K-Pg boundary

K-Pg boundary
Name	K–Pg boundary
Caption	Satellite image of the Chicxulub crater region on the Yucatán Peninsula
Epoch	Paleogene / Cretaceous
Age	~66 million years ago
Type	Geological boundary
Namedfor	Paleogene–Cretaceous transition
Coordinates	21.3°N 89.5°W

K-Pg boundary is the geological marker separating the Cretaceous and Paleogene periods, widely recognized as coincident with a mass extinction event that affected terrestrial and marine biota. The boundary is characterized by a distinct geochemical anomaly, abrupt faunal turnover, and correlation with a large bolide impact and extensive volcanism, with research involving stratigraphy, geochronology, paleontology, and geochemistry across sites such as the Gubbio sections, the Hell Creek Formation, and the El Kef section.

Nomenclature and Definition

The formal name derives from the juxtaposition of the Cretaceous and Paleogene stages as defined by stratigraphers at organizations including the International Commission on Stratigraphy and institutions like the Smithsonian Institution. The boundary horizon is conventionally placed at the lowermost part of the Paleocene following the terminal Maastrichtian, with type sections such as the sections near El Kef and reference sections in the Gubbio area used by committees and working groups of the International Union of Geological Sciences. Nomenclatural debates have involved terminology used in reports by the Geological Society of America and proposals from researchers at the Natural History Museum, London and the American Museum of Natural History.

Geological and Stratigraphic Characteristics

The boundary interval is typically a thin, fine-grained layer within sedimentary sequences recorded in marine platforms like the Western Interior Seaway margins, coastal plains such as the Hell Creek Formation exposures in the Montana badlands, and pelagic sections near the Demerara Rise. Stratigraphically, it manifests as an abrupt lithologic change, often with an iridium-enriched clay, abrupt shifts in microfossil assemblages (foraminifera, calcareous nannoplankton), and bioturbation patterns observed in cores from the Bass Strait and sections studied by teams from the University of California, Berkeley. The lithostratigraphy across continental shelves, basins like the Gulf of Mexico, and seamount settings reflects distal ejecta layers, tsunami deposits documented on the Yucatán Peninsula, and condensed pelagic successions mapped by the Geological Survey of Canada.

Impact Evidence and Geochemistry

A worldwide enrichment in platinum-group elements, most notably iridium, within a clay layer is a hallmark first reported from the Gubbio section and corroborated in samples from the El Kef and Stevns Klint exposures. Shocked quartz, microtektites, and spherules linked to high-energy impact processes were correlated with the discovery of the Chicxulub crater on the Yucatán Peninsula by teams including researchers from the Carnegie Institution for Science and the University of Arizona. Geochemical fingerprinting using isotopes of osmium, platinum, and chromium, and trace-element ratios used by laboratories at the Max Planck Institute for Chemistry and the Scripps Institution of Oceanography support an extraterrestrial component, while mercury anomalies and sulfur signals implicate contemporaneous eruptions of the Deccan Traps as recorded by chronostratigraphers from the Indian Statistical Institute and the University of Oxford.

Biotic Effects and Extinction Patterns

The boundary coincides with the abrupt disappearance of non-avian dinosaur clades in continental records such as the Hell Creek Formation and the Laramie Formation, documented by paleontologists affiliated with the American Museum of Natural History and Field Museum of Natural History. Marine extinctions affected planktonic foraminifera in Atlantic and Pacific transects sampled by expeditions of the Deep Sea Drilling Project and the International Ocean Discovery Program, with calcareous nannoplankton turnovers recorded in Mediterranean and Caribbean sections studied by teams from the University of Milan and the University of Puerto Rico. Survivorship patterns favored small-bodied mammals in faunas studied at the Djadochta Formation-style localities, opportunistic birds, and resilient benthic invertebrates, a pattern analyzed in syntheses by the Paleontological Society and the Royal Society.

Chronology and Dating Methods

High-precision age constraints combine radiometric techniques (argon–argon dating of impact melt rocks from the Chicxulub crater and ^40Ar/^39Ar work by labs at the Geological Survey of Canada), uranium–lead zircon geochronology from Deccan Traps lavas analyzed at the California Institute of Technology, and cyclostratigraphy tied to orbital tuning frameworks developed by researchers at the University of Geneva and the ETH Zurich. Integrated age models place the boundary at ~66.0 Ma within uncertainties, refined by correlations between biostratigraphy from type sections and isotopic excursions measured by groups at the Lamont–Doherty Earth Observatory.

Global Correlates and Regional Expressions

Regional expressions include tsunami and seiche deposits on the Yucatán Peninsula, glass-rich ejecta layers across the Gulf Coastal Plain, and condensed clay horizons in Stevns Klint and Gubbio. Continental sequences in North America, marine basins offshore Antarctica, and shelf settings bordering India show varying preservation of impact proxies and volcanic signatures, documented by expeditions from the British Antarctic Survey and drilling campaigns by the Ocean Drilling Program. Paleolatitudinal differences controlled survival and recovery patterns traced in records from the Patagonia sections and the Río de la Plata craton.

Research History and Debates

The initial iridium anomaly reported by researchers associated with the Gubbio sections and the University of California, Berkeley sparked the bolide hypothesis advanced in high-profile venues such as the Proceedings of the National Academy of Sciences and debated in forums including meetings of the Geological Society of America and the Royal Society. Competing and complementary models emphasize roles for the Chicxulub impact and the Deccan Traps volcanism, with contributions from numerical modelers at the NASA Goddard Institute for Space Studies and climate modelers at the National Center for Atmospheric Research. Ongoing work by interdisciplinary teams from institutions such as the Smithsonian Institution, the Natural History Museum, London, and the University of Edinburgh continues to refine extinction mechanisms, recovery dynamics, and the relative timing of impact and volcanism.

Category:Geologic boundaries