BLACK HILLS: TUFA vs. TRAVERTINE

Like most readers of this Blog, I often (always) peruse rock and mineral shops when visiting towns and cities across the nation.  This fall while camping in western South Dakota I paid a call to about all shops in the Black Hills.  Two of them had interesting and similar specimens with labels stating: 1) moss rock; and 2) coral rock.  Both rocks were labeled as collected from Cascade Springs.

Now that name brought back a flood of memories from days gone past.  I first visited the spring, located south of the city of Hot Springs, back in the mid-1960s while a student at the University of South Dakota.  I had several friends from the nearby small town of Edgemont and could tag along on their trips home.  I was first introduced to Cascade Springs in the form of lounging in the warm sun and sharing a few bottles of a cold adult beverage.  Later in life I visited the Springs on a field trip and even at a later time camped in a small tent in the area and watched the stars twinkle in a very dark sky.

At any rate, I might accept moss rock as an identification that could result in a sale to an unwary tourist, but coral rock is just basically an untruth.  All it takes is a couple of clicks on a computer to receive information that fresh water corals really do not exist in South Dakota.  Although I knew the answer to the rock identification question, I wanted relive some memories and so off we went to Cascade Springs (six miles to Cascade Springs; eight miles to Cascade Falls).

Although my mind may be used and a bit rusty, it certainly indicated “things” have changed since my last visit about three decades ago.  What I first noticed was the increase in vegetation around the Springs and the resulting stream outlet, especially the rather prominent displays of poison ivy.  I remember, at least my mind thinks it remembers, walking along the stream below the Springs without getting tangled in a mess of vegetation.  Today that is an impossible task.  Oh well, maybe that thought is true, maybe not!

Cascade Creek below the Springs. Note the massive vegetation along the edge.

Black Hills National Forest (BHNF) manages Cascade Springs as a natural area and picnic ground and per the Agency (see References) there are several rare plants growing near the springs: These species include “tulip gentian (Eustoma grandiflorum), beaked spikerush (Eleocharis rostellata), southern maidenhair fern (Adiantum capillus-veneris), and stream orchid (Epipactis gigantea).”  The plants like the availability of open water during all four seasons as the discharge temperature is a constant 67ºF---not a hot spring, as most people would testify, but not a cold one either.  However, most articles I read would characterize 67º as “warm water” and above the ambient air temperature.  The BHNF pegs its discharge rate as ~22.5 cubic feet per second, the largest spring(s) in the Black Hills.  The Springs emerge from six different outlets, now covered with rock debris and gravel, and water is captured in a concrete pool before wandering downstream in the newly formed Cascade Creek. Ultimately Cascade Creek reaches the local base level, the Cheyenne River above Angostura Reservoir. The Springs release water from the Paleozoic Madison Limestone (aka Pahasapa Limestone of Mississippian age) and the Minnelusa Formation (limestone of Pennsylvanian-Permian age), both common aquifers (collectively known as the carbonate aquifer) in the region and a source of springs, both hot and cool/ambient, in South Dakota, Wyoming and Montana.  Or, the Springs could issue from the contact of the Minnekahta Limestone [Permian age] and the Spearfish Formation [an aquitard shale of Permian-Triassic age], or from the contact of the Minnelusa and Opeche formations (a possible aquitard between the Minnekahta and Minnelusa formations).  The Minnekahta is sometimes included in the term “carbonate aquifer” noted above. 

A stratigraphic section showing aquifer units around the southern Black Hills.  Section courtesy of Gries (2009).

Ford and others (1996) opinion is that rainwater passing through surface soil horizons picks up calcium carbonate that mixes with the aquifer water and travels through the karstic solution cavities to emerge at springs or streams.  The biogenic activities in the soil horizons have high levels of calcium bicarbonate.

Tufa collected from Cascade Falls during a personal outing decades ago!  Note longitudinal views of plant debris (tubes) weathered out of specimen.  Note porous nature of tufa as compared to travertine pictured below.  Width of specimen ~13 cm.

