Dictionary:

Cretaceous

  (krĭ-tā'shəs)

1. Of or belonging to the geologic time, system of rocks, and sedimentary deposits of the third and last period of the Mesozoic Era, characterized by the development of flowering plants and ending with the sudden extinction of the dinosaurs and many other forms of life.
2. cretaceous Of, containing, or resembling chalk.

The Cretaceous Period or its system of deposits.

[From Latin crētāceus, chalky, from crēta, chalk, from Crēta (terra), Cretan (earth).]

In geological time, the last period of the Mesozoic Era, preceded by the Jurassic Period and followed by the Tertiary Period. The rocks formed during Cretaceous time constitute the Cretaceous System. Omalius d'Halloys first recognized the widespread chalks of Europe as a stratigraphic unit. W. O. Conybeare and W. Phillips (1822) formally established the period, noting that whereas chalks were remarkably widespread deposits at this time, the Cretaceous System includes rocks of all sorts and its ultimate basis for recognition must lie in its fossil remains. See also Chalk; Fossil; Jurassic; Rock age determination; Stratigraphy.

The parts of the Earth's crust that date from Cretaceous time include three components: a large part of the ocean floor, formed by lateral accretion; sediments and extrusive volcanic rocks that accumulated in vertical succession on the ocean floor and on the continents; and intrusive igneous rocks such as the granitic batholiths that invaded the crust of the continents from below or melted it in situ. The sedimentary accumulations contain, in fossils, the record of Cretaceous life. The plutonic and volcanic rocks are the chief source of radiometric data from which actual ages can be estimated, and suggest that the Cretaceous Period extended from 144 million years to 65 ± 0.5 million years before present.

There are 12 globally recognized subdivisions, or stages, in the Cretaceous, based on species development. In marine sediments the appearance and disappearance of individual, widely distributed species allows further time resolution by so-called zones. Initially these zones were largely based on ammonites, a now extinct group of cephalopods, closest to the squids and octopuses but resembling the pearly nautilus. Due to the provinciality of some ammonite species and the rarity of many, the dating of Cretaceous marine sediments now mainly devolves on microscopic fossils of calcareous plankton. The most important of these are protozoans. Their shells, in the range of 0.1–1 mm, occurring by hundreds if not thousands in a handful of chalk, have furnished about 38 pantropical zones. Next in importance are the even tinier (0.01-mm) armor plates of “nanoplanktonic” coccolithophores. Thousands may be present in a pinch of chalk, and 24 zones have been recognized. See also Cephalopoda; Coccolithophorida; Foraminiferida; Marine sediments; Micropaleontology; Paleontology; Phytoplankton; Protozoa; Zooplankton.

The Earth's magnetic field reversed about 60 times during Cretaceous time, and the resulting polarity chrons have been recorded in the remanent magnetism of many rock types. The actual process of reversal occurs in a few thousand years and affects the entire Earth simultaneously, providing geologically instantaneous time signals by which the continental and volcanic records can be linked to marine sequences and their fossil zonation. Noteworthy here is the occurrence of a very long (32-million-year) interval during which the field remained in normal polarity. See also Paleomagnetism.

During Cretaceous time the breakup of Gondwana, the great late Paleozoic-Triassic supercontinent, became complete. Laurasia had already separated from Africa by the development of Tethys and became split into North America and Eurasia by the opening of the North Atlantic. These new, deep oceanic areas continued to grow in Cretaceous time. India broke away from Australia and Australia from Antarctica. South America tore away from Africa by the development of the South Atlantic Ocean, while India brushed past Madagascar on its way north to collide with southeast Asia. As these new oceanic areas grew, comparable areas of old ocean floor plunged into the mantle in subduction zones such as those that still ring the Pacific Ocean, marked by deep oceanic trenches and by the development of mountain belts and volcanism on adjacent continental margins. See also Continents, evolution of; Plate tectonics; Subduction zones.

The face of the globe was also affected by changes in sea level. Sea level at times in the early Cretaceous stood at levels comparable to the present, but subsequently the continents were flooded with relatively shallow seas to an extent probably not attained since Ordovician-Silurian times. Maximal flooding, in the Turonian Stage, inundated at least 40% of present land area. Cretaceous seas covered most of western Europe, though old mountain belts such as the Caledonides of Scandinavia and Scotland remained dry and archipelagos began to emerge in the Alpine belt. In America, seas flooded the southeastern flank of the Appalachian Mountains, extended deep into what is now the Mississippi Valley, and advanced along the foredeep east of the rising Western Cordillera to link at times the Gulf of Mexico with the Arctic Ocean.

