On the Distribution and Age of the Weathering Crust in the Tomsk Area

B. V. Plotnikov

(Presented by the Department of General Geology)

Within the area of the city of Tomsk, the weathering crust of Paleozoic rocks is widely distributed. It is well traced in outcrops along the right bank of the Tom River, from “Boets” to the village of Kolarovo, and in the valleys of the right-bank tributaries of the Tom—the small rivers Ushaika, Basandaika, and others.

On the left bank of the Tom River, the Paleozoic clay shales and sandstones, which constitute the parent base of the weathering crust of the Tomsk area, are overlain by loose Meso-Cenozoic deposits, the thickness of which increases in the northwestern and western directions. In the same direction the weathering crust also subsides.

K. V. Radugin [4, 6], K. V. Ivanov and Yu. P. Kazanskii [1] indicate that the greatest thicknesses of the weathering crust are confined to depressions in the surface of the Paleozoic rocks and to tectonic zones of crushing of these rocks, where the thickness of the weathering crust in places exceeds 35 m.

In our study of outcrops and sections of boreholes drilled within the territory of the city of Tomsk, a weathering crust of varying thickness was also discovered. An especially thick weathering crust was exposed by boreholes No. 1245 (Fig. 1), of brick plant No. 2; 9 (borehole No. 1263, 1250, Figs. 4, 2), in the area of the Khromovskaya zaimka, in the area of the “Sibir” match factory (borehole No. 1240, Fig. 3), Tomsk II (borehole No. 1282, Fig. 5), and others. Here the thickness of the weathering crust reaches 50 m. This weathering crust is probably confined to a zone of crushing of Paleozoic rocks, having a northeastern strike, i.e., extending from Potapovskie Luzhki toward the Khromovskaya zaimka and the experimental farm “Baktin.” The width of this zone within the city limits reaches up to 1000 m.

In certain parts of the city the weathering crust is absent altogether, and on the eroded surface of fractured Paleozoic rocks lie alluvial deposits of the second, third, fourth, and fifth terraces of the Tom River, as well as of the first and second terraces of the Ushaika and Basandaika rivers.

Numerous boreholes and outcrops in the area of the city of Tomsk show that the area of distribution of the weathering crust is considerably larger than that shown on the schematic map of K. V. Ivanov and Yu. P. Kazanskii [1].

A classic exposure of the weathering crust of Paleozoic clay shales, sandstones, and diabase dikes is the area of the Camp Garden and Khromovka zaimka. In these exposures it is evident that the surface of the weathering crust is uneven. This is also confirmed by numerous works of K. V. Radugin [5, 4, 6, 7]. Its surface fluctuates from several centimeters to ten meters. The numerous depressions of the Paleozoic rocks, filled with weathering products, owe their origin not only to the selectivity of weathering, but also to the geological structures of the area, elongated in a north—northeasterly direction. Most such depressions are elongated along the strike and along zones of crushing of the Paleozoic rocks.

Fig. 1–5. Borehole sections: borehole 1245; borehole 1250; borehole 1240; borehole 1263; borehole 1282. Scale, m: 0.0, 20.0, 40.0, 60.0, 80.0, 100.0, 120.0.

Conventional symbols: clay; loam; sandy loam; clay shale and sandstone; sand; sand with pebble; weathering crust.

Having become acquainted in detail with the Paleozoic section and the weathering crust of the Camp Garden area, one may conclude that the greatest depth of alteration of the Paleozoic rocks by weathering agents is confined to areas with the highest content of organic matter and pyrite in the Paleozoic clay shales and sandstones.

In the Camp Garden area, descending springs are confined to the depressions of the weathering crust; these springs are the cause of the formation of suffosion cirques, while the redeposited Paleozoic weathering crust, consisting mainly of clays, constitutes a water-impermeable bed along which groundwater moves. In spring and summer, mudflows and landslides arise now in one cirque, now in another of the Camp Garden, moving along the Paleozoic weathering crust.

In the area of the village of Stepanovka, entire forest tracts slide down along the weathering crust, forming a “drunken forest.”

Studying the section of the weathering crust of individual parts of the Tomsk area, K. V. Radugin [7, 8, 10, 13], on the basis of an analysis of its structure, divides the weathering crust into three zones:

a) the zone of primary eluvium, which has preserved the bedding of the parent rocks, usually with a lighter color and lower hardness than the unaltered rocks;

b) the zone of structural eluvium, which has preserved not the bedding but the relict structure and texture of the bedrock, with very soft, usually completely bleached rocks;

c) a zone of structureless eluvium, devoid of relict structures and textures, with deeply altered parent rocks transformed into clays.

In addition, K. V. Radugin [8], according to the character of secondary iron compounds, distinguishes in the weathering crust an oxidation zone and, beneath it, a suboxide zone with a greenish coloration, caused by secondary ferrous iron compounds.

Depending on the composition and degree of alteration of the Paleozoic rocks by weathering agents, the color of the weathering crust ranges from white and yellow-brown to pale blue, blue, green, brown, and black tones.