Collecting pool for Cascade Springs.

Casts (the tubes) of plant debris in tufa from Cascade Falls. See longitudinal view in photo above. Diameter of tubes ~1-2 mm. 

Cascade Falls September 2016.

Cascade Falls ca 1930s.  Original postcard owned by, and courtesy of, www.neplains.com.

Cartoon showing tufa formation at Cascade Falls.  The water drops off a hard ledge of sandstone and then scours out a basin in the Skull Creek Shale.  Personal observation plus information derived from Ray and Rahn (1997).

Downstream two miles from Cascade Springs is Cascade Falls, one of the more famous “swimming holes“ in South Dakota, and the site of  several tufa layers.  It was always my impression that tufa was a calcium carbonate (CaCO₃) deposited in cool water situations as opposed to travertine (also a calcium carbonate) that forms in a warm to hot water environment.  Tufa generally is found attached to plants or plant debris, is usually quite porous and forms a carbonate layer over the plant debris that later “rots away” leaving behind tufa casts of the plants (and sometimes insects, vertebrates and mollusks).

This description of tufa and travertine has resided in my mind for decades but now was prodding my senses with a question—what is the temperature that distinguishes the formation of tufa from the formation of travertine? So, off I go to try and find the answer.

Travertine "terraces" produced by hot water springs at Hot Springs State Park, Wyoming.  See Blog posting May 13, 2011.

Ford and Pedley (1996) and Capezzuoli and others (2013) noted that travertine and tufa were often used indiscriminately as alternative names for fresh water limestone.  One person’s tufa seemed to be another person’s travertine.  This seemed especially true as I delved into the literature and found both names used for the same stratigraphic outcrops.  Capezzuoli and others (2013) defined the term travertine: “continental carbonates mainly composed of calcium carbonate deposits produced from non-marine, supersaturated calcium bicarbonate-rich waters, typically hydrothermal in origin. Travertine deposits are characterized chiefly by high depositional rates, regular bedding and fine lamination, low porosity, low permeability and an inorganic crystalline fabric. Bacteria and cyanophytes [photosynthetic bacteria] typically are the only associated organic constituents, due to the presence of unsuitable factors (for example, high temperature, high rates of deposition, pH and sulphur) for plant and tree growth (macrophytes). Aragonite rather than calcite may also be present… Such deposits are typical of tectonically active areas where geothermal heat flux (endogenic or volcanic) is high [generally higher than ~86º].”

Banded travertine collected from the Mayer "onyx" quarry in northern Arizona. Note compact nature of travertine and compare with photo of tufa above. See Blog posting March 20, 2015.

Tufa refers to “continental carbonates, composed dominantly of calcite and typical of karstic areas. These are typically produced from ambient temperature [generally less than ~68º], calcium bicarbonate-rich waters which are characterized by relatively low depositional rates producing highly porous bodies with poor bedding and lenticular profiles, but containing abundant remains of microphytes and macrophytes, invertebrates and bacteria. Secondary carbonate deposits (cements and speleothems) may also be associated. Aragonite is usually absent (except from peculiar high Mg/Ca ratio spring waters.”

It appears, then, that travertine does form in warm to hot waters heated by geothermal mechanisms, has a high rate of deposition, has low porosity and permeability, and does not contain the plant debris common in tufa.  Tufa forms in areas where ground water has traveled through rocks rich in calcium bicarbonate via fractures and caves [karstic], has poor bedding features, and contains plant and bacterial (“algal”) debris.  In addition, travertine often forms as mounds and terraces while tufa is often found in stream cascades and dams.

