Cretaceous
| Cretaceous | |||||||||
|---|---|---|---|---|---|---|---|---|---|
.png) A paleomap of Earth as it appeared 105 million years ago during the Early Cretaceous Epoch (Albian Age) | |||||||||
| Chronology | |||||||||
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| Etymology | |||||||||
| Name formality | Formal | ||||||||
| Usage information | |||||||||
| Celestial body | Earth | ||||||||
| Regional usage | Global (ICS) | ||||||||
| Time scale(s) used | ICS Time Scale | ||||||||
| Definition | |||||||||
| Chronological unit | Period | ||||||||
| Stratigraphic unit | System | ||||||||
| Time span formality | Formal | ||||||||
| Lower boundary definition | Not formally defined | ||||||||
| Lower boundary definition candidates |
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| Lower boundary GSSP candidate section(s) | None | ||||||||
| Upper boundary definition | Iridium-enriched layer associated with a major meteorite impact and subsequent K-Pg extinction event | ||||||||
| Upper boundary GSSP | El Kef Section, El Kef, Tunisia 36°09′13″N 8°38′55″E / 36.1537°N 8.6486°E | ||||||||
| Upper GSSP ratified | 1991 | ||||||||
The Cretaceous (IPA: /krɪˈteɪʃəs/ krih-TAY-shəss)[2] is a geological period that lasted from about 143.1 to 66 Ma (million years ago). It is the third and final period of the Mesozoic Era, as well as the longest. At around 77.1 million years, it is the ninth and longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, 'chalk', which is abundant in deposits from the latter half of the period. 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 on land, dinosaurs continued to dominate. The world was largely ice-free – although there is some evidence of brief periods of glaciation[3] during the cooler first half – and forests extended to the poles.
Many of the dominant taxonomic groups present in modern times can be ultimately traced back to origins in the Cretaceous. During this time, new groups of mammals and birds appeared, including the earliest relatives of placentals and marsupials (Eutheria and Metatheria respectively), with the earliest crown group birds appearing towards the end of the Cretaceous. Teleost fish, the most diverse group of modern vertebrates, continued to diversify during the Cretaceous with the appearance of their most diverse subgroup Acanthomorpha during this period. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.
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; it is widely thought to have been caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.
Etymology and history
[edit]The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822 as the Terrain Crétacé,[4] using strata in the Paris Basin[5] 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 the Latin creta, meaning 'chalk'.[6] The twofold division of the Cretaceous was implemented by Conybeare and Phillips in 1822. Alcide d'Orbigny in 1840 divided the French Cretaceous into five étages (stages): the Neocomian, Aptian, Albian, Turonian, and Senonian, later adding the Urgonian between Neocomian and Aptian and the Cenomanian between the Albian and Turonian.[7]
Geology
[edit]Subdivisions
[edit]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 into 12 stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use.
From youngest to oldest, the subdivisions of the Cretaceous Period are:[8]
| Epoch | Age/Stage | Start (base) | Definition | Etymology |
|---|---|---|---|---|
| (Ma) | ||||
| Paleocene | Danian | 66 | ||
| Late Cretaceous | Maastrichtian | 72.2 ± 0.2 | top: iridium anomaly at the Cretaceous–Paleogene boundary base:first occurrence of Pachydiscus neubergicus |
Maastricht Formation, Maastricht, Netherlands |
| Campanian | 83.6 ± 0.2 | base: last occurrence of Marsupites testudinarius | Champagne, France | |
| Santonian | 85.7 ± 0.2 | base: first occurrence of Cladoceramus undulatoplicatus | Saintes, France | |
| Coniacian | 89.8 ± 0.3 | base: first occurrence of Cremnoceramus rotundatus | Cognac, France | |
| Turonian | 93.9 ± 0.2 | base: first occurrence of Watinoceras devonense | Tours, France | |
| Cenomanian | 100.5 ± 0.1 | base: first occurrence of Rotalipora globotruncanoides | Cenomanum; Le Mans, France | |
| Early Cretaceous | Albian | 113.2 ± 0.3 | base: first occurrence of Praediscosphaera columnata | Aube, France |
| Aptian | 121.4 ± 0.6 | base: magnetic anomaly M0r | Apt, France | |
| Barremian | 125.77 ± 1.5 | base: first occurrence of |