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.