ON THE QUESTION OF THE TERTIARY WEATHERING CRUST IN THE SOUTHERN URALS

A. G. BAKIROV

This article examines the manifestation of Tertiary weathering on the territory of the Kempirsai hyperbasite massif (Novorossiisk and Stepnovsk districts of Aktobe Oblast, Kazakh SSR, 70 km south of the city of Orsk, Chkalov Oblast).

The massif is spatially confined to the Or–Ilek watershed, which is the southern continuation of the Guberlin Mountains of the Southern Urals. The watershed itself is a horst-like projection of the ancient metamorphic sequence of the Urals, with an average absolute elevation of 320 m.

The hyperbasite pluton is composed of apoperidotite and apodunite serpentinites, bordered along the periphery by gabbroids. The enclosing strata are metamorphic rocks of the Precambrian and Cambrian, and slates and effusives of the Silurian and Devonian.

In the area studied, loose deposits of marine and continental facies are fairly widespread. The former are represented by various glauconite-bearing sands, sandstones, opokas, clays, and phosphorite conglomerates of the Upper Cretaceous and Paleocene. The continental facies are represented by loose formations of the pre-Jurassic weathering crust, variegated deposits of the Jurassic–Lower Cretaceous, Eocene and Quaternary times, as well as Oligocene, Neogene, and Quaternary sandy-clayey deposits.

Characteristics of the Weathering Crust of the Hyperbasites

The weathering crust is not distributed over the entire area of the hyperbasite massif; it occurs only on the preserved elevated parts of the dissected pre-Jurassic peneplain.

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

In neighboring areas, according to P. L. Bezrukov and A. L. Yanshin [2] and A. V. Khabakov [7], the weathering crust is overlain by the continental Jurassic ($J_{2—3}$). In the area of our studies, the marine Upper Cretaceous deposits lie on the eroded surface of the weathering crust.

In the ochres, there occurs in small quantity, in the form of films and veinlets, psilomelane-wad, then bayerite and halloysite. Opal and chalcedony are recorded rarely.

Among the new formations in the nontronites, psilomelane-wad and halloysite occur; these are localized mainly in the upper horizons of the nontronites and form thin films and veinlets. In the lower horizons of the nontronites, opals occur, as well as nickel silicates.

In the leached serpentinites, a complex of secondary minerals has been established, forming two principal zones of mineralization: an upper—silicified—and a lower—carbonate—zone.

In the silicified zone, opals, chalcedonies, and quartz are widely developed. In association with the latter, nickel silicates occur (garnierite and revdinskite).

In the carbonate zone, an upper aragonite subzone, a middle calcite-dolomite subzone, and a lower magnesite-kerolite subzone are distinguished. The carbonates usually form reticulate veins, veinlets, and lens-shaped accumulations. The presence of carbonate netting is quite typical of leached serpentinites.

Variegated Deposits

Loose variegated deposits are spatially and genetically closely connected with the weathering crust.

The lower series of these deposits is represented by bauxites, oolitic brown iron ores, variegated kaolinitic clays, and redeposited ochres. The absence of calcareous formations is characteristic of the rocks of this lower series.

The average thickness of the rocks of the lower series is 8 m. In lithological features, these rocks somewhat resemble similar rocks described by P. L. Bezrukov and A. L. Yanshin [1] in the Ziren-Agach suite.

In some parts of our area, the rocks of the lower series of variegated deposits are overlain by marine Upper Cretaceous deposits (the Dzhangiz-Agach River, the Donskie chromite deposits).

We have assigned the rocks of the lower series of variegated deposits, with some reservation, to the Jurassic—Lower Cretaceous.

In the rocks of the middle series of variegated deposits, we distinguish two horizons: a lower horizon of variegated clays and an upper horizon of reddish-brown sandy clays. Characteristic features of the rocks of the middle series are the presence of lenses of quartz sands with Eocene flora in the middle part of the lower horizon, as well as marls confined approximately to the boundary between the two horizons. Both these clays and the other clays are, in individual areas, somewhat carbonatized. The reddish-brown clays and, much more rarely, the variegated clays contain bean-shaped concretions of sandy brown iron ore. The marls, as well as the quartz sands, are locally silicified. In the upper horizons of the middle series, gypsum-bearing varieties of clays are encountered, also containing aragonite. By their calcareousness, silicification, gypsum content, and also by the somewhat distinctive character of the bean-shaped concretions of brown iron ore, the rocks of the middle series of variegated deposits differ from the rocks of the lower series.

