Archive of Steve's Blog

January 2014

Leftover fruitcake from 2013. These structures are frequently lithified during the holiday season and are generally durable enough to be re-gifted the following year.

A DEVONIAN CATASTROPHE

The Devonian period was a golden age full of all the promises and possibilities of youth.  It is even now – all these years later – credited with several significant “firsts.”  Seed bearing plants first spread across the land in the Devonian, and by the end of that period great forests were commonplace.  The Devonian was the first period to witness tetrapods walking across dry ground.  The continent of Laurasia (Euramerica) was formed in the Devonian and was fast on its way to an urgent rendezvous with Gondwana.  (Their shotgun marriage in the Permian gave rise to the great supercontinent Pangea and their inevitable divorce produced the Atlantic ocean.)  The Devonian is sometimes referred to as the “Age of Fish” because of the explosive increase in diversity in the Devonian species inventory.   Much came to be in the Devonian, and that which was already there became more complex, bigger, stronger, or faster.  But the tale ends unwell.  The Late Devonian was forever besmirched by one of the top five extinction events in the earth’s history.  And this of course begs the biggest of questions:  Why?

Fast-forward to January, 1990.  A Colorado School of Mines graduate student and a UNOCAL petroleum geologist were poking around in Southern Nevada when they noticed a gravelly limestone breccia that seemed a little “off.”  While Nevada is full of limestone – and therefore full of carbonate breccias – this particular outcrop did not belong in the Devonian formation in which it occurred.  A couple days later, with CSM professor John Warme at their side, the intrepid geologists found another breccia outcrop of equivalent age some 30 kilometers eastward, this one bearing fragments several meters across.  Due to the discovery of shocked quartz grains, and several field trips and graduate students later, the scientific community had finally warmed up to the idea that this locally ubiquitous breccia was the product of a massive impact.  Found all over Southern Nevada, including near the town of Alamo, the breccia was named the Alamo Breccia by Professor Warme, who thought it a “catastrophically fitting” title.

But where was the impact crater?  In the Devonian, the entire state of Nevada was pretty much a shallow marine shelf.  It has since been squeezed, stretched, faulted and folded to the point that it bears little resemblance to its original depositional environment.  The task of disassembling the Alamo Breccia and reconstructing the impact crater from the pieces and parts remaining today is about as challenging as un-cooking a fruitcake.  But ISU Professor Leif Tapanila and his graduate students have pulled it off.  Because of their tireless work during the past 10 years, we now know the size, location, and date of the impact.

So… did this impact cause the Late Devonian mass extinction event?  That was, after all, the question that impelled professor Tapanila to the field in the first place.  To hear the answer you will need to drop in this Tuesday at the Teton County library at 6:00 pm.  Many of you will remember Professor Tapanila from his presentation last year on the great Permian mystery (Helicopron).  You may want to arrive early for a good seat.