On the Geomorphology of the Northwestern Margin of the Altai

Yu. A. Kuznetsov

In 1942–43 I had occasion to conduct geological investigations in the Northwestern Altai, within the Kurya, Krasnoshchekovo, and Zmeinogorsk districts of Altai Krai. This region proved very interesting, in particular from the geomorphological standpoint, representing an area of gradual subsidence of the Altai mountain structure beneath the loose strata of the margins of the Kulunda steppe. Geomorphological observations made in conjunction with the main work made it possible, for example, to determine with complete confidence the tectonic character of the “facies” of the Altai, and also to determine rather precisely the time and general character of the latest uplifts of the Altai.

The relief of the region described is fundamentally an ancient peneplaned surface, raised by Quaternary tectonic movements to various hypsometric levels. As a result of these recent uplifts, the structure of the relief includes, alongside forms of ancient relief, forms caused by modern erosion. Accordingly, the relief forms are distinguished by rather great diversity, and this diversity is determined, on the one hand, by the degree of uplift of one or another sector and, consequently, by the degree of dissection by modern erosion, and, on the other hand, by the properties of the rocks composing the region.

Three areas stand out quite clearly, each characterized by its own types of relief: 1) a steppe plain, representing the marginal part of the Kulunda steppe; 2) an area of flat ridges, slightly raised above this plain, which in places is replaced by the relief of typical low hills; 3) an area of middle mountains, already considerably uplifted and dissected by modern erosion. These differences in relief forms are due to the uneven uplift of the region in Quaternary time, having the character of arching, complicated by ruptures, which impart a clearly expressed stepped character to the spatial distribution of average elevations and of the principal relief forms, well reflected on the topographic map sheet N-44-V.

The entirely flat plain, composed wholly of loose deposits, occupies a small area in the northwestern part of the region. The watersheds here constitute a flat, level steppe and rise above the channels of the streams cutting through it by 40–50 meters. The slopes are gentle and inconspicuous. The modern valleys are broad, sometimes several kilometers wide.

The area of flat ridges occupies the greater part of the region’s area. It rises above the plain to a height of 100–150 m. The absolute elevations of the watersheds range within 300–400 m. This part of the region is already composed of bedrock Paleozoic rocks, which, however, even in the watershed areas are for the most part covered by a mantle of cover loams. The watersheds are very flat ridges, weakly dissected by similarly flat ravines, filled with a thick mass of deluvial–

proluvial loams. Very sharply above this semiplain rise isolated knolls and ridges of hills, crowned by rocky outcrops or ruins of the rocks most resistant to weathering.

Within the same geomorphological step one may distinguish an area that is a typical low-hill terrain, developed only within the Devonian field. Its relief is characterized by a disorderly distribution of individual hills, often with rocky summits, separated by broad flat depressions. Genetically, in our region the low-hill terrain is related to the peneplain of the gently rolling area, but prolonged denudation processes here could not produce such ultimate leveling because of the great heterogeneity of the physical properties of the Devonian rocks.

This entire area of flat ridges and low-hill terrain is dissected by flat logs and by similarly flat, broad valleys. These logs and valleys are a component of the ancient relief, which was formed under conditions of an arid climate, with the complete cessation of river activity and the dominance of deluvial-proluvial processes. The present rivers here develop along ancient valleys; moreover, the modern valleys are sharply incised into the ancient ones to a depth of as much as 15–18 m and have a relatively insignificant width.

The southeastern part of the region is the margin of a mid-mountain area, developed beyond the limits of the survey to the southwest. Here, too, the main watersheds have the gentle, rolling forms of an aged relief. But, in accordance with the greater magnitude of uplift of this area, the zone of dissection of the ancient relief by modern erosion is considerably wider here; moreover, this zone of dissected relief is developed along the right bank of the Loktevka and along the so-called Semenovka escarpment.

