Devils Tower.
Public domain photo.
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As noted in previous blogs, I am fascinated by the
Laramide igneous intrusive rocks of the northern Black Hills, South Dakota and
Wyoming (~50-58 m). I suppose this inquisitiveness
started the first time I observed Devils Tower out on the plains of northeastern
Wyoming---wow, what a monument. As a dreamer
I wanted to believe in the Native American (at least one version) story of
causative events since again I was intrigued by the Seven Sisters in the night
sky, the Pleiades.
My favorite version of Devils Tower origin goes
something like this: a few girls,
probably 7, went out to play in northeastern Wyoming and were spotted by a
giant bear that tore out after them. When trying to escape from the bear, the
girls climbed a large rock and offer a prayer to their spirit leader. The spirit leader heard their prayers and
caused the rock to rise from the ground towards the heavens---out of reach of
the bear. The angry bear, in in trying to climb the rock and reach the girls,
left deep claw marks in the side of the rock. These claw marks are easily seen
today on the sides of Devils Tower. As the rock reached higher into the heavens
the girls transformed into the star constellation we know as the Pleiades.
As a kid we learned about the Big Dipper (Ursa Major)
and Little Dipper (Ursa Minor) and yet there in the sky was this tiny dipper
that seemed to not have a connection with the other “dippers.” To this day I still find myself looking up in
the night sky for the “dippers” so as to locate the north star and get my
directions in the correct order!
Why? I don’t have the slightest
idea why I need to confirm the compass directions. That aspect is just built into my
system. That same system also confirms I
want “paper” maps during travel. Yes, I
have the requisite GPS systems, both portable and “hooked to the dash,” but I
want those paper products.
If you are unfamiliar with the “dippers” and
directions consider the following. Most
people in the U.S. can locate the bright stars of the Big Dipper in the night
sky---it just sticks out there! The Big
Dipper is also a circumpolar constellation, at least for the lower 48--it stays
in the sky all night long and seems to rotate counterclockwise around the North
Star, Polaris (actually the earth is rotating).
As the diagram shows, one needs to locate the bright stars Merak (β) and
Dubhe(α) in the Big Dipper and follow their “point” to Polaris (the North Star) in
the Little Dipper. Polaris is situated
above the geographical North Pole so is a celestial guide to north. It has been used in navigation for millennia
and even today I know which way is north—at least in a cloudless night
sky. Two hundred years ago if I were
traveling I might draw a line in the soil or line up a stick pointing north. That way come morning I would have my directions
“straight.”
One of my favorite authors is William Least
Heat-Moon who wrote a book back in 1982 entitled Blue Highways. Heat-Moon took
a long trip “trying to find himself,” or something like that, following the
blue-colored highways, the secondary highways, on the old road maps. My father ran a gasoline station (filling
station in the vernacular) in a small Kansas town and gave away those road maps
to anyone who requested such. I
collected all of these wonderful fonts of information! Even today I have numerous road atlases and
on a regular basis, sometimes daily, I pick a state or region and study the map
memorizing towns and rivers and mountains and other locations. I am easily amused! Much to chagrin of most travelers in my
vehicle, I love to travel the blue highways, rarely getting “lost”. The secret to knowing where you are located
is not a GPS but remembering all of those memorized maps, plus the fact that in
my home states on the plains the majority of the roads go east-west or
north-south following section lines (Public Land Survey System).
OK, back to Laramide intrusive rocks in the Black
Hills. I have written small postings
about Bear Butte, Crow Peak and Spearfish Peak, all major land forms in the
northern and eastern Hills. Last summer
I talked my Black Hills guide, the Junior Geologist, into hauling me to the old
mining area of Tinton. Actually, Tinton
should be described as a former (tin from cassiterite), and active, mining
area. Today most streams seem to have gold
placer mining camps, both squatters and weekenders, and some large exploration companies
have staked several hundred claims. As
best that I can tell many of the larger companies are looking for gold in
paleoplacers of the Cambrian Deadwood Formation, or in rocks similar to those
past producers over at the Homestake Mine near Lead. The companies, placer miners, and people
living near Tinton do not seem the friendliest in the world and threaten
visitors with all sorts of bodily harm for setting foot of the road. My momma didn’t raise no fools so the Junior
Geologist and I just kept trucken’ so as to stay out of the sights! I would have enjoyed taking a peek at the old
mines.
Lisenbee and Dewitt (1993) described the rocks at
Tinton as being related to one of three large intrusive centers in the northern
Black Hills---the Tinton Dome in the western Hills on the Wyoming-South Dakota
state line, the Deadwood-Lead Dome and the northwestern Bear Lodge Dome, all
related to basement (Precambrian) plutons swelling the overlying rocks and
sending up hot magma to solidify as laccoliths, sills, dikes, and other stocks.
The Tinton Dome has two major areas---the Tinton
District in South Dakota where cassiterite was the major mineral producing tin,
much of it placer tin derived from pegmatites in the area. Across the state line is the Mineral Hill
District, a locality that produced gold from the 1870’s through the 1930’s, both
placer and lode. Today, I believe,
exploration companies are purchasing leases at Mineral Hills to look for
porphyry-style gold and copper deposits.
This type of deposit is has produced much of the copper in the desert
southwest.
