Charles Lyell was a great theorist. His Principles of Geology (12 editions between 1830 and 1875) offers his readers a world in equilibrium, where organic and inorganic processes interact and balance; where things move slowly and steadily and naturally within a long timescale.
During the Ordinary General Meeting of the British Geological Society in February 1831 Adam Sedgwick, as president, criticize harshly the geological model of Lyell. Sedgwick made it clear that he accepted the unchanging nature of the fundamental physical laws, but could not believe that 'those secondary combinations' that we call geological processes had remained constant throughout time".
It is clear from such statements that Lyell's Uniformatism was not strictly applied to geology during the 19th century, the "Ice Age" hypothesis was considered an extraordinary event, but finally accepted in a brief period of time.
Surprisingly there was much more aversions against catastrophic theories in the 20th century: the explanation of the Channeled Scablands in Northamerica as result of a cataclysmic flooding was rejected until the 1960s and 1970s, the role of impacts of extraterrestrial bodies for creating craters on the Moon was underestimated until 1960, even more their impact on Earth.
Until 50 years ago it was believed that a characteristic of the Cretaceous-Palaeogene (C/Pg) transition is the apparent lack of a complete stratigraphic record, and until the decade between 1960 and 1970, with the discovery by Alvarez in the gorge of Gubbio, it was believed that the upper limit of the Cretaceous had gone eroded and lost forever.
Alvarez tried to date the sediments of the "Scaglia rossa" formation outcropping in the gorge by the accumulation of rare elements, which in a constant rate reach the earth through micrometeorites and become incorporated in the turbidite deposits. During this research he discovered the today well-known iridium anomaly.
Fig.1. The famous transition between Cretaceous (lower part) and Paleocene (upper part) Scaglia Rossa, in the gorge of Gubbio, foto rotated by 90 degrees.
The origin of this anomalous concentration of element at first remained unknown, a possible explanation, apart from a slow sedimentation resulting in an apparent concentration of micrometeorites, was the impact of a large extraterrestrial mass with a high concentration of these elements.
But there was no further evidence to confirm this hypothesis.
In 1981 Antonio Camargo-Zanoguera and Glen Penfield during a geophysical conference presented their research on a geological mystery discovered 30 years earlier during surveys on the Yucatan peninsula. The two researchers proposed a new interpretation of a circular structure revealed by seismic investigations and buried under 300 to 1.000 meters of sediment, considered of volcanic origin. But their hypothesis - that it was an impact structure - aroused little interest.
Only 10 years later, researchers from the University of Arizona began to study the crater and obtained the first absolute dates, with an age of 65 millions of years. Further research in 1993 revealed that the structure was in fact an impact crater displaying a complex structure with a central bulb surrounded by concentric circles, reaching a total diameter of 180 km.
The crater was called Chicxulub - "the devil's tail".
Alvarez, who had continued his research on the impact hypothesis, noted the temporal coincidence of the iridium anomaly and the Chicxulub-impact, and proposed that it was the searched culprit of the C-Pg event.
Based on these geological evidences, the limit between the Cretaceous and the Palaeogene in 1995 was defined at the stratotype (GSSP) of El Kef (Tunisia), coinciding with the peak of iridium and the mass extinction of foraminifera at the base of a clay layer (C / Pg boundary clay) deposited after the impact as result of decreased organic productivity in the oceans.
With the alleged importance of the impact as a time marker and the solution to the mystery of the extinction of dinosaurs, an extensive research on the Chicxulub site began. The Yaxcopoil -1 core was recuperated in Mexico during the Chicxulub Scientific Drilling Project, to study the stratigraphy of the crater and its sediment infill.
Sediments were recovered from an intervall between 400 to 1.511m depth. Up to 795m carbonatic silt- and sandstone prevail, followed by 100m of breccias, interpreted as by the impact fractured rock.
Fig.2. Yaxcopoil-1 stratigraphy with the impact breccias between the 795-895m interval (black bar on the left), after TUCHSCHERER et al. 2004.
The melt enclosed in the impact breccia has been dated by the argon method to an age of 65 million years, also zircon crystals and glass ejected from the crater and deposited on the surface of earth have been dated to an age of 65, 07 + -0.1 Ma. But the recovered core also showed two prominent stratigraphic gaps between the Upper Cretaceous (Maastrichtian) and the lower Paleocene (Danian).
More than 350 sites are today known recording the C/Pg transition, and displaying a typical sedimentary succession: a layer that varies in thickness and composition according to the distance from the point of impact.
To a distance of 500km this ejecta layer is very thick, around the crater it reaches a thickness of 100 to 80m! (Proximal facies 1).
Between 500 to 1.000km the sediments are typical for tsunami deposits - thick layers of several centimetres containing lens with clasts and spherules, transported by the waves from the impact point (proximal facies 2).
With increasing distance, the layer thins out to form the known iridium rich clay overlying a layer with small spherules (intermediate facies).
Over a distance of 5.000km the event is represented by a layer of red clay (distal facies) that still contains traces of the material ejected from the crater (at the basis of this layer the C/Pg limit of the El Kef section is defined).
This sequence is consistent with the location of the Chicxulub crater on the Yucatan Peninsula as the source of the sediments.
Fig.3. Distribution of the global offshore drilling sites that have returned sedimentological structures related to the impact of Chicxulub (asterisk).
Magenta: proximal facies 1 (distance of 500km)
Red: proximal facies 2 (distance of 1.000km)
Orange: intermediate facies (distance of 5.000km)
Yellow: distal facies (distance more than 5.000km)
Below the schematic sequence of facies associations (from SCHULTE et al. 2010).
The geological evidence supports the hypothesis that large extraterrestrial bodies can impact on earth, but it's a whole other problem how Chicxulub is related to the extinction of the Mesozoic fauna.
One of the most intriguing aspects of the impact model is the apparent explanation of the Mass Extinction at the Cretaceous-Palaeogene Boundary.
According to this model the selective extinction was caused mainly by the consequences of the impact: the release of large quantities of gas from the vaporized rocks, rich in carbonates and sulphates, changed the global climate, causing a "nuclear winter" with fire storms, followed by low temperatures, diminished solar irradiation and acid rain. Depending on the locality and the ecological niche that a species occupied, and a good dose of luck, these changes decided the specie's fate - to adapt or become extinct. Most of this scenario however is speculative - despite the strong media presence of the crater of doom explanation, the biotic effects of impacts and the response of organisms and ecosystems to such an event are still unknown or at best controversial.
As for Alvarez, the "father" of the C / Pg event, he certainly does not show modesty, so in 2009 he writes:
"Geologists could no longer doubt the reality of catastrophic impact events. The KT impact, first recognized at Gubbio, was a truly catastrophic event in Earth history, whose history is written in detail in the rock record. It correlates with, and probably caused, the great mass extinction 65 Ma. After a century and a half, the uncompromising uniformitarian gradualism of Lyell was dead."
Fig.4. "Cretaceous diary" in the little inn in the gorge of Gubbio.
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