Cretaceous 1
Cretaceous
Cretaceous Period
145.5–65.5 million years ago
Mean atmospheric O2 content over period duration ca. 30 Vol %[1]
(150 % of modern level)
Mean atmospheric CO2 content over period duration ca. 1700 ppm[2]
(6 times pre-industrial level)
Mean surface temperature over period duration ca. 18 °C[3]
(4 °C above modern level)
The Cretaceous ( /krɪˈteɪʃəs/), derived from the Latin "creta" (chalk), usually abbreviated K for its German
translation Kreide (chalk), is a geologic period and system from circa unknown operator: u'cretaceousround' [4]
± 4 to unknown operator: u'paleoceneround' [5] ± 0.3 million years (Ma) ago. In the geologic timescale, the
Cretaceous follows the Jurassic Period and is followed by the Paleogene Period of the Cenozoic Era. It is the
youngest period of the Mesozoic Era, and at 80 million years long, the longest period of the Phanerozoic Eon. The
end of the Cretaceous defines the boundary between the Mesozoic and Cenozoic eras. In many languages this period
is known as "chalk period".
The Cretaceous was a period with a relatively warm climate and high eustatic sea level. The oceans and seas were
populated with now extinct marine reptiles, ammonites and rudists; and the land by dinosaurs. At the same time, new
groups of mammals and birds as well as flowering plants appeared. The Cretaceous ended with one of the largest
mass extinctions in Earth history, the K-T extinction, when many species, including non-avian dinosaurs, pterosaurs,
and large marine reptiles, disappeared.
Although the first representatives of leafy trees and true grasses
emerged in the Cretaceous, the flora was still dominated by conifers like
Auracaria (Here: Modern Araucaria araucana in Chile).
The Cretaceous world
Paleogeography
During the Cretaceous, the
late-Paleozoic-to-early-Mesozoic supercontinent of
Pangaea completed its tectonic 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.
Cretaceous 2
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.[6]
The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period
in the Phanerozoic.[7] 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.[8] 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 (e.g., the Weald) and China (the Yixian Formation). 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 Berriasian 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.[9] Glaciation was however restricted to alpine glaciers on some high-latitude mountains, though seasonal
snow may have existed farther south. Rafting by ice of stones into marine environments occurred during much of the
Cretaceous but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga
Basin in southern Australia.[10] [11]
After the end of the Berriasian, however, temperatures increased again, and these conditions were almost constant
until the end of the period.[9] This trend was due to intense volcanic activity which produced large quantities of
carbon dioxide. The production of large quantities of magma, variously attributed to mantle plumes or to extensional
tectonics,[12] further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas.
The Tethys Sea connecting the tropical oceans east to west also helped in warming 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.[13]
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 is evidenced by widespread black shale deposition and frequent
anoxic events.[14] Sediment cores show that tropical sea surface temperatures may have briefly been as warm as 42
°C (107 °F), 17 °C (31 °F) warmer than at present, and that they averaged around 37 °C (99 °F). Meanwhile deep
ocean temperatures were as much as 15 to 20 °C (27 to 36 °F) higher than today's.[15] [16]
Cretaceous 3
Geology
Key events in the Cretaceous
view • discuss •
Maastrichtian
Campanian
Santonian
Coniacian
Turonian
Cenomanian
Albian
Aptian
Barremian
Hauterivian
Valanginian
Berriasian
Jurassic
Paleogene
Mesozoic
Cenozoic
Cretaceous 4
An approximate timescale of key Cretaceous events.
Axis scale: millions of years ago.
Research history
The Cretaceous as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using
strata in the Paris Basin[17] 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 western Europe. The name Cretaceous
was derived from Latin creta, meaning chalk.[18] The name of the island Crete has the same origin.
Stratigraphic subdivisions
The Cretaceous is divided into Early and Late Cretaceous epochs or Lower and Upper Cretaceous series. In older
literature the Cretaceous is sometimes divided into three series: Neocomian (lower/early), Gallic (middle) and
Senonian (upper/late). A subdivision in eleven stages, all origining from European stratigraphy, is now used
worldwide. In many parts of the world, alternative local subdivisions are still in use.
As with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact ages of the
system's top and base are uncertain by a few million years. No great extinction or burst of diversity separates the
Cretaceous from the Jurassic. However, the top of the system is 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.[19]
Rock formations
Drawing of fossil jaws of Mosasaurus hoffmanni,
from the Maastrichtian of Dutch Limburg, by
Dutch geologist Pieter Harting (1866).
The high eustatic sea level and warm climate of the Cretaceous meant
a large area of the continents was covered by warm shallow seas. The
Cretaceous was named for the extensive chalk deposits of this age in
Europe, but in many parts of the world, the Cretaceous system consists
for a major part of marine limestone, a rock type that is formed under
warm, shallow marine circumstances. Due to the high sea level there
was extensive accommodation space for sedimentation so that thick
deposits could form. Because of the relatively young age and great
thickness of the system, Cretaceous rocks crop out in many areas
worldwide.
