A. G. BAKIROV

ON THE GENESIS OF THE MICRORELIEF OF THE WEATHERING CRUST OF ULTRAMAFIC ROCKS OF THE KEMPIRSAI MASSIF

The origin of the microrelief of the weathering crust is considered in this work using the example of the Kempirsai hyperbasite massif (Aktobe Region, Kazakh SSR).

The massif is spatially confined to the Or–Ilek watershed, which is the southern continuation of the Guberlya Mountains of the Southern Urals. This watershed is a hilly ridge-like upland.

Within the area of the Kempirsai hyperbasite massif there are exposures of Precambrian and Cambrian metamorphic rocks, basic effusives and Silurian and Devonian shales, rocks of the gabbro–hyperbasite intrusive complex, and then loose marine and continental sediments. The latter include rocks of the pre-Jurassic weathering crust of hyperbasites, variegated deposits of the Jurassic–Lower Cretaceous and Eocene, marine Upper Cretaceous and Paleocene sediments, sandy-clayey and pebble deposits of the Oligocene and Neogene, and then Quaternary sedimentary rocks, among which variegated sandy clays occur. Within the massif, four principal directions of structural strike are rather clearly expressed: northwestern, northeastern, meridional, and nearly latitudinal.

Before giving a description of the forms of microrelief of the weathering crust of ultramafic rocks and analyzing their origin, let us briefly become acquainted with the structure of the weathering crust and the history of its formation.

Structure of the Weathering Crust and the History of Its Formation

The weathering crust of ultramafic rocks has a zonal structure. In a section of the normal profile of the crust we see (from top to bottom) ochres, nontronites, and leached serpentinites; in a section of the silicified profile—silicified and leached serpentinites. The latter in both profiles gradually pass into ordinary serpentinites.

Ochres are loose porous rocks of yellow, brownish-yellow, cherry-red, and burgundy color, consisting mainly of a mass of hydrated iron oxides.

Nontronites are loose rocks colored olive-green. Recent investigations by I. I. Ginzburg [2] have shown that “...in the nontronites, along with purely ferruginous varieties, we have isomorphous mixtures of ferrimontmorillonites and ferribeidellites with analogous aluminosilicate minerals and, in addition, a small admixture of magnesian beidellite.”

Leached or decomposed serpentinites are yellowish-greenish rocks of light tones, light in weight. They differ from unaltered ones by a lower content of magnesium and calcium and a relatively higher content of silica.

In the leached serpentinites a complex of secondary minerals has been established, forming two principal zones of mineralization: an upper silicified zone and a lower carbonate zone. In the silicified zone opals, chalcedonies, and quartz are widely developed. In the carbonate zone there are dolomite, calcite, aragonite, and magnesite.

Silicified or silicified serpentinites are cellular and massive rocks composed chiefly of chalcedony, more rarely of chalcedony and quartz. The silicified serpentinites are confined to the upper silicified zone of the mineralized leached serpentinites.

The average thickness of the loose nontronite stratum in areas of the weathering crust is approximately 15 m, the maximum being 25–30 m.

Loose variegated deposits, overlying the rocks of the crust, are spatially and genetically closely associated with the weathering crust. In the variegated deposits we distinguish three series: lower, middle, and upper.

The rocks of the lower series are represented by bauxites, oolitic brown iron ores, variegated kaolin clays, and redeposited ochres. The age of the rocks is Jurassic–Lower Cretaceous. The average thickness of these rocks is 8 m.

The middle series includes variegated and reddish-brown sandy clays with inclusions of pisoliths of brown iron ore and oolites of wad. In some areas the clays are carbonatized. In the rocks of this series there occur lenses of quartz sands with Eocene flora, as well as marls. The average thickness of these rocks, assigned to the Eocene, is 18 m.

The upper series of variegated deposits of Quaternary age is represented by reddish-brown and reddish-brownish calcareous sandy clays, up to 5 m thick. In these clays there occur pisoliths of sandy brown iron ore, as well as carbonate and marly concretions of the “zhuravchik” type.

The thickness of the loose rocks of the various series of variegated deposits is not everywhere fully maintained. In some areas of the weathering crust the lower series is often absent from the section; the middle and upper series may be weakly expressed.

