Andrew’s Outcrops

Andrew is one of the many polite teenagers that Lanark County produces by the bushel load.   He also has a keen interest in geology.  This past weekend I took the opportunity to visit outcrops that Andrew had been telling me about.  

Most of the outcrops that Andrew showed me consist of flat lying sandstone exhibiting paired vertical burrows on the top surface.   I expect that everyone would identify this sandstone as a shallow marine facies of the Nepean (upper Keeseville) Formation of the Potsdam Group.   Below are photographs of the top surfaces of  two such sandstone outcrops, each showing paired vertical burrowing.

One outcrop stood out as being particularly impressive and is likely worth a second visit.  Below are photographs of this outcrop.This outcrop can be divided into three parts, with at least two visible unconformities (separating layers 1 and 2; and separating layers 2 from 3).  Starting from the base you notice:
1.)  at least 30 vertical feet of a silicious quartz cobble conglomerate; 
2.)  followed by a foot of sandstone with layers of quartz pebbles and a pitted layer (that may represent eroded evaporite minerals?);
3.)  with a top layer showing bedded  sandstone cut by vertical burrows, some of which extend down into the middle layer (perhaps evidencing the transgression of Cambrian/Ordovician seas and the reworking of fluvial sandstone?).

The two lower layers are of continental/fluvial origin, while the upper layer is of shallow marine origin.  

The first three photographs show the basal quartz cobble conglomerate.   The fourth photograph shows two unconformities, with the basal quartz cobble conglomerate at the bottom of the photo.   The fifth photo shows vertical burrowing and an uncomformity.  The sixth photo shows vertical burrowing in the top layer (shallow marine sandstone), overlying what appears to be two fluvial events: a pebble layer, over  sandstone, over a pebble layer, over sandstone (with the pebble layers possibly being lag deposits formed by aeolian deflation?).

           
I expect that everyone would assign the top burrowed bed to a shallow marine facies of  the Nepean (upper Keeseville) Formation.    Most people would likely  assign the quartz cobble conglomerate bed  to the Covey Hill (Ausable) Formation of the Cambrian/Ordovician Potsdam Group, with some assigning it a Precambrian age.   The middle bed is likely a fluvial facies of the Nepean (lower Keeseville) formation.

Andrew also showed me a mica mine in the Precambrian rocks and a lichen covered, deeply fractured, sandstone ridge that lies on an adjoining property

The photographed outcrops fall within Lot 7 of Concession VIII in North Burgess Township (now Burgess Ward of Tay Valley Township), about four miles (6.5 kilometers) south of Perth, and west of the intersection of Stanley Road with the road to McLaren Lake.   The lichen covered sandstone ridge can be found east of the road to McLaren Lake, on Lot 8 of Concession VIII.

If you were one of the original settlers who had been granted the land upon which Andrew’s parents have their house, or granted one of the adjoining lots, I suspect you’d have complained about the lack of soil.  If you have an interest in geology, which Andrew has, you’d be pleased to be living there.

Christopher Brett
Perth, Ontario   

Suggested Reading:
David G. Lowe, R.W.C. Arnott, Godfrey S. Nowlan, A.D. McCracken, 2017
Lithostratigraphic and allostratigraphic framework of the Cambrian–Ordovician Potsdam Group and correlations across Early Paleozoic southern Laurentia; Canadian Journal of Earth Sciences, Published on the web 6 February 2017,    doi: 10.1139/cjes-2016-0151

The Cylindrical Structures in Lanark County reported in Jean Dugas’ 1952 Doctoral Thesis

In my  August 27, 2015 blog posting I mentioned that Jean Dugas,in the notes to geological  Map 1089A, reported that cylindrical or conical structures in sandstone can be found in North Elmsley Township,  Lanark County.   He commented:

“Peculiar bands showing cylindrical or conical structures noted in the Nepean formation, and best observed on lot 24, Con. VI, North Elmsley tp., are the same composition as the surrounding sandstone , but cut sharply across the beds and are themselves bedded parallel with the walls of the structures.  They are probably formed by slumping of the sand due to water action.”

Morley E. Wilson and Jean Dugas, 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.
                 
In my August 27, 2015 blog posting I also mentioned that I had not been able to find  Dugas’ occurrence on Lot 24 or his thesis.