Chalk is a rock type characteristic for (but not restricted to) the
Cretaceous. It consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that
prospered in the Cretaceous seas.
In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which
forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast. The
group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of
the southern part of the North Sea. Chalk is not easily consolidated and the Chalk Group still consists of loose
sediments in many places. The group also has other limestones and arenites. Among the fossils it contains are sea
urchins, belemnites, ammonites and sea reptiles such as Mosasaurus.
In southern Europe, the Cretaceous is usually a marine system consisting of competent limestone beds or
incompetent marls. Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on
the southern edge of the European continental shelf, at the margin of the Tethys Ocean.
Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water. In many
places around the world, dark anoxic shales were formed during this interval.[20] These shales are an important
Cretaceous 5
source rock for oil and gas, for example in the subsurface of the North Sea.
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. Some fern orders such as Gleicheniales[21] appeared as early in the fossil record as the Cretaceous,
and achieved an early broad distribution. Gymnosperm taxa like Bennettitales died out before the end of the period.
Terrestrial fauna
Tyrannosaurus rex, one of
the largest land predators
of all time, lived during
the late Cretaceous.
Up to 2 m-long
Velociraptor was
likely feathered
and roamed the
late Cretaceous.
Triceratops is one of the most
recognizable genera of the
Cretaceous.
Mammals were a minor part of
the Cretacean fauna with Eomaia
being the first Eutherian.
A pterosaur, Anhanguera
piscator
On land, mammals were a small and still relatively minor component of the fauna. Early marsupial mammals
evolved in the Early Cretaceous, with true placentals emerging in the Late Cretaceous period. The fauna was
dominated by archosaurian reptiles, especially dinosaurs, which were at their most diverse stage. 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 specialized 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, akin
to butterflies and moths, appeared. Aphids, grasshoppers, and gall wasps appeared.[22]
Cretaceous 6
Marine fauna
A scene from the early
Cretaceous: A
Woolungasaurus is
attacked by a
Kronosaurus.
Tylosaurus
was the
largest
known
Mosasaurus,
carnivorous
marine
reptiles that
emerged in
the late
Cretaceous.
Strong-swimming and toothed
predatory waterbird Hesperornis
roamed late Cretacean oceans.
Discoscaphites
iris, Owl Creek
Formation (Upper
Cretaceous),
Ripley,
Mississippi.
A plate with Nematonotus sp. ,
Pseudostacus sp., and a partial
Dercetis triqueter from
Cretaceous found in Hakel,
Lebanon
In the seas, rays, modern sharks and teleosts became common.[23] Marine reptiles included ichthyosaurs in the early
and middle of the Cretaceous, becoming extinct during the late Cretaceous, plesiosaurs throughout the entire period,
and mosasaurs appearing in the Late Cretaceous.
Baculites, an ammonite genus with a straight shell, flourished in the seas along with reef-building rudist clams. 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.[22] The Cretaceous was also an important interval in the evolution of bioerosion, the production of
borings and scrapings in rocks, hardgrounds and shells (Taylor and Wilson, 2003).
Cretaceous 7
Extinction
The impact of a meteorite or comet is today
widely accepted as the main reason for the
Cretaceous-Tertiary extinction event.
There was a progressive decline in biodiversity during the
Maastrichtian stage of the Cretaceous Period prior to the suggested
ecological crisis induced by events at the K–T boundary. Furthermore,
biodiversity required a substantial amount of time to recover from the
K-T event, despite the probable existence of an abundance of vacant
ecological niches.[24]
Despite the severity of this boundary event, there was significant
variability in the rate of extinction between and within different clades.
Species which depended on photosynthesis declined or became extinct
because of the reduction in solar energy reaching the Earth's surface
due to atmospheric particles blocking the sunlight. As is the case today,
photosynthesizing organisms, such as phytoplankton and land plants, formed the primary part of the food chain in
the late Cretaceous. Evidence suggests that herbivorous animals, which depended on plants and plankton as their
food, died out as their food sources became scarce; consequently, top predators such as Tyrannosaurus rex also
perished.[25]
Coccolithophorids and molluscs, including ammonites, rudists, freshwater snails and mussels, as well as organisms
whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought
that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the
boundary.[26]
Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased
availability of their food sources. At the end of the Cretaceous there seem to have been no purely herbivorous or
carnivorous mammals. Mammals and birds which survived the extinction fed on insects, larvae, worms, and snails,
which in turn fed on dead plant and animal matter. Scientists theorise that these organisms survived the collapse of
plant-based food chains because they fed on detritus.[27] [24] [28]
In stream communities, few groups of animals became extinct. Stream communities rely less on food from living
plants and more on detritus that washes in from land. This particular ecological niche buffered them from
extinction.[29] Similar, but more complex patterns have been found in the oceans. Extinction was more severe among
animals living in the water column, than among animals living on or in the sea floor. Animals in the water column
are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the
ocean floor feed on detritus or can switch to detritus feeding.[24]
The largest air-breathing survivors of the event, crocodilians and champsosaurs, were semi-aquatic and had access to
detritus. Modern crocodilians can live as scavengers and can survive for months without food and go into
hibernation when conditions are unfavourable, and their young are small, grow slowly, and feed largely on
invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been
linked to crocodilian survival at the end of the Cretaceous.[27]
Cretaceous 8
Numerous borings in a
Cretaceous cobble,
Faringdon, England; these are
excellent examples of fossil
bioerosion.
