The southeastern part of Wyoming contains a number
of interesting geological features, including several mountain ranges that
extend north from Colorado. One
particular part of the country is the Medicine Bow Range of both states whose
high peaks in Wyoming are known as the Snowy Range. Another is the Wyoming extension (Laramie
Range) of the Front Range of Colorado. I
have camped, hiked, collected and fished along a good part of these ranges and
have tried to pay some attention to the great exposures of Precambrian rocks.
In all of my other articles I have used the term
Precambrian in a very lose sense to indicate very old rocks of the earth’s
crust. What I have failed to indicate is
that the Precambrian represents a very, very long time span, perhaps the first four
billion years of geologic time—the vast amount of mind-boggling time before the
appearance of hard-bodied animals (animals with shells or bones). In contrast
to the Precambrian, these hard-bodied animals have only been around for less
than .5 billion years. So, the time
period of life, as we generally know it, is only about 12% of geologic time.
The Precambrian, and it is formally called a
Supereon, is divided into three Eons (remember the Paleozoic, Mesozoic, and
Cenozoic are Eons): the Hadean (4.5-3.95 Ga), Archean (3.95-2.5 Ga) and Proterozoic (2.5-.542 Ga) with the
abbreviation Ga referring to billions. The .542 Ga or 542 Ma (542 million) is
the base of the Cambrian and the time when geologists begin to find hard-bodied
animals such as trilobites and brachiopods.
Since these shelled fossils are often used to date rocks, the time since
the Precambrian has been subdivided into quite small units of time. Another mitigating factor is that many/most
Precambrian rocks have been subject to episodes of metamorphism and igneous
activity, both in the Precambrian and the later Eons. It is easy to go out and locate a
post-Precambrian sandstone or limestone.
However, very few of these sedimentary rocks are preserved as such in
the Precambrian record (except some very young ones). Most have been metamorphosed to quartzite or
schist or gneiss or marble or actually re-melted and turned into igneous granite.
For additional information on geologic time see the
Geological Society of America time scale at: www.geosociety.org/science/timescale/timescl.pdf
Sims and Finn ( 2001) have described the Precambrian
rocks (aka “the basement”) of Colorado in great detail and the following
description is from their paper. In most
of our state, the basement consists of crystalline igneous and metamorphic
rocks lying stratigraphically below the layered sedimentary rocks of the post-Precambrian
(aka Phanerozoic). In some places,
however, sequences of younger Precambrian sedimentary rocks overlie the
crystalline rocks; these sequences are included as basement.
The oldest rocks in Colorado are found in a very
small area (less than 50 acres) in far northwestern Colorado in the Uinta
Mountains (Matthews, 2009).
These rocks are termed the Owiyukuts Complex and were metamorphosed
about 2.7 Ga—in the Archean. This means
that the original rocks were older than this date; something had to be there to
metamorphose! The Owiyukuts Complex is
actually part of Wyoming –more on this later.
 |
Owiyukuts Complex (Archean) exposed in
northwestern Colorado overlain by the Proterozoic Uinta Mountain Group. Photo cropped from Matthews, 2009.
|
Most of the Precambrian rocks of Colorado, the ones
that core the north-south trending mountain ranges and are composed largely of Proterozoic
metamorphosed volcanic-sedimentary gneisses and schist, and some igneous
intrusive rocks. The radiometric dates
cluster around 1.75 Ga but again there needed to be earlier rocks to
metamorphose—geologists just don’t know where they came from but suspect
oceanic volcanic island rocks. Then
around 1.4 Ga a second major intrusive event emplaced several granitic types of
rocks such as the Sherman Granite in northern Colorado. And finally, a single large batholith (large
intrusive event) left us the Pikes Peak granite at ~1.05 Ga. In summary, readers can think of Precambrian
rocks in Colorado as being ~1.75 Ga metamorphic rocks, ~1.4 Ga granite, and ~1.05
Ga Pikes Peak granite.
The Precambrian rocks of Wyoming consist mainly of
three major geologic terranes: the Archean Wyoming
Province, the Proterozoic Trans-Hudson
Orogen , and the Proterozoic Colorado
Orogen (part of the Yavapai Terrane).
In this usage orogen refers to a belt of deformed rocks commonly
metamorphosed and intruded by igneous bodies—the rocks associated with a
tectonic or mountain building event.
The oldest rocks in Wyoming include intrusive
igneous and granite-like rocks as well as some metamorphic rocks. The Wyoming
Province is often called the Wyoming craton since it represents a very stable
part of the Precambrian “continent”.