The water from Cascade Springs has its original source percolating through soil horizons over a wide area in the southern Black Hills and then traveling through the karstic cavities of the carbonate aquifer and therefore has a dissolved CO₂ content much higher than the local atmosphere. The turbulence created by CO₂-rich water flowing over the Falls degasses the dissolved CO₂, the water chemistry equilibrium is messed up (CO₂ level drops), and precipitation of tufa (CaCO₃) takes place. I have not seen studies on Cascade Creek but in a “normal” situation after degassing and precipitation, the pH of the stream water decreases and the acidity increases.  I presumed since the water at Cascade Springs was saturated with CO₂ that the pH would strongly basic; however, Lund (2016) noted the pH was neutral at 7.0.  But again, I am far from a water chemist.

At Cascade Falls the actual waterfall started as Cascade Creek flowed over a ledge of an indurated Cretaceous sandstone, the Newcastle Sandstone.  With this turbulence of the CO₂-rich water, tufa began to form on the Newcastle and actually raised the height of the Falls.  Cascade Falls has been around for a long time since the terrace levels above the stream are composed of tufa.  I suppose this signals that Cascade Creek meandered over the valley in the geological past and CO₂-rich water degassed and forced calcium carbonate out of solution as solid CaCO_3.  Visitors can easily see the tufa at the Falls while an observant eye can locate tufa on the stream terraces. I presume this is the collecting area for the moss rock and coral rock displayed in the shops.

Tufa forming at Cascade Falls.

Why is travertine absent at Cascade Springs? Evidently, there does not seem to be a source in the immediate area that would supply heat to the aquifers in the Paleozoic limestones.  However, a few miles away the city of Hot Springs was named for their warm spring water (something like 8-9 warm/hot springs) and became an early soaking spa and advertised “disease-curing” resort.  The Mammoth Hotel and Bath House was built in the late 1880s and their spring water was ~90ºF (Lund, 2016).  For many years one of the top attractions in Hot Springs has been a large constructed swimming pool known as Evans Plunge that is fed by 87ºF springs that release about 11+ cubic feet per second.  So, why the presence of hot/warm springs at Hot Springs?  Rahn and Gries (1973), in their extensive study of large springs in the Black Hills, could not answer that question with certainty.  Their first “supposition” was heat supplied by the earth’s normal geothermal gradient; however, their studies concluded that “the unusually warm springs near the town of Hot Springs are too warm to be explained by the normal geothermal gradient.”  So, what about their other possibilities: 1) magma or some intrusive body may lie at a shallow depth under Hot Springs; however, there is little evidence for that possibility; 2) the ground water could be warmed by chemical weathering reactions of the water flowing through the rocks.  This mechanism seemed a good possibility; 3) the ground water may have been heated by radioactive decay in nearby rocks.  Locally the community of Provo had a flowing artesian well where water was about 139ºF and was evidently heated by decay of radioactive minerals.  However, the thermal waters at Hot Springs are not very radioactive; 4) Precambrian rocks under the town may have created a higher geothermal gradient.  This may be possible but not probable.  In studying their publication, I really don’t believe Rahn and Gries found a reasonable (at least one they believed in) mechanism to answer the heat question and I have been unable to locate in the literature other possible heat sources.  The best that I could come up with was to note that several deep wells in the Madison (across South Dakota, North Dakota and Wyoming) have elevated water temperatures!

One of the early resorts in Hot Springs, the Hotel Minnekahta.  Photo is from the Library of Congress collection and was taken by John Grabill ca. 1890.  

One can find travertine, or tufa, in the city of Hot Springs since it serves as a cement for the prominent beds of conglomerate along the Fall River.  In reference to the conglomerate, Gries (2009) stated that “at some time in the past, probably in late Pleistocene time, clay and gravel partly choked the [Fall River] canyon.  Then calcium carbonate, precipitated from the warm spring water, cemented them into solid rock.”  Is this travertine or tufa?   I was unable to identify with visual examination; however, the 87º water at Evans Plunge would suggest travertine.  Whatever the case, Rahn and Gries (1973) map of spring temperatures in the Black Hills has a nice anomalous, hot/warm, birdseye perched right on Hot Springs.

