On the Characteristics of the Precambrian and Lower Paleozoic Tectonics of the Mrassky Massif

L. V. PESHEKHONOV

(Presented by the scientific seminar of the Departments of General Geology, Historical Geology, and Mineral Resources)

The Mrassky (Shor) massif, within the system of massifs of the Kuznetsk Alatau, is the southernmost. By the present time, a considerable amount of comprehensive geological research has been carried out on its territory.

Information on the geology of the region is reflected, to a greater or lesser extent, in monographic, journal, and archival literature on Gornaya Shoria [2, 4, 11, 12], the Kuznetsk Alatau [3, 8], and the Sayano-Altai folded region [1, 6, 7, 10]. Analysis of these materials, together with our own observations during geological investigations in the Kamzas, Talon, Taimet, Ust-Kabyrza, Dzhelgai, and other districts, makes it possible to draw a number of definite conclusions concerning the internal tectonics of the Mrassky massif.

The internal structure of the Mrassky massif as a whole is characterized by features inherent in complexly built anticlinoria. The most general and characteristic features of its structure are reflected in the tectonic scheme (Fig. 1), on which, within the limits of the Mrassky massif, two structural stages are clearly distinguished. The identification of structural stages became possible only after analysis of the character of the orientation of folded structures within the areas of distribution of rocks of different ages. The lower, most ancient structural stage is composed of carbonate and effusive-terrigenous formations of the Late Proterozoic and partly of the Early Cambrian.

Late Proterozoic deposits are represented by green schists of the Kaurchak suite, anthraconite limestones of the Kabyrza suite, limestones and schists of the Unushkol suite, and dolomites of the West Siberian suite [9, 12, 14]. The Early Cambrian formations participating in the structure of the lower stage are represented by volcanogenic rocks of the Aryk suite, the Ust-Anzas suite, by dolomites and limestones with phosphorites of the Belka suite, and by terrigenous products of the Pyza suite and the shale suite [9, 13]. The deposits of the lower structural stage are gathered into a system of tight folds of various extent, often overturned and even thrust to the southwest and west. The orientation of the folds permits one to speak of the presence here of a single, large, arc-shaped folded structure, convex to the west and in general plan repeating the configuration of the western and northern margins of the Mrassky massif. The plan of orientation of the folded structures of the lower stage, reconstructed from the dominant elements of bedding, is characterized by a substantially submeridional direction of the fold axes. Most clearly

1. Principal directions of folded structures in the volcanogenic-sedimentary and dolomite formations of the Late Precambrian (lower or Baikalian structural stage). 2. Fields of volcanogenic and clastic rocks of the sedimentary-volcanogenic formation of the Late Precambrian—Early Cambrian, inheriting the structures of the lower (Baikalian) stage. 3. Superposed structures resulting from the manifestation of Salairian folding movements: I—Ulutag, II—Mazass, III—Uzass, IV—Kamzass, VI—Verkhizass, V—Nymzass, VII—Kultaygin brachysynclines. 4. Intrusive massifs of the gabbro-syenite complex ($D_2$—$D_3$): a) Patyn, b) Kupcha, c) Kul’-Taiga, d) Kalbuk, e) Karatag, f) Trontag. 5. Devonian effusives and red beds preserved in grabens. 6. Intrusive massifs of gabbrodiorites, granodiorites, granites, plagiogranites ($Cm_2$—$3$): a—Kamzass, b—Talonskii, c—Asyrskii, etc. 7. Steeply dipping faults and thrusts. 8. External boundaries of the Shor massif and adjacent zones of deep faults.

This is noticeable in the marginal parts of the massif, at a distance from the fields of distribution of deposits of the second structural stage. Near these fields, the structure of the lower structural stage becomes considerably more complicated. The submeridional strike of the structures changes to northwestern, northeastern, and sublatitudinal. This is explained by the fact that the general pattern of orientation of the folds of the lower stage was complicated by later (Salairian) folding movements and block displacements in Devonian and later times. The formation of the folded structures of the lower structural stage, in our opinion, is connected with the manifestation of Baikalian folding movements. Considering that the volcanogenic-sedimentary formations (Ust-Anzass and Aryk suites), formed at the early stages of Early Cambrian time, participate together with Upper Proterozoic rocks in the structure of folds of one and the same plan, it may be assumed that the Baikalian folding movements in the region described ended at the beginning of the Early Cambrian epoch.

