Rhaetian

Rhaetian
Scotese, Christopher R.; Vérard, Christian; Burgener, Landon; Elling, Reece P.; · CC BY 4.0 · source
Name	Rhaetian
Color	#F4A460
Time start	201.3
Time end	208.5
Time unit	Ma
Preceded by	Norian
Followed by	Hettangian
Era	Mesozoic
Period	Triassic

Rhaetian is the latest stage of the Triassic Period, forming the uppermost subdivision of Triassic chronostratigraphy. It marks a key transition immediately before the Jurassic and frames major biotic turnovers associated with events that affected faunas known from localities such as the Newark Basin, the Keuper, and the Karoo Basin. The Rhaetian is widely recognized in global stratigraphic charts used by institutions like the International Commission on Stratigraphy and has been the focus of studies by researchers affiliated with universities including University of Cambridge, University of Oxford, and University of Zurich.

Etymology and Naming

The stage name derives from the historical region of Raetia, once a province of the Roman Empire, and was formalized in nineteenth-century stratigraphic literature by geologists working in areas such as Switzerland, Austria, and Germany. Early proponents included figures associated with academies like the Prussian Academy of Sciences and research collections at the Natural History Museum, London. The type sections and stratotypes were discussed in journals tied to societies such as the Geological Society of London and the Deutsche Geologische Gesellschaft.

Geology and Stratigraphy

Stratigraphically, the Rhaetian overlies the Norian Stage and underlies the Hettangian Stage, representing the terminal interval of Triassic lithostratigraphic packages such as the Keuper Group, the New Red Sandstone Supergroup, and equivalents within the Alpine domain. Key reference sections are located in the Northern Calcareous Alps, the Marl Slate exposures of England, and the Stuttgart Formation outcrops near Freiburg im Breisgau. Lithologies include carbonate ramp carbonates, laminated shales, marls, and siliciclastic red beds recorded in basin fill sequences like the Tethys margin basins and intracontinental rift systems exemplified by the Central Atlantic Magmatic Province-adjacent basins. Stratigraphers from institutions including the United States Geological Survey and the Natural History Museum of Basel have refined boundary definitions using chemostratigraphic markers and magnetostratigraphy.

Paleontology and Fossil Record

The fossil record of the Rhaetian preserves diverse assemblages from marine and terrestrial realms, documented in collections at museums such as the Natural History Museum, London, the Muséum national d'Histoire naturelle, and the American Museum of Natural History. Marine faunas include ammonoids related to genera described by paleontologists working in the Paleontological Society and bivalves known from sites like the Sinemurian-age successions. Vertebrate remains feature early dinosaurs comparable to taxa studied at the Natural History Museum of Basel and archosaurs comparable to specimens housed at the Smithsonian Institution. Plant fossils, palynomorphs, and trace fossils from localities in Greenland, Norway, and the United Kingdom provide links to floras curated by institutions such as the British Museum and the University of Copenhagen. Important fossiliferous horizons include the Klausenbachtal sections, the Westbury Formation, and the Alpine Triassic sequences studied by researchers at the ETH Zurich.

Age and Correlation

The absolute age assignment for the Rhaetian has been constrained through radiometric dating campaigns using specimens and laboratories associated with the Geological Survey of Canada, the Institute of Geology, Chinese Academy of Sciences, and the Max Planck Institute for Chemistry. Correlations tie the Rhaetian to magnetostratigraphic chrons and biostratigraphic zones employed by teams from the International Stratigraphic Guide-linked working groups. Global correlation links Rhaetian sections from the Newark Supergroup in North America to contemporaneous sequences in the Siberian Platform, South China Block, and the Gondwana margins such as the Karoo Basin and Chañares Formation. Boundary definitions for the base and top of the Rhaetian have been debated at meetings of the International Commission on Stratigraphy and refined with input from researchers at the University of California, Berkeley and the University of Buenos Aires.

Depositional Environments and Paleoclimate

Depositional environments during the Rhaetian range from shallow marine carbonate platforms along the Tethys Ocean margins to fluvial-deltaic and lacustrine systems within rift basins exemplified by the Newark Basin and the Central European Basin. Climate proxies derived from isotopic data produced by laboratories like the Lamont–Doherty Earth Observatory and the GFZ German Research Centre for Geosciences indicate climatic gradients with seasonal aridity in continental interiors and greenhouse-like warmth at low latitudes. Sea-level fluctuations, volcanic inputs related to the Central Atlantic Magmatic Province, and ocean redox shifts recorded in black shale intervals influenced sedimentation patterns documented by teams at the University of Oslo and the University of Milan.

Economic Significance and Research History

Rhaetian strata have economic relevance where they host reservoir rocks, source rocks, or evaporite deposits exploited by companies such as BP, Shell, and national surveys like the Norwegian Petroleum Directorate. Research history spans 19th-century mapping by the Geological Survey of Germany to modern multidisciplinary studies integrating paleontology, geochemistry, and stratigraphy at universities including Stanford University, Harvard University, and ETH Zurich. Major conferences on Rhaetian research have been convened under the aegis of the International Palaeontological Association and the European Geosciences Union, producing white papers and working group reports that inform ongoing debates about end-Triassic biotic crises and stratigraphic subdivision.

Category:Stages of the Triassic