Mesozoic Group ERA
Mesozoic Group (ERA)
the next to last group of systems in the stratigraphic scale and the era in the geological history of the earth corresponding to it. The Mesozoic encompasses the time interval from approximately 230 million to 67 million years ago, or about 163 million years. The Mesozoic era was first identified in 1841 by the English geologist J. Phillips. It is subdivided into three systems (periods): Triassic, Jurassic, and Cretaceous. The Mesozoic era is the time when the basic contours of our present continents formed, and, probably, those of most of the ocean basins (except for the Pacific, which possibly existed earlier). The era is characterized by vegetation consisting primarily of ferns and gymnosperms and fauna with reptiles dominant among the vertebrates. It is also the era when angiosperms, mammals, and birds appeared.
General description At the end of the Paleozoic era all the ancient platforms were elevated above sea level and ringed by folded mountain systems that had formed as a result of the Hercynian folding. The Eastern European and Siberian platforms were joined by the newly developed mountain systems of the Urals, Kazakhstan, Tien-Shan, the Altai, and Mongolia. The land area increased greatly through the formation of mountain regions in Western Europe and also along the margins of the ancient platforms of Australia, North America, and South America (the Andes). In the southern hemisphere the ancient continent of Gondwana occupied an enormous area.
Thus, at the end of the Paleozoic era the continental blocks of the earth’s crust occupied enormous expanses. With the beginning of the Mesozoic they began to subside; the subsidence was accompanied by transgressions of the sea. The continent of Gondwana split and broke into the separate continents of Africa, South America, Australia, and Antarctica and peninsular India. Beginning in the Jurassic, sea waters flooded enormous areas of the ancient platforms (Eastern European, Indian, and South American) and the folded regions that had just finished forming and had become the foundations of the young platforms (the West Siberian, Scythian, Turanian, and others). In southern Europe and Southwest Asia deep troughs, the geosynclines of the Alpine folded region, began to form. Similar troughs, except for the fact that they were on the oceanic part of the earth’s crust, arose along the periphery of the Pacific Ocean. The transgression of the sea and the expansion and deepening of the geosynclinal troughs continued during the Cretaceous period. Only at the very end of the Cretaceous did the process of continental uplift and reduction in the area of the seas begin.
The marine deposits of the Triassic and Jurassic are characterized by beds of black clays and clay shales with sandstone interlayers; the beds are sometimes very thick. Around the Mediterranean and Black seas Upper Jurassic beds of limestones with coral structures and reefs are typical (Portugal, the Alps, the Crimea, the Caucasus, and elsewhere). In the early Cretaceous sandy-argillaceous lagoon deposits and, in some places, red lagoon deposits accumulated. The Upper Cretaceous is characterized by extensive sandy-glauconite deposits and carbonate rocks (limestone), especially facies of chalk. The rocks of the Cretaceous system form three vast zones on the earth’s surface. Between 30° N and 30° S lat. there are extensive carbonate deposits (sometimes red carbonate deposits), which are associated with the tropical climate zone. North and south of this zone, extending all the way to the arctic and antarctic circles, are sandy-glauconite deposits (often containing phosphorites), which are associated with the temperate climate zone. Thus, the climatic zonation of the Cretaceous period was similar to that of the present, but the tropical and temperate climate zones were broader. Evidently the Jurassic and Triassic were characterized by a very broad zone of rainy tropics.
The organic world. The primarily arid conditions of the Permian and early Triassic gave way to an increasingly wet climate in the Mesozoic. The abundant vegetation of the Carboniferous period, which included arborescent lycopods (lepidodendrons, sigillarias), giant calamites, cordaites, and other plant groups, became extinct in the epoch of arid conditions during the Permian. In the Mesozoic era the flora was renewed and vegetation developed extensively over large areas of the continents. During the Triassic the vast arid climatic zones with sparse vegetation still predominated on the continents; red sandy-argillaceous rocks and gypsum were deposited in these zones under plains and lacustrine conditions. In the temperate climatic belts significant areas were covered by forests of conifers (Voltzia and others), equisetaceous plants, ferns, arborescent lycopods (Pleuromeia), and yuccas (Vuceites), descendants of the cordaites. In the swampy areas of the forests peat beds formed, from which layers of mineral coal and coal measures later developed (in the Urals and Transbaikal regions of the USSR and in Korea, Japan, India, South Africa, and Australia). Thick tuff and lava beds associated with numerous volcanoes accumulated during the Triassic on the South American Platform (the Paraná River basin) and in the Tunguska Depression of the Siberian Platform.
