of these formations as Upper Sinyan.

Cambrian deposits, represented only by the lower division, are distributed outside the graben. The exception is...

Fig. 1. Schematic geological map of the Talanovsky graben (compiled by V. G. Kryukov, N. S. Lykhina, V. Z. Mustafin): 1—alluvial deposits. Middle Devonian: 2—andesite porphyrites; 3—basaltic, olivine-pyroxene porphyrites and clastolavas; 4—red-colored and gray-colored conglomerates, sandstones, siltstones. Lower Devonian: 5—trachyte porphyries, orthophyres; 6—trachyandesite porphyrites; 7—hornblende porphyrites; 8—labradorite-pyroxene and labradorite porphyrites; 9—lavas and clastolavas of labradorite-pyroxene porphyrites; 10—unaltered porphyrites of basic composition; 11—limestones, shales, and effusives of the Uspenskaya suite; 12—porphyrites, porphyritic albitephyres, shales, limestones, and siliceous conglomerates of the Kondomskaya suite; 13—bituminous limestones, siliceous shales, siltstones of the Yeniseiskaya suite. Devonian intrusions: 14—granosyenites; 15—syenites, alkaline syenites; 16—granodiorites and plagiogranites of the Martaiminsky complex; 17—dikes; 18—disjunctive faults; 19—geological boundaries: lithological and facies; 20—elements of bedding.

its southern part, where Cambrian limestones appear among Devonian effusives in a small erosional “window.” In the Cambrian section, the Kondoma and Usinsk formations are distinguished (from bottom to top).

Deposits of the first of these are distributed predominantly east of the graben, and those of the second—to the west of it. The Kondoma Formation consists of sedimentary-terrigenous and effusive formations, among which effusives of basic composition and various shales predominate. In addition, limestones, siltstones, sandstones, etc., are noted in it.

Apparently, the silicolite conglomerates established southwest of the graben should be assigned to the base of the Kondoma Formation. In connection with their specific external properties, they were previously taken for limestone. Unfortunately, because of the poor exposure of the southern part of the area, the relationships between the Kondoma and Yenisei formations have not been established, but the presence of silicolite conglomerates makes it possible to suppose a considerable break in sedimentation preceding the formation of the Lower Cambrian deposits.

The Cambrian age of the Kondoma deposits for the Sayan-Altai region is substantiated by finds of archaeocyathids and spores.

Deposits of the Usinsk Formation in the area described everywhere have tectonic boundaries with the volcanogenic-sedimentary formations of the Kondoma Formation. However, in neighboring regions gradual transitions between them are noted. The Usinsk Formation is dominated by reef limestones with sharply subordinate interbeds of dark-green shales. The limestones with fauna, determined by V. D. Kamelina at the Department of Historical Geology of the Tomsk Polytechnic Institute, are assigned to the Podsanashtyk-Gol—Podkameshkovka beds.

Beyond the limits of the graben, the Sinian-Cambrian strata are intruded by bodies of plagiogranites and granodiorites of the Martayga complex, belonging to a formation of granite batholiths of variegated composition [6]. Their age in the Kuznetsk Alatau is dated, on geological evidence, as Ordovician. The results of determining the absolute age of similar intrusions by the argon method are contradictory (500, 453—Cambrian; 438—Ordovician; 320 million years—Carboniferous) and cannot be accepted as reliable [2].

Devonian deposits, united into the Telbes series, within the area of the graben are represented by effusive-sedimentary formations, which in the area described were previously distinguished as undivided formations of the Lower–Middle Devonian.

Within the graben, the rocks of the Telbes series have tectonic contacts with the more ancient strata, expressed by submeridional zones of intensely brecciated, hydrothermally altered rocks 10–50 m wide. Evidently, the Devonian effusives rest with a sharp angular unconformity on the deposits of the Kondoma and Usinsk formations, as well as on the sedimentary-metamorphic formations of the Yenisei Formation that make up the bed of the Talanovsky graben.

In the area of the graben, as shown above, pre-Devonian rocks are practically absent. The effusive strata within it occur monoclinally, dipping eastward. At the same time, the dip angles of the strata near the western boundary of the graben are 30–40° and increase to 60–70° in its eastern part, which on the whole determines the asymmetry of the section of the Devonian effusives.

The work carried out by us made it possible to subdivide the sequence of effusive rocks of the Telbes series into formations of the Lower and Middle Devonian. The Lower Devonian effusives, in turn, are divided into five members, differing in petrographic features and stratigraphic—

position. In the direction from west to east, the following succession of rocks is observed in the graben.

1. Effusives of intermediate composition, forming a belt along the western boundary of the graben. The predominant rocks among them are plagioclase-amphibole porphyries, in which lens-shaped bodies of clastolavas of the same composition are observed. The unit of plagioclase-amphibole porphyries is distinguished by great petrographic persistence. Only in isolated cases does the amount of dark-colored minerals in the rocks being characterized increase significantly, and in composition they approach basaltic porphyries.

The clastolavas of the intermediate effusives are distinguished by fragmental, fluidal, and vesicular textures, with the composition of the melted fragments and the cementing mass being identical.