The formation of the weathering crust of ultrabasic rocks, with the formation of ochres, nontronites, and leached serpentinites, took place under continental conditions of the Triassic and partly of the Lower Cretaceous, established after the Permian peneplanation of the Ural folded belt.

In the Middle and Upper Jurassic and in the Lower Cretaceous, two tendencies became rather clearly manifest in the geological history of the Southern Urals, already outlined in the Lower Jurassic: humidification and cooling of the climate, and subsidence of the continent. The latter caused the transgression of the Upper Cretaceous sea, which covered significant areas of the Southern Urals.

The Upper Cretaceous sea advanced over a surface already leveled in the course of the preceding continental weathering of the Triassic, Jurassic, and Lower Cretaceous, without producing much erosion. This is evidenced by the preserved ochres and nontronites exposed from beneath the Upper Cretaceous deposits in a number of districts of the Southern Urals, and also by the covering of the loose deposits of the weathering crust by marine Cretaceous on the Shelektinsk area and the Donskie chromite deposits.

The period of submarine conservation of the crust continued throughout the Paleogene, when an even greater subsidence of the continent below sea level occurred.

At the beginning of the Eocene there was an uplift of the area. Marine conditions were replaced by continental ones. In the process of erosion, over considerable areas of the Southern Urals the marine deposits of the Upper Cretaceous and Paleogene were washed away, and the pre-Jurassic peneplain with its weathering crust was exposed and dissected.

Weathering processes in the Tertiary period, under the conditions of the hot climate of the Eocene and partly of the Oligocene, were superimposed on the pre-Jurassic crust and led to the transformation and renewal of the latter. In their early stages they were marked by the ocherization of nontronites, the leaching of serpentinites, and the formation of ochres.

In the course of weathering, silica, along with other products of weathering, was also brought into a soluble state. The bulk of the silica was “captured” by the carbonate zone of the leached serpentinites. As a result of the metasomatic replacement by silica of the dolomite and calcite of the carbonate zone of the leached serpentinites, the so-called silicified serpentinites were formed.

In the Miocene and Pliocene, and also in the Quaternary period, conditions favorable to the development of a weathering crust of the pre-Jurassic and Eocene type were no longer created.

As a result of the redeposition and reworking of the products of the weathering crust in the Middle–Upper Jurassic and Lower Cretaceous, in the Eocene and Oligocene, and then in the Quaternary period, red-colored deposits of the lower, middle, and upper series, respectively, were formed.

Microrelief of the weathering crust and its origin

The weathering crust occurs only on the preserved elevated parts of the dissected pre-Jurassic peneplain. The hypsometric elevations of the various areas of the “crust” within the massif generally vary within the limits of 420–470 m, with rare exceptions.

Areas of the weathering crust are either weakly dissected plateau-like uplands, on the surface of which loose products of the weathering crust—ochres and nontronites—come out, or else sharply defined and dissected hills with silicified serpentinites exposed on their surface.

We shall call the details of the relief within the preserved elevated parts of the pre-Jurassic peneplain with a weathering crust the microrelief of the crust. The amplitude of fluctuations in the hypsometric elevations of the forms of the crust microrelief varies from a minimum of several meters to a maximum reaching 20–30 m.

Among the elements of the microrelief one should note small valleys of slopes, troughs, closed elongated saucer-shaped depressions, watersheds of valleys and slopes, ridges, gentle elevations, etc.

In the more dissected areas of the crust, the predominant orientation of the strike of positive and negative forms of the microrelief is well expressed in meridional, northwestern (345°), northeastern (50°), and nearly latitudinal directions, coinciding with the strike of the principal structures of the pluton. In the less dissected areas, such an orientation of the strike of the microrelief forms is established only upon careful study. As we shall see below, these regularities in the orientation of the microrelief forms are by no means accidental.

Negative forms of the microrelief are confined to outcrops of loose ochre-nontronite or red-colored rocks, whereas positive forms are confined to silicified, carbonatized, or leached serpentinites that crop out directly at the earth’s surface or lie close to the surface.

