Gypsum Pseudomorphs that formed in the Sabkha Environment of the Potsdam Group

“Sabkhas are supratidal, forming along arid coastlines and are characterized by evaporite-carbonate deposits with some siliciclastics” Wikipedia . 

In my  October 7,  2016 blog posting I mentioned that I had attended the official re-opening and rededication of the Metcalfe Geoheritage Park in Almonte, Lanark County.   While I was there and admiring structures preserved in Potsdam sandstone slabs that had been rescued during the enlargement of Highway 417 in Kanata, Dr. Donaldson suggested that I should look at other rescued slabs of sandstone that had been placed in the Last Duel Park in Perth, Ontario.  Below are three photographs of slabs that now reside in the Last Duel Park.

I believe the structures shown in the first two photographs  to be relic pseudomorphs of gypsum rosettes.  The third photo shows convolute, tightly folded sandstone beds, that were held together likely as the result of microbial binding.

My  October 31st  blog posting mentioned that I had attended the second day of the Niagara Peninsula Geological Society Field Trip to Eastern Ontario and visited the Potsdam sandstone quarry at Ellisville.  While there Paul Musiol of Kingston pointed out an interesting structure that he had found in the sandstone.  Below is a photograph of Paul’s find.

I told Paul that my best guess is that his structure is a  pseudomorph of gypsum or another evaporite mineral (barite).

The addendum to my blog posting from December 29, 2015 summarizes various reports on
on evaporites in Potsdam sandstone.    Recently, in his doctoral thesis, Dave Lowe described gypsum and evaporite pseudomorphs found in Potsdam sandstone:       

David G. Lowe, 2016   
Sedimentology, Stratigraphic Evolution and Provenance of the Cambrian – Lower Ordovician Potsdam Group in the Ottawa Embayment and Quebec Basin;
Doctoral Thesis, University of Ottawa,
http://www.ruor.uottawa.ca/handle/10393/35303

Dave has recognized six siliciclastic paleoenvironments in the Potsdam:  a) braided fluvial,  b) ephemeral fluvial, c) aeolian erg, d) coastal sabkha, e) tide-dominated marine and  f) open-coast tidal flat.   He discusses the coastal sabkha environment and the formation of gypsum pseudomorphs at pages 87-92 of his thesis.  His figure 3.8 at page 90 is worth a look, as it provides photographs of  evaporite minerals, pseudomorphs and textures in coastal sabkha strata. Figure 3.8A shows impressions of sparry, nodular radiating mineral aggregates in sandstone from Kanata, which Dave compared to evaporitic desert rose nodules.  He recognized possible swallowtail twin textures suggesting that the nodules were originally formed as gypsum.  His Figure 3.8B shows cubic impressions possibly of halite in sandstone.  His Figure 3.9 shows kinked and tightly-folded sandstone intraclasts in coastal sabkha strata.

Those with an interest in microbial binding in the sabkha environment might also want to read pages 201-203 of Dave’s thesis.

Christopher Brett
Perth