The Chui Valley (depression) and the Kyrgyz Ridge framing it from the south are located within the Caledonian Ulu-Tau-North Tien Shan structural-facial zone, which serves as a peculiar structural axis of the Tien Shan.
Within the territory of the Chui Valley, two structural-geological parts are distinguished: the mountainous framing, composed mainly of Precambrian-Paleozoic rocks, and the valley itself, which has a two-part structure: a Precambrian-Paleozoic basement and a Meso-Cenozoic cover.
The Kyrgyz Ridge is mainly composed of Precambrian and Paleozoic rocks, both of sedimentary and effusive origin. A significant role is played by a thick suite of metamorphic schists, phyllites, chlorite, siliceous rocks, with interlayers of sandstones and slaty conglomerates. The slopes of the Kyrgyz Ridge, the upper reaches of the Aspara River, and the interfluve of the Kegety and Issyk-Ata rivers are composed of the Maralayskaya and Tyukskiy suites: carbonaceous quartz schists, carbonate-chlorite, and mica gneisses. The Aktyuz suite is represented by rocks of eclogite and granulite facies, i.e., thick (5000-6000 m) biotite and biotite-amphibole paragneisses, aplite-like orthogneisses, migmatites, eclogites of the oldest - Lower Proterozoic and Archean age. The suite is intensely displaced in the Caledonian era and pierced by huge batholiths of arkosic granites and small veins of porphyries. Most of the Caledonian-age granites are predominant in the Kyrgyz Ridge. There are also Hercynian intrusions.
The Paleozoic in the Kyrgyz Ala-Too is represented by all rock types. Outcrops of Cambrian rocks are noted west of the Tyo-Ashuu pass along the axis of the ridge. Here, terrigenous rocks - porphyries, sandstones, and shales are developed.
The Ordovician in the mountainous framing is marked by two areas of outcrops: the interfluve of the Shamshe and Aspara rivers to the upper reaches of the Kuragaty River. The Middle Ordovician in the eastern part is volcanogenic-carbonate. In the central part of the ridge, terrigenous materials are already present: sandstones, aleurolites, shales with interlayers of marbled limestones (Karabalta suite). The thickness of the suite is significant.
Ordovician-Silurian granitoids formed the Sokuluk, Ala-Archa, and Issyk-Ata massifs. Diorites and quartz diorites intruded in the upper reaches of Merke and Aspara (Ashmara). Crystalline schists and granites compose the axial part of the Kyrgyz Ridge.
Silurian deposits have not been found in the area, although some researchers consider it possible to relate the Jartash fine-grained suite of clayey, phyllitic shales, aleurolites, etc. to it.
Devonian rocks are found in all sections, mainly in the central part of the Kyrgyz Ridge. Here, the Devonian rock complex is represented from bottom to top by several suites (Sugandinskaya, Kastekskaya, etc.), consisting of conglomerates, gravelites, sandstones, and argillites.
Carbon is widely developed in this area. The composition of the Lower Carbon deposits is terrigenous: gray conglomerates, sandstones, aleurolites - their thickness is 700 m (Ibragimov et al., 1985). The most complete section of Visean rocks is established in the valleys of the Kegety, Ak-Suu, Kara-Balta rivers, where it consists of gray-green sandstones, tuffs, limestones (100-600 m). Namurian rocks - red-colored sandstones, aleurolites, etc., are described in detail in the basin of the Kegety River (Kegetin suite, thickness 300 m). The Middle and Upper sections are represented by the outcrop of the Orthok suite in the valley of the Shamshe River.
Permian deposits in the Kyrgyz Ridge are rare. This is understandable: in the Permian, the Northern Tien Shan experienced a geoanticlinal regime, and sediment accumulation occurred only in certain late Hercynian superimposed depressions. One of these depressions was located in the eastern part of the Kyrgyz Ridge (the valleys of the Shamshe, Konorchok, Kokjar-Suu, Ashukolter rivers), where a thickness of volcanogenic-territorial rocks was formed, with a thickness of 600-1700 m (Knauf, Korolev et al., 1972).
Mesozoic system deposits - a Triassic-Jurassic coal-bearing thickness have been found on the southwestern slope of the Chu-Ili mountains, 10 km from the village of Blagoveshchenka.
