Jurassic System Period

the second system of the Mesozoic group, corresponding to the second period of the Mesozoic era of earth history. The Jurassic, named after the Jura Mountains in France and Switzerland, follows the Triassic and precedes the Cretaceous. Based on radiological data, it began 190–195 million years ago and ended 135–137 million years ago, thus lasting 55–58 million years. The Jurassic system was identified by the French geologist A. Brongniart in 1829.

In the USSR, Jurassic beds were established in the mid-19th century by the German geologist L. von Buch, the Russian geologist K. F. Rul’e, and the French geologist A. d’ Orbigny. Later, major contributions to the study of the Jurassic in what is now the USSR were made by I. I. Laguzen, S. N. Nikitin, A. P. Pavlov, A. L. Chekanovskii, and D. N. Sokolov, as well as by V. I. Bodylevskii, A. A. Borisiak, D. I. Ilovaiskii, A. N. Krishtofovich, G. Ia. Krymgol’ts, V. F. Pchelintsev, and V. P. Rengarten.

Subdivisions. The principal subdivisions of the Jurassic system included in the international stratigraphic scale were established in France and adjacent countries. In 1839 Buch subdivided the Jurassic into three parts, later called series. The work of A. d’Orbigny (1842–52), who used the subdivisions established in various parts of Western Europe, and the research of the German geologist A. Oppel (1856–58), who contrasted the subdivisions on the basis of zonal partition, were instrumental in the identification of the various stages. The USSR has adopted the stratigraphic subdivision of the Jurassic given in Table 1. The beds of the Boreal region in the USSR, which correspond to the Tithonian, are characterized by unique organic remains and a different partition and have been singled out as the Volgian stage, parallel to the Tithonian. The Callovian stage is placed in the Middle Jurassic in many countries, and the Aalenian stage is classified as part of the Lower Jurassic.

More detailed zonal stratigraphic charts have been compiled for many regions of the USSR and foreign countries. The zonal subdivision based on ammonites established in Western Europe has been adopted as the standard. Many of the zones have widespread distribution, often intercontinental.

General description. Jurassic rocks are widespread on all continents, as well as on the ocean floor, as has been disclosed by drilling. They vary considerably with respect to composition and conditions of formation and include marine and continental sand and clay strata, sometimes altered to clay shales and slates (in geosynclinal regions), carbonate rocks (primarily limestones), and salt-bearing strata, as well as volcanic rocks, including some of marine origin (western margin of North and South America, East Asia, and certain segments of the Mediterranean belt).

Tectonic movements occurred most intensively in the Pacific geosynclinal belt, where folded mountain structures that have survived to the modern epoch formed (East Asia, the western part of North and South America, and partially the Mediterranean belt).

In the Early Jurassic, the marine basins were concentrated in the Mediterranean and Pacific geosynclinal belts. Then the seas began to expand gradually, and by the middle of the Late Jurassic, the transgression had reached its maximum, especially on the Eastern European Platform. The seas initially flooded only the northern margin of the Siberian Platform, but by the end of the Early and beginning of the Middle Jurassic, they had also flooded the eastern part (the Viliui Syneclise).

The basins of the Atlantic and Indian oceans formed in the Jurassic, apparently as a result of the drifting apart of the continents, particularly as a result of the breakup of Gondwanaland. Significant land volcanic activity was associated with fault zones (East Africa and South America). Dry land predominated on the continents of the southern hemisphere (Gondwanaland and South American Platform) during the Jurassic period.

The climate, which was little differentiated and relatively dry in the Early Jurassic, became humid in the Middle Jurassic. A clear climatic zonality emerged in the Late Jurassic.

Organic world. Land vegetation was dominated by gymno-sperms (ginkgoes, sago palms, Bennettitales, and conifers); there were also many different ferns, horsetails, and the like.

The reptiles were the dominant land vertebrates. They were represented by both herbivores (Diplodocus, Stegosaurus, and Brontosaurus) and carnivores (Ceratosaurus and Allosaurus), which reached enormous sizes. Flying lizards, pterosaurs (rham-phorhynchids and pterodactyls), were known, and ancient birds appeared (Archaeopteryx). There were also a number of small mammals. Butterflies appeared among the land arthropods.

The freshwater basins were inhabited by mollusks, often resembling contemporary ones. Certain reptiles (ichthyosaurs and plesiosaurs) lived in the seas, where they existed alongside bony fishes. Invertebrates, chiefly mollusks, predominated in the seas, including cephalopods (ammonites and belemnites), bivalves, and gastropods. There were many different corals, echinoderms, arthropods, radiolarians, and foraminifers.

