18. Parc du lac Beauchamp, Québec
Precambrian-Cambrian contact
Parc du Lac Beauchamp, Québec
Kilometre 0.6 on the park road to Lac Beauchamp, off Blvd. Maloney, Gatineau, PQ.
Unconformity between Nepean Formation quartz arenite and Precambrian granitic basement rocks. Roadcut, Parc du Lac Beauchamp, on west side of entrance road, at westward turn in the road, beside the lake.
Photo by Q. Gall.
Grenville gneiss, granite and quartzite showing typical crystalline texture, foliation, joints and fractures are unconformably overlain by Nepean sandstone, represented at this site by silica-cemented quartz arenite. The unconformity can be seen at a roadcut on the west side of the entrance road, at the westward turn in the road, beside the lake.
The Grenville basement rocks are capped by a regolith (paleosol), produced by a combination of ancient mechanical & chemical weathering. Distinctive spheroidal weathering immediately below the unconformity is regarded as the product of pre-Nepean weathering of the Precambrian land surface. Rare pebbles of vein quartz occur in thin lenses at the base of the Nepean.
Closeup of unconformity showing relict weathering at top of Precambrian granitic. Roadcut, Parc du Lac Beauchamp, on west side of entrance road, at westward turn in the road, beside the lake.
Photo by Q. Gall.
In the Nepean sandstone, the framework of well-rounded grains of quartz plus rare grains of chert and feldspar can be seen with a hand lens. These thick-bedded deposits were quarried for buildings in the early settlements of Hull (now Gatineau) and Bytown (now Ottawa). Asymmetric ripple marks are evident on the tops of some beds, and large-scale crossbedding reflects deposition as subaqueous dunes (the low angle of repose rules out deposition by wind).
Dune crossbedding can be seen in the outcrop east of an abandoned quarry (now an unpaved parking area), immediately south of Lac Beauchamp.
In the sandstone to the west of the unconformity exposure, near-vertical subcylindrical trace fossils (evidence, without the body, of animal or plant activity) are locally abundant. Some display prominent flared circular, ovoid and tear-drop-shaped burrow openings best seen on the surfaces of the largest dune units.
A minor amount of sulphides in the quartz arenites is responsible for the widespread iron oxide staining. Such staining is particularly strong on dimpled bedding surfaces which are possible relics of degraded cyanobacterial mats that served to concentrate/precipitate iron sulphides or iron oxides during quiet episodes.
Glacial polish, striae (parallel linear scratches) and chatter marks (small crescentic grooves) as well as sinous features eroded by sub-glacial hydraulic scour are apparent at the top of most outcrops of Nepean sandstone at this locality.