It seems as everyone in Hot Springs calls this "The Waterfall."  I am uncertain of the name or the source of the warm water.  Perhaps it comes from a spring above? Travertine is forming on rocks behind the falls. The cemented conglomerate is what caught my eye.

In contrast to the southern hot/warm springs the remainder of springs in the Black Hills are in the 40ºs-50ºs F range (Rahn and Gries, 1973).  One particular spring of interest is found about three miles above the junction (near community of Savoy) of Little Spearfish Creek and Spearfish Creek.  Here a spring in the Madison Limestone (aka Pahasapa) releases about 13 cubic feet per second of CO₂-rich water into Little Spearfish Creek. Little Spearfish Creek rapidly flows east toward its merger with Spearfish Creek but soon encounters a hard dolomite bed of the Ordovician Whitewood Dolomite, and thus, there is massive turbulence in the Creek and the saturated CO₂-rich water is degassed and tufa forms.  This feature is known as Roughlock Falls and is considered one of the more scenic spots in the Black Hills.  Again, the water is considered karstic in nature and flows through the quite porous Madison (Pahasapa) Limestone of Mississippian age. The water is “cold” but I could not locate an exact temperature but a nearby lodge owner told me the Falls freeze up in the winter.  It is interesting, at least to me, that the springs in the northern Hills seem cold water springs.  The northern Hills have numerous relatively young, ~50 Ma, igneous intrusions that would seem to be a good source of heat.  But then again, I am not a hard rock person or an aqueous geochemist or a hydrologist!

The upper falls at Roughlock Falls in Little Spearfish Canyon.  Note the heavy vegetation at the Falls and the formation of tufa where degassing of saturated water takes place.

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Old Postcard ca. ?? of upper and lower Roughlock Falls.  Even in the "olden days" the Falls attracted a massive amount of vegetation.

Cartoon showing how tufa forms on Roughlock falls.  Personal observation plus information derived from Ray and Rahn (1997).

Falls is the massive amount of vegetation growing along the Falls.  However, the Falls are managed by the South Dakota Parks and Recreation and they have built nice wooden platform viewing areas.  The area attracts numerous visitors wandering over from their drive up Spearfish Canyon.

I have not seen studies on South Dakota tufa and travertine but in other localities paleo-environmental studies have provided important information on climatic conditions at the time of deposition, as well as absolute dates---using carbon dating if the organic materially has not been biogenetically altered and is younger than about 50k.  Isotopic studies can help with absolute dates and often can provide information about climate at the time of deposition.  Since tufa contains plant material, at times vertebrate and arthropods fossils, as well as microfossil such as ostracods, scientists can use these fossils to articulate additional information about past environments.  For example, see Ollivier and others (2012).   But again, I have not observed environmental studies on travertine and tufa deposits in South Dakota.

REFERENCES CITED

Capezzuoli, E., A. Gandin, and M. Pedley, 2013, Decoding tufa and travertine (fresh water carbonates) in the sedimentary record: The state of the art; Sedimentology, v. 61, no. 1.

Ford, T.D. and H.M. Pedley, 1996, A review of tufa and travertine deposits of the world: Earth Science Reviews, v. 41.

Gries, J.P., 1996, Roadside Geology of South Dakota: Mountain Press Publishing Company, Missoula.

Lund, J., 1997, Hot Springs, South Dakota: Oregon Institute of Technology Geo-Heat Center Quarterly Bulletin v.18, no. 4.

Ollivier, V., P. Roiron, S. Nahapetyan, S. Joannin, and C. Chataigner, 2012, Tufa and travertine of the Lesser Caucasus: a light on the Quaternary palaeoenvironment of the Circumcaspian regions: Geophysical Research Abstracts v. 14, EGU2012-2124.

Rahn, P. H., and J. P. Gries, 1973, Large springs in the Black Hills, South Dakota and Wyoming: South Dakota Geological Survey, Report of Investigations 107.