Most of the state’s mountain ranges where the Precambrian crops out have
rocks of this age. Rocks of the
Trans-Hudson Orogeny, ~1.9 Ga, are found only in the subsurface in the eastern
part of the state, but are exposed in the nearby Black hills. The Colorado Orogen, or Colorado Province,
includes the metamorphic rocks with dates around 1.75 Ga as well as the 1.4 Ga intruded
granites in the southern Laramie and Medicine Bow ranges such as the Sherman
Granite.
 |
Landsat satellite image
Medicine Bow Mountains, Wyoming.
Cheyenne Belt trending NE-Sw below high peaks of the Snowy Range
(compare with map below). Image from
www.geology.com
|
One of the amazing features associated with the
Precambrian rocks of Wyoming is a narrow belt of highly deformed and
tectonically disturbed rocks termed the Cheyenne Belt. This zone is the tectonic suture between two
Precambrian provinces, a place where the older Archean rocks collided (plate
tectonics) with the younger Colorado Province and were welded together. This is an amazing site, at least for a
geologist!
 |
Sketch map showing location of Cheyenne Belt in
southeastern Wyoming. From Ward, 2010: www.colorado.edu/GeolSci/Resources/WUSTectonics/CheyenneBelt/index.html
|
One of the best places to see the suture zone up
close is to travel WY 130 west from Laramie through Centennial over the
Medicine Bow Mountains to Saratoga.
Popularly known as the Snowy Range Scenic Byway, the highway travels
through some of the most fantastic scenery in Wyoming. At the Nash Fork Campground the road crosses
the suture line and travelers may observe slate and phyllite that that is
complexly folded and crinkled (Hausel, 1993).
Rocks north of the Cheyenne Zone in the Medicine Bow Mountains contain the very old Archean crystalline rocks
overlain by several tens of thousands of feet of late Archean and early
Proterozoic metavolcanics, metasediments (last two terms refer to lightly
metamorphosed sediments and volcanics), quartzite, conglomerate and various
other rocks that were deposited in rivers, braided streams and shallow marine
waters in this ancient Precambrian environment—perhaps an environment similar
to the Atlantic coast of North America. The
best known geologic unit is the Snowy Pass Supergroup that includes the
Medicine Peak Quartzite, the almost white sugar sand quartzite that forms the
high peaks of the Snowy Range. Also in
the Snowy Pass Supergroup are other sedimentary rocks containing some of the
most beautiful stromatolites in the U.S. These features are composed of calcium
carbonate, cabbage-like domes deposited in shallow marine waters by cynobacteria
(aka blue-green algae). They may be seen
near the Sugarloaf Recreation area.
For a very good description of the Snowy Range,
complete with road log stops, see the Wyoming Geological Survey Information
Circular No. 32 (author: Dan Hausel) at: www.wsgs.uwyo.edu/Publications/OnlinePubs/docs/PIC/PIC-32.pdf.
Rocks south of the Cheyenne Belt are metamorphic
rocks (~1.75 Ga) intruded by granitic plutons (~1.4 Ga).
The Cheyenne Belt extends southwest and barely clips
northwestern Colorado where the Owiyukuts Complex is part of the old
Archean Wyoming Craton; hence the earlier statement that these rocks are part
of Wyoming! To the east the Belt is
buried under the Great Plains.
Chamberlain (1998) believes the Cheyenne Belt may extend as far as
northeastern Nevada.
All of this discussion on the Precambrian leads the
traveler back to WY 34 heading northeast from Bosler (north of Laramie). The highway traverses through numerous
outcrops called the Laramie anorthosite and they are worth a stop to examine
the road cuts. Anorthosite is a rather
strange igneous rock that is composed almost entirely (at least 90%) of the
feldspar mineral plagioclase, but especially common is the variety termed
laboradorite. Geologists have determined
that the igneous process forming the rock could not have been 90% enriched with
plagioclase. Therefore, the mineral must
have somehow segregated from the main magma mass (Lindsley and others, 2010). At any rate, the anorthosite was intruded
into the Laramie Mountains during the ~1.4 Ga igneous event. North of these outcrops the mountains cross
the shear zone and the rocks become older. Many rocks display the
laboradorensence of the mineral, and specimens are really nice when slabbed and
polished.
 |
|
Outcrop of anorthosite east of Bosler, Wyoming,
Laramie Range.
|
 |
|
Hand specimen of anorthosite showing laboradorensence.
|
The final tour of the Wyoming is to examine
exposures along WY 270 from Guernesy north to Manville. This road bisects a geologic structure called
the Hartville Uplift, a north-south trending Laramide (Rocky Mountain) uplift
exposing Precambrian rocks in the center surrounded by outward dipping
Paleozoic rocks (Sims and Day, 1999). The uplift is part of the Wyoming Craton and
ties in the Laramie Range to the Black Hills and also separates the Denver Basin
(east) from the Powder River Basin (west); the rocks are mostly Archean in age
but there are some Proterozoic igneous intrusions.
The Precambrian exposures are of
interest to Coloradans since the rocks contain large deposits of iron, both
banded iron formations and specular hematite.
