Cretacix types

Types.

The rocks and sediments of the Cretaceous System show considerable variation in their lithologic character and the thickness of their sequences. Mountain-building episodes accompanied by volcanism and plutonic intrusion took place in the circumPacific region and in the area of the present-day Alps. The erosion of these mountains produced clastic sediments, such as conglomerates, sandstones, and shales, on their flanks. The igneous rocks of Cretaceous age in the circum-Pacific area are widely exposed.

The Cretaceous Period was a time of great inundation by shallow seas that created swamp conditions favourable for the accumulation of fossil fuels at the margin of land areas. Coalbearing strata are found in some parts of Cretaceous sequences in Siberia, Australia, New Zealand, Mexico, and the western United States.

Farther offshore, chalks are widely distributed in the Late Cretaceous. Another rock type called the "Urgonian" limestone is similarly widespread in the Upper Barremian-Lower Aptian. This massive limestone facies, whose name is commonly associated with rudists (a reef-building bivalve of the Mesozoic), is found in Mexico, Spain, southern France, Switzerland, Bulgaria, the southern Soviet Union, and North Africa.

The mid-Cretaceous was a time of extensive deposition of carbon-rich shale with few or no benthic fossils. These so-called black shales result when there is severe deficiency of oxygen in the bottom waters of the oceans. Poor ocean circulation is suggested as the cause, and the poor circulation is thought to have resulted from the generally warmer climate that prevailed during the Cretaceous, the much smaller than present temperature difference between the poles and the equator, and the restriction of the North Atlantic, South Atlantic, and Tethys. Cretaceous black shales are extensively distributed on various continental areas, such as the western interior of North America, the Alps, the Apennines of Italy, western South America, Western Australia, western Africa, and southern Greenland. They also occur in the Atlantic Ocean, as revealed by the Deep Sea Drilling Program (a scientific program initiated in 1968 to study the ocean bottom), and in the Pacific, as noted on several seamounts.

In North America the Nevadan orogeny took place in the Sierra Nevada and Klamath Mountains from Late Jurassic to Early Cretaceous times; the Sevier orogeny produced mountains in Utah and Idaho in the mid-Cretaceous; and the Laramide orogeny, with its thrust faulting, gave rise to the Rocky Mountains and Sierra Madre Oriental during the Late Cretaceous to Early Tertiary. In the South American Andean system, mountain building reached its climax in mid-Late Cretaceous. In Japan the Sakawa orogeny proceeded through a number of phases during the Cretaceous.

In typical examples of circum-Pacific orogenic systems, regional metamorphism of the high-temperature type and large-scale granitic emplacement occurred on the inner continental side, whereas sinking, rapid sedimentation, and regional metamorphism predominated on the outer oceanic side. The intrusion of granitic rocks, accompanied in some areas by extrusion of volcanic rocks, had a profound effect on geologic history. This is exemplified by the upheaval of the Sierra Nevada, with the intermittent emplacement of

granitic bodies and the deposition of thick units of Cretaceous shales and sandstones with many conglomerate tongues in the Great Valley of California.

Volcanic seamounts of basaltic rock with summit depths of 1,300 to 2,100 metres are found in the central and western Pacific. Some of them are flat-topped, with shelves on their flanks on which reef deposits or gravels accumulated, indicating a shallow-water environment. Some of the deposits contain recognizable Cretaceous fossils. Although the seamounts were formed at various times during the late Mesozoic and Cenozoic eras, a large number of them were submarine volcanoes that built up to the sea surface during the Cretaceous. They sank to their present deep levels some time after the age indicated by their youngest shallow-water fossil.

In west-central India, the Deccan traps consist of more than 1,200 metres of basaltic lava flows that erupted from the Late Cretaceous to the Eocene Epoch of the Tertiary over an area of some 500,000 square kilometres. Volcanic activity on the western margin of the North American epicontinental sea frequently produced ashfalls over much of the western interior seaways. One of these, the "X" bentonite near the end of the Cenomanian, can be traced more than 2,000 kilometres from central Manitoba to north Texas.

Correlation.

Correlation of Cretaceous rocks is usually accomplished using fossils. Ammonites are the most widely employed fossils in terms of both frequency of use and geographic extent, but no single fossil group is capable of worldwide correlation of all sedimentary rocks. Most ammonites, for example, did not occur in all latitudes because some preferred the warmer waters of the Tethys seaway, while others resided in cooler boreal waters. Furthermore, ammonites are rarely found in sediments deposited in nonmarine and brackish environments, and they are seldom retrieved from boreholes sufficiently intact for confident identification.

