145–66 million years ago
|Mean atmospheric O|
2 content over period duration
|c. 30 vol %|
(150 % of modern level)
|Mean atmospheric CO|
2 content over period duration
|c. 1700 ppm |
(6 times pre-industrial level)
|Mean surface temperature over period duration||c. 18 °C|
(4 °C above modern level)
The Cretaceous ( // , krih-TAY-shəs) is a geological period that lasted from about 145 to 66 million years ago (mya). It is the third and final period of the Mesozoic Era. The name is derived from the Latin creta. It is usually abbreviated K, for its German translation Kreide.
The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared.
The Cretaceous (along with the Mesozoic) ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822,using strata in the Paris Basin 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.
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 originating 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 age of the system's base is 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, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico. This layer has been dated at 66.043 Ma.
A 140 Ma age for the Jurassic-Cretaceous boundary instead of the usually accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina.Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age, sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences.
From youngest to oldest, the subdivisions of the Cretaceous period are:
Maastrichtian – (66-72.1 Mya)
Campanian – (72.1-83.6 Mya)
Santonian – (83.6-86.3 Mya)
Coniacian – (86.3-89.8 Mya)
Turonian – (89.8-93.9 Mya)
Cenomanian – (93.9-100.5 Mya)
Albian – (100.5-113.0 Mya)
Aptian – (113.0-125.0 Mya)
Barremian – (125.0-129.4 Mya)
Hauterivian – (129.4-132.9 Mya)
Valanginian – (132.9-139.8 Mya)
Berriasian – (139.8-145.0 Mya)
The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms. The Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type that is formed under warm, shallow marine circumstances. Due to the high sea level, there was extensive space for such sedimentation. Because of the relatively young age and great thickness of the system, Cretaceous rocks are evident 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 leaving the deposited organic matter undecomposed. Half of the world's petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval.These shales are an important source rock for oil and gas, for example in the subsurface of the North Sea.
During the Cretaceous, the late-Paleozoic-to-early-Mesozoic supercontinent of Pangaea completed its tectonic breakup into the present-day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin mountain building (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.
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.
The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic.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. 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.
The cooling trend of the last epoch of the Jurassic continued into the first age of the Cretaceous. There is evidence that snowfalls were common in the higher latitudes and the tropics became wetter than during the Triassic and Jurassic.Glaciation was however restricted to high-latitude mountains, though seasonal snow may have existed farther from the poles. 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.
After the end of the first age, however, temperatures increased again, and these conditions were almost constant until the end of the period. °C (70-73 °F). Atmospheric CO2 and temperature relations indicate a doubling of pCO2 was accompanied by a ~0.6 °C increase in temperature. The production of large quantities of magma, variously attributed to mantle plumes or to extensional tectonics, 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 to 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.The warming may have been due to intense volcanic activity which produced large quantities of carbon dioxide. Between 70–69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric CO2 pressures with levels of 1000–1400 ppmV and mean annual temperatures in west Texas between 21 and 23
Nonetheless, there is evidence of Antarctic marine glaciation in the Turonian Age.
A very gentle temperature gradient from the equator to the poles meant weaker global winds, which drive the ocean currents, resulted in less upwelling and more stagnant oceans than today. This is evidenced by widespread black shale deposition and frequent anoxic events. 42 °C (108 °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) warmer than today's.Sediment cores show that tropical sea surface temperatures may have briefly been as warm as
Flowering plants (angiosperms) spread during this period, [ citation needed ]although they did not become predominant until the Campanian Age near the end of the period. Their evolution was aided by the appearance of bees; in fact, the development of angiosperms and insects is 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 continued to thrive, pehuéns (monkey puzzle trees, Araucaria ) and other conifers being notably plentiful and widespread. Some fern orders, such as Gleicheniales, appeared as early in the fossil record as the Cretaceous and achieved an early broad distribution.Gymnosperm taxa like Bennettitales and Hirmeriella died out before the end of the period.
