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| Tortonian | |
|---|---|
| Start | 11.63 |
| End | 7.246 |
| Unit | Stage |
| Timescale | Neogene |
Tortonian is a middle-to-late Miocene geologic stage spanning approximately 11.63 to 7.246 million years ago. It sits within the Miocene Epoch of the Neogene Period and is bounded below and above by the Serravallian and Messinian stages respectively. The interval is characterized by significant tectonic reorganizations, faunal turnovers, and climatic trends that influenced depositional basins and bioregions across Eurasia, Africa, North America, South America, and Australia.
The stage was formally defined using stratotypes and biostratigraphic markers tied to Mediterranean sections studied by European geologists. Global chronostratigraphic placement relies on magnetostratigraphy, planktonic foraminiferal bioevents, and radiometric calibrations that align with the Geologic time scale. The Tortonian corresponds to chron C5Cn.2r through C3An in marine magnetic polarity timescales and overlaps with regional land mammal ages such as the European Neogene Mammal Zones and the North American Land Mammal Ages like the Clarendonian and Hemphillian. Key biostratigraphic events include first and last occurrences of planktonic foraminifera and calcareous nannoplankton taxa documented in Mediterranean reference sections studied by researchers associated with institutions such as the University of Florence and the University of Milan.
Tortonian stratigraphic successions are preserved in classic Mediterranean basins including the Adriatic Sea margin, the Alpine Foreland Basin, and the Iberian and Apennine sections, as well as in Atlantic coastal basins like the Bay of Biscay and the Gulf of Mexico. Sediments range from deep-marine hemipelagic marls to shallow-marine carbonates, siliciclastic turbidites, and continental fluvial and lacustrine deposits. Tectonic drivers include closure and readjustment of the Tethys Sea remnants, uplift of the Alps, Apennines, and Carpathians, and strike-slip reorganization along the North Anatolian Fault and the West Anatolian extensional province. Basin inversion events and eustatic sea-level fluctuations controlled accommodation space, leading to widespread unconformities and condensed sections used as regional markers by stratigraphers.
Paleoenvironmental reconstructions for the interval indicate a trend from relatively warm, humid conditions toward progressive cooling and aridification in many continental interior regions. Marine records from the Mediterranean Sea, Pacific Ocean, and Atlantic Ocean show changes in surface water temperature, productivity, and oxygenation documented through stable isotope analyses and microfossil assemblages studied by teams at institutions like the Smithsonian Institution and the Natural History Museum, London. Climatic forcing agents implicated include changes in ocean circulation due to gateway evolution such as the proto-Isthmus of Panama connection episodes and variable atmospheric CO2 reconstructed from proxy records used by research groups at ETH Zurich and the Max Planck Institute for Chemistry. Monsoon dynamics affecting the Indian subcontinent and African hydroclimate are inferred from terrestrial pollen records and lacustrine isotopes from sites investigated by the University of Oxford and the University of Cape Town.
Floral assemblages shifted with climatic trends: subtropical and mixed mesophytic woodlands persisted in parts of Europe and East Asia, while C4 grass expansion in Africa and parts of North America promoted grassland ecosystems described in paleobotanical studies at the Royal Botanic Gardens, Kew and the National Museum of Natural History, Paris. Faunal changes include diversification and dispersal events among proboscideans, suids, cervids, and bovids in Afro-Eurasia documented in museum collections at the Natural History Museum, Vienna and the American Museum of Natural History. Marine faunas show turnover among cetaceans, pinnipeds, and marine mollusks with important fossil assemblages from the Calvert Formation, the Pisco Formation, and the Monterey Formation that have been the focus of paleontologists at institutions like the National Autonomous University of Mexico and the Geological Survey of India. Avian and herpetofaunal records from fluvial deposits yield important insights into regional biogeography published by researchers at the University of Barcelona and the Field Museum.
Tortonian-age formations of note include the Monterey Formation (California), Pisco Formation (Peru), Macedonian Neogene basin sequences, the Huesca Basin stratigraphy of the Ebro Basin (Spain), the Ghazij Formation equivalents in the Siwalik Group, and the Solimões Formation (Brazil). In Africa, Tortonian strata are exposed in the East African Rift System and the Bouri Formation-adjacent sequences studied by teams at the University of Addis Ababa and Leiden University. Correlations with North American units such as the Chesapeake Group and the Coastal Plain successions utilize combined magnetostratigraphy and biostratigraphy work performed by the United States Geological Survey and university collaborators.
Absolute and relative age control for the interval derives from integrated approaches: radiometric methods (argon-argon, potassium-argon) applied to volcanic ash layers recovered in continental basins and ocean drills by the International Ocean Discovery Program, magnetostratigraphy correlated to the Geomagnetic Polarity Time Scale, and biostratigraphic zonations using planktonic foraminifera, calcareous nannofossils, and land mammal turnovers. High-resolution astrochronology has been applied to cyclic marine sections analyzed by researchers at the University of Bremen and the University of South Carolina to refine Milankovitch-scale age models. These methods together constrain the stage boundaries and facilitate global correlation across disparate depositional settings.
Category:Miocene stages