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August 25, 2015

1

Then, Now, and Beyond

by Herb Saperstone

Field trip participants from the Northern Colorado Geologists group listen to Dr. Donald Boyd, Professor Emeritus of Geology from the University of Wyoming offer his thoughtful interpretations on the depositional origin of the Precambrian Nash Fork Formation stromatolites.

I had the privilege, once again, to participate in a unique field trip with fellow scientists and friends to visit a fascinating geologic site in southern Wyoming. To be sure, a geologic field trip is always about ancient time travel, but this was truly remarkable. Our excursion offered us an extremely distant target:  2 billion years in time yet only 2 hours on the highway from Fort Collins.  Our destination were the Precambrian-aged stromatolites of the Medicine Bow Mountains which in simple terms are– well– rocks that contain fossil slime. Fossil slime? Yes but really, really old fossil slime that was the dominant life form on our planet for more than half of earth’s entire existence!

Typical outcrop of the Precambrian Nash Fork Formation. Tectonic forces have tilted the original layers into an upright position while deep-seated physical and chemical processes caused silicification (hardening) of the original lime-mud layers.

Typical outcrop of the Precambrian Nash Fork Formation. Tectonic forces have tilted the original layers into an upright position while deep-seated physical and chemical processes caused silicification (hardening) of the original lime-mud layers.

Rock-bearing stromatolites of this age are uncommon and quite different than typical Precambrian rocks we find in Colorado (and in many other places). These stromatolites are essentially sedimentary in nature as they originally formed at or near the earth’s surface. Over time, they have been deeply buried and ‘squeezed’ into meta-sedimentary rocks and then uplifted -most likely during multiple mountain building events in this area. Finally, in a mere fraction of its 2 billion year old history, Ice Age glaciation (which ended about 10,000 years ago) uncovered the wonderful outcrops we visited today.

Convex laminae that follow the undulations of the ancient seafloor. A great variety of shapes and forms reflect both original growth patterns as well as sliding, slumping and collapsing of the microbial/sediment mats.

So now, we have a 2 billion-year old snapshot of Earth Antiquus where nothing more than simple lifeforms occupied the young Earth. A human visitor would be staring at bare rock everywhere amidst seas whose shallow margins supported the growth of thinly layered microbial mats. Over time- perhaps  days, weeks or months, the sticky surfaces trapped lime mud into delicate wavy strata resulting in undulating domes and troughs that are the tell-tale signs of once living stromatolites. How do we know this?

Modern stromatolites from western Australia. wikipedia.org/wiki/File:Stromatolites_in_Sharkbay.jpg

Modern stromatolites exist today in a few places that offer a glimpse into the mechanics of primitive algae and bacteria that were life’s rulers of Planet Earth for nearly 3 billion years.

So why do we get so excited about 2 billion year-old microbes? It is a window of understanding into the origin of our modern oxygen-rich atmosphere and perhaps– a view of what Extraterrestrial Life may actually look like.  After all, astronomers are routinely discovering sister earth “exoplanets” whose billions-year old histories almost certainly contain similar evolutionary story lines.  Just maybe…

Oblique cross section of stromatolite dome contrasting 2 billion year old life with modern flowers which made their first appearance about 150 million years ago.

Oblique cross section of stromatolite dome contrasting 2 billion year old life with modern flowers which made their first appearance a mere 150 million years ago.

So, we salute these extraordinarily ancient stromatolites as postcards of primitive Earth. Envision countless mats of slime yielding tiny bubbles of oxygen that in turn tempered Earth’s primordial atmosphere into one that could harbor more complex life forms. And maybe, just maybe these most ancient of fossils offer us a clue to our most likely alien encounter from the beyond.  A self guided walking tour of the site can be downloaded from the Wyoming State Geologic Survey.

Closer to home in Northern Colorado, these ancient stromatolites form siliceous (resistant) mounds as part of the Jurassic (150 my) Morrison Formation. The larger ‘heads’ are about 1 meter in diameter.

Wyo_Stromatolite 63

Valley margin outcrops feature glacially caused striations. Sand and gravel embedded in Ice Age valley glaciers gouge the underlying rocks in a geologic process that is a mere 10,000 (or a bit older) years old

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1 Comment Post a comment
  1. Vicky Jordan
    Sep 1 2015

    I love the connections you made to past and present. The photos really tell a story! I have such a hard time understanding why there are so many people who still question evolution when the evidence is so overwhelming. Thanks for sharing your insights.

    Reply

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