Ray, C.M. and P.H. Rahn, 1997, The origin of waterfalls in the Black Hills, South Dakota:  Proceedings of the South Dakota Academy of Science, v. 76.

USNFS: www.fs.fed.us/wildflowers/regions/Rocky_Mountain/CascadeSprings/index.shtml

For a great story about the “old” resort town of Cascade: “Of all the “ghost towns” in South Dakota, the grandest one may have been Cascade, sometimes referred to as Cascade Springs because of the nearby hot springs. Back in 1892, its heyday, the town had about 400 people and 50 businesses, including a hotel, a sanatorium and a bowling alley.”  See http://www.capjournal.com/news/dakota-life-the-life-and-death-of-cascade/article_212d861a-2ebe-11e6-876d-e7d8d36ea850.html

SCAPOLITE: A SORT OF FORGOTTEN MINERAL

Why should things be easy to understand?

Thomas Pynchon

Scapolite is one of those minerals that sort of rings a bell somewhere in the recesses of your mind; however, you cannot quite pinpoint the location! About the only thing that finally surfaced in my mind came from basic mineralogy and pointed out that scapolite is usually an alteration product of feldspar (which one?), and is a metamorphic mineral (which facies?)!  I sort of left it at that point until a few years ago when I saw some beautiful faceted gemstones labeled “scapolite.”  Perhaps my mineralogy factoids were a figment of my imagination for those faceted gems looked nothing like some less-than-spectacular specimens I remembered from class.  

Perhaps I could forgive my mind since I was a third-year college student trying to reconcile memorizing mineral crystal systems with understanding the bombing of the 16^th Street Baptist Church in Birmingham and the assassination of President Kennedy in Dallas.  In fact, the assassination of Kennedy is one of those moments in history that persons of my age have imprinted on their minds--- I was heading to Mineralogy class!  Why did the crystal systems matter when young girls and presidents were being murdered?  I guess the short answer is that I did not want to return to my home town and work in my father’s gasoline station.  And then, there were rumors about “goings-on” in southeast Asia with the military draft picking up and men of my age learning a new trade.  So, back to learning about Monoclinic and Hexagonal minerals (and I never really understood the Systems and became a paleontologist).  And, scapolite became lost!

Age is an issue of mind over matter.  If you don’t mind, it doesn’t matter.

Mark Twain 

Scapolite reappeared in my mind back in 2012 when I was working on a post describing idocrase/vesuvianite.  The latter mineral was named by the famous German mineralogist Abraham Gottlob Werner and an informal variety of scapolite is called werernite.  Long story---read the November 18, 2012 Blog posting.  At any rate, I then took scapolite from the back recesses and shoved it toward the front of my mind and four years later am finally getting around to describing some specimens that I picked up along the way!

Scapolite is a silicate but is not really an individual mineral!  It is a solid solution series between end members marialite (sodium chloride rich) and meionite (calcium carbonate rich): Na₄Al₃Si₉O₂₄Cl to Ca₄A_l6Si₆O₂₄CO₃.  The sodium and calcium are interchangeable with each other as are the chlorine and the carbonate radical, therefore leaving an infinite number of chemical compositions. In addition, the calcium may include some strontium while the sodium may include potassium. And SO₄ may substitute for some CO₃ (Evans and others, 1969).  It appears that “pure” end members never occur in nature so intermediate compositions are the norm; however, these intermediate members vary considerably in chemical composition and remain unnamed.  Members of the solid solution series are essentially indistinguishable (visual) from each other and therefore scapolite is simply used for all. 

