STRUCTURAL AND MORPHOLOGICAL CHARACTERISTICS OF LOESS DEPOSITS ON THE WESTERN SLOPE OF THE TOM–YAYA INTERFLUVE

B. F. MIKHALCHENKO

(Presented by Prof. A. M. Kuzmin)

One of the important characteristics of loess deposits is their structure. It largely predetermines the principal engineering-geological properties of these deposits: compressibility, water resistance, mechanical strength, and others. The study of the structural features of deposits is also of considerable importance in resolving the genetic aspect of the question. It is known that, for understanding the conditions of formation of deposits, not only the chemical-mineralogical composition is important, but also the size and shape of the particles composing them, the mutual arrangement of the latter, and the nature of the bonds between them. All this is united under the general term “structure,” or the fabric of the deposit. The concept of “structure” itself has no single definition. Most Soviet researchers—L. V. Pustovalov [7], M. S. Shvetsov [10], L. B. Rukhin [8], and others—have an approximately similar conception of the structure of sedimentary rocks, in which they include the principal morphological features of the component parts of a rock: the shape, size, and surface character of mineral grains, and the frequency of their occurrence.

For sedimentary, weakly cemented (without rigid bonds) rocks, the most complete engineering-petrographic definition of structure is found in V. A. Priklonsky [6]. He considers that structure is characterized by the size, shape, and surface character of the mineral particles or their aggregates composing the rock, by their mutual arrangement and ratio, and by the presence and nature of the bonds between them.

In studying the composition and structural properties of the loess deposits of the western slope of the Tom–Yaya interfluve, the understanding of structure in V. A. Priklonsky’s terminology was adopted. At the same time, we proceeded from the premise that structure is determined to a considerable degree by the conditions under which the deposits exist. According to A. K. Larionov [3], the number of aggregates, the mutual arrangement of grains and aggregates, and porosity—all these structural indicators change with the moisture content and pressure under which the deposit is found. This approach to the study of structures makes it possible to establish the relationship between engineering-geological properties and the petrographic-geological features of loess deposits [3].

The principal factors on which, in the final analysis, the structure of sedimentary deposits depends are their genesis and transformations at var...

personal stages of development (diagenesis, epigenesis, hypergenesis) [4]. To a certain extent, structure also reflects all these processes of accumulation and formation of rocks. The formation of structures in loess deposits is greatly influenced by the processes of weathering and soil formation.

Our studies to determine the structural properties of loess rocks were carried out along two lines. In the first part of the work, structural elements were studied:

a) the size, shape, roundness, and character of the surface of mineral detrital particles and microaggregates; b) the number of pores per 1 cm² of area, the shape and size of pores, and the degree of density of the pore walls were determined; c) on the basis of the results of studying the rocks under a microscope and the data on grain-size composition, the degree of mechanical sorting of the detrital material was established and the approximate ratio of the granular and aggregated portions composing the rock was identified; d) the rate and character of slaking of monolithic rock samples in water were determined.

In the second part of the work, the data obtained were analyzed and compared in order to identify groups of rocks possessing a set of similar structural elements. Having established the structural types of rocks, we set ourselves the task of clarifying the interrelation of the latter with individual stratigraphic horizons and tracing the change in the structural properties of loess rocks on individual geomorphological elements of the relief.

To carry out this work, monolithic samples of rocks of natural structure were selected from test pits at depths from 1.0 to 7.0 m. The morphological and structural properties of loess rocks were studied visually, with the aid of a binocular magnifier, and in transparent thin sections under a microscope according to the method proposed by A. K. Larionov [2, 3]. In all, 280 samples were studied; the results of the analyses are given in Table 1.

On the basis of the foregoing, among the loess deposits of the western slope of the Tom–Yaya interfluve, individual structural types of rocks are distinguished. Each of them possesses a set of similar structural elements and is closely related to a definite stratigraphic horizon. A definite dependence is observed in the change of the structural properties of loess rocks also on individual geomorphological elements of the relief (Table 1). Having traced this dependence and established the interrelation, we consider it possible to distinguish the following structural types of rocks:

I. Upper Quaternary deluvial loams ($del\ Q_3$)—the upper loess horizon: a) in the upper part of the horizon, degraded subsoil loams of granular-aggregative structure with a predominance of aggregated portions; b) below lie typical loess-like loams of granular-aggregative and granular, weakly aggregated structure with a predominance of granular portions.

II. Middle–Upper Quaternary lacustrine-alluvial ($Q_{2-3}^{al}$) weakly macroporous loess-like loams of granular-aggregative structure with an almost constant predominance of aggregated portions. Lower loess horizon.

