The Cretaceous Period is the most recent period of the Mesozoic Era, spanning 77 million years, from 142 million to 65 million years ago. In 1822, Omalius d'Halloy termed the chalky rocks (Latin: "creta") found on the English and French sides of the English Channel "Cretaceous." The name is now applied to all materials deposited above Jurassic Period rocks but below Tertiary Period rocks. During the Cretaceous more land was submerged than at any time since the Ordovician Period. Tectonic plates moved apart, causing major mountain building, and continents began to resemble those existing today.

The Cretaceous System is subdivided into the following stages from youngest to oldest: Upper Cretaceous includes Maastrichtian, Campanian, Santonian, Coniacian, Turonian, and Cenomanian; Lower Cretaceous includes Albian, Aptian, Barremian, Hauterivian, Valanginian, and Berriasian. These names, based on type localities where rocks at a given stratigraphic level were first studied, are widely used to refer to rock units or chronological times.

Rock Locations
Cretaceous rocks make up nearly 29% of the total area of Phanerozoic deposits on the Earth's landmasses. Because of the great extent of their outcrop area and presence in drill cores, Cretaceous rocks are the most intensively studied part of the Phanerozoic rock column.

At one time Cretaceous seas covered 50% of the present North American continent. Thick Cretaceous sedimentary deposits form a narrow belt of outcrops from British Columbia to Central America. They extend throughout the Rocky Mountains and western Plains states, around the north edge of the Mississippi embayment, and along the southern and eastern sides of the Appalachian Mountains. Volcanoes in the western United States spread layers of volcanic ash, now turned into bentonite, in the center of the continent. Cretaceous rocks crop out on several Caribbean islands. This group extends along the Andes Mountain belt from Colombia to Cape Horn, and onto Antarctica. Cretaceous outcrops cover parts of central and southeastern South America. A vast volcanic province in the Paraná River basin of Uruguay and Brazil is mirrored across the Atlantic Ocean in the Etendeka province of western Africa, regions that were joined before South America and Africa separated.

In Europe, Cretaceous chalk also crops out around the Paris basin. Other deposits are in Denmark, north and central Germany, central and southern France, the Pyrenees Mountains, the Alps, Italy, Slovenia, coastal Croatia, Bosnia and Hercegovina, and Yugoslavia. Russia and the southwestern Asian republics have Cretaceous rocks. There are Cretaceous rocks in west Africa and Mozambique. India's Cretaceous deposits include the Deccan traps, a large, thick sequence of volcanic rocks east of Bombay. Cretaceous rocks crop out in Thailand, Borneo, Japan, Australia, and New Zealand.

In eastern North America the continental border gradually sank, shifting the shoreline inland. Shallow seas spread over the interiors of North America, western Europe, eastern Europe, western Russia, and the northern Arctic. They occupied central Australia and eastern Africa, eventually covering one-third of the present-day landmasses. Tectonic plates moved apart after the breakup of Pangaea in the middle Mesozoic. Previously, the drift of Africa relative to Europe had created the Tethys Sea, an ancestral Mediterranean Sea, with many island arcs, basins, and microcontinental fragments. It reached its maximum width in early Cretaceous time. Movements of Africa, Europe, and the small Adriatic tectonic plate caused subduction that consumed Tethys, while seafloor spreading led to widening of the North Atlantic Ocean.

Europe remained connected to North America until 81 million years ago; Greenland separated from Europe 70 million years ago. As the Adriatic plate carrying Italy closed Tethys and collided with Austria and Switzerland, the Alps rose. The Alpine region also marked a subduction zone. The Mediterranean began to open to the south. Mountain building also took place for the Dinaric and Hellenic Alps during collisions of the Carpathian-Serbo-Macedonian and Apulian arcs, in Saudi Arabia and Oman, and in the Himalayas.

Seafloor spreading in the Atlantic led to westward movement of the North American plate. Compression around the Pacific Ocean margin started the Cordilleran and Laramide orogenies and began the creation of the present-day Rockies, Sierra Nevada, California Coast Ranges, and Andes, mountain belts that extend the length of North and South America. Enormous amounts of granite formed within the crust as subduction carried continental sediments deep within the crust around the Pacific margin and between Africa and Europe and melted. This led to volcanism in many subduction zones. Mountain building also affected Japan and the Philippines.

Marine invertebrates included many modern-looking pelecypods and gastropods. Widespread ammonite fossils have helped define Cretaceous stratigraphy. The shells, or tests, of abundant foraminifera created the chalk deposits that gave the period its name. Extensive coral reefs grew in the equatorial belt. Dinosaurs ruled the land, with sauropods, tyrannosaurs, duck-billed and horned dinosaurs prominent. Pterosaurs and toothed birds flew in the skies, and mosasaurs appeared in the seas. Primitive mammals included insectivorous marsupials. Deciduous trees and other angiosperms created modern-looking forests by the middle of the Cretaceous Period.

The End of the Cretaceous
A great extinction marked the end of the Cretaceous Period. The dinosaurs, ammonites, and many marine creatures abruptly disappeared. High concentrations of iridium and textural features of the boundary clay layer separating Cretaceous and Tertiary rocks support the idea of a meteorite impact producing a worldwide layer of debris at essentially the same moment at the very end of the Cretaceous Period. The huge Chicxulub Crater off the Yucatán Peninsula of Mexico is the likely point of impact. Some researchers favor a volcanic catastrophe or other series of events and a more gradual extinction of species and genera, but the impact theory has gained wide acceptance.

William D. Romey

Bibliography: Dott, R. H., Jr., and Prothero, D. R., Evolution of the Earth, 5th ed. (1993); Hsu, K. J., ed., Mesozoic and Cenozoic Oceans (1986); Moullade, M., and Nairn, A., eds., The Phanerozoic Geology of the World: Vol. 2, The Mesozoic (1983).