The upper series of variegated deposits is represented by reddish-brown and reddish-brownish calcareous sandstones, clays, and loams, lying with a marked unconformity on Tertiary, Cretaceous, and also older rocks.

Sequence of Mineral Formation in the Weathering Crust of Hyperbasites and in Variegated Deposits

In studying the minerals and their interrelations in the weathering crust of the hyperbasites of the Kempirsai massif and in the variegated deposits overlying the crust, it was possible to establish the following sequence in mineral formation (Fig. 1).

In analyzing this scheme, one cannot fail to note the presence of three fairly clearly distinguished stages of mineral formation.

For the first stage in the profile of the crust, an abundance of carbonates is characteristic, whereas for the second, conversely, the prevalence of chalcedony and quartz with a subordinate significance of carbonate formations is characteristic.

It is worth comparing the sequence of mineral separation in the profile of the crust in the first and second epochs of mineralization. In the first, mineral formation begins with carbonates, and in the second—with chalcedony and quartz. For the first epoch, opal cementation is characteristic with a clear predominance of carbonates; for the second—chalcedony-quartz cementation with a subordinate significance of carbonates.

Fig. 1. Scheme of the sequence of separation of minerals in the weathering crust of serpentinites of the Kempirsai hyperbasite massif and in the red-colored deposits.

The red-colored deposits overlying the weathering crust also have their own features in the character of mineralization. For the first epoch of mineral formation, oolitic brown iron ore and bauxites are characteristic; for the second—psilomelane-wads, asbolanes, pisolitic brown iron ore, dolomite, gypsum, nickel opals, cemented dolomites, and marls. The third epoch of mineralization is in general poorly represented by minerals. Gypsum, dolomite, and chalcedony of the third generations are associated with it.

The features of the epochs of mineral formation shed light on the history of the formation of the serpentinite weathering crust, which will be discussed below.

Meso-Cenozoic History of the Region

At the end of the Paleozoic and the beginning of the Mesozoic, the Southern Urals were peneplaned. After peneplanation, exceptionally favorable conditions were created for the formation of a weathering crust.

The long duration of continental weathering under peneplain conditions and favorable climatic conditions promoted the formation of a thick, loose weathering crust on all rocks of the parent substrate, regardless of their composition.

The loose products of the weathering crust on granites are represented by kaolins; on gabbroids, by ochres and kaolins; on serpentinites, by ochres and nontronites.

The formation of the weathering crust took place under conditions of an acidic environment and a fairly humid climate. The former is evidenced by the extensive development of kaolins on feldspathic rocks (kaolins, according to the latest data of Sedletsky and other researchers, form during weathering in an acidic environment), and the latter by the considerable thickness of the loose stratum of the crust, which in this form could have formed only with prolonged and constant moisture.

The zonal structure and thickness of the serpentinite weathering crust were determined by two factors: the depth of the groundwater level and the pH value of the weathering environment.

The loose ocher–nontronite stratum of the crust was a kind of cover that protected the underlying leached serpentinites from weathering. As its thickness increased, the intensity of the weathering processes in the bedrock weakened. And approximately by the end of the Lower Jurassic, there came a period when the formation of the loose nontronite stratum ceased.

From the beginning of the Middle Jurassic, two tendencies became quite clearly apparent in the geological history of the Southern Urals, already outlined in the Lower Jurassic: general subsidence of the continent, and humidification and cooling of the climate. The subsidence of the continent entailed a rise in the groundwater level, flooding and conservation of the zone of leached serpentinites and nontronites. Lakes and swamps developed.

Humidification of the climate led to the development of vegetation and to a change in the pH of the weathering environment toward increased acidity. All this, taken together, caused a change in the character of the course of the processes of surface weathering. In connection with the change in the pH of the environment, the transition of sesquioxides into a soluble state became possible, as did their transport with subsequent precipitation and the formation of bauxites and brown iron ores. The physical (mechanical) transport of loose products of the pre-Jurassic weathering crust acquired importance.

