Total height ~5.5 cm.
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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.
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).
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 :)
McCarthy, E.M., 2014, Lake Agassiz, Vast Prehistoric Lake: Online Biology Dictionary www.macroevolution.net
REFERENCES CITED
McCarthy, E.M., 2014, Lake Agassiz, Vast Prehistoric Lake: Online Biology Dictionary www.macroevolution.net
How much did you pay for that specimen may I ask ?
ReplyDeleteWhat was your rosette valued at? I recently found my own selenite pit and have a very similar piece. Beautiful!
ReplyDeleteWould love to purchase a few for me collection...possible?
DeleteNo value assigned. I just collect for personal enjoyment. Glad you found specimens. mike
ReplyDelete