Cretaceous System

Cretaceous System
Name	Cretaceous System
Period	Cretaceous
Time start	145.0
Time end	66.0
Chronology	Mesozoic Era
Used by	International Commission on Stratigraphy

Cretaceous System The Cretaceous System is the stratigraphic rock record corresponding to the Cretaceous Period within the Mesozoic Era, preserving sedimentary, igneous, and metamorphic deposits that document major tectonic, climatic, and biological developments between ~145 and 66 million years ago. It underpins correlations between regional stratigraphies such as the Western Interior Seaway, the Chalk Group, and the Gulf Coast, and informs interpretations used by organizations including the International Commission on Stratigraphy, the United States Geological Survey, and the Geological Survey of Canada.

Definition and Chronostratigraphy

The System is formally defined by chronostratigraphic boundaries tied to Global Boundary Stratotype Sections and Points ratified by the International Commission on Stratigraphy and calibrated with magnetostratigraphy, biostratigraphy, and radiometric ages from sites like GSSP localities in sections correlated to ammonite and foraminiferal turnover events. The base of the System overlies the Jurassic and is succeeded by the Paleogene, with stage names such as the Berriasian, Aptian, Cenomanian, and Maastrichtian used in global correlation. High-resolution correlation employs data from volcanic ash beds dated with U-Pb zircon geochronology, integrated with marine microfossil zonations developed from sections in the Tethys Sea, the North Atlantic, and the Western Interior Basin.

Subdivision and Regional Stages

Formal subdivision divides the System into Early and Late series and into stages used internationally (e.g., Valanginian, Albian, Turonian). Regional stage schemes—such as the British Chalk Group framework, the Russian Santonian correlations, and the South American Andean sequences—link to local lithostratigraphic units like the Wealden Group, the Burgess Shale-age comparisons, and the Dakhla Formation. Stratigraphers reconcile terms from the European Commission for Stratigraphy, the Chinese Academy of Sciences, and the Instituto Geológico y Minero de España to create composite chronologies used in basin studies across the North Sea, the Mediterranean, and the Amazon Basin.

Paleogeography and Plate Tectonics

During the System, breakup of the supercontinent Pangaea continued with seafloor spreading between fragments such as Laurasia and Gondwana, creating oceanic gateways like the proto-Atlantic Ocean and modifications to the Tethys Sea. Major plate motions involving the Farallon Plate, the Nazca Plate precursor, and the Indian Plate produced orogens referenced in provenance studies of the Andes, the Alps, and the Himalaya pre-collision basins. Marine transgressions created epicontinental seas including the Western Interior Seaway in North America and extensive platform carbonates such as the Chalk Group of Europe, while rift systems related to the Central Atlantic Magmatic Province influenced basin development and volcanic sequences mapped by the Geological Society of America.

Climate and Oceanography

Climate reconstructions for the System invoke greenhouse conditions with elevated atmospheric CO2 levels inferred from proxies tied to the Keeling Curve-era methodologies and isotopic data from foraminifera in cores recovered by the Deep Sea Drilling Project and Integrated Ocean Drilling Program. Oceanographic changes included oxygen minimum zone expansions, anoxic events like Oceanic Anoxic Event 2 linked to productivity spikes and volcanism from large igneous provinces such as the Ontong Java Plateau, and shifts in thermohaline circulation comparable to later changes studied by NOAA and NASA paleoclimate models. Sea-level highstands inundated continental interiors, influenced sedimentation in basins studied by the American Association of Petroleum Geologists.

Biota and Evolutionary Trends

The System witnessed diversification and turnover across marine and terrestrial clades: flowering plants (Angiosperms) rose to prominence in floras sampled in the Dakota Formation and Wadi Al-Hitan-region deposits, while dinosaur lineages including Tyrannosaurus, Triceratops, and ornithopods dominated many terrestrial ecosystems recorded in the Hell Creek Formation and the Nemegt Formation. Marine faunas included prolific ammonites, belemnites, and teleost fishes documented in the Gault Clay and Morrison Formation comparisons, and the evolution of modern groups such as eusuchian crocodylians and early Neornithes birds is preserved in Konservat-Lagerstätten described by researchers at institutions like the Natural History Museum, London and the Smithsonian Institution. Biogeographic patterns were influenced by continental fragmentation, island archipelagos like those inferred in the Iberian Plate setting, and dispersal events tracked using vertebrate faunas from the Gobi Desert.

Major Events and Extinction

Key events include multiple oceanic anoxic intervals, pulses of volcanism tied to large igneous provinces such as the Deccan Traps and the Carpathian magmatic episodes, and the terminal mass extinction at the System’s upper boundary coeval with the Chicxulub impact structure and the K–Pg boundary event. This extinction resulted in global turnover affecting non-avian dinosaurs, ammonites, and marine reptiles, documented by iridium anomalies, shock-metamorphosed minerals from the Yucatán Peninsula, and abrupt faunal shifts recorded in stratigraphic sections curated by the American Museum of Natural History.

Economic Geology and Resources

Cretaceous strata are economically important for hydrocarbons in provinces such as the Gulf of Mexico, the North Sea, and the Western Canada Sedimentary Basin, with reservoirs hosted in sandstone units like the Bakken Formation-analogues and carbonate platforms including the Chalk Group. Coal-bearing sequences in the Karoo Basin and phosphate deposits exploited in regions like Florida and Morocco supply raw materials for industry, while mineral occurrences associated with magmatic events provide metals studied by the United States Geological Survey and commercial firms. Aquifers in Cretaceous siliciclastic units are critical to water resources managed by entities such as the United States Bureau of Reclamation.

Category:Mesozoic geology