Regarding the genesis of the microrelief of the weathering crust of the Kempirsai hyperbasite massif, one point of view is known in the literature, belonging to I. I. Ginzburg and I. I. Savel'ev. According to it, the modern microrelief of the crust is basically a copy of the pre-Jurassic microrelief, exposed by erosion from beneath a loose cover of marine and continental depos-

of the Meso-Cenozoic. In this connection, I. I. Ginzburg [3] and I. I. Savel'ev [5] make the course of weathering processes and the character of distribution of their products dependent on the morphology of the pre-Jurassic surface. Thus, I. I. Savel'ev [5] writes that “...on the elevated parts of the ancient microrelief there occurred an accumulation of a large quantity of silicified serpentinites, while in the lowered parts—the formation of a large quantity of nontronites” (p. 76). From this it is concluded that “...the silicified rocks ended up on the elevated points of the relief (the modern one. A. B.) as a result of their formation on the elevated parts of the pre-Jurassic microrelief” (pp. 77–78). In his article I. I. Savel'ev [5] calls the hollows in the modern microrelief on areas of the weathering crust ancient hollows.

Most investigators of the crust have not expressed their opinion on the substance of the views of I. I. Ginzburg and I. I. Savel'ev. Some geologists, such as, for example, V. N. Razumova and N. P. Kheraskov, share to one degree or another the point of view of the above-mentioned authors.

The views of I. I. Ginzburg and I. I. Savel'ev on the identity of the forms of the ancient and modern microrelief of areas of the weathering crust of serpentinites are not borne out in reality.

On the weathering crust lie rocks of the red-colored deposits, which occur chiefly in depressions of the modern relief. If the latter are identified with ancient depressions, then we should have a direct dependence between the thicknesses of the loose rocks of the crust and of the red-colored deposits. In reality it is the opposite.

Silicified serpentinites are, in most cases, confined to elevated parts of the modern microrelief, but not always.

The pre-Jurassic weathering crust formed under conditions of a leveled relief. Valleys, hollows, slopes, and other depressions were those forms of relief where, owing to erosion, the least accumulation and preservation of loose products of the crust could be expected. It remains unclear what fate the pre-Jurassic microrelief underwent in the Eocene, when uplift of the continent occurred, its dissection and the transformation of the crust under conditions of Eocene weathering, and how details of the microrelief could have been preserved at the same time, moreover among such loose formations as ochres and nontronites.

In the modern macrorelief of the Southern Urals a certain inheritance of forms of the pre-Jurassic relief is noted, due to the peculiarities of the geological history of this region in the Meso-Cenozoic (materials of regional studies by A. V. Khabakov [4] and V. P. Kheraskov). But this gives no grounds whatever for drawing conclusions about the identity of the forms of the modern and pre-Jurassic microrelief, since in the latter case we are dealing with phenomena of another scale, which also have other regularities of their development.

In connection with the foregoing, we cannot agree with the views of I. I. Ginzburg and I. I. Savel'ev on the origin of the microrelief of the crust.

To understand the genesis of the microrelief of the weathering crust of ultrabasic rocks, let us examine the question of the so-called underground microrelief of the crust.

The weathering crust formed under conditions of a leveled pre-Jurassic peneplain. Flat watersheds were the most favorable areas for its formation.

The loose ochre-nontronite zone of the weathering crust has the appearance of a mantle-like cover with an uneven lower boundary. Exploration work at mineral deposits associated with the crust quite clearly reveals the presence of ochre-nontronite wells, troughs, and hollows in the leached serpentinites.

The surface separating the loose ocher-nontronite mass of the crust from the underlying leached and silicified serpentinites we shall call the underground microrelief of the weathering crust¹.

In the structure of the underground microrelief the following features are quite clearly apparent:

1) The elevated parts of the underground microrelief are associated chiefly with areas of carbonate mineralization of the leached serpentinites, at the expense of which the silicified serpentinites were formed; the latter also occur in lower places, but much more rarely;

2) Carbonate mineralization was localized mainly in fractured and tectonically weakened zones, where weathering extended to a great depth (the “Bugor” and Buranovka crust areas). The Akkermanovka area of the adjacent Khabarninskii hyperbasite massif may also be assigned here. The elevations of the underground microrelief, composed of carbonatized leached serpentinites, precisely marked these fractured and tectonically weakened sectors and zones in the ultrabasic rocks of the parent substratum².

