DEFINITION OF CRETACEOUS

The youngest of the three periods of the Mesozoic,

during which rocks of the Cretaceous System were

deposited. It ranges from 144 million years (Ma) ago until

66.4 Ma, and it is divided into the Early and Late Epochs,

and 12 ages. The name derives from the Latin creta for chalk,

in reference to the chalky terrain of England of this age.

Pangea was dispersing during the Cretaceous, and the

volume of ridges plus apparently the rate of seafloor spreading

were dramatically increased. The consequential displacement

of seawater caused global sea levels to rise, so the Late

Cretaceous was marked by high sea levels and the deposition

of shallow water limestones in many epicontinental seas

around the world. On the North American craton, the Zuni

Sequence was deposited across wide parts of the craton during

this transgression. Increased magmatic activity in the Cretaceous

may reflect more rapid mantle convection or melting,

as marked by a number of igneous events worldwide. The

South American Cordillera and the western United States saw

unusual amounts of intrusive and volcanic activity. The giant

flood basalt provinces of Parana in South America and the

Deccan of India were formed, and kimberlite pipes punctured

the lithosphere of south Africa and Greenland. The dispersal

of Pangea was associated with the opening of the Atlantic

Ocean. Africa rotated counterclockwise away from South

America, closing the Tethys Ocean in the process of opening

the Atlantic. The closure of Tethys was associated with the

emplacement of many ophiolites onto continents, including

the giant Oman (Semail) ophiolite that was thrust to the

south onto the Arabian continental margin.

Cretaceous sedimentary patterns suggest that the climate

was warming through the period and was more varied

and seasonal than in the earlier Mesozoic. The famous Cretaceous

chalks were formed by the accumulation of tests

(exoskeletons or external skeletons) of calcareous marine

algae known as coccoliths, which thrived in the shallow

warm seas. The chalks are in many places interbedded with

fossiliferous limestones with abundant brachiopods and rudist

coral fragments.

Life on the Cretaceous continents saw the development

of the angiosperms, which became the planet’s dominant

flora by the middle of the period. Invertebrate and vertebrate

species were abundant, and they included many species of

dinosaurs, giant flying Pterosaurs, and giant marine reptiles.

Dinosaurs occupied many different geological niches, and

fossil dinosaurs are found on most continents, including herbivores,

carnivores, and omnivores. Birds had appeared,

including both flying and swimming varieties. Mammals

remained small, but their diversity increased. Many life-forms

began a dramatic and progressive disappearance toward the

end of the period. These marine and land extinctions seem to

be a result of a combination of events, including climate

change, exhalations from the massive volcanism in the Indian

Deccan South American Parana flood basalt provinces, coupled

with an impact of a 6-mile (10-km) wide meteorite that

hit the Yucatán Peninsula of Mexico. The extinctions were

not all sudden—many of the dinosaur and other genera had

gone extinct probably from climate stresses before the meteorite

hit the Yucatán Peninsula. When the impact occurred, a

thousand-mile-wide fireball erupted into the upper atmosphere,

and tsunami hundreds or thousands of feet high

washed across the Caribbean, southern North America, and

much of the Atlantic. Huge earthquakes accompanied the

explosion. The dust blown into the atmosphere immediately

initiated a dark global winter, and as the dust settled months

or years later, the extra carbon dioxide in the atmosphere

warmed the Earth for many years, forming a greenhouse condition.

Many forms of life could not tolerate these rapid

changes and perished. The end Cretaceous extinction, commonly

referred to as the K-T event, is one of the most significant

mass extinction events known in the history of life.

See also FLOOD BASALT; KIMBERLITE; PANGEA.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar