K–Pg boundary

K–Pg boundary
Name	K–Pg boundary
Period	Cretaceous–Paleogene
Type	Geological boundary

K–Pg boundary is the formally recognized discontinuity between the Cretaceous and Paleogene periods marking a major geologic, paleontological, and geochemical turnover. This horizon is characterized by a thin, widely distributed layer associated with abrupt faunal and floral changes observed in stratigraphic sections from Gubbio, El Kef, Río Secco, and Raton Basin, and has been central to debates involving Alvarez hypothesis, Walter Alvarez, Luis Alvarez, Frank Asaro, and Helen Michel. The boundary has been studied through collaborations among researchers from institutions like the Smithsonian Institution, Natural History Museum, London, American Museum of Natural History, and Geological Society of America.

Geology and Stratigraphy

The boundary is preserved in marine sections such as Gubbio and El Kef and continental sequences like Hell Creek Formation, Mesa Verde, and Laramie Formation, where lithologic changes, sedimentary contacts, and taphonomic shifts demarcate the end of the Mesozoic and the onset of the Cenozoic. Stratigraphers employ principles from the International Commission on Stratigraphy, biostratigraphy using index fossils like Planktonic foraminifera, Inoceramus, and Ammonites, and magnetostratigraphy tied to the Geomagnetic Polarity Time Scale to correlate boundary sections across Europe, North America, Asia, Africa, Antarctica, and South America. Key sections display a millimetre- to centimetre-scale clay layer, often within marl or limestone successions, that coincides with abrupt changes in assemblages documented by paleontologists at institutions such as University of California, Berkeley, Yale University, and University of Chicago.

Impact Evidence and Iridium Anomaly

Discovery of an anomalously high concentration of iridium in the boundary clay led to the influential Alvarez hypothesis, linking a bolide impact with mass extinction; this signal was first reported from sections at Gubbio and El Kef and later confirmed in sections from Stevns Klint, Caravaca, Dorset, and the Río de la Plata Basin. Geochemists used techniques refined at laboratories like Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory to quantify platinum-group elements, shocked quartz, and spherules; these proxies were tied to ejecta assemblages described by researchers affiliated with Massachusetts Institute of Technology, California Institute of Technology, and Columbia University. Additional markers include microspherules, soot layers linked to widespread wildfires noted by paleoecologists at University of Texas at Austin and Pennsylvania State University, and elevated levels of nickel and chromium documented by teams from Universidad Nacional Autónoma de México.

Chicxulub Crater and Impact Dynamics

Identification of the buried Chicxulub crater on the Yucatán Peninsula provided a proximal source for the global boundary signal; geophysical surveys by groups from Purdue University, National Autonomous University of Mexico, and Universidad Nacional Autónoma de México used gravity anomalies, seismic reflection, and drilling (including the IODP and Proyecto Chicxulub) to map the multi-ring basin. Modeling of transient and final crater formation draws on theoretical frameworks from Melosh, Holsapple, and Pierazzo and numerical simulations developed at NASA Ames Research Center and Los Alamos National Laboratory to reconstruct impactor size, velocity, and angle consistent with an asteroid analogous to (10) Hygiea-scale bodies or members of the Eunomia family of the asteroid belt. Ejecta dispersal, tsunami generation in basins examined by researchers at Scripps Institution of Oceanography and global seismicity effects comparable to events recorded in Sumatra studies provide mechanisms linking the crater to stratigraphic evidence at distant sites like Stevns Klint and Dorset.

Biological Consequences and Extinction Patterns

The boundary coincides with the abrupt extinction of non-avian Dinosauria clades, major losses among Ammonoidea, many planktonic foraminifera lineages, and turnovers in marine reptiles such as Mosasaurs and Plesiosaurs, documented in fossil collections at the American Museum of Natural History and Natural History Museum, London. Survivorship patterns show differential extinction among groups including higher survival of Aves lineages, mammals with radiations recorded in Paleocene faunas from Williston Basin and Bighorn Basin, and select plant lineages studied by paleobotanists at Smithsonian Institution and Royal Botanic Gardens, Kew. Paleobiogeographic analyses using data from the Paleobiology Database and methods developed by Jack Sepkoski reveal pulses of extinction and recovery that inform debates among paleontologists at University of Michigan, University of Kansas, and University of Florida about the tempo and selectivity of the event.

Global Environmental Effects and Climate Change

Impact-induced mechanisms proposed by climate scientists and modelers from NASA Goddard Institute for Space Studies, Met Office Hadley Centre, and Max Planck Institute for Meteorology include atmospheric injection of aerosols, sulfate aerosols from vaporized evaporites in the Yucatán, and global darkness leading to photosynthetic collapse; these processes are invoked to explain signals such as soot peaks and disruptions in carbon cycling recorded in δ13C excursions at sites like Gubbio and Raton Basin. Paleoclimate proxies—oxygen isotopes, biogenic carbonate dissolution, and clay mineral assemblages—collected by teams at Lamont–Doherty Earth Observatory and Ecole Normale Supérieure indicate rapid cooling, acidification, and subsequent greenhouse warming phases paralleling experiments in taphonomy and ecosystem recovery reported by ecologists at University of California, Santa Cruz and University of Bristol.

Dating, Correlation, and Geochronology

High-precision radiometric ages from techniques like 40Ar/39Ar and U–Pb zircon geochronology applied to impact melt rocks from drilling into Chicxulub and tektite layers from Beloc and La Sierrita yield an age of approximately 66.0 million years, integrating work by geochronologists at Carnegie Institution for Science, ETH Zurich, and University of Arizona. Correlation across marine and terrestrial records uses integrated stratigraphy combining biostratigraphy, magnetostratigraphy, and chemostratigraphy standardized by the International Commission on Stratigraphy and implemented in regional syntheses by researchers at University of Copenhagen and University of Barcelona. Ongoing drilling campaigns, analytical refinements at USGS, and collaborative projects among international consortia continue to tighten the temporal resolution and refine models linking impact timing with extinction dynamics and early Paleogene recovery.

Category:Geologic formations