Over the past five years Dr. Michael Easton of the Ontario Geological Survey has been mapping the Precambrian rocks of Eastern Ontario. This is Dr. Easton's 2017 report of a previously unreported outcrop of Nepean Formation sandstone in Lanark County:
“The northeastern boundary of the geophysical anomaly may also be fault bounded because a previously undocumented inlier of Nepean Formation rocks occurs at Gillies Corners (UTM 413795E 4984105N), 4.3 km southwest of Frankto[w]n, immediately south of the northwestern boundary of the geophysical anomaly (see Figure 18.3A). This sudden appearance of a stratigraphically lower unit in an area underlain by rocks of the stratigraphically higher March Formation, suggests that if a fault is present, it is down-dropped to the north.”
Dr. Easton’s location UTM 413795E 4984105N converts to Latitude : 45.005155 deg, or 45 deg 0 min 18.558 sec. Longitude: -76.093858 deg, or -76 deg 5 min 37.889 sec. Plugging 45.005155, -76.093858 into Google will show the location of the outcrop.
In June I took County Road 10 when I was driving back to Ottawa from Perth, and stopped at Gillies Corners to look at Dr. Easton's outcrop of Nepean Formation sandstone. [While the road is mapped as Perth Road, as Drummond Concession 2 and as County Road 10, most in Perth call it the Franktown Road. Those living in Franktown call it the Perth Road. I have yet to meet anyone in Perth who calls it County Road 10, but it is identified as such by those from Ottawa and on Google Maps.]
The sandstone outcrops on both sides of Malcolms Way, with the best outcrops about 100 meters east of Gillies Corners Side Road. The better outcrop, on the north side of Malcolms Way, is about 30 yards long and 5 feet high and is flat lying, fresh sandstone. Most of the outcrop is Nepean formation white to buff sandstone. There are additional smaller, badly weathered, outcrops to the east along Malcolms Way.
Four photographs of the best outcrop are provided below [331, 339, 340, 341] :
The second photograph shows a layer of dark rock at the top, which is likely March formation (or younger Oxford formation), with an uncomformity between the underlying Nepean Formation sandstone and the overlying March Formation. (The badly weathered dark rock also outcrops further east along Malcolms Way.)
Interestingly, the Nepean formation sandstone displays three different facies. In earlier blog postings I mentioned that Dave Lowe (2016 ) has recognized six siliciclastic paleoenvironments in the Potsdam Group sandstones: a) braided fluvial, b) ephemeral fluvial, c) aeolian erg, d) coastal sabkha, e) tide-dominated marine and f) open-coast tidal flat.
At the top of the Nepean formation sandstone at this outcrop is a layer with vertical burrowing (second and fourth photo’s) – Dave Lowe’s marine facies.
Under that is a layer that extends over much of the length of the outcrop that is full of rounded pseudomorphs of evaporite minerals– Dave Lowe’s coastal sabkha facies. See the second and third photos. My third photo compares well with Dave Lowe’s (2016) photo 3.8A which he describes as “Impressions of sparry, nodular radiating mineral aggregates from Kanata, ON (locality 14),” which is behind Kanata’s Walmart.
Most of the lower two thirds of the outcrop is a fine grained, white to buff sandstone that is arguably wind blown (aeolian erg) sandstone.
Williams and Wolf’s (1984) Map P.2725 puts Gillies Corners in the March Formation, and placed the fault bounded boundary between the Nepean and March at about four kilometers west of Gillies Corners. Below is a map compiled from Williams and Wolf’s (1984) Maps P.2725 and P.2724 upon which I have plotted the location of Dr. Easton’s outcrop with a red square.
Wilson, Liberty, and Reinhardt’s (1972) geologic map covers the area north of where I have written ‘GILLIES CORNERS’ on my map. They also mapped Gillies Corners as falling within the March Formation and show the boundary between the Nepean and March at about four kilometers west of Gillies Corners. Interestingly, they also plot a small area of Nepean Sandstone (about a kilometer long and 200 meters wide) about five kilometers north of Dr. Easton’s outcrop, along Line Road 6 where it intersects Gillies Corners Side Road. I’ve marked this small area on the map with a blue oval shape.