K. V. Ivanov and Yu. P. Kazanskii [1], applying V. P. Kazarinov’s terminology [3] for deposits of the weathering crust of Western Siberia, also distinguish in the weathering crust of the Tomsk area three zones, successively replacing one another from bottom to top:

a) the zone of “saprolites” (parent eluvium according to K. V. Radugin);

b) the zone of “structural paraeluvium” (structural eluvium according to K. V. Radugin);

c) the zone of “redeposited paraeluvium” (structureless eluvium according to K. V. Radugin).

Until recently, the lithological composition of the weathering crust had not been studied in detail. In 1934, K. V. Radugin [4], while studying the geology of the loose deposits of the Tomsk–Taiga area, came to the conclusion that the Tertiary weathering crust is represented only by clays and is devoid of laterites and bauxites. The kaolin part of the weathering crust did not have favorable conditions for its preservation.

In his later works, K. V. Radugin [9, 10] draws attention to the presence in the weathering crust of clay shales of peculiar concentric flat-parallel or nonparallel bands of iron oxides, and in the weathering crust of protobases “in connection with intense chemical weathering, silica was released, and at the expense of this free silica opal—a mineral of the weathering crust—was formed.”

E. V. Shumilova [11], studying the mineralogical composition of the loose strata beneath Lagernyi Garden, notes in its section a redeposited weathering crust, which corresponds to K. V. Radugin’s structureless eluvium or to V. P. Kazarinov’s redeposited paraeluvium. Below the redeposited weathering crust, E. V. Shumilova distinguishes a horizon of eluvium of Carboniferous clay shale. This horizon will correspond to K. V. Radugin’s structural and parent eluvium, and to V. P. Kazarinov’s structural paraeluvium and saprolites.

From the lithological-mineralogical analysis of the geological section of Lagernyi Garden presented by E. V. Shumilova, the weathering crust is distinguished quite clearly.

The boundary between the weathering crust and the overlying Tertiary sequence is especially sharply delineated by mineralogical composition.

Here, in the heavy fraction of the “deposits of the weathering crust,” the percentage content of ore minerals is 72.0–82.7%, epidote—1.5–8%, hornblende up to 3.2%, zircon—4.5–17.1%, tourmaline—1.3–4.9%, rutile—0.3–1.5%, anatase—0.6–1.2%, brookite—0.1–0.4%. Of the minerals of the light fraction, quartz is most widespread, 82.2–82.7%. Feldspar occurs in smaller amounts—7.9–11.1%, muscovite—0.6–0.8%, biotite—0.3–1.2%.

In the eluvium of clay shale, E. V. Shumilova notes in the heavy fraction a reduced content of ore minerals, 29.3%, and an increased content of zircon—64.3%.

In the light fraction, the content of quartz stands out sharply—93.7%, and of muscovite—2.4%, with a very low content of feldspar—3.3%; siderite is encountered.

Thus, according to the data of E. V. Shumilova [11], in the weathering crust, as compared with the overlying sequence of Tertiary and Quaternary deposits, and also as compared with the parent basement of the weathering crust—clayey shales—the contents of ore minerals, zircon, tourmaline, rutile, anatase, quartz, and kaolinite are sharply increased.

A fairly complete mineralogical characterization of the weathering crust is given in the work of K. V. Ivanov and Yu. P. Kazanskii [1], where they indicate that the mineralogical composition of both the coarse and fine fractions of the weathering crust is quite simple. The most widespread minerals are quartz, feldspar, and subordinate clay minerals (mainly hydromicas and kaolinite); in addition, authigenic minerals—siderite, chlorite, and pyrite—are often present. Among other minerals of the coarse and fine fractions in the lower part of the weathering crust, pyrite and leucoxene are often found.

In studying the minerals of the heavy fraction, K. V. Ivanov and Yu. P. Kazanskii note that weathering processes leave their imprint on the appearance and optical properties of some accessory minerals, and that authigenic minerals formed in the course of hypergenesis play a significant role in the composition of the heavy fraction of the weathering crust.

The most resistant minerals of the heavy fraction of the weathering crust are tourmaline, epidote, rutile, zircon, brookite, anatase, and others. Less resistant minerals of the weathering crust are apatite, magnetite, pyrite, and siderite, which occur in fresh form only in the lower zones of the weathering crust.

Magnetite, pyrite, and siderite are often oxidized, forming limonite. Besides limonite, a secondary mineral of the weathering crust is leucoxene, formed during the weathering of ilmenite. The presence of unstable minerals in the Lower Carboniferous rocks created favorable conditions for enrichment of the heavy fraction of the weathering crust with minerals resistant to weathering, such as, for example, zircon, anatase, brookite, and rutile.

On the basis of chemical and mineralogical analyses, K. V. Ivanov and Yu. P. Kazanskii conclude that the weathering crust of this area may be regarded as hydromica-kaolinite, and they divide it into three zones:

1) a kaolinite-hydromica zone (corresponding to structureless eluvium);

2) a hydromica zone (which corresponds to structural eluvium);

3) a zone of unaltered rocks—corresponding to the bedrock.