Under these conditions, the rocks of the lower series of red-colored deposits were formed, represented by redeposited ochres, kaolins, oolitic iron ores, and bauxites.

The marine transgression of the Upper Cretaceous sea, which covered significant areas of the Southern Urals, was a historical inevitability in the geological life of these areas, which from the Jurassic onward began to undergo subsidence. The Upper Cretaceous sea advanced over a surface already leveled in the process 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 areas of the Southern Urals, as well as by the covering with marine chalk of loose deposits of the weathering crust in parts of the Shelektinskoye nickel deposit and the Donskie chromite deposits.

With the deposition of gravels, phosphorites, glauconitic sands, and clays of the Upper Cretaceous, which covered the partly eroded weathering crust of the serpentinites, a new period began in the life of the latter—a period of underwater preservation. It continued throughout the Paleocene as well, when the continent underwent an even greater submergence below sea level.

Formation of the Tertiary Weathering Crust

At the beginning of the Eocene the country was uplifted. Marine conditions were replaced by continental ones. In the course of erosion, over considerable areas of the Southern Urals the marine deposits of the Upper Cretaceous and Paleocene were washed away, and the pre-Jurassic peneplain, with its weathering crust, was exposed and dissected. In the Eocene a new page opened in the history of the pre-Jurassic crust—the epoch of its Tertiary development: erosion, transformation, and renewal.

Before examining the concrete manifestation of Tertiary weathering, let us touch on the history of the question of the Tertiary crust. This question is one of the debated and insufficiently illuminated ones in the Mesozoic history of the Southern Urals, despite the fact that in the Middle Urals the problem of the Tertiary crust is resolved more definitely and positively (the works of I. M. Krasheninnikov [4], E. P. Shchukina [8], and other geologists).

A defender of the Tertiary weathering crust in the Southern Urals is A. N. Aleshkov. His views are somewhat distinctive. While recognizing the Tertiary crust, he denies the pre-Jurassic one, believing that its loose products could not have been preserved during the transgression of the Upper Cretaceous sea. At the same time, he assigns great significance in the formation of the crust to hydrothermal processes, connected, in his opinion, with Alpine tectogenesis.

It should be noted that up to now none of the researchers of the Southern Urals, including A. N. Aleshkov, has presented reliable data attesting to a Tertiary weathering crust. If such data are cited, they are usually limited to references to silicified Eocene sandstones (A. V. Khabakov, I. I. Ginzburg [3], A. N. Aleshkov) and to the climatic conditions of the Eocene–Oligocene favorable for weathering. Nor has anyone thoroughly examined the question of possible Tertiary mineralization in the weathering crust of hyperbasites.

Some researchers (V. N. Razumova and N. P. Kheraskov) deny the Tertiary weathering crust in the Southern Urals. L. N. Formozova [6], on the basis of studying the chemical composition of glauconite in rocks of the Upper Jurassic, Cretaceous, and Paleogene of the Southern Urals, likewise answers the question of the Tertiary crust in the negative.

On the marine Cretaceous and Paleocene deposits, no one has yet recorded a weathering crust. This argument is the chief and sufficiently serious one among opponents of the Tertiary weathering crust.

Let us dwell on this question in greater detail.

Judging by the flora, the Eocene and Oligocene in the Southern Urals were characterized by a hot climate favorable to the formation of a weathering crust. In comparison with the pre-Jurassic epoch (Triassic–Lower Jurassic), the Eocene–Oligocene interval was relatively less prolonged, but nevertheless sufficient for the formation of loose masses of crust.

If the climate and duration of weathering processes in the Eocene–Oligocene somewhat resembled those of the pre-Jurassic epoch, the geomorphological conditions were sharply opposite. The pre-Jurassic weathering crust formed on a peneplain under conditions of leveled relief. Hence its great thickness and better preservation. The Tertiary crust, however, formed under conditions of relief dissected by erosion, when, in parallel with the formation of a crust less thick than the pre-Jurassic one, there took place—

eroded it. Hence the poor preservation of the Tertiary crust and the low probability of encountering it. The more developed and widely distributed pre-Jurassic crust could in many cases have masked the Tertiary crust, which makes the identification of the latter still more difficult.