Scapolite comes in a variety of spectral colors ranging from colorless to white and yellow, purple, blue, red, green, pink, brown, gray, orange and various mixed compositions.  However, all varieties have a white streak. The transparency ranges from completely opaque to translucent to completely transparent while the luster ranges from vitreous to dull and pearly.  As scapolite weathers to “mica” the luster becomes dull and the diaphaneity becomes opaque. The hardness of ~5.5-6.0 (Mohs) makes gemmy varieties more suitable for pendants rather than rings.  Scapolite crystals are Tetragonal and generally come in two distinct forms: short and fat, or long and prismatic.  Gemmy varieties are usually prismatic and commonly striated.  A couple of my specimens show masses of non-gemmy and opaque crystals.  Many times, crystals fluoresce under both short and long wave UV.

Scapolite is one of the few minerals that have a “square” cross-section that helps in identification.  Compare photos below of a weathered crystal from Monmouth Township, Ontario, Canada, with a crystal diagram from the Goldschmidt atlas and found on www.mindat.org and courtesy of www.smorf.nl.

Cross-sectional view of scapolite crystal, non-terminated, collected from Grenville Terrane near Bancroft, Ontario.  Note square shape of crystal and compare with sketch below.  Width of crystal ~1.6 cm; length ~2.3 cm. 

Crystal diagram of scapolite from the Goldschmidt atlas and found on www.mindat.org and courtesy of www.smorf.nl. Note square shape.

I thought scapolite was perhaps a mineral indicative of a specific metamorphic facies.  However, I have learned the “mineral” occurs in a variety of metamorphic conditions ranging from regionally metamorphosed schists and gneisses to higher temperature and pressure amphibolites and granulites (usually as an alteration of feldspar minerals and producing non-gemmy crystals).  In addition, scapolite, at times gemmy, is found in marble produced by contact metamorphism. At other times scapolite in these calc-silicate rocks contain inclusions of clinopyroxene, quartz, titanite and calcite (Ocean Drilling Program).   It is also found, at times, in pegmatites associated with contact metamorphism, and basalt ejected from volcanos.  I certainly am far from a mineralogist/petrologist but have spent numerous hours reading “lots of articles” concerning scapolite, and trying better to understand the chemistry and genesis.  I have somewhat failed in my understanding and concluded that it is a very complex mineral found in several different environments and is quite difficult to identify as to a specific mineral.

Just because we don't understand doesn't mean that the explanation doesn't exist.       Madeleine L’Engle

My collection includes two specimens composed of a non-gemmy mass of opaque crystals collected from around Bancroft, Ontario, Canada.  Also from near Bancroft is a single, squat weathered crystal. 

Above two photos are masses of opaque, non-gemmy scapolite crystals.  Note nice terminations on crystals with T pointer and nice square shape with SQ pointer..  Width FOV top ~4.0 cm, bottom ~4.3 cm.  Both specimens have tiny crystals of an amphibole (katnophorite/hornblende??) and ferroan phlogophite on reverse.

  

Map of proto-North America showing addition of crust (yellow) to continent in late Precambrian by plate collision tectonics (Grenville Orogen).  High temperature and pressure accompanies these collision events and creates large expanses of metamorphic rocks and allows for the formation of minerals like scapolite. Map from Karlstrom and others (1999).

The Bancroft area of Ontario, part of the Grenville Province, is thought to have been the margin of North America during the Proterozoic part of the Precambrian.  The rocks are composed of two tectonic elements: 1) high-grade gneisses that were part of the 1.7-1.4 Ga continental margin; and 2) a package of volcanic, plutonic, and sedimentary rocks that are thought to be a collage of arc components accreted at ca. 1.17 Ga (island arc material stuck onto the early continent by plate collision) (Keck Geology Consortium, 2011).

From the Dara-i-Pech pegmatite field, Chapa Dara District, Konar Province, Afghanistan, I have several small gemmy crystals lavender in color.  The crystals are prismatic in nature and have at least one terminated end.  The location of the crystal mine is in the northeastern part of the country where lower Paleozoic rocks are intruded by Cretaceous-Tertiary granite and granodiorite intrusions (creating contact metamorphism--cooking the limestone).   Due to political instability in Afghanistan, specifics about gemstone localities are difficult to ascertain.  

