Above I’ve provided photographs of brightly coloured and patterned beds of rock together with photographs of specimens from those beds and underlying beds. These pink banded beds can be found in outcrops along Highway 7 just north of Perth where the rocks weather grey but the pink colour and the banding in the rocks can still be seen. Fresh outcrops are found in the Tackaberry aggregate quarry on Highway 7 about a five minute drive north of Perth. The most brightly coloured of the beds lie close to the top of the sequence of rocks in the quarry, and are at the north end of the quarry. This outcrop near the top of the quarry changes with every visit, as the area appears to be being cleared for blasting. The best specimens were obtained from this location in the quarry, but samples can also be found in the blast piles throughout the quarry.
The photos show a number different features: polygonal cracks (likely mud/desiccation cracks) on the surface of beds; clearly visible coloured chemical banding that cuts across the bedding planes, that was likely caused by diagenetic processes involving the circulation of subsurface waters, and is probably Liesegang banding and Liesegang rings; and possible soft-sediment deformation. The most striking colour variation in the rocks is due to the polygonal cracks and the chemical banding. However, there are patterns in the rocks that to me suggest soft sediment deformation. It has been suggested to me by a geologist with more knowledge of sedimentary rocks than I have that I should "examine the outcrop very carefully in cross-section, looking for convincing examples where grain-size changes ... outline the ‘deformation’ features" before I can reach the conclusion that there is soft sediment deformation. I would be interested in any reader’s comments on whether the photos show this feature.
Liesegang banding is an interesting term. To geologists it means irregular concentric yellow to orange to red to brown banding and rings in rocks where the banding and rings represent the precipitation lines of iron rich and manganese minerals following the infiltration of ground waters, with the families of bands or rings separated in the direction perpendicular to the diffusion of the ground water. There are frequently different sets of precipitation bands oriented in different directions, often cross-cutting the bedding planes, often cross-cutting older dissolution patterns, and sometimes obscuring the sedimentary structures. In some rocks the banding represents numerous precipitation events over long periods of time.
The rocks along this stretch of Highway 7 are mapped by the Ontario Geological Survey as March Formation (in Quebec and New York State, Theresa Formation) and the sequence of rocks in the quarry truly represents the transition or passage beds of Sir William Logan. (Anything goes.) It would be great if the there is soft sediment deformation in the rocks. Some of my favorite papers on the geology of Eastern Ontario deal with the faulting along the St. Lawrence River and along the Ottawa River, and the theories of the St. Lawrence Rift Valley and the Ottawa-Bonnechere Graben that are evidenced by the faults along the two rivers. Soft sediment deformation, particularly seismites (sedimentary beds disturbed by siesmic waves from earthquakes) would support movement along those faults during the late Cambrian and early Ordovician and would be worth documenting. Hopefully, someone with access to thin sections and a scanning electron microscope will look at the beds before they are quarried out.
Anyone wishing to study the pink banded beds and wanting access to the best specimens would be wise to collect them early next summer, as the overburden is being cleared off, which is the step before blasting. I made the comment to an employee at Tackaberry that "rocks don’t move" and was corrected that "they do at this quarry". Here today, crushed tomorrow.
Christopher Brett
Perth, Ontario
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