On land, mammals were generally small sized, but a very relevant component of the fauna, with cimolodont multituberculates outnumbering dinosaurs in some sites.Neither true marsupials nor placentals existed until the very end, but a variety of non-marsupial metatherians and non-placental eutherians had already begun to diversify greatly, ranging as carnivores (Deltatheroida), aquatic foragers (Stagodontidae) and herbivores ( Schowalteria , Zhelestidae). Various "archaic" groups like eutriconodonts were common in the Early Cretaceous, but by the Late Cretaceous northern mammalian faunas were dominated by multituberculates and therians, with dryolestoids dominating South America.
The apex predators were 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 declined for poorly understood reasons (once thought to be due to competition with early birds, but now it is understood avian adaptive radiation is not consistent with pterosaur decline), and by the end of the period only two highly specialized families remained.
The Liaoning lagerstätte (Chaomidianzi formation) in China is a treasure chest of preserved remains of numerous types of small dinosaurs, birds and mammals, that provides a glimpse of life in the Early Cretaceous. The coelurosaur dinosaurs found there represent types of the group Maniraptora, which includes modern birds and their closest non-avian relatives, such as dromaeosaurs, oviraptorosaurs, therizinosaurs, troodontids along with other avialans. Fossils of these dinosaurs from the Liaoning lagerstätte are notable for the presence of hair-like feathers.
Insects diversified during the Cretaceous, and the oldest known ants, termites and some lepidopterans, akin to butterflies and moths, appeared. Aphids, grasshoppers and gall wasps appeared.
In the seas, rays, modern sharks and teleosts became common.Marine reptiles included ichthyosaurs in the early and mid-Cretaceous (becoming extinct during the late Cretaceous Cenomanian-Turonian anoxic event), 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 shelled, 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, hardgrounds and shells.
The impact of a large body with the Earth may have been the punctuation mark at the end of a progressive decline in biodiversity during the Maastrichtian Age of the Cretaceous Period. The result was the extinction of three-quarters of Earth's plant and animal species. The impact created the sharp break known as K–Pg boundary (formerly known as the K–T boundary). Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant ecological niches.
Despite the severity of K-Pg extinction event, there was significant variability in the rate of extinction between and within different clades. Species which depended on photosynthesis declined or became extinct as atmospheric particles blocked solar energy. 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, and all else that depended on them suffered as well. Herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, the top predators such as Tyrannosaurus rex also perished. [ citation needed ]Yet only three major groups of tetrapods disappeared completely; the non-avian dinosaurs, the plesiosaurs and the pterosaurs. The other Cretaceous groups that did not survive into the Cenozoic era, the ichthyosaurs and last remaining temnospondyls and non-mammalian cynodonts were already extinct millions of years before the event occurred.
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.
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.
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.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 seafloor. 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.
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 unfavorable, 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.
An extinction event is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.
The Jurassic is a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago (Mya) to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, and the Tithonian event at the end; neither event ranks among the "Big Five" mass extinctions, however.
The Mesozoic Era is an interval of geological time from about. It is also called the Age of Reptiles and the Age of Conifers.
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous period 298.9 million years ago (Mya), to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the following Triassic period belongs to the Mesozoic era. The concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the region of Perm in Russia.
The Paleogene is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 million years ago (Mya) to the beginning of the Neogene Period 23.03 Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The earlier term Tertiary Period was used to define the span of time now covered by the Paleogene and subsequent Neogene periods; despite no longer being recognised as a formal stratigraphic term, 'Tertiary' is still widely found in earth science literature and remains in informal use. The Paleogene is most notable for being the time during which mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period. The United States Geological Survey uses the abbreviation PE for the Paleogene, but the more commonly used abbreviation is PG with the PE being used for Paleocene.
The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 541 million years to the present, and began with the Cambrian Period when animals first developed hard shells preserved in the fossil record. The time before the Phanerozoic, called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.
The Triassic is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.9 million years ago (Mya), to the beginning of the Jurassic Period 201.3 Mya. The Triassic is the first and shortest period of the Mesozoic Era. Both the start and end of the period are marked by major extinction events. The Triassic period is subdivided into three epochs: Early Triassic, Middle Triassic and Late Triassic.
The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods,, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas, a whole class (conodonts) and 23–34% of marine genera disappeared. On land, all archosaurs other than crocodylomorphs and Avemetatarsalia, some remaining therapsids, and many of the large amphibians became extinct. Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
Plesiosauroidea is an extinct clade of carnivorous marine reptiles. They have the snake-like longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceous periods. After their discovery, some plesiosauroids were said to have resembled "a snake threaded through the shell of a turtle", although they had no shell.