We believe that, in order to identify the Tertiary weathering crust, it is necessary to proceed in another way, to choose another method than that used by previous investigators.

On serpentinites, a weathering crust forms much more readily than on other rocks, and in the profile of this crust all the stages of mineralization connected with weathering processes are rather clearly impressed. Therefore, in order to solve the problem of the Tertiary weathering crust, taking into account the specific conditions of its formation, we shall try to use an analysis of the sequence of mineral formation in the weathering crust of serpentinites and in the rocks of the red-colored deposits.

Let us now compare the pre-Jurassic weathering crust and the processes of mineral formation with the Tertiary crust and mineralization. We shall also see what happens in the case of the superposition of the Tertiary crust upon the pre-Jurassic one.

The pre-Jurassic weathering crust formed under conditions of a leveled relief. All processes of mineral formation were localized within the profile of the crust, since vertical migration of elements predominated in the profile of the weathering crust; horizontal migration within the surface areas was insignificant (Fig. 2a).

Fig. 2

The formation of the Tertiary crust took place under conditions of dissected relief. Here there occurred two parallel but spatially separated processes of migration of elements: one of them (A) in the vertical direction, the other (B) in the direction of the runoff of ground and surface waters down the slope (Fig. 2b).

With vertical migration (A), mineralization was confined to the limits of the weathering-crust profile. With migration in direction (B), however, the elements, to a considerable extent, initiated mineral formation in loose rocks formed through the erosion of products of the weathering crust and accumulating in depressions and lower parts of slopes and at the foot of uplands.

The geochemical features of the elements and the environmental conditions (the pH value) were the two factors that determined the character and association of the mineral complexes formed in the process of weathering.

What can we expect in the case of superposition of the Tertiary weathering crust upon the pre-Jurassic one, as applied to ultrabasic rocks?

First, the appearance of two mineral complexes separated by a hiatus in the time of their formation (one pre-Jurassic, the other Tertiary); second, the formation of loose red-colored deposits, spatially and genetically closely connected with the weathering crust; third, a certain duplication in the second mineral complex (the Tertiary one), owing to simultaneous and parallel processes of mineral formation in the crust profile and in the red-colored deposits.

This is precisely the picture that we observe in the weathering crust of the serpentinites of the Kempirsai massif¹.

On the basis of all that has been set forth above, we may conceive of the Tertiary history of the crust in the following form.

The weathering processes in the Tertiary period were superimposed upon the pre-Oligocene crust and led to the transformation and replenishment of the latter. In their early stages they were marked by the ocherization of nontronites, the leaching of serpentinites, and the formation of ochers and, possibly, nontronites.

In the process of weathering, calcium, magnesium, nickel, cobalt, silica, iron, and aluminum were converted into a soluble state. In the profile of the crust, silica, nickel, and partly calcium, magnesium, and iron were mainly retained. The principal mass of calcium, magnesium, iron, cobalt, and manganese, however, was carried farther by groundwaters and surface streams and was deposited together with sandy-clayey material in closed basins in the depressions of the slope relief.

In the Tertiary mineralization two features attract attention:

1) the spatial separation of silica from calcium and magnesium, important for understanding the genesis of silicified serpentinites;

2) the enrichment of the rocks of the middle series of variegated deposits with cobalt and manganese (psilomelane wads and asbolanes), which is of great importance for studying the cobalt-bearing nature of the crust, and for directing and planning prospecting and exploration work for cobalt.

The principal mass of silica was “captured” by the carbonate zone of leached serpentinites, with the formation of so-called silicified serpentinites. Calcium and magnesium migrated farther, since the conditions of the dissected relief did not permit stagnant groundwaters to form in the weathering crust with a pH at which precipitation of calcium and magnesium carbonates would have been possible. The latter were deposited in relief depressions, low areas, and drainless hollows of slopes, where, in the alkaline medium of stagnant waters, variegated rocks with psilomelane wads and asbolanes were formed.

