Cretaceous System Period

the third (last) system of the Mesozoic group, corresponding to the third period of the Mesozoic era of earth history. It follows the Jurassic and precedes the Paleogene system of the Cenozoic group. The name is derived from white chalk, the rock that is widely distributed in Europe in the upper half of the system. Using radiological methods, it has been determined that the Cretaceous period began 135–137 million years ago and ended 65–67 million years ago; thus the period lasted about 70 million years.

The Cretaceous system was first identified as an independent system by the Belgian geologist J. Omalius d’Halloy in the Paris Basin in 1822. In what is now the USSR the widespread distribution of white chalk was established by the expeditions of the Russian academicians P. S. Pallas and I. I. Lepekhin in the second half of the 18th century. The first schema for the stratigraphic subdivision of these rocks in the Volga region was proposed by P. M. lazykov in 1832. An important role in the subsequent study of the Cretaceous system was played by I. I. Laguzen, S. N. Nikitin, A. P. Pavlov, A. D. Arkhangel’skii, A. N. Krishtofovich, and V. P. Rengarten.

Subdivisions. The Cretaceous system and all its principal subdivisions that are included in the international stratigraphic scale were first identified in France and adjacent areas in Switzerland, the Netherlands, and Denmark. As a result of the work of the French stratigraphers and paleontologists A. d’Orbigny, E. Desor, A. Kokan, and E. Renevier, the Belgian geologist A. Dumont, and others, the Cretaceous has been subdivided into stages; this subdivision has remained substantially unchanged. In 1885, by a decision of the International Geologic Congress, the stages were grouped into two “divisions” (series), the Lower and Upper Cretaceous (see Table 1).

Table 1. Scheme of the stratigraphy of the Cretaceous system
Series	Stages
Upper Cretaceous K2
Lower Cretaceous Ki

The names of the stages are derived from the modern or ancient geographic names of the places where the corresponding deposits were first identified. In Soviet schemata the terms “Neocomian” and “Senonian” are used to signify superstages encompassing several stages of the Lower and Upper Cretaceous. The Danian stage (age) is classified in the Paleogene system (period) by many stratigraphers.

More detailed stratigraphic schemata have been worked out for many areas in the USSR and foreign countries; in them the stages are subdivided into substages and zones or horizons. The creation of such schemata, which are very important for preparing large-scale geological maps, helps resolve many theoretical and practical problems of geology.

General description. The rocks of the Cretaceous system are distributed on all continents of both the northern and southern hemispheres. As a result of drilling they have also been found on the floor of the oceans beneath younger sediments. Based on the prevailing composition of the rocks and their distribution, the Cretaceous system is clearly divided into two approximately equal parts that correspond to the Lower and Upper Cretaceous. These differences result from the particular characteristics of the geological history of the continents.

The tectonic movements that were intensively manifested at the end of the Jurassic in the Cordilleran and East Asian geosynclinal regions caused a vast regression of the sea. In the early Cretaceous the marine basins remained in the longitudinally extended Mediterranean region, the geosynclinal troughs in western America and East Asia, the eastern part of the Eastern European Platform, northern Siberia, and other regions. The vast areas of the platforms, above all the platforms of the southern hemisphere, were not covered by seas. Some expansion of the sea boundaries, which was particularly extensive in Australia, occurred in the Aptian and Albian ages but did not substantially change the geocratic conditions of the early Cretaceous. Various sediments, primarily sandy-argillaceous, were deposited in the seas at this time; coal measures accumulated in some places on the continents, extending as far north as Greenland and Alaska.

Approximately in the middle of the period tectonic movements began again in the circum-Pacific geosynclinal belts. They reached the greatest intensity in the East Asian geosyncline, most of which was transformed into a complexly structured folded region. At the beginning of the late Cretaceous large parts of the platforms subsided, causing one of the greatest marine transgressions in earth history. Vast areas of the Eastern European, North American, African, and Australian platforms were covered by the sea. The wide distribution of marine basins and leveling of the land reduced the delivery of clastic material. The prevailing type of marine sediment on the platforms became fine-grained calcareous and calcareous-argillaceous silts, which later became limestones, marls, and chalk. Basins within the continents filled with river, lacustrine, and sometimes coal-bearing deposits. Coal accumulated on a particularly large scale in western North America at the end of the late Cretaceous.