III. Middle Quaternary lacustrine-alluvial loessified deposits of the Rodionovo Formation ($Q_{2rd}^{a_l}$). Loams, more rarely sandy loams, of granular-aggregative, sometimes granular weakly aggregated structure.

I. Upper Quaternary deluvial loams ($del\ Q_3$). Upper loess horizon

a) Subsoil degraded loams. The loams belonging here are widely developed. They occur on various

hypsometric levels of the relief of the watershed part of the plateau, on its slopes, and on sectors of the IV terrace of the Tom River. Their thickness is not constant and varies from 1.0 to 5.0 m. In granulometric composition, as a rule, they belong to heavy, sometimes to medium silty varieties with a content of clay particles, smaller than 0.005 mm, from 16.05 to 29.35% (Table 1). Medium loams are rare and usually occur on slopes. Both varieties of subsoil loams are colored light brown, more rarely brown, dark brown, and buff; they are usually macroporous, sometimes considerably ferruginized, and dense. They do not effervesce with hydrochloric acid. The number of macropores per 1 cm² of area ranges from zero to 18. The pores are small, up to 1.0–1.5 mm; in diameter they rarely reach 2.0–3.0 mm; they are usually rounded, more rarely oval, slit-like, and irregular in form. The pore walls are dense, sometimes loose, and are often covered with brown films of iron hydroxides and, more rarely, with dark-colored crusts of humus material.

Air-dry monolithic samples of the rocks described reveal an aggregate-granular structure with a predominance of aggregated areas; usually they are strong, break in the hands only with the application of some force, and sometimes have to be broken with a hammer. When the smoothed surface of the samples is wetted dropwise with water, frequent, clearly visible microcracks appear, separating the moistened part of the rock into individual microblocks. The rate and character of soaking of the rocks in water vary. Individual varieties of loams soak slowly: after standing for several hours in distilled water they crack, then particles crumble away along the edges and small pieces slump off. Complete soaking of individual samples does not occur even within twenty-four hours.

Weakly bound, less durable varieties of degraded loams, of aggregate-granular structure with a predominance of granular areas, are rare. The latter are confined mainly to sectors of the IV terrace of the Tom River. When water is applied dropwise to their surface, rare and isolated microcracks form. In water they soak rapidly and within several minutes disintegrate, turning into dust. The results of determining the sizes, and of studying the shape and roundness of detrital particles under the microscope, and data on the granulometric composition of degraded loams (with microaggregate preparation of the samples), are summarized in Table 1.

The material presented in the table shows that watershed loams, in comparison with the others, have a more perfect mechanical sorting of detrital material. The main mass in the detrital part of the rock consists of particles of fine aleuritic (coarse-silty) size, from 0.016 to 0.056 mm in diameter. The yield of the latter in mechanical analysis with dispersive preparation of some samples (treatment of rock samples: NaCl, Na₂CO₃, NaOH) is about 44.0%. As an admixture (2.46%), coarse aleuritic particles, 0.05 to 0.1 mm in size, and isolated grains of sandy size are present (Fig. 1). On slopes the detrital material becomes more coarse-grained. Among the predominant coarse-silty particles (from 0.02 to 0.07 mm), grains of sandy size, 0.13 to 0.53 mm in diameter, are often encountered.

The silty and sandy particles are represented predominantly by quartz. In the predominant mass, the quartz grains are unrounded, angular, often sharp-angled, and irregular in shape. Larger grains, 0.06 mm in diameter, are often subrounded and rounded. Fractured fragments of quartz with striated, uneven edges are encountered. In addition to quartz, angular and subrounded grains are common

feldspars. The latter are also well rounded, usually flattened, of oval and ellipsoidal shape. In addition, isolated rounded grains of hornblende, epidote, zircon, ilmenite, and other minerals occur.

b) Typical loess-like loams. Down the section, the degraded subsoil loams are replaced by typical loess-like loams, which, together with the former deposits, constitute the upper horizon

Fig. 1. Degraded subsoil loam, characterized by a perfected sorting of clastic material. The bulk of the clastic part of the rock consists of particles of fine aleuritic size. As an admixture, isolated grains of sand size are observed. Upper loess horizon. Outcrop 6, depth 1.5 m. Watershed. Magnification 64. Without analyzer.

\((del\ Q_3)\). Their thickness is not constant and ranges from 1.5 to 6.0 m. Among them, medium loams are most developed, with heavy and light loams less common (Table 1). They are all macroporous; the number of pores per 1 cm² reaches 15–18; they are silty, highly calcareous, and effervesce vigorously with hydrochloric acid.

