ON THE LOWER BOUNDARY OF THE BELKINSKAYA FORMATION IN GORNAYA SHORIA

V. A. SIVOV

(Presented by Professor K. V. Radugin)

As is known, the phosphorite deposits of Gornaya Shoria that are most promising from an industrial standpoint are grouped into the Belkinskaya Formation. At the Belkinskoye phosphorite deposit and in other areas of its occurrence, according to G. I. Spanderashvili [2] and M. K. Vinkman [1], it lies completely conformably on the deposits of the Zapadnosibirskaya Formation of the Late Precambrian and, with a major tectono-denudational break, is overlain by deposits of the paleontologically characterized Lower Cambrian. These researchers support the conclusion of conformable occurrence of the Belkinskaya Formation on the Zapadnosibirskaya dolomites by a number of sections in which, they claim, the continuity of the sedimentation process of the indicated formations is established. In their opinion, this is evidenced by a gradual increase in the content of \(P_2O_5\) from 0.5–5% in the upper parts of the Zapadnosibirskaya Formation to 10–15% in the Belkinskaya Formation, as well as by a gradual change in the lithological composition of the formations from purely dolomitic in the former to dolomitic-limestone in the latter.

In the summer of 1965 we carried out work on compiling a geological-prognostic map of the Belkinskoye phosphorite deposit. As a result of these investigations, new data were obtained on the nature of the lower boundary of the Belkinskaya Formation, which are set forth in the present article.

In the geological structure of the Belkinskoye deposit there participate (from bottom to top) deposits of the Zapadnosibirskaya and Belkinskaya formations and undifferentiated Lower Cambrian formations (Fig. 1).

The Zapadnosibirskaya Formation is represented exclusively by dolomites, among which stromatolitic, massive, bedded, brecciated, and oolitic varieties are distinguished. Stromatolitic dolomites are the most characteristic element of the formation. Morphologically, stromatolites may be divided into two large groups: patterned structures having an irregular, capricious orientation, and parallel-banded spotted structures composed entirely of forms of Palaeomicrocystis (Fig. 2). Stromatolites of the first group form bodies of indeterminate shape, whereas the parallel-banded varieties create biostromes of rather considerable size. Thus, in the area of the Belkinskoye deposit, over a distance of 8 km, a large sheet-like deposit has been traced, almost entirely formed by stromatolites of Palaeomicrocystis. The orientation of this deposit

in general coincides with the bedding elements of the enclosing rocks, which indicates the possibility of using paleomicrocystis varieties to reveal the internal structure of the Zapadnosibirskaya Formation.

Fig. 1. Schematic geological map of the Belka area in Gornaya Shoria:
1—Quaternary deposits; 2—phosphorites of continental Meso-Cenozoic age; 3—carbonate deposits of the Lower Cambrian. Belkinskaya Formation of the lower part of the Lower Cambrian; 4—limestones of varying degrees of phosphatization; 5—bedded phosphorites. Zapadnosibirskaya Formation of the Late Precambrian; 6—massive algal dolomites, more rarely brecciated and oolitic; 7—dolomites with paleomicrocystids; 8—disjunctive faults; 9—stratigraphic boundaries between Precambrian and Cambrian subdivisions; 10—bedding elements; 11—boreholes; 12—microquartzites of metasomatic origin.

The stromatolites established in the Zapadnosibirskaya Formation—Kabyrsina granbiformis Krasn., Sibirephycus mrassiensis Krasn., Palaeomicrocystis sp. (identified by A. G. Pospelov)—indicate its Late Precambrian age.

Stratigraphically above the Zapadnosibirskaya Formation lies the Belkinskaya Formation, expressed by two types of section. The first of them, composed—

varyingly phosphatic dolomites, limestones, dolomitic breccias, and sandstones interbedded with phosphorite beds, is characteristic of the Belkinskoe deposit. The second,

Fig. 2. Paleomicrocystis dolomite.

represented by weakly phosphatic carbonate-siliceous shales, is noticeably widespread in the Kolzas, Oingol, and Uzas areas.

The Belkinskaya Formation, containing remains of Lower Cambrian problematic fossils, is everywhere overlain by mottled-banded, massive, and bedded limestones, in which remains of algae and archaeocyaths of the Lower Cambrian have been found.

The relationships between the Belkinskaya and West Siberian formations have been studied by us most fully in the Belka area, where a stratotype of the Belkinskaya Formation has been distinguished. There, steeply dipping \((50^\circ—70^\circ)\) West Siberian dolomites strike in a sublatitudinal direction, as established on the basis of a large number of measurements taken along the bedding of the dolomites and by tracing along strike a sheet-like body of paleomicrocystis dolomites that serves as a marker horizon. In turn, the axis of the gentle Belkinskaya syncline (dip angles of the limbs \(15^\circ—30^\circ\)), filled with deposits of the Belkinskaya Formation, extends in a submeridional direction and, thus, its base cuts the structure of the West Siberian Formation at an angle. To this we add that the paleomicrocystis dolomites, elongated in a sublatitudinal direction, are traced for 5 km eastward, to the eastern limb of the Belkinskaya syncline. Within the syncline itself they have not been found and reappear only immediately west of its western limb, extending there as well in a sublatitudinal direction. Moreover, in the northern part of the Belkinskaya syncline they also extend beneath the base of the Belkinskaya Formation, as established by boreholes 48, 49, 74, 72, 80, 71. The absence of beds with Palaeomicrocystis in boreholes

409, 410, 82 indicates that beneath the syncline itself these beds also have a sublatitudinal orientation.

