TRACKS/TRAILS WISCONSIN LIMESTONE

FRONT STEP AT GROUNDED SPECIALTY COFFEE.  NOTE BURROWS AND STROMATOLITES.  DIME FOR SCALE

For eight years I resided in La Crosse, Wisconsin, a picturesque city located on the Mississippi River.  For most of those years I had my morning coffee with the "7:00 am Group” at Grounded Specialty Coffee.  These fellow Wisconsinians were an eclectic group of businessmen/women who taught me much, not the least of which is that academicians need to listen closely to the townies.  As a bonus, it is not every day that a coffee imbiber gets to discuss Keynesian Economics with the chief barista (a doctorate in economics), the Zen of motorcycle maintenance with Biker Bob, local judicial matters with Mike the Barrister, La Crosse history with Carl the Philosopher, or Catholic shrines with Andy the Accountant!   At any rate, for several years I made the morning trek, climbing a single step into the shop to sample the espresso.

During my recent trip to Grounded in September, I made a startling discovery—the single step to the shop was not composed of man-made concrete but of local building stone.  In closer examination I also observed that the step was badly bioturbated and covered with trails/burrows left behind by soft bodied (probably) animals.  There are numerous vertical and horizontal burrows preserved, most likely the result of “worms”.  The normal differential weathering of the carbonate rock preserved these animal traces in raised relief.  In addition, I noticed larger structures with a dark center surrounded by concentric layers.  These interesting features are stromatolites, layered structures formed in very shallow water by cynobacteria (blue-green algae).  It seems as though these photosynthetic bacteria trapped rock and mineral grains in their mucus and cemented all together.  I was unable to observe tracks of trilobites or body fossils.

I believe the rock in question is a specimen from the early Ordovician Prairie du Chien limestone.  The Prairie du Chien is the local “caprock” and may be observed “holding up” the bluffs on both sides of the river.  It has been quarried extensively for building stone and riprap and a large quarry is easily observed at the summit of Granddads Bluff in east La Crosse.  The Prairie du Chien was deposited in a very shallow (maybe 20-30 feet) and warm sea as a carbonate mud that later hardened into a limestone (CaCO₃).  Much of the unit has been altered (post deposition) into dolomite (MgCaCO₃).

So, if you happened to travel through La Crosse, stop in at Grounded (308 Main St.), visit with Todd and the boys, and check out that great step.

BLUFF IN EAST LA CROSSE. PRAIRIE DU CHIEN CAP.  NOTE QUARRY ON RIGHT.

RHODOCHROSITE FROM ARGENTINA

Fig. 1. Massive rhodochrosite stalactites exposed in the Capillitas Mine, Argentina.  Photo courtesy of J. A. Saadi and The Giant Crystal Project.

 

The carbonates are a common and important group of minerals found in rocks of the earth’s crust.  All carbonates are distinguished by having a  CO₃ ion in their chemical composition.  Many carbonates are quite colorful such as the shades of calcite (calcium carbonate) and the blues of azurite (copper carbonate).  Most mineral collections contain numerous specimens of carbonates but three related minerals are of special interest: iron carbonate (siderite), calcium carbonate (calcite), and manganese carbonate (rhodochrosite).  The reason behind this interest is their solid solution relationship.  For example, in the popular collectable mineral rhodochrosite (MnCO₃), calcium, as well as iron, may substitute/replace the manganese; therefore, the mineral may exhibit many different shades of pink and red and the exact chemical formula varies with the differing amounts of manganese, iron, and calcium.  It is my understanding that “pure” rhodochrosite is rather rare.

The most “famous” rhodochrosite” crystals are the specimens collected from the Sweet Home Mine near Alma, Colorado.  These rose-red colored crystals are prized by collectors the world over and the Denver Museum of Nature and Science has a fantastic display.  However, also of interest to many rockhounds are the “sliced and polished stalactites” from the Capillitas Mine in Argentina. These non-crystal specimens are very recognizable and occur as the principle gangue mineral in this lead-zinc sulfide mine (Fig. 1).  The minerals are the result of hydrothermal activity in the late Tertiary and may be the largest mass of rhodochrosite ever discovered (The Giant Crystal Project, 2011).  I treasure these slices for both their color and their uniqueness (Fig. 2). 
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Fig. 2.  Rhodochrosite (stalactites) from Capillitas Mine in Argentina. Specimens ~4 X 4 cm.