Carbonates occur in the rock in the form of whitish incrustation crusts on pore walls, efflorescences, coatings along cracks, and as concretions—“crane stones,” “dolls,” and “puffs.” Calcareous neoformations, in the form of separate “dolls” and “crane stones,” are usually concentrated in the lower part, at the base of the horizon; their size reaches 1–2 cm, and sometimes 5–7 cm in cross section. The CaCO₃ content, by calcimetry, for the loess rocks of the upper horizon varies from 0.04 to 6.49%.

Air-dry samples of loess-like loams are more often fawn-colored, rarely light gray and light brown; they are usually weakly bound, not strong, break easily in the hands, and are rubbed into a fine powder. They soil the hands, leaving a thin yellowish mealy coating on the fingers.

When water is applied dropwise to the smoothed surface of the rock, isolated microcracks form. Sometimes they are numerous or are entirely absent and do not form.

Table 1

Comparative characteristics of the structural features of loess deposits on the western slope of the Tom–Yaya interfluve

Fig. 2. Loess-like loam, macroporous. The pores are slit-shaped; carbonate matter is visible on the walls. Upper loess horizon. Borehole No. 197, depth 2.5 m. Watershed slope. Magnification 64. With analyzer.

Fig. 3. Loess-like loam, macroporous. The pores are irregularly rounded in shape. The pore walls are coated with carbonate matter. Upper loess horizon. Borehole No. 7, depth 3.0 m. Watershed slope. Magnification 64. With analyzer.

Monolithic samples of rocks of natural moisture content are unstable in water and in most cases soak rapidly: within several minutes they disintegrate, turning into dust. The pores observed under the microscope have a rounded, more rarely oval, slit-like, and irregularly rounded form (Figs. 2, 3). The pore walls are loose and weakly compacted, often calcareous, and are sometimes coated with films of iron hydroxides and dark-colored crusts of humic substance.

Loess-like loams usually have a loose fabric (Fig. 4), a granular-aggregative and granular, weakly aggregated structure with a predominance of granular areas. Varieties in which granular and aggregated areas are developed in approximately

Fig. 4. Loess-like loam of loose fabric. Upper loess horizon. Borehole No. 355, depth 2.5 m. Slope of a watershed.

equal quantities are rare. Study of the structural features of the rocks of the upper horizon shows that loess-like loams developed on the surface of the flat watershed are, in most cases, of granular-aggregative structure. The loams covering the gentle slopes of the watershed are more often granular and weakly aggregated.

Rocks of granular-aggregative structure under the microscope consist of separate microaggregatic accumulations of finely ground grains of quartz, feldspars, and other clastic (syngenetic) minerals, cemented by iron hydroxides, clayey, more rarely carbonate, substance [5]. In transmitted light the microaggregates are observed as irregular, sometimes diffuse yellow-brown spots without sharp outlines. The spaces between them are filled with coarser monomineralic grains of silt size. Rocks of granular structure, in transparent sections under the microscope, prove to be entirely granular. The spaces between the grains are filled with finely dispersed iron hydroxides, clayey, more rarely carbonate, substance.

Microscopic study of the form, roundness, and sizes of clastic particles, and the results of granulometric analysis of some rock samples with dispersion preparation, show that typical loess-like loams of the upper horizon on various geomorphological elements are distinguished by very good sorting of the clastic material.

In the clastic part of the watershed loams, as on the slopes, particles of fine aleuritic (coarse-silt) size, from 0.02 to 0.06 mm, clearly predominate. According to granulometric analysis with dispersed preparation of the samples, the content of these particles is 40.4–43.0%.

Coarse-aleuritic (5.29–8.93%) and sandy (5.3–9.5%) clastic particles are present in subordinate amounts, and their content increases noticeably on the slopes. Particles of coarse-aleuritic and sandy size often show traces of mechanical working. They are frequently subrounded. Cracked grains of irregular outline with eroded, uneven edges occur. The presence of the latter indicates secondary changes in the rock associated with the dissolution of individual mineral grains. The surface of the clastic particles is smooth and rough, sometimes with pitted depressions.

II. Middle–Upper Quaternary lacustrine-alluvial weakly macroporous loesslike loams \((Q_{2-3}^{a1})\)

Lower loess horizon

Deposits of the lower loess horizon are widely distributed areally. They are exposed by numerous test pits and boreholes. They rarely crop out at the ground surface and occur only as separate patches on the slopes of gullies and in places on watershed areas. Lying at the base of the rocks of the upper loess horizon, they occur on various geomorphological elements of the relief and, together with the first horizon, cover older Quaternary and pre-Quaternary formations with a continuous mantle. Their thickness is variable and ranges from 1.3 to 9.0 m.