Large seas extended over parts of Asia, Africa, South America, and Australia. The wide spread of the chalk facies is essentially due to this deep inundation of continents, combined with the trapping of detrital sediments near their mountain-belt sources, in deltas or in turbidite-fed deep-water fans. At the same time, carbonate platforms were still widespread, and the paratropical dry belts were commonly associated with evaporite deposits. See also Paleogeography; Saline evaporites.

In parts of early Cretaceous time, ice extended to sea level in the polar regions. But during most of Cretaceous time, climates were in the hothouse or greenhouse mode, showing lower latitudinal temperature gradients. Tropical climates may have been much like present ones, and paratropical deserts existed as they do now, but terrestrial floras and faunas suggest that nearly frost-free climates extended to the polar circles as did abundant rainfall, and no ice sheets appear to have reached sea level.

On land, flowering plants (angiosperms) first appeared in early Cretaceous time, as opportunistic plants in marginal settings, and then spread to the understory of woodlands, replacing cycads and ferns. In late Cretaceous time, evergreen angiosperms, including palms, thus came to dominate the tropical rainforests. Evergreen conifers maintained dominance in the drier midlatitude settings, while in the moist higher latitudes forests of broad-leaved deciduous trees dominated. Insects became highly diverse, and many modern families have their roots in the Cretaceous. Amphibians and small reptiles were present. Larger land animals included crocodiles and crocodilelike reptiles, turtles, and dinosaurs. The mammals remained comparatively minor elements in the Cretaceous faunas. In early Cretaceous time, egg-laying and marsupial mammals were joined by placentals, but Cretaceous mammals were in general small, and lack of color vision in most modern mammalians suggests a nocturnal ancestry and a furtive existence in a dinosaurian world. Birds had arisen, from dinosaurs in Jurassic time, but their fossil record from the Cretaceous is poor and largely one of water birds. More common are the remains of flying reptiles, the pterosaurs. See also Dinosaur; Mammalia; Marsupialia; Pterosauria.

About 65 million years ago, during the reversed magnetic interval known as chron 29R, a collision with an asteroid or comet showered the entire Earth with impact debris, preserved in many places as a thin “boundary clay” enriched in the trace element iridium. This event coincided with the great wave of extinctions—the K/T crisis—which serve to bound the Cretaceous (Kreide) Period against the Tertiary.

Global effects of the impact must have included earthquake shock many orders of magnitude greater than any found in human history; associated land slips and tidal waves; a dust blackout of sunlight that must have taken many months to clear; a sharp drop in temperatures that would have brought frost to the tropics; changes in atmospheric and water chemistry; and disturbance of existing patterns of atmospheric and oceanic circulation. It is possible that earthquakes influenced volcanic eruptions. Different biotic communities were affected to different degrees. The pelagic community, sensitive to photosynthetic productivity, was severely struck, with coccolithophores and planktonic foraminiferans reduced to a few species, while ammonites, belemnites, plesiosaurs, and mosasaurs were eliminated. Dinoflagellates, endowed with the capacity to encyst under stress, suffered no great loss. Benthic life was only moderately damaged, excepting destruction of the reef community. While North American trees underwent far more extinction at the specific level than formerly believed, land floras escaped with little damage, presumably because they were generally equipped to handle stress by dormancy and seed survival. The plant-fodder-dependent dinosaurs perished, as did their predators and scavengers. The fresh-water community, buffered by ground water against temperature change and food-dependent mainly on terrestrial detritus, was little affected. While a great many individual organisms must have been killed by the immediate effects of the impact, the loss of species and higher taxa must have occurred on land and in shallowest waters mainly in the aftermath of darkness, chill, and starvation, and in the deeper waters in response to changed regimes in currents, temperatures, and nutrition. The Cretaceous crash led above all to an evolutionary outburst of the mammals, which in the succeeding tens of millions of years not only filled and multiplied the niches left by dinosaurs but also invaded the seas.

The youngest period of Mesozoic time stretching approximately from 136 to 65 million years BP.

The noun has one meaning:

Meaning #1: from 63 million to 135 million years ago; end of the age of reptiles; appearance of modern insects and flowering plants
  Synonym: Cretaceous period

The Cretaceous Period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic Period (i.e. from 145.5 ± 4.0 million years ago (Ma)) to the beginning of the Paleocene epoch of the Tertiary Period (about 65.5 ± 0.3 Ma). The youngest and longest geological period of the Mesozoic, the Cretaceous constitutes about 80 million years. The end of the Cretaceous defines the boundary between the Mesozoic and Cenozoic eras.

