Wednesday, March 14, 2012

“There is one bed of a bright emerald green, about 60 feet thick ...” -- Eliot Blackwelder, 1926



Blackwelder was describing this green quartzite, found west of Big Telephone Lake in the Medicine Bow Mountains.  Quartzite is metamorphosed sandstone, shiny and sparkly from quartz that once was sand.  It commonly is white, but can take on pastel shades with just small amounts of impurities. The green quartzite contains fuchsite, a chromium-rich version of muscovite (mica).  It is an uncommon form of the Medicine Peak quartzite that makes up much of the Snowy Range on the crest of the Medicine Bow Mountains.
The quartzite ridges of the Snowy Range rise as much as 1000 feet above the
broad crest of the Medicine Bow Mountains in southeast Wyoming, USA.
Courtesy UW Dept. Geology & Geophysics; click to view.
Sparky contemplates quartzite boulders on the backside of Medicine Bow Peak.
The Medicine Peak quartzite is one of the younger members of a group of early
Proterozoic stata whose age has been bracketed at 2.0 to 2.7 billion years.  
The sand that eventually would become the Medicine Peak quartzite was deposited on beaches and in shallow marine environments approximately 2 billion years ago.  It is quite pure, averaging 83-88% quartz, and the grains are rounded, perhaps from being tossed about by waves and currents for so long.  Deposition lasted millions of years, and the resulting formation is massive, to 6000 feet thick.  Adding to the spectacle, the quartzite beds have been tilted to almost vertical during the rough-and-tumble life of the North American continent (more below).
This vertical rock face at 12,000 feet elevation was once horizontal at sea level --
two billion years ago it was sand along the coast of ancestral North America.
When the sands of the Medicine Peak quartzite were being deposited, this part of Wyoming was beachfront property on the southern coast of North America ... but only for several hundred million years.  Then, from 1.8 to 1.6 billion years ago, North America expanded to the south and west in a series of crustal collisions.  During one of the earliest episodes, land was added to the southern continental margin in what is now Wyoming.  The evidence can be seen just a short distance south of the Medicine Peak quartzite.

The Mullen Creek - Nash Fork shear zone is one of the more intriguing geologic features of the Medicine Bow Mountains.  It represents a significant discontinuity in type and age of exposed rocks.  Those to the north are close to 2 billion years old, while rocks south of the shear zone are much younger, “only” 1.7 billion years old.  Within the shear zone itself, rocks are intensely deformed, suggesting some ancient catastrophe.

The MC-NF shear zone is part of the Cheyenne Belt -- a band of discontinuity and deformation that extends across southern Wyoming.  Between 1.7 and 1.8 billion years ago, either terranes (pieces of continents) or arcs of volcanic islands were pushed up against and joined to ancestral North America along this suture zone.  The sedimentary rocks at the continental margin were metamorphosed and tilted to near vertical, as can be seen in today’s Snowy Range.
The Cheyenne Belt in southern Wyoming; Precambrian rocks north of the
suture are predominantly Archean, those to the south are Proterozoic.  The
Medicine Peak quartzite is a member of the Lower Libby Creek Group.
Modified from Lanthier 1979 (click to view).
Highway 130 crosses the Medicine Bow Mountains west of Laramie, passing close
to the Snowy Range, visible in upper left.  Courtesy Google Maps; click to view. 


The Medicine Bow Mountains are located a short distance west of Laramie, and are easily accessible via Wyoming Highway 130, the Snowy Range Road, in the warmer months.  The highway crosses the crest of the mountains at the base of the Snowy Range, where there are great views of the Medicine Peak quartzite.




The fuchsite-bearing green quartzite is known from only two locations:  north of South Gap Lake, and at the south base of Browns Mountain west of Telephone Lakes, where there is a small quarry.  There are scattered green boulders southeast of the quarry site as far as the Towner Lake Road.  “Snowy Range green quartzite” was quarried and sold in the Laramie area in the 1960s, mainly for decorative aggregate, occasionally polished and used as decorative stone.  There are no longer roads in this area, and one has to walk about a mile through subalpine scenery to reach the "bright emerald green" bed west of Big Telephone Lake.
Toto!  I think we’re getting close to the Emerald City!!

Sources

Blackwelder, E.  1926.  Pre-Cambrian geology of the Medicine Bow Mountains.  GSA Bull. 37:615-658.

Harris, Ray E.  1991.  Decorative stones of Wyoming.  Public Information Circular 31.  The  Geological Survey of Wyoming, Laramie.

Houston, R.S., Karlstrom, K.E., Lanthier, L.R., Miller, W.R., Bigsby, P.R.  1983.  Mineral resource potential map of the Snowy Range Wilderness, Albany and Carbon counties, Wyoming.   USGS Misc. Field Studies Map 12.

Houston, R. S. Karlstrom, K.E., Graff, P.J., and Flurkey, A.J.  1992.  New stratigraphic subdivisions and redefinition of subdivisions of late Archean and early Proterozoic metasedimentary and metavolcanic rocks of the Sierra Madre and Medicine Bow Mountains, southern Wyoming.  USGS Prof. Paper 1520.

Karlstrom, K.E. and Houston, R.S.  1984.  The Cheyenne Belt:  analysis of a Proterozoic suture in southern Wyoming.  Precambrian Research 25:415-446.

Lanthier, R.  1979.  Stratigraphy and structure of the lower part of the Precambrian Libby Creek Group, central Medicine Bow Mountains, Wyoming.  Contr. Geol. 17:135-147.

2 comments:

  1. Excellent! Thanks for blogging about this. Just found it now (Sep 10, 2013) looking for info on the green quartzite, after hiking Medicine Bow Peak recently. -- By the way, there are also a few boulders of the brightest green rock on the saddle near the trail to the summit, a few thousand feet SW of South Gap Lake.

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    1. Hi Alan - thanks for reading and thanks for your comment. Very cool that you found the green quartzite.

      Cheers, Hollis

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