In mechanical composition (Table 1), the loess rocks of the lower horizon belong to heavy, more rarely medium and light, silty loams.

Heavy varieties of loams occur more often on the watershed, medium ones on the slopes. The loams of the lower horizon are sandy (the content of particles of the sandy fraction in individual samples reaches 30–36%), usually carbonate-bearing and in most cases effervesce vigorously with hydrochloric acid. Unlike the rocks of the upper loess horizon, they are relatively dense, with less distinctly expressed macroporosity. They have a characteristic color of their own: light gray and light brown, sometimes whitish. On the fracture surface of samples one can always see small yellow-brown spots of ferruginization. The rarely observed pores are more often rounded and oval in shape. The pore walls are usually dense, sometimes weakly dense and loose, impregnated with iron hydroxides, locally humified, and rarely calcareous. The loesslike loams of the lower horizon have a granular-aggregative structure with an almost constant predominance of aggregated areas. Separate, mainly light varieties of loams with a granular, weakly aggregated structure occur, in which granular areas predominate. Air-dry monolithic samples of the rocks are firmly bound and in most cases break in the hands only with the application of some effort.

When two drops of water are placed on the smoothed surface of the samples, frequent microcracks appear, breaking up the wetted surface of the rock into separate microblocks. In a few cases such microcracks are faintly visible or do not form at all. Resistance of the rocks to soaking in water is manifested differently; some monolithic loam samples soak slowly over the course of ...

some hours, while others disintegrate within a few minutes into small clods, subsequently turning into dust. Microscopic study of the sandy–silty fraction (from 0.01 to 0.25 mm) and determination of the size of clastic particles in transparent thin sections show that, among the predominant coarse-silty particles (0.02 to 0.056 mm), fragments of sandy size, from 0.1–0.3 to 0.5 mm in diameter, occur in appreciable quantities within the mass of clastic material (Fig. 5).

The imperfection of the mechanical sorting of the clastic material of the loess rocks of the lower horizon is also confirmed by the data of granulometric analysis (Table 1).

No regularity in the quantitative distribution of particles of the sandy fraction on individual geomorphological elements (according to grain-size data) is established. At the same time, study of the rocks in transparent thin sections under the microscope shows that the general coarsening of the clastic material, with a higher content of particles of sandy size, is characteristic of the loams of terrace IV of the Tom River \((Q_{2}^{IV})\).

Fig. 5. Loess-like loam, weakly macroporous. Sorting of the clastic material is imperfect. Among the predominant fine-silty particles, fragments of sandy size are often encountered. Lower loess horizon. Pit No. 7, depth 5.0 m. Watershed slope. Magn. 64. Without analyzer.

The degree of roundness of the clastic particles is not uniform and is closely dependent on their size. Particles of silt size, which constitute the bulk of the clastic material, are unrounded, angular, and irregular in shape. Coarse-silty (from 0.05 to 0.1 mm) and sandy grains (more than 0.1 mm in diameter) in most cases bear traces of mechanical abrasion.

III. Middle Quaternary lacustrine–alluvial loessified deposits of the Rodionovka Suite \((Q_{2rd}^{al})\)

The deposits of the Rodionovka Suite are everywhere overlain by the loess rocks described above. They are exposed by a large number of hand- and core-drilling boreholes, as well as by pits.

Outcrops at the present-day surface are rarely observed; in the form of separate patches they may be found in the thalwegs of ravines and, in places, on the banks of the Bobrovka River and elsewhere. The description of the principal morphological properties and structural features of the rocks was made from six samples taken from test pits exposing the upper part of the suite section. In outward appearance and in certain other properties (content of the silty and sandy fractions, effervescence with hydrochloric acid), the rocks of the upper part of the section of the Rodionovo Suite show similarity to the loess-like loams of the lower horizon. The boundary of transition to the overlying deposits of the second loess horizon is gradual, and only in certain areas (borehole 492 and others) is there an increased humus content and the presence of products of deluvial redeposition of buried soils, indicating a possible break or slowing in sediment accumulation.

In granulometric composition, the predominant rocks of the upper part of the Rodionovo Suite are heavy silty loams, rarely sandy loams. The content of the clay fraction is 9.20–27.31%, of the silty fraction 49.67–69.84%, and of the sandy fraction 8.15–41.07% (Table 1).