The direct contact of the formations was studied in boreholes 410, 409, 81, 82, 48, 42, 72, 74 and in ditches 63, 66. As a result it was established that the West Siberian dolomites in the near-contact part with the Belkinskaya Formation undergo secondary changes. In some cases metasomatic microquartzites develop on them; in others they are bleached and phosphatized. Thus, in boreholes 410, 409, 81, 82, silicification of the West Siberian dolomites is observed, the intensity of which gradually increases as the base of the Belkinskaya Formation is approached. Directly at the base, black microquartzites with relics of dolomite are observed. Stratigraphically above the microquartzites, in the indicated boreholes, a quartz–dolomite breccia has been established, constituting the basal member of the Belkinskaya Formation.

Macroscopically, the breccia is a dark-gray to black rock, whose texture more closely resembles a spotted or massive one than a brecciated one. Only in individual hand specimens are clearly outlined fragments observed. However, after firing the samples the picture changes sharply, and a distinct breccia texture appears in them. This is due to the fact that, under the influence of high temperature, the carbonaceous matter of the carbonates burns out and they acquire a light-gray to white color, against whose background black fragments of microquartzite stand out in relief (Fig. 3). These fragments are the most characteristic element of the breccia under consideration.

Fig. 3. Dolomite–microquartzite breccia at the base of the Belkinskaya Formation (black fragments are microquartzite fragments).

The overwhelming majority of them have an angular form, sharply sinuous outlines, and bear no traces of rounding. Only in some hand specimens are rounded and subrounded varieties observed. The sizes of the fragments vary from 10–15 cm and more down to 1–2 mm, with, in general, a confinement of the larger fragments to the lower stratigraphic parts of the member being established. The texture of the fragments is usually massive, but in some cases relics of the pattern of stromatolitic dolomites are observed. In addition, locally there are fragments consisting of dolomite only partly converted into microquartzite, and fragments of dolomites silicified along bedding. Along with microquartzites, fragments of dolomites are noted in the breccia, the color of which varies from grayish-white (bleaching) to dark gray.

Their texture is varied: massive, layered, stromatolitic, and oolitic varieties are noted. The outlines of the fragments are sharp and angular. Their sizes range from 2–3 mm to 5–7 cm. The presence in some fragments of relict tectonic structures, especially clearly expressed in microquartzites, should be emphasized. Their cracks

are filled with veinlets of white calcite, which do not pass into the cement of the rock. The cement of the carbonate–microquartzite breccia is represented by microquartzite–dolomite sandstone, passing into siltstone. In individual areas, bedding is observed in the cement; moreover, as a result of differentiation of the carbonate and microquartzite material, beds of dolomite sandstone and black bedded microquartzite are formed. The latter differs from the microquartzites in the West Siberian dolomites by the presence of distinct bedding, sorting of grains, and conformable contacts with beds of dolomite sandstones. In boreholes 42, 48, and 74 a different character is observed in the relationships between the West Siberian and Belkinskaya formations. Here, near the base of the Belkinskaya Formation, the West Siberian dolomites underwent not silicification, but bleaching and phosphatization, which are clearly connected with superimposed secondary processes. Thus, in bleached dolomites, relicts that have not undergone alteration are often observed. They are represented by stromatolitic and massive dolomites ranging from gray to dark gray in color. The shape of the relicts is quite irregular, and their sizes vary within wide limits. It is characteristic that paleomicrocystis varieties of dolomites are bleached more intensely than the others, which indicates a dependence of the degree of bleaching on the structural-textural features of the dolomites. In some boreholes (42, 48) it has been established that, with distance from the base of the Belkinskaya Formation downward (hypsometrically), the bleached and phosphatized dolomites gradually pass into light-gray, and then into ordinary gray dolomites. Thus, a zonal character is indicated for the change in color and degree of phosphatization of the West Siberian dolomites, depending on their spatial proximity to the deposits of the Belkinskaya Formation. The absence of bleaching in the rocks of the latter indicates that the West Siberian dolomites were subjected to this process even before deposition of the Belkinskaya Formation.

Stratigraphically above the bleached dolomites lies a phosphatic carbonate breccia, which is probably a facies analogue of the carbonate–microquartzite breccia. It consists of fragments of massive stromatolitic, more rarely bedded, dolomites, cemented by gray fine-crystalline phosphatic calcareous dolomite passing into dolomitic limestone. The color of the fragments is usually white (bleached dolomite), in places gray or dark gray. Along their periphery there are traces of ancient karstification and, in the case of gray varieties, a bleached recrystallized rim. The shape of the fragments is angular; their outlines are sharp. There is no sorting of them by size. In some fragments, relict tectonic structures are observed, represented by fractures that do not cut the cement.