Nice gemmy scapolite crystals.  length of longest crystal is ~1.1 cm.

I also have a partial violet crystal from the Marble Occurrence, Morogoro Region, Uluguru Mountains, Tanzania.  As best that I can determine, the area is the site of plate collisions in the latest Precambrian.  Metamorphism and thrust faulting left small patches of marble on older rocks (Fritz and others 2009).  If you have the inclination to read about some really complex geology, check out the Fritz article!

Partial crystal of gemmy scapolite with undetermined inclusions.  maximum width of crystal ~1.1 cm.

And finally, I have a beautiful, free form cab of crystal-clear, gemmy scapolite collected from Espirito Santo, Brazil (along with a second specimen, a nice gemmy, prismatic crystal).  Espirito Santo is a coastal Brazilian state north of Rio de Janeiro and east of the famous mineral-producing state of Minas Gerais.  It was difficult to acquire much information about the area except that really gem quality aquamarines are mined from the Mimoso do Sul Mine.  The gem bearing rocks are latest Precambrian in age (100 Ga to 54 Ga) and seem related to the Aracuai Orogeny and include a wide variety of metamorphic rocks and igneous intrusions.  The Aracuai Orogeny added crustal rock to the local Brazilian Craton. I presume, but remain uncertain, that the gem scapolite came from some of the marble units.

Prismatic, gemmy, clear with yellow tint, scapolite crystal. Length ~3.0 cm.

Gemmy, clear with yellow tint, free-form cab of scapolite. The X is beneath the cab to show the transparent nature of the crystal (thickness 6 mm.).  Length ~2.3 cm.

So, when it comes to scapolite:  I don't think I'm old enough or experienced enough to give anyone any guidance. All I would like say is that as long as you're having fun, I think you're doing the right thing.                Sania Mirza

REFERENCES CITED

Evans, B.W., D.M. Shaw, and D.R. Haughton, 1969, Scapolite stoichiometry: Contributions to Mineralogy and Petrology, v. 24, issue 4.

Fritz, H., V. Tenczer, C. Hauzenberger, E. Wallbrecher and S. Muhongo, 2009, Hot granulite nappes—Tectonic styles and thermal evolution of the Proterozoic belts in East Africa: Tectonophysics, v. 477.

Karlstrom, K.E., S.S. Harlan, M.L. Williams, J. McLelland, J.W. Geissman, Karl-Inge Åhäll, 1999, Refining Rodinia: Geologic Evidence for the Australia–Western U.S. connection in the Proterozoic:  GSA Today, v. 9, No. 10.

Keck Geology Consortium, 2011, Anatomy of a mid-crustal suture: Geology of the Central Metasedimentary Belt boundary thrust zone, Grenville Province, Ontario:  http://www.keckgeology.org/2011-ontario-canada.

Ocean Drilling Program, Unknown date, Macroscopic description of calc-silicate rocks:  http://www.odp.tamu.edu/publications/161_SR/chap_18/c18_3.htm

MORE SOUTH DAKOTA FAIRBURN AGATES

Memories, pressed between the pages of my mind
Memories, sweetened thru the ages just like wine

Elvis

As many/most readers know, I have a very soft spot in my heart for the State of South Dakota.  I suppose that comes from my attendance, in the mid-1960s, at the University of South Dakota.  During those two years of completing a graduate degree, I became fascinated with the diversity of the state’s geology from the glaciated eastern half to the Missouri River Trench to the “badlands” of the west and finally to the mountains of the Black Hills.  In “those olden days” collecting minerals from outcrops and mine dumps was fairly easy as the land was open or land owners were very accommodating to student collectors.  Today much has changed as land owners are increasingly frightened by liability lawsuits, and unscrupulous collectors (I use that term loosely) have ruined landscapes, left open pits, and swiped vertebrate fossils.  Field collecting is getting difficult. 

