Geology of Ukraine
Geology of Ukraine. The geological framework of Ukraine consists of two major regions: (1) the southwestern part of the Precambrian East European Platform (centered on the Ukrainian Crystalline Shield) with adjoining fragments of the Hercynian Platform, and (2) the Carpathian-Crimean-Caucasian Folded Zone, which is part of the Mediterranean Geosyncline. The first represents a portion of the ancestral European landmass that has displayed no tectonism since the end of the Paleozoic Era some 570 million years ago. Previously, in the Archean Era and Proterozoic Era this area was so intensively folded and subjected to magmatic intrusions that it became very rigid and resistant to subsequent orogenic movement. This craton, however, experienced epeirogenic movements, consisting of broad, reversible, and slow uplift, and subsidence. The former exposed the surface to erosion; the latter, accompanied by submergence, led to the accumulation of sediments that covered the Precambrian rocks to various depths.
The Ukrainian Crystalline Shield (I) forms the central core of the platform part of Ukraine. It extends in crescent form from Polisia south and southeastward to the vicinity of the coast of the Sea of Azov. Covered by a thin (5–15 m) veneer of Quaternary deposits, these intensively folded Precambrian metamorphic, intrusive, and extrusive rocks are exposed by streams and rivers in an area that, since the Proterozoic Era, has experienced differential uplift.
By contrast, the surrounding parts of the platform experienced various degrees of relative subsidence, downwarping, and accumulation of sedimentary deposits. To the west, southwest, and south the Precambrian rocks lie 1–3 km below the surface of the Volhynia-Podilia Platform (XII), the Bessarabian Platform (XIV), and the Black Sea Platform, whose basement slopes southward towards the Black Sea Depression (XV). West of the Volhynia-Podilia Platform, the Precambrian basement dips below the Lviv Trough (XI) to 6–7 km and continues northwestward under the Lviv-Lublin Depression (VIII). To the north and northeast the Precambrian basement lies 1 km below the Pynsk Trough (XIII) and, except for the Chernihiv Arch (IV), again drops precipitously towards the northeast to as much as 10–12 km below the surface of the Dnipro-Donets Trough (V), and then re-emerges to the northeast as the Voronezh Massif (II). Southeast of the Dnipro-Donets Trough Hercynian tectonic activity becomes evident in the Donets Suborogen (VI) and even more so in the folded structure of the Donets Basin (VII). Because this chain of troughs, extending from the Donets Basin to the Prypiat Trough, underwent some folding like a geosyncline but then again became part of the East European Platform, it is considered transitional and was called an ‘aulacogen’ by N. Shatsky.
The Carpathian-Crimean-Caucasian Folded Zone belongs to the Alpine mountain-building phase of the Tertiary Period. However, it contains rocks that reveal older tectonic activity, such as the early-to-mid-Paleozoic Caledonian orogeny, and touches the remaining fragments of the Hercynian Platform, which are also part of the Mediterranean Geosyncline. The main components of this zone are the intensively folded and raised structures of the Carpathian Mountains, Crimean Mountains, and Caucasus Mountains. They are bounded to the northeast and the north by their respective foredeeps, including the Carpathian Foredeep, the Moldavian Depression (XVa), the Karkinitska Trough (XVb), and the Azov-Kuban Depression (XVc). At both the northwestern and the southeastern ends the Carpathian Mountains and their foredeeps are offset by two Hercynian orogens—the Kielce-Sandomierz Ridge (X) in Poland and the Dobrudja orogen (XVI) south of the Danube River. Remnants of Hercynian orogens also make up the Scythian Platform, which manifests itself as the central part of the Crimea and, east of the Sea of Azov, the Azov-Kuban Depression and the Stavropol Upland.