Cretaceous hardground from
Texas with encrusting oysters
and borings. The scale bar is
10 mm.
Rudist bivalves from the
Cretaceous of the Omani
Mountains, United Arab
Emirates. Scale bar is 10 mm.
Inoceramus from the
Cretaceous of South
Dakota.
Notes
[1] Image:Sauerstoffgehalt-1000mj.svg
[2] Image:Phanerozoic Carbon Dioxide.png
[3] Image:All palaeotemps.png
[4] http:/ / toolserver. org/ ~verisimilus/ Timeline/ Timeline. php?Ma=cretaceous
[5] http:/ / toolserver. org/ ~verisimilus/ Timeline/ Timeline. php?Ma=paleocene
[6] Dougal Dixon et al., Atlas of Life on Earth, (New York: Barnes & Noble Books, 2001), p. 215.
[7] Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 280
[8] Stanley, pp. 279–81
[9] The Berriasian Age (http:/ / www. palaeos. com/ Mesozoic/ Cretaceous/ Berriasian. html)
[10] Alley, N.F. and Frakes, L.A. 2003. "First known Cretaceous glaciation: Livingston Tillite, South Australia". Australian Journal of Earth
Science 50:134–150.
[11] Frakes, L.A. and Francis, J. E. 1988. "A guide to Phanerozoic cold climates from high latitude ice rafting in the Cretaceous". Nature
333:547–549.
[12] Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy (http:/ / www. wiley. com/ WileyCDA/ WileyTitle/
productCd-1405161485. html). Wiley-Blackwell. ISBN 978-1-4051-6148-0. .
[13] Stanley, pp. 480–2
[14] Stanley, pp. 481–2
[15] "Warmer than a Hot Tub: Atlantic Ocean Temperatures Much Higher in the Past" (http:/ / www. physorg. com/ news10978. html|)
PhysOrg.com. Retrieved 12/3/06.
[16] 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 0-471-59549-7. p. 557
[17] (in Russian) Great Soviet Encyclopedia (3rd ed.). Moscow: Sovetskaya Enciklopediya. 1974. vol. 16, p. 50.
[18] Glossary of Geology (3rd ed.). Washington, D.C.: American Geological Institute. 1972. pp. 165.
[19] The official geologic timescale of the ICS (in 2008) gives 65.5 Ma as upper boundary of the Cretaceous, new callibrations by Kuiper et al.
(2008) yield 65.9 Ma
[20] See Stanley (1999), pp. 481–482
[21] C.Michael Hogan. 2010. Fern. Encyclopedia of Earth. National council for Science and the Environment (http:/ / www. eoearth. org/ article/
Fern). Washington, DC
[22] http:/ / www. ucmp. berkeley. edu/ mesozoic/ cretaceous/ cretlife. html
[23] http:/ / www. talkorigins. org/ origins/ geo_timeline. html
[24] MacLeod, N, Rawson, PF, Forey, PL, Banner, FT, Boudagher-Fadel, MK, Bown, PR, Burnett, JA, Chambers, P, Culver, S, Evans, SE,
Jeffery,
本文档为【白垩纪Cretaceous】,请使用软件OFFICE或WPS软件打开。作品中的文字与图均可以修改和编辑,
图片更改请在作品中右键图片并更换,文字修改请直接点击文字进行修改,也可以新增和删除文档中的内容。
该文档来自用户分享,如有侵权行为请发邮件ishare@vip.sina.com联系网站客服,我们会及时删除。
[版权声明] 本站所有资料为用户分享产生,若发现您的权利被侵害,请联系客服邮件isharekefu@iask.cn,我们尽快处理。
本作品所展示的图片、画像、字体、音乐的版权可能需版权方额外授权,请谨慎使用。
网站提供的党政主题相关内容(国旗、国徽、党徽..)目的在于配合国家政策宣传,仅限个人学习分享使用,禁止用于任何广告和商用目的。