Monday, February 3, 2014

RED RIVER SELENITE ROSETTES




Total height ~5.5 cm.

 During a trip to Canada, fishing not collecting, I was able to acquire (purchase) a beautiful gypsum rose collected from the Red River Floodway near Winnipeg, Manitoba.  I had read about these Canadian roses for many years since my modest collection includes several fascinating rosettes obtained from many locations in the U.S.  In fact, one of the more popular postings (1000+ reads as of today) on this Blog is an entity describing the desert roses collected near St. David, Arizona in Tertiary rocks (February 4, 2013).  However, until recently, I did not have a rosette from a non-arid locality and especially from the famous site near Winnipeg.

I had visited Winnipeg many years before as a graduate student at the University of South Dakota; however, the meeting was more of a job fair than a mineral show and I was interesting in securing a position in British Columbia climbing through the mountains.  I often think about the day that I sadly turned down the long-awaited job offer (to attend graduate school in Utah).  At any rate, I missed seeing the gypsum roses except for several displayed in the lobby of hotel that, according to the label, were dug up during building excavations in downtown Winnipeg!  But, these specimens "stuck" in my mind for all of the past years.

Winnipeg sits on an old lake bed, glacial Lake Agassiz.  This magnificent body of water was created by glacial meltwater associated with the termination of the last Pleistocene/Holocene glacial event generally termed the Wisconsin Glaciation. The Lake formed when the Laurentide Ice Sheet (Red River Lobe) receded north of a drainage divide that separated Hudson Bay (north) from the Gulf of Mexico (south). As the continental glacier melted and receded, large sections of Manitoba and parts of Ontario, Saskatchewan, Minnesota, and North Dakota were covered, at one time or another, with lake waters—perhaps 700 miles by 200 miles---over a 5000-year Lake history.  During its history, this giant lake drained north via the Mackenzie River into the Arctic Ocean, southeast into the Great Lakes, probably east into the St. Lawrence River, northeast to Hudson Bay, and south through Glacial River Warren into the Mississippi River (the only escape route that I have observed “up close”).  Today the underfit Minnesota River flows in the valley of Glacial River Warren and is an interesting site---a small river meandering around in a quite wide and deep valley.
Maximum extent of Lake Agassiz.  Courtesy of www.ndstudies.edu
It seems if Lake Agassiz drained and refilled several times, but emptied finally and completely about 8 ka, when the ice of Hudson Bay melted and allowed water from the Lake to come charging in.  Geologists estimate that drainage of the Lake raised world sea levels by three to ten feet. Lakes Manitoba, Winnipegosis, Lake of the Woods, and Winnipeg, all residual catchment basins of the Lake, remain and correspond to regions where the original lake was deepest. Though giants by modern standards, the residual lakes retain only a small fraction Agassiz's original waters (McCarthy, 2014).
 
The Red River Floodway, like most artificial floodways, was constructed around the east side of Winnipeg in order to spare the city from periodic flooding along the Red River.  Also, like most artificial floodways, this one seems pretty ugly, just a ditch cut into the flat lake flood of ancient Lake Agassiz.
 
The roses have been collected for several decades and are concentrated in groups and located in “clay” sediments of the Lake floor.  I once visited with a geology colleague from Canada and he informed me that: 1) frost heaving and freezing ground temperatures generally destroyed crystals near the surface; and 2) most collectors “dig” rather large holes (some at least 10 feet in depth) in their search for concentrations of the roses.  As far as I can tell, it seems to be a hit or miss proposition.  My colleague also told me that some “old timers” have developed a feel for the roses by probing with a small-diameter steel rod.

Whatever the case, the specimen in my collection is really beautiful, a single, clear, gemmy, twinned selenite crystal protruding from an amber-colored “mass” of selenite blades (many twinned)--a “floater.” 
Most of us, when we think of gypsum, visualize evaporating seas or a large lake, the “normal” depositional environment for massive gypsum/alabaster.  However, crystalline gypsum or selenite rarely forms in evaporatic basins, at least not in quantities.  Instead, selenite is a diagenetic or secondary mineral:  some crystals form around volcanic fumaroles where sulfur and calcium are available; in other instances selenite is formed near hydrothermal sulfide deposits where the sulfate solution reacts with available calcium; in a similar situation, decomposing pyrite (FeS2, iron sulfide) provides the sulfur for a reaction with calcium (often from available carbonate rocks).  And finally, desert roses is a specific term applied to masses of selenite crystals forming in an arid environment where percolating brines rich in the necessary calcium and sulfur, along with periodic episodes of evaporation, provide the necessary ingredients. 

So, the question—what is the source of the gypsum and calcium needed for the formation of selenite in the beds of glacial Lake Agassiz?  I hunted long and hard for a reasonable answer since it seems many references on selenite are concerned with the metaphysical properties of this mineral.  At least one “writing” stated that selenite is only found in Mexico’s Cave of the Swords or Cave of the Crystals!  That “little known fact” then indicated to the author that selenite was a powerful mineral with amazing strength!  Wow.

Some legitimate authors thought that perhaps buried deposits of Jurassic gypsum provided the necessary elements while others believed that the actual Lake Agassiz sediments provided calcium and sulfur.  Many of the answers I craved to find (at least by the titles) seemed tied up in student theses completed at Canadian universities, and copies were nearly impossible to locate. Then I ran on to a nifty little article entitled Observations on Selenite Distribution within the Lake Agassiz Clay Plain (Mantile and Betcher, 1998, Manitoba Energy and Mines, Open File Report 98-6). So, after several hours of searching I struck pay dirt and the following summary is from their report.  
        
As noted above, the roses are located in the clay plain of glacial Lake Agassiz and studies have found that the shallow groundwater in the plain is often oversaturated with gypsum.  In addition, the clays contain numerous carbonates that could provide a source of calcium. So, one school of thought is that upwelling groundwater provided the sulfate necessary for the formation of gypsum.  However, Mantile and Betcher believed that oxidation of organic sulfur in the “unsaturated zone, near or above the water table, provides the main source of sulphate throughout most of the area.”  The source of the “organic sulfur”—probably Cretaceous rocks that were eroded and dumped into the Lake.  In fact, isotopic ratios in the Lake clays indicate a Cretaceous source and these rocks would have produced abundant sulfur to bond with the locally available carbonates.  The authors also believed the larger crystals that are found deeper in the sediments are probably “thousands of years old …and would indicate a long growth period.”  The smaller crystals found at shallow depths are “much younger and possibly forming as a result of the present day groundwater conditions.” 

As usual with my curiosity, simple answers about geology questions are often not easy to locate, and are not always in “black and white.” And in fact, Mantile and Betcher noted “the conditions required to produce selenite crystals can be found throughout the Lake Agassiz clay plain, although the absence of crystals over much of the area suggests that conditions on their formation are poorly understood.”  I view the study of geology as journey on a road with many forks and turns, perhaps even a maize.  But in the end, with persistence, a little luck, and continued discovery, the answers will appear.  I absolutely love these trips and always look forward to the next one :)

REFERENCES CITED

McCarthy, E.M., 2014, Lake Agassiz, Vast Prehistoric Lake:  Online Biology Dictionary www.macroevolution.net


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