A Bad Day in the Cretaceous

We didn’t go there on our visit to the South Island, but we weren’t too far away.

And it’s an interesting story that might someday be repeated and lead to our demise.

About four kilometers south of Wharanui on the South Island is a place called Woodside Creek. It looks like almost any other rocky New Zealand gorge with a stream running down it, except the layers of rock sediment or strata are laid out in a particularly orderly manner, such as you might see in a road cut, except in this case the sediments span millions of years. At one point, if you look closely, there is a curious red clay layer a few centimeters in width between two other more homogenous layers of rock . . .

Approximate location of Woodside Creek on South Island, New Zealand

Approximate location of Woodside Creek on South Island, New Zealand (1)

In the 1970s a geologist, Walter Alvarez, was studying bands of limestone in a similar type gorge called Gola del Bottaccione in Italy. Alvarez was analyzing the layers of strata that span the Cretaceous Period and the Tertiary Period, a time span when a mass extinction occurred on earth. During this interval of time, sometimes referred to as “the Great Dying”, some 70% of the existing forms of life on earth disappeared and became extinct, including the dinosaurs, which had been the dominant land animals for 135 million years. At the juncture of the two layers Alvarez was studying was the same curious red clay layer described above.

Red clay layer at Gola del Bottaccione in Italy

Red clay layer at Gola del Bottaccione in Italy (2)

Alvarez was looking at foraminifera (called forams for short), tiny, marine fossilized creatures that once lived in the ancient seas, and were now preserved in the rock. Forams are a type of what’s called an index fossil. They are widely distributed and abundantly preserved, and their shapes vary from species to species and thus, this known variability can be used to date rocks.


Foramanifera (3)

That is, if a certain type of foram is found in a rock layer, one can say that that rock layer is so many years old. The limestone from directly below the red clay layer that Alvarez was studying was from the last stage of the Cretaceous (65 million years ago) and contained numerous, large diverse forams. Then there was the layer of red clay, which contained no forams. Finally, there was the more recent geologic layer, above the red clay layer. This contained only a handful of foram species and they were significantly smaller, different and of less variety than the forams from the layer below the red clay layer.

Up until this time, it was generally accepted that when species became extinct, it happened over a gradual period of time. You’d see fewer and fewer of the species in the fossil record and eventually none. But what Alvarez was seeing was something different—an abrupt end and disappearance of the large forams, followed by an interval of time when there were no forams and then their eventual recovery. The key seemed to lie in the curious red clay layer.

Luis Alvarez, Walter’s father and a Nobel-prizewinner in physics, became interested in his son’s studies, and they both began to work together to determine exactly how much geological time that red clay layer represented.

[A brief note on nomenclature: the Cretaceous-Tertiary boundary or border layer as described above was formerly known as the K-T boundary (K is used for an abbreviation for Cretaceous instead of C since C stands for the Carboniferous Period). Today the boundary is known as the Cretaceous-Paleogene boundary or simply the K-Pg boundary.]

Luis Alvarez suggested using iridium to date the layers. Iridium is an extremely rare element on the surface of the earth, but is much more common in meteorites and occurs in cosmic dust which falls down to earth all the time. Since cosmic dust and hence iridium fall to the earth at a relatively steady state, the Alvarezes’ plan was to measure the amount of iridium in the layer to determine how long it had taken for it to be laid down. The results astonished them; when compared to the surrounding two layers, the middle red clay layer was found to have iridium levels that were off the charts.

“In science, sometimes it’s better to be lucky than smart.”

                                                     – Walter Alvarez

At first, they thought it must be some sort of mistake. At the time, there were only two other known locations in the world where the K-Pg layer was similarly exposed: one in Denmark and the other here in New Zealand at Woodside Creek. Samples from these two locations were analyzed and similarly showed huge spikes of iridium.

Woodside Creek K-Pg layer

Woodside Creek K-Pg layer (4)

The Alvarezes didn’t know what to make of this. Finally, they speculated that an enormous asteroid had collided with the earth on an otherwise ordinary day 65 million years ago. Debris from the pulverized asteroid spread around the world leaving the iridium layer. The dust blocked out the sun turning day into night and a mass extinction occurred.

They published a paper in June 1980, “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,” which was initially met with skepticism if not downright ridicule. Again, prior to this time it was felt that species had become extinct gradually over periods of time; the Alvarezes’ views were heresy since they spoke of a catastrophic mass extinction.

Luis and Walter Alvarez at Gola del Bottaccione in Italy

Luis and Walter Alvarez at Gola del Bottaccione in Italy (5)

Jumping ahead slightly and skipping some of the other evidence, a 180 kilometer (120 miles) wide crater, now known as the Chicxulub crater, was discovered off the coast of the Yucatan Peninsula in Mexico. This was felt to be the impact site of the asteroid, 10 km (6 miles) in diameter, and travelling at 70,000 km/hr (43,500 miles/hr) that struck the earth 66 million years ago and caused the mass extinction.


Chicxulub Crater (6)

The impact delivered a destructive blast more powerful than the combined yield of all the earth’s nuclear weapons, setting off earthquakes and widespread tsunamis, creating a giant dust cloud that effectively blocked out sunlight causing the earth to cool and causing a multi-season “impact winter”. All forests were destroyed. It is postulated that everything larger than a cat died. All of the non-avian dinosaurs became extinct along with three-fourths of all birds, four-fifths of all amphibians, two-thirds of all mammal families and similarly much sea life including 95% of the plankton species. In total, 70% of all living species were wiped out forever by this cataclysmic event.

It is sobering to note that all the plants and animals that were wiped out were incredibly well-adapted and successful species. They weren’t doing anything wrong. It was simply a bad day in the Cretaceous.

Since that time, it has taken millions of years for life on earth to recover to its former level of biological diversity. We are all descendants of those creatures that happened to survive that fateful day.




Kolbert, E. (2014).The Sixth Extinction. London, England: Bloomsbury.

Misc. internet

Photo credits:

1)‪Outline Map: New Zealand – EnchantedLearning.com
www.enchantedlearning.com437 × 644Search by image

‪2)Giornate di paleontologia spi 2013 – società paleontologica …
www.scienzafacile.it1200 × 900Search by image
‪Livello a iridio della Gola del Bottaccione – Foto di Daniele Tona

3)Planktonic foraminifera: Ancient fossils the size of sand grains …
www.dailymail.co.uk634 × 476Search by image
‪planktonic foraminifera

‪4)Julian’s Blog: Global Catastophe in a thin rock layer
juliansrockandiceblog.blogspot.com1382 × 780Search by image
K-Pg boundary layer – when the Earth changed forever

‪5)Walter Alvarez – Wikipedia, the free encyclopedia
en.wikipedia.org1566 × 2100Search by image
‪Luis and Walter Alvarez (L-R) at the K-T Boundary in Gubbio, Italy 1981

‪6)One Universe: At Home in the Cosmos
research.amnh.org669 × 423Search by image
‪Formed by the impact of an asteroid or a small comet that smashed into Earth about 65 million years ago, the Chicxulub Crater in Mexico’s Yucatán Peninsula …


One thought on “A Bad Day in the Cretaceous

  1. Very interesting and very believable…. we call it The Flood and the recovery of animal life was Noah’s two by two thing…. The Flood explains so much ……I wish I was there and could see you screaming and pulling out your hair !!!! : ). 😃

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