These features in the structure of the underground microrelief of the crust are well expressed in a number of places. They can also be established from the materials of exploratory workings.

We believe that the underground microrelief of the pre-Jurassic weathering crust, which arose during its formation, was the principal and determining factor in the formation of the microrelief of the present-day surface of areas of the weathering crust.

During the transformation of the crust at the beginning of the Eocene, the ocher-nontronite zone of the crust was denuded to a considerable degree. The processes of silicification of the elevated carbonatized parts of the underground microrelief led to the formation of silicified serpentinites of chalcedony-quartz silicification.

Erosion exposed the underground microrelief of the pre-Jurassic crust. The silicified serpentinites were prepared out in the form of positive features of the microrelief of the present-day surface. The ocher-nontronite areas were washed away with the formation of negative forms. The microrelief of the surface of crust areas in the Eocene–Oligocene epoch adjusted itself to the underground microrelief of the pre-Jurassic weathering crust and, to a considerable extent, began to reproduce it. Subsequent processes of denudational-sedimentary leveling somewhat smoothed the dissected relief of the Eocene, in which form it has essentially come down to us. The present-day relief, in its microforms, in its main features reproduces the microrelief of the Eocene, differing from the latter by a certain smoothing of the forms.

Let us sum up. The view of I. I. Ginzburg and I. I. Savel’ev on the essence of the question is as follows. The present-day microrelief is essentially identical with the microrelief of the pre-Jurassic surface. The silicified serpentinites proved to be on the elevations of the present-day relief as a consequence of their formation on the elevations of the pre-Jurassic relief. The pre-Jurassic microrelief is one of the significant factors that influenced the character of the course of weathering processes and the distribution of weathering products.

Our point of view is different. We believe that the unequal degree of fracturing of the parent substrate caused the uneven course of weathering processes and the somewhat different character of mineralization in various areas of the ultramafic rocks. All this led to the formation of the subterranean microrelief of the weathering crust, which played the principal role in the formation of the microrelief¹ of the present-day surface of areas of the weathering crust. The confinement of silicified serpentinites to positive forms of the modern relief is due to erosional processes of the Tertiary and Quaternary epochs, which exposed and exhumed the silicified rocks from beneath the loose mantle of the ochre–nontronite weathering crust. Consequently, the tectonics of the parent substrate was the principal and determining factor in the formation of the modern microrelief of areas of the crust, and not the microrelief of the pre-Jurassic surface, as I. I. Ginzburg and I. I. Saveliev suppose. This is also evidenced by the coincidence in the orientation of the strike directions of the microrelief forms of the crust and of the principal structures of the Kempirsai pluton.

The study of the origin of the forms of the microrelief of the weathering crust of ultramafic rocks is of great importance for establishing the influence of tectonic conditions on the formation of relief.

LITERATURE

1. Bakirov A. G.—On the question of the Tertiary weathering crust in the Southern Urals. Izv. Tomsk Polytechnic Institute, vol. 65, no. 2, 1950.

2. Ginzburg I. I.—Nontronites of the Southern Urals. Collection dedicated to Academician D. S. Belyankin, 1946.

3. Ginzburg I. I.—The ancient weathering crust on ultramafic rocks of the Urals, part 2. Proceedings of the Institute of Geological Sciences, Academy of Sciences of the USSR, issue 81 (2), 1947.

4. Khabakov A. V.—Pre-Jurassic relief and the ancient weathering crust in the southern part of the Southern Urals. Izv. State Geographical Society, vol. 67, no. 2, 1935.

5. Saveliev I. I.—The role of microrelief in the formation of deposits of silicate ores in the Southern Urals. Proceedings of the Institute of Geological Sciences, Academy of Sciences of the USSR, issue 41 (5), 1941.

1. The amplitude of fluctuations of hypsometric elevations of the microrelief forms of the surface varies within the limits of the thickness of the loose ochre–nontronite stratum of the crust. ↩↩

2. The association of carbonatized serpentinites with positive forms of the underground microrelief indicates that, in more fractured and tectonically shattered areas of ultrabasic rocks, during the formation of the weathering crust, carbonates in most cases began to be deposited in the profile of the crust closer to the earth’s surface than in less fractured areas. ↩