Biogeographic zoning. The marine fauna and land flora of the Early Jurassic were comparatively homogeneous. The subsequent evolution of particular groups in the seas of the northern hemisphere and the subsequent change in the paleographic situation led to the emergence of the Mediterranean and Boreal regions. The former, which approximately corresponds to the equatorial zone, is characterized by the presence of reef-building corals and other groups associated with organogenic structures (calcareous algae, certain bivalve and gastropod mollusks). These groups do not occur in the more northerly latitudes, in the seas of the moderately warm Boreal region. The Notalian (or Antiboreal) region has not been identified as clearly, apparently because of the very slight development of seas within the boundaries of the modern southern continents and their subsequent displacement. The uniqueness of the fauna of the basins linked with the Pacific Ocean has made it possible to identify a Pacific province, characterized by the combination of Mediterranean, Boreal, and certain endemic elements.

The distribution of land vegetation reflects climatic zonation even more than the marine fauna.

Jurassic deposits in the USSR. Jurassic rocks are widespread in the USSR, where they are represented by formations of marine, continental, and magmatic origin. On the Eastern European Platform they include sand and clay sediments dozens of meters thick, which formed in the Late Jurassic seas. The Jurassic beds of the Crimea, the Caucasus, and the Pamirs are thicker, reaching several kilometers. The Lower and Middle Jurassic strata are composed primarily of clay shales and sandstones, while the Upper Jurassic strata are composed chiefly of organogenic limestones. In Transcaucasia the products of underwater eruptions and volcanic tuffs are significant. Intrusive bodies formed in some places on the southern coast of the Crimea, for example, Aiudag.

Jurassic deposits have been found as a result of deep drilling (800–3,000 m) over a significant part of the Western Siberian Plain, where the Lower and Middle Jurassic parts of the cross section are represented by continental sandstones, aleuroliths, and argillites, with interlayers of coal in some places. The Upper Jurassic rocks are similar in composition, but they formed at the bottom of a sea that penetrated from the north. Continental beds, some of them coal-bearing, have been identified in the intermontane depressions of the Urals, North Kazakhstan, and the Tien-Shan. Marine sediments occur in the southern part of Middle Asia (Mangyshlak, Tuarkyr, Bol’shoi Balkhan, and Kugi-tang). In the late Jurassic, gypsum- and salt-bearing strata accumulated as the sea regressed (Turkmenia, the Caucasus, and Moldavia).

On the Siberian Platform, Jurassic deposits (sands, sandstones, aleuroliths, and clays) are confined to the northern and eastern margins, which had undergone subsidence. The Upper Jurassic strata contain coal. Thick layers of sandstones, aleuroliths, and clay shales were deposited in Transbaikalia, the Far East, and Northeast Asia, as well as volcanic-sedimentary rocks with significant amounts of tuffogenic material and strata of andesites, porphyrites, basalts, and the like. Continental strata of sandstones, argillites, aleuroliths, and conglomerates, sometimes containing coal, appear in the upper horizons of the Jurassic.

Minerals. Jurassic beds account for 14 percent of the world reserves of coal and about 15 percent of the petroleum reserves. Major coal deposits have been found in Eastern Siberia (the Kansk-Achinsk Coal Basin, the Irkutsk Coal Basin, the Lena Coal Basin, and the Southern Yakut Coal Basin), in Transbaikalia, in the Far East (Bureia), and elsewhere. Outside the USSR, deposits of Jurassic coal have been found on the Balkan Peninsula, in Iran, the Mongolian People’s Republic, the People’s Republic of China, the People’s Democratic Republic of Korea, and Australia. Deposits of petroleum and gas in the USSR have been found in the Caspian region, on the Mangyshlak Peninsula, in the Caucasus, in Western Siberia, and in the eastern part of the Siberian Platform; outside the USSR they have been found in North America and the Middle East.

Oolitic iron ores formed in the shallow Jurassic seas, for example, in Lorraine. Deposits of bauxites, kaolins, iron, and nickel, as well as some diamond placers, are linked to weathering crusts. Deposits of phosphorites (Egor’evsk and others) and fuel shales (Kashpir, Undory, Obshchesyrtovskoe) have been discovered in Upper Jurassic marine beds on the Eastern European Platform. Salt-bearing (chemogenic) strata are linked to lagoon deposits in the southern USSR and the southern United States; the rock and potassium salts of eastern Turkmenia (Gaurdak) are the most important. The formation of deposits of complex ores and rare and noble metals has been linked to processes of mag-matism in the southern and, particularly, northeastern USSR, as well as in the western part of North America and South America.