1. Effusives of basic composition are the most widespread over the area of the graben, being traced almost throughout its entire extent. They are represented by fine- to medium-porphyritic and aphyric pyroxene-labrador and labrador porphyrites (aphyrites), as well as by their subordinate lavas and clastolavas. Varieties of basic effusives form, within the area of the graben, elongated belts of varying width, oriented in a N–NE direction. The most persistent among them (marker varieties) are coarse-porphyritic labrador porphyrites. Lens-shaped bodies of lavas and clastolavas, as well as bodies of well-recrystallized varieties of basic effusives, are characterized by insignificant dimensions both along strike and in thickness.

Judging from the observed relationships, the described unit is intruded by amphibole-plagioclase porphyries, which apparently are the vent facies of the basic effusives; this is also confirmed by the composition of the first unit (amphibole-plagioclase porphyries).

1. Effusives of basic-intermediate composition are traced as a belt from the southern end of the graben in a northeastern direction and are cut off by its eastern tectonic contact. Within this unit, plagiobasalt and pyroxene-plagioclase (andesitic) porphyrites and aphyrites are distinguished, as well as their amygdaloidal-porous varieties with a glassy, aphanitic, and finely recrystallized groundmass. Recrystallized rocks appear at various stratigraphic levels of the section of the unit being characterized, in the form of lenses of comparatively small size.

For effusives of basic-intermediate composition, a regular change in mineralogical and structural features is established in the direction from north to south. In the north of the area these rocks have a more acidic composition (andesitic porphyrites). Southward, their basicity increases with an increase in the amount of pyroxenes in them. In the northern parts, varieties with a poorly recrystallized (glassy) groundmass predominate; in the south, the rocks are better recrystallized.

1. Effusives of subalkaline composition gravitate toward the eastern boundary of the graben, being separated from it by units of younger effusives. They are comparatively little widespread, being represented by trachyandesite porphyrites. Among the latter, biotite and pyroxene varieties are distinguished, as well as clastolavas, tuffolavas, and tuffs of the same composition.

2. Alkaline effusives occupy comparatively small areas near the eastern boundary of the graben, forming oval (in plan) bodies in its northern, central, and southern parts. The varieties are

rocks grouped here are trachyte porphyries, orthophyres, and agglomeratic tuffs with fragments of rocks of alkaline composition.

The effusive sequences characterized above are intruded by quartz-free leucocratic syenites, which sometimes show very gradual transitions with the trachyte porphyries and which, apparently, are comagmates and a vent facies of the alkaline effusives. In addition to the intrusions of quartz-free syenites established in the northern part of the area, a dike facies is also manifested in the characterized sequences, very typical of the graben. By composition, age, and their inherent orientation, the dikes are subdivided by us (from older to younger) as follows: a) dikes of labrador porphyrites (submeridional); b) dikes of diabases and diabase porphyrites (northwestern to sublatitudinal); c) dikes of microdiorites (nearly meridional); d) dikes of syenite-porphyries (northwestern) and microsyenites (submeridional).

6. Terrigenous-sedimentary rocks are most widespread in the north of the graben, where they overlie the alkaline effusives with angular unconformity. The terrigenous deposits include green-colored and red-colored conglomerates, lenticular-bedded sandstones, siltstones, and argillites, with a significant admixture of tuffaceous material in each of the rock varieties. In rare cases, a flora of very poor preservation is noted in them.

The sedimentary rocks, alkaline effusives, and quartz-free syenites are intruded and overlain by basaltic and andesitic porphyrites, which sometimes form sill bodies in the sedimentary sequence of the northern part of the graben. Basaltic porphyrites, in addition, form covers near the eastern boundary of the graben along almost its entire extent.

All the described rock sequences are intruded by linearly elongated bodies of granosyenites, situated predominantly along the eastern tectonic boundary of the graben, which is healed by them. Dark-colored minerals in these rocks are either entirely absent or quantitatively insignificant. The quartz content in them is small, but increases substantially in the endocontact zones of the intrusions.

The granosyenites, in turn, are cut by the youngest formations in the graben—dikes of spessartites and diorite porphyrites (northwestern strike), which sometimes extend beyond the limits of the graben into the limestones of the Usinsk Suite of the Cambrian. It is interesting that, in crossing the tectonic boundary of the graben, the dikes undergo neither displacement nor even crushing.

Study of the effusives under the microscope showed that the degree and character of alteration of all rock varieties are more or less the same and correspond to the diagenetic phase of their state. This permits the supposition that in the area described there are no Cambrian effusives, which (outside the graben) have been subjected to greenschist alteration.

In accordance with existing definitions [6, 11], the effusives of the Talanovsky section may be grouped into the trachyandesite and andesite-basalt (andesite) formations.

Study of the forms of occurrence, as well as analysis of the structural-textural features of the rocks composing the named formations, make it possible to establish in each of them the following facies [5]: extrusive (sheet), sill-laccolith, dike (near-surface vein), vent, and, possibly, stock (near-surface stock).

