12. Parc Brébeuf, Gatineau
Fossiliferous limestone bedrock and glacial erratics
Parc Brébeuf, Gatineau
Rue Begin at Rue Bourget, Gatineau, PQ.
Note: Do not collect fossils.
Pebbles and crushed stone have been arranged in distinctive patterns in the walkways around the Brébeuf statue. The tapered base of this statue contains at least 500 cobblestones that are representative of the many rock types (mainly igneous and metamorphic) within the Grenville Province. The clean polished surfaces of these cobbles, rounded and smoothed by abrasion during transportation by glacial ice and meltwater before deposition in a gravel pit, provide an opportunity to study the mineralogy, textures and structures (foliation, lineation, intrusive contacts) typical of Precambrian terrane to the north.
Beds of Ordovician Limestone of the Ottawa Group are well exposed along the river shoreline. These strata contain abundant corals, stromatoporoids, cephalopods, gastropods, mollusks, brachiopods, bryozoa, rare small fragments of trilobites and trace fossils (evidence, without the body, of animal or plant activity). Limestone blocks in the retaining walls provide fine examples of the range of textures in clastic limestones: from fine- to very coarse-grained calcarenite. Primary structures include bedding, crossbedding, ripple marks (symmetric, asymmetric and interference ripple marks) and desiccation cracks.
Stratigraphy - some fossil assemblages are restricted to certain beds, reflecting marked changes in depositional environment. The lowermost limestone beds, exposed to the west along the shore of the Ottawa river, contain abundant mollusks, bryozoa and scattered patches of brachiopods. Overlying beds present excellent displays of Tetradium (tabulate coral). The next distinctive unit presents a profusion of mushroom-shaped stromatoporoids and tabulate corals, many detached and jumbled, recording a major storm event. Trace fossils in association with branched bryozoa characterize the uppermost beds of this 4-metre-thick section. These traces, abundant through at least 1.5 metres of limestone below the stone retaining walls, show a wide variety of patterns and geometries, with resemblances to numerous trace fossils such as Chondrites, Palaeophycus, Phycodes, Planolites and Thalassinoides.
Secondary structures include joints (fractures) and stylolites (non-structural fractures); these prominent features are accentuated by weathering. Several of the large stone steps, in the most westerly walkways down to the shore, display small hemispherical colonies of tabulate corals and byrozoa (Prasopora?).
Walk eastward to a large boulder of Precambrian gneiss marking the Voyageurs Portage. This boulder shows excellent folded foliation in three-dimensions, providing an example of the style of deformation that is characteristic of the Grenville Province. Immediately north, Rue Bourget has a curb along the south side consisting of two to three rows of stone paving blocks. Most are Nepean sandstone (set both on edge and parallel to bedding), but igneous rocks are also represented, including granite, syenite and diorite. Some sandstone blocks display Liesegang banding, and most are deeply iron stained (some may have been derived from the abandoned quarry at Lac Beauchamp ). Walk back along Rue Bourget to the parking area, proceeding east along the bike path (look out for speeding cyclists and rollerbladers). Just beyond the northeast fenced corner of the transformer station, take the last gravel footpath to the east (just before the T- junction in the bike path) out to the south shore of a small inlet on the Ottawa River. Excellent views of folds in limestone beds can be seen along the northeastern shore of this inlet.