BLACK HILLS: LARAMIDE INTRUSIONS

Devils Tower.  Public domain photo.

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.”

 

Locating "north".  Photo courtesy Washington University in St. Louis

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).

 

I would call this a marker tree!  Leaving Tinton heading to Wyoming.

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.

 

Signs like this are abundant near Tinton.

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.

 

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).

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.

Slab of trachyte porphyry at Cement Ridge ~13 cm width.  The majority of the rock seems to be orthoclase feldspar, both as the groundmass and the larger phenocrysts.  A few flakes of biotite are present and perhaps some slender crystals of aegirine/acmite.   

  

Cement Ridge portion of Tinton Dome.  Map from DeWitt (1989).

 A few miles east of the Wyoming-South Dakota state line is the small community of Sundance (MM 189 on I-90).  I suppose most of the readers recognize the community as the namesake of the Sundance Kid, an itinerate cowpoke (Harry Longabaugh) once jailed in the local hoosegow for borrowing someone’s horse without permission.  Of course, Robert Redford played a cool, suave cowboy (opposite Paul Newman) in the movie Butch Cassidy and the Sundance Kid and suddenly the world knows about Sundance, Wyoming.

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).  

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).

This is my favorite, Green Mountain or Little Sundance, located just east of the community.  It is a beautiful circular anticline with igneous rock pushing up the dome.  However, the stripping of the Paleozoic rocks has not yet exposed the igneous rock under the surface.  I-90 cuts across the landscape with an interchange for the community.

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.

Bridal Vail Falls flowing over the phonolite sill in Spearfish Canyon.

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.

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. 

MarketWire: http://www.marketwired.com/press-release/Golden-Predator-Acquires-Mineral-Hill-Alkalic-Gold-Property-Crook-County-Wyoming-TSX-VENTURE-GPD-1029605.htm