Curiously, an additional outcrop of Nepean sandstone appeared on Wilson,, Liberty and Reinhard’s (1964) preliminary Map 7-1964, that did not appear on the finished map. This was a small area along Gillies Corners Side Road about three kilometers north-northwest of Dr. Easton’s outcrop. Intriguingly, Reinhardt, who was the last to edit the Paleozoic compilation, was an expert on Precambrian rocks rather than Paleozoic rocks.
Jean Dugas, for his 1952 doctoral thesis, mapped the geology of the Perth map area, which included the area south of Gillies Corners. GSC Map 1089A by Wilson and Dugas (1961) replicates a map that is part of his thesis. Dr. Morley Wilson is credited as the first author as part of the map-area had been surveyed in 1930 by Dr. Morley Wilson, who mapped most of Palaeozoic area on the eastern part of the sheet. Wilson and Dugas’ map shows a number of areas of Nepean sandstone that do not appear on Williams and Wolf’s (1984) Maps P.2725 and P2724. For example, their map 1089A shows small occurrences of Nepean sandstone 4 km southwest of Gillies Corners, 4 km south of Gillies Corners, and 3 km east-southeast of Gillies Corners. The largest of these is 1.5 miles long and 1/4 mile wide and cuts across County Road 10 just west of Cockburn Creek.
It has been over seventy years since Dr. Alice E. Wilson mapped the Paleozoic geology north of Gillies Corners and ninety years since Dr. Morley E. Wilson mapped the Paleozoic geology south of Gillies Corners, and some outcrops that they found are now difficult to find. What would have been farmland when they mapped the area is now overgrown with brush and trees. Other outcrops are obscured by housing developments.
I brought Dr. Easton’s outcrop to the attention of two geologists who have worked on the Nepean Formation and March Formation and sent them the photos. One commented “The disconformable contact with the March/Theresa is very interesting — we generally only see that kind of a relationship in association with a fault.” The other noted that “A disconformable contact at the “Nepean-March” ... formation boundary has been a contentious issue over the years.”
The outcrop is worth a visit if you are interested in the Nepean Formation of the Potsdam Group.
Below I’ve provided a few of the more recent commentaries on Potsdam sandstone. I’ve also included two papers by Bernius who reported on core from the GSC’s Borehole Geophysics Test Area at Bell’s Corners, Ottawa. His papers have been overlooked What is particularly interesting is that the core contains a 50 cm thick shale layer in the Nepean Formation. Bernius also found that the upper contact of the Nepean formation with the March Formation is a disconformity.
REFERENCES AND SUGGESTED READING
Bernius, G. R., 1981,
Boreholes Near Ottawa for the Development and Testing of Borehole Logging Equipment - A preliminary Report GSC Paper 81-1C, p. 51-53
Bernius, G. R., 1996,
Borehole Geophysical Logs from the GSC Borehole Geophysics test site at Bell’s Corners, Nepean, Ontario, GSC Open File 3157, 38 pages, doi:10.4095/207617 (pdf 6427 KB)
Brett, C. P., 2016
Gypsum Pseudomorphs that formed in the Sabkha Environment of the Potsdam Group. Blog posting December 1, 2016
Brett, C. P., 2017
Why has hardly anyone referred to core from the GSC’s Borehole Geophysics Test Area at Bell’s Corners, Ottawa, when the core contains a 50 cm thick shale layer in the Nepean Formation and the core straddles the boundary between the Nepean Formation and the overlying March Formation?” Blog posting dated May 22, 2017
Brink R, Mehrtens C, Maguire H, 2019
Sedimentology and petrography of a lower Cambrian transgressive sequence: Altona Formation (Potsdam Group) in northeastern New York. Bulletin of Geosciences, 94, 369-388
Dix, G.R., Salad Hersi, O., and Nowlan, G.S., 2004,
The Potsdam–Beekmantown Group unconformity, Nepean Formation type section (Ottawa, Ontario): a cryptic sequence boundary, not a conformable transition: Canadian Journal of Earth Sciences, v. 41, no. 8, p. 897–902, https:// doi .org /10 .1139 /e04 -040 .
Dugas, Jean, 1952,
Geology of the Perth map area, Lanark and Leeds Counties, Ontario; Ph. D., McGill, 189 pages, four maps.
