SOUTH DAKOTA LAKE SUPERIOR AGATE

I recently had the opportunity to rummage through South Dakota, stopping at sites of geological interest and collecting some interesting material.  Information on the Black Hills and the western part of the state has been described in several previous blogs (for example see 7-2-13 posting).  I am certain many readers/rockhounds have fond memories of observing the many geological features out west, driving through the badlands, and even collecting a few specimens in the Hills.  I always enjoy my expeditions.

   

General geologic map of South Dakota.  Glacial sediments dominate East River.  West River has the Black Hills and numerous outcrops of Tertiary and Cretaceous rocks. X is Milbank, Y represents the Sioux Quartzite, Z is Glacial Lake Dakota.  Map courtesy of South Dakota Geological Survey.

The eastern part of the state most likely seems less interesting to travelers and I suppose most drivers set their cruise control on 80 MPH and zip through the fields of soy beans and corn on I-90.  And in fact, collectors will have a difficult time even locating bedrock unless they happen to explore areas along some of the major streams (for example, the Precambrian Sioux Quartzite cropping out along the Big Sioux River: Y on map) or “hills” of the Precambrian Milbank Granite (sticking up through the glacial drift; X on map).  Besides the Precambrian rocks there are excellent exposures of the Cretaceous Niobrara Chalk and Pierre Shale in the Missouri River Trench (southeast border of state).   What it lacks in collecting opportunities, however, is partially made up by rockhounds understanding the fascinating glacial history during the Pleistocene Epoch.  A good place to start with this understanding is by reading the Roadside Geology of South Dakota (2009) publication authored by John Paul Gries. 

 

Shaded relief map of South Dakota.  Map courtesy of Ray Sterner, John Hopkins University.

The U. S. Geological Survey places landforms of South Dakota into either the Great Plains or the Central Lowlands Physiographic Regions. Each of these regions is further subdivided into smaller sections.  However, I find it much easier to think of the state divisions as: 1) East River (the Central Lowlands) where the landscape is generally subdued and is covered by a variety of glacial sediments; 2) West River (the Great Plains) and its magnificent outcrops of Cretaceous and Tertiary rocks; and 3) the Black Hills, a Laramide mountain range that is part of the Great Plains but more closely related to the Rocky Mountain ranges.  Of course, the "river" here is the mighty Missouri River bisecting the state in a generally north-south direction.

So, eastern South Dakota is essentially covered with glacial drift or meltwater sediments and the landforms are sometimes difficult to recognize.  The James River Lowlands is currently occupied by an underfit river (small river, big valley) but during parts of the Pleistocene served as a major conduit for ice advances and meltwater releases. West of the James River is a highland area known as the Coteau de Missouri (French for Missouri Hills!).  The hills are underlain by Cretaceous bedrock but surficial rocks are mostly glacial till.  However, some small streams close to the Missouri River expose outcrops of the Cretaceous-age Pierre Shale and/or Niobrara Formation.  

East of the James River Lowlands is a region known as the Coteau des Prairies (Prairie Hills). The escarpment leading from the Lowlands to the Coteau is one of the more noticeable areas of relief in East River (~500 feet).  The bedrock again is usually the Pierre Shale but surficial rocks are glacial till.  Dakotans know this area as “Lake Country” since prairie potholes, many times the result of ice blocks left behind (kettles), are scattered across the area.

The Minnesota River Lowlands was the site of a major, really large, Pleistocene flood as waters from the massive Lake Agassiz broke a barrier and flowed southeast in the Glacial River Warren---again creating a modern underfit river. 

 

Lake Superior Agate collected from South Dakota (width ~8.5 cm).

Eastern South Dakota is not a hotbed for collectable minerals; however, the glacial deposits have some really nice cobbles of rocks from the northeast, especially those igneous rocks exposed around Lake Superior.  And, if these Lake Superior rocks were transported south and east there is always a chance to find Lake Superior Agates (Lakers). I did not realize it until much later in life that Dakotans find these agates in substantial numbers.  South Dakota’s best known rockhound, June Culp Zeitner (1916-2009), noted in her best-selling collecting book Midwest Gem, Fossil, and Mineral Trails:  “Lake Superior-type banded agates are found along many of the streams and glacial lakes in the eastern part of the state.  Pickeral Lake and Big Stone Lake are some of the places nice agates have been found especially in the northeastern part of the state.”  The specimen in my collection came from gravel deposits (Pleistocene) associated with the Big Sioux River.  This is not surprising since the river has its headwaters ~100+ miles to the north deep in the heart of glacial debris.