The differences described in the character of the relief are determined mainly by the magnitude of relative uplift. The uplift of the Altai mountain system has, in general, the character of a smooth swelling, as is indicated by the gradual rise of the watersheds bearing traces of planation in the direction from the Kulunda steppe toward the Gorny Altai. But this principal motif, which determines the general modeling of the relief, is complicated by very clearly expressed, quite young disjunctive features in the relief. A splendid example of the latter is the Semenovka escarpment described by Bublichenko. Along the latitude of the village of Batun, the strongly dissected mountain area breaks off by a steep and almost rectilinear wall toward the low flat ridges and low-hill terrain developed to the north of it. The tectonic origin of this escarpment is evident and is especially well emphasized by the character of the longitudinal profile of the streams crossing it. In their upper reaches they flow in broad valleys (remnants of the ancient relief), but as they approach the Semenovka escarpment they begin to cut vigorously into the bedrock and, over a short distance, form narrow gorges, have a steep gradient, and take on the character of mountain torrents. But immediately north of the escarpment, upon entering the low-hill zone, the gradient again becomes negligible; the channel sometimes ceases to occupy a definite position, becoming to a considerable degree wandering. At the exits into the low-hill zone of small springs and dry logs, beautifully developed alluvial fans are formed.

The amplitude of uplift along the Semenovka escarpment reaches 150 m, judging by the absolute elevations of the surfaces of ancient planation in these two zones. Apparently, the same tectonic origin is possessed by an escarpment parallel to the Semenovka escarpment, sharply separating the zone of low-hill terrain from the plain of the Kulunda margin along the line Novo-Firsovo—sopka Vostrukha. Finally, there is reason to think that a meridional fault passes along the Loktevka valley, along which the eastern part has been uplifted relative to the western. The time of formation of these faults is determined fairly precisely. The surface of ancient planation, as well as the thickness of loam-

…the loams, which occupy the ancient valleys, prove to have been clearly displaced by faults, whereas the terraces of the modern valleys (including the second one in any case) pass across the faults without changing their height. Consequently, the stepwise uplift of the Altai, accompanied by the formation of faults, took place after the deposition of the loam sequence, but before the accumulation of the second terrace of the modern valleys.

In the distribution of the modern river system there is no more or less clear dependence on the geological structure of the region, and the rivers are generally epigenetic in character. The hydrographic network apparently was laid down at the stage of a mature peneplain before the uplifts that caused the incision of valleys and the dissection of the relief. The modern river valleys are characterized by their incision, to one depth or another, into the bottoms of ancient flat valleys, by their comparatively small width, and by a small number of terraces having variable height. The depth of incision of the modern valley into the ancient one generally decreases toward the headwaters. Exceptions are rivers crossing the Semenov escarpment or a ridge of resistant rocks, where a sharp break in the longitudinal profile is observed.

A few words about Quaternary deposits and the character of the terraces. Among the Quaternary deposits two complexes of sediments are clearly distinguished: 1) ancient mantle loams and 2) terrace deposits of the modern hydrographic network.

The mantle loams cover almost the entire region with an almost continuous sheet; moreover, they are developed both in the area of flat interfluves and in the mid-mountain area, where, however, they have been preserved only in the still weakly dissected zone of watershed spaces and the tops of gullies. The thickness of the mantle loams on the watersheds is small, and sometimes they are entirely absent here, but on the slopes the thickness increases considerably, reaching 15–20 m in the valleys. Their external appearance is remarkably uniform. These are unstratified gray-yellow loams, which only in the lower horizons acquire a sandy-loam character and locally contain interbeds and small lenses of clayey sand, and at the very base are enriched with gravel of the underlying rocks. The loam sequence represents a complex of sediments of essentially deluvial–proluvial origin and was deposited under conditions of an arid climate. The ancient, sufficiently dissected relief was thereby leveled; the river valleys that had been well developed by that time were filled with loams, and surface runoff almost ceased. The modern river valleys, at least in the area of flat interfluves (for example, the Poperechnaya and Loktevka rivers), are incised into the ancient flat valleys to a depth of 15–20 m. Thus, a morphologically well-expressed upper terrace is formed, which should be regarded as a sculptural terrace, since the loams composing it entirely cannot be regarded as normal river alluvium.

The terrace (alluvial) deposits of the modern hydrographic network can be traced only as a narrow strip along the modern river valleys incised into the broad flat ancient valleys. Among them one may distinguish the deposits of: 1) the floodplain, with a height of 1–2.5 m, consisting essentially of sandy-pebble material, only locally overlain by bluish clays; and 2) the second terrace, 4–5 m high, with a very characteristic composition: below—coarse sands with lenses of pebbles; above—gray loams 1.8–2.5 m thick.

Special mention should be made of proluvial deposits, loamy in places, in places with a considerable admixture of gravel material, forming flat alluvial fans at the mouths of dry gullies and small springs in the plain of the flat-interfluve area and forming an almost continuous apron at the foot of the Semenov escarpment. These alluvial fans apparently correspond in age to the second terrace.