The age of the described effusives in the area of the graben could not be determined. Fauna has not yet been found in the sedimentary formations, and the ...

finds of flora of very poor preservation did not allow this question to be resolved. The sequence of formation of the effusives of the Talanovsky section will be considered by us later. In the present work the age of the Telbes series is determined only on the basis of comparison of the section presented with Devonian sections of neighboring regions [7, 10, 13].

The lower age boundary of the Telbes effusives in Gornaya Shoria is determined by finds of Upper Silurian—Lower Devonian fauna [3] in the strata underlying the series. Directly in the deposits of the lower horizons of the Telbes series, a Lower Devonian psilophytic flora has been collected [10]. The rocks of the series are overlain by carbonate-terrigenous Upper Devonian deposits. Thus, as a whole, its age is determined as the interval from the Lower Devonian to the beginning of the Upper Devonian.

Comparison of the Talanovsky section of the Telbes series with sections of effusive-sedimentary formations of the Eastern Sayan makes it possible to determine the age of units 1, 2, 4, and 5, parallelized with the Imir and Shunet formations [13], presumably as Lower Devonian, and the age of units 3 and 6, which are an analogue of the Ton Formation, as Middle Devonian (Eifelian?).

If the age determination of the effusives given above is correct, then the geological age of the formations intruding the effusives of the Telbes series in the graben and closely associated with them (quartz-free syenites) is determined as Lower Devonian. For the granosyenites only a lower age boundary can be established: they cut the basalt porphyrites of the Eifelian (?). Younger formations are completely absent in the area described. Meanwhile, in the Eastern Sayan the age of the intrusions cutting the Lower Devonian Imir Formation corresponds to the upper part of the Lower Devonian, while intrusions regarded as an analogue of the Ton Formation are Middle Devonian in age [12].

Both types of intrusions considered above are, apparently, representatives of the gabbro–monzonite–syenite formational type [6]. The absolute age of the latter in the Sayan–Altai region is determined at 410–384 million years—the Middle and Upper divisions of the Devonian [9].

Thus:

1. The Talanovsky graben, as a negative structure, took shape before the Eifelian (?)—before the emplacement of the granosyenite intrusions and the dikes of spessartites and diorite porphyrites, with the formation of which the tectonic development of the territory was completely completed.

2. The effusives of the section considered are an example of the trachyandesite and andesite formations characteristic of orogenic, chiefly geoanticlinal mobile zones.

3. The intrusive rocks, especially the quartz-free syenites, show the closest connection with the effusives described and are, apparently, their intrusive analogues.

4. The development of magmatism in the area of the Talanovsky graben obeys the principal regularities established [6] for the andesite and trachyandesite formations of geosynclinal mobile zones.

LITERATURE

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2. T. N. Ivanova. The history of geological development and magmatism of the Kuznetsk Alatau in the Paleozoic. In: Problems of Magmatism, Metamorphism, and Ore Formation. Gosgeoltekhizdat, 1963.

3. M. K. Korovin, K. V. Radugin. On the age of the lower boundary of the Telbes Formation. Vestnik Geolkoma, No. 1, 1927.

1. P. S. Krasnopeeva. The Algyon flora and fauna of the Saralinsk region of the Kuznetsk Alatau. Materials on the Geology of Krasnoyarsk Krai. ZSGU Publishing House, issue 8, 1940.

2. Yu. A. Kuznetsov. On the principles of distinguishing and classifying facies of magmatic rocks. In the book The Basic Ideas of M. A. Usov in Geology. Publishing House of the Academy of Sciences of the Kazakh SSR, Alma-Ata, 1960.

3. Yu. A. Kuznetsov. Principal Types of Magmatic Formations. “Nedra,” 1964.

4. I. V. Luchitskii. Volcanism and tectonics of the Devonian basins of the Minusinsk intermontane trough. Publishing House of the Academy of Sciences of the USSR, 1960.

5. A. A. Mossakovskii. Tectonic development of the Minusinsk basins and their mountain frame in the Precambrian and Paleozoic. Gosgeoltekhizdat, 1963.

6. Yu. D. Skobelev. Nepheline rocks of the Kuznetsk Alatau. Materials on the Geology of Western Siberia, No. 64, Gosgeoltekhizdat, 1963.

7. S. A. Stepanov. On the question of the age of the Telbes Formation of Gornaya Shoriya. Materials on the Geology and Mineral Resources of Western Siberia. TSU Publishing House, Tomsk, 1964.

8. N. P. Kheraskov. Geological formations (an attempt at definition). Bulletin of MOIP, Geological Section, vol. XXVII, 5, 1952.

9. A. D. Shcheglovnikov. On the comagmatic nature of certain intrusive and effusive complexes of the northwestern part of the Eastern Sayan. Materials on the Geology and Mineral Resources of Krasnoyarsk Krai, issue 3, Krasnoyarsk, 1962.

10. E. A. Shneider, B. P. Zubkus. Stratigraphy of the Lower and Middle Devonian deposits of the North Minusinsk and Sydo-Erbinsk basins. Materials on the Geology of Krasnoyarsk Krai, issue 3, 1962.