I wish to thank Ray Sterner of the John Hopkins University Applied Physics Laboratory for allowing me access to his files of state relief maps.

BLACK HILLS: A MIGHTY BIG CORAL



Boulder of Pahasapa Limestone displaying a large coral.  It is a tabulate coral and my guess is Syringopora sp.  At the very top is a U.S. quarter for scale.

I rarely collect many fossils during my current reincarnation as a geologist!  In the “old days”, the first third of my career, I was all over the central and western United States hauling students around for exciting collecting trips.  The vertebrate fossils were accessioned into museums while the invertebrates stayed in the study collections.  Life was good.

Life changed when I migrated into administration and left my research interests in paleontology.  I simply could not be a good “bench paleontologist” while tending to such items as strategic planning and program reviews!  Now I am in the “final” third of my career and have come back as an amateur mineralogist, well actually just an amateur collector.  I sort of muddle through all of those minerals but certainly I am having a great time and learning much.  Life is good.

I did run on to an interesting fossil this summer and certainly one worth collecting---if I could carry it.  But, I would have needed a front-end loader to lift the creature!

My friend, the Junior Geologist, had hauled me around to several somewhat hidden sites in the western reaches of the Black Hills, Wyoming—South Dakota.  We were looking for exposures of igneous rocks, while stopping at any accessible quarries just to “check what was going on”.  At one small quarry it appeared to me that prospectors/operators (when??) had been digging around in rocks of the Madison Group, or as they call it in the Black Hills, the Pahasapa Limestone.  I closely examined the area and could not quite figure out the prospective pay zone.  What were they after?  I could see no mineralization.  Then I thought perhaps miners were taking out dimension stone; however, conditions were not quite right for that idea.  I hope to continue my quest by acquiring a few professional publications. 

Small quarry in Pahasapa Limstone.  I don’t have the slightest idea what miners were after.

The Madison Group (upgraded from original status as a formation) is one of the best known rock units in the western U.S. The unit is limestone and dolomite and Lower and Middle Mississippian in age and was deposited in tropical marine waters extending from New Mexico/ Arizona north into western Canada.  The Antler Highlands (an active mountain-building area in central Nevada) created the western boundary while the long-lived Paleozoic Transcontinental Arch was on the east.  The Madison does not have a type locality but later geological work has indicated the unit was probably named for the Madison River, perhaps near Three Forks, Montana.

Paleographic map of the “U.S” representing Early Mississippian time.  Note the Transcontinental Arch (TA) a non-orogenic part of the Craton, the Antler Highlands (A) an orogenic (mountain building) highland, and the Craton (C) a structural/topographic high during the Paleozoic. The light blue area on the map between the Antler Belt and the Craton represent marine waters.  Map kindly provided by Ronal Blakey of CPGEOSYSTEMS.

As a side note, Three Forks played a major part in the exploration of the northwest U.S—the Louisiana Purchase---explored by Lewis and Clark.  Upon leaving St. Louis in 1804 the Corps of Discovery traversed up the Missouri River.  It was the major waterway and there were no other comparable rivers available to take a wrong turn!  However, in July 1805 the Corps reached the junction of three major uncharted streams that actually then formed the Missouri.  They named them the Gallatin, after Secretary of the Treasury Albert Gallatin, the Jefferson, after the President, and the Madison, after Secretary of State James Madison.  But the big decision was: which river to follow?  If they picked the wrong river then the Corps might need to spend the winter of 1805-1806 in the mountains—not a pleasant thought.  The leaders, against the advice of some of the Corps members, decided on the Jefferson---a wise move.  Both the Madison and Gallatin head to the south in Yellowstone National Park.  Visiting Three Forks was on my “bucket list” and I was able to do so several years ago.

Map showing location of Three Forks, Montana.