Easton, Michael, 2017
Precambrian and Paleozoic Geology of the Carleton Place Area, Grenville Province, pages 18-1 to 18-18 In Summary of Field Work and Other Activities 2017, Ontario Geological Survey, Open File Report 6333,
Landing, E., Salad Hersi, O., Amati, L., Westrop, S.R., Franzi, D.A., 2019
Early Paleozoic rifting and reactivation of a passive-margin rift: Insights from detrital zircon provenance signatures of the Potsdam Group, Ottawa graben: Comment. GSA Bulletin; March/April 2019; v. 131; no. 3/4; p. 695–698; https://doi.org/10.1130/B35104.1; published online 25 January 2019.
Lowe, David G., 2016
Lower Ordovician Potsdam Group in the Ottawa Embayment and Provenance of the Cambrian – Lower. Ordovician Potsdam Group in the Ottawa. Embayment and Quebec Basin. David G. Lowe. Doctoral Thesis submitted to the University of Ottawa
Lowe, David G, and R.W.C. Arnott, 2016
Composition and Architecture of Braided and Sheetflood-Dominated Ephemeral Fluvial Strata In the Cambrian–Ordovician Potsdam Group: A Case Example of the Morphodynamics of Early Phanerozoic Fluvial Systems and Climate Change
Journal of Sedimentary Research, v. 86, i. 6, p. 587-612, Published in June 2016, doi:10.2110/jsr.2016.39
Lowe, D.G., Arnott, R.W.C., Chiarenzelli, J.R., and Rainbird, R.H., 2018,
Early Paleozoic rifting and reactivation of a passive-margin rift: Insights from detrital zircon provenance signatures of the Potsdam Group, Ottawa graben: Geological Society of America Bulletin, v. 130, no. 7/8, p. 1377–1396, https:// doi .org /10.1130 /B31749 .1 .
Lowe, D.G., Arnott, R.W.C., Chiarenzelli, J.R., and Rainbird, R.H., 2019,
Early Paleozoic rifting and reactivation of a passive-margin rift: Insights from detrital zircon provenance signatures of the Potsdam Group, Ottawa graben: Reply. Geological Society of America Bulletin, March/April 2019, v. 131, no. 3/4, pages 699-703; published online January 25, 2019. https://pubs.geoscienceworld.org/gsa/gsabulletin/article/131/3-4/699/568492/Early-Paleozoic-rifting-and-reactivation-of-a
Lowe, David G.; Arnott, R.W.C.; Nowlan, G.; McCracken, A.D., 2017
Lithostratigraphic and allostratigraphic framework of the Cambrian-Ordovician Potsdam Group and correlations across Early Paleozoic southern Laurentia. Canadian Journal of Earth Sciences 2017, 54(5): 550-585, https://doi.org/10.1139/cjes-2016-0151
Williams, D.A., and Wolf, R.R., 1984a
Paleozoic Geology of the Carleton Place Area, Southern Ontario; Ontario Geological Survey, Map P. 2725, Geological Series-Preliminary Map, scale 1:50 000. Geology 1982.
Williams, D.A., and Wolf, R.R., 1984b
Paleozoic Geology of the Perth Area, Southern Ontario; Ontario; Geological Survey, Map P. 2724, Geological Series-Preliminary Map, scale 1:50 000. Geology 1982.
Wilson, Alice E., Liberty, B. A., and Reinhardt, E.W., 1964,
Geology Carleton Place, Ontario. Map 7-1964, Preliminary Series, Geological Survey of Canada. Scale 1:50,000 Paleozoic geology by A. E. Wilson, 1946. Paleozoic compilation by B. A. Liberty, 1963. Precambrian geology and compilation by E. W. Reinhardt, 1963.
Wilson, Alice E., Liberty, B. A., and Reinhardt, E.W., 1972,
Geology Carleton Place, Ontario. Map 1362, Geological Survey of Canada. Scale 1:50,000
Paleozoic geology by A. E. Wilson, 1946. Paleozoic compilation by B. A. Liberty, 1963, with changes and additions by E. W. Reinhardt, 1972. Precambrian geology and compilation by E. W. Reinhardt, 1963, 1969, 1972
Wilson, Morley E. and Dugas, Jean, 1961,
Map 1089A, Geology, Perth, Lanark and Leeds Counties, Ontario, Geological Survey of Canada; Geology by Morley E. Wilson, 1930 and Jean Dugas, 1949; Descriptive notes by Jean Dugas.