The Cretaceous (from Latin creta meaning 'chalk' ^[1]) as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin^[2] and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates, principally coccoliths), found in the upper Cretaceous of continental Europe and the British Isles (including the White Cliffs of Dover).

Mesozoic era
Triassic	Jurassic	Cretaceous

Dating

As with other older geologic periods, the rock beds that define the Cretaceous are well identified but the exact dates of the period's start and end are uncertain by a few million years. No great extinction or burst of diversity separated the Cretaceous from the Jurassic. However, the end of the period is most sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater in Yucatan and the Gulf of Mexico. This layer has been tightly dated at 65.5 Ma. This bolide collision is probably responsible for the major, extensively-studied Cretaceous–Tertiary extinction event.

Divisions

The Cretaceous is usually separated into Early and Late Cretaceous Epochs. The faunal stages from youngest to oldest are listed below; time is referred to as early or late, and the corresponding rocks are referred to as lower or upper:

Upper/Late Cretaceous
Maastrichtian	(70.6 ± 0.6 – 65.8 ± 0.3 Ma)
Campanian	(83.5 ± 0.7 – 70.6 ± 0.6 Ma)
Santonian	(85.8 ± 0.7 – 83.5 ± 0.7 Ma)
Coniacian	(89.3 ± 1.0 – 85.8 ± 0.7 Ma)
Turonian	(93.5 ± 0.8 – 89.3 ± 1.0 Ma)
Cenomanian	(99.6 ± 0.9 – 93.5 ± 0.8 Ma)
Lower/Early Cretaceous
Albian	(112.0 ± 1.0 – 99.6 ± 0.9 Ma)
Aptian	(125.0 ± 1.0 – 112.0 ± 1.0 Ma)
Barremian	(130.0 ± 1.5 – 125.0 ± 1.0 Ma)
Hauterivian	(136.4 ± 2.0 – 130.0 ± 1.5 Ma)
Valanginian	(140.2 ± 3.0 – 136.4 ± 2.0 Ma)
Berriasian	(145.5 ± 4.0 – 140.2 ± 3.0 Ma)

Paleogeography

During the Cretaceous, the late Paleozoic - early Mesozoic supercontinent of Pangaea completed its breakup into present day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin orogenies that had begun during the Jurassic continued in the North American Cordillera, as the Nevadan orogeny was followed by the Sevier and Laramide orogenies.

Geography of the US in the Late Cretaceous Period

Though Gondwana was still intact in the beginning of the Cretaceous, it broke up as South America, Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached to each other); thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. Broad shallow seas advanced across central North America (the Western Interior Seaway) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. At the peak of the Cretaceous transgression, one-third of Earth's present land area was submerged.^[3]

The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic.^[4] Mid-ocean ridge activity--or rather, the circulation of seawater through the enlarged ridges--enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for calcareous nanoplankton.^[5] These widespread carbonates and other sedimentary deposits make the Cretaceous rock record especially fine. Famous formations from North America include the rich marine fossils of Kansas's Smoky Hill Chalk Member and the terrestrial fauna of the late Cretaceous Hell Creek Formation. Other important Cretaceous exposures occur in Europe and China. In the area that is now India, massive lava beds called the Deccan Traps were erupted in the very late Cretaceous and early Paleocene.

Climate

The Berrasian epoch showed a cooling trend that had been seen in the last epoch of the Jurassic. There is evidence that snowfalls were common in the higher latitudes and the tropics became wetter than during the Triassic and Jurassic^[6]. Glaciation was however restricted to alpine glaciers on some high-latitude mountains, though seasonal snow may have existed further south.

After the end of the Berrasian, however, temperatures increased again, and these conditions were almost constant until the end of the period^[7]. This trend was due to intense volcanic activity which produced large quantities of carbon dioxide. The development of a number of mantle plumes across the widening mid-ocean ridges further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The Tethys Sea connected the tropical oceans east to west also helped warm the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole.^[8]

A very gentle temperature gradient from the equator to the poles meant weaker global winds, contributing to less upwelling and more stagnant oceans than today. This evidenced by widespread black shale deposition and frequent anoxic events.^[9] Sediment cores show that tropical sea surface temperatures may have briefly been as warm as 42 °C (107 °F), 17 °C warmer than at present, and that they averaged around 37 °C. Meanwhile deep ocean temperatures were as much as 15-20 °C higher than today's.^[10],^[11]

Life

Plants

Flowering plants (angiosperms) spread during this period, although they did not become predominant until the Campanian stage near the end of the epoch. Their evolution was aided by the appearance of bees; in fact angiosperms and insects are a good example of coevolution. The first representatives of many leafy trees, including figs, planes and magnolias, appeared in the Cretaceous. At the same time, some earlier Mesozoic gymnosperms like Conifers continued to thrive; pehuéns (Monkey Puzzle trees, Araucaria and other conifers being notably plentiful and widespread, although other gymnosperm taxa like Bennettitales died out before the end of the period.