As with many widespread rock units, the Madison Group has many stratigraphic correlatives.  I previously mentioned the Pahasapa in South Dakota and now add the well-known Leadville Limestone in Colorado and the Redwall Limestone in the Grand Canyon.  In many localities the Madison, and equivalents, contain numerous vugs, voids, cracks and caves.  The reason for this karst topography---after deposition of the Madison sediments and lithification the area was raised above sea level and was subjected to normal forces of nature.  Rain falling on the exposures was somewhat acidic as it brought in dissolved carbon dioxide from the atmosphere as carbonic acid (H₂CO₃).  Carbonates are soluble in acids and therefore parts of the limestones dissolve if exposed to this acidic environment.  This solution in the Madison rocks is responsible for such notable caves as Wind Cave and Jewel Cave, both national parks in the Black Hills, and Lewis and Clark Caverns, a state park in Montana.  In addition, waterways in the Madison are responsible for providing water conduits at many localities in the mountain west and adjacent plains.  See the blog posting on hot springs at Thermopolis, Wyoming (May, 13, 2011).

But, back to the fossil.  In rummaging around and checking out the rocks the Junior Geologist and I observed a really large coral well exposed in a pretty hefty boulder.  The best that we could do is photograph the specimen and hope that future visitors will find excitement in “discovering” this evidence of past marine life perched in the middle of the country.

The Junior Geologist, my guide, resting after trying to handle the boulder!  The scenery is spectacular.  

WISCONSIN LAKE SUPERIOR AGATES

I have been out fishing and hunting agates again back in the Midwest, at one time my home for several years.  As an avid fisherman I am always searching for the elusive Northern Pike and Walleye.  As a geologist I always have my eyes turned down looking for agates in shoreline gravels!  I am not the only one with this dual passion as I ran on to a website (www.hotspots.com) with combination stories of fishing and agate hunting.  My kind of place!  I am probably better at catching fish that finding Lake Superior Agates (Lakers)! However, during this recent trip I was able to procure a couple of agates from Wisconsin, not an easy task (at least for me).  

As I understand the situation, Lakers have a fairly widespread distribution with the “type” hunting areas in shoreline gravels of Lake Superior and adjacent sand/gravel pits.  But the Laker name comes, not from Lake Superior, but from the Lake Superior Till, a covering of glacial debris.

However, glacial action (Pleistocene) moved specimens into adjacent Wisconsin, Michigan, Iowa, Missouri, Nebraska, and South Dakota and there are reports of a Laker or two from Kansas.  In addition, Pabian and others (2006) noted Lake Superior-type agates have been found in deposits of the Mississippi River as far south as Louisiana.  Essentially any “gravel pit” in the glaciated region, or along the Mississippi River, has the possibility of producing a Laker.

My interest in Lakers dates “way back” to the 1970’s when I taught a combination canoeing/geology field course in Minnesota (my idea of a perfect class).  Traveling from western Kansas the class visited many interesting geological localities including some of the iron mines and taconite concentrators in northern Minnesota.  The “high point” of the class was the week we spent taking a close look at the old Precambrian rocks exposed in the Boundary Waters Canoe Area.  And did I mention sunsets, bears, campfires, and smallmouth bass?  At any rate, on the way back to Kansas we stopped and searched for agates along the north shore of Lake Superior.  And, we usually had some success.

The original source of the Lakers is from the basalts (several different layers) located in the Midcontinent Rift System (MRS).  This geological rift (think about the great East African Rift Zone) begin to form in the Precambrian (Proterozoic Era) perhaps 1.1 Ga splitting the stable part of the North American “continent” or plate (referred to by geologists as the craton).  The Rift is nearly 1400 miles long extending from northeast Kansas to Lake Superior with an eastern arm curving around and heading toward Ohio.  Hugh amounts of lava erupted along faults while adjacent rivers from the uplands dumped thousands of feet of sediments (later sedimentary sandstones and conglomerates) into the low lands of the Rift.  For some reason the rift “stopped splitting” (a failed rift in geological jargon) and the continent healed.  Most of the rocks in the rift are buried below the surface of the earth and are only known from geophysical studies and drill holes.  For example, the Midcontinent Geophysical Anomaly (MGA) in Kansas delineates the rift since the concentration of magnetite in the Rift rocks creates a magnetic “high” that is picked up by geophysical instrumentation.  However, rocks of the Rift become exposed around Lake Superior and the amygdaloidal agates erode from the basalts.  Since the Rift rocks include substantial amounts of iron, the agates have some sort of a red or orange color---oxidized iron.  Most likely the agates formed post-deposition of the basalt and are the result of percolating silica-rich groundwater filling the many vugs or vesicles in the basalt.

