The historical record in the Scaglia limestone at Gubbio: magnetic reversals and the Cretaceous-Tertiary mass extinction



The Scaglia limestone in the Umbria-Marche Apennines, well-exposed in the Gubbio area, offered an unusual opportunity to stratigraphers. It is a deep-water limestone carrying an unparalleled historical record of the Late Cretaceous and Palaeogene, undisturbed by erosional gaps. The Scaglia is a pelagic sediment largely composed of calcareous plankton (calcareous nannofossils and planktonic foraminifera), the best available tool for dating and long-distance correlation. In the 1970s it was recognized that these pelagic limestones carry a record of the reversals of the magnetic field. Abundant planktonic foraminifera made it possible to date the reversals from 80 to 50 Ma, and subsequent studies of related pelagic limestones allowed the micropalaeontological calibration of more than 100 Myr of geomagnetic polarity stratigraphy, from ca 137 to ca 23 Ma. Some parts of the section also contain datable volcanic ash layers, allowing numerical age calibration of the reversal and micropalaeontological time scales. The reversal sequence determined from the Italian pelagic limestones was used to date the marine magnetic anomaly sequence, thus putting ages on the reconstructed maps of continental positions since the breakup of Pangaea. The Gubbio Scaglia also contains an apparently continuous record across the Cretaceous–Tertiary boundary, which was thought in the 1970s to be marked everywhere in the world by a hiatus. A geochemical study of the Gubbio Cretaceous–Tertiary boundary accidentally uncovered an unexpectedly large concentration of iridium in the boundary clay, providing the first evidence for a large comet or asteroid impact at the time of the Cretaceous–Tertiary mass extinction. The sudden foraminiferal extinction in the essentially complete Gubbio Scaglia showed that the Cretaceous–Tertiary extinction was a catastrophic event. After publication of the Gubbio Cretaceous–Tertiary iridium anomaly in 1980, a decade followed in which more and more evidence for a catastrophic impact challenged the traditional uniformitarian viewpoint of geologists. In 1991, the impact site was discovered at Chicxulub, in the Yucatán Peninsula of Mexico. The Gubbio area has thus emerged as a classical geological locality. It has been critical in the development of geomagnetic reversal stratigraphy, a third system for dating Earth history, complementing biostratigraphy and radiometric age dating. Gubbio was also central to the recognition that occasional catastrophic events like great impacts require a rejection of strict uniformitarianism.