Geological history. The oldest deposits that form the foundation of the East European Platform occur at shallow depths only within the Ukrainian Crystalline Shield. Their intensive exploration and study has revealed a complex history that is not yet fully understood. The Ukrainian Crystalline Shield rocks, dating from 750 to 3,650 million years ago, are mostly metamorphic in origin consisting of gneisses (see Gneiss), migmatites, and metabasites. Less than 10 percent of them are magmatic rocks, among which granitoids (see Granite) predominate.
Archean. In the history of the formation of the Ukrainian Crystalline Shield several successive geosynclinal and platform stages are distinguished. Extensive volcanic processes and the accumulation of very thick sedimentary effusive deposits characterize the earliest recorded stages of the Archean Era. Subsequent folding, intrusive magmatism, and metamorphism produced the strata of gneiss and migmatite, which are oriented mainly in the northwesterly direction. Three complexes are distinguished in this series: the Dnipro River complex (gneisses, amphibolites, and migmatites separated by intrusions of granite in the central part of the shield), the Podilia-Berdychiv complex (gneiss), and the Teteriv River-Boh River complex (gneiss-migmatite). The Teteriv-Boh gneisses are major repositories of graphite. The ultrabasic deposits are associated with the occurrence of chrome ores, nickel ores, cobalt ores, and copper ores.
Proterozoic. After another mountain-building period the Kryvyi Rih group was deposited in a subgeosyncline that was subsequently folded with its axis in a southerly orientation. The group consists of the Inhul River-Inhulets River series of biotite, biotite-amphibole, and pyroxene-rich granitic gneisses, the metabasite series of metamorphosed volcanic rocks, and the Saksahan series of arkosic sandstones, phyllites, talc-chlorite schists, clays, marbles (see Marble), and vast jaspilitic deposits of iron ores.
Subsequently, the Ukrainian Crystalline Shield underwent erosion and peneplanation. There are no known platform sediment accumulations on it, however, except for the rare Ovruchian series. Located in the northwestern part of the shield, it contains conglomerates, quartzites, sandstones, and pyrophyllitic schists. Concurrently and subsequently, the shield experienced fracture dislocations, intrusions, and effusions. These processes gave rise to the Korosten complex (gabbro and rapakivi-like granite containing titanium-rich ilmenite) in the northwestern Korosten and central Novomyrhorod parts of the shield, and the syenite complex (parts of the Donets Ridge) near the Sea of Azov. They also caused the blocks, into which the shield was fragmented along its axis and perpendicular to it, to rise or subside relative to one another and to form a checkerboard pattern of exposed Archean and Proterozoic deposits as peneplanation continued. Mineralization along the fractures, particularly at their intersection, provided for the occurrence of lead. Weathering of crystalline rocks such as granite resulted in the formation of kaolin. Towards the end of the Proterozoic Era a transgression from the west left Riphean deposits of sandstone near Ostroh.
Paleozoic. This era was marked by the beginning of subsidence around the Ukrainian Crystalline Shield, marine transgression, and the accumulation of sedimentary deposits. After a recession in the early Cambrian, when terrigenous clays and sands were deposited, the West European Sea encroached again, accumulating clays with phosphate rock, argillites, siltstones, and sandstones in Volhynia and Podilia. These were overlain by Ordovician limestones and, after a phase of uplift and erosion, with Silurian limestones, dolomites, shales, and marls, including pelites, arkoses, and sandstones. Deposits also occurred in the geosyncline that spread through the Carpathian Mountains and Caucasus Mountains zones, but the Caledonian folding towards the end of the Silurian Period modified the deposits into the Marmara metamorphic rocks that were preserved in the Carpathian Mountains (south of Rakhiv) and the phyllites and quartzites of the western Caucasus Mountains.
