Dr. Easton describes the outcrop as follows:
“A previously unknown exposure of Covey Hill Formation strata, or possibly the Abbey Dawn Formation of Sanford and Arnott (2010), was identified on the north side of Highway 7 approximately 5 km west of Wemyss (385510E 4967646N). Here, a channel, at least 3 m deep and up to 20 m wide, was cut down into weathered calcite marble. The deeper (>3 m) west side of the channel is filled with red-weathering pebbly conglomerate containing subangular to subrounded rock fragments and quartz pebbles, which grade upward into thin-bedded reddish coarse sandstone. The shallower (~2 m) east side of the channel consists of red mudstone and siltstone beds (up to 1 cm thick) that immediately overlie the marble basement, and which are overlain, in turn, by thin-bedded reddish coarse sandstone.”
I visited the outcrop last year. Fifty percent of the outcrop is marble. I noted that Dr. Easton had described the sandstone and conglomerate at the center and east end of the outcrop. A small amount of sandstone/mudstone also occurs at the west end of the outcrop in cracks in the marble. There are vugs in the marble filled with calcite crystals. There is a small hematite gossan at the west end of the outcrop and a larger example in the center of the outcrop. At least two faults cut the outcrop. I would not have identified the sandstone/conglomerate as the Covey Hill Formation strata or the Abbey Dawn Formation. The sandstone is more likely the Hannawa Falls Member of Sanford and Arnott (2010) /the Hannawa Falls Formation of Lowe (2016).
The most interesting feature of the outcrop is the beds of siltstone/mudstone at the base of the sandstone, which drape over the marble and underlie a thin friable layer comprised of pea sized pebbles of marble.
Attached are three of the photos that I took. The first shows sandstone fissures in the marble at the west end of the outcrop. The second photo shows siltstone/mudstone layers draped over the marble and underlying a friable layer; overlain by massive sandstone beds. The base of blue ruler is along the contact with the Grenville marble. The third photo shows the hematite gossan and possibly a fault.
There have been numerous reports in the literature of tongues of sandstone extending into the underlying marble. Where the sandstone is described it is invariably a dark red sandstone, possibly what we would now identify as the eolian Hannawa Falls Formation of the Potsdam Group. Below are a few of the less often cited reports:
Helmstaedt, Gorman & McBride (1987) in a field trip guide for the Kingston, Ontario area mention a “paleokarst cave” near the entrance to the Portland Conservation Area, on the east side of Highway 38, about 2 km south of Verona, noting that a “Cut on east side of road shows a paleokarst cave in Precambrian marble filled with sandstone breccia of Nepean Formation (Fig. 11). Sandstones with local cross-bedding extend as horizontal ‘tongues' into the marble.” [Note: this guide appears to have been written when all of the sandstones of the Potsdam Group were referred to in Ontario as Nepean Formation.]
Smyth (1893, at page 104) reports that “North of Gouverneur [New York] the [crystalline] limestone and sandstone are in direct contact... From the irregular line of contact it is clear that the material of the sandstone was deposited upon a [crystalline] limestone surface that had been subject to erosion. An interesting confirmation of this conclusion is seen in the presence of narrow, irregular cracks extending several feet into the [crystalline] limestone and filled with sandstone.”
Winchell (1893, pages 107-108) reports on a trip to look at the Potsdam sandstone in Upper New York State. He also reports on hematite ore bodies in marble noting that for many occurrences the ore lies “at the base of the Potsdam.” For outcrops near Richland, New York, he noted that “The sandstone grades into soft hematite, which appears like a good ore. The upper surface of the marble is fissured in places, and the hematitic material of the sandstone extends down into these fissures (fig. 7).”
Cushing et al (1910) reported on the Geology of the Thousand Islands Region, New York State. They reported a “dark red, very thoroughly indurated and vitreous sandstone that differs from the general run of sandstone in the district” and that “all the sand-filled cracks seen in the Grenville [crystalline] limestone were filled with this type of sandstone...”.
Cushing and Newland (1923) report on sand filled cracks in marble mentioning that outcrops “may be seen in several places in the Gouverneur quadrangle” and that “on the bared [crystalline] limestone surfaces the red sandstone stands out in relief”.
Professor Bruce Selleck (2005) of Colgate University in a field trip guide mentions “Sand from the Potsdam is also found within open fractures and filling hydrothermal karst tunnels and pipes [in marble]. These fillings are often deeply colored red or maroon by abundant hematite cement, and the sand is usually tightly cemented by quartz and carbonate minerals, but rounded sand grains can usually be seen with a hand lens. Some of the conglomerates within the Potsdam contain chert clasts that are the result of silicification of marble clasts. Pebbles of jasper and clasts of laminated sandstone that had been silica-cemented, reworked and re-deposited are also present in the conglomerate and pebbly sandstone beds.
Professor Selleck commented on the association of hematite and the Potsdam as follows:“The hematite deposits also share the common presence of nearby or directly overlying inliers of Potsdam Sandstone (Chamberlain 1984). The paragenesis of the hematite deposits is generally interpreted as multi-stage with pre-Potsdam surface weathering of Proterozoic iron sulfide leading to accumulation of locally thick gossans of limonite/hematite prior to Potsdam Sandstone deposition (Chamberlain 1984). Post-Potsdam reconstitution of the iron oxides involved hydrothermal fluids that dissolved and re-deposited hematite within Potsdam Sandstone as thick botryoidal masses, specular crystalline aggregates, veins and disseminated cements in sandstone and highly altered Proterozoic basement gneiss.” There are too many other reports of hematite at the Potsdam/Grenville marble interface to mention.
Christopher Brett
Perth, Ontario
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References
Cushing, H.P., Fairchild, H.L., Ruedemann, R. And Smyth, C.H. Jr., 1910
Geology of the Thousand Islands Region, New York State Museum Bulletin No. 485 at pages 62-63
Cushing, H.P., and Newland, D. H., 1925
Geology of the Gouverneur Quadrangle, New York State Museum Bulletin No. 259 at page 49 and Plate 11
R. M. Easton, 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
Helmstaedt, H.H., Gorman W.A. & McBride, S.L. 1987
Field Tripping: Geology of the Kingston Area,
By the Department of Geological Sciences, Queen's University, Kingston,
www.whaton.uwaterloo.ca/waton/s906.html
Lowe, D.G., 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
Sanford, B.V. and Arnott, R.W.C., 2010
Stratigraphic and structural framework of the Potsdam Group in eastern Ontario, western Quebec, and northern New York State. Geological Survey of Canada, Bulletin 597, 85 pages
publications.gc.ca/collections/collection_2010 /nrcan/M42-597
Selleck, B., 2005
Exploring the root zone of an ancient fault-driven hydrothermal system in the Adirondack Lowlands, New York; NYSGA Field Trip Guidebook, 77th Annual Meeting, 12-31
Smyth, C. H., Jr., 1893
Geological Reconnoissance in the Vicinity of Gouverneur, N. Y.; Transactions of the New York Academy of Sciences, Vol. XII, pages 97-108 at 104
Winchell, N. H., 1893
The Potsdam sandstone at Potsdam, New York; in Field Observations of N. H. Winchell in 1892;
The Geological and Natural History Survey of Minnesota, Twenty-first Annual Report for the year 1892, pages 99- 111
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