Bollettino della Società Paleontologica Italiana, 58 (1), 2019, 73-75. Modena

ISSN 0375-7633 doi:10.4435/BSPI.2019.10

The Cretaceous/Palaeogene Boundary: Italian souvenirs for the youngest of the “Big Five” extinctions

Isabella Premoli Silva

I. Premoli Silva, Dipartimento di Scienze della Terra “Ardito Desio”, Università degli Studi di Milano, Via Mangiagalli 34, I-20133 Milano, Italy; isabella.premoli@unimi.it

KEY WORDS - Bottaccione gorge, Boundary Clay, Iridium anomaly, Parvularugoglobigerina eugubina Zone.

ABSTRACT - The aim of this contribution is to provide a brief introduction to the Italian section of the Bottaccione gorge, near Gubbio, and its bearing to the understanding of the end Cretaceous event. I provide an historical account of the main contributions that built the fundamental background knowledge to the famous hypothesis of an extraterrestrial cause for the extinction event, which was originally formulated grounding on observations and specimens collected in this section.

RIASSUNTO - [Il limite Cretacico/Paleogene: souvenirs dall’Italia per la più recente delle “Big Five”] - Questo contributo intende fornire una breve introduzione alla celebre sezione della gola del Bottaccione, vicino a Gubbio, e al contributo dato dagli studi qui sviluppati alla comprensione dell’evento di fine Cretacico. Si fornisce un breve resoconto storico delle ricerche che hanno costituito le basi conoscitive per l’elaborazione della famosa ipotesi di una causa extraterrestre per l’evento di estinzione, che fu originariamente formulata sulla base di osservazioni e campioni raccolti in questa sezione.

The Cretaceous/Palaeogene boundary (also known as K/Pg where K stays for Kreide and Pg for Palaeogene) represents an extreme example of event stratigraphy. It separates the Mesozoic from the Cenozoic Eratheme, the Cretaceous from the Palaeogene System, the Maastrichtian from the Danian Stages, with a numerical age of approximately 66 Ma. The K/Pg boundary coincides with the youngest of the “Big Five” extinction events (Raup & Sepkoski, 1982) that affected both the marine and terrestrial realms with the disappearance of several fossil groups that dominated in the latest part of the Cretaceous, including non-avian dinosaurs, pterosaurs, marine reptiles, ammonites, belemnites, inoceramids, rudists, globotruncanids, heterohelicids, etc.

The relevance of the K/Pg boundary and the interpretation of its quite unusual features were among the major topics of debate in the early 1960 when the Danian Stage was still considered to be the youngest stage of the Cretaceous (IGC, Copenhagen 1960). Otto Renz (1936), however, had already identified the occurrence of the boundary within the pelagic sediments of the Umbrian Scaglia Formation near Gubbio (Fig. 1), as testified by a major abrupt change of the planktonic foraminiferal fauna, which passes from globotruncanid-bearing strata to small globigerinids without globotruncanids. Later, Reichel (1952) demonstrated that a small globigerinid fauna similar to that identified by Renz in the Scaglia of the Bottaccione section occurred also in the type Danian at Stevns Klint, in Denmark.

In the early sixties Reichel’s findings stimulated a new campaign of detailed micropalaeontological investigations of the pelagic Scaglia Rossa Formation at the Bottaccione section near Gubbio (Luterbacher & Premoli Silva, 1964). We showed that there, the K/Pg boundary, already identified by Renz (1936), is well expressed lithologically in the outcrop (Fig. 2), with

Fig. 1 - Stratigraphic scheme of the Cretaceous to Palaeogene formations of the Umbria Basin according to Renz (1936).

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Bollettino della Società Paleontologica Italiana, 58 (1), 201974

the top of the Maastrichtian represented by a whitish limestone that sharply contrasts with the overlying dark red marly limestone of Danian age. In between there is a 1.5 cm-thick bipartite clayey intercalation, greenish at the base and dark red at the top, now identified as the “Boundary Clay” (Smit, 1982). Furthermore, the high resolution sampling we performed revealed that the change of planktonic fauna below and above the “Boundary Clay” is striking - visible with a simple hand lens - and is even more drastic than what was previously known (Figs 3 and 4).

Subsequently, studies across the K/Pg boundary expanded from a biostratigraphic point of view, with the Parvularugoglobigerina eugubina Zone and its minute assemblage being recognised in several land sections as well as in all the oceans, and of other stratigraphic point of views such as palaeomagnetic reversal sequence and carbon isotope stratigraphy. In 1977 the Campanian to Palaeocene succession at Bottaccione was calibrated palaeomagnetically and the K/Pg boundary turned out to fall within the magnetic anomaly 29R like in the Indian Ocean (Alvarez et al., 1977).