At the end of the Cretaceous period tectonic movements began again on the periphery of the Pacific Ocean. They were most intensive in North and South America, where the folded structures of the Rocky Mountains and Andes formed. The folding movements were accompanied by powerful volcanic activity that engulfed the East Asian and Cordilleran geosynclines and some regions of the Mediterranean geosyncline. One of the earth’s most significant volcanic areas was northeastern Asia, where lavas and tuffs of various composition cover enormous areas. In the late Cretaceous major volcanic eruptions also occurred in India and Africa, which testified to the continuing breakup of the continent of Gondwana. A vast regression of the sea, which occurred primarily on the platforms of the northern hemisphere, was associated with the movements at the end of the period.

The zonal distribution of different types of deposits and biogeographic provinces indicates the existence of climatic zones during the Cretaceous period. The tropical zone coincided roughly with the Mediterranean geosynclinal belt; the distribution of salt-bearing lagoonal deposits on both sides of this belt marked the position of regions with a dry climate. To the north was the moderately warm zone, which was distinguished by intensive coal accumulation. These climatic regions extended roughly in a latitudinal direction—that is, they more or less corresponded to our present-day zones. In the late Cretaceous, when marine transgressions were most extensive, climatic differences decreased somewhat. By isotopic paleothermometry it can be established that the temperatures of the surface waters of the ocean were much more uniform than they are today.

Organic world. The organic world at the beginning of the Cretaceous period continued to greatly resemble the preceding Jurassic stage of development. Ferns and various groups of gymnosperms, such as Cycadaceae, ginkgoes, conifers, and Bennettitales, dominated the land flora. In approximately the middle of the early Cretaceous the first angiosperms appeared, and during the end of this epoch there occurred a major change in the earth’s plant world: flowering plants became dominant. Many of the genera of angiosperms that appeared in the late Cretaceous (magnolia, laurel, plane, oak) are prominent in our modern flora. All this enables paleobotanists to draw a boundary within the Cretaceous period between the Mesophytic and the Cenophytic, geochronological subdivisions based on the evolutionary development of plants.

The fauna of land vertebrates was dominated by reptiles, represented by various types of carnivores (tyrannosaurs, tarbosaurs), herbivores (iguanodont, trachodont), and flying reptiles (pteranodons). Many of them, in particular the carnivorous dinosaurs, reached great dimensions and were the largest land predators ever to inhabit our planet. The birds differed from the ancient Jurassic forms by a higher level of organization, but they continued to have teeth. The first known toothless birds are from the end of the Cretaceous period. The appearance of the first placental mammals also dates from this time.

The seas of the Cretaceous were inhabited by a rich fauna, with the cephalopods—the ammonites and belemnites—continuing to play an important part. The shells of the ammonites sometimes reached enormous size (up to 2 m in diameter); among the ordinary forms coiled in a flat spiral there are forms that are loosely coiled, straight, towerlike, or twisted in an irregular cluster. Various gastropods and bivalves, sea urchins and sea lilies, corals, sponges, foraminiferans, and some other groups were also abundant. Many of these groups were distinguished by quite rapid evolutionary change, and their remains are successfully used as index fossils for stratigraphic correlations.

Among the marine vertebrates bony fishes became wide-spread; from this time they were the most common representatives of the class. The large reptiles—ichthyosaurs (until the end of the early Cretaceous) and plesiosaurs—continued to exist. In the late Cretaceous marine lizards appeared—mosasaurs, which reached a length of 12 m.

Unlike land vegetation, the animal world did not undergo sharp changes between the early and late Cretaceous. Major changes occurred at the very end of the period, when many groups of animals typical of the Mesozoic became extinct—for example, ammonites, belemnites, Inoceramus, rudistids, dinosaurs, and plesiosaurs. The marked changes in the composition of marine and land fauna at the end of the Cretaceous period served as the basis for drawing the boundary between the Mesozoic and Cenozoic groups.

