Cretaceous

Cretaceous
Name	Cretaceous
Caption	Reconstruction of terrestrial ecosystems during the late Cretaceous
Period	Mesozoic
Time start	145.0
Time end	66.0
Major events	Formation of chalk deposits; breakup of Pangaea; end-Cretaceous mass extinction

Cretaceous

The Cretaceous was the final period of the Mesozoic Era, spanning from about 145 million to 66 million years ago, and concluded with one of Earth's major mass extinctions. It witnessed dramatic developments in biodiversity, continental arrangement, and sedimentation that shaped subsequent Paleogene landscapes and faunas, influencing the trajectory of groups discussed in works by Charles Darwin and explored in modern syntheses like publications from the Royal Society and the Geological Society of America.

Definition and Timeframe

The Cretaceous is formally defined in the international stratigraphic charts maintained by the International Commission on Stratigraphy and subdivided into Early and Late epochs with stage names such as the Berriasian, Valanginian, Aptian, Albian, Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian. Chronostratigraphic boundaries were calibrated using radiometric dates from volcanic ash beds analyzed by laboratories associated with institutions like the United States Geological Survey and the Max Planck Society, and correlated to biostratigraphic markers such as ammonite zones recognized in studies by researchers from the British Geological Survey and the Smithsonian Institution.

Geology and Paleoenvironments

Extensive chalk and limestone deposits characterize marine sequences preserved on continental shelves studied near locales like the White Cliffs of Dover and the Sierra Nevada (United States), reflecting widespread carbonate sedimentation under conditions investigated by teams from universities including Harvard University and the University of Cambridge. Continental deposits composed of fluvial, lacustrine, and floodplain sediments occur in basins such as the Western Interior Seaway, the Neuquén Basin, the Gobi Desert basins, the Iberian Basin, and the Shaanxi Basin, each yielding vertebrate remains curated in museums like the American Museum of Natural History and the Natural History Museum, London. Hydrocarbon-bearing sequences formed in rift basins documented by geologists at the Norwegian Petroleum Directorate and the Petroleum Museum reflect active sedimentary processes tied to paleogeographic change.

Climate and Sea-Level Changes

Global climate during the period trended warmer than present with reduced polar ice documented by oxygen isotope studies conducted by researchers at the Lamont–Doherty Earth Observatory and the Scripps Institution of Oceanography, while episodic cooling and greenhouse fluctuations appear in records from the Antarctic Peninsula and the Greenland Ice Sheet Project proxies. High global sea levels produced epicontinental seas such as the Western Interior Seaway that split continents and are mapped in synthesis volumes from the University of Texas and the Canadian Journal of Earth Sciences. Ocean anoxic events and repeated transgressions and regressions are recorded in organic-rich shales and black shales studied in localities like the Kashmir Basin and the South China Sea, with isotope excursions interpreted via collaborations involving the California Institute of Technology and the University of Tokyo.

Flora and Fauna

Terrestrial vegetation shifted markedly with the rise and diversification of angiosperms documented in floral assemblages from the Yixian Formation, the Liaoning Province, the Hell Creek Formation, and the Santana Formation, studied by botanists affiliated with the Royal Botanic Gardens, Kew and the Missouri Botanical Garden. Gymnosperms, ferns, cycads, and bennettitaleans persisted alongside early flowering plants that influenced herbivore evolution recorded in vertebrate collections at the Field Museum and the National Museum of Natural History (France). Vertebrate faunas included diverse dinosaurs such as hadrosaurs, ceratopsians, tyrannosaurids, and titanosaurians described in monographs from the American Museum of Natural History and field reports from expeditions financed by the National Geographic Society. Marine ecosystems hosted mosasaurs, plesiosaurs, diverse teleost fishes, ammonites, and rudists, with fossil material studied by researchers at the Smithsonian Institution and the Natural History Museum, London; insect diversification and early pollinator relationships are recorded in amber deposits curated by the Hungarian Natural History Museum and the Royal Belgian Institute of Natural Sciences.

Tectonics and Paleogeography

The breakup of Pangaea continued through rifting and seafloor spreading that formed oceanic basins such as the North Atlantic Ocean and influenced the opening of the South Atlantic Ocean, reconstructed in plate-tectonic models by scientists at the Potsdam Institute for Climate Impact Research and the Geological Survey of Brazil. Continental drift produced isolated provinces like Madagascar, India, and Antarctica with distinctive endemic faunas noted in faunal studies published by the Indian Statistical Institute and the South African Museum. Mountain-building events tied to subduction zones affected regions including the Andes, the Alps, and the Rocky Mountains with sediment dispersal patterns documented by research groups at the University of Buenos Aires and the University of Zurich.

Biotic Events and Extinctions

Biotic turnovers during the period include multiple regional extinctions, faunal radiations, and the terminal mass extinction at the Cretaceous–Paleogene boundary preserved in a global iridium anomaly investigated by teams tied to the Carnegie Institution for Science and the University of California, Berkeley. The bolide impact hypothesis centered on the Chicxulub crater was advanced by collaborative work involving the Geological Society of America and drilling campaigns funded by the International Ocean Discovery Program, while alternative and complementary mechanisms such as Deccan Traps volcanism were evaluated by volcanologists at the Indian Institute of Science and the Geological Survey of India. Post-boundary studies by researchers at the University of Copenhagen and the National Museum of Natural History (France) trace ecological recovery and the reshaping of terrestrial and marine ecosystems leading into the Paleogene.

Category:Geologic periods