Friday, October 7, 2011

Stromatolite Hike


Sparky and stromatolites,  Medicine Bow Mountains, WY
It’s a crisp but glorious Indian Summer day in the Holocene Epoch, and Sparky is standing next to Precambrian stromatolites ... the fossilized remains of a 1.7 billion-year old mat of cyanobacteria (aka blue-green algae) that grew in a warm shallow sea.  This is what I love about geology, the opportunity to look far into the past and unravel a tiny bit of the surprising mysteries around us.  Uncovered Earth said it so well when he wrote:  “To appreciate a thing, you must first come to know it”.

For my knowledge and appreciation of the stromatolites of the Nash Formation, I thank one of the  great legends of Wyoming geology, Dr. Samuel H. Knight.  It was because of his “Precambrian stromatolites, bioherms and reefs in the lower half of the Nash Formation, Medicine Bow Mountains” (1968) that Sparky and I were able to hike along Precambrian reefs, for Doc Knight had drawn a detailed map of reef locations (click to view).
The early Proterozoic metasedimentary formations of the Medicine Bow Mountains, including the stromatolitic Nash Formation, were steeply folded ca 1.8 billion years ago, a time of terrane or arc accretion along the Cheyenne belt (suture zone) in southeast Wyoming.  The near-vertical orientation of these strata can be seen in this photo of the Snowy Range -- the highest peaks of the Med Bows.  The bedding planes of the Medicine Peak quartzite form sheer faces below the ridge crest.  The quartzite is so resistant that cirques typical of alpine landscapes are absent.  Instead there is a eight-mile-long ridge with 1000 feet of vertical relief.
Snowy Range, southeast face
The stromatolitic Nash Formation also was severely deformed, but the rocks underwent only low-grade metamorphism, hence the wonderful preservation of stromatolites.  In spite of multiple episodes of uplift and folding, the original orientation of outcrops often can be determined from the stromatolites themselves.  In the next photos, the V shapes (as well as the black arrow and shoe) point to the ground at time of formation (the Vs are gaps between adjacent algal mounds).



The Nash Formation is crossed by sill-like masses of gabbro that had not been dated as of 1992. 
Gabbro intrusion in Nash Formation in foreground, with beautiful subalpine lake for scale.

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