On the Precambrian (Pre-Tarzhulian) Hiatus in Sedimentation and the Associated Weathering Crust in the Kuznetsk Alatau

G. A. Ivankin, I. I. Koptev, V. L. Shipitsyn

Until quite recently, many researchers considered the process of sedimentation and the section of Precambrian and Cambrian deposits of the Kuznetsk Alatau to be continuous [8, 14, 16]. By the present time a number of hiatuses have been identified in the Precambrian sections of this region, and these have received broad recognition [1–7, 11]. Still under discussion are estimates of the duration of these hiatuses and of their significance in the geological development of the region. The prospects of such stratigraphic levels for the discovery of useful minerals, the formation of which is associated with the weathering crust and with the redeposition of its products, have been entirely overlooked.

In the present article the authors set themselves the aim of drawing the attention of geologists, for the time being, to only one hiatus in sedimentation in the upper Proterozoic of the eastern part of the Kuznetsk Alatau—the pre-Tarzhulian.

The Tarzhulian Formation was distinguished as an independent stratigraphic subdivision in the basin of the middle course of the Bely Iyus River [5–7]. It is characterized by an exceptionally uniform lithological composition and is composed of bedded, more rarely massive, dolomites. The formation occurs in the upper part of the Precambrian section and is overlain in a number of places by paleontologically characterized deposits of the Lower Cambrian. The assemblage of organic remains (stromatolites, oncolites, catagraphs) contained in the rocks of the Tarzhulian Formation is compared with the Yudomian assemblage of the Siberian Platform.

Owing to its uniform and stable composition and to the presence of characteristic forms of phytolites, the Tarzhulian Formation is well recognized and everywhere occupies the same stratigraphic position. It is very widely distributed. For example, identical to the Tarzhulian Formation of the Bely Iyus River area are the Gidrin Formation of the Sarala region [3–4], the Bagzas Formation of the headwaters of the Tom River [9–10], the West Siberian Formation (formation) of Gornaya Shoria [9], the upper part of the Martyukhin Formation of the Batenev Ridge, the Ovsyankov Formation of the Eastern Sayan, etc.

Within the eastern slope of the Kuznetsk Alatau and the Batenev Ridge, as a result of large-scale prospecting carried out in 1960–1967, ...

by employees of TPI, a hiatus in sedimentation was established that preceded the time of formation of the Tarzhul Formation.

It is well recorded in the area of the Kulbyurstog River, in the basin of the Son River, and at other localities. A number of features indicating unconformable occurrence of the basal beds of the formation on the underlying strata (an angular unconformity, with which the formation overlaps older rocks; occurrence of the basal beds on strata occupying different stratigraphic levels in the section of the older deposits; the presence in a number of places of a weathering crust beneath the basal beds of the formation, etc.) served as a reliable basis for establishing the hiatus, which was also confirmed by paleontological materials.

For example, in the area of the village of Katyushkina (Son River basin, Batenev Range), in the centroclinal closure of the Katyushkino syncline, the Tarzhul Formation rests on the underlying rocks through a weathering crust. On the right side of the Karasuk log, in its mouth part, residual products of the weathering crust fill pockets and cavities on the uneven and complex surface of the underlying limestones and silicilites of the Tyurim Formation. They are represented mainly by unsorted psammitic-aleuritic material, in which angular fragments of black limestones and silicilites, cherry-colored sandstones and siltstones, and more rarely yellowish dolomites are scattered. At the base of the weathering crust there are accumulations of large blocks, up to 1.0 m across, of limestones and silicilites, cemented by dark-cherry clayey-ferruginous material. The thickness of the crust is about 10 m. The color of the weathering products is bright cherry, more rarely greenish-gray and black. The bright-cherry varieties are densely saturated with finely disseminated hematite, which locally segregates into nests up to 0.10 m in size.

Five kilometers farther south, on the left side of the Karasuk log, the Tarzhul Formation already rests on dark-lilac andesitic porphyrites of the Kulbyurstog Formation. Its basal member has the following structure (from bottom to top):

1. Varigrained, non-bedded and unsorted sandstones, composed of angular fragments of effusive rocks and grains of kaolinized feldspars. The sandstones are characterized by a high hematite content, which colors the rock bright cherry. Because of the massive textures and the presence of unrounded feldspar grains in the sandstones, the boundary with the underlying porphyrites is diffuse and is recognizable only by the change in color. The sandstones should be regarded as material composing the disintegration zone of the pre-Tarzhul weathering crust. Thickness 4.0 m.

2. Interbedding of cherry-colored bedded sandstones and siltstones and yellowish dolomites. The number and thicknesses of the carbonate interbeds increase toward the top. Thickness 9.0 m.

3. Medium-grained, non-bedded sandstones of dirty greenish-gray color. Thickness 0.9 m.

Higher in the section follow dark and light bedded dolomites, in the lower part having cherry and pinkish coloration. Among the dolomites there are beds of ferruginous rocks; of these, the thickest (more than 40 m) occurs 70 m above the base of the formation. The beds are composed of cherry-brown siliceous dolomites, silicilites, and breccias.