A further geochemical study of the Bottaccione K/Pg boundary documented an unexpected high concentration of Iridium (the Iridium anomaly) in the “Boundary Clay”, finally leading in 1980 to the interpretation that the mass extinction was a catastrophic event possibly related to a bolide impact (Alvarez et al., 1980; Fig. 5). Since 1980 additional evidence of extraterrestrial materials has been

Fig. 2 - Photograph of the outcrop of the Cretaceous/Palaeogene boundary at Bottaccione. Author pointing to the “Boundary Clay”.

collected from around the world (see Smit, 1999 and references therein), but only ten years later the impact site was discovered at Chicxulub in the Yucatan Peninsula, Mexico (Hildebrandt et al., 1991). The bolide that would have caused the devastating extinction had a diameter estimated at least at 10 km leaving a crater larger than 130 squared km. Tectites, tsunamiites, impactites were recovered in various localities in the Gulf of Mexico, but far from the impact location no lithologic break is noticed and the “Boundary Clay” can well be overlooked falling within a monotonous succession within a single formation (i.e., Smit, 1999).

How the Chicxulub impact event is linked to the mass extinction and mass mortality, especially in the marine realm, is still debated, even though Smit (1999, p. 109) reported that “the original suggestion of Alvarez et al. (1980), that a short period of photosynthesis shutdown followed by a food-chain collapse in combination with a prolonged period (> 3 k.y.) of global warming, would explain most of the biotic events.” (see also Coccioni & Galeotti, 1994).

Fig. 3 - Microfacies of samples (a) G-97C, base of the Danian, and (b) G-97B, top of the Maastrichtian, Bottaccione section (modified from Luterbacher & Premoli Silva, 1964). Magnification ×30.

75I. Premoli Silva - Memories from the Bottaccione Gorge

REFERENCES

Alvarez L.W., Alvarez W., Asaro F. & Michel H.V. (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208: 1095-1108.

Alvarez W. (2009). The historical record in the Scaglia limestone at Gubbio: magnetic reversals and the Cretaceous-Tertiary mass extinction. Sedimentology, 56: 137-148.

Alvarez W., Arthur M.A., Fischer A.G., Lowrie W., Napoleone G., Premoli Silva I. & Roggenthen W.M. (1977). Upper Cretaceous-Paleocene magnetic stratigraphy at Gubbio, Italy; V, Type section for the Late Cretaceous-Paleocene geomagnetic reversal time scale. Geological Society of America Bulletin, 88: 383-389.

Coccioni R. & Galeotti S. (1994). K/T boundary extinction: Geologically instantaneous or gradual event? Evidence from deep-sea benthic foraminifera. Geology, 22: 779-782.

Hildebrand A.R., Penfield G.T., Kring D.A., Pilkington M., Camargo-Zanoguera A., Jacobsen S.B. & Boynton W.V. (1991). Chicxulub crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology, 19: 867-871.

Luterbacher H.P. & Premoli Silva I. (1964). Biostratigrafia del limite Cretaceo-Terziario nell’Appennino centrale. Rivista Italiana di Paleontologia e Stratigrafia, 70: 67-117.

Fig. 4 - Succession of planktonic foraminiferal assemblages across the Maastrichtian/Palaeocene boundary plotted against the Bottaccione lithostratigraphic log (modified from Luterbacher & Premoli Silva, 1964).

Fig. 5 - Plot of Iridium content across the Cretaceous/Palaeocene boundary interval from the Bottaccione section and nearby sections showing the peak of Iridium (anomaly) within the “Boundary Clay” (modified from Alvarez et al., 1980).

Raup D.M. & Sepkoski J.J. jr (1982). Mass extinction in the marine fossil record. Science, 215: 1501-1503.

Reichel M. (1952). Remarques sur les Globigérines du Danien de Faxe (Danemark) et sur celles des couches de passage du Crétacé au Tertiaire dans la Scaglia de l’Apennin. Eclogae Geologicae Helvetiae, 45: 341-340.

Renz O. (1936). Stratigraphische und mikropaleontologische Untersuchung der Scaglia (Obere Kreide-Tertiar) im zentralen Apennin. Eclogae Geologicae Helvetiae, 29: 1-149.

Smit J. (1982). Extinction and evolution of planktonic foraminifera after a major impact at the Cretaceous/Tertiary boundary. Geological Society of America Special Papers, 190: 329-352.

Smit J. (1999). The Global Stratigraphy of the Cretaceous-Tertiary Boundary Impact Ejecta. Annual Review of Earth Planetary Science, 27: 75-113.

Manuscript received 11 April 2019Manuscript revised accepted 14 April 2019Published online 30 April 2019Guest Editors Massimo Bernardi & Giorgio Carnevale