Blasting of Outcrops Along Highway 15, including at Elgin

Since the spring the Province of Ontario has been widening Highway 15 between Crosby and the turn off at Hwy 32 to Gananoque, and has been blasting most of the outcrops along Highway 15.   A lot of fresh rock has been exposed.  Unfortunately, most of the blasted rock has been used to construct the roadbed for the new highway (rather than being saved for geologists and rock hounds).
               
The geology is complicated.  There are good outcrops of Potsdam Group sandstone of late Cambrian age, and outcrops of Precambrian plutonic igneous rock and Precambrian high grade metamorphic rock.   The Precambrian basement rocks are part of the Central Metasedimentary Belt of the Grenville Province.   I mentioned in my last posting that the Central Metasedimentary Belt is subdivided into a number of terranes.   Each of these terranes is characterized by distinct rock assemblages and mineral deposits.   This stretch along Highway 15 falls within the Frontenac terrane of the Central Metasedimentary Belt.   The Frontenac terrane comprises metamorphosed sedimentary rocks (metaquartzite, quartzofeldspathic gniesses, marble), lacks volcanic rocks, and was affected by the intrusion of plutonic rocks and intense metamorphism in the period of about  1190-1160 Ma.   The Frontenac Terrane is preserved at upper amphibolite to granulite facies.

Wynne-Edwards (1967), who conducted the most comprehensive study of these rocks, concluded that the “metamorphic rocks most reasonably represent a sequence of marine sediments deposited in a stable area of relatively shallow water, and brought by deep burial into a metamorphic environment with high temperature and confining pressure [where they were intensely deformed, folded and refolded.]  ...The metasedimentary rocks of the Westport map-area are dominantly stratiform quartz-biotite-feldspar gneisses, coarsely crystalline calcite marble ... and smaller amounts of quartzite...   Granitic material in the form of pods, layers or lenses is present in all these rocks. ...  In addition to this granitic material, large concordant plutons of quartz monzonite and monzonite occur at certain places, particularly in belts of crystalline limestone.  These monzonites post-date the main period of metamorphism and have altered gabbroic rocks previously emplaced at the same sites.  Diabase dykes and younger porphyritic andesite dykes cut all these rocks. ...
    An extended period of erosion before the deposition of the Paleozoic rocks exposed the highly metamorphic rocks at the surface and established much of the existing topography.   In some places the ancient land surface is preserved as a deep zone of oxidation and weathering below the Nepean sandstone.”

(Wynne-Edwards, H. R., 1967,  Westport map Area, Ontario, With Special Emphasis On the Precambrian Rocks, Geological Survey of Canada, Memoir 346, 142 pages at pages 108, 4 and 5
http://geoscan.nrcan.gc.ca/starweb/geoscan/servlet.starweb?path=geoscan/fulle.web&search1=R=100533  )

Below I’ve provided an extract from the Geological Survey of Canada’s Map 1182A, Westport, Ontario,  compiled by H.R. Wynne-Edwards.  This shows the geology along Highway 15 from Crosby (at the top left  corner of the map) to south of Morton.  The large pink oval shape at the bottom right corner of this map extract is the Lydnhurst quartz monzonite pluton.

Below is part of the legend for Wynne-Edwards’ geologic map.

The complete map can be downloaded from http://geoscan.nrcan.gc.ca/starweb/geoscan/servlet.starweb?path=geoscan/fulle.web&search1=R=108032
   
Those that have driven along Highway 15 this past summer will have noted that the outcrops of Potsdam Group sandstone at Elgin have changed.   In some ways you can see more as the rock is fresh, but some of the  weathered surfaces are gone.  Below are photographs of three sandstone outcrops near Elgin (the first two showing the unconformity) .
Sam_0544  - Northeast corner of Hwy15 and the main road into Elgin
Sam_0547 - other side of Hwy 15
Sam_0552  - just south of Elgin, west side of road

 (If you visit, you might consider taking a stiff  broom to clear off the outcrops..)

Below is a photograph of an outcrop of Nepean Formation, Potsdam Group, sandstone.  This outcrop is north of Morton on the east  side of Highway 15.  The ruler is  along an inch thick bed of rounded quartz pebble conglomerate that extends the length of the outcrop.

Below I’ve provided photographs of outcrops of gneiss at and just south of Morton, together with close up photographs of pods of granitic material in the gneiss.

 South of Morton there are also fresh outcrops of Wynne-Edwards’ Lydnhurst quartz monzonite pluton


I’ve found that looking at the outcrops on the weekends works best, as you do not have to dodge the construction vehicles. 

Christopher Brett
Perth, Ontario

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Addendum: 

Wynne-Edwards (1967)  did not place the Precambrian rocks that he mapped within a plate tectonics setting, as the theory had not been extended to the Precambrian at the time he conducted his field work and published his memoir.   Since that time  the general adoption of the theory has led to these rocks being placed in a plate tectonics setting.   See, for example:

Davidson, A., and van Breemen, O. (2000), Age and extent of the Frontenac plutonic suite in the Central metasedimentary belt, Grenville Province, southeastern Ontario;  Geological Survey of Canada, Current Research 2000-F4,

James M. McLelland, Bruce W. Selleck and M.E. Bickford (2010), Review of the Proterozoic evolution of the Grenville Province, its Adirondack outlier, and the Mesoproterozoic inliers of the Appalachians, The Geological Society of America, Memoir 206, p. 1-29.

Jeff Chiarenzelli, Sean Regan, William H. Peck, Bruce W. Selleck, Brian Cousens, Graham B. Baird and Catherine H. Shrady (2010), Shawinigan arc magmatism in the Adirondack Lowlands as a consequence of closure of the Trans-Adirondack backarc basin,  Geosphere 2010;6;900-916

All are available online.