The upper structural stage is composed of carbonate, more rarely volcanogenic formations formed in the Early Cambrian and partly in the Middle Cambrian epoch. These are deposits of the Adiak, Mrass, Mazass, Karchit, Uzass, and Ulutag suites. In the area of distribution of these deposits the fold forms are the simplest. As a rule, these are synclines of brachyform outlines, with axes oriented close to the latitudinal direction, often complicated by intrusive bodies and disjunctive structures cutting through them. Structures of this type occupy an area of less than one third of the Mrass massif and are confined to its southern half. The largest of them are the Ulutag, Mazass, Uzass, Kamzass, Nymzass, Verkhizass, and Kultaygin brachysynclines and graben-synclines.

1. The Ulutag brachysyncline is composed of limestones of the Mazass (Karchit) and terrigenous formations of the Ulutag suite. The former make up the limbs, and the latter fill the trough of the brachysyncline. In the central part, the Ulutag brachysyncline is complicated by a disjunctive structure of northeastern trend. In connection with this, a difference is observed in the orientation of bedding in the western and eastern parts of the brachysyncline. In the western part of the brachysyncline, somewhat rotated relative to the eastern part, a southwestern and northeastern strike of bedding predominates. The dip angles of the bedding vary widely, from 30 to 70°.

1. The Mazass brachysyncline is composed of conglomerates of the Adiak Formation, limestones of the Mazass Formation, and sandy-shaly deposits of the Ulutag Formation. The deposits participating in the structure of the brachysyncline are characterized by a predominantly latitudinal strike of bedding, with varied dips of the stratification from 20 to 80°. In isolated cases, small folds are established with overturning of the beds to the southwest. The limbs of the fold are complicated by disjunctive faults, so that it is more correct to regard this structure as a graben-syncline.

2. The Uzas brachysyncline is composed of the Ust-Kolzas sequence (Belka Formation) of siliceous, argillaceous-siliceous, and siliceous-argillaceous shales with dolomitic sedimentary breccias at the base, Karakol limestones (Karchit Formation), Uzas pyroxene porphyrites, and limestones of the Mazass Formation. The southern limb of the brachysyncline is connected along a disjunctive fault with the northern limb of the Mazass synclinal structure. The northeastern limb of the brachysyncline is complicated by the Kalbuk gabbro-syenite stock. The axis of the brachysyncline is oriented from west to northeast.

3. The Kamzass brachysyncline is composed of deposits of the Adiak Formation. The southern limb of the brachysyncline is truncated by a disjunctive fault of latitudinal strike, so that it is directly in contact with limestones of the Mazass Formation, commonly distributed in the trough part of the brachysyncline. The axis of the fold is oriented from west to northeast. The more detailed structure of the Kamzass brachysyncline is shown in Fig. 1.

4. The Nymzass brachysyncline is composed of deposits of the Karchit and Mazass formations. Its northern limb is complicated by a thrust-type fault, along which rocks of the lower structural level are thrust over the deposits composing the Nymzass brachysyncline. Within the preserved southern limb, the rocks dip northward at an angle of 40–60°.

5. The Verkhnekizass brachysyncline is composed of deposits of the Karchit, Mazass, and Ulutag formations. Like the Nymzass brachysyncline, it is complicated by the same thrust, along which rocks of the lower structural level have been thrust onto the northern limb of the Verkhnekizass brachysyncline. The eastern limb of this structure is truncated by faults of the Balyksinsk mobile zone. In the preserved part of the structure described, the latitudinal strike of bedding predominates, dipping northward.

6. The Kultaygin brachysyncline is characterized by an isometric-oval form. Deposits of the Mrass Formation take part in its structure (according to G. I. Spanderashvili). This structure is complicated by small massifs of granites (Asyr massif) and gabbro-syenites (Kul-Taiga).

The structures of the upper structural level characterized above, away from the contact zones, are usually oriented almost at a right angle to the folded structures of the lower structural level (Fig. 1). This circumstance makes it possible to regard the tectonic movements that created two discordant types of differently aged structures as two independent foldings. In conclusion, we note that the structures of the upper level are the result of the manifestation in the Cambrian of the Salair folding, identified by A. M. Kuzmin as early as 1928, which, in his opinion, played a major role in the formation of a number of folded edifices of the Altai-Sayan region [8].

References

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