In the beginning of the Devonian Period sand was deposited in lagoons to become the Old Red Sandstone of Podilia. Continued marine transgression in the middle Devonian, however, provided for the accumulation of dolomites, limestones, and sandstones in Volhynia and Podilia. Particularly thick limestone deposits were laid down in the Carpathian Mountains Zone; limestones, clays, sandstones, and conglomerates in the Crimean Mountains and Caucasus Mountains zones; and sandstones, dolomites, limestones, gypsum, and rock salt, which formed into domes that served as traps for petroleum (see Petroleum deposits) and natural gas (see Natural gas deposits), in the Dnipro-Donets Trough. Iron-bearing sandstones and shales were deposited in the Donets Basin. Towards the end of the Devonian Period, the Hercynian folding known as the Breton phase was associated with volcanism in the Dnipro-Donets Trough and the Donets Basin, and with the intrusions of granite in the Caucasus Mountains.
At the beginning of the Carboniferous Period marine transgression from the east into the Dnipro-Donets Trough and from the west into the Lviv Trough contributed to thick shale and limestone deposits. By the middle Carboniferous Period marine sedimentation changed to lagoon deposition alternating with terrestrial deposition of plant material that turned into coal. Many layers of shale, sandstones, limestones, and coal seams produced up to 12 km of Carboniferous deposits in the Donets Basin. A less complete record survived in the Dnipro-Donets Trough and the Lviv Trough in the west.
In the Permian Period Ukraine was mostly dry land. Submergence was confined mostly to the south. Marine transgression in the Donets Basin and the Dnipro-Donets Trough contributed to the deposition of what became sandstone, limestone, dolomite, gypsum, and rock salt, which later domed upward into Mesozoic strata, forming traps for petroleum and natural gas. The Lower Permian deposits consisted of conglomerates, sandstones, and dolomites in the Caucasus Mountains, and outliers of conglomerates, dolomites, quartzites, and sandstones in the Crimean Mountains and Carpathian Mountains zones.
Mesozoic. Beginning with the Triassic Period, continental conditions prevailed. Only in the south did marine transgression provide for the accumulation of a series of conglomerates, gray-green phyllites, and limestone-dolomites in the Carpathian Mountains Zone, dark gray mica schists and sandstones with limestone lenses in the Crimean Mountains Zone, and shales, sandstones, and limestones in the Caucasus Mountains Zone. Lacustrine colored clays, alluvial sandstones, and marls were deposited in the Dnipro-Donets Trough. The end of the Triassic Period was marked in the Donets Basin, Crimea, and Caucasia by a crustal disturbance known as the Old Cimmerian folding. Upwarping of the dome-like anticline in the Donets Basin triggered the release of mercury-bearing fluids from the magma below through fissures into the porous layers of Carboniferous sandstone and produced commercial concentrations of mercury.
In the Jurassic Period the sea in the south deepened and widened until by the Late Jurassic Period it also covered part of the Donets Basin and the entire Dnipro-Donets Trough. Clays, later interbedded with limestone and oolitic iron-bearing sandstone, were deposited here. In the Carpathian Mountains-Dobrudja region the continental deposits were replaced with marine shales and limestones. In the Crimean Mountains and Caucasus Mountains zones, however, volcanic activity added tuff and lava to the marine deposits of shales, limestones, and dolomites. Volcanism was associated with the New Cimmerian folding, which towards the end of the Jurassic Period affected both the Crimean and Caucasus zones and the Donets Basin.
Among the Mesozoic deposits the most widespread were the Cretaceous ones. They involved the deposition of conglomerates, sandstones, shales, and marls (collectively known as flysh) into the geosyncline of the Carpathian Mountains Zone. In the Crimean Mountains and Caucasus Mountains zones, however, limestone was deposited. By the Middle Cretaceous Period the sea invaded the Dnipro-Donets Trough from the north and then inundated Podilia and Volhynia, leaving as major islands the Ukrainian Crystalline Shield and the Donets Ridge. Under these conditions thick strata of carbonate-rich sediments consisting of chalk, marls, limestone, and phosphate-rich sediments were deposited in deeper water, while glauconite sands were laid down along the shores of the remaining islands. The Cretaceous Period was brought to a close with folding in the Donets Basin, which also marked the beginning of the Alpine folding in the Carpathian-Crimean-Caucasian Zone and led to the regression of the sea.