Paleogene-Neogene deposits (according to S.S. Shchults) are represented by two complexes: the Kyrgyz red-colored (K+T) and the Tyynan orogenic (N+Q1), located in the foothills (shelves) of the Kyrgyz Ridge. To the north, they are submerged under thick deposits of Quaternary sediments. Despite a high degree of study (O.K. Chediya et al., 1973), there is still no consensus among researchers on some key issues of their stratigraphy. The main reason for the disagreements lies in the sharp facies difference of the most powerful section along the valley of the Noruz River, the so-called Serafimov anticline, with sections from other parts of the depression. In the Serafimov section, 6 suites were distinguished, named: Kokturpak, Kokomerensky, Serafimov, Jeldysuysky, Saryagach, and Chui. In other sections in the territory of the depression, the red-colored complex includes the Serafimov, Jeldysuysky, and Saryagach suites, but the Sharpyldak suite is added, and the Kyrgyz suite replaces the Kokomerensky, with the overall sequence of suites expressed as follows: Kokturpak, Kyrgyz, Chui, and Sharpyldak. According to L.I. Turbin (1973), in the Serafimov section, the Kyrgyz suite splits into 4 suites (starting from Kokomerensky and above). According to O.K. Chediya et al. (1973), the Kokomerensky, Serafimov, and Jeldysuysky suites are analogs of the Kyrgyz suite, while the Serafimov is analogous to the Chui. However, in general, the volumes and, importantly, the dating of the suites defined by these authors are convincing. It seems justified to distinguish the Suluterek and Shamsyn suites, the first of which corresponds to the Kokturpak, and the second to the Kyrgyz.
In the Kyrgyz Ala-Too, red clays are found everywhere, especially in the eastern part in the valleys of the Issyk-Ata, Kegety, Shamshe, Kyzyl-Suu rivers, etc. These deposits lack any boulder-gravel formations indicating mountainous relief. The red clays formed on land under conditions of a hot dry climate, more or less flat relief, as indicated by their clayey composition, red color, and gypsum content.
Neogene deposits are exclusively distributed in the foothills of the Kyrgyz Ridge, where coarse-layered boulder-dislocated conglomerates with interlayers of clays and sandstones prevail.
The Suluterek suite (K P2st) lies in the valley of the Suluterek River, a left tributary of the Chu River, on a peneplain surface of Precambrian and Paleozoic rocks. In a narrow bordering strip in the northern part of the depression, a well recorded a pack of pink, cherry-pink patty and loams, lying according to Jurassic deposits. Its composition in the southern foothills includes breccias, conglomerates, gravelites, patty, loams, and turbidite rocks with lenses, nodules, and interlayers of gypsum, marls, with a thickness of about 150 m.
The Shamsyn suite (RZ - Nsm) is developed in the basin of the Shamshe River. In the lower part of the suite, alluvial sediments (sandstones, aleurolites, etc.) prevail, while in the upper part, proluvial-deluviual (breccias, conglomerate-breccias, coarse-grained and coarse-layered sandstones, gravelites). These rocks are colored pink, gray-pink, gray-brown. Thickness - 550 m. Within the Serafimov structure, the suite splits into four suites. Their composition includes salt deposits (mirabilite, glauberite, halite), gypsum, limestones, and marls with a total thickness of more than 3000 m.
The Chui suite (N21-2) is found in the basin of the Noruz River and is represented by two parts: the lower - fine-grained; and the upper - coarse-grained (variegated conglomerates, gravel-sandy patty, sandstones). The color is pale, with a thickness of about 800 m.
The Sharpyldak suite (N23 - Q21) has a coarse-grained composition everywhere and is almost always gray, greenish-gray in color with rare interlayers of brownish-gray and pinkish-gray clays. The Chui suite marks the beginning of orogenic processes, while the Sharpyldak suite indicates intensive orogenesis. The thickness of the suite is 300-600 m.
Quaternary deposits in the Chui depression are widespread, and in its mountainous framing, they form an intermittent cover on a Paleozoic foundation. A characteristic feature is the often-changing thickness. Thus, in the mountainous part, it ranges from fractions of a meter to several meters, while within the depression, it increases to hundreds of meters.
Quaternary deposits are mainly represented by terraced complexes, moraines along river valleys, as well as coarse-grained accumulations of various genesis in the depression (alluvial, proluvial, fluvioglacial, deluvial, etc.). The study of Quaternary deposits is of great practical interest, as it is closely related to the formation of the accumulative relief of the valley, which is most favorable for agricultural development.
At the interdepartmental meeting on the study of the Quaternary period, held in Tashkent in 1961, a stratigraphic scheme of Quaternary deposits of Central Asia and Southern Kazakhstan was approved, which includes the scheme by P.G. Grigorenko (1961) for the Chui depression. According to this scheme, Quaternary deposits are divided into four complexes.
Lower Quaternary complexes of deposits (Q1) have survived only as small remnants on the tops of the highest flat-topped mountain ranges and are represented by boulder-gravelites, gravelites, and gravel deposits, covered by a mantle of loess-like loams, with a thickness of 40 m.
