The Cretaceous as a separate period was first defined by Belgian geologist
Jean d'Omalius d'Halloy in 1822,
strata in the
 and named for the extensive beds of
calcium carbonate deposited by the shells of marine
coccoliths), found in the upper Cretaceous of
 The name Cretaceous was derived from
Latin creta, meaning chalk.
The Cretaceous is divided into
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
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
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
Normandian coast. The
group is found in England, northern France, the
low countries, northern
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
arenites. Among the fossils it contains are
ammonites and sea reptiles such as
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
Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and 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.