As usual, my early fall trip to the Black Hills of South Dakota included a couple of jaunts to the agate beds---locations where collecting is still possible.  There are several postings on this Blog that emphasize the geology of the beds so I will not repeat that information here.  I essentially want to post a few photos to emphasize that the agates are still out there, both in the source beds at Teepee Canyon, and out on the plains where rocks were transported by ancient streams.  Collecting the former requires some large crack hammers, arm strength, gloves and eye protection while collecting on the plains emphasizes “lots of” walking away from the main roads.

Teepee Canyon is located approximately18 miles west of Custer, South Dakota, about 2 miles west of Jewel Cave National Monument off U. S. 16.  As soon as travelers leave the Monument they should look to the west, up slope, to spot piles of broken rocks.  Sawmill Spring Road, (FS 456) leads off to the west and about a mile further West Teepee Canyon road takes off.  My best advice is to follow one of these roads/tracks and look for quarries where past prospectors have tried their luck.  The land is managed by the U.S. Forest Service and there are mining claims---I think.  It is best if rockhounds stop in the USFS office in Custer and discuss your plans with one of the friendly employees. 

The agates are encased in chert nodules housed within the lower Minnelusa Formation (Paleozoic: Pennsylvanian).  I suppose these nodules are the result of silica-rich meteoric waters circulating through the unit with resulting diagenesis producing the chert.  Why some nodules are agatized—I don’t have the slightest idea.  Just as I am uncertain how/why agates really form!  The formation of agates in several types of rocks is extremely complicated, even for the “experts”.

Teepee Canyon agate.  Width ~3.1 cm.

Teepee Canyon agate.  Width ~3.9 cm.

Teepee Canyon agate.  Width ~3.2 cm.

Fairburn agates are perhaps the “most famous” agates found in the Great Plains and are valued for their colorful fortification patterns with an abundance of reds (iron oxide), oranges (iron oxide) and blacks (manganese oxides).  There are several localities where agate hunters have collected a variety of stones but the easiest spot for collectors to locate is the “original Fairburn Beds” near the small community of Fairburn, located south along I-90, ~25 miles, of Rapid City near SD 79.  After reaching the community of Fairburn, agate hunters should travel east along French Creek Road (good gravel road) for about 12 miles to a sign locating the original collecting area managed by the Buffalo Gap National Grasslands.  Although known to collectors for decades, these Fairburn beds still yield an occasional agate, and as many colorful specimens of jasper, quartz and chalcedony as can be carried out in your collecting bag.  There are also occasional pieces of petrified wood, and brachiopods replaced by silica.

Fairburn Agate (obverse).  Width bands ~1.1 cm.

Fairburn Agate (reverse).  Width bands ~1.4 cm.

The really interesting story about the Fairburns involves their relationship with the Teepee Canyon Agates described above.  Most geologists now believe that the Minnelusa Formation in the Black Hills is the source of the Fairburn Agates and the siliceous pebbles were transported out to the plains by Tertiary streams draining the Hills.  The agates may be found within conglomerate beds of the Chamberlain Pass Formation and overlying Chadron Formation (both are Eocene in age and part of the White River group). Perhaps agates are most easily observed in the lag gravels covering many outcrops where the finer sediments have eroded away from the Eocene formations leaving behind a veneer of pebbles. 

At any rate, 2016 was again a successful season of locating a few agates, and that was all I asked.  Just hobbling around in the agate fields brought back a flood of pleasant memories from 50 years ago. See below.

I love those random memories that make me smile no matter what is going on in my life right now.  Unknown

I would suggest that if you are interested in Fairburn Agates---pick up a book or three written by Roger Clark, the premier expert on the agates.  Book one, South Dakota's Fairburn Agate, is only available on the used book circuit.  Book number two, Fairburn Agate: Gem of South Dakota, and number three,  Fairburn Agate: South Dakota State Gemstone are in print and published by Silverwind Agates.