They associate the formation of the weathering crust with deep chemical decomposition of poorly resistant minerals during a period of prolonged continental regime.

The question of the age of the weathering crust of the Paleozoic clayey shales is debatable.

K. V. Radugin [4] considered the age of the weathering crust to be Lower Tertiary, V. P. Kazarinov [3]—Cretaceous. K. V. Ivanov and M. P. Yudin [2] regarded it as Lower Tertiary and Cretaceous, the former being superimposed upon the latter.

On the basis of the latest investigations, K. V. Ivanov and Yu. P. Kazanskii [1] come to the conclusion “that the weathering crust on the rocks of the Kolyvan-Tomsk folded zone formed no later than the Upper Cretaceous, probably within the Late Cretaceous epoch.”

It seems to us that to confine the age of the weathering crust of the Tomsk area within the limits of a single “Upper Cretaceous” epoch is entirely incorrect. Let us consider this question.

On the right bank of the Tom River, one may say, deposits of the middle and upper divisions of the Carboniferous period, deposits of the Permian period, and those of the Mesozoic era are completely absent. This was probably a period of continental regime, which lasted about 200 million years. The Devonian and Lower Carboniferous deposits folded into structures were destroyed under the action of weathering agents. In each period of destruction, a weathering crust characteristic of that period was formed.

During this time the relief of the locality was now dissected, now smoothed. Evidently, on the right bank of the Tom River, in the valleys of the Malaya and Bolshaya Kirgizka rivers, one may find such depressions where a pre–Upper Cretaceous weathering crust has been preserved—perhaps Permian, Triassic, or Jurassic.

In another case, if on the weathering crust, as is the case in the area of the Lagernyi Garden, the Khromovka quarry, and other sites, there lie sediments formed in the middle of Tertiary time, why then should this weathering crust be “Upper Cretaceous” and not Lower Tertiary? Lower Tertiary time, a time of warm and humid climate, existed for about 10 million years. This time, it seems to us, is quite sufficient for the formation of a Lower Tertiary weathering crust.

Beneath the first terrace of the Tom River on the right bank, basal parts of Tertiary deposits have been discovered, and beneath them—a weathering crust. The age of this weathering crust will undoubtedly be pre-Tertiary or Upper Cretaceous.

If we consider the geological section west of Tomsk (i.e., on the left bank of the Tom River), then at the base of the loose deposits lie Cretaceous sediments, and in the remaining areas even sediments of Jurassic age, and beneath them—a weathering crust. In this case the age of the weathering crust must be pre-Cretaceous, i.e., Jurassic (and pre-Jurassic or Triassic).

Since west of Tomsk the Paleozoic rocks covered by the weathering crust subside, and, as K. V. Radugin [5] indicates, Meso-Cenozoic sediments may adjoin them and overlie them, the conclusion suggests itself that each succeeding series of deposits, younger than the preceding one, overlaps the weathering crust corresponding in age to the preceding overlain sediments; i.e., with transgressive sedimentation there is a (regressive rejuvenation) increase in the age of the buried weathering crust.

On the basis of what has been set forth, we believe that the age of the weathering crust for the entire Tomsk area is not only Upper Cretaceous; in different parts of the area it may prove to be of different ages. Therefore, for the Tomsk area as a whole, the age of the weathering crust should be considered Meso-Cenozoic.

LITERATURE

1. Ivanov K. V., Kazanskii Yu. P. Material on the study of the weathering crust of the Tomsk area. Vestnik ZS and NGU, issue 3, 1958.

2. Ivanov K. V. and Yudin M. I. Geology and mineral resources of the Tom–Yaya interfluve. O. F. ZSGU, 1950.

3. Kazarinov V. P. Mesozoic and Cenozoic deposits of Western Siberia. Gostoptekhizdat, 1958.

4. Radugin K. V. Materials on the geology of loose deposits of the Tomsk–Taiga area. Publ. ZSGGGT, issue 9, 1934.

1. Radugin K. V. Do the Tertiary deposits of the city of Tomsk overlie the Paleozoic, or do they adjoin it? TPI.

2. Radugin K. V. The interrelation between the relief of the weathering crust and groundwater flows. TPI.

3. Radugin K. V. The occurrence of the footwall of the Tertiary deposits of the city of Tomsk. TPI.

4. Radugin K. V. Structural and bedrock eluvium of proterobases and diabases of the city of Tomsk. TPI.

5. Radugin K. V. Bands of iron oxides in the weathering crust beneath the Lagerny Garden of the city of Tomsk. TPI.

6. Radugin K. V. Opal (and precious opal) of the weathering crust near Boiko, beneath the Lagerny Garden of the city of Tomsk. TPI.

7. Shumilova E. V. Lithology of the loose stratum in the section beneath the Lagerny Garden near the city of Tomsk. Bulletin of the West Siberian Geological Trust, issue 4, 1936.