Cenozoic. The Tertiary Period was marked by the emergence of the Carpathian Mountains and Caucasus Mountains. By the end of the period the principal topographic landforms and drainage patterns of Ukraine emerged. But during the Paleogene (the earlier epoch of the Tertiary Period), flysh, including oil-bearing shale, was still being deposited in the Carpathian Zone. In the flysh were trapped many useful minerals, including oil (see Petroleum deposits) and natural gas (see Natural gas deposits). Along the Black Sea Platform (the Crimean and Caucasus zones) sandstones and shales, with occasional lenses of phosphate rock, alternated with the prevailing limestone deposits. The Dnipro-Donets Trough was accumulating alternating layers of clays and sands with occasional marls. In the late Eocene (the latter half of the Paleogene Period) a shallow sea even covered much of the central part of the Ukrainian Crystalline Shield, reworking in places the weathered surface to form small concentrations of bauxite, and leaving behind a veneer of sand, clay, sandstone, and marl.
By the Neogene Epoch of the Tertiary Period the Carpathian Mountains emerged as overlapping folds, leaving a narrow body of water, which gradually retreated to the southwest, in the foredeep. In the permeable strata the folds retained commercial petroleum deposits. Clays, sands, and limestones with rock salt, potassium salts, gypsum, anhydrite, oil, ozokerite, and sulfur were deposited in the intramontane basins and the foredeep. On the south side of the Carpathian Mountains the Pannonian Massif sank below the sea, while a ring of volcanoes released great masses of effusives (andesites, dacites, and rhyolites) to form the inner side of the Carpathian Arc. Volcanic eruptions ejected tuff, which was deposited along with clays in the Pannonian Basin. The Crimean Mountains, already established during the New Cimmerian folding at the end of the Jurassic Period, were raised to higher elevations. Large quantities of iron ores were deposited near Kerch (see Kerch Iron-ore Basin). The Caucasus Mountains, an archipelago in the Paleogene Period, emerged from the sea as a mountain range. Petroleum deposits and natural gas deposits were trapped in the folds. Sediments from the Caucasus Mountains, chiefly in the form of clays and sands, were deposited in the Azov-Kuban Depression. The shallow sea retreated from the southeastern part of the Ukrainian Crystalline Shield, leaving behind bottom deposits of limestone, sand, and clay, bog deposits of lignite, and, along the shore, major concentrations of manganese and titanium (see Titanium ores).
In the Quaternary Period the Black Sea assumed its present configuration. When drainage into the Mediterranean Sea through the Dardanelles and Bosporus was established, the base level of erosion dropped and deep valleys, which are evident in Ukraine’s landscape today, began to be carved by widely meandering streams. As the climate cooled, a series of glacial and interglacial stages provided the last major imprint on the surface deposits of Ukraine. Of the last four glacial ages (Günz, Mindel, Riss, and Würm), all were associated with glacial scour and deposition in the Caucasus Mountains, and the last two with minor mountain glaciation in the Carpathian Mountains. During the glacial stages deep loess deposits covered nearly three-fourths of Ukraine. During the Riss stage the continental ice sheet modified the surface with scouring and left moraine deposits in the north and down the Dnipro Lowland to Kremenchuk. Its meltwaters left outwash deposits in Polisia. Meanwhile, epeirogenic movements slowly raised the Carpathian Mountains, Crimean Mountains, and, particularly, the Caucasus Mountains, as well as the Podolian Plateau, the Pokutian-Bessarabian Upland, and the Right-Bank Upland, thus accelerating the erosion processes. At the same time the Black Sea and the Black Sea Lowland continued to subside. This led to the formation of limans. In Polisia peat bogs and small deposits of bog iron ore formed because of impeded drainage.
BIBLIOGRAPHY
Geologiia SSSR, 5, 8, 18 (Moscow 1958, 1974, 1986)
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Ihor Stebelsky
[This article originally appeared in the Encyclopedia of Ukraine, vol. 2 (1988).]