Iron was first produced from the Sunrise mine, and later the Chicago,
Central, and Good Fortune mines, near the towns of Hartville and Sunrise in the
late 1800’s. These mines then shipped
this hematite ore to the Colorado Fuel and Iron Corporation open-hearth
furnaces
in Pueblo, Colorado. (Sims and Day,
1999). At the time when mining ceased at the Sunrise mine in 1980, the Hartville
district had produced about 45 million tons of iron ore (Hausel, 1989).
 |
Sunrise Mine ca. 1907. Photo courtesy of Wyoming
Tales and Trails.
|
The September 13, 1907 edition of the Mines
and Mining reported: Sunrise is a company
town in the fullest sense. Everything, and may it be said everybody, is owned
by the Colorado Fuel and Iron Company. No special brand is necessary, for the
fact impresses itself indelibly on all who come here. Visitors are not
especially welcomed, which a glance at the passenger accommodations on the
train that meets the Colorado & Southern at Hartville Junction forces
itself on all comers.
From Hartville Junction the spur to Sunrise via Guernsey, a
distance of about fifteen miles, belongs and is operated by the Colorado Fuel
and Iron Company. It is a fine piece of railroad engineering with its high
grades and frequent curves and one would not mind paying two prices for
transportation, as he must. If only the accommodations were adequate, but, as
has been said, the company seems not to care for that sort of traffic. Having
constructed the line for its own convience, no doubt it considers itself an
accommodator of the public by attaching a caboose to its trains of ore cars,
which caboose has poor seating capacity for about eight people, through several
times that number travel over the route as a rule.
The employees were
forced not only to depend on the favor of the Company for the opportunity to
earn a living, but to live in such houses as the Company furnished, to buy such
food, clothing and supplies as the Company sold them, to accept for their
children such instruction as the companies wished to provide, and to conform
even in their religious worship to the Company's wishes.
In summary, southeastern Wyoming has a number of interesting geological
features and rocks associated with the Precambrian. The southern Medicine Bow and Laramie ranges
have rocks that belong to the Colorado Orogen and date to the younger part of
the Precambrian termed the Proterozoic.
Metamorphic rocks have dates ~1.75 Ga and are intruded by granites, such
as the Sherman, with dates clustering around ~1.4 Ga. The northern boundary of these rocks is a
shear zone termed the Cheyenne Belt and represents the suturing of the Colorado
Orogen to the much older Wyoming Craton (rocks of the older Precambrian termed
the Archean). These Archean rocks are
exposed in the northern part of these ranges as well as in the Hartville
Uplift. In addition, in areas around the
suture zone in the Laramie Range (northeast of Bosler) large plutons of
anorthosite crop out.
Travelers should make every effort to travel these secondary as
they offer many more chances to examine the geology than say, I-25! We also need to remember that the total
environment back in the Precambrian was so much different than what we see at
the present. Physical environments were
similar in that the land contained streams and the oceans had different marine
zones; however, plants and animals as we know them did not exist. In addition, the atmosphere contained much
less oxygen and the ozone layer did not exist.
Enjoy the travel and remember the words of J. W. Schopf: For four-fifths of our history, our planet was populated by pond scum!
REFERENCES CITED
Chamberlain, K. R., 1998, Timing
of Deformation and Model of Crustal Structure Produced During Continent-arc Collision,
ca. 1.78 Ga, Southeastern Wyoming: Rocky
Mountain Geology, v. 33; no. 2.
Hausel, W.D., 1989, The Geology of Wyoming’s
Precious Metal Lode and Placer Deposits: Geological Survey of Wyoming Bulletin
68.
Karlstrom, K. E. and E. D. Humphreys, 1998,
Persistent Influence of Proterozoic Accretionary Boundaries in the Tectonic Evolution
of Southwestern North America: Interaction of Cratonic Grain and Mantle Modification
Events: Rocky Mountain Geology v. 33, no. 2.
Lindsley,
D. H., B. R. Frost, C.. R. Frost, and J. S. Scoats, 2010, Petrology,
Geochemistry, and Structure of the Chugwater Anorthosite, Laramie Anorthosite
Complex, Southeastern Wyoming: The Canadian Mineralogist, v. 48.
Matthews,
V., 2009, Messages in Stone: Colorado Geological Survey, Denver.
Sims, P. K. and W. C. Day (compliers), 1999,
Geologic Map of Precambrian Rocks of the Hartville Uplift, Southeastern Wyoming
with a section on Mineral Deposits in
the Hartville Uplift by Terry Klein:
U. S. Geological Survey Map I-2661.
Sims, P.K., and Finn, C.A., 2001, Precambrian Basement
Map of Colorado—A Geologic Interpretation of the Aeromagnetic Anomaly Map: U.S.
Geological Survey Open-file Report 01-364.