Many ammonites are very good index fossils, but they are not perfect. When Cretaceous stage boundaries were proposed by the above-mentioned international group of geologists in Copenhagen in 1983, the problems of correlating the boundary between the Campanian Stage and the underlying Santonian were examined. The ammonite Placenticeras bidorsatum, the index species to the oldest Campanian zone of the "classic" zonation, is extremely rare in the type area of western France and appears to be restricted to northwestern Europe. Other ammonites discussed as possible alternatives were Submortaniceras, a genus known from Spain, the KwaZulu/Natal province of South Africa, Madagascar, Mexico, and the Gulf and Pacific coasts of the United States and British Columbia but not from the type area, and the Scaphites hippocrepis, a species described from North America. In Europe, Scaphites aquisgranensis, a late form of S. hippocrepis, occurs together with P. bidorsatum. This S. hippocrepis form occurs widely in the Gulf and Atlantic coastal plains of North America, western Germany, Belgium, and The Netherlands, as well as in the North American western interior and the Aquitane basin of France. The belemnite species Gonioteuthis granulataquadrata is widely used in western Germany for definition of the boundary, but it has a restricted boreal distribution (see below Cretaceous environment ). The extinction level of the free-living crinoid Marsupites testudinarius, the appearance of the coccolith Aspidolithus parcus, and the first occurrence of the

foraminiferans Bolivinoides strigillatus and Globotruncana arca are all used to define the boundary in some regions but not in others.

It was generally agreed that a boundary level close to the currently used appearance of the belemnite species G. granulataquadrata in the boreal realm--i.e., temperate paleobiogeographic region--would be desirable because this boundary could be correlated with a number of other events.

It is desirable to have a reference section for the boundaries of all Cretaceous stages, and the Campanian example above serves to illustrate the variety of fossil groups used to define boundaries and the complexity of the definition problem. The boundaries of the other stages have similar problems of restricted distribution for fossils in the classic type areas. Other fossil types useful for defining Cretaceous stage boundaries are inoceramid bivalves, echinoids, larger foraminiferans, and calpionellids.

On a more local scale, correlation can be achieved using a variety of fossil groups. Rudist, inoceramid, and exogyrid bivalves have been used in many areas to subdivide or zone the Cretaceous Period for the purpose of correlation. Rudist bivalves, for example, have been employed in conjunction with larger foraminiferans to zone sediments of the Tethyan regions of France and Yugoslavia. Echinoids and belemnites have been used together to zone the Late Cretaceous of eastern England. Angiosperm pollen provides for recognition of zones for the Late Cretaceous of the North American Atlantic Coastal Plain. (see also Index: pollen stratigraphy) Some fossil groups are useful for correlation between several regions because of their nektonic or planktonic life habit. Principal among these are ammonites, belemnites, planktonic foraminiferans, calcareous nannofossils, and radiolarians. In North America, for instance, Late Cretaceous strata in Texas, Arkansas, Mexico, and the Caribbean have been correlated using planktonic foraminiferans. Occasionally ostracods (small bivalved crustaceans) are useful; e.g., they have been used to correlate Early Cretaceous strata of northwestern Europe with those of the Russian Platform.

The epicontinental sea of the North American western interior has been particularly well studied, primarily because it can be zoned to great precision. Sixty ammonite zones, to cite a case in point, are recognized in the rocks deposited between the late Albian and the late Maastrichtian. In addition, frequent bentonite beds resulting from the volcanic ash of the Sevier orogenic events provide radiometric dates with which to independently verify the synchronicity of the ammonite zones. This detailed resolution of about 0.5 million years per zone is unusual for the Cretaceous Period. Interestingly, the youngest Cretaceous biozone of the North American western interior is recognized regionally by the occurrence of the dinosaur genus Triceratops, because the last approximately one million years in that area are characterized by nonmarine sediments.

For some of the geologic record, more detailed subdivisions within zones can be developed on the basis of magnetic reversals. The Cretaceous Period, however, has a dearth of magnetic reversals. Specifically, only 16 reversals are noted for latest Jurassic to Aptian time, none for Aptian to late Santonian time, and just 9 from the late Santonian to the

Cenozoic boundary (see Table 20). Magnetic reversals occur far more frequently in Cenozoic rocks.

The Mesozoic Era.