I  recently located and read Jean Dugas’ doctoral thesis:

Dugas, Jean, 1952,
 Geology of the Perth map area, Lanark and Leeds Counties, Ontario; Ph. D., McGill, 189 pages, four  maps.          

In his thesis he includes five clear  photographs of the conical and cylindrical structures, and good written descriptions of the structures, together with a review of the leading papers discussing theories for their origin.  He concludes that his structures, because of their conical shape, were formed by slumping, and suggests that “as most of the structures are underlain by [crystalline] limestone and it is common to find solution cavities in the [crystalline] limestone” the slumping was likely into cavities in the limestone.

Interestingly, while the the captions to his photographs place the structures on lot 24, the  text of Jean Dugas’ thesis mentions that the structures are found on lot 23.   Both the text and the captions place the structures in concession VI.  In the text of his thesis he comments (at page 99):
      
"Good exposures of an unusual structure in the Nepean sandstone can be seen on lot 23, concession VI of North Elmsley township. A section of the sandstone shows a decisive break in bedding. Across the stratification, two conical structures occur with the apex down, about six feet high and five feet at the base of the cone (cf. photos nos. 29,  30,  31, p. 100). There is a distinct irregular banding, the bands being slightly twisted (cf. photos nos. 31, p. 100, and 32, p. 101). In horizontal section the structure is obviously concentric, forming a circle or a spiral (cf. photo. no. 33 , p. 101).  Though no other exposures of vertical sections have been found, concentric layers were observed at other places in the sandstone.”

Lots 23 and 24 of Concession VI in North Elmsley Township are a few kilometers west of Rideau Ferry and lie south of County Road 1.   Perhaps armed with the photographs I will have better luck locating the outcrops, particularly if I look in both lots.

Christopher Brett
Lanark County

A Possible Conical Fossil Near the Base of the Potsdam Group

In my blog posting for  March 2, 2017 I reported on a sandstone outcrop in Lanark County on the north side of Highway 7 approximately 5 km west of Wemyss.  This is an outcrop first reported by Dr. Easton (2015).   For that blog posting I attached three photos.   The second photo shows siltstone/mudstone layers draped over the Grenville marble and underlying a friable layer; overlain by massive  sandstone beds.

While at the outcrop I collected a loose specimen that had obviously fallen off the outcrop.  The specimen is comprised of the siltstone/mudstone layers and the friable layer.  Below are two photographs of that specimen.

In the first photograph I’ve placed a magenta box around the texture that might be a fossil.  I believe it to be a fossil (rather than dessication cracks)  because the two sides of the item are the same width and arguably represents the cross-section of a conical shape.     Further, the conical shape also arguably ends at an upper cap like shape.

In Eastern Ontario the base of the Potsdam Group is believed to be middle Cambrian in age. 
There are  numerous conical shaped fossils in the Cambrian (e.g., Volborthella, cephalopods, hyoliths), and the specimen may not be distinct enough to be identified.    

Below I’ve provided a reference to a fairly recent paper by Hagadorn  and Waggoner (2002) that is available over the internet and  which contains a discussion of the fossil Volborthella.

I’ve also provided two references to a very recent paper on hyoliths.

Anyone wanting the specimen for research purposes should send me an email.  Unfortunately it is one of those specimens where every time you pick it up another piece falls off.

Christopher Brett
Perth, Lanark County

References

Bettam, S.,  2017
U of T undergrad leads team of paleontologists, classifying mysterious ancient cone-shaped sea creatures, U of T News, Global Lens Breaking Research, January 11, 2017
https://www.utoronto.ca/news/u-t-undergrad-leads-team-paleontologists-classifying-mysterious-ancient-cone-shaped-sea
          
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  

Hagadorn, J.W., and Waggoner, B.M., 2002
The Early Cambrian problematic fossil Volborthella: New insights from the Basin and Range, in F. A. Corsetti, ed.,  Proterozoic-Cambrian of the Great Basin and Beyond, Pacific Section  SEPM Book 93, p. 135-150.

Moysiuk, J.,  Smith, M. R.  and  Caron, J.-B., 2017
Hyoliths are Palaeozoic lophophorates,  Nature 541,   394–397  (19 January 2017)   doi:10.1038/nature20804
http://www.nature.com/nature/journal/v541/n7637/abs/nature20804.html