This past summer I’ve been collecting and photographing trace fossils in the sedimentary rocks of Lanark County, and the adjacent counties, in Eastern Ontario. The sedimentary rocks in Lanark County are generally considered to be of Upper Cambrian to Lower Ordovician in age. The fossils that I found fall within three main groups of rocks:
1) Fossils from the Potsdam Group/ Nepean Sandstones
2) Fossils from the March Formation (Mostly Sandstone and Dolostone, but also some shale)
3) Fossils found in rocks in Drummond Township that are mapped as the March Formation, but appear to be much older (herein, the Drummond Sequence) - Mostly Sandstone and Carbonate, but also shale and mudstone.
Group 1 - Fossils of the Potsdam Group/ Nepean Sandstones (First Two Photos)
There are numerous examples of vertical worm burrowing in sandstone outcrops within a fifteen minutes drive of Perth, Ontario, including paired burrows (Diplocraterion, Arenicolites) and single burrows (Skolithos). In addition I found a few example of horizontal burrowing.
Good examples of vertical worm burrows can be seen at outcrops at
1.1 Dodds & Erwin parking lot, 2870 Rideau Ferry Road, County Road 1, in Perth.
1.2 Waste Management Site, 666 Wild Life Road - Southern boundary of Perth
1.3 Bowes Side Road, just south of where it crosses the Tay River - 10 minutes west of Perth
1.4 South West Corner of Narrows Lock Road (County Road 14) and Powers Road - 15 minutes Southwest of Perth
Group 2 - March Formation (third photo)
My best specimens from the March Formation have been sampled from the Phillipsville bluff, which is just south of the Town of Phillipsville, in the Township of Rideau Lakes, United Counties of Leeds and Grenville, about an hour southwest of Perth. The bluff has been mapped both as Nepean Formation and as March Formation, but most often as March Formation.
The bluff is about 20 meters high at its maximum height and runs for a few hundred meters along Hughes Road. It is mainly composed of horizontal beds of sandstone which display vertical worm holes and bioturbation. There are a few easily accessed one cm to five cm thick beds of shale and dolostone where I found a few fossils of the brachiopod Lingula and abundant fossil shards (that I assume to be of same brachiopod). Trace fossils found in the shale beds are of three main shapes: pyramids (photo 3) , bottle caps and egg shapes. The pyramid shapes are possibly tracks, while the egg shapes might be resting traces.
Group 3 - Drummond Sequence (last four photos)
Directly north of Perth (in Concessions III, IV, V, etc. in Drummond Township) is an area that has most recently been mapped as March Formation, but the rocks differ from other areas that have been mapped as March Formation. The block is bounded by faults that separate it on the west from Nepean Sandstone, on the southeast from March Formation, and on the Northeast from Nepean Sandstone. It is, admittedly, inter-bedded carbonate and sandstone, the key characteristics of March Formation. However, I have not yet found any shelled fossils in the Drummond Sequence, the trace fossils I have found differ from those that I have found in the Nepean Formation and in beds that I accept as March Formation, and the Drummond Sequence does not look like typical March Formation.
Factors that separate the Drummond Sequence from March Formation include:
1. Absence of shelled fossils.
2. Abundant, multicoloured, desiccation cracks in sandstone, carbonate and shale.
3. The carbonate layers are more brightly coloured, especially on fresh surfaces where the rocks appear as rose & white, and brown & white. While the carbonates weather grey, the rose streaks can be seen on the weathered surfaces, and the rock can be mapped on this basis.
4. Abundant horizontal burrowing.
5. The more abundant range and selection of trace fossils that I have found (including, many of which I cannot identify).
In addition, Sir William Logan found Climactichnites and Protichnites at a quarry in Lot 6, Concession 3, Drummond Township that falls within the Drummond Sequence. Both Climactichnites and Protichnites are thought to be Late Cambrian, Potsdam Group fossils that have not been found in the March Formation.
It is possible that Drummond Sequence is a thin veneer of March Formation carbonates overlaying Nepean Sandstone. However, the trace fossils I’ve been finding suggest that it might be older than Upper Cambrian, the age of the Nepean Formation of the Potsdam Group.
Regards,
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