Cretaceous–Paleogene boundary

Cretaceous–Paleogene boundary
Name	Cretaceous–Paleogene boundary
Caption	Chicxulub crater, Yucatán Peninsula
Type	Geological boundary
Age	~66 million years
Period1	Cretaceous
Period2	Paleogene
Namedfor	Iridium anomaly at Gubbio
Region	Global

Cretaceous–Paleogene boundary The Cretaceous–Paleogene boundary is a global stratigraphic horizon marking a major biostratigraphic and geochemical turnover between the Cretaceous and Paleogene periods; it coincides with a mass extinction event that terminated many Mesozoic lineages and reshaped Paleogene ecosystems. Interdisciplinary research by teams from institutions such as the Smithsonian Institution, Natural History Museum, London, Geological Society of America, United States Geological Survey, Royal Society, Max Planck Society, CNRS, University of Cambridge, Harvard University, Caltech, University of Tokyo, and Universidad Nacional Autónoma de México integrates field stratigraphy, geochemistry, paleontology, and geochronology to interpret boundary records from localities including Gubbio, El Kef, Hell Creek Formation, Río Colorado, Pondicherry, Demerara Rise, Bighorn Basin, Deccan Traps, and Chicxulub crater.

Geological context and definition

The boundary is defined at a specific lithostratigraphic contact traditionally correlated with the global iridium anomaly first reported from Gubbio and subsequently recognized at sites such as Caravaca de la Cruz, El Kef, Zumaya, Stevns Klint, and Sierra de Gúdar; it separates uppermost Cretaceous Maastrichtian strata from lowermost Danian strata of the Paleogene and serves as the base of the Paleocene. Regional chronostratigraphic schemes by organizations including the International Commission on Stratigraphy and the Subcommission on Quaternary Stratigraphy place the boundary within a framework tied to magnetic polarity chrons such as Chron C29r and biostratigraphic markers including extinction of foraminifera species documented by researchers at University of Copenhagen, Universidad de Barcelona, and University of Oslo.

Global stratigraphy and boundary markers

Stratigraphic correlation relies on multiple marker suites: geochemical anomalies (notably the global iridium spike discovered by a team including scientists from University of California, Berkeley and Università di Padova), shocked minerals such as planar deformation features identified by analysts from Imperial College London and ETH Zurich, spherules and microtektites recovered from Caribbean Sea and Indian Ocean cores collected by expeditions of ODP and IODP, and biostratigraphic turnovers in calcareous nannoplankton, planktonic foraminifera, and palynomorph assemblages studied at Bayerische Staatssammlung für Paläontologie, Natural History Museum, Vienna, and Field Museum. Boundary sections at Stevns Klint (designated a World Heritage Site), El Kef (type section recognized by UNESCO), and Rødvig preserve laminated pelagic sequences used in cyclostratigraphic work by groups at University of Oslo and ETH Zurich. Lithologic markers include the pelagic clay or marl layer often termed “K–Pg clay” that overlies Maastrichtian chalks of the Selandian shelf settings documented in cores from North Atlantic drilling campaigns.

Impact event and ejecta evidence

Evidence for a bolide impact includes the Chicxulub structure documented by geophysical surveys coordinated by Petróleos Mexicanos and drilling projects led by consortia including International Ocean Discovery Program and IODP Expedition 364; high-pressure minerals (coesite, stishovite) reported by teams at Carnegie Institution for Science and Geological Survey of Canada; shocked quartz studies by groups from USGS and University of Texas; global microspherule layers analyzed by laboratories at MIT, University of Arizona, and University of Sao Paulo; and a worldwide iridium anomaly measured by mass spectrometrists at Scripps Institution of Oceanography and Lawrence Berkeley National Laboratory. Geochemical fingerprinting of impactor components has engaged researchers at Institut de Physique du Globe de Paris, Max Planck Institute for Chemistry, and National Institute of Standards and Technology to compare platinum-group element ratios and osmium isotope excursions across cores from Atlantic Ocean, Pacific Ocean, and Gulf of Mexico sites.

Biotic effects and mass extinction

The extinction pulse at the boundary eliminated many taxa including non-avian Dinosauria clades known from formations such as Hell Creek Formation and Laramie Formation, marine reptiles documented in Cretaceous collections at Natural History Museum, London and American Museum of Natural History, and numerous planktonic foraminifera and calcareous nannoplankton species catalogued at British Antarctic Survey and Institut Català de Paleontologia Miquel Crusafont. Studies by paleontologists at Yale University, University of Kansas, University of Alberta, University College Dublin, and University of Bergen describe patterns of selective extinction and survivorship among mammals (e.g., basal placentals and marsupials), birds (e.g., early neoaves), squamates, crocodyliforms, and plants recorded in palynological records from sites investigated by teams at Smithsonian Tropical Research Institute and Royal Botanic Gardens, Kew. The extinction mechanism debate has involved proponents from Arizona State University, University of Chicago, University of Colorado Boulder, and Brown University, integrating volcanic hypotheses centered on Deccan Traps eruptions with impact-driven scenarios associated with Chicxulub.

Recovery and early Paleogene ecosystems

Early Paleogene recovery of terrestrial and marine ecosystems is documented in successions such as the Williston Basin, Bighorn Basin, Halls Creek, Río del Peñón Blanco, and Danian sections studied by researchers at University of New Mexico, University of Saskatchewan, University of Madrid, University of Leiden, and University of Michigan. Radiations of mammals (notably early Placentalia and Marsupialia), diversification of teleost fishes recorded at Natural History Museum of Los Angeles County, and the rise of new plant assemblages tracked by palynologists at University of Göttingen and University of Paris reflect ecological restructuring; paleoecologists from University of California, Santa Cruz and Princeton University use isotope geochemistry, taphonomy, and functional morphology to reconstruct trophic webs, climate shifts during the Paleocene–Eocene Thermal Maximum studied by teams at Lamont–Doherty Earth Observatory and GEOMAR.

Dating methods and age constraints

Absolute age constraints combine high-precision radiometric dating (40Ar/39Ar, U–Pb zircon) performed by laboratories at California Institute of Technology, University of Geneva, Arizona State University, and University of Colorado, magnetostratigraphy tied to chrons (e.g., Chron C29r) developed by researchers at Scripps Institution of Oceanography and Paleomagnetic Laboratory "Fort Hoofddijk", and cyclostratigraphy applied by groups at ETH Zurich and University of Bergen. Integrated age models place the boundary at approximately 66.022–65.98 million years ago, refined through U–Pb dates on impact-related melt rocks from Chicxulub and 40Ar/39Ar ages on volcanic ash beds within Deccan Traps sequences analyzed at Geological Survey of India and Monash University. Ongoing work by consortia including IUGS and International Ocean Discovery Program continues to improve temporal resolution and reconcile regional discrepancies.

Category:Geology