Cassiterite (SnO2) granules were mined
(placer) at Tinton in the late 1800’s and early 1900’s. These granules were taken from a small stream
near Tinton in the late 1960’s.
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The three large Laramide intrusive
domes in the Black Hills: Bear Lodge, Tinton, Lead/Deadwood (BH). Inyan Kara is a separate laccolith. Map from DeWitt(1989).
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The rocks at Mineral Hill are exceedingly complex
with ring dikes, multi-phase alkaline intrusive rocks including trachyte
porphyry, Precambrian rocks, amphibolite, and sedimentary rocks, especially the
Cambrian Deadwood Formation, and perhaps a central brecciated diatreme
pipe. The epithermal style gold veins (warm
water at a somewhat shallow depth) were the targets of the early miners (2009,
Market Wired).
The trachyte porphyry (mostly orthoclase feldspar of
two different generations—see photo) at Mineral Hill occurs as sills and dikes
and is part of the Laramide intrusive suite in the northern Hills. The southwestern section of Mineral Hill is
known as Cement Ridge where the resistant trachyte forms the highest elevation (6674
feet) of the dome, and the second highest peak in the Wyoming section of the
Black Hills and is home to a USFS Fire Lookout Tower.
USFS Fire Lookout Tower at Cement
Ridge. Outcrop in foreground is part of
the Laramide intrusive.
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North of Sundance lies the Bear Lodge Mountains, a
major section of the Wyoming Black Hills.
Again, Lisenbee and DeWitt (1993) described the Bear Lodge as one of the
major domes of the northern Black Hills where basement plutons and rising magma
have pushed up overlying rocks into a dome.
The Bear Lodge Mountains are now under exploration for Rare Earth
Minerals.
Sundance Mountain, a laccolith on
the edge of the community. The rock
exposed is a rhyolite (Lisenbee and DeWitt, 1993).
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Immediately southwest of Sundance, actually on the
edge of town, is a nice laccolith know as Sundance Mountain (elevation 5810). This igneous rock is a magnesium rich
rhyolite (Lisenbee and DeWitt, 1993).
My favorite laccolith in the Black Hills area is
Green Mountain just southeast of Sundance (elevation 3806 feet). It is a perfect circular dome with hogbacks
surrounding the feature but without igneous rocks exposed in the center. The
magma domed the overlying sedimentary rocks; however, they were not completely
stripped off. So, it actually is a nice
little anticline and is mapped as such on maps.
Another easily available exposure of Laramide intrusive rocks in the northern Black Hills is near Bridal Vail Falls in Spearfish Canyon (US Alt. 14). At this locality a large sill was intruded between the Cambrian Deadwood Formation and rocks of Precambrian age. The falls are directly related to the sill since a small stream in Rubicon Gulch could not cut through the hard igneous rocks and therefore tumbles over the sill into local base level, Spearfish Creek.
Another easily available exposure of Laramide intrusive rocks in the northern Black Hills is near Bridal Vail Falls in Spearfish Canyon (US Alt. 14). At this locality a large sill was intruded between the Cambrian Deadwood Formation and rocks of Precambrian age. The falls are directly related to the sill since a small stream in Rubicon Gulch could not cut through the hard igneous rocks and therefore tumbles over the sill into local base level, Spearfish Creek.
Another easily available exposure of Laramide
intrusive rocks in the northern Black Hills is near Bridal Vail Falls in
Spearfish Canyon (US Alt. 14). At this
locality a large sill was intruded between the Cambrian Deadwood Formation and
rocks of Precambrian age. The falls are
directly related to the sill since a small stream in Rubicon Gulch could not
cut through the hard igneous rocks and therefore tumbles over the sill into
local base level, Spearfish Creek.
A laccolith somewhat more difficult to spot is Elkhorn Peak located along I-90 between Whitewood and Spearfish along the eastern flanks of the Black Hills. The igneous rocks intruded between the Precambrian and the Cambrian deadwood Formation and domed up the overlying sedimentary rocks: the Minnekahta Limestone (Permian) is exposed as a dip slope while the Pennsylvanian-Permian Minnelusa Formation covers the dome.
Elkhorn Peak, a covered laccolith,
located along I-90 near Spearfish.
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As for Devils Tower, the rock is a phonolite (nephaline
seyenite—see posting on Spearfish Peak) that was intruded as hot magma into the
overlying sedimentary tocks during the Laramide intrusive event. The “claw marks” defining columns are
actually contraction furrows propagated as the magma cooled. Over several million years the sedimentary
rocks eroded away leaving behind the resistant igneous rocks. The monolith rises ~867 feet from its base, ~1,267
above the Belle Fouche River, and ~5,117 above sea level. In 1906 Devils Tower
was designated as our first National Monument.
REFERENCES
CITED
DeWitt, E., 1989, geologic Map of the Black Hills Area,
South Dakota and Wyoming: U.S. geological Survey Miscellaneous Investigations Map
I-1910.
Lisenbee, A.L. and E. DeWitt, 1993, Laramide
Evolution of the Black Hills Uplift in Snoke, A.W., J.R. Steidtmann and S.M.
Roberts, eds. Geology of Wyoming: Geological Survey of Wyoming Memoir no. 5.
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