The events in Asia of the Mesozoic (245 to 66.4 million years ago) may be summarized as follows: events in the Tethysides, events in the Altaids, events in the continental nuclei, and events in the circum-Pacific orogenic belts.

Mesozoic events in the Tethysides.

As the Cimmerian continent was moving across the Tethyan realm--eliminating the Paleo Tethys Ocean in front of itself while enlarging the Neo-Tethys behind it--it also began falling apart internally. Thus, a northern fragment (consisting of the Farah block in Afghanistan, the central Pamirs, and the western Ch'iang-t'ang block in Tibet) became separated from a southern fragment (including the Helmand block in Afghanistan, the southern Pamirs, and the Lhasa block in southern Tibet) by an ocean whose ophiolitic remnants are today encountered in the mountain ranges of eastern Iran, along the Farah River in Afghanistan and in the T'ang-ku-la Mountains in Tibet continuing to Mandalay in Myanmar. This ocean opened in the Permian and closed early in the Cretaceous (i.e., earlier than about 125 million years ago).

The northern fragment of the Cimmerian continent, including much of modern-day Iran and the Black Sea mountains of northern Turkey, collided with the Altaid collage along a suture zone that passes north of the Elburz Mountains and south of the Kopet-Dag Range in northern Iran, through the Hindu Kush range in Afghanistan, south of the northern Pamirs and the Kunluns in northern Tibet, and then follows the Chin-sha River and continues through western Thailand and into the Malay Peninsula. The collision occurred late in the Triassic in Iran (about 220 million years ago), in the Early Jurassic (about 200 million years ago) between Iran and Indochina, and again in the Late Triassic in Southeast Asia. This collision created a massive wall of mountains along the southern border of Asia, called the Cimmeride Mountains (the name taken from the ancient people the Cimmerians, in whose homeland north of the Black Sea the first pieces of evidence for this chain were found at the beginning of the 20th century). These mountains extended from Turkey well into Southeast Asia. The large, rich tin-bearing granite belt of western Thailand and Malaysia was formed during this collision.

The southern fragment of the Cimmerian continent soon caught up with the northern fragment; and, following the emplacement late in the Jurassic (163 to 144 million years ago) of a part of the floor of the intervening ocean onto the Lhasa block in the form of a giant ophiolite sheet, the southern fragment also collided with Asia, thus eliminating the entire Paleo Tethys and its marginal basins. Widespread aridity in much of Central Asia during the Late Jurassic was probably a result of the rain shadow that formed behind the wall of the Cimmeride Mountains to the south.

The interval from the Late Triassic to the Late Jurassic (about 230 to 144 million years ago) was also the time when the Yangtze paraplatform and the Huan-an, Tung-nanya, and Annamia blocks collided with one another and also with the eastern end of the Cimmerian continent and the rest of Asia. This created the multibranched Cimmeride mountain ranges of eastern and southeastern Asia, including the Tsinling Mountains that separate North

China from South China. Some of the metamorphic rocks in the eastern end of the Tsinling range in the Ta-pieh Mountains were buried to depths reaching 60 miles (100 kilometres) during the collision of the Yangtze and the North China paraplatforms. These collisions formed another vast tin-bearing granite province in southern China.

In the Middle East the rifting of the Cimmerian continent opened the eastern Mediterranean in the Late Triassic (between 230 and 208 million years ago), with Turkey moving away from Africa. In the Early Jurassic (208 to 187 million years ago) the Turkish part of the Cimmerian continent continued to disintegrate and to open a number of new Tethyan branches, the northernmost of which connected with the opening central Atlantic Ocean in the middle of the Jurassic (about 165 million years ago) via the mountain chains of southern and southwestern Europe and northwestern Africa.

The disintegration of the southern supercontinent Gondwanaland accelerated in the middle Mesozoic. This disintegration led to the opening of the central and the southern Atlantic and Indian oceans that was partially compensated by the beginning closure of the NeoTethys. In Asia the main subduction zones consuming Neo-Tethyan ocean floor began forming along the northern margin of the ocean in Iran and in the Himalayas in the Late Jurassic. A unified subduction zone--extending from northern Turkey, along the south of the Pontic Mountains, through southwestern Iran (the present northern slope of the Zagros Mountains) and Makran, north of the Salt Range in Pakistan to the present-day Himalayan suture zone along the valleys of the Indus and Brahmaputra rivers, and from there to Myanmar and Sumatra--came into being during the Early Cretaceous (i.e., about 120 to 100 million years ago). The vast Late Cretaceous granitic intrusions of the Trans-Himalaya and the Karakoram ranges and the andesitic volcanics that occupy a thin strip from northern Turkey through Iran and Pakistan to the Karakorams and extend beyond the Himalayas into Myanmar, Sumatra, and Borneo are the result of the rapid destruction of the Neo-Tethyan ocean floor.