The Wisconsin agates came from a gravel pit in Trempealeau County, a few miles north of my “headquarters” in La Crosse.  They are not large but do have that distinctive Laker Look to them.  Eckert (2000) notes that all of the Wisconsin counties along the river have produced Lakers from sand and gravel quarries—and elsewhere.  I visited with an “ole timer” in La Crosse who found a Laker in a local gravel parking lot.  It pays to keep your eyes on the ground, except when crossing a busy street.

This find prodded me into checking up on the official State Gemstone of Minnesota and found the following proclamation:  Be it enacted by the Legislature of the State of Minnesota: The Lake Superior agate is adopted as the official gemstone of the State of Minnesota.  Approved May 15. 1969. 

A pair of Wisconsin Lakers, both ~2 cm.

I then got to thinking about other state symbols and wondering about official rocks, minerals, etc.  It was only then I realized that not only did several states have some sort of an official gem, several states adopted a variety of agate as their official rock, stone, gemstone, or mineral.  In fact, agates are the most common mineral of the many state symbols.  Consider the following:

Florida Stone: Agatized Coral is so designated.  Of course calling it a “stone” gets out of the “what is it” debate: mineral or rock.  They could have made it the State Gem except that slot is occupied by moonstone.  Yes, Florida has also given us a State Beverage (orange juice).

Kentucky Rock: the official State Rock is the Kentucky Agate but of course agate is really a mineral.  However, the official mineral of Kentucky is occupied by Coal, a sedimentary rock.  I think the “symbol namers” were partaking at the official State Bourbon Festival (Kentucky Bourbon Festival, Inc.).

Louisiana Gemstone: Agate, which is interesting in that the specimens are alluvial and found in gravels and are non-native.  Are these really Lake Superior Agates transported via the Mississippi River from Minnesota?  Maybe Louisianans had tired of drinking their State Drink (milk) and skipped over to Kentucky for some adult refreshments.

Minnesota Gemstone: Lake Superior Agate.  I am particularly partial to the Minnesota State Muffin (blueberry).

Montana Gemstone: Montana is the only state with two official gemstones—sapphire and agate.  The agates are multisource specimens so just about any ole agate from Montana will do.  I actually thought their official motto was “Big Sky Country”.  Nope, The Treasure State.

Nebraska Gemstone: Blue Agate (more of a blue chalcedony) formed in fault and joint planes in wind deposited siltstones and claystones of the Tertiary age Chadron Formation (24-37 my) ---the formation common in the badlands of Nebraska and South Dakota.

Nebraska Rock:  Prairie Agate.  Here we go again—calling a mineral a rock.  I am really uncertain about this one as the official state website notes:  “Agate is variegated quartz noted for its layered varieties. In most specimens, the bands are coarse and differ in color and translucency, as well as in compactness and porosity. The prairie agate, distinguished from most other agates because it seldom has these bands.”  If it does not exhibit bands then why call it an agate? 

Oregon Rock:  Thunder eggs, known to all collectors far and wide.

South Dakota Gemstone: the well known Fairburn Agate collected on the plains surrounding the Black Hills.  These alluvial agates are the same as Tepee Canyon Agates collected from the Minnelusa Formation in situ.   Fairburns are among the more expensive of the agates but are among the more beautiful. 

Tennessee Rock: the state has two official rocks, Paint Rock Agate and limestone.  Of course, they need to change agate to the state gem except that slot is occupied by Tennessee River Pearls.

So there you have, agates are the official “something” of nine states.   In my opinion, Rhode Island wins the prize for “Least Unrecognized Official State Mineral’ which is Bowenite (a variety of serpentine).  Now does anyone recognize that name?  However, they make up for it by having a great piece of barbeque meat as their State Bird, the Rhode Island Red!  Second place goes to Massachusetts with another rather unfamiliar mineral, Babingtonite (calcium iron manganese silicate).  The award for “Lack of Creativity in Selecting a State, Mineral, Rock or Gemstone” goes to Nevada and its State Rock---Sandstone.  Not a specific rock, like Yule Marble or Salem Limestone, but plain old sandstone.  I found this out on the website of Nevada’s First Lady where she also a hot link to:  “Nevada’s Early BIGFOOT Sightings.”  It is also sad to report that my home state of Kansas evidently does not believe in symbols of a geological nature.  I need to get those school kids working!

REFERENCES CITED

Eckert, A. W., 2000, Earth Treasures Volume 1: The Northeastern Quadrant; iUniverse Incorporated.

Pabian, R., B. Jackson, P. Tandy, J. Cromartie, 2006, Agates: Treasures of the Earth:  Firefly Books, Limited.