Biogeographic regionalization. In the early Cretaceous the same zoogeographic regions as those existing in the Jurassic were clearly expressed; these are the boreal, Mediterranean, and southern (Australian). The seas of the boreal region were inhabited by the fauna of moderately warm waters, representatives of which were the aucellas, belemnites, and some genera of ammonites. In the basins of the Mediterranean (equatorial) region there lived a richer and more varied thermophilic fauna of rudistids, large oysters, corals, sea urchins, and other genera of ammonites and belemnites. The population of the seas in the southern region resembled the boreal fauna in some features. In the late Cretaceous, apparently in connection with the wide distribution of marine transgressions and the general evening out of climatic conditions, the differences among these regions noticeably decreased. The tropical Mediterranean zone was still clearly distinguished by the prevalence of thermophilic forms. There was also some differentiation in the composition of Cretaceous land vegetation.

Deposits in the USSR. In the USSR, Cretaceous rocks are very widespread and are represented by different types of marine, lagoonal, continental, and volcanic accumulations. They occupy vast areas on the Eastern European Platform and in the mountainous regions of the Carpathians, Crimea, and Caucasus. Lower Cretaceous clays, sands, and sandstones, of relatively small thickness and in places containing phosphorite concretions, are exposed primarily in the eastern and central parts of the platform. Shells of ammonites and bivalves are found in these Lower Cretaceous rocks in addition to an occasional plant imprint. Cretaceous deposits are much more fully represented in the Crimea, the Caucasus, and the Caspian basin, where in places they are more than 1,000 m thick. The Upper Cretaceous is widely found in the southern half of the platform and in the folded regions that surround it on the south. Calcareous and calcareous-argillaceous deposits are sharply predominant, only occasionally replaced by siliceous and sandy sediments. Extensive development of white chalk is very typical for the Eastern European Platform. In the Carpathians and on the southern slope of the Greater Caucasus flysch strata up to 4,000–5,000 m thick date to the Upper Cretaceous. Volcanic rocks play an important role in the composition of the Upper Cretaceous in the Lesser Caucasus. On most of the West Siberian Plain there are strata of sandy-argillaceous, and occasionally siliceous, sediments with horizons of limestone and marl. In the northern and central parts of the lowland marine facies usually predominate; on the periphery they give way to marine coastal, continental, and sometimes red lagoonal deposits. The accumulation of iron ores is associated with coastal shallow-water sediments of the Turonian stage in the marginal parts of the basin.

A narrow strip between the Southern Urals and Central Kazakhstan joins the area of distribution of Cretaceous rocks in the West Siberian Plain with the Aral Sea region and the deserts and mountain ranges of Middle Asia, where deposits of the Upper Cretaceous are more fully represented. Cretaceous limestones, marls, and sandstones with abundant ammonite, Inoceramus, and sea urchin fossils are exposed on Mangyshlak, in the scarps of the Ustiurt Plateau, in the Kopetdag Mountains, and elsewhere. In eastern Middle Asia marine sediments are replaced by red lagoonal and continental deposits.

On the Siberian Platform deposits of the Cretaceous system (sandy-argillaceous, primarily continental, coal-bearing strata up to 2,000 m thick) are distributed along the northern margin, in the lower course of the Lena River, and in the Viliui basin. Vast areas in the Northeast and Far East of the USSR are covered by Cretaceous rocks of different composition and origin (sandy-argillaceous marine and continental coal-bearing deposits, lavas, and tuffs).

Minerals. Large deposits of hard and brown coal are associated with the rocks of the Cretaceous system (in the USSR on the Siberian Platform, in Transbaikalia, the Far East, and Northeast; abroad in Mongolia, North America, and elsewhere). In a number of regions of the USSR (Ciscaucasia, Middle Asia, West Siberia) and in foreign countries (the United States, Mexico, Canada) commercially important oil and natural gas deposits are found in Cretaceous beds. Deposits of salts and occasionally bauxites (southern Europe) are associated with the red lagoonal facies; sedimentary iron ores are associated with coastal marine facies. There are deposits of gold, silver, tin, lead, mercury, and other ore minerals in the volcanic rocks, which are particularly developed on both sides of the Pacific Ocean, in the East Asian and Cordilleran folded regions. Cretaceous concretionary phosphorites occur on the Eastern European Platform, in Western Europe, and North America. Upper Cretaceous strata contain enormous masses of various carbonate rocks.