Eight kilometers along strike to the northeast, on the right side of the valley of the Son River, the Tarzhul Formation also rests on effusive rocks of the Kulbyurstog Formation, but through basal conglomerates. The rocks of the well-rounded pebbles of the conglomerates are represented by green, cherry-colored, and

lilac porphyrites, volcanomictic sandstones, and siltstones. The thickness of the conglomerate bed increases from 0.4 m to 3.0 m eastward over a distance of 100 m. Above the conglomerates lie bright cherry-colored argillites and siltstones with lens-shaped interbeds of coarse-grained sandstones and yellowish argillites. The thickness of the bed ranges from 3.0 to 6.0 m. The argillites are abruptly replaced by light-gray massive and bedded dolomites. The total thickness of the basal member, composed of clastic rocks, is 6.0–7.5 m.

On the geological map of the area under consideration, compiled by E. V. Shirokova and D. M. Bondareva, the Kul’byurstorg effusives are assigned to the Berikul’ Formation of the Middle Cambrian [15], with which it is difficult to agree. The greater antiquity of the effusive sequence is indicated by: first, the gentle overlap of the dolomite sequence onto the effusives (the beds dip northward at angles of 20–40°, and bedding was measured both in the dolomites and in the basal conglomerates); second, the presence in the basal conglomerates of volcanomictic rocks of the same composition as in the underlying effusive sequence; third, the occurrence of basal conglomerates on different horizons—on mandelphyres in some places, on porphyrites in others, and on tuffaceous sandstones elsewhere.

On the right side of the valley of the Son River, the dolomites of the Tarzhul Formation and the effusives of the Kul’byurstorg Formation are separated by a 100-meter sequence of dark-gray massive limestones with lenses and interbeds of black siliceous siltstones. The thickness of this sequence increases northeastward to 260 m in the section along Podtemnyi Log (after 8 km).

The Tarzhul Formation in this section, as everywhere, is composed of massive and bedded organogenic–algal dolomites containing the Udom complex of microphytolites [1, 12, 15]. The basal member consists of interbedded vari-grained sandstones, grits, and conglomerates. The clastic material consists of well-rounded pebbles of the underlying rocks—sandstones, limestones, oncolitic dolomites, as well as green and cherry-colored porphyrites of the Kul’byurstorg Formation.

Thus, in the NW part of the Batenev Range, a break at the base of the Tarzhul Formation is clearly established; it is expressed by basal conglomerates, the presence of a weathering crust, and structural unconformity.

The structural unconformity in itself indicates a sufficiently long duration of the break and the large scale of the geological events that preceded the accumulation of the dolomitic sediments of the Tarzhul Formation.

The duration of the break can be estimated only approximately, on the basis of some radiological data and analysis of the geological map. Over most of its area of distribution the Tarzhul Formation lies on the Kul’byurstorg Formation, and the age of the latter, determined by several methods, is about 1000 million years [1, 2, 11]. The lower age limit of distribution of the Udom complex of microphytolites, with which the Tarzhul Formation abounds from its very base, is determined at 680–700 million years [13]. If the latter figure is accepted, then the break in pre-Tarzhul time in the NW part of the Batenev Range amounts to no less than 300 million years.

One may question the duration of the break estimated at 300 million years, but there is no doubt that it was sufficient for the formation of a thick weathering crust, as a result of the redeposition of whose products large deposits could have formed. It is precisely at this level that we have discovered sedimentary iron ores.

confined to the boundary of the Tarzhul and Kul’byurstug formations and occurring everywhere within the belt bordering the field of outcrops of the Tarzhul dolomites in the area of the settlements of Chalgystag and Katyushkina.

In this area (within the Katyushkina syncline), two types of iron mineralization associated with the pre-Tarzhul weathering crust have been established.

The first type consists of residual products of the weathering crust, densely saturated with finely disseminated hematite. The content of iron oxides in them is 15–30%, and alumina is 20%.

The erosion and redeposition of products of the weathering crust lead to enrichment in iron and silica of the rocks of the Tarzhul Formation. Thus, in the composition of its lower part there occur highly ferruginous dolomitic-siliceous and siliceous rocks, representing the second type of mineralization. The content of iron oxides in these rocks is 5–15%, but beds with contents up to 50% are distinguished.

In the Tarzhul Formation of the Katyushkina syncline a complete facies series is represented, from continental formations of the weathering crust to shallow-water marine deposits, both in vertical section and in the lateral direction—along strike; i.e., there are prerequisites for also discovering highly aluminous rocks.

Confirmation of the bauxite prospects of the characterized stratigraphic level may also be provided by the fact that in the basin of the Belyi Iyus–Karatan rivers there is a known deposit (the Amara deposit) of sillimanite-cordierite schists, formed at the contact with a granitoid intrusion at the expense of red-colored sandy-clayey rocks. The latter, in their stratigraphic position and age, correspond to the basal beds of the Tarzhul Formation. Approximately the same geological setting (sillimanite-cordierite schists at the base of the Yenisei sequence) is noted in the area of Bol. Berchikul’ (Margaita); the Bokson bauxite deposit (Eastern Sayan) is confined to this same level.

Thus, the establishment of the pre-Tarzhul hiatus and of the weathering crust associated with it makes it possible not only to present in a new way the history of the geological development of the Kuznetsk Alatau in the Late Precambrian, but also to identify promising areas for useful minerals associated with the weathering crust—iron ores, allites, etc.

LITERATURE

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