On the basis of the factual material presented, in our opinion, the presence of a major hiatus between the West Siberian and Belkinskaya formations is demonstrated quite clearly. This is substantiated both by structural mapping (truncation at the base of the Belkinskaya Formation of the occurrence of paleomicrocystis dolomites, discordance of the elements of bedding of the formations) and by the presence at the base of the Belkinskaya Formation of carbonate–microquartzite and phosphatic–carbonate basal breccia. Its fragments, often with clearly expressed relict tectonic structures, with bleaching rims and traces of karstification, arose through erosion mainly of rocks of the West Siberian Formation. The latter, before accumulation of the sediments of the Belkinskaya Formation, were subjected to folding, brecciation, bleaching, and especially silicification. This was accompanied by the formation, in the dolomites of the West Siberian Formation, of microquartzites and bedded silicified dolomites; moreover, these its

rocks, as a rule, are confined to the base of the Belkinskaya Formation. Macroscopically, the microquartzites are cryptocrystalline and have a dark-gray to black color. The coloration of the rock is due to the presence in it of a very fine admixture of carbonaceous matter, dispersed uniformly throughout. In hand specimens, relicts of unreplaced silicified dolomite are observed. They are represented by dolomite, silicified to varying degrees, having predominantly a massive texture. Only in rare cases are microquartzites with relicts of a stromatolitic pattern observed.

As one moves away from the base of the Belkinskaya Formation (hypsometrically downward), the intensity of silicification of the dolomites regularly decreases, and the microquartzites gradually pass into quartz-bearing, and then into ordinary dolomites. Thus, in borehole 82, immediately below the basal carbonate–quartzite breccia, the following is observed:

interval 100.7—106.0 m — black secondary microquartzite after dolomite, graphitized in places;

interval 106.0—108.0 m — gray fine-crystalline silicified dolomite;

interval 108—114.75 m — gray microcrystalline dolomite. The rock is intensely silicified, to the point of formation of secondary microquartzite:

interval 114.75—116 — gray fine-crystalline dolomite, intensely laminated, silicified in places to the point of formation of secondary microquartzite;

interval 116—130.4 — black-gray massive, distinctly bedded, silicified dolomite;

interval 130.4—150, gray, fine-crystalline, massive dolomite.

The formation of microquartzites only in those parts of the West Siberian dolomites that are spatially confined to the base of the Belkinskaya Formation clearly indicates the time and setting in which silicification took place. Most probably, it was caused by infiltration processes that occurred under conditions of a continental regime preceding the deposition of the Belkinskaya sediments. In this case, microquartzites, arising along different stratigraphic horizons of the West Siberian Formation, would constantly underlie the base of the Belkinskaya Formation. On the other hand, under exogenous conditions silicification could hardly have penetrated to a depth greater than 100—150 m. Therefore, the absence of microquartzites in parts of the dolomites remote from the base of the Belkinskaya Formation can be explained by the fact that there the level of the denudation surface exceeded the depth of silicification.

The absence of silicification in the rocks of the Belkinskaya Formation itself is explained by the fact that this process took place before its deposition. The latter is convincingly confirmed by the presence, in the basal breccia of the Belkinskaya Formation, of fragments of microquartzites. The reason for the discontinuous distribution of silicified dolomites at the contact of the West Siberian and Belkinskaya formations (they are absent in a whole number of areas—Onzas, Dalnii, the north of the Belka area) is not yet entirely clear. In our view, it is due to the structural-textural features of the West Siberian dolomites. Thus, our work has established that secondary microquartzites develop in the overwhelming majority of cases after bedded and massive varieties of dolomites and usually do not affect stromatolitic dolomites. The latter, in the near-contact part with the Belkinskaya Formation, underwent processes of bleaching and phosphati-

—zation, which, like the formation of microquartzites, occurred under conditions of the pre-Belkinskaya hiatus.

From the foregoing it follows that, along with the unconformable relationship of the Belkinskaya and West Siberian formations, a pre-Belkinskaya weathering crust has been established in the area, represented by metasomatic quartzites and bleached phosphatized dolomites. In our opinion, further study of the pre-Belkinskaya weathering crust will provide substantial assistance in developing more targeted criteria for prospecting bedded phosphorites, as well as as-yet-undiscovered deposits of iron, aluminum, and other useful minerals associated with weathering crusts.

REFERENCES

1. A. B. Gintsinger, M. K. Vinkman. On the stratigraphic position of the phosphorite-bearing and manganese-bearing deposits of Gornaya Shoria and the Kuznetsk Alatau. Transactions of SNIIGGiMS, issue 24, 1962.

2. G. I. Spanderashvili. Phosphorites of Gornaya Shoria. In: Phosphorites of Western Siberia. Nedra Publishing House, issue 65, 1965.