Middle Quaternary deposits (QII) in the Chui Valley are represented by two complexes:
a) lower complex (QH1) Oiarchinsky,
b) upper complex (QII2) Aflatunsky.
The rocks of the Onarchinsky complex form the highest terraces, corresponding moraines, and proluvial deposits (alternation of boulder-gravelites and sandstones, with a thickness of up to 30 m) are found in relatively small areas on the terraces of the third tier at the exits from the mountains of the Sokuluk, Jylamys, Alamyudun, and Noruz rivers. Proluvial deposits are widely distributed, represented at the base by boulder-gravelites and accumulations of gravel with lenses of sand, covered by loess-like loams and loams.
Middle Pleistocene moraines, with a thickness of 150-300 m, are found in the gorges of all major rivers at altitudes of 1900-3000 m.
The upper Aflatunsky complex is located on the deposits of the lower complex and is represented by the lower terraces of the third tier. Alluvial deposits are developed in all mountain valleys, with boulder-gravelites lying in the lower part of the section, and loess-like loams in the upper part.
Alluvial-proluvial deposits (gravelites, loams, and sands with a loess-like cover) are distributed in small areas at the exits of river valleys from the mountains. Proluvial deposits make up large areas of the cones of discharge and the continuous fan of the foothills of the plain part of the Chui depression. The lower part of the section consists of gray boulder-gravelites, while the upper part consists of loess-like rocks.
Upper Quaternary deposits in the Chui depression occupy extensive areas of the second-tier terraces and are represented by lower, middle, and upper complexes. They correspond to the era of Kyrgyz glaciation. The lower complex (QIII1), Alamudunsky, includes the accumulative covers of terraces and cones of discharge of the upper parts of the second tier and unites the moraines of the first stage (Aksayskaya) of the Kyrgyz glaciation. In the mountainous framing, they are embedded in middle Quaternary deposits. Alluvial-proluvial rocks are found in places where small river channels cross cones and are represented by gravelly-boulder and loamy accumulations with a thickness of up to 15 m.
Proluvial deposits, exposed in the foothill area, are covered in the Chui depression by younger formations. They are widely distributed and are exposed at a depth of 100-150 m.
The lower part of the sections is composed of gray boulder-gravelites and gravelites, while the upper part consists of loess-like rocks. The thickness of the proluvium ranges from 10 to 80 m.
Glacial deposits are represented by the moraines of the first stage of the Kyrgyz glaciation and are found in the eastern part of the northern slope of the Kyrgyz Ridge at altitudes of 2000-3000 m.
The middle complex of Upper Quaternary deposits (QIII2), Belovodsky, includes accumulative covers of terraces, cones of discharge of the second tier and unites the moraines of the second (Turkestan) stage of the Kyrgyz glaciation. Alluvial deposits are very widespread, forming the terraces of the middle tier of the second tier. The lower part of the section is represented by polymictic boulder-gravelites with interlayers of gravel, while the upper part consists of pale loess-like loams.
Alluvial-proluvial deposits occupy about 70% of the area of the Chui depression and have a two-part structure. The lower part of the sections is represented by boulder-gravelites, which are replaced by sandy-gravelly, and then sandy-loamy sediments as one moves away from the mountains; the upper part consists of loess-like deposits, in which buried soils are found. The thickness of alluvial-proluvial deposits ranges from 8 to 72 m.
Glacial deposits in the form of moraines of the second stage of the Kyrgyz glaciation are found at altitudes of 2200-3500 m.
The upper complex of Upper Quaternary deposits (QIII3), Kamyshanovsky, includes accumulative covers of terraces and cones of discharge of the upper tier of the second tier and unites the moraines of the third stage of the Kyrgyz glaciation. Alluvial deposits form the terraces of the lower tier of the second tier. The lower part is represented by sandy-gravelites, while the upper part consists of sandy-clayey deposits. Alluvial-proluvial deposits are developed only in places where rivers exit from the mountainous part. Proluvial deposits are found isolated on the cones of discharge. The lower part of the section consists of boulder-gravelites, while the upper part consists of loess-like loams.
Modern deposits (QIV) combine the deposits of terraces and cones of discharge of the first tier of modern glaciation and corresponding slope and aeolian accumulations.
Alluvial deposits of boulder-gravelites with sandy filling and a small layer of loam and loess in the upper part form floodplain terraces. Aeolian deposits are widespread only in the northern part of the Chui depression, in the area of the "Manas" airport. They consist of gray coarse-grained sand and form a hummocky-ridge relief.
Detailed geological-geomorphological studies conducted in the Chui and Issyk-Kul depressions by K.E. Abdrakhmatov and A.K. Trofimov (1983) have allowed for the clarification and supplementation of the stratigraphic scheme of Quaternary deposits of the Chui depression.