Macroscopically, the named varieties of rocks display a light-gray, more rarely light-brown to brown color; usually they are dense, sometimes weakly macroporous, and effervesce with hydrochloric acid.

The rarely encountered isolated macropores are round and oval in form, with dense walls. The pore diameter does not exceed 1.0–1.5 mm. Air-dry monolithic samples of the rocks, in most cases of a granular-aggregate, more rarely granular, weakly aggregated structure, have low strength; they break in the hands without special effort, and only certain varieties have to be broken with a hammer. When water is applied dropwise to the smoothed surface of samples with undisturbed structure, microcracks form, separating the moistened portion of the rock into individual microblocks.

When the rocks are studied in thin sections under the microscope, imperfect mechanical sorting of the clastic material is observed. The measurements of clasts under the microscope given in the table show that, among the prevailing unrounded particles from 0.02 to 0.056 mm, larger clasts of the sandy fraction, from 0.1–0.2 to 0.4 mm in diameter, are often encountered in the mass of clastic material (Fig. 6). The latter, in most cases, have irregular outlines of form and bear traces of mechanical abrasion. Well-rounded grains of round and oval form are rarely encountered among them.

No regularity is revealed in the distribution of the clastic particles of the rocks by size on individual geomorphological elements of the relief. The section of the deposits of the cover complex ends with the Rodionovo Suite.

The factual material presented above shows that the varieties of loess rocks distinguished by structural and morphological properties have a close interrelationship with individual stratigraphic horizons and reflect, to a considerable degree, the conditions of their formation.

The deluvial loess-like loams of the upper horizon \((del\ Q_3)\) are characterized by a high degree of sorting of the clastic material. In the clastic part of these rocks, particles of fine-aleuritic (coarse-silty) size clearly predominate. The degree of roundness of the clastic particles is in direct dependence on their size. Particles of coarse-silty size, as a rule, are unrounded, angular, and irregular in form. Larger clasts of sandy size (more than 0.05 mm in diameter), contained in the rocks of the upper loess horizon as an insignificant admixture, are to varying degrees

rounded, and sometimes traces of point pitting and dimpled depressions are visible on their surfaces. The presence of the latter is possibly connected with the activity of air currents, which may have partly winnowed the clastic material of the deposits of the upper loess horizon.

The lacustrine-alluvial loess-like loams of the lower horizon \((Q^{a}_{2-3})\) have a comparatively dense structure and less pronounced macroporosity. The mechanical sorting of the clastic material,

Fig. 6. Heavy loess-like loam, characterized by an increased content of clastic particles of sand size. Deposits of the Rodionovka suite \((Q^{al}_{2rd})\). Outcrop 6, depth 6.0 m. Watershed. Magn. 64. Without analyzer.

as in the underlying loams of the Rodionovka suite, becomes less perfect.

In the terrigenous clastic part of these rocks, along with the predominant particles of silt size, the content of fragments of the sand fraction increases. The presence of subrounded and rounded particles with smooth, more rarely rough, surfaces indicates mechanical reworking of the clastic material during the period of its redeposition and accumulation.

LITERATURE

1. D. N. Botvinkina. Bedding of sedimentary rocks. Transactions of the Geological Institute, Academy of Sciences of the USSR, vol. 59, 1962.

2. A. K. Larionov. Structure of loess rocks. Conference on the study and use of clays. Lviv, 1957.

3. A. K. Larionov, V. A. Priklonskii, V. P. Anan’ev. Loess rocks and their engineering properties. Gosgeoltekhizdat, 1959.

4. Methods for the study of sedimentary rocks. Edited by N. M. Strakhov, vols. 1–2, Moscow, Gosgeoltekhizdat, 1957.

5. B. F. Mikhal’chenko. Characteristics of the mineralogical composition and structures of loess rocks of the Bogashevo area. Transactions of NIIIZhT, issue XXVIII, 1962.

6. V. A. Priklonskii. Soil science. 2nd ed. Gosgeoltekhizdat, 1955.

7. L. V. Pustovalov. Petrography of sedimentary rocks. Gostoptekhizdat, vol. 2, 1940.

8. L. B. Rukhin. Fundamentals of lithology. Gostoptekhizdat, 1953.

9. Reference guide to the petrography of sedimentary rocks, vols. 1–2. Leningrad, Gostoptekhizdat, 1958.

10. M. S. Shvetsov. Petrography of sedimentary rocks. 2nd ed. Moscow–Leningrad, Gosgeolizdat, 1948.