In the Early Cretaceous other entirely intraoceanic subduction zones also formed just north of the former Gondwanian continental margins in Turkey, Iran, and Oman. The attempted subduction of these margins resulted in the emplacement of vast portions of the NeoTethyan ocean floor on top of these margins in the form of giant ophiolite sheets, such as the Semail Nappe in Oman. These ophiolite nappes (i.e., thrust sheets) are major sources of chromite deposits. Also in the Early Cretaceous a small sliver of continental crust that now forms much of southwestern Sumatra rifted from northwestern Australia and collided with the rest of Sumatra in the Late Cretaceous, opening the northeastern segment of the Indian Ocean behind it.

Mesozoic events in the Altaids.

Most of the Mesozoic events in the Altaids were the echoes of the Cimmeride collisions farther south. In places these collisions split the old Altaid edifice at high angles to the collision front, creating such extensional basins as the Turgay plain (Porte of Turgay), just north of the Aral Sea, and the West Siberian Plain, which contains little-deformed Jurassic and younger shallow-water and continental sedimentary rocks with significant hydrocarbon reserves. In other places closer to the collision front, the basement was uplifted along major thrust faults, creating mountain ridges (e.g., in the Tupqaraghan Peninsula on the east coast of the Caspian Sea and the Kyzylkum Desert

of southern Kazakstan). Between these, large compressional basins formed (e.g., the Turkmenian basins) or older ones became accentuated (the Tarim and Dzungarian), within which large sedimentary thicknesses and important hydrocarbon reserves accumulated. The compressional structures were connected in places with extensional structures through large strike-slip fault systems, the best-known of which runs through the Fergana Valley in southern Central Asia.

Mesozoic events in the continental nuclei.

The Angaran platform was also affected by the Cimmeride collisions but reacted more mildly than the Altaids. The vast Tunguska trap basalts erupted in the transition between the Permian and Triassic periods, and the eruptions lasted well into the Triassic. They were related to the rifting of the West Siberian Plain and were coeval with basaltic eruptions in the Turgay plain. Suggestions that the Tunguska trap volcanism is responsible for the major worldwide extinction wave of marine organisms at the Permian-Triassic boundary have generated much controversy among geologists but have found little support. The old Proterozoic rifts on the Angaran platform were compressed at the end of the Jurassic, probably in response to the ongoing shortening of the Cimmeride continent.

Major Late Jurassic-Early Cretaceous extension and basaltic volcanism affected especially the northern part of the Arabian platform. This extensional event was part of a much wider extensional province in north-central Africa. Yet another such event occurred in the northern and eastern parts of the platform in the Late Cretaceous, creating deep shelf basins.

During the Mesozoic, India separated from Gondwanaland. Its eastern margin formed early in the Cretaceous (about 140 million years ago), when India separated from Australia. The Early Cretaceous rifting event that affected the eastern margin of the Indian platform also led to some rejuvenation of the older Gondwanan rifts. India separated from Madagascar some 85 million years ago. Another rifting along this margin, about 66 million years ago, removed the Seychelles and Saya de Malha banks in the present western Indian Ocean from India and also gave rise to the huge Deccan trap basalt eruptions which involved about 50 distinct flows in probably less than a million years. As is the case with the Tunguska traps, the Deccan eruptions may have been responsible for the mass extinctions (including that of the dinosaurs) at the Cretaceous-Tertiary boundary some 66.4 million years ago.

The subduction of the floor of the Pacific Ocean dominated the evolution of the Pacific margin of Asia, especially during the second half of the Mesozoic Era. Large subduction-accretion complexes formed in Japan and in Borneo, and the Kolyma block--forming present-day northeastern Asia--collided with the Angaran platform during the Late Jurassic-Early Cretaceous interval. This collision produced the 375mile-wide Verkhoyansk fold-and-thrust belt, in the front of which coal was deposited in postcollisional molasse basins.

A major magmatic arc flanked Asia between Japan and Indochina in the Late Jurassic to Late Cretaceous interval and joined the Neo-Tethyan arc system in Borneo. Late Cretaceous to Early Tertiary extensional tectonics along this arc formed many of the offshore basins along the Chinese continental margin.