Editing Cretaceous (section)

=== Geologic formations ===
[[File:MosasaurusHoffmann.jpg|thumb|left|Drawing of fossil jaws of ''[[Mosasaurus|Mosasaurus hoffmanni]]'', from the [[Maastrichtian]] of [[Limburg (Netherlands)|Dutch Limburg]], by Dutch geologist [[Pieter Harting]] (1866)]]
[[File:9119 - Milano, Museo storia naturale - Scipionyx samniticus - Foto Giovanni Dall'Orto 22-Apr-2007.jpg|thumb|''[[Scipionyx]]'', a [[theropod]] dinosaur from the Early Cretaceous of Italy]]
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 [[Lake sediment|marine]] [[limestone]], a rock type that is formed under warm, shallow marine conditions. Due to the high sea level, there was extensive [[Accommodation (geology)|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 [[coccolith]]s, microscopically small [[calcite]] skeletons of [[coccolithophore]]s, a type of [[algae]] that prospered in the Cretaceous seas.

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 [[shale]]s were formed during this interval,{{sfn|Stanley|1999| pp=481–482}} such as the [[Mancos Shale]] of western North America.<ref>{{cite journal |last1=Weimar |first1=R.J. |title=Upper Cretaceous Stratigraphy, Rocky Mountain Area |journal=AAPG Bulletin |date=1960 |volume=44 |pages=1–20 |doi=10.1306/0BDA5F6F-16BD-11D7-8645000102C1865D}}</ref> These shales are an important [[source rock]] for [[Fossil fuel|oil and gas]], for example in the subsurface of the North Sea.

==== Europe ====
{{see also|:Category:Cretaceous System of Europe}}
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 [[France|French]] [[Normandy|Normandian]] coast. The [[group (stratigraphy)|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 [[Consolidation (soil)|consolidated]] and the Chalk Group still consists of loose sediments in many places. The group also has other [[limestone]]s and [[arenite]]s. Among the fossils it contains are [[sea urchin]]s, [[belemnite]]s, [[ammonite]]s and sea reptiles such as ''[[Mosasaurus]]''.

In southern Europe, the Cretaceous is usually a marine system consisting of [[Competence (geology)|competent]] limestone beds or incompetent [[marl]]s. Because the [[Alpine orogeny|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]].

==== North America ====
{{see also|:Category:Cretaceous System of North America}}
[[File:Map of North America with the Western Interior Seaway during the Campanian (Upper Cretaceous).png|thumb|Map of North America During the Late Cretaceous]]
During the Cretaceous, the present North American continent was isolated from the other continents. In the Jurassic, the North Atlantic already opened, leaving a proto-ocean between Europe and North America. From north to south across the continent, the [[Western Interior Seaway]] started forming. This inland sea separated the elevated areas of [[Laramidia]] in the west and [[Appalachia (landmass)|Appalachia]] in the east. Three dinosaur clades found in Laramidia (troodontids, therizinosaurids and oviraptorosaurs) are absent from Appalachia from the Coniacian through the Maastrichtian.<ref name=Brownstein2018>{{cite journal |last=Brownstein |first=Chase D |year=2018 |title=The biogeography and ecology of the Cretaceous non-avian dinosaurs of Appalachia |url=https://palaeo-electronica.org/content/2018/2123-appalachia-biogeography |journal=[[Palaeontologia Electronica]] |volume=21 |pages=1–56 |doi=10.26879/801|doi-access=free }}</ref>