During the Jurassic period the sea engulfed vast areas of Europe, western and eastern Siberia, North Africa, and the eastern coast of the Pacific Ocean. Within the continents large flat areas formed; they were irrigated by rivers, covered with lush vegetation growing under temperate and rainy tropical climatic conditions, and contained vast lakes and swamps. In these areas the peat accumulated that was later transformed into seams of mineral coal. Gymnosperms—conifers, ginkgoes, bennettites, and cycads—were the most widely distributed plants in the tropical and subtropical zones. Ferns predominated among the spore plants. The equisetaceous plants and lycopods began to play a subordinate part. In the northern temperate zone coniferous, ginkgo, and Czekanowskiales forests predominated.
In the Triassic the age of giant amphibians (Stegocephalia) came to an end and already by the late Triassic reptiles had assumed the dominant role among the vertebrates. The giant reptiles flourished particularly during the Jurassic and Cretaceous periods. They became extremely varied and were divided into water forms (plesiosaurs and ichthyosaurs), land forms— dinosaurs (Iguanodon, trachodonts, Stegosaurus, and others), and flying forms (pterosaurs). The vast tropical forests and lakemarsh depressions where they fed on water plants were an especially favorable environment for their development. During the Jurassic small mammals and the first toothed bird, Archaeopteryx, appeared.
There was a major change in the composition of vegetation in the middle of the Cretaceous period: the angiospermous plants, which had first appeared in the early Cretaceous, assumed the position of dominance that they have preserved to the present. The moisture-loving Jurassic and early Cretaceous vegetation gradually assumed a subordinate role, although relicts of this flora exist to the present in some tropical and subtropical regions —for example, in New Zealand. Among the Cretaceous flora of angiosperms the predominant plants were the planes, laurels, rubber plants (Ficus), Magnoliaceae, and legumes. Common among the conifers were pines, yews, sequoias, and taxodiums. Bennettites became extinct by the end of the Cretaceous and only one species of ginkgo remained. The ferns and Cycadophyta began to play a subordinate role. In the tropical and temperate zones there were vast forests, and peat beds that were the beginning of coal seams continued to accumulate. The development of the angiosperms in the middle of the Cretaceous period promoted the spread of insects (pollinators) and this, in its turn, led to extensive development of the class Aves and, later, of mammals, which supplanted the reptiles. At the end of the Mesozoic era the large reptiles (dinosaurs) became extinct.
During the Mesozoic era the composition of the flora and fauna of the seas also changed greatly. The Paleozoic genera and species disappeared completely by the beginning of the Triassic and were replaced by new ones. Lamellibranchs and gastropods developed and the brachiopods, which had predominated in the Paleozoic, became secondary. Among the cephalopods the ammonites and belemnites reached their peak, as did the sea urchins and sea lilies among the Echinodermata. Bony fish developed and assumed a dominant position. In the late Mesozoic era the gigantic marine reptiles (ichthyosaurs) became extinct, as did invertebrates such as the ammonites and belemnites.
Minerals The deposits of the Mesozoic group include beds of brown and hard coal (the Asiatic USSR, China, and the United States), deposits of petroleum, sedimentary iron ores (USSR, France), bauxites (USSR, France, Hungary, Rumania), and phosphorites and rock salt (USSR). Gold ore deposits (Alaska, California, and the Verkhoiansk region) are associated with the intrusive rocks of the folded regions in the Pacific Ocean belt, as are ore deposits of silver, copper, lead, zinc, and tin.
REFERENCESStrakhov, N. M. Osnovy istoricheskoi geologii, 3rd ed., parts 1–2. Moscow-Leningrad, 1948.
Gignoux, M. Stratigraficheskaia geologiia. Moscow, 1952. (Translated from French.)
Krishtofovich, A. N. Paleobotanika, 4th ed. Leningrad, 1957.
Paleozoiskie i mezozoiskie flory Evrazii i fitogeografiia etogo vremeni. Moscow, 1970.
M. V. MURATOV and V. A. VAKHRAMEEV