Terrestrial fauna

Pterosaur

On land, mammals were a small and still relatively minor component of the fauna. The fauna was dominated by archosaurian reptiles, especially dinosaurs, which were at their most diverse. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they faced growing competition from the adaptive radiation of birds, and by the end of the period only two highly specialised families remained.

The Liaoning lagerstätte (Chaomidianzi formation) in China provides a glimpse of life in the Early Cretaceous, where preserved remains of numerous types of small dinosaurs, birds, and mammals have been found. The coelurosaur dinosaurs found there represent types of the group maniraptora, which is transitional between dinosaurs and birds, and are notable for the presence of hair-like feathers.

During the Cretaceous, insects began to diversify, and the oldest known ants, termites and some lepidopterans appeared. Aphids, grasshoppers, and gall wasps appeared. Numerous exceptionally preserved insects have been found in the Lower Cretaceous Siberian lagerstätte of Baissa.

Marine fauna

In the seas, rays, modern sharks and teleosts became common. Marine reptiles included ichthyosaurs in the early and middle of the Cretaceous, plesiosaurs throughout the entire period, and mosasaurs in the Late Cretaceous.

Baculites, a genus of straight-shelled form of ammonite, flourished in the seas. The Hesperornithiformes were flightless, marine diving birds that swam like grebes. Globotruncanid Foraminifera and echinoderms such as sea urchins and starfish (sea stars) thrived. The first radiation of the diatoms (generally siliceous, rather than calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the Miocene. The Cretaceous was also an important interval in the evolution of bioerosion, the production of borings and scrapings in rocks and shells (Taylor and Wilson, 2003).

Extinction

Main article: Cretaceous–Tertiary extinction event

In the extinction event that defines the end of the Cretaceous, a significant number of species (~50%) and known families (~25%) disappeared. Plants were nearly unscathed, while marine organisms were hit the hardest. These include a large number (~95%) of types of planktic foraminifers (excepting the Globigerinida), an even larger number of Coccolithophores, all the ammonite and belemnite cephalopods, and all reef-forming rudist molluscs and inoceramid clams), as well as all marine reptiles except turtles and crocodiles. Dinosaurs are the most famous victims of the Cretaceous extinction. Dinosaurs that were unique to the very end of the period (such as Tyrannosaurus rex, Triceratops, and Ankylosaurus) were wiped out. The last of the pterosaurs became extinct and the vast majority of birds did as well, including the Enantiornithes and Hesperornithiformes.

The intensive mid-Cretaceous insect extinction began during the Albian.

See also

• Chalk Formation
• List of fossil sites (with link directory)
• Western Interior Seaway

References

• Neal L Larson, Steven D Jorgensen, Robert A Farrar and Peter L Larson. Ammonites and the other Cephalopods of the Pierre Seaway. Geoscience Press, 1997.
• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
• Ovechkina, M.N. and Alekseev, A.S. 2005. Quantitative changes of calcareous nannoflora in the Saratov region (Russian Platform) during the late Maastrichtian warming event. Journal of Iberian Geology 31 (1): 149-165. PDF
• Rasnitsyn, A.P. and Quicke, D.L.J. (2002). History of Insects. Kluwer Academic Publishers. ISBN 1-4020-0026-X.  — detailed coverage of various aspects of the evolutionary history of the insects.
• Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 3rd ed. New York: John Wiley & Sons, Inc., 1995. ISBN 0-471-60618-9}
• Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6
• Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103.[1]

Notes

External links

Wikimedia Commons has media related to:

Cretaceous

• UCMP Berkeley Cretaceous page

Numerous borings in a Cretaceous cobble, Faringdon, England; these are excellent examples of fossil bioerosion.

• Bioerosion website at The College of Wooster

Cretaceous period
Lower/Early Cretaceous	Upper/Late Cretaceous
Berriasian | Valanginian | Hauterivian Barremian | Aptian | Albian	Cenomanian | Turonian | Coniacian Santonian | Campanian | Maastrichtian

zh-yue:白堊紀

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)