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series. The Cretaceous is named after the white limestone known as chalk which occurs widely in northern France and is seen in the white cliffs of south-eastern England, and which dates from this time.
The South Polar region of the Cretaceous comprised the continent of East Gondwana–modern day Australia and Antarctica–a product of the break-up of Gondwana. The southern region, during this time, was much warmer than it is today, ranging from perhaps 4–8 °C (39–46 °F) in the latest Cretaceous Maastrichtian in what is now southeastern Australia. This prevented permanent ice sheets from developing and fostered polar forests, which were largely dominated by conifers, cycads, and ferns, and relied on a temperate climate and heavy rainfall. Major fossil-bearing geological formations that record this area are: the Santa Marta and Sobral Formations of Seymour Island off the Antarctic Peninsula; the Snow Hill Island, Lopez de Bertodano, and the Hidden Lake Formations on James Ross Island also off the Antarctic Peninsula; and the Eumeralla and Wonthaggi Formations in Australia.
Oceanic anoxic events or anoxic events (anoxia conditions) were intervals in the Earth's past where portions of oceans became depleted in oxygen (O2) at depths over a large geographic area. During some of these events, euxinia, waters that contained hydrogen sulfide, H
2S, developed. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia".
The Maastrichtian is, in the ICS geologic timescale, the latest age of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned the interval from. The Maastrichtian was preceded by the Campanian and succeeded by the Danian.
Dallas–Fort Worth sits above Cretaceous-age strata ranging from ≈145-66 Ma. These Cretaceous-aged sediments lie above the eroded Ouachita Mountains and the Fort Worth Basin, which was formed by the Ouachita Orogeny. Going from west to east in the DFW Metroplex and down towards the Gulf of Mexico, the strata get progressively younger. The Cretaceous sediments dip very gently to the east.
The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K-T) boundary, is a geological signature, usually a thin band of rock. K, the first letter of the German word Kreide (chalk), is the traditional abbreviation for the Cretaceous Period and Pg is the abbreviation for the Paleogene Period.
The Paleocene, or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek palæo- meaning "old" and the Eocene Epoch, translating to "the old part of the Eocene".
The Niobrara Formation, also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member overlying the Fort Hays Limestone Member. The chalk formed from the accumulation of coccoliths from microorganisms living in what was once the Western Interior Seaway, an inland sea that divided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains of the US and Canada. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, and pterosaurs as well as several primitive aquatic birds. The type locality for the Niobrara Chalk is Knox County in northeastern Nebraska.
The Tropic Shale is a Mesozoic geologic formation. Dinosaur remains are among the fossils that have been recovered from the formation, including Nothronychus graffami. The Tropic Shale is a stratigraphic unit of the Kaiparowits Plateau of south central Utah. The Tropic Shale was first named in 1931 after the town of Tropic where the Type section is located. The Tropic Shale outcrops in Kane and Garfield counties, with large sections of exposure found in the Grand Staircase-Escalante National Monument.
The prehistory of the United States comprises the occurrences within regions now part of the United States of America during the interval of time spanning from the formation of the Earth to the documentation of local history in written form. At the start of the Paleozoic era, what is now "North" America was actually in the southern hemisphere. Marine life flourished in the country's many seas, although terrestrial life had not yet evolved. During the latter part of the Paleozoic, seas were largely replaced by swamps home to amphibians and early reptiles. When the continents had assembled into Pangaea drier conditions prevailed. The evolutionary precursors to mammals dominated the country until a mass extinction event ended their reign.
The Cretaceous–Paleogene (K–Pg) extinction event, also known as the Cretaceous–Tertiary(K–T)extinction, was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the exception of some ectothermic species such as the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms survived. It marked the end of the Cretaceous period, and with it the end of the entire Mesozoic Era, opening the Cenozoic Era that continues today.
Si logramos publicar esos nuevos resultados, sería el primer paso para cambiar formalmente la edad del Jurásico-Cretácico. A partir de ahí, la Unión Internacional de la Ciencias Geológicas y la Comisión Internacional de Estratigrafía certificaría o no, depende de los resultados, ese cambio.
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