Turonian
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| Turonian | |
|---|---|
 | |
| Name | Turonian |
| Color | #A0C0FF |
| Time start ma | 93.9 |
| Time end ma | 89.8 |
| Time start uncertainty | 0.8 |
| Time end uncertainty | 0.3 |
| Preceding | Cenomanian |
| Following | Coniacian |
| Era | Mesozoic |
| Period | Cretaceous |
Turonian The Turonian is an age and stage of the Cretaceous chronostratigraphic scale situated between the Cenomanian and the Coniacian. The interval is widely recognized in international stratigraphy through global correlations involving stratotypes, ammonite zonation, radiometric dates, and isotopic excursions tied to events recorded in formations such as the Bohemian Cretaceous Basin, Western Interior Seaway, and Gabon deposits.
Definition and Chronostratigraphy
The base of the Turonian is formally defined at the first appearance of an index ammonite faunal assemblage correlated with sections in the GSSP framework originally proposed from the Southern France and later formalized with radiometric calibration from volcanic ash beds in the New Jersey Coastal Plain and New Mexico exposures. Chronostratigraphic subdivision uses ammonite zones first established by researchers working in the Bohemian Massif, England, Morocco, Egypt, and Colombia, and integrates biostratigraphy from foraminifera and calcareous nannofossils described by teams associated with institutions like the Natural History Museum, London, the Smithsonian Institution, and the Royal Society. High-precision U-Pb zircon ages from ash layers in the Western Interior Basin and isotopic correlations with work by groups at Caltech and the Max Planck Society constrain the numerical age to approximately 93.9–89.8 million years ago.
Global Stratotype and Regional Correlations
The Global Stratotype Section and Point (GSSP) debates for the Turonian involved candidate sections in France, Italy, Austria, and Spain before consensus tied correlations to markers present in Egyptian and Moroccan sequences that match radiometrically dated bentonite layers from the Western Interior Seaway. Regional correlations rely on ammonite faunas described from the Bohemian Cretaceous Basin, echinoid and rudist assemblages recorded in the Mediterranean Basin, and planktonic foraminiferal turnovers documented in cores from the South Atlantic and North Sea. Stratigraphic charts published by the International Commission on Stratigraphy and national surveys such as the United States Geological Survey and British Geological Survey integrate these regional schemes with global stage boundaries.
Paleogeography and Climate
During the Turonian continental configurations reflected the breakup of Pangaea into major plates such as the Laurentia, Gondwana, Eurasia, and microcontinents like the Amasia-proximal fragments; seaways like the Western Interior Seaway expanded, influencing the Tethys Ocean circulation. Paleoclimate reconstructions indicate a greenhouse world with elevated atmospheric CO2 inferred from proxies developed by researchers at institutions including Columbia University, ETH Zurich, and the University of Oxford, and a prominent oceanic anoxic event (OAE 2 aftermath) leading to widespread organic-rich black shales in basins such as the Atlantic Coastal Plain, South China Sea rift basins, and the Sao Paulo Basin. Studies from the Max Planck Institute for Chemistry and the University of Mainz document warm, equable conditions with reduced polar ice, high sea level shown on maps produced by teams at the Paleogeographic Atlas Project and University of Chicago.
Biostratigraphy and Notable Fossils
Biostratigraphy of the Turonian is dominated by ammonite zonation developed from classic localities in the Bohemian Massif, Scotland, Morocco, Texas, and Colombia. Key fossil groups include ammonites (families described by researchers at the Natural History Museum, London and Muséum national d'Histoire naturelle, Paris), planktonic foraminifera documented by researchers at the Scripps Institution of Oceanography, calcareous nannofossils studied at Cambridge University, rudists recorded from the Caribbean and Mediterranean, and diverse marine vertebrates from formations such as the Smoky Hill Chalk Member and the Western Interior Seaway yielding mosasaurs, plesiosaurs, and pelagic sharks described in papers in journals like Nature and the Journal of Vertebrate Paleontology. Important terrestrial records with dinosaur remains and angiosperm floras occur in strata studied in Uzbekistan, Brazil, and North America.
Sedimentology and Depositional Environments
Turonian deposits include chalks, marls, black shales, and carbonate platforms formed in epicontinental seas; classic sedimentary sequences are exposed in the White Chalk Group of England, the Bohemian Cretaceous Basin limestones, and the Gabon and Peruvian marine successions. Depositional models incorporate work on turbidites and hemipelagites from the North Atlantic margins investigated by the Norwegian Geological Survey and carbonate platform buildups with rudist reefs in the Tethyan realm studied by teams at the University of Barcelona. Sequence stratigraphy applied by researchers at the University of Texas at Austin and the University of Southampton documents highstand systems tracts, transgressive-regressive cycles, and condensed sections associated with global sea-level fluctuations.
Major Geological Events and Extinctions
The Turonian records the aftermath of the Cenomanian–Turonian boundary events, including sustained high sea levels and the protracted environmental effects following Oceanic Anoxic Event 2 as investigated by groups at the Institut de Physique du Globe de Paris, ETH Zurich, and Lamont–Doherty Earth Observatory. Biotic turnovers affected ammonites, planktonic foraminifera, and reef builders with regional extinctions and radiations documented in stratigraphic work from Morocco, Texas, and East Greenland. Volcanism linked to large igneous provinces and enhanced nutrient fluxes is discussed in literature involving the IUGS community and geochronologists at Massachusetts Institute of Technology.
Economic Resources and Paleontological Sites
Turonian strata host hydrocarbon source rocks such as organic-rich shales in the Gabon Basin, Atlantic Coastal Plain, and Western Interior Basin evaluated by petroleum geologists from the Society of Petroleum Engineers and national surveys like the U.S. Energy Information Administration. Chalk and limestone from the White Chalk Group and Karst-related carbonates are quarried for construction and industrial uses examined by the British Geological Survey and EuroGeoSurveys. Notable paleontological localities include the Bentonites and fossiliferous exposures of the Niobrara Formation, Moroccan phosphogenic outcrops yielding ammonites and marine reptiles, and South American sites in Colombia and Argentina where museums such as the Museo Argentino de Ciencias Naturales curate Turonian collections.