Ontario Geological Survey Remapping the Perth Map Sheet

I was driving into Perth in August when I noticed an individual taking a sledge hammer to an outcrop of granite along Scotch Line (County Road 10) within the town limits of Perth.    My initial thought was that it must be a geologist taking a sample to age date the granite, so I stopped, introduced myself, and asked the individual if that was what he was doing.   I was glad that I’d stopped.   It was Dr.  Michael  Easton of the Ontario Geological Survey and he was collecting a sample in order to later determine its age.   I suspect that this was the first time he has been asked the question of whether he was collecting a sample in order to age date the rock.    (I often get asked in an accusing tone to explain what I’m doing when I just stop to look at an outcrop.)  

Dr. Easton  was good enough to tell me that he was working on the Perth map sheet and that his initial report had been published in the Summary of Field Work and Other Activities, 2015.   The following is a link to that report.

Easton, R. M.,  2015
Project Unit 15-014. Precambrian and Paleozoic Geology of the Perth Area, Grenville Province; in Summary of Field Work and Other Activities, 2015. Ontario Geological Survey, OFR 6313
at pages 18-1 to 18- 13
http://www.mndm.gov.on.ca/en/news/mines-and-minerals/summary-field-work-and-other-activities-2015   

Dr. Easton has recently sent to me electronic copies of two recently published  reports from this year’s field season that deal with rocks in the Perth area:
               
Easton, R.M.,  2016a.
Precambrian and Paleozoic geology of the Perth area, Grenville Province; in Summary of Field Work and Other Activities, 2016, Ontario Geological Survey, Open File Report 6323, p.17-1 to 17-13.

Easton, R.M.,  2016b.
Metasomatism, syenite magmatism and rare earth element and related metallic mineralization in Bancroft and Frontenac terranes: A preliminary deposit model; in Summary of Field Work and Other Activities, 2016, Ontario Geological Survey, Open File Report 6323, p.18-1 to 18-9.

Both reports can be downloaded from:
http://www.mndm.gov.on.ca/en/news/mines-and-minerals/summary-field-work-and-other-activities-2016
       
In his reports from 2015 and 2016a, Dr. Easton discusses A) the medium-pressure granulite- and upper amphibolite-facies rocks of the Frontenac terrane, B) the upper greenschist- to lower amphibolite-facies rocks of the Sharbot Lake domain, C) separated  by the Maberly shear zone, and includes maps showing the locations of the terranes and shear zone.   Easton 2015 discusses the Maberly Shear Zone and directs one to a “spectacular, 150 m long roadcut on the south side of Highway 7   (385925E, 4967751N) [which is] a microcosm of the Maberly shear zone, and consists of alternating panels, 10 to 15 m wide, of marble breccia interlayered with thin-layered, highly flattened, compositional silicate tectonites.”   For anyone that has not previously driven out to look at the shear zone, the outcrop is worth a look.  Below is a photograph of the outcrop.

The ruler in the photo is one meter long.

Easton (2015 and 2016a) divided the Frontenac terrane in the Perth map area into 3 subdomains, and reports on differences in the marbles present in the 3 subdomains of the Frontenac terrane.    His petrographic study of rocks in the Frontenac terrane suggests metamorphic pressures and temperatures higher than previously reported for the Perth Map sheet.  He found assemblages suggesting “pressures and temperatures greater than 8 kilobars and higher than 670◦C” and other assemblages “indicating metamorphic pressures as high as 11 to 14 kilobars and temperatures approaching 1000◦C.”    He notes that “a significant metamorphic pressure change occurs along a north-northeast-trending fault located southeast of Smiths Falls”, a “fault that trends north-northeast from Chaffey’s Lock to Portland to Glen Elm just south-southeast of Smiths Falls”, which he names the Chaffey’s Lock fault.  He puts the Perth map area in bathozone 6 of  Carmichael, noting that bathozone 4 conditions exist 20 km south of the Perth map area near Lyndhurst, adding that a "difference of 2 kilobars between the Perth and Lyndhurst areas would involve at least a vertical displacement of 7 km” across the fault but because the fault “places rocks of the Nepean Formation against rocks of the upper March and Oxford formations”  there was “no more than 100 m of post-Ordovician displacement across the fault.”

In his reports from 2015 and 2016, Dr. Easton also discusses the metallic mineral potential of the Perth map sheet, principally the potential for Kiruna-Type magnetite-apatite mineralization and the  rare earth potential of numerous mica-apatite deposits.   Dr. Easton mentions finding “a strontium equivalent of the mineral haunghoite (BaCe(CO3)2F). A strontium equivalent of haunghoite has not been previously identified, and this may represent a new mineral species.” 

He also discusses the industrial mineral potential (sandstone as potential source for silica; marble as a carbonate source; vermiculite), and includes analyses of sandstones and marbles.

While Dr. Easton’s reports concentrate on Precambrian rocks, he also mapped the Paleozoic rocks (the Cambrian  to Lower Ordovician Covey Hill and Nepean formations, the Lower Ordovician March and Oxford formations, and the Middle Ordovician Rockcliffe Formation). In his reports Dr. Easton mentions that he has found a number of  previously unknown exposures of Potsdam Group sandstone  and conglomerate (both Covey Hill and Nepean formations).   A previously unknown exposure of Potsdam group sandstone and conglomerate that he “identified on the north side of Highway 7 approximately 5 km west of Wemyss (385510E 4967646N)” is worth looking at.


The 2017 Annual meeting of the Geological Association of Canada/Mineralogical Association of Canada will be held in Kingston, Ontario from May 14-18, 2017 and  will coincide with the 175th anniversary of the founding of the Geological Survey of Canada by the legislature of the Province of Canada in 1842, in Kingston, Canada West.  As part of the conference Dr. Michael Easton of the Ontario Geological survey will be leading a one day field trip on May 14th  entitled New Insights into the Tectonic and Metamorphic Architecture of the Composite Arc Belt and the Frontenac-Adirondack Belt near Perth, Ontario, Grenville Orogen which highlights the results of his recent mapping, geochemistry, petrology and new geophysical data collected in the Perth area.   See:  http://www.kingstongacmac.ca/en/field-trips/


Christopher Brett
Perth, Ontario

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