Wednesday, April 30, 2014

A Peculiar Set Of Geologic Circumstances

Fossilization has been on my mind for several reasons, not the least of which is my having recently located a 270–ish million year old ammonite (read about the adventure here!).

IMG_6493Permian LimestonesNEFWFR
Permian limestones - where fossils are found

In addition to a lone ammonite we found lounging languidly in the Permian limestones of Black Rock Canyon, we noticed a lot of “fossil hash.”

IMG_6466Crinoid StemInHashNEFWFR
Nice crinoid stem in the fossil hash

Yellow arrow points to crinoid stem of previous picture
We also strolled past probable remnants of stromatolites, cyanobacterial mats that trapped sediment in the shallow Permian seas and grew into large, cabbage–like pillars and mounds. 

Follow the red arrow to one of many stromatolites

Of course, I got to thinking. WHY? HOW? What is it about a peculiar set of geologic circumstances that bring this miscellany of an ancient existence into our modern lives? 

Fossilization is indeed a rare occurrence. The odds of any animal part becoming a fossil are exceedingly miniscule – the phenomenon depends on circumstances that are almost as rare as hen’s teeth. 

In order for an animal or its pieces parts to become a fossil, it of course has to die first. In a shallow sea scenario, our critter would eventually sink to the bottom. 

Then, our animal needs to become a dead carcass in mud that is not consumed by scavengers or decayed into oblivion. 

IMG_6474Sponge FossilNEFWFR
I'm thinking sponge.
Next, it needs to be buried rapidly, such as by storm sediments or an underwater landslide.
As if all this were not enough to deal with, after death our animal still needs to survive. While its organic components (generally the shell or skeleton or other hard parts) undergo mineral replacement they would need to survive the pressures of tens or hundreds or even thousands of feet of overlying rock and sediment. In turn, these rocks would have to be preserved for tens or hundreds or even (in the case of certain stromatolites) thousands of millions of years without suffering metamorphism (FYI: rocks do not suffer metamorphism, they enjoy it!), yielding to tectonic rearrangement, or succumbing to erosion.

And we are not done yet. Our rocks have to be uplifted over geologic time from whatever geologic depths they had found themselves. Once uplifted, our fossil must be exposed by the weathering and erosion of any overlying rocks. 

Only then, 270 million years after burial, might the ammonites, the crinoids, the sponges, and the stromatolites be discovered by a chance group of hikers and geologists looking for interesting stuff in a random desert canyon. 

IMG_6496 AmmoniteNEFWFR
The lovely ammonite is nearly the size of my rock hammer

No comments:

Post a Comment