As we see, the character of mineralization in the profile of the crust and in the variegated deposits differed somewhat from each other, both quantitatively and qualitatively. In the crust, the formation of chalcedony, quartz, and partly nickel silicates took place chiefly. Carbonates and minerals of hydrous oxides of iron, aluminum, manganese, and cobalt occupied a sharply subordinate position. In the variegated deposits, on the contrary, processes of carbonatization, the formation of oolitic iron ore, psilomelane wad, and asbolane were predominantly developed. Processes of silicification, selectively associated with carbonate rocks or with quartz sands and remains of Eocene flora, were of subordinate importance.

One cannot fail to note a certain lithological inconsistency of the rocks of the middle series of variegated deposits, indicating variability of climatic conditions during the period of formation of the Tertiary weathering crust. This was also promoted by the very position of the Southern Urals, which in the Eocene–Oligocene lay at the boundary of two different botanical-geographical regions: the “evergreen Poltava” and the deciduous Turgai, possessing all the features of a temperate climate (Kryshtofovich).

The presence of carbonates (marls and dolomites), sulfates (gypsum), well-cemented quartz sandstones, redeposited nontronites, and preserved remains of drought-resistant flora testifies to alkaline weathering under conditions of semideserts or hot steppes. Ocher, oolitic brown iron ores, and variegated clays, however, characterize humid climatic conditions and an acidic weathering environment, favorable to the migration of iron.

In the sequence of mineral formation in the weathering crust of serpentinites, two epochs of mineralization are expressed quite distinctly: the pre-Jurassic, associated with the formation of the crust, and the Tertiary, associated with the transformation and renewal of the pre-Jurassic crust under conditions of continental weathering in the Eocene–Oligocene¹.

At the Akkermanovka and Khalilovo nickel deposits (data of D. G. Ulyanov, G. S. Gritsaenko, and G. A. Krutov), two epochs of mineral formation are also indicated. The authors mentioned, in 1932, and M. A. Tsibulnik in 1938, attempted to connect the first epoch of mineralization with hydrothermal activity, and the second with weathering processes. In our opinion, in the above-mentioned deposits, as also in the Kempirsay deposits, we are dealing with pre-Jurassic and Tertiary mineral formation. The commonality of the geological history experienced by these deposits, and the similarity in the character of mineralization, make it possible to draw such an analogy.

LITERATURE

1. Bezrukov P. L. and Yanshin A. L.—Jurassic deposits and bauxite deposits in the Southern Urals, Transactions of the Scientific-Research Institute of Geology and Mineralogy, issue 7, 1934.

2. Bezrukov P. L. and Yanshin A. L.—Jurassic deposits and deposits of aluminous ores in the Primugodzhar steppes, Transactions of the Scientific-Research Institute of Mineral Raw Materials, vol. 1, part 1, issue 110, 1937.

3. Ginzburg I. I.—The ancient weathering crust on ultrabasic rocks of the Urals, vol. I, Types and morphology of the ancient weathering crust. Transactions of the Institute of Geological Sciences of the USSR Academy of Sciences, issue 80 (II), 1946.

4. Krasheninnikov I. M.—The ancient weathering crust of the forest-steppe Trans-Urals, Proceedings of the Dokuchaev Soil Committee, No. 3, 1915.

5. Ulyanov D. G. and Shishulina M. G., Gritsaenko G. S., Krutov L. L., Petrova A. S.—Deposits of silicate-nickel ores of the Orsk–Khalilovo district, Transactions of the Scientific-Research Institute of Mineral Raw Materials, issue 117, 1937.

6. Formozova L. N.—On the question of the age of the ancient weathering crust in the Southern Urals, Bulletin of the Moscow Society of Naturalists, Geological Section, vol. XXII (2), 1947.

7. Khabakov A. V.—Pre-Jurassic relief and ancient weathering crust in the southern part of the Southern Urals, Proceedings of the State Geographical Society, vol. 67, issue 2, 1935.

8. Shchukina E. N.—Geology and geomorphology of the weathering crust of the Middle Urals, Bulletin of the Moscow Society of Naturalists, Geological Section, XXI, 5, 1946.

1. The presence in the weathering crust of the serpentinites of the Kempirsay massif of two mineral complexes, associated with pre-Jurassic and Tertiary mineralization, was noted by us as early as 1941 in the work “Weathering Crust and Nickel Deposits.” ↩↩